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Ma, Weiguang
Publications (10 of 19) Show all publications
Xue, D. & Ma, W. (2019). Magnetic Switching of a Stoner-Wohlfarth Particle Subjected to a Perpendicular Bias Field. Electronics, 8(3), Article ID 366.
Open this publication in new window or tab >>Magnetic Switching of a Stoner-Wohlfarth Particle Subjected to a Perpendicular Bias Field
2019 (English)In: Electronics, ISSN 2079-9292, Vol. 8, no 3, article id 366Article in journal (Refereed) Published
Abstract [en]

Characterized by uniaxial magnetic anisotropy, the Stoner-Wohlfarth particle experiences a change in magnetization leading to a switch in behavior when tuned by an externally applied field, which relates to the perpendicular bias component (h(perp)) that remains substantially small in comparison with the constant switching field (h(0)). The dynamics of the magnetic moment that governs the magnetic switching is studied numerically by solving the Landau-Lifshitz-Gilbert (LLG) equation using the Mathematica code without any physical approximations; the results are compared with the switching time obtained from the analytic method that intricately treats the non-trivial bias field as a perturbation. A good agreement regarding the magnetic switching time (t(s)) between the numerical calculation and the analytic results is found over a wide initial angle range (0.01 < (0) < 0.3), as h(0) and h(perp) are 1.5 x K and 0.02 x K, where K represents the anisotropy constant. However, the quality of the analytic approximation starts to deteriorate slightly in contrast to the numerical approach when computing t(s) in terms of the field that satisfies h(perp) > 0.15 x K and h(0) = 1.5 x K. Additionally, existence of a comparably small perpendicular bias field (h(perp) << h(0)) causes t(s) to decrease in a roughly exponential manner when h(perp) increases.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
Stoner-Wohlfarth particle, Landau-Lifshitz-Gilbert equation, anisotropy, bias field
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:umu:diva-159633 (URN)10.3390/electronics8030366 (DOI)000465642600010 ()
Available from: 2019-06-13 Created: 2019-06-13 Last updated: 2019-06-13Bibliographically approved
Hausmaninger, T., Zhao, G., Ma, W. & Axner, O. (2018). Depletion of the vibrational ground state of CH4 in absorption spectroscopy at 3.4 μm in N2 and air in the 1-100Torr range. Journal of Quantitative Spectroscopy and Radiative Transfer, 205, 59-70
Open this publication in new window or tab >>Depletion of the vibrational ground state of CH4 in absorption spectroscopy at 3.4 μm in N2 and air in the 1-100Torr range
2018 (English)In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 205, p. 59-70Article in journal (Refereed) Published
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. 

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2018
Keywords
Absorption spectroscopy, Cavity enhanced, Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy, Methane, Depletion, Optical saturation
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-143714 (URN)10.1016/j.jqsrt.2017.10.007 (DOI)000417665000008 ()
Available from: 2018-01-08 Created: 2018-01-08 Last updated: 2018-11-06Bibliographically approved
Zhao, G., Hausmaninger, T., Ma, W. & Axner, O. (2017). Differential noise-immune cavity-enhanced optical heterodyne molecular spectroscopy for improvement of the detection sensitivity by reduction of drifts from background signals. Optics Express, 25(23), 29454-29471
Open this publication in new window or tab >>Differential noise-immune cavity-enhanced optical heterodyne molecular spectroscopy for improvement of the detection sensitivity by reduction of drifts from background signals
2017 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 25, no 23, p. 29454-29471Article in journal (Refereed) Published
Abstract [en]

The detection sensitivity of noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is often limited by background signals that bring in drifts. A novel realization of NICE-OHMS, termed differential NICE-OHMS, that both reduces such drifts and enlarges the molecular signal is presented. It is based on simultaneous detection of NICE-OHMS signals in reflection and transmission, followed by a subtraction of the former (properly weighted) from the latter. An Allan plot analysis shows that the instrumentation could demonstrate a noise equivalent absorption per unit length (NEAL) of 4.7 × 10−14 cm−1, obtained for an integration time of 170 s.

Place, publisher, year, edition, pages
Optical Society of America, 2017
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-142459 (URN)10.1364/OE.25.029454 (DOI)000415136700127 ()
Available from: 2017-12-04 Created: 2017-12-04 Last updated: 2018-11-06Bibliographically approved
Zhao, G., Tan, W., Hou, J., Qiu, X., Ma, W., Li, Z., . . . Jia, S. (2016). Calibration-free wavelength-modulation spectroscopy based on a swiftly determined wavelength-modulation frequency response function of a DFB laser. Optics Express, 24(2), 1723-1733
Open this publication in new window or tab >>Calibration-free wavelength-modulation spectroscopy based on a swiftly determined wavelength-modulation frequency response function of a DFB laser
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2016 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 24, no 2, p. 1723-1733Article in journal (Refereed) Published
Abstract [en]

A methodology for calibration-free wavelength modulation spectroscopy (CF-WMS) that is based upon an extensive empirical description of the wavelength-modulation frequency response (WMFR) of DFB laser is presented. An assessment of the WMFR of a DFB laser by the use of an etalon confirms that it consists of two parts: a 1st harmonic component with an amplitude that is linear with the sweep and a nonlinear 2nd harmonic component with a constant amplitude. Simulations show that, among the various factors that affect the line shape of a background-subtracted peak-normalized 2f signal, such as concentration, phase shifts between intensity modulation and frequency modulation, and WMFR, only the last factor has a decisive impact. Based on this and to avoid the impractical use of an etalon, a novel method to pre-determine the parameters of the WMFR by fitting to a background-subtracted peak-normalized 2f signal has been developed. The accuracy of the new scheme to determine the WMFR is demonstrated and compared with that of conventional methods in CF-WMS by detection of trace acetylene. The results show that the new method provides a four times smaller fitting error than the conventional methods and retrieves concentration more accurately.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-117836 (URN)10.1364/OE.24.001723 (DOI)000369066300134 ()26832551 (PubMedID)
Available from: 2016-04-05 Created: 2016-03-04 Last updated: 2018-06-07Bibliographically approved
Ma, W., Silander, I., Hausmaninger, T. & Axner, O. (2016). Doppler-broadened NICE-OHMS beyond the cavity-limited weak absorption condition – I. Theoretical Description. Journal of Quantitative Spectroscopy and Radiative Transfer, 168, 217-244
Open this publication in new window or tab >>Doppler-broadened NICE-OHMS beyond the cavity-limited weak absorption condition – I. Theoretical Description
2016 (English)In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 168, p. 217-244Article in journal (Other academic) Published
Abstract [en]

Doppler-broadened (Db) noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) is conventionally described by an expression (here referred to as the CONV expression) that is restricted to the case when the single-pass absorbance, α0L, is much smaller than the empty cavity losses, π/F [here termed the conventional cavity-limited weak absorption (CCLWA) condition]. This limits the applicability of the technique, primarily its dynamic range and calibration capability. To remedy this, this work derives extended descriptions of Db NICEOHMS that are not restricted to the CCLWA condition. First, the general principles of Db NICEOHMS are scrutinized in some detail. Based solely upon a set of general assumptions, predominantly that it is appropriate to linearize the Beer–Lambert law, that the light is modulated to a triplet, and that the Pound–Drever–Hall sidebands are fully reflected, a general description of Db NICE-OHMS that is not limited to any specific restriction on α0L vs. π/F, here referred to as the FULL description, is derived. However, this description constitutes a set of equations to which no closed form solution has been found. Hence, it needs to be solved numerically (by iterations), which is inconvenient. To circumvent this, for the cases when α0Loπ/F but without the requirement that the stronger CCLWA condition needs to be fulfilled, a couple of simplified extended expressions that are expressible in closed analytical form, referred to as the extended locking and extended transmission description, ELET, and the extended locking and full transmission description, ELFT, have been derived. An analysis based on simulations validates the various descriptions and assesses to which extent they agree. It is shown that in the CCLWA limit, all extended descriptions revert to the CONV expression. The latter one deviates though from the extended ones for α0L around and above 0.1π/F. The two simplified extended descriptions agree with the FULL description for a larger range of α0L than the CONV expression, viz. for the ELET description for α0L up to 0.3π/F and for ELFT for α0L up to 0.6 or 1.0 π/F (depending on the mode of detection). It is then demonstrated that the conventional view of Db NICE-OHMS, which states that the out-of-phase and the in-phase signals can be referred to as a pure absorption and dispersion signal, respectively, breaks down when the CCLWA condition does not hold. In this case, the out-of-phase signal is additionally affected by the phase shifts of the laser components (i.e. dispersion) while the in-phase signal is also influenced by their attenuation. Access to new descriptions broadens considerably the dynamic range of Db NICE-OHMS and facilitates calibration using standard references samples, and thereby its applicability

Keywords
NICE-OHMS, Cavity enhanced spectroscopy, Dynamic range, Frequency modulation spectroscopy, Theoretical model
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-110271 (URN)10.1016/j.jqsrt.2015.09.007 (DOI)000366007000018 ()
Available from: 2015-10-19 Created: 2015-10-19 Last updated: 2018-11-06Bibliographically approved
Hausmaninger, T., Silander, I., Ma, W. & Axner, O. (2016). Doppler-broadened NICE-OHMS beyond the cavity-limited weak absorption condition – II: experimental verification. Journal of Quantitative Spectroscopy and Radiative Transfer, 168, 245-256
Open this publication in new window or tab >>Doppler-broadened NICE-OHMS beyond the cavity-limited weak absorption condition – II: experimental verification
2016 (English)In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 168, p. 245-256Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
NICE OHMS, Frequency modulation spectroscopy, Cavity enhanced spect, Experimental verification
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-110273 (URN)10.1016/j.jqsrt.2015.09.008 (DOI)000366007000019 ()
Available from: 2015-10-19 Created: 2015-10-19 Last updated: 2018-11-06Bibliographically approved
Zhao, G., Tan, W., Jia, M., Hou, J., Ma, W., Dong, L., . . . Jia, S. (2016). Intensity-Stabilized Fast-Scanned Direct Absorption Spectroscopy Instrumentation Based on a Distributed Feedback Laser with Detection Sensitivity down to 4 x 10(-6). Sensors, 16(9), Article ID 1544.
Open this publication in new window or tab >>Intensity-Stabilized Fast-Scanned Direct Absorption Spectroscopy Instrumentation Based on a Distributed Feedback Laser with Detection Sensitivity down to 4 x 10(-6)
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2016 (English)In: Sensors, ISSN 1424-8220, E-ISSN 1424-8220, Vol. 16, no 9, article id 1544Article in journal (Refereed) Published
Abstract [en]

A novel, intensity-stabilized, fast-scanned, direct absorption spectroscopy (IS-FS-DAS) instrumentation, based on a distributed feedback (DFB) diode laser, is developed. A fiber-coupled polarization rotator and a fiber-coupled polarizer are used to stabilize the intensity of the laser, which significantly reduces its relative intensity noise (RIN). The influence of white noise is reduced by fast scanning over the spectral feature (at 1 kHz), followed by averaging. By combining these two noise-reducing techniques, it is demonstrated that direct absorption spectroscopy (DAS) can be swiftly performed down to a limit of detection (LOD) (1 sigma) of 4 x 10(-6), which opens up a number of new applications.

Keywords
direct absorption spectroscopy, DFB laser, intensity stabilization, fast scanning, relative intensity noise, polarization rotator
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-127635 (URN)10.3390/s16091544 (DOI)000385527700199 ()
Available from: 2016-11-30 Created: 2016-11-16 Last updated: 2018-06-09Bibliographically approved
Silander, I., Hausmaninger, T., Ma, W., Harren, F. J. M. & Axner, O. (2015). Doppler-broadened mid-infrared noise-immune cavity-enhanced optical heterodyne molecular spectrometry based on an optical parametric oscillator for trace gas detection. Optics Letters, 40(4), 439-442
Open this publication in new window or tab >>Doppler-broadened mid-infrared noise-immune cavity-enhanced optical heterodyne molecular spectrometry based on an optical parametric oscillator for trace gas detection
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2015 (English)In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 40, no 4, p. 439-442Article in journal (Refereed) Published
Abstract [en]

An optical parametric oscillator based Doppler-broadened (Db) noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) system suitable for addressing fundamental vibrational transitions in the 3.2-3.9 mu m mid-infrared (MIR) region has been realized. An Allan-Werle analysis provides a detection sensitivity of methane of 1.5 x 10-9 cm(-1) with a 20 s integration time, which corresponds to 90 ppt of CH4 if detected at the strongest transition addressed at 40 Torr. This supersedes that of previous Db MIR NICE-OHMS demonstrations and suggests that the technique can be suitable for detection of both the environmentally important (CH4)-C-13 and CH3D isotopologues. It also opens up for detection of many other molecular species at ppt and sub-ppt concentration levels.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-101400 (URN)10.1364/OL.40.000439 (DOI)000349848400001 ()25680119 (PubMedID)
Available from: 2015-07-02 Created: 2015-03-30 Last updated: 2018-11-06Bibliographically approved
Silander, I., Hausmaninger, T., Ma, W., Ehlers, P. & Axner, O. (2015). Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry down to 4 x 10-13 cm-1 Hz-1/2: implementation of a 50,000 finesse cavity. Optics Letters, 40(9), 2004-2007
Open this publication in new window or tab >>Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry down to 4 x 10-13 cm-1 Hz-1/2: implementation of a 50,000 finesse cavity
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2015 (English)In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 40, no 9, p. 2004-2007Article in journal (Refereed) Published
Abstract [en]

We report on the realization of a Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) instrumentation based on a high-finesse (50,000) cavity with a detection sensitivity of 4 x 10(-13) cm(-1) Hz(-1/2). For the P-e(11) transition targeted at 1.5316 mu m, this corresponds to a C2H2 concentration of 240 ppq (parts-per-quadrillion) detected at 100 Torr. The setup was originally affected by recurrent dips in the cavity transmission, which were attributed to excitation of high-order transverse mode by scattering from the mirrors. The effect of these was reduced by insertion of a small pinhole in the cavity.

Place, publisher, year, edition, pages
Optical Society of America, 2015
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-106284 (URN)10.1364/OL.40.002004 (DOI)000353924600031 ()25927769 (PubMedID)
Available from: 2015-07-10 Created: 2015-07-09 Last updated: 2018-06-07Bibliographically approved
Axner, O., Ehlers, P., Hausmaninger, T., Silander, I. & Ma, W. (2014). Noise-immune cavity-enhanced analytical atomic spectrometry — NICE-AAS: a technique for detection of elements down to zeptogram amounts. Spectrochimica Acta Part B - Atomic Spectroscopy, 100, 211-235
Open this publication in new window or tab >>Noise-immune cavity-enhanced analytical atomic spectrometry — NICE-AAS: a technique for detection of elements down to zeptogram amounts
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2014 (English)In: Spectrochimica Acta Part B - Atomic Spectroscopy, ISSN 0584-8547, E-ISSN 1873-3565, Vol. 100, p. 211-235Article in journal (Refereed) Published
Abstract [en]

Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is a powerful technique for detection of molecular compounds in gas phase that is based on a combination of two important concepts: frequency modulation spectroscopy (FMS) for reduction of noise, and cavity enhancement, for prolongation of the interaction length between the light and the sample. Due to its unique properties, it has demonstrated unparalleled detection sensitivity when it comes to detection of molecular constituents in the gas phase. However, despite these, it has so far not been used for detection of atoms, i.e. for elemental analysis. The present work presents an assessment of the expected performance of Doppler-broadened (Db) NICE-OHMS for analytical atomic spectrometry, then referred to as noise-immune cavity-enhanced analytical atomic spectrometry (NICE-AAS). After a description of the basic principles of Db-NICE-OHMS, the modulation and detection conditions for optimum performance are identified. Based on a previous demonstrated detection sensitivity of Db-NICE-OHMS of 5×10−12 cm−1 Hz−1∕2 (corresponding to a single-pass absorbance of 7×10−11 over 10 s), the expected limits of detection (LODs) of Hg and Na by NICE-AAS are estimated. Hg is assumed to be detected in gas phase directly while Na is considered to be atomized in a graphite furnace (GF) prior to detection. It is shown that in the absence of spectral interferences, contaminated sample compartments, and optical saturation, it should be feasible to detect Hg down to 10 zg/cm3 (10 fg/m3 or 10-5 ng/m3), which corresponds to 25 atoms/cm3, and Na down to 0.5 zg (zg = zeptogram = 10-21 g), representing 50 zg/mL (parts-per-sextillion, pps, 1:1021) in liquid solution (assuming a sample of 10 µL) or solely 15 atoms injected into the GF, respectively. These LODs are several orders of magnitude lower (better) than any previous laser-based absorption technique previously has demonstrated under atmospheric pressure conditions. It is prophesied that NICE-AAS could provide such high detection sensitivity that the instrumentation should not, by itself, be the limiting factor of an assessment of elemental abundance; the accuracy of an assessment would then instead be limited by concomitant species, e.g. originating from the handling procedures of the sample or the environment.

Place, publisher, year, edition, pages
Elsevier, 2014
Keywords
Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS), Frequency modulation spectroscopy (FMS), Cavity enhanced (CE) spectroscopy, Ultra-trace element detection, Individual atom detection
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-92508 (URN)10.1016/j.sab.2014.08.016 (DOI)000343853400028 ()
Funder
Swedish Research Council, 621-2008-3674Swedish Research Council, 621-2011-4216
Available from: 2014-08-27 Created: 2014-08-27 Last updated: 2018-06-07Bibliographically approved
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