Showing papers in "Applied Physics B in 2018"

Laser spectroscopy for breath analysis: towards clinical implementation

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

Optical remote sensing for monitoring flying mosquitoes, gender identification and discussion on species identification

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.

Two-dimensional temperature and carbon dioxide concentration profiles in atmospheric laminar diffusion flames measured by mid-infrared direct absorption spectroscopy at 4.2 μm

Abstract: We present a multi-line flame thermometry technique based on mid-infrared direct absorption spectroscopy of carbon dioxide at its $$v_3$$ fundamental around 4.2 μm that is particularly suitable for sooting flames. Temperature and concentration profiles of gas phase molecules in a flame are important characteristics to understand its flame structure and combustion chemistry. One of the standard laboratory flames to analyze polycyclic aromatic hydrocarbons (PAH) and soot formation is laminar non-premixed co-flow flame, but PAH and soot introduce artifact to most non-contact optical measurements. Here we report an accurate diagnostic method of the temperature and concentration profiles of CO2 in ethylene diffusion flames by measuring its $$v_3$$ vibrational fundamental. An interband cascade laser was used to probe the R-branch bandhead at 4.2 μm, which is highly sensitive to temperature change, free from soot interference and ambient background. Calibration measurement was carried out both in a low-pressure Herriott cell and an atmospheric pressure tube furnace up to 1550 K to obtain spectroscopic parameters for high-temperature spectra. In our co-flow flame measurement, two-dimensional line-of-sight optical depth of an ethylene/N2 laminar sooting flame was recorded by dual-beam absorption scheme. The axially symmetrical attenuation coefficient profile of CO2 in the co-flow flame was reconstructed from the optical depth by Abel inversion. Spatially resolved flame temperature and in situ CO2 volume fraction profiles were derived from the calibrated CO2 spectroscopic parameters and compared with temperature profiles measured by two-line atomic fluorescence.

Nonlinear optical studies of sodium borate glasses embedded with gold nanoparticles

Abstract: Optical glasses possessing large third-order optical nonlinear susceptibility and fast response times are promising materials for the development of advanced nonlinear photonic devices. In this context, gold nanoparticle (NP)-doped borate glasses were synthesized via the melt-quench method. The nonlinear optical (NLO) properties of thus prepared glasses were investigated at different wavelengths (i.e., at 532 nm using nanosecond pulses, at 750 nm, 800 nm, and 850 nm wavelengths using femtosecond, MHz pulses). At 532 nm, open aperture (OA) Z-scan signatures of gold NP-doped borate glasses demonstrated reverse saturable absorption (RSA), attributed to mixed intra-band and interband transitions, while in the 750‒850 nm region, the OA Z-scan data revealed the presence of saturable absorption (SA), possibly due to intra-band transitions. The NLO coefficients were evaluated at all the spectral regions and further compared with some of the recently reported glasses. The magnitudes of obtained NLO coefficients clearly demonstrate that the investigated glasses are potential materials for photonic device applications.

Mid-infrared laser absorption tomography for quantitative 2D thermochemistry measurements in premixed jet flames

Abstract: A tomographic laser absorption spectroscopy technique, utilizing mid-infrared light sources, is presented as a quantitative method to spatially resolve species and temperature profiles in small-diameter reacting flows relevant to combustion systems. Here, tunable quantum and interband cascade lasers are used to spectrally resolve select rovibrational transitions near 4.98 and 4.19 $$\upmu$$ m to measure CO and $${\mathrm{CO}_{2}}$$ , respectively, as well as their vibrational temperatures, in piloted premixed jet flames. Signal processing methods are detailed for the reconstruction of axial and radial profiles of thermochemical structure in a canonical ethylene–air jet flame. The method is further demonstrated to quantitatively distinguish between different turbulent flow conditions.

Dual-wavelength operation of a diode-pumped Nd:YVO 4 laser at the 1064.1 & 1073.1 nm and 1064.1 & 1085.3 nm wavelength pairs

Abstract: Two dual-wavelength operation regimes of a diode-pumped Nd:YVO4 laser were demonstrated at 1064.1 & 1073.1 and 1064.1 & 1085.3 nm with two intracavity birefringent plates. The output power ratio of the dual-wavelength output could be freely adjusted. The highest total output power was 1.44 W under the dual-wavelength condition with 1:1 power ratio for both wavelength pairs. The slope efficiency was more than 16% and optical-to-optical efficiency more than 13% with respect to the absorbed pump power.

A discretely tunable dual-wavelength multi-watt Yb:CALGO laser

Abstract: A discretely tunable dual-wavelength diode-pumped Yb:CALGO laser using a single birefringent filter (BRF) plate which covered a wavelength range of approximately 1020–1070 nm was demonstrated. A detailed study was conducted for BRF plates with thickness of 0.5, 2, 4 and 6 mm using different output couplers. This simple design was capable of delivering multi-watt dual-wavelength output power and the frequency offset discretely varied from approximately 1.3 to 12.5 THz. The maximum dual-wavelength output power was 4.1 W using a 6-mm-thick BRF plate with 5% output coupler.

Thermal lensing in Nd:GdVO 4 laser with direct in-band pumping at 912 nm

Abstract: We report on the thermal lensing effect study in a 1063-nm Nd:GdVO4 laser with in-band diode pumping at 912 nm. The thermal lensing focal power was determined using the experimentally measured values of the laser beam width and beam quality factor (M2). The maximum observed thermal lens dioptric power was 9 m−1 at 11.3 W of output power (18.87 W of absorbed pump power) for the pump spot size diameter of ~ 394 µm. Finite element analysis simulation was also performed and strongly supported the experimental data. The effect of experimental conditions on the thermal lensing strength and comparison with previous values obtained with traditional 808 nm pumping were discussed. It was shown that pumping with 808 nm suffers from higher thermal lensing. A comparison with a well-known Nd:YVO4 laser crystal was also made.

A high-sensitivity temperature sensor based on Sagnac interferometer employing photonic crystal fiber fully filled with ethanol

Abstract: A high-sensitivity temperature sensor based on photonic crystal fiber Sagnac interferometer is proposed and studied. All holes of the PCF are filled with ethanol with capillarity. The cladding air holes are uniform arrangements. The two air holes around the core are removed to form new core modes with high birefringence. The sensitivities of the temperature can be up to −8.7657 and 16.8142 nm/ $$^{\circ }$$ C when temperature rises from 45 to 75 $$^{\circ }$$ C and the fiber length is 5.05 cm. And when temperature rises from 10 to 45 $$^{\circ }$$ C, the sensitivity can reach −7.848 and 16.655 nm/ $$^{\circ }$$ C with fiber length 2.11 cm. The performance of the selective-filled and the fully-filled PCF with temperature from 45 to 75 $$^{\circ }$$ C and fiber length 5.05 cm are analyzed and compared. The fully filling can better achieve PCF’s sensing performance. The simple structure and high sensitivities make the temperature sensor easy to achieve. The temperature sensor with high sensitivities and good linearity has great application value for environmental temperature detecting.

Multi-pass cell-assisted photoacoustic/photothermal spectroscopy of gases using quantum cascade laser excitation and heterodyne interferometric signal detection

Abstract: A heterodyne interferometer-based signal retrieval in photoacoustic/photothermal gas spectrometer using mid-infrared quantum cascade laser excitation is demonstrated. This new method for all-optical photoacoustic/photothermal signal detection allows for sensitivity enhancement using standard multi-pass cells, commonly used in absorption-based spectrometers. Two types of multi-pass cell are examined: a Herriott type with up to 33 passes and a White type with up to 46 passes. Good agreement between experimental results and numerical analysis is obtained.

3D transformations of light fields in the focal region implemented by diffractive axicons

Abstract: Three-dimensional transformations of light fields were investigated, including such characteristics as intensity distribution and polarisation state, in the focal region at tight focusing and in the paraxial regime due to the lens apodisation by a diffraction axicon with different structures (linear, binary, spiral axicons) Theoretical analysis demonstrates the different possibilities of such transformations and significant differences in results depending on the focusing modes (sharp or paraxial) The experimental results obtained were in good agreement with the simulation results, demonstrating that optical systems employing lens–axicon combinations may be useful in the laser trapping of nano- and microparticles, optical microscopy, and improvement of high-capacity information storage techniques

An anatomy of strong-field ionization-induced air lasing

Abstract: It is known that in an intense laser field of a sufficiently high strength combined with a sufficiently long wavelength, i.e., in the regime of the Keldysh parameter γ < 1, photoionization of atoms and molecules can be realized through a quantum tunnel process. The tunnel ionization preferentially occurs from the orbital with the lowest ionization energy, thus the majority of the generated ions will stay on the ground state. It is surprising that tunnel ionization of nitrogen molecules with mid- and near-infrared intense laser fields can initiate strong laser-like emissions, indicating generation of stimulated emissions in molecular nitrogen ions. The physical mechanism behind the observation is still under debate. Here, we review the major progresses we made in the past a few years. The focus is placed on investigations on the lasing action at 391 nm wavelength initiated by either mid-infrared strong laser fields in the wavelength range from 1.2 to 2 µm or near-infrared intense laser fields around 800 nm wavelength. We reveal that the mechanisms of lasing actions are different for the pump lasers in the above two spectral regions. We also show that the coherent wavepackets of molecular nitrogen ions generated in the intense laser fields uniquely allow for efficient nonlinear interaction with light at resonance frequencies.

Wavelength modulation spectroscopy near 5 $$\upmu$$m for carbon monoxide sensing in a high-pressure kerosene-fueled liquid rocket combustor

Abstract: A laser absorption sensor was developed for carbon monoxide (CO) sensing in high-pressure, fuel-rich combustion gases associated with the internal conditions of hydrocarbon-fueled liquid bipropellant rockets. An absorption feature near 4.98 $$\upmu$$ m, comprised primarily of two rovibrational lines from the P-branch of the fundamental band, was selected to minimize temperature sensitivity and spectral interference with other combustion gas species at the extreme temperatures (> 3000 K) and pressures (> 50 atm) in the combustion chamber environment. A scanned wavelength modulation spectroscopy technique (1f-normalized 2f detection) is utilized to infer species concentration from CO absorption, and mitigate the influence of non-absorption transmission losses and noise associated with the harsh sooting combustor environment. To implement the sensing strategy, a continuous-wave distributed-feedback (DFB) quantum cascade laser (QCL) was coupled to a hollow-core optical fiber for remote mid-infrared light delivery to the test article, with high-bandwidth light detection by a direct-mounted photovoltaic detector. The method was demonstrated to measure time-resolved CO mole fraction over a range of oxidizer-to-fuel ratios and pressures (20–70 atm) in a single-element-injector RP-2-GOx rocket combustor.

Phase-stabilized 100 mW frequency comb near 10 μm.

Abstract: Long-wavelength mid-infrared (MIR) frequency combs with high power and flexible tunability are highly desired for molecular spectroscopy, including investigation of large molecules such as C60. We present a high power, phase-stabilized frequency comb near 10 μm, generated by a synchronously pumped, singly resonant optical parametric oscillator (OPO) based on AgGaSe2. The OPO can be continuously tuned from 8.4 to 9.5 μm, with a maximum average idler power of 100 mW at the center wavelength of 8.5 μm. Both the repetition rate (frep) and the carrier-envelope offset frequency (fceo) of the idler wave are phase-locked to microwave signals referenced to a Cs clock. We describe the detailed design and construction of the frequency comb, and discuss potential applications for precise and sensitive direct frequency comb spectroscopy.

Qualitative and quantitative analyses of copper ores collected from Baluchistan, Pakistan using LIBS and LA-TOF-MS

Abstract: We present a compositional analysis of different copper ore samples collected from Baluchistan, Pakistan. The qualitative as well as quantitative analysis was performed using laser-induced breakdown spectroscopy (LIBS) and laser ablation time-of-flight mass spectrometer (LA-TOF-MS). The laser-produced plasma on the surface of the samples was generated by focusing a high-power Nd: YAG laser and the time-integrated optical emission spectra were recorded using a set of four spectrometers coupled and equipped with charged coupled device (CCD). We used the Calibration-free laser-induced breakdown spectroscopy (CF-LIBS) for the quantitative determination of major and trace elements present in different ore samples. The quantitative analysis of these samples was also performed using LA-TOF-MS. Fifteen elements including lithium, sodium, magnesium, aluminum, silicon, potassium, calcium, manganese, titanium, iron, copper, zinc, silver, strontium and barium with varying concentrations have been detected in the optical emission spectra. Both the techniques, LIBS through CF and LA-TOF-MS, independently yield quantitative results that are in good agreement. It is demonstrated that LIBS and LA-TOF-MS are complementary, non-destructive and efficient techniques that can be used to analyze any solid material in a short time.

Drone-based area scanning of vegetation fluorescence height profiles using a miniaturized hyperspectral lidar system

Abstract: We have developed a compact hyperspectral lidar system based on a continuous-wave (CW) 445 nm diode laser and a double Scheimpflug imaging arrangement. The light-weight construction allows the integration of the system on a commercial drone. Airborne, range-resolved spatial imaging of vegetation fluorescence is demonstrated.

Half-Watt average power femtosecond source spanning 3–8 µm based on subharmonic generation in GaAs

Abstract: Frequency combs with a wide instantaneous spectral span covering the 3–20 µm molecular fingerprint region are highly desirable for broadband and high-resolution frequency comb spectroscopy, trace molecular detection, and remote sensing. We demonstrate a novel approach for generating high-average-power middle-infrared (MIR) output suitable for producing frequency combs with an instantaneous spectral coverage close to 1.5 octaves. Our method is based on utilizing a highly-efficient and compact Kerr-lens mode-locked Cr2+:ZnS laser operating at 2.35-µm central wavelength with 6-W average power, 77-fs pulse duration, and high 0.9-GHz repetition rate; to pump a degenerate (subharmonic) optical parametric oscillator (OPO) based on a quasi-phase-matched GaAs crystal. Such subharmonic OPO is a nearly ideal frequency converter capable of extending the benefits of frequency combs based on well-established mode-locked pump lasers to the MIR region through rigorous, phase- and frequency-locked down conversion. We report a 0.5-W output in the form of an ultra-broadband spectrum spanning 3–8 µm measured at 50-dB level.

Nanosecond nonlinear optical and optical limiting properties of hollow gold nanocages

Abstract: Gold nanocages (NCs) were prepared using the galvanic replacement reaction. Transmission electron microscopy images confirmed the porous morphology and completely hollow interior of the gold NCs. The nanosecond nonlinear optical and optical limiting (OL) properties of the NCs were characterized using the open-aperture Z-scan technique with 8-ns laser pulses at 532 nm. The gold NCs exhibited intensity-dependent transformation from saturable absorption to reverse-saturable absorption. The nonlinear absorption coefficient and saturable energy of the NCs were 5 × 10− 12 m/W and 2.5 × 1010 W/m2, respectively. Meanwhile, the gold NCs were found to display strong OL properties towards nanosecond laser pulses. The OL threshold of the gold NCs was lower than that of solid gold nanoparticles and comparable with that of a carbon nanotube suspension. Input fluence and angle-dependent scattering measurements indicated that nonlinear scattering plays an important role in the OL behavior of the gold nanostructures at high laser excitation. The improved OL response in gold NCs was discussed from the viewpoint of structural characteristic. The ultrathin and highly porous walls of the gold NCs can effectively transfer the photon-induced heat to the surrounding solvent, resulting in enhanced OL properties compared with those of solid gold nanoparticles. The intensity-dependent transformation from saturable absorption to reverse-saturable absorption and excellent OL response indicate that the smart gold NCs with ultrathin and highly porous walls can be considered as potential candidate in pulse shaping, passive mode locking, and eye protection against powerful lasers.

High-energy and ultra-wideband tunable terahertz source with DAST crystal via difference frequency generation

Abstract: We have demonstrated a high-energy and broadly tunable monochromatic terahertz (THz) source based on difference frequency generation (DFG) in DAST crystal. A high-energy dual-wavelength optical parametric oscillator with two KTP crystals was constructed as a light source for DFG, where the effect of blue light was first observed accompanying with tunable dual-wavelength pump light due to different nonlinear processes. The THz frequency was tuned randomly in the range of 0.3–19.6 THz. The highest energy of 870 nJ/pulse was obtained at 18.9 THz under the intense pump intensity of 247 MW/cm2. The THz energy dips above 3 THz have been analyzed and mainly attributed to the resonance absorption induced by lattice vibration in DAST crystal. The dependence of THz output on the input energy was studied experimentally, and THz output saturation was observed. Furthermore, tests of transmission spectroscopy of four typical samples were demonstrated with this ultra-wideband THz source.

Mid-infrared supercontinuum generation in fluoride fiber amplifiers: current status and future perspectives

Abstract: The quest for a compact and efficient broadband laser source able to probe the numerous fundamental molecular absorption lines in the mid-infrared (3–8 µm) for various applications has been going on for more than a decade. While robust commercial fiber-based supercontinuum (SC) systems have started to appear on the market, they still exhibit poor energy conversion into the mid-infrared (typically under 30%) and are generally not producing wavelengths exceeding 4.7 µm. Here, we present an overview of the results obtained from a novel approach to SC generation based on spectral broadening inside of an erbium-doped fluoride fiber amplifier seeded directly at 2.8 µm, allowing mid-infrared conversion efficiencies reaching up to 95% and spectral coverage approaching the transparency limit of ZrF4 (4.2 µm) and InF3 (5.5 µm) fibers. The general concept of the approach and the physical mechanisms involved are presented alongside the various configurations of the system to adjust the output characteristics in terms of spectral coverage and output power for different applications.

A comprehensive investigation on core optoelectronic and laser properties of ZTS single crystals: an effect of Mg 2+ doping

Abstract: The synthesis and large-sized single crystals of pure and Mg2+-doped zinc (tris) thiourea sulfate has been developed using slow evaporation methods at 300 K. The size of the grown crystal for pure, 2% Mg and 5% Mg-doped ZTS is found to be ~ 15 mm × 17 mm, ~ 20 mm × 18 mm, and 22 mm × 10 mm, respectively. The crystal structural and vibrational modes are identified by powder X-ray diffraction, FT-IR and FT-Raman analyses. The grown crystal with 5% Mg doping possesses higher optical transparency (67%). Optical energy band gap is found ~ 4.36 eV for 2% Mg and ~ 4.41 eV for 5% Mg-doped crystals. Enhancement in PL intensity of UV band was observed due to doping. The third-order nonlinear susceptibility, χ3 is found to be enhanced due to doping which is in the order of 10−3 esu. DSC study confirms that the 5% MgZTS crystals possess higher thermal stability than pure as well as 2% MgZTS. Furthermore, the dielectric study confirms that the grown crystals possess low defects. The microhardness is also found to be enhanced due to doping. All the properties of ZTS are found to be enhanced by Mg doping and make it more suitable for optoelectronic and nonlinear applications compared to pure.

Multi-species trace gas sensing with dual-wavelength QCLs

Abstract: Instrumentation for environmental monitoring of gaseous pollutants and greenhouse gases tends to be complex, expensive, and energy demanding, because every compound measured relies on a specific analytical technique. This work demonstrates an alternative approach based on mid-infrared laser absorption spectroscopy with dual-wavelength quantum cascade lasers (QCLs). The combination of two dual- and one single-DFB QCL yields high-precision measurements of CO (0.08 ppb), CO2 (100 ppb), NH3 (0.02 ppb), NO (0.4 ppb), NO2 (0.1 ppb), N2O (0.045 ppb), and O3 (0.11 ppb) simultaneously in a compact setup (45 × 45 cm2). The lasers are driven time-multiplexed in intermittent continuous wave mode with a repetition rate of 1 kHz. The individual spectra are real-time averaged (1 s) by an FPGA-based data acquisition system. The instrument was assessed for environmental monitoring and benchmarked with reference instrumentation to demonstrate its potential for compact multi-species trace gas sensing.

A circular polarization converter based on in-linked loop antenna frequency selective surface

Abstract: In this paper, we report the design, fabrication and measurement of a circular polarization converter based on an in-linked loop-antenna frequency selective surface. The building unit cell is the in-linked loop-antenna module, which consists of same front and back planar loop antennas in-linked by a pair of through-via holes passing through a sandwiched perforated metal ground plane. The proposed device can achieve transmission polarization conversions from right- or left-handed circularly polarized waves to left- or right-handed ones, respectively, or vice versa. Simulation and experimental results show that it has relative conversion ratio of near unity at resonant frequency and very low Joule insertion loss in the operating frequency band. The proposed circular polarization converter may be applied to wireless systems where circular polarization diversity is needed.

An “in-fiber” Whispering-Gallery-Mode bi-sphere resonator, sensitive to nanometric displacements

Abstract: A double-cavity, Whispering-Gallery-Mode (WGM) resonator, composed of two polystyrene microspheres, is formed inside the capillary of a microstructured optical fiber. The spectra are compared with the corresponding WGM patterns of the isolated microspheres. We collect scattering spectra from the contact area between the two beads, as the spacing between them is varied, and the evolution of the modes with sphere separation is discussed. Potential applications of the presented optical design are also proposed.

Effects of the turbulent atmosphere and the oceanic turbulence on the propagation of a rotating elliptical Gaussian beam

Abstract: Based on the extended Huygens–Fresnel integral, the average intensity, the gradient force and the effective beam width in the x- and y-directions of a rotating elliptical Gaussian (REG) beam in the turbulent atmosphere and the oceanic turbulence are explored analytically and numerically. It is shown that the intensity patterns of the REG beam in the atmospheric and the oceanic turbulences spin anticlockwise within a certain propagation range and always keep unimodal distributions during the propagation. Meanwhile, the numerical experiments of the REG beam are carried out to verify the theoretical analytical results. By comparative analysis, the effective beam widths can be adjusted by varying the turbulent characteristic parameters and the initial beam waist of the REG beam. The intensity modulation of the REG beam through the turbulent atmosphere and the oceanic turbulence can be realized.

Dual-wavelength DFB quantum cascade lasers: sources for multi-species trace gas spectroscopy

Abstract: We report on the design, fabrication, and performance of dual-wavelength distributed-feedback (DFB) quantum cascade lasers (QCLs) emitting at several wavelengths in the mid-infrared (mid-IR) spectrum. In this work, two new designs are presented: for the first one, called “Neighbour” DFB, two single-mode DFB QCLs are fabricated next to each other, with minimal lateral distance, to allow efficient beam-coupling into multi-pass gas cells. In addition, the minimal distance allows either laser to be used as an integrated heater for the other, allowing to extend the tuning range of its neighbour without any electrical cross-talk. For the second design, the Vernier effect was used to realize a switchable DFB laser, with two target wavelengths which are distant by about 300 cm $$^{-1}$$ . These devices are promising laser sources for Tunable Diode Laser Absorption Spectroscopy applications targeting simultaneous detection of multiple gasses, with distant spectral features, in compact and mobile setups.

Quartz tuning fork-based photodetector for mid-infrared laser spectroscopy

Abstract: In this paper, we report a new type of photoelectric detector based on a standard quartz crystal tuning fork (QCTF) with resonant frequency of ~ 32 kHz for spectroscopic applications. Analogous to the photoelectric effect of traditional semiconductor detectors, we utilize the piezoelectric effect of the QCTF to gauge the light intensity. To explore the capabilities of this technique, the impact of incident light beam excitation positions with respect to QCTF on signal amplitude, resonant frequency and Q factor, as well as the dependence on incident light intensity, ambient pressure and temperature, was investigated in detail. Finally, the QCTF-based photodetector was successfully demonstrated for qualitative analysis of gasoline components by combing a broadband tunable external cavity quantum cascade laser.

LIISim: a modular signal processing toolbox for laser-induced incandescence measurements

Abstract: Evaluation of measurement data for laser-induced incandescence (LII) is a complex process, which involves many processing steps starting with import of data in various formats from the oscilloscope, signal processing for converting the raw signals to calibrated signals, application of models for spectroscopy/heat transfer and finally visualization, comparison, and extraction of data. We developed a software tool for the LII community that helps to evaluate, exchange, and compare measurement data among research groups and facilitate the application of this technique by providing powerful tools for signal processing, data analysis, and visualization of experimental results. A common file format for experimental data and settings simplifies inter-laboratory comparisons. It can be further used to establish a public measurement database for standardized flames or other soot/synthetic nanoparticle sources. The open-source concept and public access to the software development should encourage other scientists to validate and further improve the implemented algorithms and thus contribute to the project. In this paper, we present the structure of the LIISim software including the materials database concept, signal-processing algorithms, and the implemented models for spectroscopy and heat transfer. With two application cases, we show the operation of the software how data can be analyzed and evaluated.

Tunable enhanced Goos–Hänchen shift of light beam reflected from graphene-based hyperbolic metamaterials

Abstract: The tunable and enhanced Goos–Hanchen (GH) shift for TM-polarized reflected beam from the graphene-based hyperbolic metamaterials (GHMM) is theoretically investigated. It is demonstrated that the lateral shift of the reflected beam can be tunable by Fermi energy and thickness of dielectric, and the largest GH shifts can be hundreds of wavelengths due to the enhanced effect by the GHMM. The minimum reflected angle (Brewster angle) moves to larger angle of incidence with the Fermi energy and thickness of dielectric increasing. Numerical simulation results for Gaussian incident beams coincide with the theoretical results from the stationary-phase method. The GH shift from the GHMM, maybe, open a new way for photoelectronic device application in future.

Properties of a random electromagnetic multi-Gaussian Schell-model vortex beam in oceanic turbulence

Abstract: A new kind beam called the random electromagnetic multi-Gaussian Schell-model (REMGSM) vortex beam has been introduced. Based on the Huygens–Fresnel principle, the elements of the cross-spectral density matrix of the REMGSM vortex beam propagation in oceanic turbulence have been derived. The average intensity and spectral degree of polarization properties of the REMGSM vortex beam propagating in oceanic turbulence are illustrated and analyzed using numerical examples. The results show that the REMGSM vortex beam propagating in stronger oceanic turbulence will evolve into flat-topped beam and Gaussian-like beam more rapidly as the propagation distance increases.