umu.sePublications
Change search
Refine search result
1234567 51 - 100 of 344
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 51.
    Ehlers, Patrick
    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.
    Silander, Isak
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Use of etalon-immune distances to reduce the influence of background signals in frequency-modulation spectroscopy and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy2014In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 31, no 12, p. 2938-2945Article in journal (Refereed)
    Abstract [en]

    The detection sensitivity of phase-modulated techniques such as frequency-modulation spectroscopy (FMS) and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is often limited by etalon background signals. It has previously been shown that the impact of etalons can be reduced by the use of etalon-immune distances (EIDs), i.e., by separating the surfaces that give rise to etalons by a distance of q. L-m, where L-m is given by c/2n nu(m), where, in turn, n and nu(m) are the index of refraction between the components that make up the etalon (thus most often that of air) and the modulation frequency, respectively, and where q is an integer (i.e., 1, 2, 3,.) or half-integer (i.e., 1/2, 1, 3/2,.) for the dispersion and absorption modes of detection, respectively. An etalon created by surfaces separated by an EID will evade detection and thereby not contribute to any background signal. The concept of EIDs in FMS and NICE-OHMS is in this work demonstrated, scrutinized, and discussed in some detail. It is shown that the influence of EIDs on the absorption and dispersion modes of detection is significantly different; signals detected at the dispersion phase are considerably less sensitive to deviations from exact EID conditions than those detected at the absorption phase. For example, the FM background signal from an etalon whose length deviates from an EID by 2.5% of L-m (e.g., by 1 cm for an L-m of 40 cm), detected in dispersion, is only 9% of that in absorption. This makes the former mode of detection the preferred one whenever a sturdy immunity against etalons is needed or when optical components with parallel surfaces (e.g., lenses, polarizers, or beam splitters) are used. The impact of the concept of EIDs on NICE-OHMS is demonstrated by the use of Allan-Werle plots.

  • 52.
    Ehlers, Patrick
    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: optimum modulation and demodulation conditions, cavity length, and modulation order2014In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 31, no 9, p. 2051-2060Article in journal (Refereed)
    Abstract [en]

    Doppler broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry (Db-NICE-OHMS) has been scrutinized with respect to modulation and demodulation conditions (encompassing the modulation frequency,nu(m), the modulation index, beta, and the detection phase, theta), the cavity length, L, and the modulation order, k (defined as nu(m)/nu(FSR), where nu(FSR) is the free-spectral range of the cavity), primarily in the Doppler limit but also for two specific situations in the Voigt regime (for equal Doppler and homogeneous width and for purely Lorentzian broadened transitions), both in the absence and presence of optical saturation (the latter for the case in which the homogeneous broadening is smaller than the modulation frequency). It is found that, for a system with a given cavity length, the optimum conditions (i.e., those that produce the largest NICE-OHMS signal) for an unsaturated transition in the Doppler limit comprise nu(m)/Gamma(D) = 1.6 (where Gamma(D) is the half-width at half-maximum of the Doppler width of the transition), beta = 1.3, and theta = 0.78 pi. It is also found that the maximum is rather broad; the signal takes 95% of its maximum value for modulation frequencies in the entire 0.4 less than or similar to nu(m)/Gamma(D) less than or similar to 2.4 range. When optical saturation sets in, theta shifts toward the dispersion phase. The optimum conditions encompass k > 1 whenever L > 0.35L(D) and 2.6L(D) for the dispersion and absorption modes of detection, respectively [where L-D is a characteristic length given by c/(2 Gamma(D))]. Similar conditions are found under pressure broadened conditions. 

  • 53.
    Ehlers, Patrick
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Silander, Isak
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wang, Junyang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry instrumentation for Doppler-broadened detection in the 10-12 cm-1 Hz-1/2 region2012In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 29, no 6, p. 1305-1315Article in journal (Refereed)
    Abstract [en]

    A fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry (FL-NICE-OHMS) system for white-noise-limited Doppler-broadened detection down to 5.6 x 10(-12) cm(-1) Hz(-1/2) is demonstrated. The system is based on a previous FL-NICE-OHMS instrumentation in which the locking of the laser frequency to a cavity mode has been improved by the use of an acousto-optic modulator (AOM) and provision of a more stable environment by the employment of a noise-isolating enclosed double-layer table, a temperature regulation of the laboratory, and an ultra-high-vacuum (UHV) gas system. White-noise behavior up to 10 s provides the instrument with a minimum detectable on-resonance absorbance per unit length of 1.8 x 10(-12) cm(-1) and a relative single-pass absorption (Delta I/I) of 7.2 x 10(-11). The system was applied to detection of acetylene on a transition at 1531.588 nm, yielding a detection sensitivity of C2H2 in atmospheric pressure gas of 4 ppt (measured over 10 s). (C) 2012 Optical Society of America

  • 54.
    Ehlers, Patrick
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Silander, Isak
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wang, Junyang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry incorporating an optical circulator2014In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 39, no 2, p. 279-282Article in journal (Refereed)
    Abstract [en]

    To reduce the complexity of fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry, a system incorporating a fiber-coupled optical circulator to deflect the cavity-reflected light for laser stabilization has been realized. Detection near the shot-noise limit has been demonstrated for both Doppler-broadened and sub-Doppler signals, yielding a lowest detectable absorption and optical phase shift of 2.2 x 10(-12) cm(-1) and 4.0 x 10(-12) cm(-1), respectively, both for a 10 s integration time, where the former corresponds to a detection limit of C2H2 of 5 ppt. (C) 2014 Optical Society of America

  • 55.
    Ehlers, Patrick
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wang, Junyang
    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 NICE-OHMS beyond the triplet formalism: assessment of optimum modulation index2014In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 31, no 7, p. 1499-1507Article in journal (Refereed)
    Abstract [en]

    The dependence of Doppler broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) on the modulation index, beta, has been investigated experimentally on C2H2 and CO2, both in the absence and the presence of optical saturation. It is shown that the maximum signals are obtained for beta that produce more than one pair of sidebands: in the Doppler limit and for the prevailing conditions (unsaturated transition and the pertinent modulation frequency and Doppler widths) around 1 and 1.4 for the dispersion and absorption detection phases, respectively. The results verify predictions given in an accompanying work. It is also shown that there is no substantial broadening of the NICE-OHMS signal for beta < 1. The use of beta of unity has yielded a Db-NICE-OHMS detection sensitivity of 4.9 x 10(-12) cm(-1) Hz(-1/2), which is the lowest (best) value so far achieved for NICE-OHMS based on a tunable laser. The number of sidebands that needs to be included in fits of the line-shape function to obtain good accuracy has been assessed. It is concluded that it is enough to consider three pairs of sidebands whenever the systematic errors in a concentration assessment should be below 1% when beta < 2 are used and <1 parts per thousand for beta < 1.5.

  • 56.
    Eriksson, Mirjam
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tunable diode laser absorptionspectroscopy of atomic potassium in a KOH-seeded flat flame2018Independent thesis Advanced level (degree of Master (Two Years)), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    Potassium (K) is the main ash-forming element released from biomass during thermochemical conversion. A better understanding of K chemistry and monitoring of K species is needed to optimize combustion systems. Since K species are highly reactive and prevailing concentrations depend on the conversion conditions, accurate quantification requires in situ measurement techniques. Tunable diode laser absorption spectroscopy with a single-mode distributed feedback laser is used to probe the D1 transition of atomic potassium, K(g), at 769.9 nm. The large current tuning range of the diode laser (5 cm-1) enables monitoringthe wings of the absorption profile. Fitting to the acquired line shape wings is used as astrategy to enhance the dynamic range of the sensor and measure K(g) concentrations even under optically thick condition. A potassium-rich combustion environment is simulated by converting KOH salt in a premixed methane/air flat flame. Quantitative measurements of K(g) are made at 75 positions in the flame. This yields radial K(g) profiles at three different heightsin the plume above the KOH salt and an axial profile at the burner center. The acquired average K(g) concentrations are corrected for effective plume size, i.e. the absorption pathlength determined from the radial profiles. Knowledge of the K(g) distribution in flames can lead to a better understanding of K release and primary reaction kinetics.

  • 57.
    Fakhri, Maryam
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Near-infrared optical frequency comb Vernier spectroscopy in air and in a flame2017Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    A Vernier spectrometer is built with a near-infrared mode-locked Er:doped fiber laser, a Fabry-Perot cavity with finesse of 1000, a diffraction grating and a photo detector. The optical cavity provides high sensitivity in absorption detection by enhancing the interaction length of the light with molecular species contained in the cavity. Coupling an optical frequency comb to the cavity provided a broadband spectral bandwidth with high precision to measure the absorption of several molecular species simultaneously. Also, by using the optical cavity as a filter, transmission of some bunch comb lines was achieved. This comb filtering together with a simple grating and a photodiode formed the Vernier detection technique to provide very fast measurements while it kept the setup very simple and compact. The system allows to detect carbon dioxide in the air and water vapor and OH radicals in the flame in a spectrum spanning from 1550 nm to 1590 nm, approximately. The retrieved spectrum has a resolution of 9.3 GHz being acquired in 0.05 s.

  • 58. Fatehi, Hesameddin
    et al.
    Schmidt, Florian M.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Bai, Xue-Song
    Gas phase combustion in the vicinity of a biomass particle during devolatilization: model development and experimental verification2018In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 196, p. 351-363Article in journal (Refereed)
    Abstract [en]

    A numerical and experimental study on the devolatilization of a large biomass particle is carried out to quantify the effect of homogeneous volatile combustion on the conversion of the particle and on the temperature and species distribution at the particle vicinity. A global chemical kinetic mechanism and a detailed reaction mechanism are considered in a one dimensional numerical model that takes into account preferential diffusivity and a detailed composition of tar species. An adaptive moving mesh is employed to capture the changes in the domain due to particle shrinkage. The effect of gas phase reactions on pyrolysis time, temperature and species distribution close to the particle is studied and compared to experiments. Online in situ measurements of average H2O mole fraction and gas temperature above a softwood pellet are conducted in a reactor using tunable diode laser absorption spectroscopy (TDLAS) while recording the particle mass loss. The results show that the volatile combustion plays an important role in the prediction of biomass conversion during the devolatilization stage. It is shown that the global reaction mechanism predicts a thin flame front in the vicinity of the particle deviating from the measured temperature and H2O distribution over different heights above the particle. A better agreement between numerical and experimental results is obtained using the detailed reaction mechanism, which predicts a wider reaction zone.

  • 59. Fitting, H.-J.
    et al.
    Cornet, N.
    Salh, Roushdey
    Physics Department, University of Rostock, Universit¨atsplatz 3, D-18051 Rostock, Germany.
    Guerret-Piécourt, C.
    Goeuriot, D.
    von Czarnowski, A.
    Electron beam excitation in thin layered samples2007In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 159, p. 46-52Article in journal (Refereed)
  • 60. Fitting, H.-J.
    et al.
    Fitting-Kourkoutis, L.
    Salh, Roushdey
    Physics Department, University of Rostock, Universitätsplatz 3, D-18051 Rostock, Germany.
    Schmidt, B.
    EFTEM EELS and Cathodoluminescence in Si-implanted SiO2 layers2009In: Microscopy and Microanalysis, ISSN 1431-9276, E-ISSN 1435-8115, Vol. 15, no S2, p. 1104-1105Article in journal (Refereed)
  • 61. Fitting, H.-J.
    et al.
    Fitting-Kourkoutis, L.
    Salh, Roushdey
    Physics Department, University of Rostock, Universitätsplatz 3, D-18051 Rostock, Germany.
    Zamoryanskaya, M. V.
    EFTEM, EELS, and Cathodoluminescence of Si nanoclusters in silica2009In: Instrumentation and Methodology, Vol. 1, p. 129-130Article in journal (Refereed)
  • 62. Fitting, H.-J.
    et al.
    Salh, Roushdey
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Multimodal luminescence in ion-implanted silica2007In: Fundamental and Applied Spectroscopy, ISSN 0094-243X, Vol. 25-28, p. 18-25Article, book review (Refereed)
  • 63. Fitting, H.-J.
    et al.
    Salh, Roushdey
    Institute of Physics, University Rostock, Rostock, Germany.
    Barfels, T.
    Schmidt, B.
    Multimodal Luminescence Spectra of Ion-implanted Silica2005In: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396X, Vol. 202, p. R142-R144Article in journal (Refereed)
  • 64. Fitting, H.-J.
    et al.
    Salh, Roushdey
    Physics Department, University of Rostock, Universitätsplatz 3, D-18051 Rostock, Germany.
    Fitting Kourkoutis, L.
    Cathodoluminescence of defects interstitial molecules and clusters in silica2007In: Microscopy and Microanalysis, ISSN 1431-9276, E-ISSN 1435-8115, Vol. 13, no S02, p. 1378-1379Article in journal (Refereed)
  • 65. Fitting, H.-J.
    et al.
    Salh, Roushdey
    Physics Dept., Rostock University, Universitaetsplatz 3, D-18051 Rostock, Germany.
    Kourkoutis, L.
    Schmidt, B.
    Cluster growth and luminescence in ion-implanted silica2008In: EMC 2008 14th European Microscopy Congress 1–5 September 2008, Aachen, Germany Volume 2: Materials Science / [ed] Silvia Richter and Alexander Schwedt, 2008, Vol. 2, p. 17-18Conference paper (Refereed)
  • 66. Fitting, H.-J.
    et al.
    Salh, Roushdey
    Institute of Physics, University of Rostock, Universitätsplatz 3, D-18051 Rostock, Germany.
    Schmidt, B.
    Multimodal electronic-vibronic spectra of luminescence in ion-implanted silica layers2007In: Journal of Luminescence, ISSN 0022-2313, E-ISSN 1872-7883, Vol. 122-123, p. 743-746Article in journal (Refereed)
    Abstract [en]

    Thermally oxidized SiO2 layers of 100 and 500 nm thickness have been implanted by oxygen and sulfur ions with a dose of 3×1016 and 5×1016 ions/cm2, respectively, leading to an atomic dopant fraction of about 4 at.% at the half depth of the SiO2 layers. The cathodoluminescence spectra of oxygen and sulfur implanted SiO2 layers show besides characteristic bands a sharp and intensive multimodal structure beginning in the green region at 500 nm over the yellow-red region extending to the near IR measured up to 820 nm. The energy step differences of the sublevels amount on average 120 meV and indicate vibration associated electronic states, probably of O2-interstitial molecules, as we could demonstrate by a respective configuration coordinate model.

  • 67. Fitting, H.-J.
    et al.
    Salh, Roushdey
    Institute of Physics, University of Rostock, D-18051 Rostock, Germany.
    Schmidt, B.
    Multimodal Luminescence Spectra of Ion-implanted Silica2007In: Semiconductors, Vol. 41, p. 453-457Article in journal (Refereed)
    Abstract [en]

    The main luminescence bands in silica SiO2are the red luminescence R (650 nm, 1.9 eV) of thenon-bridging oxygen hole center, and the blue band B (460 nm, 2.7 eV) and ultraviolet luminescence UV (290 nm,4.3 eV), both commonly related to oxygen-deficient centers. In the present work, we will enhance or replaceeither the first or second constituent of SiO2, i.e., silicon or oxygen, isoelectronically by additional implantationof the respective ions. Thus, thermally oxidized SiO2layers have been implanted by different ions of theIV group (C, Si, Ge, Sn, Pb) and of the VI group (O, S, Se) with doses up to 5×1016cm–2, leading to an atomicdopant fraction of about 4 at % at the half depth of the SiO2layers. Very surprisingly, the cathodoluminescencespectra of oxygen- and sulfur-implanted SiO2layers show, besides the characteristic bands, a sharp and intensivemultimodal structure beginning at the green region at 500 nm up to the near infrared. The energy step differencesof the sublevels equal on average 120 meV, and indicate vibration associated electronic states, probablyof interstitial molecules.

  • 68. Fitting, H.-J.
    et al.
    Salh, Roushdey
    German Institute for Polymers (DKI), Department of Physics, Schlossgartenstrasse 6, D-64289 Darmstadt, Germany.
    Schmidt, B.
    Thermal decomposition and new luminescence bands in wet, dry, and additional oxygen implanted silica layers2008In: Journal of Non-Crystalline Solids, ISSN 0022-3093, E-ISSN 1873-4812, Vol. 354, p. 1697-1702Article in journal (Refereed)
  • 69. Fitting, H.-J.
    et al.
    Ziems, T.
    Salh, Roushdey
    Physics Department, University of Rostock, Rostock, Germany.
    von Czarnowski, A.
    Schmidt, B.
    Luminescent Defect Dynamics in Amorphous SiO2:H2005In: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 2, p. 693-698Article in journal (Refereed)
  • 70. Fitting, H.-J.
    et al.
    Ziems, T.
    Salh, Roushdey
    Institute of Physics, Rostock University, Rostock, Germany.
    Zamoryanskaya, M. V.
    Kolesnikova, K. V.
    Schmidt, B.
    von Czarnowski, A.
    Cathodoluminescence of wet, dry and hydrogen-implanted silica films2005In: Journal of Non-Crystalline Solids, ISSN 0022-3093, E-ISSN 1873-4812, Vol. 351, p. 2251-2262Article in journal (Refereed)
  • 71.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry2009Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Noise-immune cavity-enhanced optical heterodyne molecular spectro-metry (NICE-OHMS) is one of the most sensitive laser-based absorption techniques. The high sensitivity of NICE-OHMS is obtained by a unique combination of cavity enhancement (for increased interaction length with a sample) with frequency modulation spectrometry (for reduction of noise). Moreover, sub-Doppler detection is possible due to the presence of high intensity counter-propagating waves inside an external resonator, which provides an excellent spectral selectivity. The high sensitivity and selectivity make NICE-OHMS particularly suitable for trace gas detection. Despite this, the technique has so far not been often used for practical applications due to its technical complexity, originating primarily from the requirement of an active stabilization of the laser frequency to a cavity mode.

    The main aim of the work presented in this thesis has been to develop a simpler and more robust NICE-OHMS instrumentation without compro-mising the high sensitivity and selectivity of the technique. A compact NICE-OHMS setup based on a fiber laser and a fiber-coupled electro-optic modulator has been constructed. The main advantage of the fiber laser is its narrow free-running linewidth, which significantly simplifies the frequency stabilization procedure. It has been demonstrated, using acetylene and carbon dioxide as pilot species, that the system is capable of detecting relative absorption down to 3 × 10-9 on a Doppler-broadened transition, and sub-Doppler optical phase shift down to 1.6 × 10-10, the latter corresponding to a detection limit of 1 × 10-12 atm of C2H2. Moreover, the potential of dual frequency modulation dispersion spectrometry (DFM-DS), an integral part of NICE-OHMS, for concentration measurements has been assessed.

    This thesis contributes also to the theoretical description of Doppler-broadened and sub-Doppler NICE-OHMS signals, as well as DFM-DS signals. It has been shown that the concentration of an analyte can be deduced from a Doppler-broadened NICE-OHMS signal detected at an arbitrary and unknown detection phase, provided that a fit of the theoretical lineshape to the experimental data is performed. The influence of optical saturation on Doppler-broadened NICE-OHMS signals has been described theoretically and demonstrated experimentally. In particular, it has been shown that the Doppler-broadened dispersion signal is unaffected by optical saturation in the Doppler limit. An expression for the sub-Doppler optical phase shift, valid for high degrees of saturation, has been derived and verified experimentally up to degrees of saturation of 100.

  • 72.
    Foltynowicz, Aleksandra
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Schmidt, Florian M.
    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 signals from optically saturated transitions under low pressure conditions2008In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 25, no 7, p. 1156-1165Article in journal (Refereed)
    Abstract [en]

    The influence of optical saturation on noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) signals from purely Doppler-broadened transitions is investigated experimentally. It is shown that the shape and the strength of the dispersion signal are virtually unaffected by optical saturation, whereas the strength of the absorption signal decreases as (1+G+-1)-1/2, where G+-1 is the degree of saturation induced by the sideband of the frequency modulated triplet, in agreement with theoretical predictions. This implies, first of all, that Doppler-broadened NICE-OHMS is affected less by optical saturation than other cavity enhanced techniques but also that it exhibits nonlinearities in the power and pressure dependence for all detection phases except pure dispersion. A methodology for assessments of the degree of saturation and the saturation power of a transition from Doppler-broadened NICE-OHMS signals is given. The implications of optical saturation for practical trace species detection by Doppler-broadened NICE-OHMS are discussed.

  • 73.
    Foltynowicz, Aleksandra
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Schmidt, Florian M
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wavelength modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy signal line shapes in the Doppler limit2009In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 26, no 7, p. 1384-1394Article in journal (Refereed)
    Abstract [en]

    A thorough analysis of the shape and strength of Doppler-broadened wavelength modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy signals is presented and their dependence on modulation frequency, modulation amplitude and detection phase is investigated in detail. The conditions that maximize the on-resonance signal are identified. The analysis is based on the standard frequency modulation spectroscopy formalism and the Fourier description of wavelength modulation spectroscopy and verified by fits to experimental signals from C2H2 and CO2 measured at 1531 nm. In addition, the line strengths of two CO2 transitions in the v2→3v1+v2+v3 hot band [Pe(7) and Pe(9)] were found to differ by ~20% from those given in the HITRAN database.

  • 74.
    Foltynowicz-Matyba, Aleksandra
    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.
    Reduction of background signals in fiber-based NICE-OHMS2011In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 28, no 11, p. 2797-2805Article in journal (Refereed)
    Abstract [en]

    Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) based on a fiber-coupled electro-optic modulator (EOM) provides a compact and versatile experimental setup. It has, however, been limited by background signals originating from an imbalance of the phase modulated triplet created by a cross-coupling between the principal axes of the polarization maintaining fibers and the extraordinary axis of the EOM. Two strategies for reducing these background signals are investigated: (i) using an EOM with a titanium diffused waveguide, in which the balance of the triplet is controlled by active feedback, and (ii) using an EOM with a proton exchanged waveguide that does not support light propagation along the ordinary axis. It is shown that both approaches significantly reduce drifts and noise in the system. Using a cavity with a finesse of 5700, an absorption sensitivity of 3: 2 x 10(-12) cm(-1) in 1 min of integration time (i.e., 1: 8 x 10(-11) cm(-1) Hz(-1/2)) is demonstrated for Doppler-broadened detection, the lowest reported so far for Doppler-broadened NICE-OHMS. For sub-Doppler detection, a minimum detectable optical phase shift of 1: 3 x 10(-12) cm(-1) in 400s of integration time is obtained. (C) 2011 Optical Society of America

  • 75.
    Forsling, Robin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Time constants of optical turbulence in the atmosphere2016Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 76. Froelich, P.
    et al.
    Dalgarno, A.
    Jonsell, Svante
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Saenz, A.
    Voroni, A.
    Zygelman, B.
    Antihydrogen in interaction with atoms and surfaces2005In: AIP Conference Proceedings vol. 770, 2005, p. 51-67Conference paper (Refereed)
    Abstract [en]

    An overview of the antiatom interactions with matter is given in the context of the ongoing efforts to trap and cool antihydrogen at CERN. We show how the atom‐antiatom scattering can, in addition to annihilation, result in a number of collisional reactions such as elastic scattering, rearrangement to protonium and positronium, and even radiative association leading to formation of unusual short‐lived atom‐antiatom molecules. We also show that sufficiently cold antiatoms can be reflected from material walls. The rates for these reactions are of interest for the experiments with antihydrogen atoms at CERN, they probe the feasibility of collisional cooling of antihydrogen and they contribute to the understanding of stability of matter in contact with antimatter in general. © 2005 American Institute of Physics

  • 77. Fuller, Franklin D
    et al.
    Gul, Sheraz
    Chatterjee, Ruchira
    Burgie, E Sethe
    Young, Iris D
    Lebrette, Hugo
    Srinivas, Vivek
    Brewster, Aaron S
    Michels-Clark, Tara
    Clinger, Jonathan A
    Andi, Babak
    Ibrahim, Mohamed
    Pastor, Ernest
    de Lichtenberg, Casper
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Hussein, Rana
    Pollock, Christopher J
    Zhang, Miao
    Stan, Claudiu A
    Kroll, Thomas
    Fransson, Thomas
    Weninger, Clemens
    Kubin, Markus
    Aller, Pierre
    Lassalle, Louise
    Brauer, Philipp
    Miller, Mitchell D
    Amin, Muhamed
    Koroidov, Sergey
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, California, USA.
    Roessler, Christian G
    Allaire, Marc
    Sierra, Raymond G
    Docker, Peter T
    Glownia, James M
    Nelson, Silke
    Koglin, Jason E
    Zhu, Diling
    Chollet, Matthieu
    Song, Sanghoon
    Lemke, Henrik
    Liang, Mengning
    Sokaras, Dimosthenis
    Alonso-Mori, Roberto
    Zouni, Athina
    Messinger, Johannes
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Department of Chemistry–Ångström, Molecular Biomimetics, Uppsala University, Uppsala, Sweden.
    Bergmann, Uwe
    Boal, Amie K
    Bollinger Jr, J Martin
    Krebs, Carsten
    Hogbom, Martin
    Phillips Jr, George N
    Vierstra, Richard D
    Sauter, Nicholas K
    Orville, Allen M
    Kern, Jan
    Yachandra, Vittal K
    Yano, Junko
    Drop-on-demand sample delivery for studying biocatalysts in action at X-ray free-electron lasers2017In: Nature Methods, ISSN 1548-7091, E-ISSN 1548-7105, Vol. 14, p. 443-449Article in journal (Refereed)
    Abstract [en]

    X-ray crystallography at X-ray free-electron laser sources is a powerful method for studying macromolecules at biologically relevant temperatures. Moreover, when combined with complementary techniques like X-ray emission spectroscopy, both global structures and chemical properties of metalloenzymes can be obtained concurrently, providing insights into the interplay between the protein structure and dynamics and the chemistry at an active site. The implementation of such a multimodal approach can be compromised by conflicting requirements to optimize each individual method. In particular, the method used for sample delivery greatly affects the data quality. We present here a robust way of delivering controlled sample amounts on demand using acoustic droplet ejection coupled with a conveyor belt drive that is optimized for crystallography and spectroscopy measurements of photochemical and chemical reactions over a wide range of time scales. Studies with photosystem II, the phytochrome photoreceptor, and ribonucleotide reductase R2 illustrate the power and versatility of this method.

  • 78.
    Ghorbani, Ramin
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Real-time breath gas analysis of carbon monoxide: laser-based detection and pulmonary gas exchange modeling2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Breath gas analysis is a promising approach for non-invasive medical diagnostics and physiological monitoring. Real-time, breath-cycle resolved biomarker detection facilitates data interpretation and has the potential to improve the diagnostic value of breath tests as exhalation profiles carry spatiotemporal information about biomarker origin and gas exchange in the respiratory tract. This thesis presents and scrutinizes a novel methodology for the analysis of real-time breath data, where single-exhalation profiles are simulated using a pulmonary gas exchange model and least-squares fitted to measured expirograms to extract airway and alveolar contributions and diffusing capacities. The methodology is demonstrated on exhaled breath carbon monoxide (eCO), a candidate biomarker for oxidative stress and respiratory diseases. The thesis mainly covers (1) the construction of a compact optical sensor based on tunable diode laser absorption spectroscopy (TDLAS) in the mid-infrared region (4.7 μm) for selective and precise real-time detection of CO in breath and ambient air (detection limit 9 ± 5 ppb at 0.1 s), (2) the design of an advanced online breath sampling system, (3) the implementation of a trumpet model with axial diffusion (TMAD) to simulate the CO gas exchange, and (4) the application of extended eCO analysis in clinical studies to establish the healthy non-smoker baseline of the eCO parameters and to study the response to CO and wood smoke exposure. It is shown that the TMAD adequately describes the gas exchange during systemic CO elimination for different breathing patterns, and that there is no difference between eCO parameters from mouth- and nose exhalations. Expirogram shape and eCO parameters exhibit a dependence on the exhalation flow rate, but for a given breathing maneuverer, the parameters lie in a narrow range. Airway CO is close to and correlates with ambient air CO, indicating negligible airway production in the healthy population. The alveolar diffusing capacity is independent of endogenous CO, even after exposure to elevated exogenous CO, and could be used to assess lung diffusion abnormalities. Compared to CO exposure, no clear additional effect of exposure to wood smoke particles on eCO is observed. The discrimination between endogenous and exogenous CO sources remains a challenge.

  • 79.
    Ghorbani, Ramin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Blomberg, Anders
    Umeå University, Faculty of Medicine, Department of Public Health and Clinical Medicine, Medicine.
    Schmidt, Florian M.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Extended breath CO analysis: baseline and diurnal variation of pulmonary gas exchange parametersManuscript (preprint) (Other academic)
  • 80.
    Ghorbani, Ramin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Blomberg, Anders
    Umeå University, Faculty of Medicine, Department of Public Health and Clinical Medicine, Medicine.
    Schmidt, Florian M.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Modeling pulmonary gas exchange and single-exhalation profiles of carbon monoxide2018In: Frontiers in Physiology, ISSN 1664-042X, E-ISSN 1664-042X, Vol. 9, article id 927Article in journal (Refereed)
    Abstract [en]

    Exhaled breath carbon monoxide (eCO) is a candidate biomarker for non-invasive assessment of oxidative stress and respiratory diseases. Standard end-tidal CO analysis, however, cannot distinguish, whether eCO reflects endogenous CO production, lung diffusion properties or exogenous sources, and is unable to resolve a potential airway contribution. Coupling real-time breath gas analysis to pulmonary gas exchange modeling holds promise to improve the diagnostic value of eCO. A trumpet model with axial diffusion (TMAD) is used to simulate the dynamics of CO gas exchange in the respiratory system and corresponding eCO concentrations for the first time. The mass balance equation is numerically solved employing a computationally inexpensive routine implementing the method of lines, which provides the distribution of CO in the respiratory tract during inhalation, breath-holding and exhalation with 1 mm spatial and 0.01 s temporal resolution. Initial estimates of the main TMAD parameters, the maximum CO fluxes and diffusing capacities in alveoli and airways, are obtained using healthy population tissue, blood and anatomical data. To verify the model, mouth-exhaled expirograms from two healthy subjects, measured with a novel, home-built laser-based CO sensor, are compared to single-exhalation profiles simulated using actual breath sampling data, such as exhalation flow rate (EFR) and volume. A very good agreement is obtained in exhalation phases I and III for EFRs between 55 and 220 ml/s and after 10 s and 20 s of breath-holding, yielding a unique set of TMAD parameters. The results confirm the recently observed EFR dependence of CO expirograms and suggest that measured end-tidal eCO is always lower than alveolar and capillary CO. Breath-holding allows the observation of close-to-alveolar CO concentrations and increases the sensitivity to the airway TMAD parameters in exhalation phase I. A parametric simulation study shows that a small increase in airway flux can be distinguished from an increase in alveolar flux, and that slight changes in alveolar flux and diffusing capacity have a significantly different effect on phase III of the eCO profiles.

  • 81.
    Ghorbani, Ramin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Schmidt, Florian M.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Fitting of single-exhalation profiles using a pulmonary gas exchange model: application to carbon monoxide2019In: Journal of Breath Research, ISSN 1752-7155, E-ISSN 1752-7163, Vol. 13, no 2, article id 026001Article in journal (Refereed)
    Abstract [en]

    Real-time breath gas analysis coupled to gas exchange modeling is emerging as promising strategy to enhance the information gained from breath tests. It is shown for exhaled breath carbon monoxide (eCO), a potential biomarker for oxidative stress and respiratory diseases, that a weighted, nonlinear least-squares fit of simulated to measured expirograms can be used to extract physiological parameters, such as airway and alveolar concentrations and diffusing capacities. Experimental CO exhalation profiles are acquired with high time-resolution and precision using mid-infrared tunable diode laser absorption spectroscopy and online breath sampling. A trumpet model with axial diffusion is employed to generate eCO profiles based on measured exhalation flow rates and volumes. The concept is demonstrated on two healthy non-smokers exhaling at a flow rate of 250 ml s−1 during normal breathing and at 120 ml s−1 after 10 s of breath-holding. The obtained gas exchange parameters of the two subjects are in a similar range, but clearly distinguishable. Over a series of twenty consecutive expirograms, the intra-individual variation in the alveolar parameters is less than 6%. After a 2 h exposure to 10 ± 2 ppm CO, end-tidal and alveolar CO concentrations are significantly increased (by factors of 2.7 and 4.9 for the two subjects) and the airway CO concentration is slightly higher, while the alveolar diffusing capacity is unchanged compared to before exposure. Using model simulations, it is found that a three-fold increase in maximum airway CO flux and a reduction in alveolar diffusing capacity by 60% lead to clearly distinguishable changes in the exhalation profile shape. This suggests that extended breath CO analysis has clinical relevance in assessing airway inflammation and chronic obstructive pulmonary disease. Moreover, the novel methodology contributes to the standardization of real-time breath gas analysis.

  • 82.
    Ghorbani, Ramin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Schmidt, Florian M.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    ICL-based TDLAS sensor for real-time breath gas analysis of carbon monoxide isotopes2017In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 25, no 11, p. 12743-12752Article in journal (Refereed)
    Abstract [en]

    We present a compact sensor for carbon monoxide (CO) in air and exhaled breath based on a room temperature interband cascade laser (ICL) operating at 4.69 µm, a low-volume circular multipass cell and wavelength modulation absorption spectroscopy. A fringe-limited (1σ) sensitivity of 6.5 × 10−8 cm−1Hz-1/2 and a detection limit of 9 ± 5 ppbv at 0.07 s acquisition time are achieved, which constitutes a 25-fold improvement compared to direct absorption spectroscopy. Integration over 10 s increases the precision to 0.6 ppbv. The setup also allows measuring the stable isotope 13CO in breath. We demonstrate quantification of indoor air CO and real-time detection of CO expirograms from healthy non-smokers and a healthy smoker before and after smoking. Isotope ratio analysis indicates depletion of 13CO in breath compared to natural abundance.

  • 83.
    Ghorbani, Ramin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Schmidt, Florian M.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Real-time breath gas analysis of CO and CO2 using an EC-QCL2017In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 123, no 5, article id 144Article in journal (Refereed)
    Abstract [en]

    Real-time breath gas analysis is a promising, non-invasive tool in medical diagnostics, and well-suited to investigate the physiology of carbon monoxide (CO), a potential biomarker for oxidative stress and respiratory diseases. A sensor for precise, breath-cycle resolved, simultaneous detection of exhaled CO (eCO) and carbon dioxide (eCO2) was developed based on a continuous wave, external-cavity quantum cascade laser (EC-QCL), a low-volume multi-pass cell and wavelength modulation spectroscopy. The system achieves a noise-equivalent (1σ) sensitivity of 8.5 × 10−8 cm−1 Hz−1/2 and (2σ) detection limits of 9 ± 2 ppbv and 650 ± 7 ppmv at 0.14 s spectrum acquisition time for CO and CO2, respectively. Integration over 15 s yields a precision of 0.6 ppbv for CO. The fact that the eCO2 expirograms measured by capnography and laser spectroscopy have essentially identical shape confirms true real-time detection. It is found that the individual eCO exhalation profiles from healthy non-smokers have a slightly different shape than the eCO2 profiles and exhibit a clear dependence on exhalation flow rate and breath-holding time. Detection of indoor air CO and broadband breath profiling across the 93 cm−1 mode-hop-free tuning range of the EC-QCL are also demonstrated.

  • 84. Glimtoft, Martin
    et al.
    Svanqvist, Mattias
    Ågren, Matilda
    Nordberg, Markus
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Östmark, Henric
    Digital micromirror devices in Raman trace detection of explosives2016In: Detection and Sensing of Mines, Explosive Objects, and Obscured Targets XXI / [ed] Steven S. Bishop, Jason C. Isaacs, SPIE - International Society for Optical Engineering, 2016, article id 982312Conference paper (Refereed)
    Abstract [en]

    Imaging Raman spectroscopy based on tunable filters is an established technique for detecting single explosives particles at stand-off distances. However, large light losses are inherent in the design due to sequential imaging at different wavelengths, leading to effective transmission often well below 1 %. The use of digital micromirror devices (DMD) and compressive sensing (CS) in imaging Raman explosives trace detection can improve light throughput and add significant flexibility compared to existing systems. DMDs are based on mature microelectronics technology, and are compact, scalable, and can be customized for specific tasks, including new functions not available with current technologies. This paper has been focusing on investigating how a DMD can be used when applying CS-based imaging Raman spectroscopy on stand-off explosives trace detection, and evaluating the performance in terms of light throughput, image reconstruction ability and potential detection limits. This type of setup also gives the possibility to combine imaging Raman with non-spatially resolved fluorescence suppression techniques, such as Kerr gating. The system used consists of a 2nd harmonics Nd:YAG laser for sample excitation, collection optics, DMD, CMOS-camera and a spectrometer with ICCD camera for signal gating and detection. Initial results for compressive sensing imaging Raman shows a stable reconstruction procedure even at low signals and in presence of interfering background signal. It is also shown to give increased effective light transmission without sacrificing molecular specificity or area coverage compared to filter based imaging Raman. At the same time it adds flexibility so the setup can be customized for new functionality.

  • 85. Gonoskov, A.
    et al.
    Bastrakov, S.
    Efimenko, E.
    Ilderton, A.
    Marklund, M.
    Meyerov, I.
    Muraviev, A.
    Sergeev, A.
    Surmin, I.
    Wallin, Erik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Extended particle-in-cell schemes for physics in ultrastrong laser fields: Review and developments2015In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 92, no 2, article id 023305Article in journal (Refereed)
    Abstract [en]

    We review common extensions of particle-in-cell (PIC) schemes which account for strong field phenomena in laser-plasma interactions. After describing the physical processes of interest and their numerical implementation, we provide solutions for several associated methodological and algorithmic problems. We propose a modified event generator that precisely models the entire spectrum of incoherent particle emission without any low-energy cutoff, and which imposes close to the weakest possible demands on the numerical time step. Based on this, we also develop an adaptive event generator that subdivides the time step for locally resolving QED events, allowing for efficient simulation of cascades. Further, we present a unified technical interface for including the processes of interest in different PIC implementations. Two PIC codes which support this interface, PICADOR and ELMIS, are also briefly reviewed.

  • 86. Grivel, Jean-Claude
    et al.
    Alexiou, Aikaterini
    Namazkar, Shahla
    Pitillas, Andrea
    Effect of Platinum Group Metal Doping in Magnesium Diboride Wires2016In: IEEE transactions on applied superconductivity (Print), ISSN 1051-8223, E-ISSN 1558-2515, Vol. 26, no 3, article id 6200505Article in journal (Refereed)
    Abstract [en]

    The effect of some platinum group metals (PGM = Rh, Pd, and Pt) on the microstructure and critical current density of Cu/Nb-sheathed MgB2 wires has been studied using Mg1-x PGM(x)B(2) powders with low doping levels. It was found that Pt and Pd do not enter the MgB2 lattice and have only limited influence on T-c. In contrast, some Rh can be substituted and induces a decrease of T-c. Secondary phases are formed when the solubility limit is exceeded, but they have different morphologies depending on the dopant. For some of these PGM elements, flux pinning improvements have been observed at low fields. The results are discussed in comparison with previous investigations using other transition metals for doping on the Mg site.

  • 87. Guillaume, E.
    et al.
    Döpp, A.
    Thaury, C.
    Ta Phuoc, K.
    Lifschitz, A.
    Grittani, G.
    Goddet, J.-P.
    Tafzi, A.
    Chou, S.-W.
    Veisz, László
    Max-Planck-Institut für Quantenoptik, Garching, Germany.
    Malka, V.
    Electron Rephasing in a Laser-Wakefield Accelerator2015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, no 15, article id 155002Article in journal (Refereed)
    Abstract [en]

    An important limit for energy gain in laser-plasma wakefield accelerators is the dephasing length, after which the electron beam reaches the decelerating region of the wakefield and starts to decelerate. Here, we propose to manipulate the phase of the electron beam in the wakefield, in order to bring the beam back into the accelerating region, hence increasing the final beam energy. This rephasing is operated by placing an upward density step in the beam path. In a first experiment, we demonstrate the principle of this technique using a large energy spread electron beam. Then, we show that it can be used to increase the energy of monoenergetic electron beams by more than 50%.

  • 88.
    Guo, Qinda
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Construction of a free-space DFB-laser-based frequency modulated laser radar for measurement of distance and velocity2016Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    The aim of this experiment is to construct a frequency modulated laser radar for measuring the distance and velocity of an object. The system is based on a DFB laser and a heterodyne Michelson interferometer constructed by free space components. Using the laser radar we measure the position of an object in a 60 cm range with a maximum error of 2.6 mm, and the velocity of an object in 3 to 20 mm/s with a maximum error of 0.11 mm/s. The acquisition times are 60 ms and 120 ms for the position and velocity measurements respectively. Using Allan variance we estimate to have an absolute position detection sensitivity of 20 μm after ~200s of averaging.

  • 89.
    Guo, Wenli
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Design of laser tweezers set-up and evaluation of its stiffness using equipartition theorem2017Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    Optical tweezers can hold and move micro particles by using only a laser beam. The objective of this thesis work was first to design and build an optical tweezers setup and trap one micro-meter spherical particle. Second, we evaluated the trap stiffness using the equipartition theorem method. We found that the optical design allowed for trapping the particles and we measured the trap stiffness to 2.8 ± 0.9 pN/µm in the x direction and 2.6 ± 0.8 pN/µm in the y direction. The uneven trapping strength is due to the alignment of the lenses.

  • 90.
    Hagman, Henning
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Dion, Claude
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sjölund, Peder
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Petra, Stefan J.H.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Kastberg, Anders
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Influence of the lattice topography on a three-dimensional, controllable Brownian motor2008In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 81, no 3, p. 33001-Article in journal (Refereed)
    Abstract [en]

    We study the influence of the lattice topography and the coupling between motion in different directions, for a three-dimensional Brownian motor based on cold atoms in a double optical lattice. Due to controllable relative spatial phases between the lattices, our Brownian motor can induce drifts in arbitrary directions. Since the lattices couple the different directions, the relation between the phase shifts and the directionality of the induced drift is non-trivial. Here is therefore this relation investigated experimentally by systematically varying the relative spatial phase in two dimensions, while monitoring the vertically induced drift and the temperature. A relative spatial phase range of 2pi x 2pi is covered. We show that a drift, controllable both in speed and direction, can be achieved, by varying the phase both parallel and perpendicular to the direction of the measured induced drift. The experimental results are qualitatively reproduced by numerical simulations of a simplified, classical model of the system.

  • 91.
    Han, Dongmei
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Design of an optical trap and calibration of its trap stiffness using the equipartition theorem2016Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    The aim of this work was to construct an optical tweezers instrumentation and to calibrate the stiffness of the system using the equipartition theorem. The beam expansion part of optical tweezers system was designed using Zemax. The trapping part of the system was built using various optics, among other things, a 0.36 mW He-Ne laser and an oil-immersion objective with numerical aperture (NA) of 1.3 for trapping of polystyrene bead with 1μm. The equipartition theorem was applied to calibrate the trap stiffness using position sensitive detector (PSD). The calibration coefficient was obtained by measuring the relative displacement recorded by the PSD with respect to the displacement of particle on the specimen plane in x and y direction. The stiffness we measured in x direction was (3.4±0.2) pN/μm, and in y direction was (4.0±0.1) pN/μm. By improving the alignment with a pin hole and making sure the center of the beam went through the objective, a symmetrical optical trap was achieved. And by preserving the laser power with using as fewer optics as possible to get larger trapping force, the 1μm bead was trapped.

  • 92.
    Hansson, Tobias
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wallin, Erik
    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.
    Scalar Wigner theory for polarized light in nonlinear Kerr media2013In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 30, no 6, p. 1765-1769Article in journal (Refereed)
    Abstract [en]

    A scalar Wigner distribution function for describing polarized light is proposed in analogy with the treatment of spin variables in quantum kinetic theory. The formalism is applied to the propagation of circularly polarized light in nonlinear Kerr media, and an extended phase-space evolution equation is derived along with invariant quantities. The formalism is additionally used to analyze the modulational instability. (C) 2013 Optical Society of America

  • 93. Harvey, C. N.
    et al.
    Gonoskov, A.
    Marklund, M.
    Wallin, Erik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Narrowing of the emission angle in high-intensity Compton scattering2016In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 93, no 2, article id 022112Article in journal (Refereed)
    Abstract [en]

    We consider the emission spectrum of high-energy electrons in an intense laser field. At high intensities (a0∼200) we find that the QED theory predicts a narrower angular spread of emissions than the classical theory. This is due to the classical theory overestimating the energy loss of the particles, resulting in them becoming more susceptible to reflection in the laser pulse.

  • 94.
    Hashemloo, Avazeh
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Numerical simulation of the dynamics of a trapped molecular ion2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis explores the dynamics of a heteronuclear diatomic molecular ion, possessing a permanent electric dipole moment, µ, which is trapped in a linear Paul trap and can interact with an off-resonance laser field. To build our model we use the rigid-rotor approximation, where the dynamics of the molecular ion are limited to its translational and rotational motions of the center-of-mass. These dynamics are investigated by carrying out suitable numerical calculations.

    To introduce our numerical methods, we divide our research topic into two different subjects. First, we ignore the rotational dynamics of the ion by assuming µ = 0. By this assumption, the system resembles an atomic ion, which mainly exhibits translational motion for its center of the mass when exposed to an external trapping field. To study this translational behavior, we implement full-quantum numerical simulations, in which a wave function is attributed to the ion. Finally, we study the quantum dynamics of the mentioned wave packet and we compare our results with those obtained classically.

    In the latter case, we keep the permanent dipole moment of the ion and we study the probable effects of the interaction between the dipole moment and the trapping electric field, on both the translational and the rotational dynamics of the trapped molecular ion. In order to study these dynamics, we implement both classical and semi-classical numerical simulations. In the classical method, the rotational and the translational motions of the center of mass of the ion are obtained via classical equations of motion. On the other hand, in the semi-classical method, while the translational motion of the center-of-mass is still obtained classically, the rotation is treated full-quantum mechanically by considering the rotational wave function of the ion. In the semi-classical approach, we mainly study the probable couplings between the rotational states of the molecular ion, due to the interaction of the permanent dipole moment with the trapping electric field. In the end, we also present a semi-classical model, where the trapped molecular ion interacts with an off-resonance laser field.

  • 95.
    Hashemloo, Avazeh
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Dion, Claude
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Quantum stability of an atomic ion in a Paul trap revisited2017In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 115, no 15-16, p. 1927-1933Article in journal (Refereed)
    Abstract [en]

    We study the quantum stability of the dynamics of atomic ions in a Paul trap. We revisit the results of Wang et al [Phys. Rev. A 52, 1419 (1995)], which showed that quantum trajectories did not have the same region of stability as their classical counterpart, contrary to what is obtained from a Floquet analysis of the motion in the periodic trapping field. Using numerical simulations of the full wave-packet dynamics, we confirm that the classical trapping criterion are fully applicable to quantum motion, when considering both the expectation value of the position of the wave packet and its width.

  • 96.
    Hashemloo, Avazeh
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Dion, Claude
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rotational dynamics of a diatomic molecular ion in a Paul trap2015In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 143, article id 204308Article in journal (Refereed)
    Abstract [en]

    We present models for a heteronuclear diatomic molecular ion in a linear Paul trap in a rigid-rotor approximation, one purely classical and the other where the center-of-mass motion is treated classically, while rotational motion is quantized. We study the rotational dynamics and their influenceon the motion of the center-of-mass, in the presence of the coupling between the permanent dipole moment of the ion and the trapping electric field. We show that the presence of the permanent dipole moment affects the trajectory of the ion and that it departs from the Mathieu equation solution found for atomic ions. For the case of quantum rotations, we also evidence the effect of the above-mentioned coupling on the rotational states of the ion.

  • 97.
    Hashemloo, Avazeh
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Dion, Claude
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rahali, Ghosne
    Umeå University, Faculty of Science and Technology, Department of Physics. Jazan Univ, Dept Phys, Jazan, Saudi Arabia.
    Wave packet dynamics of an atomic ion in a Paul trap: Approximations and stability2016In: International Journal of Modern Physics C, ISSN 0129-1831, Vol. 27, no 2, article id 1650014Article in journal (Refereed)
    Abstract [en]

    Using numerical simulations of the time-dependent Schrödinger equation, we study the fullquantum dynamics of the motion of an atomic ion in a linear Paul trap. Such a trap is based on atime-varying, periodic electric ̄eld and hence corresponds to a time-dependent potential for theion, which we model exactly. We compare the center-of-mass motion with that obtained fromclassical equations of motion, as well as to results based on a time-independent e®ective po-tential. We also study the oscillations of the width of the ion's wave packet, including close tothe border between stable (bounded) and unstable (unbounded) trajectories. Our results con- ̄rm that the center-of-mass motion always follows the classical trajectory, that the width of thewave packet is bounded for trapping within the stability region, and therefore that the classicaltrapping criterion is fully applicable to quantum motion.

  • 98. Hattne, Johan
    et al.
    Echols, Nathaniel
    Tran, Rosalie
    Kern, Jan
    Gildea, Richard J.
    Brewster, Aaron S.
    Alonso-Mori, Roberto
    Gloeckner, Carina
    Hellmich, Julia
    Laksmono, Hartawan
    Sierra, Raymond G.
    Lassalle-Kaiser, Benedikt
    Lampe, Alyssa
    Han, Guangye
    Gul, Sheraz
    DiFiore, Doerte
    Milathianaki, Despina
    Fry, Alan R.
    Miahnahri, Alan
    White, William E.
    Schafer, Donald W.
    Seibert, M. Marvin
    Koglin, Jason E.
    Sokaras, Dimosthenis
    Weng, Tsu-Chien
    Sellberg, Jonas
    Latimers, Matthew J.
    Glatzel, Pieter
    Zwart, Petrus H.
    Grosse-Kunstleve, Ralf W.
    Bogan, Michael J.
    Messerschmidt, Marc
    Williams, Garth J.
    Boutet, Sebastien
    Messinger, Johannes
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Zouni, Athina
    Yano, Junko
    Bergmann, Uwe
    Yachandra, Vittal K.
    Adams, Paul D.
    Sauter, Nicholas K.
    Accurate macromolecular structures using minimal measurements from X-ray free-electron lasers2014In: Nature Methods, ISSN 1548-7091, E-ISSN 1548-7105, Vol. 11, no 5, p. 545-548Article in journal (Refereed)
    Abstract [en]

    X-ray free-electron laser (XFEL) sources enable the use of crystallography to solve three-dimensional macromolecular structures under native conditions and without radiation damage. Results to date, however, have been limited by the challenge of deriving accurate Bragg intensities from a heterogeneous population of microcrystals, while at the same time modeling the X-ray spectrum and detector geometry. Here we present a computational approach designed to extract meaningful high-resolution signals from fewer diffraction measurements.

  • 99.
    Hausmaninger, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mid- and near-infrared NICE-OHMS: techniques for ultra-sensitive detection of molecules in gas phase2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) is a technique for ultra-sensitive detection of molecular absorption and dispersion. For highest performance, the technique combines cavity enhancement (CE) with frequency modulation (FM); while the former increases the effective interaction length between the light and the analyte by several orders of magnitudes, the latter removes the in-coupling of 1/f noise and makes the signals background free. The combination of CE and FM also gives the technique an immunity to amplitude noise caused by the jitter of the laser frequency relative to the cavity resonance frequencies. All these properties make the technique suitable for ultra sensitive trace gas detection in the sub-parts-per-trillion (ppt) range. The aim of this thesis is to improve the performance of the NICE-OHMS technique and to increase its range of applications.

    The work in this thesis can be divided into three areas:Firstly, a mid-infrared (MIR)-NICE-OHMS instrumentation was developed. In a first realization an unprecedented white-noise equivalent absorption limit for Doppler broadened (Db) detection in the MIR of 3×10-9 cm-1Hz-1/2was demonstrated. This was subsequently improved to 2.4×10-10 cm-1Hz-1/2allowing for detection methane and its two main isotopologues (CH3D and 13CH4) at their natural abundance.Secondly, further development of an existing near-infrared NICE-OHMS system was performed. This resulted in an improved longtime stability and the first shot-noise limited NICE-OHMS system for Db detection with a noise equivalent absorption limit of 2.3×10-14 cm-1detected over 200 s. Thirdly, models and theoretical descriptions of NICE-OHMS signals under strong absorption conditions and from methane under high laser power were developed. It was experimentally verified that the models allow for a more accurate evaluation of NICE-OHMS signals under a wide range of conditions.

  • 100.
    Hausmaninger, Thomas
    et al.
    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.
    Model for molecular absorption spectroscopy in the 1-100 Torr range in the presence of vibrational depletion - Applied to CH4 in N2 and dry airManuscript (preprint) (Other academic)
    Abstract [en]

    When molecules whose collision induced vibrational decay rates are small are probed by molecular absorption spectroscopic (MAS) techniques the absorption signal can, under certain conditions, be reduced and distorted. The reason has been attributed to the fact that a substantial fraction of the molecules in the interaction region will reside in excited vibrational states, which leads to a depletion of the vibrational ground state. One type of molecule in which this can take place is methane. A model for this phenomenon, based on CH4 in trace concentrations in either N2 or dry air in a cylindrical gas cell, detected by mid-infrared light in the 1 - 100 Torr pressure range, is presented. Due to a fast collisional coupling between various rotational states and velocity groups we suggest that depletion in MAS can be modeled adequately by a simple three-level system to which the transport of molecules in the system is coupled as diffusion according to Fick's law, applied to each level individually. The model is verified in a separate work [Hausmaninger T et al., J Quant Spectrosc Radiat Tr. 2017;205:59-70] with good agreement. It predicts that depletion has a strong pressure dependence in the 1 - 30 Torr range, that it is significantly more pronounced in N2 than in air, and that considerable degrees of depletion can be obtained for mW powers of light (> 10% for powers > 20 mW). The findings indicate that, unless precautions are taken, depletion can adversely affect quantitative assessments performed by MAS. Means of how to reduce depletion are given.

1234567 51 - 100 of 344
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf