Absorption Of Laser Radiation Research Articles

Field emission from point diamond cathodes under continuous laser irradiation

The presented study investigates the impact of continuous laser irradiation in the visible range on the field emission properties of diamond needle-like micro-sized crystallites with a nanometer tip radius. The measurements were carried out in a vacuum diode configuration with a flat metal anode using DC voltage source. It was found that the field emission current increased under illumination, showing a direct correlation with the radiation power. At a maximum power density of about 400 W/cm2 the relative increase in current under the action of laser irradiation was 13%. The relative increase in current is determined by the parameters of the dark current-voltage characteristic and reaches its maximum value in the region corresponding to the minimum increase in dark current with voltage. It is shown that the most likely mechanism for the increase in current is a change in the electrical resistance of the diamond microneedle as a result of absorption of laser radiation in the presence of electron levels located in the band gap of the diamond associated with impurities or structural defects in the near surface layer of the diamond microneedle.

Robustness investigation of an in-situ absorption measurement system for laser processing

The laser-material interaction Katrin Wudy for laser-material processes, such as laser welding, where several phase changes occur, still needs to be fully understood. The high energy input in the material in a short time and small space renders the laser-material interaction a complex and highly dynamic mechanism. This interaction and the resulting radiation absorption govern the process behavior during manufacturing and, in turn, the quality of parts. In situ monitoring of the laser radiation absorption can be facilitated to evaluate the process. The absorption can be determined using an integrating sphere. To employ such a measuring system reliably without intense calibration effort for high throughput experiments, the robustness of the measuring signal toward changes to the system is crucial. This work evaluates said robustness by a series of experiments for which the signal of the integrating sphere diode and the back-reflection sensor of the laser are considered. The measuring system was altered by varying the electronic gain and the distance between the sphere and the specimen. Multiple experiments with varying laser powers were conducted for every configuration of the measuring system. Additionally, the stability of the reflective properties of the coated inner sphere was evaluated by analyzing the change in the measurements over time. The experimental results show the robustness of the integrating sphere signal trend toward the changes in the measurement parameters and degradation of the inner sphere coating. These are properties of a robust measuring system, which has excellent use in high throughput experiments for fundamental research in laser-material interaction.

In English

When metal nanoparticles (MNPs) are illuminated with a monochromatic laser wave, the frequency of which is far from the plasmon frequency (the frequency of plasmon resonances), under certain conditions (depending on the frequency of the wave, its polarization, the size and shape of the MNPs), absorption of light by MNPs can be dominated by magnetic absorption (absorption caused by the magnetic component of the electromagnetic field of the light (laser) wave). This work is focused on studying the features of absorption caused by the influence of the magnetic component of laser radiation. This issue is rather poorly studied for MNPs of non-spherical shape. Therefore, how the shape of the particle manifests itself in its absorption of laser radiation (laser pulses) is one of the goals of our research. In this work, we will study the features of magnetic absorption of light (laser radiation) depending on the shape of the particles. In this paper, we will investigate the influence of spheroidal MNPs on this process. Calculations will be carried out using the kinetic equation method, because we will consider the case when the size of the MNP is smaller than the length of free path of the electron in the MNP. Note that the kinetic approach makes it possible to obtain correct results for the case when the size of the particle is greater than the length of the free path. For non-spherical MNPs, we have developed a theory that makes it possible to calculate the energy of magnetic absorption by a particle when it is irradiated with laser pulses. The dependence of magnetic absorption on the ratio of the radii of curvature of spheroidal MNPs and the vector of the magnetic field of an electromagnetic (laser) wave was constructed and theoretically investigated. An interesting result is the absorption of energy by a spheroidal MNP as its disco similarity increases. We now use to estimate the relative contributions of electric We and magnetic Wm absorption to the total absorption. For example, let us take a gold MNP’s, then ωp ≈ 5·1015 s–1, ν ≈ 1013 s–1, R = 3·10–6 sm, ω ≈ 2·1014 s–1 (carbon dioxide laser), ε' ≈ –600, ε'' ≈ 30 we received the next ratio We/Wm ≈ 2. We can see that for the given set of parameters magnetic absorption is twice as large as electric. Obviously, for different parameters of the particle and a different frequency range electric absorption can be either larger or smaller than magnetic absorption. Hence, when studying the dependence of optical absorption by MNP’s on particle form, we must allow for both electric and magnetic absorption. For an asymmetric MNP’s (for example ellipsoidal particles), apart from everything else, the ratio of the electric and magnetic contributions to absorption (as fixed frequency) is strongly dependent on the degree of particle asymmetric and wave polarization.

Nondestructive testing of local microcracking in laboratory mineral samples using an acoustic technique with a laser source of ultrasound and its verification with x-ray computer tomorgaphy

An acoustic technique of nondestructive testing of a local microcracking degree in laboratory mineral samples is proposed based on laser generation of ultrasound. A spectral power of broadband signals of longitudinal ultrasonic waves is measured, which are generated by absorption of pulsed laser radiation in a special material served as a laser source of ultrasound and further scattered on microcracks in samples (the so called structural noise power). The direct relationship between an increase in microcracking and a growth in the structural noise power in scanning regions of the samples has been obtained for feldspars of two types. For the first time, an independent method of X-ray computer tomography of the complex-shaped samples confirmed the reliability of the results of acoustic measurements. The established relationship between the local microcracking and structural noise power can be used in the monitoring systems for observing the crack formation in rocks and minerals under different external loadings.

Multifunctional Core/Shell Diamond Nanoparticles Combining Unique Thermal and Light Properties for Future Biological Applications.

We report the development of multifunctional core/shell chemical vapor deposition diamond nanoparticles for the local photoinduced hyperthermia, thermometry, and fluorescent imaging. The diamond core heavily doped with boron is heated due to absorbed laser radiation and in turn heats the shell of a thin transparent diamond layer with embedded negatively charged SiV color centers emitting intense and narrowband zero-phonon lines with a temperature-dependent wavelength near 738 nm. The heating of the core/shell diamond nanoparticle is indicated by the temperature-induced spectral shift in the intensive zero-phonon line of the SiV color centers embedded in the diamond shell. The temperature of the core/shell diamond particles can be precisely manipulated by the power of the incident light. At laser power safe for biological systems, the photoinduced temperature of the core/shell diamond nanoparticles is high enough to be used for hyperthermia therapy and local nanothermometry, while the high zero-phonon line intensity of the SiV color centers allows for the fluorescent imaging of treated areas.

Competing van der Waals and dipole-dipole interactions in optical nanocells at thicknesses below 100 nm

To reliably register the absorption or selective reflection of laser radiation in a nanocell filled with a vapor of Cesium or Rubidium at thicknesses below 100 nm, it becomes necessary to increase the atomic density by heating the cell. It is shown that both atom-wall van der Waals (vdW) interactions and Dipole-Dipole (DD) Lorentz-Lorenz interactions cause a similar “red” shift in the frequency of resonances of about the same order, which largely increases the total frequency shift. Previously, unaccounted-for DD interaction led to an overestimation of the value of the C3 coefficient of the vdW interaction. We show for the first time that varying the nanocell thickness between 130 nm and 30 - 50 nm leads to a decrease of the C3 computed from the vdW “red” shift, i.e. we observe experimentally the so-called “retardation” of the vdW effect recently predicted in theoretical papers. The measurements were made using our unique nanometric-thin cells.

From Localized Laser Energy Absorption to Absorption Delocalization at Volumetric Glass Modification with Gaussian and Doughnut-Shaped Pulses

Volumetric modification of transparent materials by femtosecond laser pulses is successfully used in a wide range of practical applications. The level of modification is determined by the locally absorbed energy density, which depends on numerous factors. In this work, it is shown experimentally and theoretically that, in a certain range of laser pulse energies, the peak of absorption of laser radiation for doughnut-shaped (DS) pulses is several times higher than for Gaussian ones. This fact makes the DS pulses very attractive for material modification and direct laser writing applications. Details of the interactions of laser pulses of Gaussian and doughnut shapes with fused silica obtained by numerical simulations are presented for different pulse energies and compared with the experimentally obtained data. The effect of absorbed energy delocalization with increasing laser pulse energy is demonstrated for both beam shapes, while at relatively low pulse energies, the DS beam geometry provides stronger local absorption compared to the Gaussian geometry. The implications of a DS pulse action for post-irradiation material evolution are discussed based on thermoelastoplastic modeling.

Quartz-Enhanced Photoacoustic Spectroscopy in the Terahertz Spectral Range

Infrared laser photo-acoustic spectroscopy provides very high sensitivity of a gas sample analysis when high-power tunable laser radiation sources and resonant photo-acoustic detectors (PADs) are used. In the resonant PAD, the acoustic signal generated by absorbed laser radiation in a measurement chamber is amplified proportionally to a Q-factor of the acoustic resonator. But, compact tunable high-power lasers (with power above 100 mW) still are not widely spread in the terahertz spectral range. One of the ways to achieve an acceptable sensitivity of terahertz photo-acoustic spectroscopy is using PADs with a very high Q-factor. The latter can be achieved using PAD with a quartz tuning fork. The current state in this field is presented in the review.

On Possible Mechanism of Enhancement of Absorption of Powerful Laser Radiation by Metals

In this work, it is shown that the dependence of the mass of conductive band electrons in a metal on their energy can be a reason of enhancement of the absorption of powerful laser radiation by the metal. To do this, a problem of response (current) of the electron placed in one-dimension periodic potential (lattice) to an electric field periodic in time (electromagnetic wave) is solved. The solution shows that for sufficiently large amplitude of the wave the dependence of the electron current on the wave amplitude becomes non-linear. Within a certain rangе of parameters, this dependence can be described by a simple formula that corresponds to the dependence of the electron mass on its energy. The formula was used for solving the problem of penetration of electromagnetic wave into a metal with the approach of modified Drude model. The non-linearity results in the enhancement of the wave absorption and generation of wave with frequencies close to those of plasma penetrating deep into the metal. The discussed effects manifest themselves in electric fields about. 1 V/Angstrom. The obtained results can be used in the interpretation of experiments data and in the creation of mathematical modeling of the interaction of powerful laser radiation with metal.

Plasma formation in holmium:YAG laser lithotripsy.

Duringholmium:yttrium-aluminum-garnet(holmium:YAG) laser lithotripsy to break urinary stones, urologists frequently see flashes of light. As infrared laser pulses are invisible, what is the source of light? Here we studied the origin, characteristics, and some effects of flashes of light in laser lithotripsy. Ultrahigh-speed video-microscopy was used to record single laser pulses at 0.2-1.0 J energy lasered with 242 µm glass-core-diameter fibers in contact with whole surgically retrieved urinary stones and hydroxyapatite (HA)-coated glass slides in air and water. Acoustic transients were measured with a hydrophone. Visible-light and infrared photodetectors resolved temporal profiles of visible-light emission and infrared-laser pulses. Temporal profiles of laser pulses showed intensity spikes of various duration and amplitude. The pulses were seen to produce dim light and bright sparks with submicrosecond risetime. The spark produced by the intensity spike at the beginning of laser pulse generated a shock wave in the surrounding liquid. The subsequent sparks were in a vapor bubble and generated no shock waves. Sparks enhanced absorption of laser radiation, indicative of plasma formation and optical breakdown. The occurrence and number of sparks varied even with the same urinary stone. Sparks were consistently observed at laser energy >0.5 J with HA-coated glass slides. The slides broke or cracked by cavitation with sparks in 63 ± 15% of pulses (1.0 J, N = 60). No glass-slide breakage occurred without sparks (1.0 J, N = 500). Unappreciated in previous studies, plasma formation with free-running long-pulse holmium:YAG lasers can be an additional physical mechanism of action in laser procedures.

A perspective on Faraday isolators for advanced lasers

Advances in the development of lasers with high output power raise the problem of increasing the operating power of Faraday isolators, since these devices can be indispensable in a number of applications. Due to the relatively high absorption of laser radiation in magneto-active media that are the basis of isolators, their use in laser systems is limited. In this Perspective, we overview the approaches that can be used to develop Faraday isolators with a high level of operating power and summarize the related achievements made in recent years, which have become possible due to the production of a wide range of new magneto-active media and the study of their properties. The issues and challenges on the path of further progress, as well as promising directions for it, are discussed.

Simulation of the depth of the melted layer on the surface of a semiconductor using JAVA cross-platform application

The paper describes a method for obtaining p-n transitions due to the laser recrystallization of the surface of CdTe semiconductor samples, as well as a software application developed in Java that allows the simulation of thermal processes at the boundary of the epitaxial layer-substrate with laser irradiation of the semiconductor surface. It allows to make predictions regarding the thickness of the melted layer, which will affect the parameters of the devices made on the basis of the obtained barrier layers. The theoretical modeling of the processes taking place at absorption of laser radiation by the surface layer of a semiconductor is carried out.

Oscillations of convective flow around a continuous optical discharge in high-pressure xenon

Laser-sustained plasma or continuous optical discharge (COD), predicted and first obtained in pioneering works initiated and inspired by Professor Raizer et al (1970 JETP Lett. 11 120–3; 1970 JETP Lett. 11 302–4; 1972 Sov. Phys.—JETP 34 763–9), is still a unique method of sustaining localized dense plasma in the laboratory and some special devices such as high-brightness broadband radiation sources [6–10]. Although the parameters determining the COD state can be stabilized, in practice, COD tends to regular oscillations. As stated in the paper, the oscillations may be attributed to the regular pulsing of thermal gravitational convective plumes around the COD. Application problems arising from instability of this type is pulsing of the emission from promising radiation sources utilizing COD plasma. This paper presents a detailed analysis of the experimental data on COD convective plume radii and oscillation frequencies depending on the plasma-forming gas pressure. The analysis leads to a similarity relation for the oscillation frequency. The similarity relation turns out to be common for optical discharges and laminar flames under conditions of prevailing buoyancy forces. This indicates the hydrodynamic nature of the instability considered, regardless of whether the energy input method is in the form of absorbed laser radiation or a chemical reaction. To support the idea of a hydrodynamic origin of the pulsations, a numerical simulation of a convective plume from a concentrated heat source was carried out. The thermal dissipation power of the heat source was equal to that of the COD. The results of numerical simulations demonstrate good agreement with those of the experiments. The dynamic temperature, density and gas velocity distributions obtained show that the direct cause of the oscillations is the dynamics of the toroidal vortices developed in the convection plume. The pulsing frequency equal to the frequency of the vortex formation may be increased under incoming additional forced gas flow that may also suppress the convective oscillations. The results obtained may be useful for studying optical discharges, and improving the parameters of high-brightness broadband laser-plasma radiation sources based on them.

Saturable Absorption of Free-Electron Laser Radiation by Graphite near the Carbon K-Edge.

The interaction of intense light with matter gives rise to competing nonlinear responses that can dynamically change material properties. Prominent examples are saturable absorption (SA) and two-photon absorption (TPA), which dynamically increase and decrease the transmission of a sample depending on pulse intensity, respectively. The availability of intense soft X-ray pulses from free-electron lasers (FELs) has led to observations of SA and TPA in separate experiments, leaving open questions about the possible interplay between and relative strength of the two phenomena. Here, we systematically study both phenomena in one experiment by exposing graphite films to soft X-ray FEL pulses of varying intensity. By applying real-time electronic structure calculations, we find that for lower intensities the nonlinear contribution to the absorption is dominated by SA attributed to ground-state depletion; our model suggests that TPA becomes more dominant for larger intensities (>1014 W/cm2). Our results demonstrate an approach of general utility for interpreting FEL spectroscopies.

Distribution of laser radiation in opaque porous media

The work is aimed at studying the interaction of laser radiation and powder material, in particular, the nature of propagation and absorption of laser radiation in a porous medium. Measurements of the nature of propagation and interaction of laser radiation with a powder material were carried out using a low-power laser in the “before reflow” mode. The paper presents the results of measuring the intensity of the passage of a laser beam through layers of powder material made of an aluminum alloy AlSi10Mg with an average fraction of 30 μm of various thicknesses. The results of measuring the heating of the powder material under the action of laser radiation and the dependence of the depth of penetration of the laser beam into the powder material are presented.

Thermal lens effect induced in a Cr,Tm,Ho:YAG laser rod: A comparison between under lasing and non-lasing conditions

In this paper, transient thermal lensing effects in a typical flashlamp-pumped CTH:YAG laser crystal under lasing and non-lasing conditions are studied and compared in a low repetition-rate regime. It is shown that in the lasing regime (for pumping energies higher than 90J) the time-resolved behavior and the strength of the three mode thermal lenses of the rod are different from the non-lasing condition. According to this research, it can be referred that higher-order processes such as excited state absorption of both pump and laser radiation and energy transfer up-conversion may have a more significant contribution to the laser dynamics in the lasing regime. The analysis of the results also shows that the energy transfer between different manifolds in Cr3+, Tm3+, and Ho3+ ions may depend on coolant temperature.

Active Brownian motion of strongly coupled charged grains driven by laser radiation in plasma

The systems of active Brownian grains can be considered as open systems, in which there is an exchange of energy and matter with the environment. The collective phenomena of active Brownian grains can demonstrate analogies with ordinary phase transitions. We study the active Brownian motion of light-absorbing and strongly interacting grains far from equilibrium suspended in gas discharge under laser irradiation when the nature and intensity of the active motion depend on the effect of radiation. Active Brownian motion is caused by photophoresis, i.e., absorption of laser radiation at the metal-coated surface of the grain creates radiometric force, which in turn drives the grains. We experimentally observed the active Brownian motion of charged grains in the transition of the grain monolayer from the solid to liquid state. An analysis of the character of motion, including the mean-square and linear displacement and persistence length at various values of the randomization (coupling parameter) of the grain structure, was presented.

Solid particle deposition of indoor material combustion products

Heavy smoke emission during fires hampers evacuation of people and movement of rescue teams to a fire source. It is often pyrolysis and combustion products that cause deaths from fires in closed spaces. Fire sprinkler system activation does not always lead to efficient smoke deposition. Sometimes, a fire suppression liquid supply, on the contrary, deteriorates fire evacuation conditions. The study of smoke generation and deposition characteristics is an essential prerequisite for improving rescue technologies during fires. The purpose of this research was to define the characteristics of deposition of solid combustion product particles of a group of typical construction and finishing materials in closed spaces: wood, linoleum, rubber, etc. Solid particles (pyrolysis products) have been distributed by size and concentration. The smoke generation and extinction coefficients, as well as deposition rates of smoke aerosol particles with and without using water aerosol have been determined. The ranges of optical properties of smoke aerosol have been specified. The time of dispersed water impact on smoke aerosol was varied to evaluate its effect on the mass rate of deposition of solid combustion product particles. The characteristics of absorption of laser radiation with different wavelengths in flue gases have been defined to identify the most advantageous colors of reflector materials used to manufacture direction signs for the evacuation of people from the combustion zone.

Nonlinear phenomena in femtosecond laser systems based on Yb: YAG thin-rod amplifiers

Nonlinear effects in a high-power laser amplification system based on thin-rod active elements are experimentally investigated. It is shown that the most important nonlinear effect is the Kerr effect leading to self-focusing and self-phase modulation of laser radiation. The observed luminescence in Yb : YAG crystals in the visible region of the spectrum is caused by the charge transfer luminescence effect, which, in turn, is due to multiphoton absorption of laser radiation. Second-order nonlinearity in the crystal is used to effectively convert femtosecond pulses to second harmonic and sum-frequency waves which exhibit radiations of green and UV spectra.

Simulation of Heat Transfer in Single-Crystal Lithium Niobate in Interaction with Continuous-Wave Laser Radiation

The paper presents the simulation results of heat transfer in single-crystal lithium niobate (LiNbO3) in the form of cylinder of diameter mm and height mm in interaction with continuous-wave laser radiation with the output power of W and the wavelength of nm. The density of the LiNbO3 crystal is kg/m3; the thermal conductivity along the [001] direction is W/(m×K); the thermal conductivity in the (001) plane is W/(m×K); the specific heat at constant pressure is J/(kg×K); the absorption coefficient is %/cm @ 1064 nm. The laser beam propagates along the optical axis of the crystal. The laser beam intensity profile is represented as a Gaussian function, and the absorption of laser radiation of the single-crystal lithium niobate is described by Beer-Lambert’s law. The numerical solution of the non-stationary heat conduction problem is obtained by meshless scheme using anisotropic radial basis functions. The time interval of the non-stationary boundary-value problem is 2 h 30 min. The results of numerical calculations of the temperature distribution inside and on the surface of the single-crystal lithium niobate at times s are presented. The time required to achieve the steady-state heating mode of the LiNbO3 crystal, as well as its temperature range over the entire time interval, have been determined. The accuracy of the approximate solution of the boundary-value problem at the n-th iteration is estimated by the value of the norm of relative residual . The results of the numerical solution of the non-stationary heat conduction problem obtained by meshless method show its high efficiency even at a small number of interpolation nodes.