高斯波束对双层粒子的辐射俘获力

从广义米理论出发，将入射高斯波束用矢量球谐函数展开.根据对电磁场动量的讨论，得出了高斯波束对多层球形粒子的辐射俘获力的表示式，并就单高斯波束对在轴双层有吸收粒子受到的辐射俘获力进行了数值模拟，讨论了束腰半径、吸收系数、内外层相对厚度对俘获情况的影响. 关键词： 辐射俘获力 多层球形粒子 光镊 高斯波束     

Relativistic ponderomotive forces in the field of intense laser radiation

The motion of a relativistic charged particle in the presence of the field of high-power laser radiation represented in the form of a Gaussian beam of arbitrary mode is analyzed. The vector potential of the radiation field is expanded in terms of a small parameter (the ratio of the wavelength to the Gaussian beam waist). A specific feature of averaging with respect to the phases of the high-mode Gaussian beams is demonstrated. The averaged equations for the motion of particle and a general expression for the ponderomotive relativistic force for the circularly polarized radiation are derived. It is demonstrated that relativistic effects suppress the averaged action of high-power laser radiation on the particle.     

Scattering of Gaussian beam by a spherical particle with a spheroidal inclusion

A generalized Lorenz-Mie theory framework (GLMT) is applied to the study of Gaussian beam scattering by a spherical particle with an embedded spheroid at the center. By virtue of a transformation between the spherical and spheroidal vector wave functions, a theoretical procedure is developed to deal with the boundary conditions. Numerical results of the normalized differential scattering cross section are presented.     

Wavelength-dependent intensity distribution of a Gaussian beam scattered by a spherical particle

The wavelength-dependent intensity of a light beam scattered by an isotropic homogeneous particle illuminated with an on-axis monochromatic polarized Gaussian beam is calculated in this paper. The vector spherical harmonics expansion and T transformation matrix are used to form the theoretical basis. Numerical results show that the angular intensity distribution is symmetric for an on-axis polarized Gaussian beam illumination upon an aqueous spherical particle at 473, 532 and 660 nm. For the specified aqueous spherical particle, the scattered intensity distribution decreases with increase in wavelengths.     

Radiation forces of a dielectric medium plate induced by a Gaussian beam

Based on the angular spectrum method, we investigate the radiation forces on a finite-size dielectric medium plate induced by a Gaussian beam. The formulas for the radiation force along longitudinal and transverse direction are derived, and numerically calculation is performed. It is shown that for the finite-size dielectric medium plate, the radiation forces exerted by the Gaussian beam is dependent upon the angle and position of a single ray striking on the plate and the intensity of light. Our numerical results indicate that if we choose the appropriate parameters there will be enough transverse forces to overcome the gravity force, making the plate move upwards.     

Acoustic radiation force on a rigid cylinder near rigid corner boundaries exerted by a Gaussian beam field

Acoustic manipulation is one of the well-known technologies of particle control and a top research in acoustic field. Calculation of acoustic radiation force on a particle nearby boundaries is one of the critical tasks, as it approximates realistic applications. Nevertheless, it is quite difficult to solve the problem by theoretical method when the boundary conditions are intricate. In this study, we present a finite element method numerical model for the acoustic radiation force exerting on a rigid cylindrical particle immersed in fluid near a rigid corner. The effects of the boundaries on acoustic radiation force of a rigid cylinder are analyzed with particular emphasis on the non-dimensional frequency and the distance from the center of cylinder to each boundary. The results reveal that these parameters play important roles in acoustic manipulation for particle-nearby complicated rigid boundaries. This study verifies the feasibility of numerical analysis on the issue of acoustic radiation force calculation close to complex boundaries, which may provide a new idea on analyzing the acoustic particle manipulation in confined space.     

Transformation of laser radiation by Gaussian apertures

The transformation of simple Gaussian light beams by lenses with a complex focal length is investigated in an approximation of paraxial optics. The results obtained are compared with the corresponding results of geometric optics into which they go over in limiting cases. A. N. Sevchenko Scientific-Research Institute of Physical Problems, 7, Kurchatov St., Minsk, 220064, Belarus. Translated, from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 4, pp. 471–478, July–August, 1997.     

Scattering of Gaussian beam by a spheroidal particle with a spherical inclusion at the center

An analytic solution to electromagnetic scattering by a spheroidal particle having a spherical inclusion at the center, for oblique incidence of a Gaussian beam, is obtained within the framework of the generalized Lorenz–Mie theory (GLMT). By virtue of a transformation between the spheroidal and spherical vector wave functions, a theoretical procedure is developed to deal with the boundary conditions. Numerical results of the normalized differential scattering cross section are evaluated.     

Method for converting a Gaussian laser beam into a uniform beam

The use of a phase filter to convert a gaussian laser beam into a beam with more uniform irradiance distribution is discussed. This phase filter is calculated by employing an iterative procedure on a computer. A computer-generated hologram of this phase function is used to experimentally verify the feasibility of this method.     

Flat-top laser beam generated by coherent beam combining of Gaussian lasers

We present a fiat-top laser beam generation scheme using coherent beam combining of hexagonally arranged Gaussian lasers. To produce a beam with a flat-top profile, we optimize the amplitude and phase of each unit laser using the least-square method. Simulation results show that with 13 unit lasers, a beam with the flatness of less than 1% in the optimizing region can be achieved. The main lobe contains over 95% of the total power. The scheme requires no external beam shaping element and has the potential to be designed for high-power applications.     

The electromagnetic field of a guided Gaussian beam

An electromagnetic wave traveling along a dielectric slab between two parallel, plane conductors is analyzed for a special case that is particularly relevant to guided laser beams employed in integrated optics. The beam is assumed to have an approximate gaussian field dependence along the y-direction, defined to be transverse to the direction of propagation and to the normal to the conductor planes. Approximate expressions for the field in a well collimated beam are derived. Asymptotic approximations, valid far away from the focus of a beam that is not necessarily well collimated, are also derived. It is found that the TE and TM guided gaussian beams are not linearly polarized. This is unlike the case for the usual waveguide modes whose fields are constant along the y direction. It is also found that the guided gaussian beam must have a focus and that the divergence from focus is more rapid than for a similar gaussian beam in free space.     

Nonlinear absorption,scattering, and extinction of laser radiation by two-layered spherical system-gold nanoparticle and vapor shell in water

Nonlinear absorption, scattering and extinction of laser radiation with wavelengths 532, 633 nm by spherical gold nanoparticles (NPs) with radii in the range of 5–100 nm placed in water and heated by laser radiation with formation and expansion of vapor nanoshells is theoretically investigated. Decrease of absorption, decrease and subsequent increase of scattering and extinction with increasing of shell radius beginning from the initial period of shell expansion is established. Optical indicatrixes and nonlinear behavior of scattered radiation are investigated including the examination of these characteristics during the adiabatic expansion of vapor shell. Formation of vapor nanoshells (bubbles) as a result of the action of short laser pulses on NPs placed in tissue was proposed for cutting of tissue.     

Nonlinear frequency conversion of radiation of a focused Gaussian beam

Nonlinear conversion of radiation of a Gaussian beam focused by a lens possessing coma aberration is theoretically analyzed for nonlinear crystals of 6, 4, 4ˉ symmetry group. With the help of computer modeling, spatial-angular and energetic structures of fundamental radiation on the crystal face and of the second optical harmonic in a lithium iodate crystal are obtained. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 14–18, December, 2007.     

高斯光束中吸收双层球形微粒的横向光俘获

为了研究吸收双层球形微粒的横向光俘获,基于几何光学模型提出了双层带吸收球形微粒的光俘获模型,对TEM₀₀模式高斯光束照射下外层有光吸收的双层电介质球形微粒受到的横向光俘获力进行了数值模拟,取得了光俘获力特性的一系列结果.结果显示,双层球形微粒的外层吸收系数对包括稳态俘获位置,峰值强度,稳态俘获的刚度等光俘获特性有很大影响.此外,内外径的比率对吸收双层球形微粒的光俘获特性也有调制性的影响.在一定条件下,带吸收的双层球形微粒可以被俘获在光轴上,也可能被俘获在中心在光轴上的圆环上. 关键词： 光俘获 几何光学模型 高斯光束 吸收双层球     

Heating of nonequilibrium electrons by laser radiation in solid transparent dielectrics

A computer simulation of the heating of nonequilibrium electrons by an intense high-frequency electromagnetic field leading to the bulk damage of solid transparent dielectrics under single irradiation has been carried out. The dependences of the avalanche ionization rate on threshold field strength have been derived. Using the Fokker-Planck equation with a flux-doubling boundary condition is shown to lead to noticeable errors even at a ratio of the photon energy to the band gap ∼0.1. The series of dependences of the critical fields on pulse duration have been constructed for various initial lattice temperatures and laser wavelengths, which allow the electron avalanche to be identified as a limiting breakdown mechanism. The ratio of the energy stored in the electron subsystem to the excess (with respect to the equilibrium state) energy of the phonon subsystem by the end of laser pulse action has been calculated both with and without allowance for phonon heating. The influence of phonon heating on the impact avalanche ionization rate is analyzed.     

Study on a Bessel–Gaussian beam laser

A new concept and the method are presented to obtain a laser beam output with high luminance and quality. Instead of using the conventional concept of “obtaining a single transverse mode through compressing the oscillating mode volume using a small aperture diaphragm”, the large multimode volume and the high output power are obtained by studying the physical mechanism of the expansion and coupling between a Bessel beam and a Gaussian beam. A high quality light beam (close to the diffraction limit) with high luminance and large intensity difference between the center and the edge is achieved simultaneously.