位相分辨偏振灵敏光学相干层析术:组织双折射特性的成像与定量分析

我们研制了一种基于光纤的位相分辨偏振灵敏光学相干层析成像系统。该系统中的偏振状态控制设量在参考臂而非光源臂上,因而使得光抵达样品的传输效率大大提高。鉴于光源的部分偏振性,入射于样品上的光含有任意偏振状态的分量,通过对参考光偏振状态的调制,就可相干地提取对应于入射光四种正交偏振状态并经样品后向散射的光信号。基于斯托克斯矢量夹角在无损光纤系统传输的变换不变性,我们能利用测量臂中光信号的斯托克斯参数来确定双折射样品深度分辨的位相延迟信息。利用所研制的偏振灵敏光学相干层析成像系统,不仅确认了韧带和软骨的双折射性质,而且定量分析了不同条件下韧带的双折射变化．研究结果表明：韧带松弛可使其双折射特性明显减弱,而韧带经拉伸后,其双折射特性的变化却不明显。     

Experimental and theoretical investigations on the validity of the geometrical optics model for calculating the stability of optical traps.

We have developed a computer program based on the geometrical optics approach proposed by Roosen to calculate the forces on dielectric spheres in focused laser beams. We have explicitly taken into account the polarization of the laser light and thd divergence of the laser beam. The model can be used to evaluate the stability of optical traps in a variety of different optical configurations. Our calculations explain the experimental observation by Ashkin that a stable single-beam optical trap, without the help of the gravitation force, can be obtained with a strongly divergent laser beam. Our calculations also predict a different trap stability in the directions orthogonal and parallel to the polarization direction of the incident light. Different experimental methods were used to test the predictions of the model for the gravity trap. A new method for measuring the radiation force along the beam axis in both the stable and instable regions is presented. Measurements of the radiation force on polystyrene spheres with diameters of 7.5 and 32 microns in a TEM00-mode laser beam showed a good qualitative correlation with the predictions and a slight quantitative difference. The validity of the geometrical approximations involved in the model will be discussed for spheres of different sizes and refractive indices.     

Plasmonic Lens with Multiple-Turn Spiral Nano-Structures

In this paper, we investigate the focusing properties of a plasmonic lens with multiple-turn spiral nano-structures, and analyze its field enhancement effect based on the phase matching theory and finite-difference time-domain simulation. The simulation result demonstrates that a left-hand spiral plasmonic lens can concentrate an incident right-hand circular polarization light into a focal spot with a high focal depth. The intensity of the focal spot could be controlled by altering the number of turns, the radius and the width of the spiral slot. And the focal spot is smaller and has a higher intensity compared to the incident linearly polarized light. This design can also eliminate the requirement of centering the incident beam to the plasmonic lens, making it possible to be used in plasmonic lens array, optical data storage, detection, and other applications.     

Plasmon-Enhanced Second Harmonic Generation: from Individual Antennas to Extended Arrays

We analyze the emission yield of the second harmonic generation (SHG) from dense ordered arrays of L-shaped Au nanoantennas within a well-defined collection angle and compare it to that of the isolated nanostructures designed with the same geometrical parameters. Thanks to the high antenna surface density, arrays display one order of magnitude higher SHG yield per unit surface with respect to isolated nanoantennas. The difference in the collected nonlinear signals becomes even more pronounced by reducing the collection angle, because of the efficient angular filtering that can be attained in dense arrays around the zero order. Albeit this key-enabling feature allows envisioning application of these platforms to nonlinear sensing, a normalization of the SHG yield to the number of excited antennas in the array reveals a reduced nonlinear emission from each individual antenna element. We explain this potential drawback in terms of resonance broadening, commonly observed in densely packed arrays, and angular filtering of the single antenna emission pattern provided by the array 0th order.

     

Metalens Focusing the Co-/cross-polarized Lights in Longitudinal Direction

Recently, metasurface has attracted lots of attentions because of its great capability in phase engineering for the transmitted cross-polarization light, and many functional optical elements have been designed and investigated. Commonly, the co-polarization and cross-polarization lights will coexist in the transmitted fields. Here, we propose a planar metalens composed of L-shaped nanoholes, which can focus an incident plane wave to two different focal spots in longitudinal direction for the co-polarized and cross-polarized transmitted lights respectively. In our design, the focal length of the transmitted cross-polarized lights can be tuned easily according to Fermat principle. Meanwhile, the focal length of the co-polarized transmitted lights can also be modulated by the ring number of the designed metalens. Because of the polarization-independent characteristic of the L-shaped nanoantenna, the designed planar metalens can also be suitable for both linear and circular polarized incident lights.     

Polarization Management of Terahertz Extraordinary Optical Transmission through Ultracompact L-Shaped Subwavelength Patterns on Metal Films

Compact and efficient terahertz (THz) polarization conversion components are of importance for applications where the small dimension of the laser device/system is critical. Here, we propose an ultracompact L-shaped subwavelength patterns on metal films to realize the THz polarization management. By optimizing the geometric parameters of single-layered and double-layered patterns, the linear-polarized THz incidence can be converted to elliptical polarized output or rotated by 90° efficiently due to the THz extraordinary optical transmission phenomenon. The physical mechanism is explored by mode analysis using numerical and analytical modeling.     

Efficient Broadband Transmissive 90° Polarization Rotation Using a Single Layer of L-shaped Particles Inside a Glass Cube

We realize 90° polarization rotation with both high polarization conversion efficiency and broad bandwidth by using a single layer of L-shaped particles inside a glass cube. The simulation results show that Fabry-Perot resonance effect enhances the transmission for both co-polarization and cross-polarization light in the L-shaped layer. And the co-polarization electric field component has been suppressed in the far field because of the destructive interference. The simulation results also show that the spectral band of peak polarization rotation can be shifted by changing the size parameters. This broadband polarization rotation mechanism may be very useful in designing polarization rotators.     

Polarization Dependent of Plasmonic Lenses with Variant Periods on Superfocusing

A plasmonic lens with variant periods was investigated for optical behavior at near-field by means of numerical computational method. To study influence of incident light on different polarization modes, we considered linear polarization, circular polarization, elliptical polarization, radial polarization (RP), and azimuthally polarization in our computational analyses. A finite difference and time domain algorithm is employed in the numerical study. Our computational numerical calculation results demonstrate that focusing performance for the plasmonic lens illuminated under radial polarization is best in comparison to that of the illumination with the other four polarization states. The plasmonic lens with RP illumination can realize superfocusing with ultra-long depth of focus. It is possible to be used as an optical probe or a type of plasmonic lens for imaging with high resolution in the near future.     

Characteristics of Plasmonic at a Metal/Chiral Sculptured Thin Film Interface

The propagation of surface plasmon polariton at an interface of metallic thin film and chiral sculptured thin film theoretically has been investigated using the transfer matrix method in the Kretschman configuration. The optical absorption of structure as a function of polar incident angle for linear polarization P and S has been calculated at different structural parameters. The results show that exist multiple plasmon peaks for P polarization, while there are the weak plasmon peaks when incident of light is S-polarized plane wave.     

Design Optimisation of Plasmonic Metasurfaces for Mid-Infrared High-Sensitivity Chemical Sensing

In this paper, we report on a general method to optimise the optical characteristics of 2D-arrays of plasmonic gold nanoantennas performing as band-pass filter functionalised metasurfaces to be used as high-sensitivity mid-infrared spectroscopic sensors. We demonstrate that it is possible to increase their sensitivity in the detection of chemical and biological substances when the sensors are used in the surface-enhanced infrared absorption (SEIRA) technique. This technique allows revealing the presence of a substance adsorbed on the nanoantennas by measuring its optical absorption under the conditions for which the maximum value of the functionalised metasurface reflectivity occurs at the same wavelength of the substance maximum absorption peak. In particular, numerical simulations based on finite element method of the metasurface detection response demonstrate the possibility to increase the sensor sensitivity of more than four orders of magnitude with respect to that one achievable if the same amount of the substance is deposited on an unstructured planar metal surface. These results can be obtained by acting on the 2D-array periodicity, nanoantenna shape (i.e. rod and cross), size and thickness independently from the wavelength at which the substance absorption occurs. Moreover, in the case of cross-shaped nanoantennas, we report a complete numerical characterisation of the dependence of the metasurface maximum reflectivity and peak wavelength on the variation of the geometrical parameters of both the nanoantennas and the 2D-array.     

Determination of the orientation distribution of adsorbed fluorophores using TIRF. I. Theory.

The spectroscopic technique total internal reflection fluorescence can be used for determination of the orientation of adsorbed fluorescent molecules. The underlying theory is presented in general terms and elaborated in detail for the case that the fluorescent group is a porphyrin ring. It is shown that order parameters of the orientation distribution can be obtained if both the fluorescence intensity and its polarization are measured as functions of the polarization of the incident laser beam. From these order parameters an approximation of the orientation distribution can be derived by the maximum-entropy method.     

Polarization Splitter with Optical Bistability in Metal Gap Waveguide Nanocavities

A polarization splitter is proposed and numerically investigated. It is composed of two same structures with different arrangement, which is a kind of metal-dielectric nanocavity filling a piece of nonlinear optical material into metal gap waveguides for each. This device with optical bistability based on surface plasmon provides a new way to manipulate light by tuning the incident light intensity and will be essential for the coming optical information processes.     

双光子激发荧光各向异性度的成像

荧光各向异性度 (fluorescence anisotropy) 测量可以获得荧光分子的转动速度信息,进而了解分子质量、结构、以及与周边环境的相互作用情况 . 围绕一台双光子激发扫描荧光成像系统,通过改变外光路和图像记录与处理程序,从而实现了双光子激发荧光各向异性度成像,并针对一些典型样品和体系,展示了该方法的应用 . 实验中观察了 FITC 荧光分子、 FITC 结合的 CD44 抗体分子及与肿瘤细胞表面受体结合的 FITC-CD44 抗体分子 . 测量结果表明,不同分子质量、不同微观环境状态下的荧光分子,其各向异性度大小不同,在各向异性度图中能够被明显区分 . 荧光各向异性度成像能够定量测量样品微区的各向异性度值,并以二维图像的形式直观表达,是各向异性度测量与成像技术的良好结合 .     

Enhancing the Strength of an Optical Trap by Truncation

Optical traps (tweezers) are beginning to be used with increasing efficacy in diverse studies in the biological and biomedical sciences. We report here results of a systematic study aimed at enhancing the efficiency with which dielectric (transparent) materials can be optically trapped. Specifically, we investigate how truncation of the incident laser beam affects the strength of an optical trap in the presence of a circular aperture. Apertures of various sizes have been used by us to alter the beam radius, thereby changing the effective numerical aperture and intensity profile. We observe significant enhancement of the radial and axial trap stiffness when an aperture is used to truncate the beam compared to when no aperture was used, keeping incident laser power constant. Enhancement in trap stiffness persists even when the beam intensity profile is modulated. The possibility of applying truncation to multiple traps is explored; to this end a wire mesh is utilized to produce multiple trapping that also alters the effective numerical aperture. The use of a mesh leads to reduction in trap stiffness compared to the case when no wire mesh is used. Our findings lead to a simple-to-implement and inexpensive method of significantly enhancing optical trapping efficiency under a wide range of circumstances.     

Efficient Surface Plasmon Polariton Excitation with the Beam Generated by Micro Triangular Prism

The optical beam generated by a micro triangular prism is presented to excite surface plasmon polaritons (SPPs) on a single silver nano slit. The electromagnetic fields generated by the micro triangular prism and the excited surface plasmon polaritons are simulated with finite-difference time-domain method. Compared with directly normal incident beam, the efficiency of SPPs’ excitation with the beam generated by the micro triangular prism is highly improved.     

Using Neutron Spin Echo Resolved Grazing Incidence Scattering to Investigate Organic Solar Cell Materials

The spin echo resolved grazing incidence scattering (SERGIS) technique has been used to probe the length-scales associated with irregularly shaped crystallites. Neutrons are passed through two well defined regions of magnetic field; one before and one after the sample. The two magnetic field regions have opposite polarity and are tuned such that neutrons travelling through both regions, without being perturbed, will undergo the same number of precessions in opposing directions. In this case the neutron precession in the second arm is said to "echo" the first, and the original polarization of the beam is preserved. If the neutron interacts with a sample and scatters elastically the path through the second arm is not the same as the first and the original polarization is not recovered. Depolarization of the neutron beam is a highly sensitive probe at very small angles (<50 μrad) but still allows a high intensity, divergent beam to be used. The decrease in polarization of the beam reflected from the sample as compared to that from the reference sample can be directly related to structure within the sample.In comparison to scattering observed in neutron reflection measurements the SERGIS signals are often weak and are unlikely to be observed if the in-plane structures within the sample under investigation are dilute, disordered, small in size and polydisperse or the neutron scattering contrast is low. Therefore, good results will most likely be obtained using the SERGIS technique if the sample being measured consist of thin films on a flat substrate and contain scattering features that contains a high density of moderately sized features (30 nm to 5 µm) which scatter neutrons strongly or the features are arranged on a lattice. An advantage of the SERGIS technique is that it can probe structures in the plane of the sample.     

基于旋转偏振角的线偏振扫描成像方法研究

提出一种基于旋转偏振角的新的偏振光成像方法:改变入射线偏振光偏振角和检偏角,采集样品图像系列,总结出背向散射光2个正交偏振分量的光强差关于入射线偏振光偏振角和检偏角的函数关系式.通过对模拟散射介质,猪肉脂肪,猪肉骨骼肌和牛肉骨骼肌等样品进行实验,论证了偏振差函数式中各个参数与样品光学特性之间的联系,并从中提取出2个相互独立的参数,分别表征样品的纤维方向角和光学各向异性度,从而实现对样品浅表层光学特性进行定量测量.和普通光强图像相比,用这些独立的光学信息生成不同基色的图像,能更直观明了地区分组织结构差异,具有潜在的临床医学应用价值,如成为一种皮肤疾病、皮肤损伤的检测方法.     

Observations on charging effects of non-conductive and uncoated materials by ion beam pre-bombardment in scanning electron microscopy

The specimen charging defects of non-conductive materials in scanning electron microscopy are discussed with reference to the surface electric field generated by the illuminating electron beam dose. If the charge density depends on the relaxation time constant as defined by a product of the permittivity and resistivity when known or available, the electric field can be evaluated by the incident dose stored when illuminated by an electron scanning beam.It was found by observation that uncoated or non-conductive materials pre-bombarded by a positive ion beam, which contributes to the generated negative field, together with the charging effects, could be eliminated at the optimum time of neutralization.In the normal process of double fixation and staining of biological specimens, the local electric field produces increased contrast due to polarization effects. The dark and bright images of secondary and backscattered electrons, respectively, can be analysed by taking into account local polarization, in addition to voltage contrast.     

Identification of cell asymmetry and orientation by light scattering.

Light scattering from chicken red blood cells has been used as a model system to identify the asymmetry of cells. The histogram for forward angle light scattering for these cells is bimodal, the signal size being dependent on the cell orientation. A dual orthogonal scatter system is used to conclusively demonstrate this orientational variation in signal. A third scattering system, using a single incident beam with two orthogonal detectors, is used to further characterize the orientational variation of the scatter signal. In this third system it is shown that the signal in a detector set 90 degrees from the incident beam collects light reflected from the cell surface. The optical selection of cells in specific orientations using these systems may circumvent the need to physically orient cell in flow systems.     

Numerical Investigation on Elliptic Cylindrical Nanowire Hybrid Plasmonic Waveguide–Based Polarization Beam Splitter

A novel design of elliptic cylindrical nanowire hybrid plasmonic waveguide (ECNHPW)–based polarization beam splitter (PBS) is proposed. In the proposed design, the ECNHPW arm acts as an input port and a bar port; on the other hand, a regular silicon wire (RSW) arm acts as a cross port. By selecting the physical parameters of the proposed PBS accurately, the transverse electric (TE) mode is merely satisfied with the phase-matching condition. In contrast, the transverse magnetic (TM) mode does not propagate to the RSW arm. Consequently, the TM input mode goes directly to the ECNHPW arm, while the TE input mode in ECNHPW is coupled with RSW arm. As a result, the two different polarization modes are meritoriously separated, and they pass through two different arms. For the proposed PBS, the insertion loss (IL) of both polarizations lies below 1 dB. For TE input, the value of the polarization extinction ratio (PER) is 27.2 dB, and for TM input, it is 23.9 dB at 1550 nm operating wavelength. Further optimization is implemented by varying the wavelength, thickness of SiO₂, and the gap between the waveguides using the finite element method (FEM). The proposed PBS is designed with 150 nm bandwidth, high PER, and low IL, which can be suitable for photonic integrated circuits (PICs).