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  • 101.
    Hausmaninger, Thomas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Silander, Isak
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Doppler-broadened mid-infrared NICE-OHMS system based on an optical parametric oscillator2016In: Laser Applications to Chemical, Security and Environmental Analysis, 2016Conference paper (Refereed)
    Abstract [en]

    An OPO-based NICE-OHMS instrument for trace gas detection addressing fundamental vibration transitions in the mid-IR (3.2-3.9 µm) range has been developed. It shows a detection sensitivity for CH4 of 2.4×10−10cm−1Hz−1∕2 (corresponding to low ppt concentrations).

  • 102.
    Hausmaninger, Thomas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Silander, Isak
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry in the mid-IR region down to 10−10 cm−1 Hz−1/2Manuscript (preprint) (Other academic)
  • 103.
    Hausmaninger, Thomas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Silander, Isak
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics. Shanxi University, Taiyuan 030006, China.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Doppler-broadened NICE-OHMS beyond the cavity-limited weak absorption condition – II: experimental verification2016In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 168, p. 245-256Article in journal (Refereed)
    Abstract [en]

    Doppler-broadened (Db) noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) is normally described by an expression, here termed the conventional (CONV) description, that is restricted to the conventional cavity-limited weak absorption condition (CCLWA), i.e. when the single pass absorbance is significantly smaller than the empty cavity losses, i.e. when α0L<<π/F. To describe NICE-OHMS signals beyond this limit two simplified extended descriptions (termed the extended locking and extended transmission description, ELET, and the extended locking and full transmission description, ELFT), which are assumed to be valid under the relaxed cavity-limited weak absorption condition (RCLWA), i.e. when α0L<π/Fα0L<π/F, and a full description (denoted FULL), presumed to be valid also when the α0L<π/Fα0L<π/F condition does not hold, have recently been derived in an accompanying work (Ma W, et al. Doppler-broadened NICE-OHMS beyond the cavity-limited weak absorption condition - I. Theoretical Description. J Quant Spectrosc Radiat Transfer, 2015, http://dx.doi.org/10.1016/j.jqsrt.2015.09.007, this issue). The present work constitutes an experimental verification and assessment of the validity of these, performed in the Doppler limit for a set of Fα0L/πFα0L/π values (up to 3.5); it is shown under which conditions the various descriptions are valid. It is concluded that for samples with Fα0L/πFα0L/π up to 0.01, all descriptions replicate the data well. It is shown that the CONV description is adequate and provides accurate assessments of the signal strength (and thereby the analyte concentration) up to Fα0L/πFα0L/π of around 0.1, while the ELET is accurate for Fα0L/πFα0L/π up to around 0.3. The ELFT description mimics the Db NICE-OHMS signal well for Fα0L/πFα0L/π up to around unity, while the FULL description is adequate for all Fα0L/πFα0L/π values investigated. Access to these descriptions both increases considerably the dynamic range of the technique and facilitates calibration using certified reference gases, which thereby significantly broadens the applicability of the Db NICE-OHMS technique.

  • 104.
    Hausmaninger, Thomas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Zhao, Gang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Depletion of the vibrational ground state of CH4 in absorption spectroscopy at 3.4 μm in N2 and air in the 1-100Torr range2018In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 205, p. 59-70Article in journal (Refereed)
    Abstract [en]

    A model presented in an accompanying work predicts that mid-IR absorption signals from methane in trace concentrations in various buffer gases detected at pressures in the 1-100Torr range can be reduced and distorted due to depletion of the vibrational ground state if the molecules are exposed to laser powers in the tens of mW range or above. This work provides experimental evidence of such depletion in a resonant cavity under a variety of conditions, e.g. for intracavity laser powers up to 2W and for buffer gases of N-2 or dry air, and verifies the applicability of the model. It was found that the degree of depletion is significantly larger in N-2 than dry air, and that it increases with pressure for pressures up to around 10Torr (attributed to a decreased diffusion rate) but decreases with pressure for pressures above 20Torr (caused by an increased collisional vibrational decay rate). The maximum degree of depletion (similar to 80%) was obtained for methane in N-2 at around 15Torr. This implies that absorption spectrometry of methane can experience significant non-linear dependencies on laser power, pressure, as well as buffer gas composition. It is shown that depletion takes place also in (CH4)-C-13, which verifies the applicability of the model also for this isotopologue, and that NICE-OHMS signals detected in absorption phase are less affected by depletion than in dispersion. It was concluded that the absorption mode of detection can provide concentration assessments that are virtually free of influence of depletion for intracavity powers below 0.8 W. 

  • 105.
    He, Hai
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Design of an optical trap and calibration of its trap stiffness using the power spectrum method2016Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    Optical tweezers are used to trap small particles using the radiation pressure. The aim of this experiment was to design an optical tweezers system and calibrate the stiffness of optical tweezers using the power spectrum method. The experimental setup included a trapping system using He-Ne laser, an oil immersion objective (numerical aperture (NA) =1.3) and optics for beam-expansion, a microscope system for observing the micro-sized particles and a data recording and processing system. In this experiment, the free beads were trapped successfully using the optical optical trap and the stiffness of the trap was calibrated by the power spectrum method. The corner frequency was fc= (17 ± 2) Hz in the x direction and fc= (15 ± 2) Hz in the y direction. The stiffnesses of the trap were κx = (2.3 ± 0.2 pN m) in the x direction and κy = (2.0 ± 0.2)pNm in the y direction. The errors come from the the radius of the bead and the temperature of the fluid. By ignoring the noise through the power spectrum, a reliable optical trap was achieved.

  • 106.
    He, Xunjie
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Schlieren photography as a tool for measurement of temperature in a candle plume2017Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    The Schlieren technique is widely used for studying transparent media due to its noninvasion and sensitivity to changes in refractive index. In this report, we make use of a Z-type Schlieren imaging system for measurement of gas temperature distribution in a candle plume. The index of refraction gradients were corresponding to the intensity of each pixel of the image. We get the temperature distribution of air surrounding the candle by the relationship between temperature and refractive index. In the experiment, we obtain the temperature distribution of air surrounding the candle, ranging from 1272 K to 287 K with respect to the radial distance of candle plume. Compare with the measured results from thermal couple, the Schlieren technique for measure temperature of medium is much feasible.

  • 107.
    He, Zhenfei
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Realization of a free-space frequency modulated DFB-laser-based laser for measurement of distances and velocities2015Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    The report describes the realization and performance test for distance and velocity measurement of two frequency modulated continuous wave (FMCW) laser radar setups(referred to as basic and advanced system), both setups are based on free-space-optics.With a different sweep frequency the measurement ability of the free-space system was different. For the basic system, the result of measurement was better with 100Hz sweep frequency while the 50 Hz was the most suitable sweep frequency of the advanced system to measure distances and velocities. Comparing the results of measurements, the advanced system was more accurate than the basic system. The standard deviation of the advanced system was smaller than the basic system, but the basic system was easier to construct. For the most accurate results, the smallest shift in the distance we can assess was 0.01 mm for the advanced system and 0.2 mm for the basic system. The velocity resolution was 0.01 mm/s for the basic system and 3x10-3  mm/s for the advanced system.

  • 108. Hedman, Daniel
    et al.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics. Department of Physics, University of California and the Lawrence Berkeley National Laboratory, USA.
    Rosen, Arne
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Larsson, J. Andreas
    On the Stability and Abundance of Single Walled Carbon Nanotubes2015In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, article id 16850Article in journal (Refereed)
    Abstract [en]

    Many nanotechnological applications, using single-walled carbon nanotubes (SWNTs), are only possible with a uniform product. Thus, direct control over the product during chemical vapor deposition (CVD) growth of SWNT is desirable, and much effort has been made towards the ultimate goal of chirality-controlled growth of SWNTs. We have used density functional theory (DFT) to compute the stability of SWNT fragments of all chiralities in the series representing the targeted products for such applications, which we compare to the chiralities of the actual CVD products from all properly analyzed experiments. From this comparison we find that in 84% of the cases the experimental product represents chiralities among the most stable SWNT fragments (within 0.2 eV) from the computations. Our analysis shows that the diameter of the SWNT product is governed by the well-known relation to size of the catalytic nanoparticles, and the specific chirality is normally determined by the product's relative stability, suggesting thermodynamic control at the early stage of product formation. Based on our findings, we discuss the effect of other experimental parameters on the chirality of the product. Furthermore, we highlight the possibility to produce any tube chirality in the context of recent published work on seeded-controlled growth.

  • 109.
    Hedström, Lucas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Classifying the rotation of bacteria using neural networks2019Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Bacteria can quickly spread throughout the human body, making certain diseases hard or impossible to cure. In order to understand how the bacteria can initiate and develop into an infection, microfluidic chambers in a lab environment are used as a template of how bacteria reacts to different types of flows. However, accurately tracking the movement of bacteria is a difficult task, where small objects has to be captured with a high resolution and digitally analysed with computationally heavy methods. Popular imaging methods utilise digital holographic microscopy, where three-dimensional movement is captured in two-dimensional images by numerical reconstruction of the diffraction of light. Since numerical reconstructions become computationally heavy when a good accuracy is required, this master's thesis work focus on evaluating the possibility of using convolutional neural networks to quickly and accurately determine the spatial properties of bacteria. By thorough testing and analysis of state of the art and old networks a new network design is presented, designed to eliminate as many imaging issues as possible. We found that there are certain network design choices that help with reducing the overall error of the system, and with a well chosen training set with sensible augmentations, some networks were able to reach a 60% classification accuracy when determining the vertical rotation of the bacteria. Unfortunately, due to the lack of experimental data where the ground-truth is known, not much experimental testing could be performed. However, a few tests showed that images of high quality could be classified within the expected range of vertical rotation.

  • 110. Heissler, P.
    et al.
    Barna, A.
    Mikhailova, J. M.
    Ma, Guangjin
    Khrennikov, K.
    Karsch, S.
    Veisz, László
    Max-Planck-Institut für Quantenoptik, Garching, Germany.
    Földes, I. B.
    Tsakiris, G. D.
    Multi‑μJ harmonic emission energy from laser‑driven plasma2015In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 118, no 2, p. 195-201Article in journal (Refereed)
    Abstract [en]

    We report on simultaneous efficiency and divergence measurements for harmonics from solid targets generated by the relativistic oscillating mirror mechanism. For a value of the normalized vector potential of aL≃1.5aL≃1.5, we demonstrate the generation of 30 μJ high-harmonic radiation in a 17±317±3 mrad divergence cone. This corresponds to a conversion efficiency of ≳≳ 10−4 in the 17–80 nm range into a well-confined beam. Presuming phase-locked harmonics, our results predict unprecedented levels of average power for a single attosecond pulse in the generated pulse train. Results of PIC simulations raise the prospect of attaining efficiencies of a few percent at higher laser intensities.

  • 111. Henderson, Ben
    et al.
    Khodabakhsh, Amir
    Metsälä, Markus
    Ventrillard, Irène
    Schmidt, Florian M.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Romanini, Daniele
    Ritchie, Grant A. D.
    te Lintel Hekkert, Sacco
    Briot, Raphaël
    Risby, Terence
    Marczin, Nandor
    Harren, Frans J. M.
    Cristescu, Simona M.
    Laser spectroscopy for breath analysis: towards clinical implementation2018In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 124, no 8, article id 161Article in journal (Refereed)
    Abstract [en]

    Detection and analysis of volatile compounds in exhaled breath represents an attractive tool for monitoring the metabolic status of a patient and disease diagnosis, since it is non-invasive and fast. Numerous studies have already demonstrated the benefit of breath analysis in clinical settings/applications and encouraged multidisciplinary research to reveal new insights regarding the origins, pathways, and pathophysiological roles of breath components. Many breath analysis methods are currently available to help explore these directions, ranging from mass spectrometry to laser-based spectroscopy and sensor arrays. This review presents an update of the current status of optical methods, using near and mid-infrared sources, for clinical breath gas analysis over the last decade and describes recent technological developments and their applications. The review includes: tunable diode laser absorption spectroscopy, cavity ring-down spectroscopy, integrated cavity output spectroscopy, cavity-enhanced absorption spectroscopy, photoacoustic spectroscopy, quartz-enhanced photoacoustic spectroscopy, and optical frequency comb spectroscopy. A SWOT analysis (strengths, weaknesses, opportunities, and threats) is presented that describes the laser-based techniques within the clinical framework of breath research and their appealing features for clinical use.

  • 112.
    Hjartarson, Örn
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Separation of lobes in Multispectral Digital Holography2013Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Through a holographic recording a property from the third dimension, the depth, is obtained in the form of a phase map of the incident light. One wavelength holography will have a unique phase for the depth range corresponding to the wavelength of the light and outside this range the real depth can not be resolved. By introducing more wavelengths to the measurement the unique phase combination of the waves will have a wider range and larger objects can be resolved. Up to six wavelengths can be simultaneous recorded by making them occupy different spatial frequencies. A set of spatial frequencies together describing a property of the wave is referred to as a lobe. For more than 6 wavelengths and a larger depth range produced by a more seldom repeated unique phase combination the individual waves will occupy the same frequencies, i.e. the lobes overlap. The separation of overlapping lobes is essential in order to make precise and time independent measurements of large and/or moving objects. To separate the lobes the complex fields, i.e. the phases together with the amplitudes, were simulated to propagate a distance and again recorded. The propagation leads to a phase shift of the spatial frequencies which reveals the complex fields in the case of two overlapping wavelengths. For three overlapping wavelengths the resolution, i.e spatial frequencies describing the object, has to be reduced in order to determine the individual complex fields. Since the propagation is a linear transformation for the frequencies that do not overlap, only the overlapping elements whose propagation is nonlinear produce new information. The new information gained is therefore independent of the number of wavelengths used which limits the exact determination of the fields to two wavelengths.

    Through the holographic recording another property of the complex field is obtained which is the superimposed individual intensities. This bounds the complex fields to certain values, i.e. restricts the possible amplitude of the waves. The recording in the two planes produces two intensity distributions which both must be satisfied by the complex fields. The optimization model for this was formulated and a simple optimization algorithm was implemented. Instead of an equality constraint of the intensities the inequality constraint was implemented, mainly due to that the optimization process was out of the scope of the thesis and the inequality constraint resulted in a simple implementation. The result pointed out important properties even though the optimization could not separate the fields satisfactorily for more than three wavelengths. The inequality constraint contains enough information to solve the case of three overlapping wavelengths.

  • 113.
    Hjältén, Adrian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Modeling the cavity dispersion in cavity-enhanced optical frequency comb Fourier transform spectroscopy2018Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Cavity enhanced optical frequency comb spectroscopy is a technique that allows

    for quick and sensitive measurements of molecular absorption spectra. Locking

    the comb lines of an optical frequency comb to the cavity modes of an enhancement

    cavity and then extracting the spectral information with a Fourier

    transform spectrometer grants easy access to wide segments of absorption spectra.

    One of the main obstacles complicating the analysis of the measurements is

    the inevitable dispersion occurring inside the cavity. In this project, absorption

    measurements of CO2 were performed using an existing and well established

    setup consisting of a near-infrared optical frequency comb locked to a Fabry-

    Pérot enhancement cavity using the Pound-Drever-Hall technique, and a Fourier

    transform spectrometer. The purpose was to improve theoretical models of the

    measured absorption spectra by creating and verifying a model for the cavity

    dispersion, stemming mostly from the cavity mirrors but also from the normal

    dispersion of the intracavity medium. Until now, the cavity dispersion has been

    treated as an unknown and was included as a fitting parameter together with

    the CO2 concentration when applying fits to the absorption measurements. The

    dispersion model was based on previously performed precise measurements of

    the positions of the cavity modes. The model was found to agree well with

    measurements. In addition, pre-calculating the dispersion drastically reduced

    computation time and seemed to improve the overall robustness of the fitting

    routine. A complicating factor was found to be small discrepancies between

    the locking frequencies as determined prior to the measurements and the values

    yielding optimum agreement with the model. These apparent shifts of the locking

    points were found to have a systematic dependence on the distance between

    the locking points. The exact cause of this was not determined but the results

    indicate that with the locking points separated by more than about 10nm the

    shifts are negligible.

  • 114.
    Holkundkar, Amol
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Proton acceleration by circularly polarized traveling electromagnetic wave2012In: Physical Review Special Topics. Accelerators and Beams, ISSN 1098-4402, E-ISSN 1098-4402, Vol. 15, no 9, p. 091301-Article in journal (Refereed)
    Abstract [en]

    The acceleration of charged particles, producing collimated monoenergetic beams, over short distances holds the promise to offer new tools in medicine and diagnostics. Here, we consider a possible mechanism for accelerating protons to high energies by using a phase modulated circularly polarized electromagnetic wave propagating along a constant magnetic field. It is observed that a plane wave with dimensionless amplitude of 0.1 is capable to accelerate a 1 keV proton to 386 MeV under optimum conditions. Finally, we discuss possible limitations of the acceleration scheme.

  • 115.
    Hong, Yu
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Design and Calibration of Optical Tweezers Using the Equipartition Theorem2015Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    The aim was to construct an optical tweezers system for moving small polystyrene particles and apply the equipartition theorem for the calibration of trap stiffness. The construction of the optical tweezers system was based on simulations using a ray-tracing program to achieve desired expansion of the beam and limited amounts of aberrations. A position sensitive detector (PSD) is used to record the positions of the bead trapped in the optical tweezers. The calibration between the position of the particle and the relative position on the PSD was based on the power spectrum method. We got the stiffness of the optical tweezers 0.28 pN/μm and 0.29 pN/μm in the x and the y directions with 1 μm beam using a 10 mW He-Ne laser.

  • 116.
    Huang, Zixi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Development of a Schlieren set up to provide bi-directional imaging by color coding2017Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    The aim of the experiment is to construct Z-type Schlieren set up and use dierent kinds of color filters to simultaneously achieve bi-directional Schlieren images. Four types of color filters are compared and the best combination is identified as consisting of a red, a green and a blue filter.

  • 117.
    Ilderton, Anton
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Johansson, Petter
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pair annihilation in laser pulses: Optical versus x-ray free-electron laser regimes2011In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 84, no 3, p. 032119-Article in journal (Refereed)
    Abstract [en]

    We discuss the theory and phenomenology of pair annihilation, within an ultrashort laser pulse, to a single photon. The signature of this process is the unidirectional emission of single photons with a fixed energy. We show that the cross section is significantly larger than for two-photon pair annihilation in vacuum, with x-ray free-electron laser parameters admitting a much clearer signal than optical beams.

  • 118.
    Iqbal, Javed
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Department of Chemistry, University of Agriculture, Faisalabad 38040, Pakistan.
    Enevold, Jenny
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Larsen, Christian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wang, Jia
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Revoju, Srikanth
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Eliasson, Bertil
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    An arylene-vinylene based donor-acceptor-donor small molecule for the donor compound in high-voltage organic solar cells2016In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 155, p. 348-355Article in journal (Refereed)
    Abstract [en]

    A donor-acceptor-donor (D-A-D) molecule has been designed and synthesized for use as the electron donating material in solution-processed small-molecule organic solar cells (OSCs). The D-A-D molecule comprises a central electron-accepting (2Z,2'Z)-2,2'-(2,5-bis(octyloxy)-1,4-phenylene)bis(3-(thiophen-2-yl)acry lonitrile) (ZOPTAN) core, which is chemically connected to two peripheral and electron-donating triphenylamine (TPA) units. The ZOPTAN-TPA molecule features a low HOMO level of -5.2 eV and an optical energy gap of 2.1 eV. Champion OSCs based on a solution-processed and non-annealed active material blend of [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) and ZOPTAN-TPA in a mass ratio of 2:1 exhibits a power conversion efficiency of 1.9% and a high open-circuit voltage of 1.0 V. 

  • 119.
    Islam, Md Mazharul
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Printed transparent conducting electrodes based on carbon nanotubes (CNTs), reduced graphene oxide (rGO), and a polymer matrix.2019Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The main focus of this project was to prepare transparent and conductive electrodes (TCEs). TCEs were made out of multi-walled carbon nanotubes (MWCNTs), reduced graphene oxide (rGO), and polyvinylpyrrolidone (PVP). Based on the theoretical aspect, MWCNTs has emerged as a promising nanofiller in the polymer matrix due to its high electrical conductivity. As a nanofiller, MWCNTs were used with a small ratio of rGO with PVP as a polymer matrix in this project to prepare TCEs having low sheet resistance with high transparency. An appropriate amount of PVP has been shown to be a good combination with MWCNTs and rGO in the solvent to keep MWCNTs dispersed for a long time. Carboxyl group (-COOH) functionalized MWCNTs (FMWCNTs) was produced in a controlled oxidative procedure due to enabling good dispersion of FMWCNTs in water and ethanol solvents. In contrast, water dispersible rGO was chemically prepared by using GO and sodium borohydride where GO was produced from graphite by using improved Hummer's method. Drop casting and spray coating methods were applied to fabricate TCEswhere only water was used as the solvent for drop casted TCEs and a mixing ratio of water and ethanol was 70:30 as solvent for spray coated TCEs.

    It was also determined in this project that the spray coating method was more suitable for preparing TCEs rather than thedrop casting method due to easy fabrication, large area coating possibility, and the smoothness of the coated film surface. The sheet resistance was obtained as 5026 Ω/ ⃣  where the transparency was 65% in the case of the drop casted electrode for the ratio of rGO:FMWCNTs:PVP was 1.2:60:1 with 0.02 mg FMWCNTs. In the case of spray coated electrode at the same ratio of rGO:FMWCNTs:PVP, the sheet resistance was measured as 5961 Ω/ ⃣  where the transparency was 73%. But in the case of 60:1 mass ratio of FMWCNTs:PVP with 0.02 mg FMWCNTs, the sheet resistance was 7729 Ω/ ⃣  and transparency was 77% for spray coated electrode. So, it is clear that the sheet resistance was improved by adding a small mass ratio of rGO with FMWCNTs:PVP.

  • 120. Jahn, Olga
    et al.
    Leshchenko, Vyacheslav E.
    Tzallas, Paraskevas
    Kassel, Alexander
    Krueger, Mathias
    Muenzer, Andreas
    Trushin, Sergei A.
    Tsakiris, George D.
    Kahaly, Subhendu
    Kormin, Dmitrii
    Veisz, Laszlo
    Umeå University, Faculty of Science and Technology, Department of Physics. Max-Planck-Institut für Quantenoptik, Garching, Germany.
    Pervak, Vladimir
    Krausz, Ferenc
    Major, Zsuzsanna
    Karsch, Stefan
    Towards intense isolated attosecond pulses from relativistic surface high harmonics2019In: Optica, ISSN 2334-2536, Vol. 6, no 3, p. 280-287Article in journal (Refereed)
    Abstract [en]

    Relativistic surface high harmonics have been considered a unique source for the generation of intense isolated attosecond pulses in the extreme ultra-violet and x-ray spectral ranges. Their practical realization, however, is still a challenging task and requires identification of optimum experimental conditions and parameters. Here, we present measurements and particle-in-cell simulations to determine the optimum values for the most important parameters. In particular, we investigate the dependence of harmonics efficiency, divergence, and beam quality on the pre-plasma scale length as well as identify the optimum conditions for generation of isolated attosecond pulses by measuring the dependence of the harmonics spectrum on the carrier - envelope phase of the driving infrared field. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

  • 121.
    Jin, Xu
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China; School of Mechanical Engineering, Dongguan University of Technology, Dongguan 523808, China.
    Sandström, Andreas
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB, Linnaeus Vag 24, SE-901 87 Umeå, Sweden.
    Lindh, E. Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Yang, Wei
    Tang, Shi
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB, Linnaeus Vag 24, SE-901 87 Umeå, Sweden.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB, Linnaeus Vag 24, SE-901 87 Umeå, Sweden.
    Challenging conventional wisdom: finding high-performance electrodes for light-emitting electrochemical cells2018In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 10, no 39, p. 33380-33389Article in journal (Refereed)
    Abstract [en]

    The light-emitting electrochemical cell (LEC) exhibits capacity for efficient charge injection from two air stable electrodes into a single-layer active material, which is commonly interpreted as implying that the LEC operation is independent of the electrode selection. Here, we demonstrate that this is far from the truth and that the electrode selection instead has a strong influence on the LEC performance. We systematically investigate 13 different materials for the positive anode and negative cathode in a common LEC configuration with the conjugated polymer Super Yellow as the electroactive emitter and find that Ca, Mn, Ag, Al, Cu, indium tin oxide (ITO), and Au function as the LEC cathode, whereas ITO and Ni can operate as the LEC anode. Importantly, we demonstrate that the electrochemical stability of the electrode is paramount and that particularly electrochemical oxidation of the anode can prohibit the functional LEC operation. We finally report that it appears preferable to design the device so that the heights of the injection barriers at the two electrode/active material interfaces are balanced in order to mitigate electrode-induced quenching of the light emission. As such, this study has expanded the set of air-stable electrode materials available for functional LEC operation and also established a procedure for the evaluation and design of future efficient electrode materials.

  • 122.
    Johansson, Alexandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Etalon immune distances for reduction of etalon background signals in frequency modulation spectrometry2014Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Frequency modulation spectrometry (FMS) is a sensitive laser based spectroscopic technique often used for investigation of molecules and atoms in gas phase. There is an ambition to reduce background signals and noise in spectroscopic systems and thereby increasing sensitivity in detection of gases which would extend the applicability of these types of techniques. This ambition was the starting point for this thesis.

    FMS is based on modulation of monochromatic light. At not too large modulation indices, the modulation creates a frequency modulated (FM) triplet with one carrier wave at the laser frequency and two equally frequency detuned sidebands that are exactly out of phase. This property of the FM light can be used for reduction of background signals from so called etalons.

    Etalons are background signals originating from multiple reflections between surfaces in the experimental setup. Since these signals can give rise to drifts and noise in detection systems it is of great importance to reduce this background. This thesis studies a method to reduce etalons called etalon immune distances (EID). The basic idea of the technique is to use the FM triplet that contains two sidebands that are completely out of phase. By matching the modulation frequency to the free spectral range (FSR) of the surfaces it is possible, in theory, to completely eliminate etalon background signals. Thus by placing the optical components at specific physical distances, the etalon signals can be reduced. The method of EID is examined in order to gain understanding of the behavior of etalon signals when FM light is used. The purpose of the thesis is to examine to what extent the method can be used for reduction of etalon background signals when FMS is used.

    The experiential work shows that EID is in fact a good method for reduction of these background signals. By placing the surfaces at EID it is possible to reduce the etalons significantly. However, the results also show that the method is very sensitive to deviations from perfect placing of the surfaces. For the modulation frequency of 396.2 MHz that was used in the experiment, a displacement of 2.7 mm from EID will give rise to 5% of the maximum measured etalon fringe amplitude in absorption phase. For dispersion phase the corresponding value is 25.3 mm. This indicates that detection in dispersion phase is much less sensitive to displacements of the surfaces than it is in absorption phase. Results thus show that it is important to be aware that the placing of the optical components on the optical table is of great importance in reduction of the etalon background signals. 

  • 123.
    Johansson, Alexandra C.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Optical Frequency Comb Fourier Transform Spectroscopy2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Fourier transform spectroscopy (FTS) based on optical frequency combs is an excellent spectroscopic tool as it provides broadband molecular spectra with high spectral resolution and an absolutely calibrated frequency scale. Moreover, the equidistant comb mode structure enables efficient coupling of the comb to enhancement cavities, yielding high detection sensitivity. This thesis focuses on further advances in comb-based FTS to improve its performance and extend its capabilities for broadband precision spectroscopy, particularly in terms of i) spectral resolution, ii) accuracy and precision of molecular parameters as well as concentrations retrieved from fitting models to spectra, and iii) species selectivity.

    To improve the spectral resolution we developed a new methodology to acquire and analyze comb-based FTS signals that yields spectra with a resolution limited by the comb linewidth rather than the optical path difference of the FTS, referred to as the sub-nominal resolution method. This method enables measurements of narrow features, e.g. low-pressure absorption spectra and modes of enhancement cavities, with frequency scale accuracy and precision provided by the comb. Using the technique we measured low-pressure spectra of the entire 3ν13 carbon dioxide (CO2) band at 1575 nm with sufficient signal-to-noise ratio and precision to observe collision narrowing of the absorption lineshape, which was for the first time with a comb-based spectroscopic technique. This allowed retrieval of spectral line parameters for this CO2 band using the speed-dependent Voigt profile.

    Using the sub-nominal resolution method, we measured the transmission modes of a Fabry-Perot cavity over 15 THz of bandwidth with kHz resolution and characterized the cavity modes in terms of their center frequency, linewidth, and amplitude. From the mode center frequencies, we retrieved the group delay dispersion of cavity mirror coatings and intracavity gas with an unprecedented combination of spectral bandwidth and resolution. By measuring both the mode broadening and frequency shift simultaneously we performed broadband cavity-enhanced complex refractive index spectroscopy (CE-CRIS), which allows for simultaneous and calibration-free assessment of the absorption and dispersion spectra of intracavity gas. In this first demonstration we measured the absorption and dispersion spectra of three combination bands of CO2 in the 1525 to 1620 nm range.

    Another comb-based FTS technique is noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS), which combines phase modulation and cavity-enhancement to obtain broadband and highly sensitive absorption spectra. In this thesis we improved the NICE-OFCS technique in terms of stability, sensitivity and modeling of the NICE-OFCS signal. We implemented a model of the NICE-OFCS signal with multiline fitting for assessment of gas concentration. We also identified the optimum operating conditions of the NICE-OFCS systems for accurate gas concentration assessment.

    Finally, to improve the species selectivity we combined comb-based FTS with the Faraday rotation spectroscopy (FRS) technique. In this first demonstration of optical frequency comb Faraday rotation spectroscopy (OFC-FRS), we measured background and interference-free spectra of the entire Q- and R-branches of the fundamental vibrational band of nitric oxide at 5.3 μm showing good agreement with the theoretical model.

  • 124.
    Johansson, Alexandra C.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Filipsson, Anna
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rutkowski, Lucile
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Maslowski, Piotr
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    CO2 Line Parameter Retrieval Beyond the Voigt Profile Using Comb-Based Fourier Transform Spectroscopy2018In: Conference on Lasers and Electro-Optics, Optical Society of America, 2018Conference paper (Refereed)
    Abstract [en]

    We measure absorption spectra of the CO213 band at 1.57 μm using optical frequency comb Fourier transform spectroscopy with sub-nominal resolution and retrieve line shape parameters using multiline fitting with the speed-dependent Voigt profile.

  • 125.
    Johansson, Alexandra C.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rutkowski, Lucile
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Filipsson, Anna
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hausmaninger, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Zhao, Gang
    Umeå University, Faculty of Science and Technology, Department of Physics. 2 State Key Laboratory of Quantum Optics and Optics Devices, Institute of Laser Spectroscopy, Shanxi University, Taiyuan 030006, China.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Broadband calibration-free cavity-enhanced complex refractive index spectroscopy using a frequency comb2018In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 26, no 16, p. 20633-20648Article in journal (Refereed)
    Abstract [en]

    We present broadband cavity-enhanced complex refractive index spectroscopy (CE-CRIS), a technique for calibration-free determination of the complex refractive index of entire molecular bands via direct measurement of transmission modes of a Fabry-Perot cavity filled with the sample. The measurement of the cavity transmission spectrum is done using an optical frequency comb and a mechanical Fourier transform spectrometer with sub-nominal resolution. Molecular absorption and dispersion spectra (corresponding to the imaginary and real parts of the refractive index) are obtained from the cavity mode broadening and shift retrieved from fits of Lorentzian profiles to the individual cavity modes. This method is calibration-free because the mode broadening and shift are independent of the cavity parameters such as the length and mirror reflectivity. In this first demonstration of broadband CE-CRIS we measure simultaneously the absorption and dispersion spectra of three combination bands of CO2 in the range between 1525 nm and 1620 nm and achieve good agreement with theoretical models. This opens up for precision spectroscopy of the complex refractive index of several molecular bands simultaneously. 

  • 126.
    Johansson, Alexandra C.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rutkowski, Lucile
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Khodabakhsh, Amir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Signal line shapes of Fourier-transform cavity-enhanced frequency modulation spectroscopy with optical frequency combs2017In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 34, no 2, p. 358-365Article in journal (Refereed)
    Abstract [en]

    We present a thorough analysis of the signal line shapes of Fourier-transform-based noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS). We discuss the signal dependence on the ratio of the modulation frequency, f(m), to the molecular linewidth, G. We compare a full model of the signals and a simplified absorption-like analytical model that has high accuracy for low f(m)/G ratios and is much faster to compute. We verify the theory experimentally by measuring and fitting the NICE-OFCS spectra of CO2 at 1575 nm using a system based on an Er: fiber femtosecond laser and a cavity with a finesse of similar to 11000. 

  • 127.
    Johansson, Alexandra C.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Westberg, Jonas
    Khodabakhsh, Amir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rutkowski, Lucile
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wysocki, Gerard
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Faraday Rotation Spectroscopy Using an Optical Frequency Comb2017In: 2017 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), IEEE, 2017Conference paper (Refereed)
    Abstract [en]

    Summary form only given. The mid-infrared (MIR) part of the optical spectrum (3-12 μm) houses the fundamental absorption bands of a multitude of environmentally important molecules, but the abundance of water absorption often causes interference with the target species and makes concentration measurement inaccurate. The broad spectral coverage of optical frequency comb spectroscopy (OFCS) provides access to entire ro-vibrational bands and allows more accurate concentration quantification and retrieval of sample temperature. To further improve detection sensitivity of paramagnetic species in the presence of interfering species, we combine a MIR optical frequency comb with the Faraday rotation spectroscopy (FRS) technique [I], which is insensitive to interferences from diamagnetic molecules, such as H 2 O, CO 2 , and CO. In FRS, the rotation of the polarization induced by an external magnetic field in the vicinity of paramagnetic molecular transitions is translated to an intensity change by the use of a polarization analyzer, which effectively removes the influence of any non-paramagnetic species. In the proof of principle demonstration of OFC-FRS we detect nitric oxide (NO) in the presence of water at 5.3 μm using a Fourier transform spectrometer.

  • 128.
    Johansson, Alexandra C.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Westberg, Jonas
    Wysocki, Gerard
    Foltynowicz, Aleksandra
    Optical frequency comb Faraday rotation spectroscopy2018In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 124, no 5, article id 79Article in journal (Refereed)
    Abstract [en]

    We demonstrate optical frequency comb Faraday rotation spectroscopy (OFC-FRS) for broadband interference-free detection of paramagnetic species. The system is based on a femtosecond doubly resonant optical parametric oscillator and a fast-scanning Fourier transform spectrometer (FTS). The sample is placed in a DC magnetic field parallel to the light propagation. Efficient background suppression is implemented via switching the direction of the field on consecutive FTS scans and subtracting the consecutive spectra, which enables long-term averaging. In this first demonstration, we measure the entire Q- and R-branches of the fundamental band of nitric oxide in the 5.2–5.4 μm range and achieve good agreement with a theoretical model.

  • 129. Johari, G. P.
    et al.
    Andersson, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Effects of stacking disorder on thermal conductivity of cubic ice2015In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 143, no 5, article id 054505Article in journal (Refereed)
    Abstract [en]

    Cubic ice is said to have stacking disorder when the H2O sequences in its structure (space group Fd (3) over barm) are interlaced with hexagonal ice (space group P6(3)/mmc) sequences, known as stacking faults. Diffraction methods have shown that the extent of this disorder varies in samples made by different methods, thermal history, and the temperature T, but other physical properties of cubic and hexagonal ices barely differ. We had found that at 160 K, the thermal conductivity, kappa, of cubic ice is similar to 20% less than that of hexagonal ice, and this difference varies for cubic ice samples prepared by different methods and/or subjected to different thermal history. After reviewing the methods of forming cubic ice, we report an investigation of the effects of stacking disorder and other features by using new data, and by analyzing our previous data on the dependence of kappa on T and on the pressure. We conclude that the lower kappa of cubic ice and its weaker T-dependence is due mainly to stacking disorder and small crystal sizes. On in situ heating at 20-50 MPa pressure, kappa increases and cubic ice irreversibly transforms more sharply to ice Ih, and at a higher T of similar to 220 K, than it does in ex situ studies. Cooling and heating between 115 and 130 K at 0.1 K min(-1) rate yield the same kappa value, indicating that the state of cubic ice in these conditions does not change with time and T. The increase in kappa of cubic ice observed on heat-annealing before its conversion to hexagonal ice is attributed to the loss of stacking faults and other types of disorders, and to grain growth. After discussing the consequences of our findings on other properties, we suggest that detailed studies of variation of a given property of cubic ice with the fraction of stacking faults in its structure may reveal more about the effect of this disorder. A similar disorder may occur in the mono-layers of H2O adsorbed on a substrate, in bulk materials comprised of two dimensional layers, in diamond and in Zirconium and in numerous other crystals.

  • 130. Johari, G. P.
    et al.
    Andersson, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Structural relaxation and thermal conductivity of high-pressure formed, high-density di-n-butyl phthalate glass and pressure induced departures from equilibrium state2017In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 146, no 23, article id 234505Article in journal (Refereed)
    Abstract [en]

    We report a study of structural relaxation of high-density glasses of di-n-butyl phthalate (DBP) by measuring thermal conductivity, κ, under conditions of pressure and temperature (p,T) designed to modify both the vibrational and configurational states of a glass. Various high-density glassy states of DBP were formed by (i) cooling the liquid under a fixed high p and partially depressurizing the glass, (ii) isothermal annealing of the depressurized glass, and (iii) pressurizing the glass formed by cooling the liquid under low p. At a given low p, κ of the glass formed by cooling under high p is higher than that of the glass formed by cooling under low p, and the difference increases as glass formation p is increased. κ of the glass formed under 1 GPa is ∼20% higher at ambient p than κ of the glass formed at ambient p. On heating at low p, κ decreases until the glass to liquid transition range is reached. This is the opposite of the increase in κ observed when a glass formed under a certain p is heated under the same p. At a given high p, κ of the low-density glass formed by cooling at low p is lower than that of the high-density glass formed by cooling at that high p. On heating at high p, κ increases until the glass to liquid transition range is reached. The effects observed are due to a thermally assisted approach toward equilibrium at p different from the glass formation p. In all cases, the density, enthalpy, and entropy would change until the glasses become metastable liquids at a fixed p, thus qualitatively relating κ to variation in these properties.

  • 131.
    Kaihovirta, Nikolai
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Larsen, Christian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Improving the Performance of Light-Emitting Electrochemical Cells by Optical Design2014In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 6, no 4, p. 2947-2954Article in journal (Refereed)
    Abstract [en]

    The organic light-emitting electrochemical cell (LEG) has emerged as an enabling technology for a wide range of novel and low-cost emissive applications, but its efficiency is still relatively modest. The focus in the field has so far almost exclusively been directed toward limiting internal loss mechanisms, whereas external losses resulting from poor light-outcoupling have been overlooked. Here, we report a straightforward procedure for improving the efficiency and emission quality of LECs. We find that our high-performance glass-encapsulated LECs exhibit a near-ideal Lambertian emission profile but that total internal reflection at the glass/air interface and a concomitant edge emission and self-absorption represent a significant loss factor. We demonstrate a 60% improvement in the outcoupled luminance in the forward direction by laminating a light-outcoupling film, featuring a hexagonal array of hemispherical microlenses as the surface structure, onto the front side of the device and a large-area metallic reflector onto the back side. With this scalable approach, yellow-emitting LEC devices with a power conversion efficiency of more than 15 lm W-1 at a luminance of 100 cd m(-2) were realized. Importantly, we find that the same procedure also can mitigate problems with spatial variation in the light-emission intensity, which is a common and undesired feature of large-area LECs.

  • 132. Karhu, Juho
    et al.
    Tomberg, Teemu
    Vieira, Francisco Senna
    Umeå University, Faculty of Science and Technology, Department of Physics. University of Helsinki, Department of Chemistry, Helsinki, Finland.
    Genoud, Guillaume
    Hanninen, Vesa
    Vainio, Markku
    Metsala, Markus
    Hieta, Tuomas
    Bell, Steven
    Halonen, Lauri
    Broadband photoacoustic spectroscopy of 14CH4 with a high-power mid-infrared optical frequency comb2019In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 44, no 5, p. 1142-1145Article in journal (Refereed)
    Abstract [en]

    We report a photoacoustic spectroscopy setup with a high-power mid-infrared frequency comb as the light source. The setup is used in broadband spectroscopy of radiocarbon methane. Owing to the high sensitivity of a cantilever-enhanced photoacoustic cell and the high-power light source, we can reach a detection limit below 100 ppb in a broadband measurement with a sample volume of only a few milliliters. The first infrared spectrum of (CH4)-C-14 is reported and given a preliminary assignment. The results lay a foundation for the development of optical detection systems for radiocarbon methane. 

  • 133.
    Kastberg, Anders
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Dion, Claude
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hagman, Henning
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Zelan, Martin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    A three-dimensional Brownian motor, realised with symmetric optical lattices2009In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 246, no 5, p. 999-1005Article in journal (Refereed)
    Abstract [en]

    A three-dimensional Brownian motor is realised using lasercooled caesium atoms trapped in a system of two static, and individually symmetric, optical lattices; a so-called double optical lattice. Isotropic fluctuations, emanating from light scattering, are rectified, and the diffusion of the ensemble of atoms is biased, with a resulting constant velocity that is controllable both in direction and magnitude. The working principle of the Brownian motor can be seen as a pulsation between two different potentials, both symmetric but around different points. The correlation between interferometric spatial offsets, and imbalance in optical pumping rates, leads to a spatio-temporal asymmetry sufficient for generating a controlled, directed motion.

  • 134.
    Khodabakhsh, Amir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Fourier transform and Vernier spectroscopy using optical frequency combs2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Optical frequency comb spectroscopy (OFCS) combines two previously exclusive features, i.e., wide optical bandwidth and high spectral resolution, enabling precise measurements of entire molecular bands and simultaneous monitoring of multiple gas species in a short measurement time. Moreover, the equidistant mode structure of frequency combs enables efficient coupling of the comb power to enhancement resonant cavities, yielding high detection sensitivities. Different broadband detection methods have been developed to exploit the full potential of frequency combs in spectroscopy, based either on Fourier transform spectroscopy or on dispersive elements.There have been two main aims of the research presented in this thesis. The first has been to improve the performance of mechanical Fourier transform spectrometers (FTS) based on frequency combs in terms of sensitivity, resolution and spectral coverage. In pursuit of this aim, we have developed a new spectroscopic technique, so-called noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS), and achieved a shot-noise-limited sensitivity and low ppb (parts-per-billion, 10−9) CO2 concentration detection limit in the near-infrared range using commercially available components. We have also realized a novel method for acquisition and analysis of comb-based FTS spectra, a so-called sub-nominal resolution method, which provides ultra-high spectral resolution and frequency accuracy (both in kHz range, limited only by the stability of the comb) over the broadband spectral range of the frequency comb. Finally, we have developed an optical parametric oscillator generating a frequency comb in the mid-infrared range, where the strongest ro-vibrational molecular absorption lines reside. Using this mid-infrared comb and an FTS, we have demonstrated, for the first time, comb spectroscopy above 5 μm, measured broadband spectra of several species and reached low ppb detection limits for CH4, NO and CO in 1 s.The second aim has been more application-oriented, focused on frequency comb spectroscopy in combustion environments and under atmospheric conditions for fast and sensitive multispecies detection. We have demonstrated, for the first time, cavity-enhanced optical frequency comb spectroscopy in a flame, detected broadband high temperature H2O and OH spectra using the FTS in the near-infrared range and showed the potential of the technique for flame thermometry. For applications demanding a short measurement time and high sensitivity under atmospheric pressure conditions, we have implemented continuous-filtering Vernier spectroscopy, a dispersion-based spectroscopic technique, for the first time in the mid-infrared range. The spectrometer was sensitive, fast, robust, and capable of multispecies detection with 2 ppb detection limit for CH4 in 25 ms.

  • 135.
    Khodabakhsh, Amir
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Abd Alrahman, Chadi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Noise-immune cavity-enhanced optical frequency comb spectroscopy2014In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 39, no 17, p. 5034-5037Article in journal (Refereed)
    Abstract [en]

    We present a new method of optical frequency comb spectroscopy that combines cavity enhancement with frequency modulation to obtain immunity to laser frequency-to-amplitude noise conversion by the cavity modes and, thus, high absorption sensitivity over a broad spectral range. A frequency comb is locked to a cavity with a free spectral range (FSR) equal to 4/3 times the repetition rate of the laser, and phase-modulated at a frequency equal to the cavity FSR. The transmitted light is analyzed by a Fourier transform spectrometer with a high bandwidth detector. Phase-sensitive detection of the interferogram yields a noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS) signal. In the first demonstration, we record NICE-OFCS signals from the overtone CO2 band at 1575 nm with absorption sensitivity of 4.3 x 10(-10) cm(-1) Hz(-1/2) per spectral element, close to the shot noise limit.

  • 136.
    Khodabakhsh, Amir
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Badarla, Venkata R.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rutkowski, Lucile
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Johansson, Alexandra C.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lee, Kevin F.
    Jiang, J.
    Mohr, C.
    Fermann, Martin E.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Optical Frequency Comb Spectroscopy at 3.3 and 5.2 mu m by a Tm:fiber-Laser-Pumped Optical Parametric Oscillator2016In: Proceedings Conference on Lasers and Electro-Optics, 2016Conference paper (Refereed)
    Abstract [en]

    Using a doubly-resonant femtosecond optical parametric oscillator in combination with a multipass cell and a Fourier transform spectrometer we measure broadband CH4 and NO absorption spectra at fundamental transition wavelengths.

  • 137.
    Khodabakhsh, Amir
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Johansson, Alexandra C.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Noise-immune cavity-enhanced optical frequency comb spectroscopy: a sensitive technique for high-resolution broadband molecular detection2015In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 119, no 1, p. 87-96Article in journal (Refereed)
    Abstract [en]

    Noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS) is a recently developed technique that utilizes phase modulation to obtain immunity to frequency-to-amplitude noise conversion by the cavity modes and yields high absorption sensitivity over a broad spectral range. We describe the principles of the technique and discuss possible comb-cavity matching solutions. We present a theoretical description of NICE-OFCS signals detected with a Fourier transform spectrometer (FTS) and validate the model by comparing it to experimental CO2 spectra around 1,575 nm. Our system is based on an Er:fiber femtosecond laser locked to a cavity and phase-modulated at a frequency equal to a multiple of the cavity free spectral range (FSR). The NICE-OFCS signal is detected by a fast-scanning FTS equipped with a high-bandwidth commercial detector. We demonstrate a simple method of passive locking of the modulation frequency to the cavity FSR that significantly improves the long-term stability of the system, allowing averaging times on the order of minutes. Using a cavity with a finesse of ~9,000, we obtain absorption sensitivity of 6.4 × 10−11 cm−1 Hz−1∕2 per spectral element and concentration detection limit for CO2 of 450 ppb Hz−1/2, determined by multiline fitting.

  • 138.
    Khodabakhsh, Amir
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ramaiah-Badarla, Venkata
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rutkowski, Lucile
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Johansson, Alexandra C.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lee, Kevin F.
    Jiang, Jie
    Mohr, Christian
    Fermann, Martin E.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Fourier transform and Vernier spectroscopy using an optical frequency comb at 3-5.4 μm2016In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 41, no 11, p. 2541-2544Article in journal (Refereed)
    Abstract [en]

    We present a versatilemid-infrared frequency comb spectroscopy system based on a doubly resonant optical parametric oscillator tunable in the 3-5.4 mu m range and two detection methods: a Fourier transform spectrometer (FTS) and a continuous-filtering Vernier spectrometer (CF-VS). Using the FTS with a multipass cell, we measure high precision broadband absorption spectra of CH4 at 3.3 mu m and NO at 5.25 mu m, the latter for the first time with comb spectroscopy, and we detect atmospheric species (CH4, CO, CO2, and H2O) in air in the signal and idler ranges. Multiline fitting yields minimum detectable concentrations of 10-20 ppbHz-1/2 for CH4, NO, and CO. For the first time in the mid-infrared, we perform CF-VS using an enhancement cavity, a grating, and a single detector, and we measure the absorption spectrum of CH4 and H2O in ambient air at similar to 3.3 m mu, reaching a 40 ppb concentration detection limit for CH4 in 2 ms. 

  • 139.
    Khodabakhsh, Amir
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rutkowski, Lucile
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Morville, Jerome
    Johansson, Alexandra C.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Soboń, Grzegorz
    Umeå University, Faculty of Science and Technology, Department of Physics. Faculty of Electronics, Wrocław University of Science and Technology, Wrocław, Poland.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Cavity-Enhanced Continuous-Filtering Vernier Spectroscopy at 3.3 mu m using a Femtosecond Optical Parametric Oscillator2017In: 2017 CONFERENCE ON LASERS AND ELECTRO-OPTICS EUROPE & EUROPEAN QUANTUM ELECTRONICS CONFERENCE (CLEO/EUROPE-EQEC), IEEE , 2017, p. CH_2_2-Conference paper (Refereed)
    Abstract [en]

    Optical frequency comb spectroscopy in the mid-infrared fingerprint region combines broad spectral bandwidth with high detection sensitivity and allows simultaneous detection of trace amounts of many molecular species. We have recently demonstrated a continuous-filtering Vernier spectrometer based on a mid-infrared optical frequency comb and an enhancement cavity for fast and sensitive detection of CH4 [1]. Here we present an improved, fully automatized and frequency calibrated continuous-filtering Vernier spectrometer, schematically shown in Fig. 1(a). The comb source is a doubly resonant optical parametric oscillator (DROPO) based on an orientation-patterned GaAs crystal synchronously pumped by a Tm:fiber femtosecond laser (125 MHz repetition rate, frep). The signal comb (3.1–3.4 µm, 30 mW) is mode matched to a 60-cm long Vernier enhancement cavity with a finesse of ~350 at 3.25 μm, placed in an enclosure that can be filled with the gas sample. The output mirror is attached to a PZT and mounted on a translation stage. When the cavity free spectral range is perfectly matched to twice the frep (250 MHz) every other signal comb mode is transmitted through the cavity. By detuning the cavity length from this perfect match position the cavity resonances act as a filter and transmit groups of comb modes called Vernier orders [2]. A diffraction grating mounted on a galvo-scanner separates these orders after the cavity and the chosen order is sent to the detection system. The Vernier order is tuned across the signal comb spectrum by scanning the cavity length (at 20 Hz) and the grating is rotated synchronously to fix the order in space and allow acquisition of the entire spectrum in 25 ms. Any residual mismatch between the cavity length scan and the grating rotation is compensated by a feedback loop acting on the frep of the pump laser and the PZT of the Vernier cavity [2]. A Fabry-Perot etalon is used for frequency calibration of the spectrometer. Figure 1(b) shows in black the normalized transmission spectrum of a sample containing 5.0 ppm CH4 and 160 ppm water. The red and blue curves show the corresponding fit of the Vernier spectrum [3] of CH4 and water, respectively, calculated using Voigt profiles, line parameters from the HITRAN database, and the experimentally determined cavity finesse. The figure of merit of the spectrometer is 1×10−9cm−1 Hz−1∕2 per spectral element and multiline fitting yields minimum detectable concentration of CH4 of 2 ppb in 25 ms, translating into 400 ppt Hz−1∕2 Since the spectrum of the signal comb covers the fundamental C-H stretch transitions we expect low detection limits for other hydrocarbons as well. In conclusion, mid-infrared comb-based continuous-filtering Vernier spectroscopy allows fast and highly sensitive measurement of broadband absorption spectra using a robust and compact detection system.

  • 140.
    Khodabakhsh, Amir
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rutkowski, Lucile
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Morville, Jérôme
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mid-infrared continuous-filtering Vernier spectroscopy using a doubly resonant optical parametric oscillator2017In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 123, no 210Article in journal (Refereed)
    Abstract [en]

    We present a continuous-filtering Vernier spectrometer operating in the 3.15-3.4 mu m range, based on a femtosecond doubly resonant optical parametric oscillator, a cavity with a finesse of 340, a grating mounted on a galvo scanner, and two photodiodes. The spectrometer allows acquisition of one spectrum spanning 250 nm of bandwidth in 25 ms with 8 GHz resolution, sufficient to detect molecular lines at atmospheric pressure. An active lock ensures good frequency and intensity stability of the consecutive spectra and enables continuous signal acquisition and efficient averaging. The relative frequency scale is calibrated using a Fabry-Perot etalon or, alternatively, the galvo scanner position signal. We measure spectra of a calibrated CH4 gas sample as well as dry and laboratory air and extract CH4 and -H2O concentrations by multiline fitting of model spectra. The figure of merit of the spectrometer is 1.7 x 10(-9) cm(-1) Hz(-1/2) per spectral element and the minimum detectable concentration of CH4 is 360 ppt Hz(-1/2), averaging down to 90 ppt after 16 s.

  • 141.
    Khodabakhsh, Amir
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rutkowski, Lucile
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Morville, Jérôme
    Johansson, Alexandra C.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Soboń, Grzegorz
    Umeå University, Faculty of Science and Technology, Department of Physics. Faculty of Electronics, Wrocław University of Science and Technology, Wrocław, Poland.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Continuous-Filtering Vernier Spectroscopy at 3.3 mu m Using a Femtosecond Optical Parametric Oscillator2017In: 2017 conference on lasers and elecro-optics (CLEO): Science and innovations, IEEE , 2017, article id SW1L.5Conference paper (Refereed)
    Abstract [en]

    Using a cavity-enhanced continuous-filtering Vernier spectrometer based on a femtosecond optical parametric oscillator we measure broadband spectra of atmospheric water and CH4 around 3.3 mu m reaching 4 ppb detection limit for CH4 in 15 ms.

  • 142. Khrennikov, K.
    et al.
    Wenz, J.
    Buck, A.
    Xu, Jiancai
    Heigoldt, M.
    Veisz, Laszlo
    MPI für Quantenoptik, Garching, Germany.
    Karsch, S.
    Tunable All-Optical Quasimonochromatic Thomson X-Ray Sourcein the Nonlinear Regime2015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 114, no 19, article id 195003Article in journal (Refereed)
    Abstract [en]

    We present an all-laser-driven, energy-tunable, and quasimonochromatic x-ray source based on Thomson scattering from laser-wakefield-accelerated electrons. One part of the laser beam was used to drive a few-fs bunch of quasimonoenergetic electrons, while the remainder was backscattered off the bunch at weakly relativistic intensity. When the electron energy was tuned from 17–50 MeV, narrow x-ray spectra peaking at 5–42 keV were recorded with high resolution, revealing nonlinear features. We present a large set of measurements showing the stability and practicality of our source.

  • 143.
    Kinnunen, Jussi
    et al.
    Department of Physics and Mathematics, University of Eastern Finland, Joensuu, Finland.
    Kokkonen, Harri
    Department of Applied Physics, University of Eastern Finland, Kuopio, Finland.
    Kovanen, Vuokko
    Department of Health Sciences, University of Jyväskylä, Jyväskylä, Finland.
    Hauta-Kasari, Markku
    School of Computing, University of Eastern Finland, Joensuu, Finland.
    Vahimaa, Pasi
    Department of Physics and Mathematics, University of Eastern Finland, Joensuu, Finland.
    Lammi, Mikko
    Institute of Biomedicine, University of Eastern Finland, Kuopio, Finland.
    Töyräs, Juha
    Department of Applied Physics, University of Eastern Finland, Kuopio, Finland.
    Jurvelin, Jukka
    Department of Applied Physics, University of Eastern Finland, Kuopio, Finland.
    Nondestructive fluorescence-based quantification of threose-induced collagen cross-linking in bovine articular cartilage2012In: Journal of Biomedical Optics, ISSN 1083-3668, E-ISSN 1560-2281, Vol. 17, no 9, p. 97003-, article id 22975679Article in journal (Refereed)
    Abstract [en]

    Extensive collagen cross-linking affects the mechanical competence of articular cartilage: it can make the cartilage stiffer and more brittle. The concentrations of the best known cross-links, pyridinoline and pentosidine, can be accurately determined by destructive high-performance liquid chromatography (HPLC). We explore a nondestructive evaluation of cross-linking by using the intrinsic fluorescence of the intact cartilage. Articular cartilage samples from bovine knee joints were incubated in threose solution for 40 and 100 h to increase the collagen cross-linking. Control samples without threose were also prepared. Excitation-emission matrices at wavelengths of 220 to 950 nm were acquired from the samples, and the pentosidine and pyridinoline cross-links and the collagen concentrations were determined using HPLC. After the threose treatment, pentosidine and lysyl pyridinole (LP) concentrations increased. The intrinsic fluorescence, excited below 350 nm, decreased and was related to pentosidine [r=−0.90, 240/325nm (excitation/emission)] or LP (r=−0.85, 235/285nm) concentrations. Due to overlapping, the changes in emission could not be linked specifically to the recorded cross-links. However, the fluorescence signal enabled a nondestructive optical estimate of changes in the pentosidine and LP cross-linking of intact articular cartilage.

  • 144. Kluczynski, P.
    et al.
    Jahjah, M.
    Naehle, L.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Belahsene, S.
    Fischer, M.
    Koeth, J.
    Rouillard, Y.
    Westberg, Jonas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Vicet, A.
    Lundqvist, S.
    Detection of acetylene impurities in ethylene and polyethylene manufacturing processes using tunable diode laser spectroscopy in the 3-mu m range2011In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 105, no 2, p. 427-434Article in journal (Refereed)
    Abstract [en]

    Using recently developed GaInAsSb/AlGaInAsSb DFB lasers, tunable diode laser spectroscopy (TDLS) has been extended into the 3-mu m wavelength region for the detection of acetylene impurities in hydrocarbon compounds encountered in ethylene manufacturing. Measurements of acetylene in pure polymer grade ethylene and in a gas mixture of ethylene and ethane typical of the process stream around a hydrogenation reactor have been performed. Using a procedure incorporating subtraction of a hydrocarbon background spectrum a detection limit of 5 ppb m was achieved under ordinary laboratory conditions. Under forced temperature cycling conditions, the detection limit deteriorated to 180 ppb m, due to temperature drift caused by optical interferences generated by reflections in the laser TO8 can.

  • 145. Kluczynski, Pawel
    et al.
    Lundqvist, Stefan
    Westberg, Jonas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Faraday rotation spectrometer with sub-second response time for detection of nitric oxide using a cw DFB quantum cascade laser at 5.33 µm2011In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 103, no 2, p. 451-459Article in journal (Refereed)
    Abstract [en]

    A Faraday modulation spectrometer for sensitive and fast detection of nitric oxide at 5.33 μm utilizing a room temperature continuous wave distributed feedback quantum cascade laser and a Peltier cooled MCT detector is presented. The magnetic field was modulated at 7.4 kHz whereas the laser wavelength was scanned at 20 Hz across the most favorable rotational-vibrational transition for FAMOS, Q3/2(3/2), at 5.33 μm. Using a 15 cm optical path and lineshape fitting, the spectrometer provides a detection limit of 4.5 ppb for a response time of 1 s. An Allan variance analysis demonstrates that the system has an excellent stability, up to several hours of operation.

  • 146.
    Kobyakov, Dmitry
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hydrodynamics of Binary Bose-Einstein Condensates and Hydro-elasticity of the Inner Crust of Neutron Stars2014Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In the present thesis, “Hydrodynamics of Binary Bose-Einstein Condensates and Hydro-elasticity of the Inner Crust of Neutron Stars”, the hydrodynamic effects, instabilities and superfluid turbulence in binary immiscible ultracold gases, and hydro-elastic macroscopic coupled modes and microscopic structure of the inner layers of the crust of neutron stars, are studied. The ultracold gas dynamics can be realized in the laboratory. The excitation modes of the inner crust determine a number of observable properties such as elasticity, thermal properties and mass transport properties. Here we focus on expanding the details, rather than repeating the results presented in the published articles.

    In the part of the thesis related to atomic ultracold gases, we utilize the physical parameters in the experimentally realizable parameter region. We numerically simulate the coupled non-linear Schrödinger equations, and calculate observable quantities, such as phase and modulus of the order parameter, conditions needed for observation of the Rayleigh-Taylor instability and for turbulence generation. The numerical calculations are accompanied by analytical description of the processes. The dispersion relation for capillary-gravitational waves at the interface between two ultracold gases, is derived straightforwardly from the superfluid Lagrangian. The equations of motion for centre-of-mass of the superfluids are derived, and then used in our model of the quantum swapping of immiscible superfluids pressed by a strong external force. By numerical simulation, we find that the Kelvin-Helmholtz instability which occurs at the non-linear stage of the Rayleigh-Taylor instability, can generate quantum turbulence with peculiar properties. We find that two-dimensional superfluid systems with weak inter-component repulsion are different from previously studied strongly repulsive binary superfluids, because the quantum Kelvin-Helmholtz instability in weakly repulsive superfluids rolls up the whole interface forming a vortex bundle, similarly to dynamics of the shear fluid layers in the classical hydrodynamics. Production of vortex bundles favours the Kolmogorov spectrum of turbulence, and we find that the Kolmogorov scaling indeed is present in a freely decaying turbulence.

    In the part of the thesis related to neutron stars, we study the inner crust of neutron stars, where the fully ionized atomic nuclei coexist with a superfluid of neutrons. The interaction between superfluid neutrons and the crystallized Coulomb plasma is due to the interaction between density perturbations (interaction of the scalar type), and between the current - the non-dissipative entrainment effect (interaction of the vector type). We calculate velocities of the collective modes of the crystal coupled to superfluid neutrons. As an input we use the results of microscopic nuclear calculations in the framework of the compressible liquid drop model (the Lattimer and Swesty equation of state), and more recent effective Thomas-Fermi calculations with shell corrections (N. Chamel, and the Brussels theoretical nuclear physics group). Knowledge of velocities as functions of the matter density in the inner crust is important for calculation of a number of dynamic and transport properties. The heat transport properties of the inner crust are directly observable in accreting binary systems (low-mass x-ray binaries). The mass transport properties of the inner crust are directly linked to the rotational evolution, being a key physical ingredient of the pulsar glitch phenomenon. The elastic properties are related to the vibrational modes of the star, and to the breaking stress of the crust.

    In the second part of our work on neutron stars we investigate the microscopic structure of the inner crust treating the structure as an anisotropic crystal coupled to s-wave superfluid neutron liquid. As we analyse dynamics of the elementary excitations at higher wavenumbers (smaller scales), we reach the edge of the first Brillouin zone. The Lattimer-Swesty data is applicable for wavenumbers much smaller than the edge of the first Brillouin zone. We extrapolate the data through the whole first Brillouin zone to calculate the fastest growth rate of the unstable modes. The crucial step is to calculate the mode velocities in anisotropic crystal incorporating both the induced neutron-proton interactions, and the electron screening properties. We find that the combined influence of these two effects leads to softening of the longitudinal phonon of the lattice above about the Thomas-Fermi screening wavenumber of the electrons. The critical wavenumber when the frequency becomes purely imaginary is about  1/5 - 2/3  of the reciprocal lattice vector, thus validating our assumption. The imaginary mode frequency implies instability at finite wavenumbers. Our calculations suggest that the mode at the first Brillouin zone edge is the most unstable, and thus the structure experiences a displacive phase transition when the central ion of a unit cell of the body-cubic-centred lattice, is displaced to the cube face. Thus, the electronic structure of matter at densities above the neutron drip [1], is richer than previously appreciated, and new microscopic calculations of nuclear structure are necessary which take into account the high-wavenumber physics. Such calculations will provide crucial input to models interpreting the quasi-periodic oscillations in Soft Gamma Repeaters as magnetar x-ray flares, and to the theory of glitches of neutron stars.

    [1] The neutron drip density is ~3×1011 g cm-3.

  • 147.
    Kobyakov, Dmitry
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bezett, A.
    Institute for Theoretical Physics, Utrecht University, The Netherlands.
    Lundh, Emil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Department of Applied Physics, Division of Condensed Matter Theory, Chalmers University of Technology.
    Bychkov, Vitaliy
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Turbulence in binary Bose-Einstein condensates generated by highly nonlinear Rayleigh-Taylor and Kelvin-Helmholtz instabilities2014In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 89, p. 013631-Article in journal (Refereed)
    Abstract [en]

    Quantum turbulence (QT) generated by the Rayleigh-Taylor instability in binary immiscible ultracold 87Rb atoms at zero temperature is studied theoretically. We show that the quantum vortex tangle is qualitatively different from previously considered superfluids, which reveals deep relations between QT and classical turbulence. The present QT may be generated at arbitrarily small Mach numbers, which is a unique property not found in previously studied superfluids. By numerical solution of the coupled Gross-Pitaevskii equations we find that the Kolmogorov scaling law holds for the incompressible kinetic energy. We demonstrate that the phenomenon may be observed in the laboratory.

  • 148. Kolliopoulos, G.
    et al.
    Bergues, B.
    Schröder, H.
    Carpeggiani, P. A.
    Veisz, Laszlo
    Max-Planck-Institut für Quantenoptik, Garching, Germany.
    Tsakiris, G. D.
    Charalambidis, D.
    Tzallas, P.
    Revealing quantum path details in high-field physics2014In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 90, no 1, article id 013822Article in journal (Refereed)
    Abstract [en]

    The fundamental mechanism underlying harmonic emission in the strong-field regime is governed by tunnel ionization of the atom, followed by the motion of the electron wave packet in the continuum, and finally by its recollision with the atomic core. Due to the quantum nature of the process, the properties of the electron wave packet strongly correlate with those of the emitted radiation. Here, by spatially resolving the interference pattern generated by overlapping the harmonic radiation emitted by different interfering electron quantum paths, we have succeeded in unravelling the intricacies associated with the recollision process. This has been achieved by mapping the spatial extreme-ultraviolet (EUV)-intensity distribution onto a spatial ion distribution, produced in the EUV focal area through the linear and nonlinear processes of atoms. By in situ manipulation of the intensity-dependent motion of the electron wave packets, we have been able to directly measure the difference between the harmonic emission times and electron path lengths resulting from different electron trajectories. Due to the high degree of accuracy that the present approach provides, we have been able to demonstrate the quantum nature of the recollision process. This is done by quantitatively correlating the photoemission time and the electron quantum path-length differences, taking into account the energy-momentum transfer from the driving laser field into the system. This information paves the way for electron-photon correlation studies at the attosecond time scale, while it puts the recollision process from the semiclassical prospective into a full quantum-mechanical context.

  • 149. Kolliopoulos, G.
    et al.
    Tzallas, P.
    Bergues, B.
    Carpeggiani, P. A.
    Heissler, P.
    Schröder, H.
    Veisz, László
    Max-Planck-Institut für Quantenoptik, Garching, Germany.
    Charalambidis, D.
    Tsakiris, G. D.
    Single-shot autocorrelator for extreme-ultraviolet radiation2014In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 31, no 5, p. 926-938Article in journal (Refereed)
    Abstract [en]

    A novel single-shot second-order autocorrelation scheme for extreme-ultraviolet radiation (XUV) is proposed. It is based on an ion-imaging technique, which provides spatial information of ionization products in the focal volume of the XUV beam. Using simple analytical and detailed numerical modeling, an evaluation toward selecting an optimum configuration has been performed. The implementation of the concept to characterize attosecond pulses is discussed, and the proposed setups are assessed.

  • 150. Kormin, Dmitrii
    et al.
    Borot, Antonin
    Ma, Guangjin
    Dallari, William
    Bergues, Boris
    Aladi, Mark
    Földes, Istvan B.
    Veisz, Laszlo
    Umeå University, Faculty of Science and Technology, Department of Physics. Max-Planck-Institut für Quantenoptik, D-85748 Garching, Germany.
    Spectral interferometry with waveform-dependent relativistic high-order harmonics from plasma surfaces2018In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, article id 4992Article in journal (Refereed)
    Abstract [en]

    The interaction of ultra-intense laser pulses with matter opened the way to generate the shortest light pulses available nowadays in the attosecond regime. Ionized solid surfaces, also called plasma mirrors, are promising tools to enhance the potential of attosecond sources in terms of photon energy, photon number and duration especially at relativistic laser intensities. Although the production of isolated attosecond pulses and the understanding of the underlying interactions represent a fundamental step towards the realization of such sources, these are challenging and have not yet been demonstrated. Here, we present laser-waveform-dependent high-order harmonic radiation in the extreme ultraviolet spectral range supporting well-isolated attosecond pulses, and utilize spectral interferometry to understand its relativistic generation mechanism. This unique interpretation of the measured spectra provides access to unrevealed temporal and spatial properties such as spectral phase difference between attosecond pulses and field-driven plasma surface motion during the process.

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