Reflection Of Laser Radiation Research Articles

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.

Reflection of Laser Radiation with a Gaussian Intensity Distribution from a Rough Surface

Reflection of Laser Radiation with a Gaussian Intensity Distribution from a Rough Surface

Новый метод измерения показателя преломления текущей жидкости

The necessity of developing a new method for measuring the refractive index of a liquid in a turbulent mode of its flow in a pipeline is substantiated. A new method for measuring the refractive index of a medium n is presented. For its practical implementation, a new design of a flow-through refractometer has been developed, which makes it possible to measure n in both laminar and turbulent fluid flow. A new design of the optical part of the refractometer is considered. A new way of placing it on the vertical section of the pipeline for measuring n is proposed. All this makes it possible to make insignificant the influence on the measurement result of n flowing liquid of errors associated with multiple reflections of laser radiation from optical elements, with the formation of voids or vortex flows in the pipeline and with temperature fluctuations. In the design of a refractometer with a new method for measuring n, there are no restrictions on determining the value of n of a flowing liquid, in contrast to the used industrial flow-through refractometers, the principle of which is based on the phenomenon of total internal reflection of laser radiation at the interface of two media. The results of experimental studies of various media are presented.

Estimation of the influence of the Nonequilibrium State of the Electronic Subsystem and the Dispersion Properties of a Solid-State Plasma Medium in the conditions of Decay Instability on the reflection of Laser Radiation

Estimation of the influence of the Nonequilibrium State of the Electronic Subsystem and the Dispersion Properties of a Solid-State Plasma Medium in the conditions of Decay Instability on the reflection of Laser Radiation

Reflection of laser radiation from an optically anisotropic crystal with preservation of polarization extinction

Reflection of laser radiation from an optically anisotropic crystal with preservation of polarization extinction

Model of the polarization extinction ratio change due to multiple reflection of laser radiation from the faces of the terbium-gallium garnet crystal in Faraday rotator

In this work we investigated the phenomena in the Faraday rotator, that can affect the polarization state of transmitting laser radiation. We considered two factors that can cause deterioration of radiation extinction ratio and, therefore, isolation properties of Faraday cell: multiple reflections of radiation from the faces of the TGG crystal and imperfection of the laser beam input relative to the central axis of the crystal. We were able to calculate the impact of these factors on polarization extinction ratio of laser radiation transmitting through the crystal.

Magnetically Induced Atomic Transitions of the Potassium D2 Line

Magnetically induced (MI) transitions of the D2 line of 39K atom in an external magnetic field of 10–600 G have been experimentally and theoretically investigated for the first time using circularly polarized σ+ and σ− radiations. According to the selection rules, the transitions between the sublevels of the ground and excited levels of a hyperfine structure with Fe – Fg = ΔF = ±2 are forbidden in the zero magnetic field, whereas their probabilities increase radically in a magnetic field. For the Fg = 1 → Fe = 3 (ΔF = +2) and Fg = 2 → Fe = 0 MI transitions, the highest probabilities are achieved using the σ+ and σ– radiations, respectively. The atomic transitions have been spectrally resolved using selective reflection of laser radiation from a nanocell filled with potassium atomic vapor, which makes it possible to investigate the behavior of the MI transitions. The experimental and theoretical data are shown to be in good agreement.

Biospeckle local contrast analysis for surface roughness study of articular cartilage

Diffuse reflection of laser radiation by a biological tissue always gives rise to the formation of a biospeckle pattern. The biospeckle pattern depends on the surface irregularities and carries significant information about the surface roughness. Biospeckle imaging has been extensively applied to medicine, biology and agriculture for evaluating different types of biological tissues, since it is safe, simple and non-destructive optical method. In consequence of the texture properties of biospeckle pattern, various texture analysis methods have been presented in literature for extracting the information involved in biospeckle patterns. In this work, first, an experimental setup to simultaneously capture multiple biospeckle patterns from many cross-section areas of bovine articular cartilage tissue surface is investigated. Next, a new model based on local contrast measurement is proposed on the captured biospeckle patterns for final and proper estimation of the average roughness of the various cross-section areas of articular cartilage surface. The proposed model computes the local contrast for pairs of pixels at different neighborhood separation distances in order to obtain three contrast features. The obtained results showed that the contrast features can efficiently differentiate between the average roughness of the different cross-section areas of articular cartilage surface with 0.02 μm accuracy.

Магнитоиндуцированныe атомные переходы D-=SUB=-2-=/SUB=--линии калия

For the first time, magnetically induced (MI) transitions of the D2 line of the 39K atom in an external magnetic field of 10–600G using circularly polarized radiation σ + and σ – have been investigated. According to the selection rules in a zero magnetic field, transitions between sublevels of the ground and excited levels of the hyperfine structure with Fe - Fg = delta F = ± 2 are prohibited, while in the magnetic field there is a giant increase in their probabilities. For MI transitions, Fg = 1-> Fe = 3(delta F = +2) the highest probability is achieved when using σ+ radiation, and for Fg = 2->Fe = 0 the highest probability is achieved when using σ-  radiation. For the atomic transitions spectral separation the process of selective reflection of laser radiation from nanocell, filled with potassium atoms is used. This which allowed us to study the behavior of the MI transitions. The experiment well consistent with the theory.

Selective Reflection of Laser Radiation from Ultrathin Layers of Cesium Atomic Vapors Confined in a Nanocell

The effect of the van der Waals interaction of cesium atoms with the sapphire windows of a nanocell was experimentally investigated using the selective reflection process. The distance L between the windows varied in the range of 50–2000 nm and the nanocell was filled with the vapors of cesium atoms. For the Cs atoms (the transition 6S1/2 → 6P1/2), the C3 coefficient of the van der Waals interaction with the sapphire windows of the nanocell is measured. It is shown that it is possible to determine the magnetic fields with the spatial resolution of 70 nm using the selective reflection spectrum and, consequently, both the homogeneous and highly gradient magnetic fields can be measured.

Conservative finite-difference scheme for the 2D problem of femtosecond laser pulse interaction with kink structure of high absorption in semiconductor

ABSTRACTThe problem of high-intense laser pulse interaction with a semiconductor under the condition of a light energy nonlinear absorption, which results in high absorption domains formation, is considered. Such interaction allows reaching a construction of an element for all-optical data treatment. For its adequate description we propose new mathematical model taking into account the longitudinal and transverse diffraction effects. The longitudinal diffraction induced a reflection of laser radiation from boundaries of the high absorption domains that results in changing of their spatial structure. The conservative finite-difference scheme (FDS) is developed for numerical computation of the complicated nonlinear processes. The property of the FDS conservatism is proved. For the proposed FDS realization the two-stage iteration method is proposed. Computer simulation results are presented. We show the uniform boundedness of the mesh functions at the iterations and the convergence of the iteration process. We show also positiveness and boundedness of the mesh function corresponding to free-charged particles. We discuss some properties of differential problem including the problem invariants, positiveness of the free-charged concentrations.

Selective Reflection of Potassium Vapor Nanolayers in a Magnetic Field

The selective reflection of laser radiation from the interface between a dielectric window and the atomic vapors confined in a nanocell of thickness L ≈ 350 nm is used to develop effective Doppler-broadening- free spectroscopy of potassium atoms. A small atomic line width and a relation between the signal intensity and the transition probability allowed us to resolve four lines of atomic transitions responsible for the D1 lines of the 39K and 41K isotopes. Two groups containing four atomic transitions form in an applied magnetic field upon pumping by radiation with circular polarization σ+ or σ–. Different intensities (probabilities) of transitions for the σ+ and σ– excitations are detected in magnetic field B0 ≈ A hfs /μB ≈ 165 G (A hfs is the magnetic dipole constant for the ground state and μB is the Bohr magneton). A substantially different situation is observed at B ≫ B0, since high symmetry appears for the two groups formed by radiation with circular polarization σ+ or σ–. Each group is the mirror image of the other group with respect to the frequency of the 42S1/2–42P1/2 transition, which additionally proves the occurrence of the complete Paschen–Back regime of the hyperfine structure at B ≈ 2.5 kG. A developed theoretical model well reproduces the experimental results. Possible practical applications are described. The results obtained can also be applied to the D1 lines of 87Rb and 23Na.

ВЛИЯНИЕ ЭЛЕКТРОСТАТИЧЕСКОГО ПОЛЯ НА ПРОЦЕСС ЛАЗЕРНОЙ ТЕРМООБРАБОТКИ МЕТАЛЛОВ

Modern methods of laser processing of the materials are actively introduced into production. However, their wide application in engineering is hampered by the high energy intensity of the processes and the unexplored complex high-speed processes of such processing technologies. This paper covers the hybrid laser technologies for processing materials, in particular, laser-field hardening of metals. The theoretical study of laser interaction with metal is carried out; it is shown that the laser radiation reflection index and the depth of its penetration depend on the electrical conductivity of a skin layer. The study determined the main interrelations between the quality parameters of a treated layer and the parameters of the laser-field technological complex. The paper gives the results of the study on hybrid laser-field hardening of steels widely used in engineering (Steel 10, Steel 45, and Steel 65G) in the electrostatic field. It is shown, that the electrostatic field superposition on a treatment zone leads to the increase in the depth and hardness of a hardened layer through the directed motion of electrons deep into metals. The authors offer a mathematical model for the temperature field distribution in metal under the influence of laser radiation that considers the electrostatic field superposition and allows investigating the dynamics of the hybrid laser hardening. The limitation of the increase in the rate of material cooling by the electrons directional motion in the electrostatic field is mathematically substantiated. The specific values of the electrostatic field influence coefficient are calculated. It is shown, that when superposing the external field, the threshold, critical value of the density of the laser radiation power causing the melting of the treated surface increases. The authors offer the specific mathematical models to be used when preparing a production for determining the required laser radiation power and the strength of the electrostatic field. Based on the results of the study, the principal process scheme is offered, and the laser-field technological complex facility is designed and manufactured.

Selective reflection of laser radiation from submicron layers of Rb and Cs atomic vapors: Applications in atomic spectroscopy

We studied selective reflection (SR) of laser radiation from a window of a nanocell with thickness L ~ λ1,2/2 filled with Rb and Cs atoms, where λ1 = 780 nm and λ2 = 852 nm are the wavelengths resonant with the D2 laser lines for Rb and Cs, respectively. It is demonstrated that the negative derivative of the SR signal profile for L > λ/2 changes to the positive one for L < λ/2. It is shown that the real-time formation of the SR signal profile derivative (SRD) with the spectral width 30–40 MHz and located at the atomic transition is, in particular, a convenient frequency marker of D2 transitions in Rb and Cs. The amplitudes of SRD signals are proportional to the atomic transition probabilities. A comparison with the known saturated absorption (SA) method demonstrated a number of advantages, such as the absence of cross-over resonances in the SRD spectrum, the simplicity of realization, a low required power, etc. An SRD frequency marker also operates in the presence of the Ne buffer gas at a pressure of 6 Torr, which allowed us to determine the Ne–Rb collisional broadening, whereas the SA method is already inapplicable at buffer gas pressures above 0.1 Torr. The realization simplicity makes the SRD method a convenient tool for atomic spectroscopy. Our theoretical model well describes the SRD signal.

Decoupling of hyperfine structure of Cs D_1 line in strong magnetic field studied by selective reflection from a nanocell

Decoupling of total electronic and nuclear spin moments of Cs atoms in external magnetic field for the case of atomic $D_1$ line, leading to onset of the hyperfine Paschen-Back regime has been studied theoretically and experimentally. Selective reflection of laser radiation from an interface of dielectric window and atomic vapor confined in a nanocell with 300 nm gap thickness was implemented for the experimental studies. The real time derivative of selective reflection signal with a frequency position coinciding with atomic transitions was used in measurements, providing $\sim$ 40 MHz spectral resolution and linearity of signal response in respect to transition probability. Behavior of 28 individual Zeeman transitions in a wide range of longitudinal magnetic field (0 - 6 kG) has been tracked under excitation of Cs vapor by a low-intensity $\sigma^+$- polarized cw laser radiation. For $B\ge 6~$kG, only 8 transitions with nearly equal probabilities and the same frequency slope remained in the spectrum, which is a manifestation of the hyperfine Paschen-Back regime. The obtained experimental results are consistent with numerical modeling. Due to small divergence of selective reflection signal, as well as sub-wavelength thickness and sub-Doppler spectral linewidth inherent to nanocell, the employed technique can be used for distant remote sensing of magnetic field with high spatial and $B$-field resolution.

Mesh-ray model and method for calculating the laser radiation absorption

The present paper puts forward a mathematical model of laser radiation absorption in a laser target, which combines approximations of geometrical and wave optics and the corresponding numerical algorithm. This model depends on the principles of geometrical optics in the range of weak variation of the plasma refractive index on the scale of the wave length. This enables one to describe the refraction of the radiation. A transition to wave approximation is carried out near the surface of critical density, where the approximation of the geometrical optics is a fortiori inapplicable. For this aim, the plasma medium is approximately represented as a set of plane layers, on which the one-dimensional Helmholtz equation is solved. This makes it possible to construct a simple and relatively accurate method for calculating the absorption and reflection of laser radiation near the critical density surface in order to effectively take into account the dependence of the interaction of radiation with matter on the polarization direction, etc. The proposed model is adapted for implementation in the radiation gas dynamics (RGD) code. A numerical computation subroutine is presented based on the analytical solution of the differential equations corresponding to the optical ray model of the laser radiation energy flux. This solution is obtained under the assumption that the squared gradient of the refractive index is constant in any cell of the mesh. The convergence rates of the proposed algorithms are estimated using the data obtained in the numerical experiments.

Experimental Analysis of Instrumental Uncertainty in the Measurement Channel of an Optoelectronic System for Monitoring Surfaces of Complex Shape

We consider the results of studies of the instrumental uncertainty in the measurement channel of an optoelectronic triangulation system. We analyze the selection and effect of the design characteristics of the system, the angles of incidence and reflection of laser radiation on the measurement results for this uncertainty in monitoring surfaces of complex shape.

Dynamics of ultrathin laser targets with optimal parameters

The set of equations describing the motion of a thin (compared to the wavelength) target in the field of a laser pulse that takes into consideration separate motion of the electron and ion layers is derived. In the case of strong Coulomb coupling between the layers, the set of equation of motions of the layers is reduced to the well-known light-sail equation containing a self-consistent coefficient of nonlinear reflection of laser radiation by a moving target. The optimal thickness of the laser target at which the target acquires maximum energy for given laser-pulse parameters is determined. It is shown that this thickness depends not only on laser intensity, but also on laser-pulse duration and the ratio of electron and ion masses. The growth rates of transverse instability of optimal targets under their intense acceleration are analyzed. It is demonstrated that instability does not develop in the currently experimentally accessible range of laser intensities and pulse durations between 100 and 200 fs.

Reflection and refraction of laser radiation at interface between two uniaxial crystals

Analogues of Fresnel formulas for interface between two uniaxial nonmagnetic transparent crystals with arbitrary orientation of optical axes are obtained. For isotropic media the formulas are reduced to conventional Fresnel formulas, thus confirming the validity of mathematical calculations. Computer simulations of laser radiation reflection and refraction at the interfaces between a layer of nerve fibers and a stack of photoreceptor disks as well as between bleached and dark-adapted photo-receptor disks have been carried out. The results do not contradict experimental data.

Laser radiation scattering from the wires and fibers of imploding arrays on the Angara-5-1 facility

A method is developed for measurements of laser radiation scattering by wires and fibers in different types of imploding arrays in the initial stage of plasma production at discharge currents per wire of up to 2 kA for aluminum arrays and up to 8 kA for tungsten arrays. The experiments were carried out on the Angara-5-1 facility at a current density in the wires of 108 A/cm2 and current growth rate of ∼1013 A/s. It is found that the indicatrix of laser radiation reflected from the wires (fibers) in cylindrical and conical arrays is modified at currents of 0.1–10 kA per wire (fiber). The experimental data on the reflection and scattering of laser radiation from wires and fibers are compared with the results of numerical simulations of their electric explosion in vacuum. It is proposed that the change in the reflection indicatrix of laser radiation is caused by the onset of thermal instabilities. The typical size of density and temperature inhomogeneities on the wire surface is in a range of 10–20 μm, which probably results in a transition from specular to diffuse reflection of laser radiation. A simultaneous abrupt (over 2–3 ns) reduction in the reflection intensity from several wires of an array indicates a homogeneous distribution of the discharge current over the irradiated wires. This closes the issue of the quality of the contact between the wires and the electrodes. The obtained experimental information is of considerable importance for the development of numerical codes for simulations of the implosion of wire arrays and the refinement of the wire parameters in the initial stage of plasma production.