Laser Wavenumber Research Articles

Resonance Raman studies on phonon softening, phonon lifetime, Fano resonance, and multipeak analysis of MoS2 nanoflakes

AbstractIn this study, the resonance Raman spectra of hydrothermally produced 2H‐MoS2 nanoflakes excited by a 633‐nm laser were examined. Spectral observations include both fundamental MoS2 modes and additional Raman lines, which arise from alterations in the energy states of the semiconductor owing to the incidence of laser radiation. Phonon softening and alterations in the phonon lifetime were computed for different laser powers. The Fano resonance, which causes asymmetry in the Raman spectral lines, was analyzed at different laser powers. The Fano line‐shape function is used to fit the asymmetry in the in‐plane vibrational mode whereas multi‐peak fitting using the Fano‐Lorentzian function is used to fit the out‐of‐the‐plane fundamental mode, which is coupled with “b” mode. A direct study of the electron–phonon interaction was carried out with the “b” mode. The shift in laser wavenumber was then investigated using the 2LA(M) modes observed in the resonance Raman spectra. These findings provide new optoelectronic device designers with an understanding of the intricate electron–phonon interactions in transition metal dichalcogenides.

Modeling of generation of characteristic x-ray radiation under vacuum heating of electrons of nanocylinders

A model is developed for the generation of hot electrons near the surfaces of ionized cylinders by a laser field of nonrelativistic intensity, which allows one to go beyond the electrostatic approximation and takes into account the absorption of the laser field energy by the generated electrons. A model of Kα x-ray generation in a copper substrate, when the cylinders are located on the substrate obliquely and parallel to each other, and the laser field propagates perpendicularly to the substrate, is also considered. It is revealed that the Kα radiation yield depends rather strongly on the angle of inclination of the cylinders. The optimal parameters, the cylinder radius multiplied by the laser wavenumber, the angle of inclination of the cylinders, and direction of the linearly polarized laser electric field, are determined at the laser field amplitude aL = 0.2. With these parameters, the yield of Kα radiation from a copper substrate covered with cylinders is 2.7 times higher than the maximum yield of Kα radiation from the substrate covered with ionized clusters under the same irradiation conditions and 4 times higher than the maximum yield of Kα radiation from a flat copper target irradiated by a p-polarized laser field of the same amplitude. An increase in the yield of Kα radiation from the substrate covered with nanocylinders as compared to the yield of Kα radiation from the substrate covered with ionized clusters is due to an increase in the number of accelerated electrons.

Evaluating stability of a Raman spectrometer for long‐time experiments

AbstractRaman spectroscopic measurements spanning several hours to days are increasingly common and important. The stability of a Raman spectrometer becomes crucial when comparing spectra recorded at different occasions in such time‐intensive experiments. In this work, series of Raman spectra of benzene and emission spectra of neon were acquired using a standard Raman spectrometer operating in a typical air‐conditioned laboratory, with synchronized measurements of ambient parameters (pressure, temperature, humidity, etc.) subject to continuous unsystematic environmental drifts. The time‐dependent fluctuations of positions and intensities of selected bands in the series of spectra recorded over tens of hours displayed variations of up to ±0.3 cm−1 in the band positions and up to 2% in the associated Raman intensities, with experiments over longer time periods prone to more pronounced fluctuations. After accounting for change in the laser power fluctuations, the intensity variations were reduced to about 1%. Both the variations in the band positions and intensities showed practically no correlation with either of the ambient parameters or the refractive index of air. Tiny drifts in the laser wavenumber (on the order of 0.01 cm−1), attributed to the change in the refractive index of the laser cavity associated with the variations in the ambient pressure, were too small to account for the observed fluctuations in the band positions.

Electron affinity of lead

A beam of Pb− ions produced by a cesium sputtering ion source is photodetached in the presence of an electric field, inside a linear optical cavity. Amplification of the light flux by the resonant cavity makes it possible to record exploitable photoelectron interferograms, even though the Pb− current does not exceed a few pA. The laser wavenumber is set either just above the first 3P1 fine-structure excited threshold of neutral Pb, or above the higher 3P2 threshold. The photoelectron kinetic energy is deduced from the electron interferograms with a precision high enough to provide a new experimental value of the electron affinity of lead, 8 times more precise and slightly lower than the one measured in 2016: or 0.356 721(2) eV, instead of 287 733(13) m−1 or 0.356 743(16) eV.

Quantum cascade laser-based photoacoustic sulfuryl fluoride sensing

Although sulfuryl fluoride (SO2F2) is an efficient fumigant that does not react with the surface of indoor materials and does not reduce the stratospheric ozone shield, there are some concerns about its use. It is a toxic gas that attacks the central nervous system, and its global warming potential (GWP) value is 4780 for 100 years’ time. Therefore, it is a clear necessity of implementing detection methods for tracing such a molecule. In this work a sensitive photoacoustic setup was built to detect SO2F2 at concentrations of parts per billion by volume (ppbv). The symmetric S–O stretching mode was excited by a continuous-wave quantum cascade laser with radiation wavenumber ranging from 1275.7 to 1269.3 cm−1. The photoacoustic signal was generated by modulating the laser wavenumber at the first longitudinal mode of the photoacoustic cell with amplitude depth of 5 × 10−3 cm−1. The detection of a minimum SO2F2 concentration of 20 ppbv was achieved.

Infrared spectroscopy of molecular ions in selected rotational and spin-orbit states.

First results are presented obtained with an experimental setup developed to record IR spectra of rotationally state-selected ions. The method we use is a state-selective version of a method developed by Schlemmer et al. [Int. J. Mass Spectrom. 185, 589 (1999); J. Chem. Phys. 117, 2068 (2002)] to record IR spectra of ions. Ions are produced in specific rotational levels using mass-analyzed-threshold-ionization spectroscopy. The state-selected ions generated by pulsed-field ionization of Rydberg states of high principal quantum number (n ≈ 200) are extracted toward an octupole ion guide containing a neutral target gas. Prior to entering the octupole, the ions are excited by an IR laser. The target gas is chosen so that only excited ions react to form product ions. These product ions are detected mass selectively as a function of the IR laser wavenumber. To illustrate this method, we present IR spectra of C2H2 (+) in selected rotational levels of the (2)Πu,3/2 and (2)Πu,1/2 spin-orbit components of the vibronic ground state.

Nanoscale Chemical Imaging via AFM coupled IR Spectroscopy

Conventional infrared spectroscopy is one of the most widely used tools in science and industry to identify materials via vibrational resonances of chemical bonds, but optical diffraction limits its spatial resolution to the scale of many microns. Atomic force microscopy (AFM) enjoys excellent spatial resolution and can measure mechanical, electrical, magnetic and thermal properties of materials, but has historically lacked the ability to perform robust chemical analysis. Two techniques, (1) AFM-based infrared spectroscopy (AFM-IR) and (2) scattering scanning near field optical microscopy (s-SNOM) have been developed which couple AFM with an IR source allowing the chemical identification capabilities of IR spectroscopy to extend to the nanoscale. As complementary techniques, AFM-IR and s-SNOM together provide an unrivaled capability to perform nanoscale chemical analysis on a diverse range of organic, inorganic, photonic and electronic materials. The AFM-IR technique achieves nanoscale spatial resolution by using the AFM probe as a local sensor of the IR absorption [1]. A sample is irradiated with light from a pulsed, tunable infrared laser. When the IR laser is tuned to a wavelength where the sample has an absorption peak, a portion of the incident IR light is absorbed and converted into heat resulting in a rapid thermal expansion of the absorbing region. This generates an impulse force on the AFM tip, inducing resonant oscillations of the AFM cantilever. IR absorption spectra can then be obtained by measuring the cantilever oscillation amplitude as a function of laser wavenumber. Because the amplitude of induced cantilever oscillation is directly proportional to the IR absorption of the sample [2], absorption spectra obtained by AFM-IR compare very well to conventional FTIR without the use of modelling or reference samples. More recent extensions of this technique have allowed chemical measurements to be performed on samples as thin as individual monolayers [3]. Figure 1 shows an example application of the AFM-IR measurement, nanoscale chemical measurements of the environmental degradation of a biomedical material. One challenge to implantable devices is the possible degradation of the material due to exposure to the in vivo environment. Polyurethane was exposed to this simulated environment for sufficient time to allow the initiation of surface degradation. The chemical mechanism of this degradation was then analyzed and mapped using the AFM-IR technique. The AFM-IR spectra clearly show the chain scission which occurs in the ether bonds of the polyurethane by the reduction in the peak at 1108 cm -1 . It also shows the formation of alcohol groups by the increase in the 1176 cm -1 absorption band and carboxylate groups (COO-) shown by the increase in the broad bands at 1652 and around 1400 cm -1 . These absorption bands can be used to map the degradation and determine how the degradation initiates and extends into the sample, information that was previously not achievable at this length scale. The s-SNOM technique has been used in the SPM field for more than two decades [4]. It uses a metallized AFM tip to enhance and scatter radiation from a nanometer scale region of the sample. The scattered radiation is detected in the far field and carries information about the complex optical properties of the nanoscale region of the sample under the metallized tip. Both the optical amplitude

Intra-cavity photodetachment microscopy and the electron affinity of germanium

A beam of Ge− ions produced by a cesium sputtering ion source is photodetached, in the presence of an electric field, inside a linear optical cavity injected with a single mode ring Ti:Sa laser. The laser wavenumber can be set either above the highest fine-structure excitation threshold or just above the , intermediate fine-structure threshold of the ground-term of Ge I. A single-electron interferogram is produced, according to the principles of photodetachment microscopy, which contains both photoelectron energies produced by the two opposite wave vectors contained in the optical cavity. This makes the Doppler-free measurement of the photodetachment threshold even more direct than with the usual double-spot photodetachment microscopy method, at the expense of some reduction of the contrast in the interferograms. Both methods concur in producing a revised value of the electron affinity of germanium: or , one order of magnitude more precise and significantly smaller than the last measured value or . This new technique can be applied to weaker detachment thresholds or p-wave detachment.

Instability of a thin conducting foil accelerated by a finite wavelength intense laser

We derive a theoretical model for the Rayleigh–Taylor (RT)‐like instability for a thin foil accelerated by an intense laser, taking into account finite-wavelength effects in the laser wave field. These finite-wavelength effects lead to the diffraction of the electromagnetic wave off the periodic structures arising from the instability of the foil, which significantly modifies the growth rate of the RT-like instability when the perturbations on the foil have wavenumbers comparable to or larger than the laser wavenumber. In particular, the growth rate has a local maximum at a perturbation wavenumber approximately equal to the laser wavenumber. The standard RT instability, arising from a pressure difference between the two sides of a foil, is approximately recovered for perturbation wavenumbers smaller than the laser wavenumber. Differences in the results for circular and linear polarization of the laser light are pointed out. The model has significance for radiation pressure acceleration of thin foils, where RT-like instabilities are significant obstacles.

Effective improvement of depth resolution and reduction of ripple error in depth-resolved wavenumber-scanning interferometry

The bottleneck problem hindering the development of Depth-Resolved Wavenumber-Scanning Interferometry (DRWSI) is a limited depth resolution due to a finite range of the laser wavenumber scanning. A robust Complex Number Least Squares Algorithm (CNLSA) is proposed in the present study to take full advantage of phase and amplitude information over the entire range of frequencies, and the algorithm performs much better than any window function. Experimental results as well as simulations show that the phase ripple error is reduced and the depth resolution is significantly refined. A CNLSA is likely to become practical and suitable for DRWSI applications.

Studies of spectral modification and limitations of the modified paraxial equation in laser wakefield simulations

Laser pulses propagating through plasma undergo spectral broadening through local energy exchange with driven plasma waves. For propagation distances on the order of the energy depletion length, spectral shifts can be comparable to the laser central frequency and wavenumber, a result of approximate action conservation. Here, we examine the local spectral shift, energy depletion, and action conservation of nonlinear laser pulses using the modified paraxial simulation code WAKE. Breakdown of the modified paraxial equation (MPE), which is based on the assumption of slow temporal variation of the pulse envelope, is monitored via consideration of the wave action. Although action is theoretically conserved for the continuous MPE, we observe that for large red shifts, action decays for the discrete implementation. Numerical analysis of the propagation algorithm verified the observed behavior. Increasing resolution improves action conservation up to a time, which is identified to be the validity limit for the MPE. Increased resolution also leads to increased simulation times and eliminates the strength of the MPE solver–efficiency.

Simultaneous wavenumber measurement and coherence detection using temporal phase unwrapping

Wavelength scanning interferometry and swept-source optical coherence tomography require accurate measurement of time-varying laser wavenumber changes. We describe here a method based on recording interferograms of multiple wedges to provide simultaneously high wavenumber resolution and immunity to the ambiguities caused by large wavenumber jumps. All the data required to compute a wavenumber shift are provided in a single image, thereby allowing dynamic wavenumber monitoring. In addition, loss of coherence of the laser light is detected automatically. The paper gives details of the analysis algorithms that are based on phase detection by a two-dimensional Fourier transform method followed by temporal phase unwrapping and correction for optical dispersion in the wedges. A simple but robust method to determine the wedge thicknesses, which allows the use of low-cost optical components, is also described. The method is illustrated with experimental data from a Ti:sapphire tunable laser, including independent wavenumber measurements with a commercial wavemeter. A root mean square (rms) difference in measured wavenumber shift between the two of ~4 m⁻¹ has been achieved, equivalent to an rms wavelength shift error of ~0.4 pm.

Raman spectroscopy study of lichens using three spectrometers under different experimental conditions: analyses of the results with relevance for extraplanetary exploration

We have carried out analyses on three extremophile lichens from the Tabernas Desert (Spain) under different experimental conditions: dry and wet samples in the laboratory and wet specimens in the field, using three spectrometers: one portable, one FT-Raman and one dispersive micro-Raman instruments. Apart from pigment characterization, the information obtained from the spectra is compared and the differences analysed. The fact that no results were achieved on dry lichens using the handheld spectrometer is of special relevance for those looking for life in hazardous environments. Since in some extreme habitats, life could be “dormant” and only activates under appropriate conditions, miniaturized instruments (such as those included for extraplanetary exploration missions) could not detect bio-markers and, then, life signals will not be noticed when, actually, life is there. Furthermore, we show in this work, using the miniaturized Raman spectrometer, that not only laser wavelength, spectral resolution and wavenumber region of the spectrum are important for the bio- or geo-marker recognition but also spot-size is of vital relevance for the unambiguous biomolecular characterization. In our opinion, mini-Raman instruments are useful for assessing in situ the eventual presence of bio-markers in complex natural samples, such as organisms, but are not accurate enough for precise molecular identification.

Experimental observation of the dependence of Autler-Townes splitting on the probe and coupling laser wave-number ratio in Doppler-broadened open molecular cascade systems

We have investigated the effects of inhomogeneous Doppler line broadening on the Autler-Townes (AT) splitting in a three-level open molecular cascade system. For moderate Rabi frequencies in the range of $300--600\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, the fluorescence line shape from the uppermost level \ensuremath{\mid}3⟩ in this system depends strongly on the wave-number ratio of the two laser fields. However, the fluorescence spectrum of the intermediate level \ensuremath{\mid}2⟩ appears as expected. We provide a description of the conditions for optimally resolved AT splitting in terms of the ratio of the probe and coupling laser wave numbers, and the laser propagation geometry based on our theoretical analysis of the Doppler integral. The low threshold Rabi frequency region for Autler-Townes splitting is important for high-resolution Autler-Townes spectroscopy since the laser E-field amplitudes are modest in continuous-wave laser experiments.

Parasitic diffuse reflection in a Fourier transform spectrometer yielding subharmonic ghosts and line-shape distortion

When using a high-resolution Fourier transform spectrometer (FTS) in a cube-corner configuration, subharmonic ghosts are observed in the spectrum. These ghosts are attributable to parasitic diffuse reflections on the mirrors of the FTS arm. The reflected beams skip a part of the interferometer and travel a different path from the main beam thus experiencing a smaller optomechanical gain. These reflections are present in the reference laser channel as well as on the measurement channel, and each affect the estimated spectrum differently. The sampling grid generated by the reference laser has periodic errors that are synchronized with the fringe signal. The measured spectrum can therefore exhibit sampling jitter ghosts at submultiples of the reference laser wavenumber in addition to its own additive subharmonics. The diffuse reflection experiencing the nominal optomechanical gain, such as in a plane-mirror configuration, will impact directly on the instrument line shape and on the radiometric accuracy of the spectrometer since some radiation is not propagating at the expected angles in the instrument.

Classical field descriptions for ultrashort tightly-focused laser pulses

The wave equations for an ultrashort tightly-focused laser pulse in Hermite–Gaussian (0,0) mode are solved approximately. We obtain the analytical field expressions, which are exact up to the second order of the parameters 1/(k0L) and 1/(k0w0) (k0 is the laser wave number, w0 the laser beam waist, and L the laser pulse length). Our solutions can be reduced to usual paraxial ones naturally and more precise compared with the usual paraxial ones.

Method to synthesize polynomial current waveforms and intensity compensation functions for DFB lasers in digital sweep integration gas analyzers

With analysis methods using digital sweep integration of the absorbance function, linear current ramps can produce non-linear laser intensity and wavenumber functions from distributed feedback lasers. These non-linear functions produce offset and gain errors in mole fraction estimates on tunable diode laser gas analyzers. A method is described to synthesize polynomial current waveforms and laser intensity compensation functions to give linear wavenumber functions and to minimize the offset error. Quantitative and qualitative results are presented to evaluate reduction in mole fraction errors.

Accurate description of Gaussian laser beams and electron dynamics

Abstract In this paper, the higher order corrections to the description of a Gaussian laser field are derived and expressed as power functions of the parameter s=1/kw0, where k is the laser wave number and w0 the beam width at the focus center. Using the test particle simulation programs, the electron dynamics obtained using the paraxial approximation, the fifth-order correction, and the seventh-order correction are compared. Special attention is given to electron acceleration in vacuum by intense laser beams. The results reveal that, when kw0≳50, the paraxial approximation field is good enough to reproduce all the electron dynamic characteristics. In the range of 40≲kw0

Ionization instabilities of an electromagnetic wave propagating in a tenuous gas

A theory is developed to study the scattering instability that occurs when a laser pulse propagates through and ionizes a gas. The instability is due to the intensity dependence of the ionization rate, which leads to a transversely structured free electron density. The instability is convective in the frame of the laser pulse, but can have a relatively short growth length scaling as Lg∼k0/kp2, where k0 is the laser wave number, kp2=ωp2/c2 and ωp is the plasma frequency. The most unstable perturbations correspond to a scattering angle for which the transverse wave number is around the plasma wave number, kp. The scattered light is frequency upshifted. The comparison between simple analytic theory and numerical simulation shows good agreement.

Nonparaxial propagation of ultrashort laser pulses in plasma channels

The propagation characteristics of an ultrashort laser pulse in a preformed plasma channel are analyzed. The plasma channel is assumed to be parabolic and unperturbed by the laser pulse. Solutions to the wave equation beyond the paraxial approximation are derived that include finite pulse length effects and group velocity dispersion. When the laser pulse is mismatched within the channel, betatron oscillations arise in the laser pulse envelope. A finite pulse length leads to a spread in the laser wave number and consequently a spread in betatron wave number. This results in phase mixing and damping of the betatron oscillation. The damping distance characterizing the phase mixing of the betatron oscillation is derived, as is the dispersion distance characterizing the longitudinal spreading of the pulse.