Evanescent Bessel beam generation through filtering highly focused cylindrical vector beams with a defect mode one-dimensional photonic crystal

We propose a simple setup for generating evanescent Bessel beams using the defect mode of one-dimensional (1D) photonic crystal. The angular selectivity provided by the defect mode mimics the role of an axicon for Bessel beam generation. When an azimuthally polarized beam is strongly focused onto a 1D defect mode photonic crystal interface, an evanescent Bessel beam of the first-order is produced, while an evanescent Bessel beam of the zeroth-order will be created under a radially polarized beam illumination. Switching between a donut shape and a solid focal distribution can be easily realized by controlling the polarization of the illumination. Such a versatile evanescent Bessel beam generation may find potential applications in optical trapping.     

Experimental Research on Focusing Property of Radially Polarized Beam Based on Solid Immersion Lens

Realization of a near-field optical virtual probe based on an evanescent Bessel beam is strongly dependent on a radially polarized beam; this makes it essential to study the focusing property of the beam. In this paper, two experimental setups based on a fiber device and a liquid crystal device, respectively, are built to generate a radially polarized beam. This beam and an annular radially polarized beam are focused by means of a high numerical aperture objective and a solid immersion lens (SIL). Near-field distribution of the focus spot, the evanescent Bessel field, is experimentally measured with a scanning near-field optical microscope (SNOM). The full width at half maximum (FWHM) of the central peak of the evanescent Bessel field is about 200 nm in the close vicinity of the bottom surface of SIL. This has potential for use as a near-field optical virtual probe.     

Formation of an electron beam with a duration shorter than 100 fs during photoemission of electrons by femtosecond laser pulses

Irradiation of a thin metal target by 38-fs laser pulses at a wavelength of 800 nm is shown to generate a beam of photoelectrons that contains a component whose duration is shorter than 100 fs. The ensemble of photoelectrons is formed by photoemission of a gold film about 10 nm thick sputtered on the base of a prism made of fused silica. The laser beam irradiates a dielectric-metal interface and propagates inside the prism at an angle of 45° to a normal to the interface. The photoelectron beam is formed by accelerating photoelectrons in a spatially inhomogeneous electrostatic potential. The ultrashort component of the photoelectron beam is found to be formed under the action of a ponderomotive potential. It is shown that the ultrashort electron component can be separated from the remaining part of the photoelectron beam with the help of an inhomogeneous electrostatic field.     

Quasi-Gaussian beam analytical basis and comparison with an alternative approach (L)

A superposition of propagating Bessel beams was recently numerically demonstrated to approximate a Gaussian beam and was used to evaluate the scattering by a sphere centered on the focal point of the beam. An analytical beam synthesis used in optics by Agrawal and Pattanayak [J. Opt. Soc. Am. 69, 575-578 (1979)] is found here to be recovered as the weak focusing limit of the quasi-Gaussian beam when evanescent contributions are omitted from the analytical synthesis. The propagating-wave part of the analytical synthesis has similarities to, and differences from, the recent quasi-Gaussian approximation.     

Acoustic scattering of a high-order Bessel beam by an elastic sphere

The exact analytical solution for the scattering of a generalized (or “hollow”) acoustic Bessel beam in water by an elastic sphere centered on the beam is presented. The far-field acoustic scattering field is expressed as a partial wave series involving the scattering angle relative to the beam axis and the half-conical angle of the wave vector components of the generalized Bessel beam. The sphere is assumed to have isotropic elastic material properties so that the nth partial wave amplitude for plane wave scattering is proportional to a known partial-wave coefficient. The transverse acoustic scattering field is investigated versus the dimensionless parameter ka(k is the wave vector, a radius of the sphere) as well as the polar angle θ for a specific dimensionless frequency and half-cone angle β. For higher-order generalized beams, the acoustic scattering vanishes in the backward (θ = π) and forward (θ = 0) directions along the beam axis. Moreover it is possible to suppress the excitation of certain resonances of an elastic sphere by appropriate selection of the generalized Bessel beam parameters.     

Scattering of an unpolarized Bessel beam by spheres

The scattering process of an unpolarized Bessel beam through spherical scatterers is investigated. We derive the analytical solutions of scattered fields of x-and y-polarized Bessel beams using a sphere, after which the dimensionless scattering function for an unpolarized Bessel beam is obtained. The dimensionless scattering function is applicable to spherical scatterers of any size on the beam axis or near it. Through numerical simulations, we demonstrate that extreme points exist in the direction or neighboring direction of the conical angle for spherical scatterers on the beam axis, whereas the existence of extreme points depends on the ratio between the spherical scatterers size and central spot size of the Bessel beam.     

非相干光源无衍射光的自重建

基于Hankel 波理论分析了非相干光源产生Bessel光束的自重建特性, 利用光学设计软件ZEMAX模拟了Bessel光束经过轴上圆形障碍物后的截面光强分布. 由于发光二级管(LED) 具有一定的频谱宽度且不像激光具有很高的相干度, 因此我们采用一定频宽范围的连续谱来描述. 从模拟结果可以直观地看出Bessel光束被轴上圆形障碍物遮挡后逐步完成自重建, 说明用LED非相干光作为光源具有自重建特性.实验上采用LED和轴棱锥元件产生Bessel光束, 然后通过轴上圆形障碍物、轴上方形障碍物, 并拍摄了不同位置处的截面光强分布图, 证实了非相干光源产生Bessel光束的自重建特性.实验结果和模拟结果相符合. 关键词： 无衍射光束 LED光源 轴棱锥 自重建     

Cherenkov phase-matching in Raman-seeded four-wave mixing by a femtosecond Bessel beam

It is demonstrated experimentally that the angle vs. wavelength dependence of the emission generated by multi-step four-wave mixing process seeded by stimulated Raman scattering in water under femtosecond Bessel beam excitation is determined by the longitudinal phase-matching from IR to near UV. It is shown that if on-axis phase velocity of the pump Bessel beam is equal to the phase velocity of the Stokes axial wave, then, similar to Cherenkov radiation, all the other anti-Stokes beams too acquire that axial velocity.     

The characteristics of laser micro drilling using a Bessel beam

A Bessel beam is suitable for laser micro-fabrication because it possesses both a micron-sized focal spot and a deep focal depth. As the influence of focusing aberrations is much smaller than that of a convex lens, the generation of a Bessel beam using an axicon is very practical. In the case of laser micro drilling of austenitic stainless steel with a Bessel beam, suitable processing conditions were investigated. The threshold fluence to make a through hole increased with an increase in the sample thickness. For samples with the same thickness, the threshold fluence increased with an increase in the crossing angle. A small crossing angle Bessel beam can drill a deep hole with a small threshold fluence. A through hole with a diameter smaller than 10 μm can be made on a stainless steel sheet 20 μm thick by using a Bessel beam with a large crossing angle. The taper of the hole drilled with the Bessel beam is smaller than that with the focused beam from the convex lens. PACS 52.38.Mf; 42.62.Cf; 42.79.Bh     

贝塞尔光束与激光打孔

当高斯激光束用于打小孔时，焦深很小。截断的贝塞尔光束的主极大具有很长的焦深，可降低调焦精度，用于打小孔时优于高斯光束，但是分析表明两者的成孔质量和所能实现的深径比差不多。本文通过计算指出环状分布的光束经透镜后可在焦平面产生适用于激光打孔的贝塞尔－高斯光束，这是一种高效、简单实用的方法。     

Second-harmonic generation of the nth-order Bessel beam

We investigate the second-harmonic generation of the nth-order Bessel beam in the nonlinear medium. The analysis is based on the Khokhlov-Zabolotskaya-Kuznetsov wave equation under the second-order approximation in nonlinear acoustics. The theory indicates that for an nth-order Bessel beam, the second-harmonic beam is nearly diffraction-free in the radial direction and behaves as a Bessel beam of the order 2n, and that the axial pressure amplitude is proportional to the square root of propagation distance. A variety of applications in many fields of nonlinear acoustics and nonlinear optics is expected.     

Enhancement of cross-polarized beam components at a metamaterial surface

Beam-interface interactions under incidence of a TE polarized elegant Hermite–Gaussian beam are considered in order to obtain efficient excitation of higher-order beam modes at a homogeneous, isotropic metamaterial surface. It is shown by numerical simulations that two transmitted TE and TM polarization beam components are enhanced more efficiently at a metamaterial surface than in the case of a dielectric interface. The mixed Poynting vector contribution to the incident energy flux is evaluated to explain this enhancement. The numerical results obtained are verified against energy balance at the interface.     

Review of non-diffracting Bessel beam experiments

The theory of non-diffracting Bessel beam propagation and experimental evidence for nearly-non-diffracting Bessel beam propagation are reviewed. The experimental results are analysed to show that the observed propagation distances are characteristic of the optical systems used to transport the beams, and are not unique to the Bessel beam intensity profiles. The results indicate that, despite the innovative techniques employed to generate finite radius Bessel beam distributions, nearly-non-diffracting Bessel beam propagation has not yet been conclusively demonstrated.     

Scattering of a Bessel beam by a sphere

The exact scattering by a sphere centered on a Bessel beam is expressed as a partial wave series involving the scattering angle relative to the beam axis and the conical angle of the wave vector components of the Bessel beam. The sphere is assumed to have isotropic material properties so that the nth partial wave amplitude for plane wave scattering is proportional to a known partial-wave coefficient. The scattered partial waves in the Bessel beam case are also proportional to the same partial-wave coefficient but now the weighting factor depends on the properties of the Bessel beam. When the wavenumber-radius product ka is large, for rigid or soft spheres the scattering is peaked in the backward and forward directions along the beam axis as well as in the direction of the conical angle. These properties are geometrically explained and some symmetry properties are noted. The formulation is also suitable for elastic and fluid spheres. A partial wave expansion of the Bessel beam is noted.     

Laser beam shaping with magnetic fluid-based liquid deformable mirrors

A new controllable laser beam shaping technique is demonstrated, where a magnetic fluid-based liquid deformable mirror is proposed to redistribute the laser phase profile and thus change the propagation property of the beam. The mirror is driven by an inner miniature actuator array along with a large outer actuator. The inner actuator array is used for deforming the magnetic fluid surface, while the outer actuator is used to linearize the fluid surface response and amplify the magnitude of the deflection. In comparison to other laser beam shaping techniques, this technique offers the advantages such as simplicity, low cost, large shape deformation, and high adaptability. Based on a fabricated prototype of the liquid deformable mirror, an experimental AO system was set up to produce a desired conical surface shape that shaped the incident beam into a Bessel beam. The experimental results show the effectiveness of the proposed technique for laser beam shaping.     

Manipulation of Particles with Counter-Propagating Evanescent Waves

Two counter-propagating evanescent beams are used to align and manipulate polystyrene particles on a prism surface. Since the radiation pressure transferred laterally from the evanescent wave is negated on both sides, particles can be stably aligned. By projecting a circular and a linear beam spot onto the interface, both multiple and single arrays of particles are achieved. Arrays of particles trapped on the interface can be easily moved adjusting the intensity of incident beams on either side. We also simulate electromagnetic distribution of scattering light that is converted from the evanescent wave using the FDTD method. The results show that scattering light converts from an evanescent wave propagating through a particle array and has a distance longer than that propagating from a normal evanescent wave.     

一种演示全反射消逝波的简便方法

建立了一套演示光全内反射产生的消逝波的实验装置。在罗丹明B溶液中的棱镜表面处,观察到了由532纳米的激光束全内反射产生的消逝波激发的荧光。     

像散Bessel光束自重建特性的理论和实验研究

基于菲涅耳衍射积分理论和巴比涅原理,推导出像散Bessel光束经圆形障碍物后的光强分布一般表达式.数值模拟了像散Bessel光束经圆形障碍物遮挡后光场的自重建过程,并设计相关实验进行验证,实验结果与理论模拟基本符合.结果表明：零阶像散Bessel光束经过轴上和离轴障碍物后均会发生光束重建现象.随着传输距离的增加,像散Bessel光束的外轮廓尺寸变大、中心光点阵列数增多,逐渐重建出不同于障碍物前的完整光束.并且观察到光束在重建过程中横向和纵向的重建速度并不一致,存在一定的速度差.利用螺旋相位板产生高阶像散Bessel光束,验证了高阶像散Bessel光束经障碍物遮挡后同样具有自重建特性.研究结果对像散Bessel光束在多层面粒子操纵方面的应用具有参考价值.     

Efficient conical emission of stimulated Raman Stokes light generated by a Bessel pump beam

Stimulated Raman scattering in H(2) gas was studied with a Bessel pump beam, which was generated with a binary phase grating. The first Stokes light was emitted along the surface of a cone, the angle of which was equal to the cone angle of the Bessel pump beam multiplied by the ratio of the wavelengths of the Stokes and pump beams. High conversion efficiency into the first Stokes light of as much as 60% was achieved, indicating that the complete Bessel J(0) mode of the pump beam is involved in the stimulated scattering process.     

Sidelobe suppression in light-sheet fluorescence microscopy with Bessel beam plane illumination using subtractive imaging

The fluorescence from the out-of-focus region excited by the sidelobes of a Bessel beam is the major concern for light-sheet fluorescence microscopy(LSFM) with Bessel beam plane illumination. Here, we propose a method of applying the subtractive imaging to overcome the limitation of the conventional LSFM with Bessel beam plane illumination. In the proposed method, the sample is imaged twice by line scanning using the extended solid Bessel beam and the ring-like Bessel beam. By subtracting between the two images with similar out-of-focus blur, the improved image quality with the suppression of the Bessel beam sidelobes and enhanced sectioning ability with improved contrast are demonstrated.