离轴矢量光束的紧聚焦特性

基于理查德-沃尔夫矢量衍射积分理论,数值研究了离轴矢量光束经过高数值孔径透镜后的紧聚焦特性.通过对比分析偏振奇点离轴的径向、角向矢量光束紧聚焦后,焦点附近的强度和相位分布规律,提出了一种由广义柱矢量光束设计紧聚焦场的方法.此外,通过分析矢量光束自旋分量的聚焦行为和自旋轨道耦合所诱导的自旋角动量分布变化,讨论了偏振奇点离轴对称破缺对高阶矢量光束紧聚焦场空间结构,特别是其自旋角动量空间分布的影响.该研究结果不仅对矢量光场的紧聚焦特性进行了补充,也为改进和完善焦场分布提供了理论参考.     

轴对称矢量光束聚焦特性研究现状及其应用

轴对称矢量光束是一种空间非均匀偏振光束, 中心光强为零, 经物镜聚焦后能在焦点附近产生空间场分量. 在高变迹系数光学系统成像情况下, 与线偏光、圆偏光相比, 径向偏振光与光瞳滤波技术及图像复原技术结合, 能获得较小焦斑, 提高横向分辨力. 介绍了轴对称矢量光束的特性, 基于电偶极子辐射模型和矢量衍射理论研究了轴对称矢量光束经高数值孔径物镜聚焦后的特性, 系统介绍了基于轴对称矢量光束实现光斑紧聚焦的几种方法, 并简述了轴对称矢量光束在差动共焦超分辨成像领域的研究设想. 关键词： 差动共焦显微技术 紧聚焦 光瞳滤波 轴对称矢量光束     

离轴高斯涡旋光束的深聚焦特性

基于德拜矢量衍射积分理论,对离轴高斯涡旋光束经过大数值孔径透镜后聚焦场的特性进行了研究,获得了离轴高斯涡旋光束深聚焦后复振幅分布函数,在此基础上对离轴高斯涡旋光束深聚焦场的光强和相位分别进行了分析.数值模拟结果表明:离轴距离的改变对高斯涡旋光束在焦平面上的光强分布和相位分布会产生影响,离轴距离的增加会加剧聚焦场光强在y轴方向上分布的差异,而离轴距离的符号决定了光强集中区域的方向.另一方面,与一阶离轴涡旋光束不同,高阶离轴涡旋光束经过深聚焦后会发生暗核分裂现象,出现多个相位奇点,奇点个数等于原始光束对应的拓扑荷数,且分裂后的奇点具有明显的对称性.研究表明,这种暗核分裂现象由大数值孔径透镜深聚焦引起.     

紧聚焦条件下的轴向双焦点波带片EI北大核心CSCD

基于紧聚焦条件下的矢量衍射理论推出了一个解析解形式的环半径公式,利用此公式设计得到的二元相位波带片可以使入射光的紧聚焦区域呈现轴向双焦点分布。并且,公式中的两个焦点的轴向距离和环半径直接相关,可以设计一系列的具有不同轴向距离的二元相位波带片,用来调制高数值孔径物镜,使其产生轴向可调的双焦点。以径向偏振贝塞尔-高斯光束为例,数值模拟了在不同轴向偏移距离的二元相位波带片的调制下的紧聚焦场的空间强度分布。数值模拟结果表明,基于此类二元相位波带片可以成功地实现轴向距离可调的双焦点。此外,利用一些具有特殊轴向距离的二元相位波带片,还可产生"光泡"与"光针"等特殊的紧聚焦场分布。因此,此类二元相位波带片有望应用于微粒子的动态操控与捕获。     

柱矢量涡旋光束强聚焦特性的修正研究

对现有柱矢量涡旋光束强聚焦公式的不足进行修正,数值模拟了不同拓扑核柱矢量涡旋光束的强聚焦场分布.结果表明:调节偏振旋转角、数值孔径以及遮挡通光孔径可以有效地构造平顶光束与暗通道.调节偏振旋转角能改变涡旋光束径向部分与角向部分的相对强度,实现平顶聚焦;增加孔径遮挡值可以在相对较小的数值孔径下形成平顶光束.当孔径遮挡较大时可以在高数值孔径下实现更窄的暗通道.     

紧聚焦条件下的轴向双焦点波带片

基于紧聚焦条件下的矢量衍射理论推出了一个解析解形式的环半径公式,利用此公式设计得到的二元相位波带片可以使入射光的紧聚焦区域呈现轴向双焦点分布。并且,公式中的两个焦点的轴向距离和环半径直接相关,可以设计一系列的具有不同轴向距离的二元相位波带片,用来调制高数值孔径物镜,使其产生轴向可调的双焦点。以径向偏振贝塞尔-高斯光束为例,数值模拟了在不同轴向偏移距离的二元相位波带片的调制下的紧聚焦场的空间强度分布。数值模拟结果表明,基于此类二元相位波带片可以成功地实现轴向距离可调的双焦点。此外,利用一些具有特殊轴向距离的二元相位波带片,还可产生"光泡"与"光针"等特殊的紧聚焦场分布。因此,此类二元相位波带片有望应用于微粒子的动态操控与捕获。     

高阶矢量光束高数值孔径聚焦特性的研究

本文利用Richards-Wolf矢量衍射积分模型对-2到4阶的贝塞尔型 矢量光束经高数值孔径聚焦后焦平面处的光强分布进行了理论仿真和特性研究, 并对矢量光束的聚焦场在真空下俘获玻璃小球的梯度力分布进行了仿真. 结果表明聚焦场的形状、尺度及各分量的变化与矢量光束的阶数有密切关系, 且具有特定的规律; 此外, 聚焦场的梯度力分布与矢量光束的阶数也有密切关系,并且具有多种不同形状和尺度的光陷阱.     

强聚焦混合偏振矢量光束在瑞利粒子上产生的辐射力

理论研究了强聚焦混合偏振矢量光束作用在瑞利粒子的辐射力,推导出混合偏振矢量光束深聚焦在焦平面处产生辐射力的计算表达式,数值模拟了焦平面附近轴向光束强度分布及数值孔径与径向系数对辐射力分布的影响.结果表明,强聚焦混合偏振矢量光束深聚焦后在焦平面附近产生的辐射力在一定情况下能够实现对瑞利粒子的三维捕获,并且受数值孔径和径向系数的影响,其中径向系数影响较为明显.大量数据整合结果表明当径向系数大于3时,才能实现与光阑数值孔径相匹配,完成对焦平面附近瑞利粒子的三维捕获.     

矢量非傍轴离轴高斯光束的传输

基于矢量瑞利-索末菲衍射积分公式，得出了波动方程的一个解，它代表矢量非傍轴离轴高斯光束，其在自由空间的传输方程表示为解析的结果.矢量非傍轴离轴高斯光束的轴上和远场公式，矢量非傍轴高斯光束的传输方程，以及傍轴的结果都可作为一般表达式的特例而得出.研究表明，f参数对光束的非傍轴特性有重要影响，而离心参数也影响非傍轴行为.与共轴情况不同的是，对离轴情况，在y方向存在场的纵向分量. 关键词： 激光光学 矢量非傍轴离轴高斯光束 瑞利-索末菲衍射积分 f参数 离轴参数     

厄米-高斯光束在热非局域介质中传输的数值模拟研究

基于非线性薛定谔方程和热扩散的泊松方程,采用分步傅里叶算法以及多重网格法对厄米-高斯光束在不同形状的热非局域介质铅玻璃中的传输进行了数值模拟研究. 结果表明,低阶厄米-高斯光束可以较为稳定地在铅玻璃中传输. 高阶厄米-高斯光束在铅玻璃中传输变得不稳定,并且阶数越高,稳定性越差. 样品的形状对于厄米-高斯光束的影响很大. 在正方形样品中,厄米-高斯光束的传输与Snyder-Mitchell模型符合得相对较好. 在矩形样品中厄米-高斯光束在传输过程中的强度分布将发生较大的变化. 关键词： 非局域非线性介质 厄米-高斯光束 光束传输     

High-sensitivity vibrational imaging with frequency modulation coherent anti-Stokes Raman scattering (FM CARS) microscopy

We demonstrate a new approach to coherent anti-Stokes Raman scattering (CARS) microscopy that significantly increases the detection sensitivity. CARS signals are generated by collinearly overlapped, tightly focused, and raster scanned pump and Stokes laser beams, whose difference frequency is rapidly modulated. The resulting amplitude modulation of the CARS signal is detected through a lock-in amplifier. This scheme efficiently suppresses the nonresonant background and allows for the detection of far fewer vibrational oscillators than possible through existing CARS microscopy methods.     

采用圆偏振啁啾飞秒激光脉冲操控CS2分子的相干拉曼散射过程

采用两束圆偏振啁啾飞秒激光脉冲,非共线相干激发三原子分子CS₂液体. 在相位匹配的方向上,探测到由CS₂频率为397 cm^-1的振动模式产生的强度对称分布的相干反斯托克斯拉曼散射(CARS)信号和相干斯托克斯拉曼散射(CSRS)信号. 当调整两束激发光的圆偏振状态时,CARS,CSRS信号的强度、偏振、波长均发生规律性的改变:CARS,CSRS信号的强度分布反映了CS₂ 在不同极化状态下的受激拉曼散射截面大小;信号光的 关键词： 啁啾脉冲 相干反斯托克斯拉曼散射(CARS) 相干斯托克斯拉曼散射(CSRS) 2')" href="#">CS₂     

Broadband coherent anti-Stokes Raman scattering light generation in BBO crystal by using two crossing femtosecond laser pulses

As broad as 12000 cm(-1) coherent anti-Stokes Raman scattering (CARS) light from ultraviolet to infrared was generated in a BBO crystal by using two crossing femtosecond laser pulses with 30% conversion efficiency. More than fifteenth-order anti-Stokes and second-order Stokes Raman sidebands were observed with nice Gaussian spatial mode. The effect of the crossing angle between two input beams on the spectrum and emitting angle of the Raman sidebands was studied in detail. Calculation shows that the phase-matching condition determines the frequencies and angles of the sidebands.     

超衍射极限相干反斯托克斯拉曼散射显微成像技术及其探测极限分析

理论上提出一种突破衍射极限限制的相干反斯托克斯拉曼散射显微成像方法, 并对其探测极限进行分析.通过引入环形附加探测光与艾里斑周边的声子作用, 实现点扩展函数的改造, 提高相干反斯托克斯拉曼散射显微成像系统的横向空间分辨率. 随着分辨率的提高, 信号强度也随之降低, 尤其当应用于生物学、医学研究时, 样品分子数密度通常很低, 这将导致信号探测更加困难. 因此分析系统的探测极限, 确定超分辨体积元内的最小可探测分子数是展开超衍射极限相干反斯 托克斯拉曼散射显微成像实验研究的重要前提. 当泵浦光、斯托克斯光、探测光光强均达到极大值, 分辨率约40 nm三维空间内, 超衍射极限相干反斯托克斯拉曼散射显微成像系统的散粒噪声信噪比由曝 光时间与样品分子数密度决定. 曝光时间若取20 ms, 探测极限约为10³, 样品分子数目只有大于探测极限, 才能保证信号可以从噪声背景中提取出来. 关键词： 突破衍射极限 相干反斯托克斯拉曼散射 非线性光学 探测极限     

High-resolution four-photon intracavity laser spectroscopy with the external phase conjugation mirror

The phase conjugation regime is experimentally implemented in a cell with a liquid (water, ethanol, carbon tetrachloride) at the four-wave mixing of counter-propagating beams of diode lasers. Intracavity enhancement of the conjugate wave and the wave of the coherent anti-Stokes Raman scattering (CARS) arising in the interaction of the conjugate wave and one of the pumping waves in CARS scheme is detected. The latter process occurs in a narrow spectral range when the geometry of the wave vectors of the interacting waves in CARS scheme ensures arrival of the CARS signal at the laser aperture. It is found out that the quality of the four-photon phase conjugate mirror in the given interaction scheme is very sensitive to any perturbations (mechanical, acoustic, thermal, etc.) of the induced grating in the sample, which manifests itself in the distortion of the signal spectrum.     

纳米分辨相干反斯托克斯拉曼散射显微成像

采用附加探测光声子耗尽法来实现超衍射极限相干反斯托克斯拉曼散射显微成像. 此方法引入一束环形分布的附加探测光来消耗点扩展函数周边的相干声子, 实现点扩展函数的改造, 从而达到超越衍射极限的空间分辨率. 为了获得更高的空间分辨率和更佳的相位匹配条件, 通常需采用高数值孔径物镜对抽运光、斯托克斯光和探测光进行聚焦, 此时标量衍射理论不再成立. 基于矢量衍射理论, 分析了线偏振光、圆偏振光先后经过螺旋相位片和高数值孔径物镜后的光强分布, 结果表明: 圆偏振光在高数值孔径物镜后焦平面的光强分布呈中心对称状, 较线偏振环形光更适合作为附加探测光. 此外, 采用全量子理论分析了附加探测光声子耗尽法. 结果表明: 当附加探测光与探测光强度比为80时, 成像系统的横向空间分辨率可以达到45 nm; 继续提高附加探测光强度, 空间分辨将进一步提高.     

Coherent anti-Stokes Raman scattering imaging with a laser source delivered by a photonic crystal fiber

We demonstrate laser-scanning coherent anti-Stokes Raman scattering (CARS) imaging with two excitation laser beams delivered by a large-mode-area photonic crystal fiber. The group-velocity dispersion and self-phase modulation effects are largely suppressed due to the large mode area of the fiber and the use of picosecond pulses. The fiber delivery preserves the signal level and image spatial resolution well. High-quality images of live spinal cord tissues are acquired using the fiber-delivered laser source. Our method provides a basic platform for developing a flexible and compact CARS imaging system.     

Coherent anti-Stokes Raman spectroscopy (CARS)

By putting the non-linear susceptibility tensors in irreducible spherical form it is found that orientation averaging, selection rules, and the angular dependence of intensity on field polarizations all follow in a straightforward way. These spherical tensor techniques are used to treat coherent anti-Stokes Raman scattering (CARS) in detail and a comparison is made with conventional Raman scattering. The selection rules for both processes are shown to be the same. The paper concludes with a brief discussion of the rotational structure in CARS.     

Molecular vibration imaging in the fingerprint region by use of coherent anti-Stokes Raman scattering microscopy with a collinear configuration

We have developed a new coherent anti-Stokes Raman scattering (CARS) microscopy system with a collinear configuration for use in the fingerprint region. The system consists of a picosecond laser system and a transmission-type laser scanning microscope without a pinhole in front of the detector. The observable Raman-shift region is 900-1750 cm(-1), the spectral resolution is 30cm(-1), and the spatial resolution is smaller than 1 mum in the lateral direction and 3.2 mum in the depth direction, with objectives with a numerical aperture of 0.65. CARS spectra and images of polystyrene beads are demonstrated, and CARS imaging of a viable yeast cell is attempted.     

Coherent anti-Stokes Raman scattering and normal anti-Stokes Raman scattering due to LO phonons in GaP using cw diode lasers

Coherent anti-Stokes Raman scattering (CARS) and normal anti-Stokes Raman scattering (NARS) have been measured in (001) GaP at room temperature due to the 403 cm⁻¹ LO phonons using a continuous wave (CW) 785.0 nm fixed-wavelength pump laser and a CW Stokes laser tunable in the 800-830 nm wavelength range. CARS measurements are normally made using pulsed lasers. The use of CW diode lasers allows a more accurate comparison between the measured and calculated values of the CARS signal. The pump and Stokes laser beams were linearly polarized perpendicular to each other, same as the pump and normal Stokes/anti-Stokes scattered light for the GaP sample used in this work. The pump and Stokes laser powers incident upon the GaP sample, located in the focal plane of a 20 mm effective focal length lens, were <20 and 50 mW, respectively. The diameter of the laser beams in the focal plane of the focusing lens was determined to 40±5 μm. The pump and Stokes laser beam intensities incident upon the 0.3 mm thick GaP sample were <2 and 5 kW cm², respectively. The powers of the CARS and NARS signals were measured using a Raman spectrometer. The signal output of the Raman spectrometer was calibrated using the pump laser and several neutral density filters. The Raman linewidth (full-width at half-maximum) of the LO phonons was determined to be 0.95±0.05 cm⁻¹, using the variation of the CARS signal with the wavelength of the Stokes laser. The measured powers of the CARS and NARS signals are about a factor of 5 and 1.5, respectively, smaller than those calculated from the corresponding theoretical expressions.