A novel phase-sensitive scanning near-field optical microscope

Phase is one of the most important parameters of electromagnetic waves. It is the phase distribution that determines the propagation, reflection, refraction, focusing, divergence, and coupling features of light, and further affects the intensity distribution. In recent years, the designs of surface plasmon polariton(SPP) devices have mostly been based on the phase modulation and manipulation. Here we demonstrate a phase sensitive multi-parameter heterodyne scanning near-field optical microscope(SNOM) with an aperture probe in the visible range, with which the near field optical phase and amplitude distributions can be simultaneously obtained. A novel architecture combining a spatial optical path and a fiber optical path is employed for stability and flexibility. Two kinds of typical nano-photonic devices are tested with the system. With the phase-sensitive SNOM, the phase and amplitude distributions of any nano-optical field and localized field generated with any SPP nano-structures and irregular phase modulation surfaces can be investigated. The phase distribution and the interference pattern will help us to gain a better understanding of how light interacts with SPP structures and how SPP waves generate, localize, convert, and propagate on an SPP surface. This will be a significant guidance on SPP nano-structure design and optimization.     

Image edge-enhancement in optical microscopy with a phase mismatched spiral phase plate

We present a spiral phase filtering system with a large tolerance for edge enhancement of both phase and amplitude objects in optical microscopy.The method is based on a Fourier 4-f spatial filtering system.A phase mismatched spiral phase plate (SPP) fabricated by electron beam lithography is employed as the radial Hilbert transform for image edge enhancement.Compared with holography,SPP is simple,economical,reliable,and easy to integrate.     

Spin-controlled directional launching of surface plasmons at the subwavelength scale

In this paper, we demonstrate a spin-controlled directional launching of surface plasmons at the subwavelength scale.Based on the principle of optical spin's effect for the geometric phase of light, the nanostructures were designed. The inclination of the structures decides the spin-related geometric phase and their relative positions decide the distance-related phase. Hence, the propagation direction of the generated surface plasmon polaritons(SPPs) can be controlled by the spin of photons. Numerical simulations by the finite difference time domain(FDTD) method have verified our theoretical prediction. Our structure is fabricated on the Au film by using a focused ion beam etching technique. The total size of the surface plasmon polariton(SPP) launcher is 320 nm by 180 nm. The observation of the SPP launching by using scanning near-field optical microscopy is in agreement with our theory and simulations. This result may provide a new way of spin-controlled directional launching of SPP.     

纳米集成光路中的光源、光波导和光增强

使用近场光学显微术(scanning near-field optical microscopy, SNOM)研究了ZnO亚微米线端面出射性质，不同空间形貌Ⅱ-Ⅵ族半导体荧光器件光波导特性，二维光子晶体、准晶光子晶体对LED的出射增强作用以及表面等离激元(surface plasmon polariton, SPP)与半导体纳米荧光器件的相互作用，对纳米集成光路中的光源、光波导、光增强三个重要问题做了实验和理论上的分析.研究发现半导体微纳米线端面出射光束的质量与样品的直径有密切关系.通过合理地设计其直径和     

纳米集成光路中的光源、光波导和光增强

使用近场光学显微术(scanning near-field optical microscopy, SNOM)研究了ZnO亚微米线端面出射性质,不同空间形貌Ⅱ-Ⅵ族半导体荧光器件光波导特性,二维光子晶体、准晶光子晶体对LED的出射增强作用以及表面等离激元(surface plasmon polariton, SPP)与半导体纳米荧光器件的相互作用,对纳米集成光路中的光源、光波导、光增强三个重要问题做了实验和理论上的分析.研究发现半导体微纳米线端面出射光束的质量与样品的直径有密切关系.通过合理地设计其直径和 关键词： 纳米集成光路 扫描近场光学显微术 光波导 光增强     

Cherenkov terahertz radiation from Dirac semimetals surface plasmon polaritons excited by an electron beam

We demonstrate a physical mechanism for terahertz(THz) generation from surface plasmon polaritons(SPPs). In a structure with a bulk Dirac semimetals(BDSs) film deposited on a dielectric substrate, the energy of the asymmetric SPP mode can be significantly enhanced to cross the light line of the substrate due to the SPP-coupling between the interfaces of the film. Therefore, the SPPs can be immediately transformed into Cherenkov radiation without removing the wavevector mismatch. Additionally, the symmetric SPP mode can also be dramatically lifted to cross the substrate light line when a buffer layer with low permittivity relative to the substrate is introduced. In this case, dual-frequency THz radiation from the two SPP modes can be generated simultaneously. The radiation intensity is significantly enhanced by over two orders due to the field enhancement of the SPPs. The radiation frequency can be tuned in the THz frequency regime by adjusting the beam energy and the chemical potential of the BDSs. Our results could find potential applications in developing room temperature, tunable, coherent, and intense THz radiation sources to cover the entire THz band.     

What spatial light modulators can do for optical microscopy

With the availability of high‐resolution miniature spatial light modulators (SLMs) new methods in optical microscopy have become feasible. The SLMs discussed in this review consist of miniature liquid crystal displays with micron‐sized pixels that can modulate the phase and/or amplitude of an optical wavefront. In microscopy they can be used to control and shape the sample illumination, or they can act as spatial Fourier filters in the imaging path. Some of these applications are reviewed in this article. One of them, called spiral phase contrast, generates isotropic edge enhancement of thin phase samples or spiral‐shaped interference fringes for thicker phase samples, which can be used to reconstruct the phase topography from a single on‐axis interferogram. If SLMs are used for both illumination control and spatial Fourier filtering, this combination for instance allows for the generalization of the Zernike phase contrast principle. The new SLM‐based approach improves the effective resolution and avoids some shortcomings and artifacts of the traditional method. The main advantage of SLMs in microscopy is their flexibility, as one can realize various operation modes in the same setup, without the need for changing any hardware components, simply by electronically switching the phase pattern displayed on the SLMs.     

Anisotropic edge enhancement in optical scanning holography with spiral phase filtering

Anisotropic edge enhancement is simulated using a spiral phase plate(SPP) in optical scanning holography(OSH). We propose to use a delta function and an SPP as the pupil functions to realize anisotropic edge enhancement. The interference of these two pupils is used to two-dimensionally scan an object to record its edge-only information. This is done in three ways: first, by shifting the SPP, second, by using two offset SPPs of same charge, and finally, by using two oppositely charged SPPs. Our computer simulations show the capability of selectively enhancing the edges of a given object.     

Characterization of polarization states of surface plasmon polariton modes by Fourier-plane leakage microscopy

We demonstrate that the polarization states of guided wave surface plasmon polariton (SPP) modes can be unambiguously identified by introducing a linear polarizer in the optical path of the light within a leakage-based microscope. We show the use of Fourier-plane leakage-based microscopy as a polarization characterization method to study the polarization states of SPP modes excited in plasmonic waveguides. Our results indicate that the inclusion of a linear polarizer provides additional image processing capabilities to leakage-based microscopes.     

Surface-plasmon-polaritons-assisted nanolithography with dual-wavelength illumination for high exposure depth

We propose a new direct writing nanolithography approach using a plasmonic focusing device and a nano silver mirror with dual-wavelength illumination for high exposure depth. Arrays of pyramid aperture are used to focus the incident light beams into 80 nm light spots. The pyramid combined with a thin silver film coated on the substrate constructs a surface plasmon polaritons (SPP) coupling cavity, which amplifies the intensity of the light field in it by SPP effect and resonance. The transmission depth of the standing wave formed by forward and reflected light could reach hundreds of nanometers. Two lasers with different wavelengths are used as illumination sources to homogenize the light field through complementation between the two standing waves. Simulation results show by using 355 nm and 441 nm wavelengths, a space of 44 nm at the bottom of the photoresist could be obtained after exposure and development. The feature size of resist patterns could be further scaled down, depending on the optimization of parameters of photoresist exposure and development, illumination wavelengths, etc.     

Dynamic generation of plasmonic Moiré fringes using phase-engineered optical vortex beam

Dynamic generation of plasmonic Moiré fringes using a phase engineered optical vortex (OV) beam is experimentally demonstrated. Owing to the unique helical phase carried by an OV beam, the initial phase of surface plasmon polaritons (SPPs) emanating from a metallic grating can be adjusted dynamically by changing the phase hologram displayed on a spatial light modulator. Plasmonic Moiré fringes are readily achieved by overlapping two SPP standing waves with certain angular misalignment, excited by the positive and negative topological charge components, respectively, of a cogwheel-like OV beam. The near-field scanning optical microscopy measurement result of SPP distributions has shown a good agreement with the numerical predictions.     

激光直写制作高阶螺旋相位板及其性能

将拓扑荷为4的螺旋相位光束与平面光干涉的计算全息图输入到空间光调制器中,得到含有多个衍射级次的高阶涡旋光束。为提高衍射效率,利用激光直写技术制作拓扑荷为4的高阶螺旋相位板,经测定,相位板深度理论数值为1.073 m,测量数值为1.082 m,相位板制作误差在0.83%以内。平行光束通过此相位板时,在夫琅和费衍射场获得一个高质量的高阶光学涡旋,光强分布与理论数值基本吻合,衍射效率达到86%。     

Generation of optical vortices with tunable intensity modulation

We analyze vortex properties of the optical beams generated by a spiral phase plate (SPP) which cannot modulate the phase of the incident beam range from 0 to 2π nicely, and find that the vortices have intensity modulation (IM) with central bright speckle. We construct an improved SPP to produce high quality optical vortices with definite IM. Theoretical analysis and real experiments show that this improved SPP can be used to produce optical vortices with configurable intensity modulation degree and without central bright spot.     

Dual-channel phase-shifting interferometry for microscopy with second wavelength assistance

Dual-channel phase-shifting interferometry for simultaneous phase microscopy is presented. Red and blue light beams are used for microscope illumination. A 45 tilted beamsplitter replicates the object and reference waves in red light together with the object wave in blue light into two parallel beams. The two resulting quadrature phase-shifting interferograms in red light and the object waves in blue light are generated in the two channels. The two interferograms are recorded simultaneously by a color charge-coupled device (CCD) camera, and can be separated via RGB components of the recorded color patterns without crosstalk. As a result, the phase of tested specimen can be retrieved. The feasibility of the proposed method is demonstrated by test performed on a microscopic specimen.     

基于双表面等离子激元吸收的纳米光刻

光刻技术(lithography)是微纳结构制备的关键技术之一.受限于光的衍射极限,传统光刻方法进一步缩小特征尺寸变得越来越难.表面等离子激元(surface plasmon polariton,SPP)作为光与金属表面自由电子密度振荡相互耦合形成的一种特殊电磁形式,具有波长短、场密度大、异常色散等特点,在突破传统光学衍射极限的研究和应用中具有重要的学术和实用价值.本文针对SPP在光刻胶中的非线性吸收及其在大视场纳米光刻中的应用进行了理论和实验探索.在回顾SPP概念的基础上,阐述了双SPP吸收的概念及其应用于纳米光刻的优势,明确了该效应具有与传统双光子吸收不同的内涵和特性.在800和400nm飞秒激光的作用下,实现了基于双SPP吸收效应的周期干涉条纹,同时验证了双SPP吸收的闽值效应,通过控制曝光计量实现了图形线宽的调控,最小线宽小于真空光波长的1/10.利用SPP波长短、场增强的特点,并结合非线性吸收的闽值效应,单次曝光区域比纳米图形尺度大4 5个数量级,曝光区域的直径可达1.6 mm.同时制备出较为复杂的同心圆环结构.基于双SPP吸收独有的特性以及SPP丰富的模式,有望进一步在大光刻视场、超小尺度图形光刻技术上获得突破.     

Dual mode near-field scanning optical microscopy for near-field imaging of surface plasmon polariton

In this paper, we theoretically and experimentally demonstrate that metal coated apertured probes are efficient near-field probes on surfaces with high reflectivity for the scattering as well as for the collection mode near-field scanning optical microscopy (NSOM). We show that a blunt apertured metal coated tip is very effective in suppressing image dipoles which affect strongly the signals scattered from frequently used sharp metal tips or gold nanoparticle attached probes. By using a simultaneous collection and scattering mode (dual mode) NSOM we measure the near-field images of surface plasmon polariton (SPP) launched from a slit. The collection mode measures propagating SPP along lateral distance in a long scan range with high signal-to-noise ratio, and the scattering mode measures the polarization resolved near-field of SPP. Comparisons of the measured data obtained in the dual mode enable to easily characterize SPP and to separate the measured near-field into the propagating SPP and the directly transmitted light.     

A general phase retrieval algorithm based on a ptychographical iterative engine for coherent diffractive imaging

Coherent diffractive imaging (CDI) is a lensless imaging technique and can achieve a resolution beyond the Rayleigh or Abbe limit. The ptychographical iterative engine (PIE) is a CDI phase retrieval algorithm that uses multiple diffraction patterns obtained through the scan of a localized illumination on the specimen, which has been demonstrated successfully at optical and X-ray wavelengths. In this paper, a general PIE algorithm (gPIE) is presented and demonstrated with an He-Ne laser light diffraction dataset. This algorithm not only permits the removal of the accurate model of the illumination function in PIE, but also provides improved convergence speed and retrieval quality.     

A general phase retrieval algorithm based on ptychographicaliterative engine for coherent diffractive imaging

Coherent diffractive imaging (CDI) is a lensless imaging technique and can achieve a resolution beyond the Rayleigh or Abbe limit. The ptychographical iterative engine (PIE) is a CDI phase retrieval algorithm that uses multiple diffraction patterns obtained through the scan of a localized illumination on the specimen, which has been demonstrated successfully at optical and X-ray wavelengths. In this paper, a general PIE algorithm (gPIE) is presented and demonstrated with an He-Ne laser light diffraction dataset. This algorithm not only permits the removal of the accurate model of the illumination function in PIE, but also provides improved convergence speed and retrieval quality.     

基于偏振滤波图像增强和动态散斑照明的宽场荧光层析显微镜

提出了一种基于偏振滤波图像增强和动态散斑照明的新型宽场荧光层析显微镜. 该显微镜采用了一种新型的偏振滤波图像增强技术, 基于激发光与荧光偏振态的差异, 利用偏振器件滤除激发光; 并利用动态散斑照明实现宽场层析. 该荧光层析显微镜具有结构简单、低成本、响应速度快、容易操作等特点. 实验研究结果表明, 本文提出的滤波方案能够显著地提高图像质量, 利用动态散斑照明实现宽场层析具有较高的纵向分辨能力. 研究丰富了在荧光显微镜中, 从强激发光中提取弱荧光信号的技术手段, 为今后发展具有快速响应, 波长可调谐的多光谱荧光层析等高端的显微镜具有重要参考意义.     

Mode characteristics of silver-coated inverted-wedge silica microdisks

The characteristics of whispering gallery modes(WGM) in silver-coated inverted-wedge silica microdisks are theoretically investigated by using finite element method. Dielectric TE mode always exists in silver-coated inverted-wedge resonators; dielectric TM mode tends to couple with SPP modes; only pure interior surface plasmonic polariton(SPP) mode but not pure exterior SPP mode is observed in contrast to the metal-coated cylindrical and toroidal resonators. The dependence of quality factor of different kinds of WGMs on the radius of the resonator and the thickness of the coated silver layer are systematically analyzed. We find that the quality factors of the hybrid WGMs associated with SPP mode can reach 104. The maximum light intensity enhancement in ambient for a hybrid mode consisting of a dielectric TM mode and an exterior SPP mode can be obtained when a silver film of thickness ~40 nm is deposited. The silver-coated inverted-wedge silica resonators may be widely applied in sensing and surface enhanced Raman scattering.