Laser Radiation Research Articles

On the status of channeling radiation and laser based radiation sources

A unified description of channeling radiation (CR) in oriented crystals (OC) and radiation in the field of an intense plane wave (laser radiation sources – LRS) is proposed in terms of two Lorentz-invariant parameters. The case of planar positron channeling (quasi-channeling) and linearly polarized laser wave is considered in detail up to TeV energies. The crucial difference between LRS and CR is that in the former case both invariants are independent, while in channeling they are linearly related to each other. This leads to a strong limitation on the range of possible values of these invariants in OC. On the other hand, OC make it possible to study QED processes in strong non-uniform external fields.

Способ обработки спектра излучения лазера на основе экспериментально записанной формы пропускания интерферометра Фабри – Перо

At present, to create quantum frequency standards, the phenomenon of coherent population trapping (CPT) is widely used, in which, when a certain intensity is reached and the frequency interval between the spectral components coincides with the frequency of the hyperfine transition in an atom (most often in rubidium or cesium), a narrow resonance is observed in the absorption spectrum and the transmission of a cell with alkali metal vapor increases sharply. Since the amplitude of the CPT resonance critically depends on the intensity and position of the spectral components of the pump radiation, it is necessary to control the parameters of these components, which are usually recorded in digital form. This paper describes a fundamentally new method for processing the laser emission spectrum recorded with a scanned Fabry – Perot interferometer (FPI), based on using the experimentally recorded transmission form of the scanned FPI as a reference spectral component. The paper presents examples of processing the experimentally recorded radiation spectra of a diode laser with an external resonator under microwave modulation of its injection current. The error of processing by the proposed method turned out to be approximately an order of magnitude smaller than when processing by the well-known Airy formula describing the transmission of the FPI. This is explained by the fact that the Airy formula is valid for light beams whose wave front curvature is consistent with the curvature of the IFP mirrors, which is often not true in the case of a real experiment. The method can be used to process the spectra of modulated radiation of various types of lasers, which is an important task in detecting CPT resonances and further creating quantum frequency standards based on them.

Control of melt dynamics in laser cutting based on a resonant nozzle cavity

Instabilities of the melt flow dynamics on the laser cutting front lead to quality losses, due to the formation of striations and burr at the cut flank, especially when using cost-efficient 1 μm laser radiation. This paper summarizes fundamental studies utilizing the concept of a resonant cavity as basis for the nozzle design in a novel approach to improve the quality of cut flanks. The nozzle is designed to excite acoustic waves for promoting gas flow oscillations in the interaction zone and to ideally match the acoustic resonance of the gas column in the kerf. The influence of the cavity length on the oscillation frequency is analyzed using schlieren and acoustic techniques. Further, it is experimentally demonstrated that the average surface roughness of the resulting cut flanks is reduced up to 50 % in the upper third section, where the coupling of the gas flow oscillation to the melt is most pronounced, while the assist gas consumption is significantly decreased compared to a standard 3 mm diameter conical nozzle.

Numerical study of generation mechanisms of narrow spectral modes, emerging in a raman fiber laser with randomly distributed feedback

The present paper discusses possible influence of nonlinear effects, namely – the degenerate four-wave mixing process – upon the build-up and attenuation of narrow spectral modes, emerging in radiation of a Raman fiber laser with randomly distributed feedback.

Modification of Diamond Surface by Femtosecond Laser Pulses

The basic mechanisms of laser interaction with synthetic diamond are reviewed. The characteristics of the main regimes of diamond surface etching are considered. In addition to the well-known graphitization and ablation processes, nanoablation and accumulative graphitization, which have attracted relatively recent attention, are described in detail. The focus is on femtosecond (fs) laser exposure, which allows for the formation of a dense cold electron–hole plasma in the focal zone and minimal overheating in the surrounding area. This potentially opens the way to the development of unique laser-based technologies that combine physical and chemical processes for precise surface treatment and functionalization. The physical limitations that determine how precisely the diamond surface can be treated by short-pulsed laser radiation and possible ways to overcome them with the ultimate goal of removing ultrathin layers of the material are discussed. Special attention is paid to the novel possibility of inducing the local formation of point active defects—nitrogen vacancy (NV) complexes in the laser-irradiated zone. Such defects have been at the forefront of solid-state physics for the past thirty years due to continuous attempts to exploit their unique properties in quantum optics, quantum computing, magnetometry, probing, and other fields. Both regimes of NV center formation with and without graphitization of the diamond lattice are considered. Thus, it is shown that intense pulsed laser irradiation is a perfect tool for the processing of synthetic diamonds at the micro-, nano-, and even at the atomic level, which can be well controlled and managed.

Manifestation of the laws of vibrational motion of molecules of the condensed medium under the influence of laser radiation in the Raman spectrum

The vibrational spectra of polyatomic C4H8O2 dioxane molecules which is symmetric in the polarizability tensor have been studied by the method of Raman scattering in a low-frequency range (0–1400 cm−1). The possibilities of their use for obtaining information about the features of interactions of molecules, their are shown in vibrational and orientational motion dynamics. For the dioxan molecule, 8 spectra associated with vibrations of C-H atoms were observed and it was found that their vibration frequencies correspond to approximately 400–1300 cm−1. The values of the relaxation parameters characterizing the laws of interaction of atoms related to hydrogen bonding were found. Manifestation of these spectra was analyzed on the basis of the change in the polarizability of the molecule

The Effect of Mechanotherapy and Virtual Reality on Cardiovascular Activity and the Severity of Shortness of Breath in Patients with Post COVID-19 Syndrome: A Prospective Randomized Study

INTRODUCTION. At present, there is an urgent need to develop modern rehabilitation programs for COVID-19 patients with long-lasting residual symptoms of the disease, primarily from the cardiopulmonary system.
 AIM. To assess the impact of a comprehensive medical rehabilitation program using virtual reality technologies, robotic mechanotherapy with biofeedback and interactive balance therapy on the severity of hypoxic syndrome, shortness of breath and cardiovascular activity in patients who have undergone COVID-19 and have manifestations of post-COVID syndrome (long COVID).
 MATERIAL AND METHODS. The study sample consisted of 120 patients aged 40 to 70 years who had suffered COVID-19 within 16 months prior to inclusion in the study and had manifestations of SPCN, who were randomized into 2 groups. In the main group (n = 60), a new complex of medical rehabilitation was prescribed against the background of basic treatment: 1) sessions on the robotic biofeedback simulator for lower limb muscle training Con-Trex, #10; 2) balance therapy (sensorimotor training) using the biofeedback simulator Stabilan 012, #10; 3) training on the rehabilitation interactive markerless system with virtual reality technology Nirvana, #10. In the control group (n = 60) patients received only basic treatment: 1) a special complex of therapeutic gymnastics performed in the hall with a physical therapy instructor, № 10; 2) low-intensity laser radiation, № 10; 3) speleo-exposure, № 10; 4) medical massage on the chest area, № 10. Research methods included clinical examination, anamnesis collection and cardiopulmonary testing and assessment of dyspnea severity before the beginning of rehabilitation and immediately after its completion.
 RESULTS AND DISCUSSION. In the period from 1 to 6 months after COVID-19, 90.0 % of patients had dyspnea, as well as a decrease in peak oxygen consumption and high values of heart rate (HR), systolic and diastolic blood pressure (BP) at peak exercise. The application of a new complex of medical rehabilitation in patients with post-COVID disorder syndrome contributed to a more significant, than in the standard method of rehabilitation, increase in cardiorespiratory endurance, which is manifested in the increase in the maximum work performed by 12.9 % and oxygen consumption of the first ventilation threshold by 6.3 % and in the decrease in diastolic BP at peak load by an average of 7 mm Hg. hg according to cardiopulmonary testing, as well as an increase in the proportion of patients with mild (up to 60.0 %) or no dyspnoea (up to 26 %) and a decrease in the number of patients with moderate dyspnoea (up to 13.3 %), severe and very severe dyspnoea (up to 0 %). Most of the authors in their studies also note a high frequency of shortness of breath, along with such manifestations of post- COVID-19 syndrome as general weakness, poor exercise tolerance.
 CONCLUSION. A new comprehensive medical rehabilitation program using virtual reality technologies, robotic mechanotherapy with biofeedback and interactive balance therapy are recommended to reduce the severity of hypoxic syndrome and dyspnea and improve cardiovascular performance in patients with post-COVID syndrome.

Nonlinear intensity dependence of ratchet currents induced by terahertz laser radiation in bilayer graphene with asymmetric periodic grating gates

We report on the observation of a nonlinear intensity dependence of the terahertz radiation-induced ratchet effects in bilayer graphene with asymmetric dual-grating gate lateral lattices. These nonlinear ratchet currents are studied in structures of two designs with dual-grating gates fabricated on top of boron nitride encapsulated bilayer graphene and beneath it. The strength and sign of the photocurrent can be controllably varied by changing the bias voltages applied to individual dual-grating subgates and the back gate. The current consists of contributions insensitive to the radiation’s polarization state, defined by the orientation of the radiation electric field vector with respect to the dual-grating gate metal stripes, and the circular ratchet sensitive to the radiation helicity. We show that intense terahertz radiation results in a nonlinear intensity dependence caused by electron gas heating. At room temperature, the ratchet current saturates at high intensities of the order of hundreds to several hundreds of kW cm−2. At T=4 K, the nonlinearity manifests itself at intensities that are one or two orders of magnitude lower; moreover, the photoresponse exhibits a complex dependence on the intensity, including a saturation and even a change of sign with increasing intensity. This complexity is attributed to the interplay of the Seebeck ratchet and the dynamic carrier-density redistribution, which feature different intensity dependencies and nonlinear behavior of the sample’s conductivity induced by electron gas heating. The latter is demonstrated by studying the THz photoconductivity. Our study demonstrates that graphene-based asymmetric dual-grating gate devices can be used as terahertz detectors at room temperature over a wide dynamic range, spanning many orders of magnitude of terahertz radiation power. Therefore, their integration together with current-driven read-out electronics is attractive for the operation with high-power pulsed sources.

A novel laser melting sampler for discrete, sub-centimeter depth-resolved analyses of stable water isotopes in ice cores

Abstract We developed a novel laser melting sampler (LMS) for ice cores to measure the stable water isotope ratios (δ18O and δD) as temperature proxies at sub-centimeter depth resolutions. In this LMS system, a 2 mm diameter movable evacuation nozzle holds an optical fiber through which a laser beam irradiates the ice core. The movable nozzle intrudes into the ice core, the laser radiation meanwhile melts the ice cylindrically, and the meltwater is pumped away simultaneously through the same nozzle and transferred to a vial for analysis. To avoid isotopic fractionation of the ice through vaporization, the laser power is adjusted to ensure that the temperature of the meltwater is always kept well below its boiling point. A segment of a Dome Fuji shallow ice core (Antarctica), using the LMS, was then demonstrated to have been discretely sampled with a depth resolution as small as 3 mm: subsequent analysis of δ18O, δD, and deuterium excess (d) was consistent with results obtained by hand segmentation within measurement uncertainties. With system software to control sampling resolution, the LMS will enable us to identify temperature variations that may be detectable only at sub-centimeter resolutions in ice cores.

Multiphysics modelling of powder bed fusion for polymers

ABSTRACT Polymeric materials for powder bed fusion additive manufacturing have been attracting extensive research interest due to their vast potential for fabricating end-use functional parts. Here, a high-fidelity multiphysics approach combining the discrete element model with the computational fluid dynamics model has been developed to simulate the printing process of polymers in powder bed fusion, involving powder recoating, melting, and coalescence. The developed approach considers particle flow dynamics, the reflection, absorption, and transmission of infrared laser radiation, and the viscous flow of polymer melt. The pore formation mechanisms due to lack of fusion and gas entrapment in polyamide 12 parts printed via selective laser sintering are studied. The simulation results reveal that lower polymer viscosity would be beneficial to the densification rate of the printed parts. Excessively small powder particles would degrade powder bed quality due to the agglomeration of polymer powder, thus leading to high porosity in the printed parts.

Precise determination of adiabatic ionisation energies of large polyatomic molecules: p-diaminobenzene

We present a procedure, based on established principles, to accurately determine the ionisation energies of medium-sized to large polyatomic molecules in supersonic beams. The method combines single-photon excitation from the electronic ground state using tunable vacuum-ultraviolet laser radiation, the delayed pulsed-field ionisation of high Rydberg states with sequences of pulsed electric fields, and the modelling of the lineshapes from calculations of the field-ionisation dynamics of high Rydberg states and of the rotational contours of the photoelectron bands. The method is illustrated by a measurement of the adiabatic ionisation energy of p-diaminobenzene (p-DB), for which we obtain the value of 54,617.47(15) cm , which represents a more than 20-fold accuracy improvement over previous determinations.

Femtosecond Laser Fabrication of Silver Microstructures in Nanoporous Glasses

This paper presents the results of studying the process of laser formation of microstructures from silver nanoparticles in nanoporous quartz glasses. Glass samples were impregnated with organometallic molecules Ag(hfac)COD in a supercritical carbon dioxide environment. The formation of point and linear microstructures was carried out by high-frequency (70 MHz) femtosecond laser radiation with a wavelength of 525 nm and energy in the pulse up to 1 nJ. It was found that the formation of microstructures occurs due to photo- and thermal decomposition of precursor molecules with the formation of plasmonic silver nanoparticles. It is shown that the developed temperatures can exceed the melting point of glass, which leads to the appearance of microstructures with altered refractive index. A qualitative model explaining the individual stages of cluster formation in the glass volume under point laser impact is presented.

Assessment of the Microcirculation in Superficial and Deep Biotissue with Photobiomodulation

Background Quantitative measurement of blood flow in specific biotissue targets is crucial for photobiomodulation therapy (PBMT). PBMT has been applied on many clinical applications. However, the conclusive confirmation of its effects on microcirculation and laser wavelength dependency in superficial and deep tissues is lacking. Objective This study examines the microcirculatory response and laser wavelength dependency of red and near infrared (NIR) lasers in biotissue. Laser Doppler flowmetry (LDF) was used to assess the flux and velocity of erythrocytes in buccal tissue when exposed to 660 nm and 830 nm laser radiation. Discussion The microcirculatory response of subjects during and after laser radiation were analyzed. Each laser has an energy density of 73.17 J/cm2 . Both the 660 nm and 830 nm lasers demonstrated the capability to enhance microcirculation in both superficial and deep tissues. Furthermore, we observed that laser wavelengths at 660 nm and 830 nm respectively enhanced the microcirculatory response in superficial and deep tissues. Importantly, this cumulative effect persisted for at least 10 minutes. Conclusions The present results indicate that PBM can effectively improve local blood perfusion in specific target areas and peripheral biotissues. These findings have potential applications in enhancing wound healing and local microcirculation within the target areas.

A Molecular Dynamics Model for Studying the Influence of High Temperatures under Laser Irradiation on Changes in a BCC Crystal Structure

The paper discusses the changes in the structures of BCC crystals subjected to high-temperature exposure. The interest in the study is explained by the processes occurring in the liquid surface layer and their subsequent impact on layer crystallization. They will further affect various physical and geometric characteristics of the material surface as a whole. The presented model helps observe the imperfections of the structure caused by the appearance of pores on the surface layers of the metal. The computational cell temperature in the designed model is distributed according to the solution of the linear problem of heat conduction. The model allows for revealing a surface layer continuity violation when the excess free volume localizes in the form of a group of spherical pores. The dimensions of such imperfections, as well as the duration of their existence, differ when modeling different laser radiation energy densities. Further research reveals the conditions for the pores to remain stable throughout the entire simulation time, as well as the relationship between the crystallographic orientation of the “solid-liquid” interphase boundary and the sizes of the formed pores” interphase boundary and the sizes of the pores formed. Keywords: bcc crystal, molecular dynamics model, interface boundary, porosity.

Photovoltaic AlGaAs/GaAs devices for conversion of high-power density laser (800–860 nm) radiation

Photovoltaic converters of high-power (λ = 800–860 nm, ELR = 150–550 W/cm2) laser radiation (PhotoVoltaic Laser Power Converters – PVLPCs) based on AlGaAs/GaAs heterostructures grown by metalorganic vapor-phase epitaxy have been developed. To increase the output voltage, the space charge region of p-GaAs/n-AlхGa1-хAs heterojunction was shifted to the n-AlхGa1-хAs wide-gap layer with a gradual “x”. The technology for embedding a rear reflector based on TiOx/SiO2/Ag into the photovoltaic converter structure by “transferring” the heterostructure to a supporting substrate and the method of bonding to form a monolithic structure of the PVLPC has been elaborated. To operate the heterostructures at an increased power density of laser radiation, they have been designed to eliminate possible potential barriers at the heterointerfaces and the frontal contact topology with a high density of the metal grid (50 μm and 125 μm contact pitch are under consideration) has been employed. PVLPCs with efficiency of 62% were obtained at the power density of monochromatic radiation (λ = 850 nm) ELR = 170 W/cm2. The finger pitch of 50 μm allows keeping efficiency more than 56% even with a fivefold increase in laser radiation (up to ELR = 500 W/cm2).

VUV lasing in diffuse discharges formed by runaway electrons

The parameters of stimulated emission in diffuse discharges formed in a sharply inhomogeneous electric field by runaway electrons in mixtures of rare gases with the addition of H2 and F2 at pressures up to 10 atm are studied. Efficient VUV lasing was obtained at wavelengths from 148 to 193 nm on the transitions of H2, F2 and exciplex ArF* molecules. It was shown that the addition of He buffer gas increases the pulse duration, while Ne addition improves the output energy of the VUV laser on the H2 Lyman band. A laser pulse duration over 10 ns and an output of 0.12 mJ were obtained. The diffuse discharge in mixtures of He with F2 was found to form as a result of successive ionization waves. It was shown that the laser pulse at 157 nm has three peaks, which correspond to the maxima of the diffuse discharge current. Therewith, the first or second peak of the laser radiation has the maximum intensity, depending on the amplitude of the conduction current in the primary ionization wave. A maximal F2* laser electrical efficiency of η 0 = 0.18% and an output of Q 157 = 3.8 mJ were obtained in a He–F2 gas mixture at pressure of 10 atm, which exceeds the efficiency of lasers of this type pumped by transverse volume discharges with UV preionization. Long-pulse operation of the ArF* laser was achieved in a He–Ne–Ar–F2 gas mixture. Lasing at 193 nm continued during two periods of the diffuse discharge current. The total duration of the laser pulse was as long as 40 ns, and the radiation energy at 193 nm was as high as 2 mJ from an active volume of 20 cm3.

Extinction of high-power laser radiation under adverse weather conditions.

The propagation of laser radiation over long distances can be significantly affected by atmospheric extinction due to precipitation as well as aerosol particles and molecules. The knowledge of the contribution of precipitation is critical to the operation of a variety of laser-based systems. The study of high-power laser transmission around 1µm is of particular interest because several atmospheric transmission windows are located in this region. To investigate the effect of adverse weather conditions on laser transmission, free-space laser transmission experiments are conducted on the DLR test range in Lampoldshausen, Germany. A high-power laser with a wavelength of 1.03µm is used for the transmission measurements in combination with calibrated power monitors. Local weather conditions are continuously monitored by meteorological instruments during the experiments. Extinction coefficients are derived from transmission measurements showing that the extinction for snow is 7 times higher than for rain, and the extinction for drizzle/rain is 4 times higher than for rain at a given precipitation rate of 1mm/h. For a mixture of rain and snow, the extinction is comparable to that of rain, indicating that the water content strongly influences the optical properties and thus the extinction of laser radiation in mixed precipitation. A good relationship is found between the measured extinction coefficient and visibility for drizzle and rain and a slightly larger scatter of the data for snow. Furthermore, measured extinction coefficients are compared to the extinction coefficients based on the experimental size distributions of precipitation particles and geometric optics. A reasonable agreement is obtained for rain, with no improvement taking the forward-scattering into the detector aperture into account, and a much better agreement is obtained for snow when the forward-scattering contribution is included.

UV laser formation of complex topologies for sensitive elements of navigation sensors based on surface acoustic waves

A method utilizing a UV laser to form complex electrode structures on the surface of the sensitive element prototype for a solid-state gyroscope (SSG) based on surface acoustic waves (SAW) was tested. The prospects of using the method are demonstrated in terms of the flexibility of changing the topology, the speed of navigation sensors prototypes manufacturing, and reducing the cost. Methods for forming surface structures on piezoelectric materials using an IR and UV laser are compared. The parameters of laser formation of a surface pattern for configuring the topology of a sensitive element by evaporation of a thin-film coating 5000 nm and 200 nm thick deposited on lithium niobate substrates are determined. The amplitude-frequency characteristics of the manufactured samples of SAW-based SSG sensing elements are obtained. Additionally a finite-element modeling was performed to obtain theoretical prediction of dependencies of the sensing element characteristics on the thickness of the metallization forming the electrodes. It was found that when using a fiber UV laser with a wavelength of 355 nm, it is necessary to impose more stringent requirements on the quality of metallization and on the calculation of laser radiation regimes for processing coatings with a thickness of 200 nm. It is shown that when using a UV laser, it is possible to manufacture more compact sensitive elements of navigation sensors on surface acoustic waves than when using IR lasers.

A compact optical sensor for risk assessment from high-power laser scattering

The outdoor operation of multi-kilowatt high-power lasers involves the potential hazard of exposing the human eye and skin to scattered radiation. In present laser safety standards and regulations, the effects of atmospheric off-axis scattering and the dynamic interaction between the laser beam and the target object are not adequately represented. Therefore, experimental studies are dedicated to providing quantitative information on scattered radiation for comprehensive laser safety evaluations. With this end in view, a mobile detection system has been developed that relies on the technical specifications (i.e. entrance aperture and field-of-view) of laser safety regulations. The mobile detection system is deployed at the German Aerospace Center (DLR) laser test range at Lampoldshausen. Atmospheric scattering experiments are performed by placing the calibrated detection system under specified scattering angles relative to the direction of the laser beam and determining the scattered radiation, i.e. absolute values for scattered laser power. In addition, the detection system is used for target scattering experiments. In these experiments, the sensor’s versatility is demonstrated by measuring the specularly and diffusely scattered laser radiation from metallic target objects. The dependence of the scattering power as a function of the scattering angle, distance, and laser output power is investigated during laser-matter interaction to demonstrate the capabilities of the system.

EFFECTS OF MLS-LASER ON NEUROPATHIC PAIN IN DIABETIC SENSОMOTOR NEUROPATHY

About 20-50 % of patients with diabetes and about 60 % of those with diabetic neuropathy develop neuropathic pain, which is characterized by tingling, burning, sharp, shooting, or stabbing sensations, and even electric shock-like sensations. Painful diabetic neuropathy can significantly affect the quality of life of patients with diabetes, the ability to perform daily activities and negatively affect mood. According to the 2021 consensus of an international panel of experts regarding the treatment of painful distal symmetric polyneuropathy, non-pharmacological forms of treatment should also be considered due to unsatisfactory pharmacotherapy. A number of studies have demonstrated the role of photobiomodulation as a non-pharmacological method of treating painful diabetic neuropathy. The purpose of this placebo-controlled, longitudinal study was to investigate the effect of high-energy MLS-laser therapy on neuropathic pain. Material and methods: A total of 69 cases of patients with type 2 diabetes and painful diabetic neuropathy of the lower extremities were followed, divided into two groups: an experimental - 41 patients received high-energy laser radiation and a control (placebo) group - 28 patients, with a "mock" laser treatment. For objectification of pain, the Bulgarian version of the short form of the McGill pain questionnaire, version SF-MPQ-2, which includes the visual analog scale (VAS), was used. Comparisons between groups were performed with parametric or non parametric tests depending on the distribution of the variables, the number of the compared groups and the study pre test – post test design. Results: The pain index, reported by the McGill questionnaire, in the experimental group, decreased by - 63.2 % at the end of the nine-day treatment and by - 56.1 % at the 90th day after the start of treatment compared to the value before therapy. In the control group, there was a minimal change at the end of treatment (day 21), which did not persist until the end of the observation period (day 90). Conclusions: MLS laser therapy significantly increases the pain threshold and should be considered a safe, non-pharmacological adjunct to standard therapy in patients with painful diabetic peripheral neuropathy.