Incident Laser Radiation 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.

Surface-enhanced optical absorption and induced heating in tapered silicon nanoprobes

Subject of study. This study investigates the correlation between the heating temperature and the mesoscopic shape of the Si probe of an atomic force microscope subjected to medium-intensity laser irradiation (5mW/cm2) in the presence of a rough metal substrate. Aim of study. This study aims to quantitatively assess the dependence of optical-field enhancement and induced heating of the tip of a tapered Si nanoprobe, under laser irradiation, on the radius of curvature and cone angle of the probe tip, the distance between the tip and substrate, and the surface roughness of an Au substrate. Method. The localization of the electromagnetic field in the gap between a Si nanoantenna and a defect on the surface of an Au substrate was simulated using the finite-difference time-domain method. A thin Au coating (thickness up to 50 nm) on a glass substrate was used as a plasmonic surface. Owing to the excitation of surface plasmon resonance, such a coating enhances the absorption of light and increases the heating temperature of the Si optical antenna. Main results. The influence of the polarization angle of the incident laser radiation on the distribution of the electric field near the tip of the probe was examined. Only the component of the incident light field along the direction of the probe axis was enhanced near the tip of the Si cantilever. The influence of various parameters, including the radius of curvature, cone angle of the tip of the Si nanoantenna, distance between the probe and substrate, and surface roughness of the Au substrate, on the maximum temperature in the tip region of the Si probe was investigated. The probe temperature was found to decrease with decreasing cone angle of the probe, whereas the temperature of the cantilever tip decreased as the cone angle of the probe tip increased. The dependence of temperature on the radius of curvature of the Si nanoantenna tip in the presence of an Au substrate was evaluated. With an increase in the surface roughness of the Au film, the temperature of the Si antenna tip increased, gradually approaching a limit value. Practical significance. The results of this study are useful for selecting the optimal parameters of an experiment in which a heated probe is used. Controlled heating of a Si probe can facilitate the study of phase transitions in various nanomaterials and that of local thermochemical nanocatalysis for creating new structural materials with specific properties.

Cascade Bowl Multicavity Structure for In Situ Surface-Enhanced Raman Scattering Detection of Organic Gas Molecules.

With the increasing emphasis on atmospheric environmental protection, it is crucial to find an efficient, direct, and accurate method to identify pollutant species in the atmosphere. To solve this problem, we designed and prepared the cascade multicavity (CMC) structure composed with silver nanoparticles (Ag NPs) as a surface-enhanced Raman scattering (SERS) substrate with favorable light transmittance and flexibility. The multicavity structure distributed on the surface introducing the homogeneous connecting holes endows the structure to more fully utilize the incident light while slowing the gas movement rate. Theoretical and experimental results have demonstrated that the Ag NPs/cascade multicavity (Ag-CMC) SERS substrate is a highly sensitive SERS substrate that can be used for in situ detection of gases under non-perpendicularly incident laser irradiation or bending of the substrate. We believe that the SERS substrate can provide a more efficient and feasible way for in situ detection of gaseous pollutants.

Aktivnoe brounovskoe dvizhenie pylevykh chastits v kvaziodnomernykh (tsepochechnykh) strukturakh v tleyushchem razryade

We present the experimental results of our study of the formation and dynamics of chain structures by active Brownian particles in a DC glow discharge. The mechanism of active particle motion is associated with the conversion of laser radiation by particles into the energy of their own (nonthermal) motion. Through our analysis of the motion parameters (the trajectories, the root-mean-square displacement, the kinetic energy) as a function of the intensity of laser radiation incident on the particles, we have shown that the particles are active Brownian ones. It is possible to control their motion without changing the discharge parameters. It has been experimentally found that the formation of chain structures and their stable state are not violated under kinetic heating of the particles as their kinetic energy increases by more than an order of magnitude. This suggests the realization of a mechanism for the formation of chains with a strong coupling between the particles that is not explained by the simple (ion) wake behind the upstream particle.

Simulation of x-ray bremsstrahlung generation under vacuum heating of solid target electrons

An extended Brunel model of hot electron generation in nonrelativistic laser-plasma interactions is considered. The model takes into account the permittivity of the surface plasma and the energy absorbed by electrons accelerated by the electric field components perpendicular and parallel to the target. A model for the generation of x-ray bremsstrahlung in the case of a Gaussian laser beam is presented. It is shown that the influence of electron motion parallel to the plasma surface on the absorbed intensity becomes distinguishable only at relatively low absolute values of the permittivity. Calculations of the dependences of the yield of hard bremsstrahlung x-rays on the angle of incidence of laser radiation and on the energy interval in which the yield of bremsstrahlung is measured are in qualitative agreement with the experimental data, if we assume that the electron concentration in the skin layer is relatively low (approximately 5–7 times the critical concentration).

Thermal Lattice Field during Ultra-Short Laser Pulse Irradiation of Metal Targets: A Fokker–Planck Analytical Model

The ultrafast fs laser pulse heating of thin metal films is studied for the first time using the two-temperature model on the basis of the Fokker–Planck formalism. The incident laser radiation is multi-modal, while the electron temperature is described during the first 2 fs. The predictions are intended for use by experimentalists in optoelectronics, photonics, laser processing, electronics, and bio- and nanomedicine. The crucial role of the nano-sized spatial dimensions of the metal sample is highlighted. A significant result of this study is the interdependence between the target’s size, the phonon/lattice characteristics, and the coefficient β (the quotient of non-diffusive phenomena), which varies between zero (pure diffusive case) and one (pure non-diffusive case).

Partial crystallization in a Zr-based bulk metallic glass in selective laser melting

Metals and alloys in amorphous state are promising for the use as structural and functional materials due to their superior properties related to the absence of such defects as grain boundaries and dislocations. Obtaining the amorphous state requires quenching from liquid state with a high cooling rate. In conventional technologies, this is the reason for a considerable size limitation hindering application of amorphous alloys. Additive manufacturing (AM) is free of the size limitation. The so-called bulk metallic glass (BMG) alloys have extremely low critical cooling rates and can be used in AM. Recent studies on AM from Zr-based BMGs by selective laser melting (SLM) has proved the possibility of attaining the amorphous state and revealed partial crystallization to be the principal drawback of this process. The present work aims to analyze the conditions for partial crystallization and attempts to control it by optimizing the process parameters. A comprehensive parametric analysis is accomplished in single-track SLM experiments with Zr-based BMG alloy Vit 106. The observed microstructures are related to the temperature fields and thermal cycles estimated by an analytic heat-transfer model in the laser-impact zone. In the cross section of a laser track, a central bright domain is identified as the remelted zone. An annular darker crystallization zone encircles the remelted zone. The model fits the experimentally obtained dependencies of the remelted zone size versus laser power and scanning speed and indicates that 43 ± 2% of the incident laser energy is transferred into the substrate thermal energy. The principal energy losses are reflection of the incident laser radiation and material evaporation. Primary crystalline inclusions existing in the substrate before laser processing dissolve at the laser melting. The mean cooling rate in the remelted zone is up to 4 orders of magnitude greater than the critical cooling rate. Therefore, homogeneous nucleation is not expected. Nevertheless, the theoretically estimated crystallization times are sufficient for a considerable crystal growth in the heat-affected zone where primary crystalline inclusions and nuclei are not completely dissolved.

Competition between nonlinear processes excited by picosecond laser pulses in a disodium ditungstate Raman crystal for two excitation polarizations

Two nonlinear processes, namely, two-photon absorption (TPA) and stimulated Raman scattering (SRS) in Na2W2O7 crystal at a wavelength of 523.5 nm, are studied theoretically and experimentally depending on the polarization of the incident picosecond laser radiation. It was found that, due to the anisotropy of the band gap structure, the TPA coefficient at 523.5 nm strongly depends on the pump radiation polarization. For radiation along the c axis with polarization parallel to the a axis, the TPA coefficient was measured to be 1.28 cm GW−1, and no SRS process was registered. However, for polarization parallel to the b axis, no measurable TPA was detected and SRS with a gain of 6.5 cm GW−1 was observed. To the best of our knowledge, this is the first demonstration of the competition of two nonlinear processes in one crystal depending on its orientation.

Model-based simulations of pulsed laser ablation using an embedded finite element method

A model of thermal ablation with application to multi-pulsed laser lithotripsy is presented. The approach is based on a one-sided Stefan–Signorini model for thermal ablation, and relies on a level-set function to represent the moving interface between the solid phase and a fictitious gas phase (representing the ablated material). The model is discretized with an embedded finite element method, wherein the interface geometry can be arbitrarily located relative to the background mesh. Nitsche’s method is adopted to impose the Signorini condition on the moving interface. A bound constraint is also imposed to deal with thermal shocks that can arise during representative simulations of pulsed ablation with high-power lasers. We report simulation results based on experiments for pulsed laser ablation of wet BegoStone samples treated in air, where Begostone has been used as a phantom material for kidney stone. The model is calibrated against experimental measurements by adjusting the percentage of incoming laser energy absorbed at the surface of the stone sample. Simulation results are then validated against experimental observations for the crater area, volume, and geometry as a function of laser pulse energy and duration. Our studies illustrate how the spreading of the laser beam from the laser fiber tip with concomitantly reduced incident laser irradiance on the damaged crater surface explains trends in both the experimental observations and the model-based simulation results.

Cross beam energy transfer and backward stimulated Brillouin scattering in double-cone ignition experiment

<sec>In the research of direct-drive laser fusion, laser irradiation of a target pellet can stimulate various laser plasma instabilities, such as stimulated Brillouin scattering (SBS) and cross-beam energy transfer (CBET), which significantly reduce the energy coupling efficiency between the laser and target pellet as well as the laser irradiation uniformity, leading the implosion quality to degrade. In the double-cone ignition (DCI) scheme of laser fusion scheme, the diagnosis of SBS and CBET is important owing to the different target configurations and oblique incident laser irradiation from the traditional spherically symmetric direct-drive central ignition scheme. In this paper, a simple and reliable backscattering diagnostic system is developed and applied to the diagnosis of the time-resolved backscattering spectrum at wavelength near 351 nm in a DCI experiment on the Shenguang-II upgrade (SG-IIU) facility. We use the system to carry out an experimental study of the SBS process and CBET process in DCI.</sec><sec>The backscattering diagnostic system collects the backscattered light signal through the scattered light by reflector mirror via an optical fiber. The signal is dispersed by a spectrometer and then recorded by a streak camera. The signal contains both the laser reference signal from the frequency doubling crystal and the backscattered light. With the help of the reference signal, the diagnostic system can reliably give the energy fraction of backscattered light. The experimental results show that the energy fraction of backscattered light around 351 nm is not higher than 3%, which is significantly lower than the experimental result of the spherically symmetric irradiation direct-drive central ignition scheme.</sec><sec>By analyzing the correlation between the backscattered signal and the laser irradiation conditions and combining the results of a set of comparative experiments, we determine that the backscattered signal contains both CBET and SBS. There is a significant difference in the CBET fraction between the backscattered signal of the #5 laser and the backscattered signal of the #7 laser. By combining the polarisation state of the laser beams, we confirm that this phenomenon is related to the polarisation angle between the laser beams. This finding provides a reference for designing subsequent large-scale laser fusion devices.</sec>

Optical bistability in layered InSe crystal

In this study, we experimentally study the dependence of the intensity of laser radiation incident on and transmitted through a layered InSe crystal. A picosecond YAG:Nd3+ laser with a parametric light generator was used as a light source, making it possible to generate light pulses with a duration of [Formula: see text]30 ps, tunable in frequency within the range of 0.75–1.5 μm. Optical bistability in the region of exciton resonance was detected in an InSe crystal at room-temperature under the picosecond YAG:Nd laser. It is shown that the decrease in absorption in the exciton resonance region is associated with the process of exciton–exciton interaction, shielding of excitons, and the Mott transition.

AAg3Ga8Se14 (A = Rb, Cs): second-harmonic generation responses realized through the parallel arrangement of AgSe4 and GaSe4 tetrahedrons.

Two new quaternary selenides AAg3Ga8Se14, (A = Rb, 1; Cs, 2) were synthesised via solid-state reaction in sealed silica tubes. Compounds 1 and 2 crystallised in the monoclinic space group Cm (no. 8) and their three-dimensional [Ag3Ga8Se14]- anionic frameworks were comprised of AgSe4 and GaSe4 tetrahedrons. Their UV-Vis-near infrared diffuse reflectance spectra showed that 1 and 2 possessed wide band gaps of 2.17 and 2.10 eV, respectively. Notably, under incident laser irradiation at 1910 nm, compounds 1 and 2 presented moderate second-harmonic generation responses of 0.6 and 0.7 × AgGaS2, respectively, with phase-matching behaviours due to the parallel arrangement of nonlinear optical (NLO) functional tetrahedral AgSe4 and GaSe4 units. The laser-induced damage thresholds of 1 and 2 were estimated to be 25.4 and 18.0 MW cm-2, respectively, which were 2.1 and 1.5 times the threshold of AgGaS2. This study revealed that the title selenides, which were constructed from tetrahedral units arranged in a parallel array, are promising infrared NLO materials.

A GaInP-based photo-converter of laser radiation with an efficiency of 46.7% at a wavelength of 600 nm

GaInP-based laser power converters (LPC) structure grown by MOVPE and device chip design have been optimized for operation under high-power lasers of the green-red spectral range. Light I-V curves records have shown the performance of the LPC at up to 40-50 W/cm2 of incident power densities. The highest level data were obtained for 532, 600, and 633 nm power laser lines: 44.3%, 46.7%, and 40.6% under 13-16 W/cm2, respectively. LPC demonstrated an efficiency of more than 40% at the incident laser radiation power density elevated up to 40-50 W/cm2. Keywords: laser photoconverter, MOVPE, efficiency, spectral response.

Импульсная проводимость в Ag-Pd-резисторах, индуцированная импульсами лазера

Pulsed EMF in resistive thick-film Ag - Pd elements is experimentally studied when their surface is irradiated with a laser beam. The dependences of the pulsed EMF on the coordinate of the action of the laser beam on the film surface are obtained. It is shown that applying a constant bias voltage to a thick-film resistive element can significantly increase the amplitude of unipolar EMF pulses induced by laser pulses. It was found that the amplitude and frequency of the EMF signals depend, respectively, on the power and frequency of laser pulses. It is shown that by the value of the pulsed EMF it is possible to control the parameters of the high-power laser radiation incident on the surface in real time. The design of a receiver for recording the parameters of laser radiation based on an Ag - Pd sensitive element has been developed. The sensitivity of the device for measuring the power and pulse repetition rate is estimated.

Pulse Conductivity in Ag-Pd Resistors Induced by Laser Pulses

Pulsed EMF in resistive thick-film Ag-Pd elements is experimentally studied when their surface is irradiated with a laser beam. The dependences of the pulsed EMF on the coordinate of the action of the laser beam on the film surface are obtained. It is shown that applying a constant bias voltage to a thick-film resistive element can significantly increase the amplitude of unipolar EMF pulses induced by laser pulses. It was found that the amplitude and frequency of the EMF signals depend, respectively, on the power and frequency of laser pulses. It is shown that by the value of the pulsed EMF it is possible to control the parameters of the high-power laser radiation incident on the surface in real time. The design of a receiver for recording the parameters of laser radiation based on an Ag-Pd sensitive element has been developed. The sensitivity of the device for measuring the power and pulse repetition rate is estimated. Keywords: measurement of laser parameters, thick-film resistor, thermo EMF.

Influence of the Characteristics of Multilayer Interference Antireflection Coatings Based on Nb, Si, and Al Oxides on the Laser-Induced Damage Threshold of ZnGeP2 Single Crystal

In this work, the effect of the defect structure and the parameters of antireflection interference coatings based on alternating layers of Nb2O5/Al2O3 and Nb2O5/SiO2 layers on the laser-induced damage threshold of ZGP crystals under the action of Ho:YAG laser radiation at a wavelength of 2.097 μm was determined. Coating deposition was carried out using the ion-beam sputtering method. The laser-induced damage threshold of the sample with a coating based on alternating layers Nb2O5 and SiO2 was W0d = 1.8 J/cm2. The laser-induced damage threshold of the coated sample based on alternating layers of Nb2O5 and Al2O3 was W0d = 2.35 J/cm2. It has been found that the presence of silicon conglomerates in an interference antireflection coating leads to a decrease in the laser-induced damage threshold of a nonlinear crystal due to local mechanical stresses and the scattering of incident laser radiation.

Condition monitoring of current media using a differential refractometer

A new design of a differential flow refractometer has been developed to monitor the condition of flowing media in a pipeline. A new method of refractive index measurement has been implemented, taking into account the specifics of flowing and closed cuvette arrangement, as well as the angles of incidence of laser radiation on their walls. The effect of changes in the optical density in the flowing liquid on the refractive index measurement result is determined. The results of experimental investigations of different media are presented.

Pulsed laser deposition of plasmonic structures in air by irradiation through the substrate

This work presents results from a method for single step fabrication of noble metal nanostructures in air that express plasmonic properties. A special configuration of the laser irradiation through the substrate is applied. Due to the interaction of the deposited material with the incident laser radiation, the resulting structure could be a dense nanoparticle monolayer instead of three-dimensional structure, which is usually obtained at pulsed laser deposition in air. The proposed geometry of irradiation is characterized by several key parameters that are defined and their influence on the properties of the fabricated structures is studied. The change of the processing conditions may result in significant change of the optical properties of the deposited structures. This effect is more expressed in the case of silver. The deposition of gold at the presented method leads to a saturation of the amount of the deposited material, and respectively to constant optical properties with the increase of the deposition time at fixed other parameters. Conditions where formation of a monolayer of well-defined separated nanoparticles are also presented. It is demonstrated that using this method composite gold/silver structures with tunable optical properties can be obtained. On a basis of numerical calculations, the optical properties of the obtained structure and the temperature evolution of the formed material are obtained. These data are used to explain the observed dependences and the formation mechanism. The method is simple and can be applied for efficient deposition of plasmonic systems with potential application in the design of different optical and sensor elements.

Excitation of high-intensity terahertz surface waves under action of two-frequency laser radiation

The excitation of terahertz (THz) surface mode at oblique incidence of two-frequency laser radiation on semi-bounded plasma is studied. It is shown that if frequency difference of laser fields coincides with the surface eigenmode frequency, the significant increase surface wave (SW) field occurs. It is found that additional amplification of SW field occurs when the s- and p-polarized laser radiation is incident at the angle of total reflection. The SW energy flux density increases most significantly for p-polarized laser radiation when it is incident at the angle of total reflection on the near-critical plasma with rare electron collisions. It is shown that in this case the SW energy flux density can significantly exceed the laser intensity.

Sub-Doppler spectra of sodium [formula omitted] lines in a wide range of magnetic field: Theoretical study

We compute the interaction of a sodium vapor with a static magnetic field ranging from 0 up to 1 Tesla, which allows to obtain the behavior of all Zeeman transitions as a function of the magnetic field for any polarization of incident laser radiation: π,σ±. We use these results combined with a Fabry-Pérot microcavity model to describe the transmitted and reflected signal of a sodium vapor confined in a nanometric-thin cell. This allows us to present for the first time high resolution absorption spectra, and provide a complete description of the Zeeman transitions, along with some important peculiarities such as the appearance of guiding transitions and magnetically-induced circular dichroism. The obtained theoretical results can be used in the upcoming experiments with sodium vapor nanocells.