Polarization of light backscattered by small particles

We study scattering of light by wavelength-scale spherical, cubic, and spheroidal particles as well as clusters of spherical particles for equal-volume-sphere size parameters 4≤x≤10 and refractive indices 1.1≤m≤2.0. Such particles exhibit three specific features in the regime of backscattering: first, the intensity shows a backscattering peak; second, the degree of linear polarization for unpolarized incident light is negative; and, third, the depolarization ratio is double-lobed. We find that the overall characteristics of the scattering-matrix elements can be explained by an internal field composed of waves propagating in opposite directions near the particle perimeter and forming standing waves, as well as a wave propagating forward with the wavelength of the internal medium. When moving from the central axis of the particle toward its perimeter, the internal field changes from a forward-propagating wave with a wavelength dictated by the particle refractive index toward a standing wave with an apparent wavelength of the surrounding medium. The mapping of the internal field to the scattered far field is like an interference dial where rotation of the dial by a quarter of a wavelength on the particle perimeter results in a change from a destructive to constructive interference feature in the angular patterns (or vice versa). The dial is a manifestation of a well-known rule of thumb: the number of maxima or minima in the scattering-matrix elements is given by the size parameter. We explain the backscattering peak as deriving from the backward-propagating internal wave near the particle perimeter. Negative polarization follows from the spatial asymmetry of the internal fields: inside the particle, the fields are amplified near the central plane perpendicular to the polarization state of incident light, resulting in more pronounced interference effects for the perpendicular polarization than for the parallel polarization. The double-lobe feature in the depolarization results from the same internal-field structure with leading cross-polarized fields located slightly different from the copolarized fields. We discuss practical implications of these findings for the retrieval of particle sizes, shapes, and refractive indices from observations and laboratory experiments.     

Polarization characteristics of backscattering of turbid media based on Mueller matrix

Mueller matrix is one approach to characterizing optical polarization of the turbid media. We have simulated the two-dimensional images of Mueller matrix based on single-scattering approximation model and implemented experiments to verify the simulations. By comparing the experimental results to the theoretical simulations, we have obtained some conclusions. When the particle size is smaller than the wavelength, the linearly polarized light propagating through the turbid media of Rayleigh scatterers has better polarization-maintaining ability. Whereas when the particle size is larger than the wavelength, the circularly polarized light propagating through the turbid media of Mie scatterers has better polarization-maintaining ability. Moreover, the radial dependence of the element patterns becomes weak as the transport mean free path decreases. This study can help us understand to the fundamental principle of optical polarization.     

Theory of gyrotropic ring resonators with counterpropagating modes coupling

The Faraday effect in ring resonators leads to the resonance splitting of clockwise and counter-clockwise propagating modes. In this work we investigate how this resonance splitting influences the ring resonator's behavior if both modes are coupled by a sidewall corrugation of the resonator. Coupled mode theory is used to evaluate the transmission and reflection characteristics of a ring resonator coupled to a waveguide. Based on these evaluations, we characterize a novel ring resonator switch. The resonance of the corrugated gyrotropic resonator coupled to a waveguide can be switched off by the application of the external magnetic field. In this case, the resonance is not shifted, as what typically occurs at small perturbations, but suppressed.     

渤海近岸水体悬浮颗粒物对后向散射特性的影响研究

进行海洋水体悬浮颗粒物的空间分布及粒径构成对后向散射特性变化影响,研究对于加深水体散射性质的认识、提高水色遥感反演参数的精度及海洋环境监测等具有重要意义。结合渤海海域2017年6月、9月的现场数据,获得了夏季、秋季各站位总悬浮颗粒物浓度(SPM)、颗粒粒径以及后向散射系数等参数并进行研究。结果表明,后向散射系数b_bp在可见光波段随波长的增加而减小;同时,夏季和秋季,大多数站位b_bp(550)的变化趋势与SPM的变化趋势基本一致;在进行两者之间的相关性研究时发现,相关系数R2并不高,仅为0.24。该研究还建立了平均粒径(D_A)、中值粒径(D₅₀)与b_bp的关系,得出D_A与b_bp呈线性关系,受季节及水体颗粒构成的影响,夏季,水体颗粒物以小颗粒为主,两者之间的相关性高于秋季,两个季节的R2分别为0.7和0.3;同时得出当水体以小颗粒为主时,b_bp随D_A增大而增大,以大颗粒为主的时,b_bp随D_A增大而减小。D₅₀与b_bp的关系则不同,两者之间呈现非常好的幂指数关系,在同一季节里,b_bp均随着D₅₀的增大而减小,即水体中颗粒物粒径越小,其后向散射系数越大,夏季和秋季两者之间的相关系数分别为0.66和0.5;并且发现如果不分季节差异,现场水体颗粒物的粒径构成对b_bp的影响很难确定。     

光源偏振对水体颗粒后向散射系数测量的影响

颗粒后向散射系数是水体主要的固有光学特性,也是海洋水色的决定因子和海洋水色卫星遥感反演的基础参数。当前,利用光学仪器现场原位测量是获取水体颗粒后向散射系数的主要方法。由于光源自身和仪器光路中镜面反射和折射,其出射光源可能存在一定的偏振,进而会影响水体后向散射系数测量的精度。目前,关于水体后向散射系数测量仪器的光源偏振性及其对颗粒后散射系数测量精度影响的研究尚为空白。针对该问题,以应用广泛的水体后向散射测量仪HydroScat6(HS-6)为例,对其出射光源的偏振特性进行了系统测量,并进一步开展了光源偏振对水体颗粒后向散射系数测量精度影响的实验研究。结果表明,HS-6六个光学通道除590 nm通道出射光源偏振度略低外(~15%),其他通道出射光源中心波长偏振度均在20%~30%。因此,HS-6出射光源具有显著的偏振特性。光源偏振对颗粒后向散射系数测量具有不可忽略的影响,且影响程度随着波段,线偏振角度及悬浮泥沙浓度而变化。不同悬浮泥沙浓度下,光源偏振引起的420,442,470,510,590和670 nm六个波段的平均偏差可达15.49%,11.27%,12.79%,14.43%,13.76%,12.46%。因此,利用光学仪器现场测量颗粒后向散射系数需要考虑光源偏振的影响,并需尽可能降低出射光源的偏振度。     

激光同步辐射作为阿秒X射线辐射源的特性研究

 研究了逆流相对论电子与激光脉冲相互作用获得激光同步辐射的频率上移、微分散射截面等特性。发现逆流相对论电子与短脉冲激光相互作用，可以获得阿秒X射线辐射脉冲。短脉冲激光条件下得到的后向散射光的频率上移与长脉冲激光条件下得到的后向散射光的频率上移是完全一致的，同时发现随着入射电子初始能量的增加，散射光的准直性越来越好，后向散射光脉冲的脉宽越来越短。     

Contribution of backscattered electrons to the total electron yield produced in collisions of 8–28 keV electrons with tungsten

It is shown experimentally that under energetic electron bombardment the backscattered electrons from solid targets contribute significantly (∼80%) to the observed total electron yield, even for targets of high backscattering coefficients. It is further found that for tungsten (Z = 74) with a backscattering coefficient of about 0.50, about 20% of the total electron yield is contributed by the total secondary electrons for impact energies in the range of 8–28 keV. The yield of true backscattered electrons at normal incidence (η ₀), total secondary electrons (δ) and the total electron yield (δ _tot) produced in collisions of 8–28 keV electrons with W have been measured and compared with predictions of available theories. The present results indicate that the constant-loss of primary electrons in the target plays a significant role in producing the secondary electrons and that it yields a better fit to the experiment compared to the power-law.      

佛克脱线型HeⅠ 1 083.0 nm的有效后向散射截面的理论研究

考虑到碰撞展宽导致的洛伦兹线型,理论上研究了佛克脱线型(高斯线型和洛伦兹线型的卷积)HeⅠ 1 083.0 nm辐射线的后向散射截面和有效后向散射截面,HeⅠ 1 083.0 nm辐射线实际上由三条非常接近的1 083.034,1 083.025和1 082.908 nm辐射线组成,文章给出了此3条辐射线的有效后向散射截面的曲线及其峰值,得到的10-16数量级与预言的数量级相一致。此研究比以前假定HeⅠ 1 083.0 nm辐射线为高斯线型更接近实际,从而为共振雷达探测热成层亚稳态He提供了更全面翔实的理论基础。文章还给出了HeⅠ 388.9 nm和HeI 318.8 nm辐射线的有效后向散射截面的曲线及其峰值,得到它们的有效后向散射截面比HeI 1 083.0 nm辐射线的小,此结论与实验结果相一致。     

高能激光计后向散射能量损失补偿方法研究

针对锥形腔高能激光计后向散射能量损失补偿问题,系统分析了均匀分布激光入射强漫反射面情况下的锥形吸收腔的后向散射问题,进而针对不同高能量激光的输出光斑形状,建立了锥形吸收腔开口处光功率密度分布和后向散射总功率的数学模型,在此基础上对测量结果进行了补偿和修正,有效改善了高能量激光能量测量准确度.