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Publisher: Springer-Verlag (Total: 2351 journals)

 Applied Physics B: Lasers and Optics   [SJR: 0.983]   [H-I: 104]   [24 followers]  Follow         Hybrid journal (It can contain Open Access articles)    ISSN (Print) 0946-2171 - ISSN (Online) 1432-0649    Published by Springer-Verlag  [2351 journals]
• Interband cascade laser-based ppbv-level mid-infrared methane detection
using two digital lock-in amplifier schemes
• Authors: Fang Song; Chuantao Zheng; Di Yu; Yanwen Zhou; Wanhong Yan; Weilin Ye; Yu Zhang; Yiding Wang; Frank K. Tittel
Abstract: A parts-per-billion in volume (ppbv) level mid-infrared methane (CH4) sensor system was demonstrated using second-harmonic wavelength modulation spectroscopy (2f-WMS). A 3291 nm interband cascade laser (ICL) and a multi-pass gas cell (MPGC) with a 16 m optical path length were adopted in the reported sensor system. Two digital lock-in amplifier (DLIA) schemes, a digital signal processor (DSP)-based DLIA and a LabVIEW-based DLIA, were used for harmonic signal extraction. A limit of detection (LoD) of ~ 13.07 ppbv with an averaging time of 2 s was achieved using the DSP-based DLIA and a LoD of ~ 5.84 ppbv was obtained using the LabVIEW-based DLIA with the same averaging time. A rise time of 0→2 parts-per-million in volume (ppmv) and fall time of 2→0 ppmv were observed. Outdoor atmospheric CH4 concentration measurements were carried out to evaluate the sensor performance using the two DLIA schemes.
PubDate: 2018-02-23
DOI: 10.1007/s00340-018-6916-y
Issue No: Vol. 124, No. 3 (2018)

• Deposition of nanocomposite Cu–TiO 2 using heterogeneous colliding
plasmas
• Authors: Pramod K. Pandey; Raj K. Thareja; Ravi Pratap Singh; John T. Costello
Abstract: The formation of CuTiO2 nanocomposites has been observed in an experiment in which laser plasma plumes of Cu and Ti collide and stagnate in an oxygen atmosphere. The inherent advantage of this technique lies in its simplicity and flexibility where laser, target composition and geometry along with ambient atmosphere are all controllable parameters through which the stoichiometry of the deposited nanocomposites may be selected. The experiment has been performed at three oxygen ambient pressures 10−4, 10−2, 100 mbar and we observe its effect on stoichiometry, and morphology of the deposited nanocomposites. Here, we show how the stoichiometry of deposited nanocomposites can be readily controlled by changing just one parameter, namely the ambient oxygen pressure. The different peaks of photoluminescence spectra $$\lambda =390{\text{ nm}}\;\left( {E=3.18{\text{ eV}}} \right)$$ corresponding to the anatase phase of TiO2, along with the peaks at λ = 483 nm (E = 2.56 eV) and 582 nm (E = 2.13 eV) of deposited nanocomposites, shows the doping/blending effect on the band gaps which may potentially be of value in solar cell technology. The technique can, in principle, be extended to include nanocomposites of other materials making it potentially more widely applicable.
PubDate: 2018-02-21
DOI: 10.1007/s00340-018-6919-8
Issue No: Vol. 124, No. 3 (2018)

• Randomly spaced chirped grating-based random fiber laser
• Authors: Ziyang Guo; Jingxuan Song; Yimin Liu; Zhaoxin Liu; Ping Shum; Xinyong Dong
Abstract: A random fiber laser is demonstrated using a randomly spaced chirped-fiber Bragg grating (CFBG) array, which was fabricated along a 1-m-long erbium-doped fiber using a UV laser with a random separation ranging from 30 to 100 mm. Random distributed feedback is effectively realized through reflection of the CFBGs and further enhanced by changing chirp directions of the gratings. When pumped with a 980-nm laser diode, laser with multi-wavelength output was achieved with a pump threshold of 15 mW. Laser performance was studied as a function of pump power and time.
PubDate: 2018-02-20
DOI: 10.1007/s00340-018-6918-9
Issue No: Vol. 124, No. 3 (2018)

• Spectroscopic analysis of high protein nigella seeds (Kalonji) using
laser-induced breakdown spectroscopy and inductively coupled
plasma/optical emission spectroscopy
• Authors: Imran Rehan; M. Zubair Khan; Irfan Ali; Kamran Rehan; Sabiha Sultana; Sher Shah
Abstract: The spectroscopic analysis of high protein nigella seeds (also called Kalonji) was performed using pulsed nanosecond laser-induced breakdown spectroscopy (LIBS) at 532 nm. The emission spectrum of Kalonji recorded with an LIBS spectrometer exposed the presence of various elements like Al, B, Ba, Ca, Cr, K, P, Mg, Mn, Na, Ni, S, Si, Cu, Fe, Ti, Sn, Sr, and Zn. The plasma parameters (electron temperature and electron density) were estimated using Ca–I spectral lines and their behavior were studied against laser irradiance. The electron temperature and electron density was observed to show an increasing trend in the range of 5802–7849 K, and (1.2–3.9) × 1017 cm− 3, respectively, in the studied irradiance range of (1.2–12.6) × 109 W/cm2. Furthermore, the effect of varying laser energy on the integrated signal intensities was also studied. The quantitative analysis of the detected elements was performed via the calibration curves drawn for all the observed elements through typical samples made in the known concentration in the Kalonji matrix, and by setting the concentration of P as the calibration. The validity of our LIBS findings was verified via comparison of the results with the concentration of every element find in Kalonji using the standard analytical tool like ICP/OES. The results acquired using LIBS and ICP/OES were found in fine harmony. Moreover, limit of detection was measured for toxic metals only.
PubDate: 2018-02-20
DOI: 10.1007/s00340-018-6915-z
Issue No: Vol. 124, No. 3 (2018)

• Modeling of dispersion engineered chalcogenide rib waveguide for ultraflat
mid-infrared supercontinuum generation in all-normal dispersion regime
• Authors: H. Ahmad; M. R. Karim; B. M. A. Rahman
Abstract: A rigorous numerical investigation has been carried out through dispersion engineering of chalcogenide rib waveguide for near-infrared to mid-infrared ultraflat broadband supercontinuum generation in all-normal group-velocity dispersion regime. We propose a novel design of a 1-cm-long air-clad rib waveguide which is made from $$\text {Ge}_{11.5}\text {As}_{24}\text {Se}_{64.5}$$ chalcogenide glass as the core with either silica or $$\text {Ge}_{11.5}\text {As}_{24}\text {S}_{64.5}$$ chalcogenide glass as a lower cladding separately. A broadband ultraflat supercontinuum spanning from 1300 to 1900 nm could be generated when pumped at 1.55 $$\upmu \text {m}$$ with a low input peak power of 100 W. Shifting the pump to 2  $$\upmu \text {m}$$ , the supercontinuum spectra extended in the mid-infrared region up to 3400 nm with a moderate-input peak power of 500 W. To achieve further extension in mid-infrared, we excite our optimized rib waveguide in both the anomalous and all-normal dispersion pumping regions at 3.1  $$\upmu \text {m}$$ with a largest input peak power of 3 kW. In the case of anomalous dispersion region pumping, numerical analysis shows that supercontinuum spectrum can be extended in the mid-infrared up to 10  $$\upmu \text {m}$$ , although this contains high spectral amplitude fluctuations over the entire bandwidth which limits the supercontinuum sources in the field of high precision measurement applications. On the other hand, by optimizing a rib waveguide geometry for pumping in all-normal dispersion region, we are able to generate a smooth and flat-top coherent supercontinuum spectrum with a moderate bandwidth spanning the wavelength range 2–5.5  $$\upmu \text {m}$$ with less than 5 dB spectral fluctuation over the entire output bandwidth. Our proposed design is highly suitable for making on-chip SC light sources for a variety of applications such as biomedical imaging, and environmental and industrial sensing in the mid-infrared region.
PubDate: 2018-02-17
DOI: 10.1007/s00340-018-6914-0
Issue No: Vol. 124, No. 3 (2018)

• Optical remote sensing for monitoring flying mosquitoes, gender
identification and discussion on species identification
• Authors: Adrien P. Genoud; Roman Basistyy; Gregory M. Williams; Benjamin P. Thomas
Abstract: Mosquito-borne diseases are a major challenge for Human health as they affect nearly 700 million people every year and result in over 1 million deaths. Reliable information on the evolution of population and spatial distribution of key insects species is of major importance in the development of eco-epidemiologic models. This paper reports on the remote characterization of flying mosquitoes using a continuous-wave infrared optical remote sensing system. The system is setup in a controlled environment to mimic long-range lidars, mosquitoes are free flying at a distance of ~ 4 m from the collecting optics. The wing beat frequency is retrieved from the backscattered light from mosquitoes transiting through the laser beam. A total of 427 transit signals have been recorded from three mosquito species, males and females. Since the mosquito species and gender are known a priori, we investigate the use of wing beat frequency as the sole predictor variable for two Bayesian classifications: gender alone (two classes) and species/gender (six classes). The gender of each mosquito is retrieved with a 96.5% accuracy while the species/gender of mosquitoes is retrieved with a 62.3% accuracy. Known to be an efficient mean to identify insect family, we discuss the limitations of using wing beat frequency alone to identify insect species.
PubDate: 2018-02-17
DOI: 10.1007/s00340-018-6917-x
Issue No: Vol. 124, No. 3 (2018)

• Detecting the propagation effect of terahertz wave inside the two-color
femtosecond laser filament in the air
• Authors: J. Zhao; X. Zhang; S. Li; C. Liu; Y. Chen; Y. Peng; Y. Zhu
Abstract: In this work, to decide the existence of terahertz (THz) wave propagation effect, THz pulses emitted from a blocked two-color femtosecond laser filament with variable length were recorded by a standard electric–optic sampling setup. The phenomenon of temporal advance of the THz waveform’s peak with the increasing filament length has been observed. Together with another method of knife-edge measurement which aims at directly retrieving the THz beam diameter, both the experimental approaches have efficiently indicated the same filament range within which THz wave propagated inside the plasma column. At last, a preliminary two-dimensional near-field scanning imaging of the THz spot inside the cross section of the filament has been suggested as the third way to determine the issue of THz wave propagation effect.
PubDate: 2018-02-16
DOI: 10.1007/s00340-018-6913-1
Issue No: Vol. 124, No. 3 (2018)

• Impersonation attack on a quantum secure direct communication and
authentication protocol with improvement
• Authors: Ali Amerimehr; Massoud Hadain Dehkordi
Abstract: We analyze the security of a quantum secure direct communication and authentication protocol based on single photons. We first give an impersonation attack on the protocol. The cryptanalysis shows that there is a gap in the authentication procedure of the protocol so that an opponent can reveal the secret information by an undetectable attempt. We then propose an improvement for the protocol and show it closes the gap by applying a mutual authentication procedure. In the improved protocol single photons are transmitted once in a session, so it is easy to implement as the primary protocol. Furthermore, we use a novel technique for secret order rearrangement of photons by which not only quantum storage is eliminated also a secret key can be reused securely. So the new protocol is applicable in practical approaches like embedded system devices.
PubDate: 2018-02-14
DOI: 10.1007/s00340-018-6907-z
Issue No: Vol. 124, No. 3 (2018)

• Calibration curves for commercial copper and aluminum alloys using
handheld laser-induced breakdown spectroscopy
• Authors: B. N. Bennett; M. Z. Martin; D. N. Leonard; E. Garlea
Abstract: Handheld laser-induced breakdown spectroscopy (HH LIBS) was used to study the elemental composition of four copper alloys and four aluminum alloys to produce calibration curves. The HH LIBS instrument used is a SciAps Z-500, commercially available, that contains a class-1 solid-state laser with an output wavelength of 1532 nm, laser energy of 5 mJ/pulse, and a pulse duration of 5 ns. Test samples were solid specimens comprising copper and aluminum alloys and data were collected from the samples’ surface at three different locations, employing a 12-point-grid pattern for each data set. All three data sets of the spectra were averaged, and the intensity, corrected by subtraction of background, was used to produce the elemental calibration curves. Calibration curves are presented for the matrix elements, copper and aluminum, as well as several minor elements. The surface damage produced by the laser was examined by microscopy. The alloys were tested in air and in a glovebox to evaluate the instrument’s ability to identify the constituents within materials under different environmental conditions. The main objective of using this HH LIBS technology is to determine its capability to fingerprint the presence of certain elements related to subpercent level within materials in real time and in situ, as a starting point for undertaking future complex material characterization work.
PubDate: 2018-02-13
DOI: 10.1007/s00340-018-6909-x
Issue No: Vol. 124, No. 3 (2018)

• Flame thermometry using laser-induced-grating spectroscopy of nitric oxide
• Authors: Andrew Luers; Anna-Lena Salhlberg; Simone Hochgreb; Paul Ewart
Abstract: A systematic study of laser-induced thermal-grating scattering (LITGS) using nitric oxide as an absorbing species is presented as a means of thermometry in air-fed combustion. The relative contributions to the scattered signal from degenerate four-wave mixing, DFWM, and from laser-induced thermal-grating scattering, LITGS, are studied in the time domain for NO in N2 buffer gas up to 4 bar, using a pulsed laser system to excite the (0,0) γ-bands of NO at 226.21 nm. LITGS signals from combustion-generated NO in a laminar, pre-mixed CH4/O2/N2 flame on an in-house constructed slot burner were used to derive temperature values as a function of O2 concentration and position in the flame at 1 and 2.5 bar total pressure. Temperature values consistent with the calculated adiabatic flame temperature were derived from averaged LITGS signals over 50–100 single shots at 10 Hz repetition rate in the range 1600–2400 K with a pressure-dependent uncertainty of ± 1.8% at 1 bar to ± 1.4% at 2.5 bar. Based on observed signal-to-noise ratios, the minimum detectable concentration of NO in the flame is estimated to be 80 ppm for a 5 s measurement time at 10 Hz repetition rate.
PubDate: 2018-02-13
DOI: 10.1007/s00340-018-6912-2
Issue No: Vol. 124, No. 3 (2018)

• High-repetition-rate interferometric Rayleigh scattering for flow-velocity
measurements
• Authors: Jordi Estevadeordal; Naibo Jiang; Andrew D. Cutler; Josef J. Felver; Mikhail N. Slipchenko; Paul M. Danehy; James R. Gord; Sukesh Roy
Abstract: High-repetition-rate interferometric-Rayleigh-scattering (IRS) velocimetry is demonstrated for non-intrusive, high-speed flow-velocity measurements. High temporal resolution is obtained with a quasi-continuous burst-mode laser that is capable of operating at 10–100 kHz, providing 10-ms bursts with pulse widths of 5–1000 ns and pulse energy > 100 mJ at 532 nm. Coupled with a high-speed camera system, the IRS method is based on imaging the flow field through an etalon with 8-GHz free spectral range and capturing the Doppler shift of the Rayleigh-scattered light from the flow at multiple points having constructive interference. The seed-laser linewidth permits a laser linewidth of < 150 MHz at 532 nm. The technique is demonstrated in a high-speed jet, and high-repetition-rate image sequences are shown.
PubDate: 2018-02-10
DOI: 10.1007/s00340-018-6908-y
Issue No: Vol. 124, No. 3 (2018)

• Two-dimensional tungsten photonic crystal selective emitter: effects of
geometrical parameters and temperature
• Authors: Ali Rostamnejadi; Meysam Daneshvar
Abstract: In this paper, we have studied the effects of structural parameters and temperature on the emissivity of a square array of cylindrical nano/microcavities on tungsten slab by finite difference time domain method. It has been shown that the physical nature of the emissivity enhancement depends on the structural parameters of the nano/microcavities. In the case of narrow and shallow nanocavities with radius r ≤ 150 nm and depth d ≤ 150 nm; the emissivity has the same behavior as that of flat tungsten. Thermally excited surface plasmon polaritons cause a sharp peak in the emissivity of nanocavities with 150 ≤ d ≤ 250 nm and 150 ≤ r ≤ 350 nm at wavelength in the order of periodicity, λ ~ a. In the case of wide and deep microcavities with r ≥ 350 nm and d ≥ 250 nm; there are anomalous peaks in the emissivity which are well matched with the modified resonant wavelengths of a microcavity. At wavelengths shorter than periodicity, the Bragg diffraction from the surface of periodic microcavities reduces the emissivity. The obtained results show that to have a favorable selective thermal emitter from 2D W nano/microcavities with emission efficiency more than 90%, the periodicity should be as small as possible, the cavity depth should be large enough and its radius should be selected according to the working temperature.
PubDate: 2018-02-09
DOI: 10.1007/s00340-018-6910-4
Issue No: Vol. 124, No. 3 (2018)

• Investigation of optical properties of an overdense magnetized plasma lens
in the interaction with high-intensity Gaussian laser pulses
• Authors: M. Ghorbanalilu; B. Shokri
Abstract: Self-focusing of a high-intensity circularly polarized Gaussian laser pulse by an overdense magnetized thin plasma lens is numerically investigated. The quasi-static axial magnetic field can be produced by inverse Faraday effect (IFE) mechanism in laser–plasma interaction. It has been shown that the inclusion of self-transparency, ponderomotive force, and magnetic field effects significantly affect the self-focusing properties. When the strength of the magnetic field increases, the self-focusing property is enhanced for the right and is weakened for the left-handed circularly polarized laser pulse. The ponderomotive force repels electrons from the axis and drives electron cavitation and as a result further lowers the plasma frequency. When the influence of the ponderomotive force is taken into account, self-focusing for both polarizations is strongly affected. The clear difference between the effects of the right- and left-handed circularly polarized pulses may lead us to use them for different experimental applications.
PubDate: 2018-02-09
DOI: 10.1007/s00340-018-6904-2
Issue No: Vol. 124, No. 3 (2018)

• Opposite change trend of electrical behavior curves near the threshold
between GaAs- and GaN-multi-quantum-well laser diodes
• Authors: Liefeng Feng; Shupeng Wang; Yang Li; Xiufang Yang; Ding Li; Cunda Wang
Abstract: The opposite and abrupt change trends of the electrical behavior between narrow and wide bang-gap multi-quantum-well (MQW) laser diodes (LDs) in the ‘threshold region’, which corresponds to the current region between two kinks in the IdV/dI–I curves, were confirmed from the apparent properties measured directly and junction properties extracted by our ac-IV method, as well as simulation calculations. In the threshold region, negative capacitance and series resistance curves in narrow bandgap LDs (wavelengths are 780 and 650 nm) drop down, while in wide-bandgap LDs (wavelengths are 450 and 405 nm), they jump up; the junction voltage curves in narrow bandgap LDs jump up, while in wide-bandgap LDs, they drop down. We qualitatively interpreted the opposite change trend of these electrical parameters, and concluded that different stimulated emission mechanisms caused this opposite change trend of LDs with these two types of materials.
PubDate: 2018-02-09
DOI: 10.1007/s00340-018-6911-3
Issue No: Vol. 124, No. 3 (2018)

• High-sensitivity 308.6-nm laser absorption diagnostic optimized for OH
measurement in shock tube combustion studies
• Authors: Shengkai Wang; Ronald K. Hanson
Abstract: We report the development of a high-sensitivity laser absorption diagnostic optimized for measurement of the hydroxyl radical (OH) at temperatures relevant to combustion studies, and demonstrated here in shock tube experiments. This diagnostic utilizes a narrow-linewidth CW UV laser that is tunable over the A2Σ – X2Π (0,0) band of the OH rovibronic transitions. First, we identified the strongest absorption transition of OH, over the current temperature range of interest, to be the Q1(5) transition near 308.61 nm. We then measured the OH absorption coefficients behind reflected shock waves over temperatures of 1656–2993 K and pressures of 0.88–4.09 atm, and determined the pressure-broadening and pressure-shifting coefficients in argon bath gas. Compared to the previous diagnostic targeting the OH R1(5) transition, the current diagnostic has approximately 2.2 times the sensitivity. Finally, we demonstrated the excellent sensitivity of the current OH diagnostic in a set of highly-diluted C3H8 oxidation experiments in a shock tube, where a 1 − σ detection limit of less than 0.15 ppm OH was successfully achieved.
PubDate: 2018-02-08
DOI: 10.1007/s00340-018-6902-4
Issue No: Vol. 124, No. 3 (2018)

• Theoretical modeling on the laser-induced phase deformation of liquid
crystal optical phased shifter
• Authors: Zhuangqi Zhou; Xiangru Wang; Rusheng Zhuo; Xiaoxian He; Liang Wu; Xiaolin Wang; Qinggui Tan; Qi Qiu
Abstract: To improve the working condition of liquid crystal phase shifter on incident laser power, a theoretical model on laser induced phase distortion is built on the physics of heat deposition and heat transfer. Four typical factors (absorption, heat sink structure, cooling fluid rate, and substrate) are analyzed to evaluate the influence of phase distortion when a relative high-power laser is pumped into the liquid crystal phase shifter. Flow rate of cooling fluid and heat sink structure are the most important two factors on improving the limit of incident laser power. Meanwhile, silicon wafer is suggested to replace the back glass contacting the heat sink, because of its higher heat transfer coefficient. If the device is fabricated on the conditions that: the total absorption is 5% and it has a strong heat sink structure with a flow rate of 0.01 m/s, when the incident laser power is 110W, the laser-induced phase deformation on the center is diminished to be less than 0.06, and the maximum temperature increase on the center is less than 1K degree.
PubDate: 2018-02-07
DOI: 10.1007/s00340-018-6905-1
Issue No: Vol. 124, No. 3 (2018)

• Propagation and interaction of finite-energy Airy–Hermite–Gaussian
beams in photorefractive media
• Authors: Qichang Jiang; Yanli Su; Hexian Nie; Ziwei Ma; Yonghong Li
Abstract: We investigate numerically the propagation and interaction characteristics of finite-energy Airy–Hermite–Gaussian beams in biased photorefractive media. For the case of first-order Hermite polynomial, two main lobes of the initial input beam can form breathing solitons with two components. The interval between two soliton components in the $$y{\text{-direction}}$$ increases gradually with the propagation distance, while the central position of two soliton components in the $$x{\text{-direction}}$$ is almost unchanged during propagation. Moreover, in interaction situations, four main lobes of the two Airy–Hermite–Gaussian beams can also form breathing solitons with four components under the in-phase and out-of-phase conditions, respectively.
PubDate: 2018-02-07
DOI: 10.1007/s00340-018-6906-0
Issue No: Vol. 124, No. 3 (2018)

• CH PLIF and PIV implementation using C-X (0,0) and intra-vibrational band
filtered detection
• Authors: Stephen D. Hammack; Aaron W. Skiba; Tonghun Lee; Campbell D. Carter
Abstract: This study demonstrates advancement in a low-pulse energy methylidyne (CH) planar laser-induced fluorescence (PLIF) method that facilitates its application alongside flows seeded for particle image velocimetry (PIV) or other particle scattering based methods, as well as in high scattering environments. The C-X (0,0) R-branch excitation and filtered detection are carefully selected such that the laser line frequency is heavily attenuated by an edge filter while allowing transmission of most of the (0,0) band fluorescence. There are strong OH A-X (0,0) lines in the vicinity, but they can be avoided or utilized through dye laser tuning. As a demonstration of efficacy, PIV is performed simultaneously with the PLIF imaging. Using the edge filter, particle scattering signal is reduced to sub-fluorescence levels, allowing for flame-front analysis. This achievement enables flame-front tracking at high repetition rates (due to the low-pulse energy required) in combination with a scattering method such as PIV or use in high scattering environments such as enclosed combustors or near burner surfaces.
PubDate: 2018-02-03
DOI: 10.1007/s00340-017-6883-8
Issue No: Vol. 124, No. 2 (2018)

• The phase interrogation method for optical fiber sensor by analyzing the
fork interference pattern
• Authors: Riqing Lv; Liqiang Qiu; Haifeng Hu; Lu Meng; Yong Zhang
Abstract: The phase interrogation method for optical fiber sensor is proposed based on the fork interference pattern between the orbital angular momentum beam and plane wave. The variation of interference pattern with phase difference between the two light beams is investigated to realize the phase interrogation. By employing principal component analysis method, the features of the interference pattern can be extracted. Moreover, the experimental system is designed to verify the theoretical analysis, as well as feasibility of phase interrogation. In this work, the Mach–Zehnder interferometer was employed to convert the strain applied on sensing fiber to the phase difference between the reference and measuring paths. This interrogation method is also applicable for the measurements of other physical parameters, which can produce the phase delay in optical fiber. The performance of the system can be further improved by employing highlysensitive materials and fiber structures.
PubDate: 2018-02-01
DOI: 10.1007/s00340-018-6901-5
Issue No: Vol. 124, No. 2 (2018)

• Saturation spectroscopy of an optically opaque argon plasma
• Authors: Ben Eshel; Christopher A. Rice; Glen P. Perram
Abstract: A pure argon (Ar) plasma formed by a capacitively coupled radio-frequency discharge was analyzed using Doppler-free saturation spectroscopy. The expected line shape was a characteristic of sub-Doppler spectra in the presence of velocity-changing collisions, a narrow Lorentzian centered on a Doppler pedestal, but the observed line shapes contain a multi-peak structure, attributed to opacity of the medium. Laser absorption and inter-modulated fluorescence spectroscopy measurements were made to validate opacity as a driving factor of the observed line shapes. Spectral line shapes are further complicated by the spatial dependence of the pump laser, probe laser and of the absorbing medium, as well as the large absorbance of the transition under investigation. A numerical line shape was derived by accounting for the spatial variation of the pump and probe with a saturated line shape obtained from the rate equations for an equivalent two-level system. This simulated line shape shows good qualitative agreement with the trends observed in the data.
PubDate: 2018-02-01
DOI: 10.1007/s00340-017-6869-6
Issue No: Vol. 124, No. 2 (2018)

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