Elliptical Laser Beam Research Articles

Влияние скорости сканирования на формирование ТЛИПСС радиально-симметричным и эллиптическим гауссовым фемтосекундным лазерным пучком

Влияние скорости сканирования на формирование ТЛИПСС радиально-симметричным и эллиптическим гауссовым фемтосекундным лазерным пучком

Anisotropic thermal conductivity measurement using a new Asymmetric-Beam Time-Domain Thermoreflectance (AB-TDTR) method.

Anisotropic thermal properties are of both fundamental and practical interests, but remain challenging to characterize using conventional methods. In this work, a new metrology based on asymmetric beam time-domain thermoreflectance (AB-TDTR) is developed to measure three-dimensional anisotropic thermal transport by extending the conventional TDTR technique. Using an elliptical laser beam with controlled elliptical ratio and spot size, the experimental signals can be exploited to be dominantly sensitive to measure thermal conductivity along the cross-plane or any specific in-plane directions. An analytic solution for a multi-layer system is derived for the AB-TDTR signal in response to the periodical pulse, elliptical laser beam, and heating geometry to extract the anisotropic thermal conductivity from experimental measurement. Examples with experimental data are given for various materials with in-plane thermal conductivity from 5 W/m K to 2000 W/m K, including isotropic materials (silicon, boron phosphide, and boron nitride), transversely isotropic materials (graphite, quartz, and sapphire), and transversely anisotropic materials (black phosphorus). Furthermore, a detailed sensitivity analysis is conducted to guide the optimal setting of experimental configurations for different materials. The developed AB-TDTR metrology provides a new approach to accurately measure anisotropic thermal phenomena for rational materials design and thermal applications.

Generation of electron plasma wave and particle acceleration by beating of two intense cross focused elliptical laser beams in collisionless plasma

This paper presents the propagation of two intense cross-focused elliptical laser beams (ELBs) in a collisionless plasma and their effect on the excitation of electron plasma waves (EPWs) and particle acceleration, when relativistic and ponderomotive nonlinearities are operative. Due to the mutual interaction of the two ELBs, cross-focusing takes place in the plasma. An EPW is generated on account of the beating of the two cross-focused ELBs of frequencies ω1 and ω2. Nonlinear differential equations have been set up for the beamwidth parameters of the ELBs, the power of the EPW and the energy gained by the electrons using WKB and paraxial-ray approximations. Numerical simulations have been carried out to investigate the effect of various laser and plasma parameters such as incident laser intensity and plasma density on the focusing of ELBs in plasmas and its effect on the power of excited EPWs and particle acceleration. Numerical results show that the focusing and intensity of both beams in plasma increase with increasing values of incident laser intensity and plasma density. It is observed that the cross-focusing of two ELBs enhances the power of generated EPWs and the yield of energy gain. This study is useful for determining the propagation dynamics of ELBs in plasma and the results are helpful for various applications requiring multiple laser beams.

Mitigation of stimulated Raman backscattering by elliptical laser beam in collisionless plasma

This paper presents theoretical and numerical study of the propagation of an elliptical laser beam in collisionless plasma with relativistic-ponderomotive nonlinearities and its effect on the excitation of electron plasma wave (EPW) and resulting stimulated Raman backscattering (SRBS) process. Nonlinear interaction between an intense elliptical laser beams with the electron plasma wave leads stimulated Raman backscattering. Following Wentzel Kramers Brillouin (WKB) and paraxial-ray approximations, non-linear differential equations for the beam width parameters of elliptical laser beam, electron plasma wave and back-scattered beam are set up and solved numerically. Numerical simulations have been carried out to investigate the effect of laser and plasma parameters on the focusing of elliptical laser beam in plasma and further its effect on the excitation of electron plasma wave and backreflectivity of stimulated Raman scattering (SRS). The focusing of an elliptical laser beam, EPW and scattered wave are reduced at higher values of laser intensity in plasma. The amplitude of excited electron plasma wave, which depends on the focusing of main laser beam and electron plasma wave reduce for higher values of incident laser intensity. The results indicate that the focusing of waves mitigates the backreflectivity of SRS. The results are also compared with only relativistic nonlinearity and the Gaussian profile of laser beam. It is observed that an elliptical laser beam mitigates the backreflectivity of SRS at higher values of laser intensity. This study is relevant to inertial confinement fusion scheme, where SRS plays very important role.

Investigation of Weakly Relativistic Ponderomotive Effects on Self-Focusing During Interaction of High Power Elliptical Laser Beam with Plasma

This paper presents an investigation of weakly relativistic ponderomotive effects on self-focusing during interaction of high power elliptical laser beam with plasma. The nonlinear differential equations for the beam width parameters of elliptical laser beam have set up by using Wentzal–Krammers–Brillouin (WKB) and paraxial approximations. These equations have been solved numerically by using fourth order Runge–Kutta method to study the variation of these beam width parameters against normalized distance of propagation. Effects of variation in laser beam intensity, plasma density and electron temperature on the beam width parameters are also analyzed.

Specifics of the Crystallization of Lanthanum Borogermanate Glass by a Femtosecond Laser Beam

A method of nucleating a crystalline seed by means of a stationary laser beam with increasing pulse energy, which considerably reduces and stabilizes the formation time of microcrystals, is proposed. A significant improvement in the uniformity of the crystalline structures formed by irradiation with a focused femtosecond elliptic laser beam as compared with an ordinary Gaussian beam is demonstrated. It is demonstrated by means of confocal Raman scattering spectroscopy that the crystal channels obtained consist of LaBGeO5 crystals oriented along the direction of laser scanning.

Z-scan analysis of elliptical Gaussian beams in Kerr media through the variational method: Toward integrative analytic solutions of Z-scan

Previous studies on Z-scan have paid little attention to Z-scan transmittance about elliptical Gaussian beams in thick Kerr media. The purpose of this study is twofold. First, it proposes a new analytical approach to the Z-scan on-axis transmittance for elliptical Gaussian beams through the variational method. The analytical approach yields an explicit analytical solution (not numerically analyzed solution) for the Z-scan on-axis transmittance in arbitrary thick media, and then the explicit solution is generalized to circularly or elliptically symmetric Gaussian beams. With the analytical solutions, we explain the mechanism of the elliptical beam waist changes in both positive and negative Kerr media. Second, the proposed solutions are experimentally tested in the photorefractive LiNbO3:Fe crystal with a cw Ti:sapphire elliptical laser beam (λ=800nm), showing congruence between experimental data and the predicted values from the solutions.

Analytical solutions for three-dimensional modeling of temperature rise inside solid material induced by laser irradiation

In this paper, analytical solutions for three-dimensional(3-D) modeling of temperature rise inside solid material induced by laser irradiation is presented. Firstly, the theoretical physical model of temperature rise for a material surface irradiated by laser is established based on classical heat conduction theory. Then, integral transform method is used to solve the heat conduction equation and its analytical solutions are obtained. In the end, the square and elliptical Gaussian laser beam is selected as examples, and temperature distributions of silicon material irradiated by the two lasers are simulated. Results show that, when silicon material is irradiated by laser, the shape of temperature distributions on the surface of material are similar to that of the laser intensity due to direct absorption mechanism of laser energy on the material surface. The temperature distributions in a certain depth below the material surface also contain the shape information of laser intensity on the material surface.

Effect of self-focusing of elliptical laser beam on second harmonic generation in collisionless plasma

In the present paper, effect of self-focusing of elliptical laser beam on second harmonic generation is investigated in collisionless plasma by making use of paraxial ray approximation. There is redistribution of carriers from high field region to low field region by ponderomotive force and hence transverse density gradient gets established in plasma. This transverse density gradient in turn generates the electron plasma wave at pump wave frequency. The electron plasma wave so generated interacts with pump wave there by producing second harmonic. Effect of variation in laser beam intensity and plasma density on second harmonic yield is also analyzed.

Nonlinear Interaction of High Power Elliptical Laser Beam with Cold Collisionless Plasma

In the present paper, nonlinear interaction of high power elliptical laser beam with cold collisionless plasma is investigated by taking in to account ponderomotive nonlinearity. Nonlinear differential equations for the beam width parameters of semi-major axis and semi-minor axis of elliptical laser beam have been set up and solved numerically to study the variation of beam width parameters with normalized distance of propagation. Effects of variation in laser intensity, plasma density and plasma temperature on the beam width parameters are also analyzed.

Nonlinear Interaction of Elliptical Laser Beam with Collisional Plasma: Effect of Linear Absorption

In the present work, nonlinear interaction of elliptical laser beam with collisional plasma is studied by using paraxial ray approximation. Nonlinear differential equations for the beam width parameters of semi-major axis and semi-minor axis of elliptical laser beam have been set up and solved numerically to study the variation of beam width parameters with normalized distance of propagation. Effects of variation in absorption coefficient and plasma density on the beam width parameters are also analyzed. It is observed from the analysis that extent of self-focusing of beam increases with increase/decrease in plasma density/absorption coefficient.

Mitigating the relativistic laser beam filamentation via an elliptical beam profile.

It is shown that the filamentation instability of relativistically intense laser pulses in plasmas can be mitigated in the case where the laser beam has an elliptically distributed beam profile. A high-power elliptical Gaussian laser beam would break up into a regular filamentation pattern-in contrast to the randomly distributed filaments of a circularly distributed laser beam-and much more laser power would be concentrated in the central region. A highly elliptically distributed laser beam experiences anisotropic self-focusing and diffraction processes in the plasma channel ensuring that the unstable diffractive rings of the circular case cannot be produced. The azimuthal modulational instability is thereby suppressed. These findings are verified by three-dimensional particle-in-cell simulations.

An Integrated Flexible Self-calibration Approach for 2D Laser Scanning Range Finders Applied to the Hokuyo UTM-30LX-EW

Abstract. The paper presents a flexible approach for the geometric calibration of a 2D infrared laser scanning range finder. It does not require spatial object data, thus avoiding the time-consuming determination of reference distances or coordinates with superior accuracy. The core contribution is the development of an integrated bundle adjustment, based on the flexible principle of a self-calibration. This method facilitates the precise definition of the geometry of the scanning device, including the estimation of range-measurement-specific correction parameters. The integrated calibration routine jointly adjusts distance and angular data from the laser scanning range finder as well as image data from a supporting DSLR camera, and automatically estimates optimum observation weights. The validation process carried out using a Hokuyo UTM-30LX-EW confirms the correctness of the proposed functional and stochastic contexts and allows detailed accuracy analyses. The level of accuracy of the observations is computed by variance component estimation. For the Hokuyo scanner, we obtained 0.2% of the measured distance in range measurement and 0.2 deg for the angle precision. The RMS error of a 3D coordinate after the calibration becomes 5 mm in lateral and 9 mm in depth direction. Particular challenges have arisen due to a very large elliptical laser beam cross-section of the scanning device used.

Simple and sensitive technique for alignment of the pinhole of a spatial filter of a high-energy, high-power laser system

In this paper, a new method for alignment of the pinhole of a spatial filter (SF) has been proposed and demonstrated experimentally. The effect of the misalignment of the pinhole on the laser beam profiles has been calculated for circular and elliptical Gaussian laser beams. Theoretical computation has been carried out to illustrate the effect of an intensity mask, placed before the focusing lens of the SF, on the spatial beam profile after the pinhole of the SF. It is shown, both theoretically and experimentally, that a simple intensity mask, consisting of a black dot, can be used to visually align the pinhole with a high accuracy of 5% of the pinhole diameter. The accuracy may be further improved using a computer-based image processing algorithm. Finally, the proposed technique has been demonstrated to align a vacuum SF of a compact 40 J Nd:phosphate glass laser system.

Single-molecule imaging of transcription factor binding to DNA in live mammalian cells

Imaging single fluorescent proteins in living mammalian cells is challenging due to out-of-focus fluorescence excitation by common microscopy schemes. We report the development of a novel fluorescence microscopy method, reflected light sheet microscopy (RLSM), which allows selective plane illumination throughout the nucleus of living mammalian cells, for reducing out-of-focus fluorescence signal. Generation of a thin light sheet parallel to the imaging plane and close to the sample surface is achieved by reflecting an elliptical laser beam incident from the top by 45° with a small mirror. The thin light sheet allows for an increased signal-to-background ratio superior to previous illumination schemes and enables imaging of single fluorescent proteins with up to 100 Hz time resolution. We demonstrate the sensitivity of RLSM by measuring the DNA-bound fraction of glucocorticoid receptor (GR) and determine the residence times on DNA of various oligomerization states and mutants of GR and estrogen receptor (ER), enabling us to resolve different modes of DNA binding of GR. Finally, we demonstrate two-color single molecule imaging by observing the spatio-temporal co-localization of two different protein pairs. The combination of our single molecule measurements and statistical analysis reveals dynamic properties of transcription factors in live mammalian cells.

反射式椭圆高斯光束平顶整形系统

In order to convert elliptical Gaussian laser beam to circle flat-top beam,a two-mirror unobscured reflective optical system is designed.Based on the energy conservation law,the Flattened Lorentzian function is chosen as the distribution expression of the output flat-top beam to build the relationship between the input and output aperture coordinates.The equations in integral form that describe the reflective surfaces of the shaping system are derived by applying the law of reflection expressed in vector.Through the numerical calculation of the surface data and optical design software simulation,the circle flat-top collimated output beam is achieved and the illumination uniformity of the target area is above 95%.It indicates that the shaping system could improve the flatten degree of the elliptical Gaussian laser beam apparently.

A common-path interferometer including integrated single-prism beam shaping

In this work, a common-path interferometer including integrated single-prism beam shaping is presented. Both theoretical considerations and experimental results on single-prism shaping of elliptical and circular laser beams were performed. Based on a single-prism beam shaper, a common-path interferometer without any classical beam splitter device was assembled and characterised. It is shown that the presented setup provides well-evaluable interference patterns with a Michelson contrast of 0.42–0.45.

Self-Focusing of Elliptical Laser Beam in Collisional Plasma and Its Effect on Stimulated Brillouin Scattering Process

This paper presents an investigation of self-focusing of elliptical laser beam in a collisional plasma and its effect on stimulated Brillouin scattering (SBS) process. The non-linearity arising through non-uniform heating leads to redistribution of carriers, which modifies the background plasma density profile in a direction transverse to pump beam axis. This modification affects the incident laser beam, ion-acoustic wave and back scattered beam. Non-linear differential equations for the beam width parameters of the pump laser beam, ion-acoustic wave and back scattered beam are set up and solved numerically. It is observed from the analysis that the focusing of waves greatly enhances the SBS back-reflectivity.

Stimulated Brillouin scattering of elliptical laser beam in collisionless plasma

This paper presents an investigation of self-focusing of elliptical laser beam in collisionless plasma and its effect on stimulated Brillouin scattering. The pump beam interacts with a pre-excited ion-acoustic wave leading to Brillouin back-scattered process. The transverse intensity gradient of a pump beam generates a ponderomotive force, which modifies the background plasma density profile in a direction transverse to pump beam axis. This modification in density effects the incident laser beam, ion-acoustic wave and back-scattered beam. Non-linear differential equations for the beam width parameters of pump laser beam, ion-acoustic wave and back-scattered beam are set up and solved numerically. It is observed from the analysis that the focusing of waves enhances the SBS back-reflectivity.

Direct laser patterning of self-assembled monolayer using elliptical laser beams: A theoretical parametric study

A theoretical quantitative analysis of processing parameters for application of an elliptical laser beam to achieve maximum patterning area is the focus of this study. Direct laser patterning (DLP) of self-assembled monolayers (SAM) is achieved by localized heating of the sample above the SAM desorption temperature. Through use of elliptical laser beams in the present work, three goals are achieved by analyzing the heat diffusion model and related thermo-kinetics model: (1) optimal working conditions (combination of laser power, scanning velocity and aspect ratio) for DLP to produce maximum feature size, or highest processing velocity at a given power; (2) identification of conditions that reduces the potential thermal damage to the substrate; (3) shedding light on issues related to uniformity or homogeneity of heating a substrate using an elliptical laser beam. A heat diffusion model is employed to provide the resulting surface temperature caused by elliptical laser beams, and the coupled thermo-kinetics model is used to determine the final SAM coverage generated by DLP. Parametric analysis revealed that 70–150 mW can be used to pattern feature sizes in the range of 2–10 times of equivalent circular beam size. It is also found that each elliptical laser beam has a unique optimal aspect ratio to result in the widest feature size for a given laser power and scanning velocity. The edge transition width increases with an increase of the aspect ratio. Keeping the aspect ratio of elliptical laser beam small (i.e. β<20), a sharp edge definition could be obtained; if an aspect ratio larger than 30 is used, a surface with gradual edge definition could be obtained.