Enhance the resolution of photonic crystal negative refraction imaging by metal grating

The resolution of imaging is limited by the missing of high-frequencies information. The superlens employing negative refraction can compensate for these components. But for the directional coupling of Bloch waves and the low coupling efficiency of large-angle waves, the resolution of subwavelength imaging is not satisfactory. However, the subwavelength metallic grating can produce high-order diffracted waves carrying a lot of high-frequencies information. Therefore, this structure is used to inhibit the zero-order diffraction and enhance the high-order diffraction to achieve super-resolution.     

Near-field terahertz imaging with a dynamic aperture

By introduction of an optical gating beam on a semiconductor wafer, near-field terahertz (THz) imaging with a dynamic aperture has been realized. The spatial resolution is determined by the focus size of the optical gating bean and the near-field diffraction effect. THz imaging with subwavelength spatial resolution (better than 50mum) is demonstrated.     

有孔探针近场拉曼光谱和成像技术进展

基于有孔探针SNOM的近场拉曼光谱和成像技术的出现使得拉曼光谱的分辨率突破了光学衍射极限,从而提供了一个有力的工具对样品亚波长尺度之下的化学信息进行表征。文章讨论了探针性质对实现近场拉曼光谱的影响,并全面地介绍了有孔探针近场拉曼光谱发展十余年来在纳米尺度化学分辨成像、液-液界面性质研究、微观层面解释SERS增强机理、图像化反映SERS热点分布等诸多领域的研究进展。     

Three-dimensional subwavelength imaging by a photonic-crystal flat lens using negative refraction at microwave frequencies

We experimentally demonstrate subwavelength resolution imaging at microwave frequencies by a three-dimensional (3D) photonic-crystal flat lens using full 3D negative refraction. The photonic crystal was fabricated in a layer-by-layer process. A subwavelength pinhole source and a dipole detector were employed for the measurement. By point-by-point scanning, we obtained the image of the pinhole source shown in both amplitude and phase, which demonstrated the imaging mechanism and subwavelength feature size in all three dimensions. An image of two pinhole sources with subwavelength spacing showed two resolved spots, which further verified subwavelength resolution.     

Quantum imaging with incoherent photons

We propose a technique to obtain subwavelength resolution in quantum imaging with potentially 100% contrast using incoherent light. Our method requires neither path-entangled number states nor multiphoton absorption. The scheme makes use of N photons spontaneously emitted by N atoms and registered by N detectors. It is shown that for coincident detection at particular detector positions a resolution of lambda/N can be achieved.     

Dipole‐like backscatter stimulated Raman scattering for in vivo imaging

Stimulated Raman scattering (SRS) scanning microscopy has the potential to enable label‐free in vivo imaging for research and clinical medicine. Volume SRS from focus occurs in the forward scattered direction. Therefore, multiple scattering events are required to direct the light out of the tissue, reducing imaging depth and resolution. Here, a method called Stokes interference SRS (SISRS) is introduced that operates by the addition to the standard pump and stimulated emission probe beams a third beam called the donut beam. The donut is close in wavelength to the probe beam and, after passage through a π phase plate, forms an annular beam in the focal plane with bright nodes above and below focus. The donut beats with the probe beam, and when they destructively interfere with each other, the microscope's 3‐D stimulated emission focal spot is reduced to subwavelength dimensions. A subwavelength focal volume emits a dipole pattern of SRS with forward and backscatter lobes, enabling high‐resolution single‐backscatter imaging from deep within tissues. The reduction of the focal volume also increases the resolution of the scanning image creating imaging beyond the diffraction limit. SISRS imaging may provide in vivo label‐free Raman images comparable with that achieved in stained in vitro tissues in all planes of section. Copyright © 2014 John Wiley & Sons, Ltd.     

Demagnifying high-resolution imaging based on a new system for nanolithography

A novel subdiffraction-limited photolithography design with demagnification and high contrast features has been presented in this paper. Alternating phase shift mask (Alt-PSM) is not only used for shifting phase between adjacent aperture pairs through the phase shifter, but also it enhances the transmission by the interference of diffracted evanescent waves from subwavelength apertures at the surface. Research results show that the combination of Alt-PSM and planar hyperlens can realize subwavelength plane-to-plane imaging beyond the diffraction limit. Furthermore, with a well-proportioned reduction factor of 2, the resolution down to 60 nm can be easily attained from an Alt-PSM with 255 nm period at a working wavelength of 365 nm. Actually finer resolution can be obtained if we properly optimize the parameters of the mask and the planar hyperlens.     

A method for uniform demagnification imaging beyond the diffraction limit: cascaded planar hyperlens

Metamaterial composed of multistacked metal and dielectric films provides the access to ray tracing in subwavelength regions and to form a variety of transformation optical devices for manipulating light beyond the diffraction limit, such as hyperlens and cloak. In this paper, this method is employed to design planar hyperlens for demagnification imaging lithography. Variant ray route configurations are considered and compared for the sake of imaging quality as well as device structure complexity. It is found that specifically designed trajectory route from the object plane to the image plane help to yield imaging devices with uniform demagnification ratio and improved image quality. Then multiple similar imaging devices could be cascaded for further demagnification and reduce of structure complexity from the viewpoint of application. The imaging results with about 1/23 wavelength (16 nm) half-pitch resolution in the measure of electric field intensity are demonstrated with numerical simulations. Also presented are the imaging characteristic analyses including light intensity, demagnification ratio and resolution.     

Near-sighted superlens

We consider the problem of subwavelength imaging via a slab of a left-handed medium (LHM) in the presence of material losses. We derive the analytical expression for the resolution limit of a LHM-based lens and demonstrate that the area of its subwavelength performance is usually limited to the near-field zone.     

Negative refraction and imaging beyond the diffraction limit by a two-dimensional left-handed metamaterial

We report our experimental results on two-dimensional left-handed metamaterials (LHM) exhibiting negative refraction and subwavelength imaging. Transmission and reflection spectra of LHM are studied and a left-handed transmission band is observed at the frequencies where both dielectric permittivity and magnetic permeability are negative. Impedance matching is verified both with the experiments and simulations. The two-dimensional LHM structure is verified to have a negative refractive index. We employed three different methods to observe negative refraction; refraction through prism-shaped LHM, beam shifting method, and phase shift experiments. We further demonstrated subwavelength imaging and resolution using LHM superlenses. The effect of thickness on the resolving power is investigated experimentally.     

Imaging enhancement of a photonic crystal superlens due to a surface mode with a specific dispersion

We study the imaging process for a photonic crystal slab lens with a surface defect by the finite-difference-time-domain (FDTD) method. We demonstrate an odd surface mode with a specific dispersion curve in this system. The dispersion curve has an extreme point, which is corresponding to a slow light. If the working frequency is chosen at this extreme point the subwavelength resolution of image will be enhanced. Moreover, the subwavelength resolution of image is very sensitive to the position of this extreme point in the dispersion diagram. Longer interaction time and better field distribution may give a qualitative physical understanding for the enhancement of imaging quality.     

衍射极限尺度下的亚波长电磁学

作为波的本性之一,衍射是现代物理学的重要研究内容.衍射导致自由空间中波的能量不能被无限小地聚集,从而为成像、光刻、光存储、光波导等技术设定了一个原理性的障碍——衍射极限.对于电磁波和光波而言,尽管通过提高介质的折射率可以压缩衍射效应,但由于自然界中材料的折射率有限,该方法存在很大限制.近年来,随着表面等离子体光学的兴起,表面等离子体在超越传统衍射极限方面的能力和应用前景受到了学术界的关注.本文从亚波长电磁学的角度出发,介绍衍射极限研究的历史,综述了突破衍射极限的理论方法.首先,利用金属介质表面等离子体激元的短波长特性,可将等效波长压缩一个数量级以上,在纳米尺度实现光波的聚焦或定向传输;更进一步,通过人为设计超构材料和超构表面,利用结构化金属和介质中的局域谐振、耦合等特殊电磁响应,可实现亚波长局域相位调制、超宽带色散调控、近完美吸收、光子自旋轨道耦合等,从而突破传统理论的诸多局限,为下一代电磁学和光学功能器件奠定重要基础.     

Super-resolution by annular pupil inserted scattered light fluorescence microscopes

Scattered light fluorescence (SLF) microscopy has been developed to take images through scattering media with subwavelength resolution. Here, we investigate the effect of an annular pupil on SLF microscopes via theory and simulations. As opposite to subwavelength resolution achieved by the conventional SLF microscopes, super-resolution can be achieved by annular pupil inserted scattered light fluorescence (API-SLF) microscopes. In fact, the resolution of an API-SLF microscope is usually relevant to one of its component, the scattering medium. By showing the average resolution of API-SLF microscopes that composed of different scattering media is about 0.4 times of the incident wavelength, we demonstrate that an API-SLF microscope with appropriate scattering medium is able to beat the diffraction limit.     

Spatially shifted beam approach to subwavelength focusing

Although negative-refractive-index metamaterials have successfully achieved subwavelength focusing, image resolution is limited by the presence of losses. In this Letter, a metal transmission screen with subwavelength spaced slots is proposed that focuses the near-field beyond the diffraction limit and, furthermore, is easily scaled from microwave frequencies to the optical regime. An analytical model based on the superposition of shifted-beam patterns is developed that agrees very well with full-wave simulations and is corroborated by experimental results at microwave frequencies.     

Simulation of photodetection using finite-difference time-domain method with application to near-field subwavelength imaging based on nanoscale semiconductor photodetector array

Simulation of detecting photoelectrons using multi-level multi-electron (MLME) finite-difference time-domain (FDTD) method with an application to near-field subwavelength imaging based on semiconductor nanophotodetector (NPD) array is reported. The photocurrents from the photodiode pixels are obtained to explore the resolution of this novel NPD device for subwavelength imaging. One limiting factor of the NPD device is the optical power coupling between adjacent detector pixels. We investigate such power coupling in the presence of absorbing media as well as the spatial distributions of the electric field and photoelectron density using the MLME FDTD simulation. Our results show that the detection resolution is about one tenth of the operating wavelength, which is comparable to that of a near-field scanning optical microscope based on metal clad tapered fiber.     

周期性和孤立的纳米结构的超透镜亚波长成像

超透镜光刻技术是一种很有前景的纳米结构成像技术,由于其具有可以克服衍射极限的能力,直到2005年,张翔和他的同事在365 nm紫外线波长下成功的对一排纳米线和刻在高分子膜上的四个字母NANO实现了超分辨成像,分辨率高达1/6入射波长。通过传递矩阵方法优化出超透镜结构,并通过选择适当的材料和设计在超透镜结构中的每个层的厚度以及合理的优化实验等方法制备一个新的超透镜结构,利用这种超透镜结构实现了周期性纳米结构及孤立纳米结构的亚波长成像。实验结果表明,对于周期性的纳米结构,其图像分辨率达到100 nm,而孤立结构的分辨率低于50 nm,小于入射波长的1/7。     

周期性和孤立的纳米结构的超透镜亚波长成像

超透镜光刻技术是一种很有前景的纳米结构成像技术,由于其具有可以克服衍射极限的能力,直到2005年,张翔和他的同事在365 nm紫外线波长下成功的对一排纳米线和刻在高分子膜上的四个字母NANO实现了超分辨成像,分辨率高达1/6入射波长。通过传递矩阵方法优化出超透镜结构,并通过选择适当的材料和设计在超透镜结构中的每个层的厚度以及合理的优化实验等方法制备一个新的超透镜结构,利用这种超透镜结构实现了周期性纳米结构及孤立纳米结构的亚波长成像。实验结果表明,对于周期性的纳米结构,其图像分辨率达到100 nm,而孤立结构的分辨率低于50 nm,小于入射波长的1/7。     

Near-field time-reversal amplification

The spatial resolution of the focused field of a classical time-reversal mirror has a wavelength-order lambda diffraction limit. Previously reported results for subwavelength focus require either the full knowledge of the original source or the evanescent waves in the near field. Here it is shown that subwavelength focusing can be achieved without a priori knowledge of the original probe source. If the field is recorded at a few wavelengths away from the probe source, where the amplitude of the near field is too low for subwavelength focusing, it is shown that the low amplitude near field can be amplified and the spatial resolution improved, using the near-field time reversal (NTR) procedure introduced here. The NTR is performed from the phase of the spatial spectrum of the field recorded on an array around the original probe source using an analytical continuation for the amplitude of the spatial spectrum. Following theory, lambda/20 resolution is experimentally demonstrated with audible acoustic wavefields in the air.     

聚焦径向调制Bessel-Gaussian光束实现亚波长尺寸纵向偏振光束

亚波长尺寸纵向偏振光束在粒子加速,单个分子偶极矩测量,二次谐波成像和纵向偏振共焦显微镜等方面有非常广泛的应用.本文提出径向调制的Bessel-Gaussian光束模型,经高数值孔径显微物镜聚焦可以获得高纯度的纵向偏振光束.利用矢量衍射理论数值模拟了焦点附近光强分布、磁场分布和能流密度空间分布.结果表明光斑半高全宽度可以达到0.4λ.该方法无需环形孔径或环形相位滤波器,光能利用率高,分辨率好,达到改进各种应用的效果. 关键词： Bessel-Gaussian光束 纵向偏振光束 聚焦 亚波长     

Subwavelength imaging enhancement through a three-dimensional plasmon superlens with rough surface

In this Letter we investigate the subwavelength imaging of a three-dimensional plasmon superlens based on the full vector wave simulations of optical wave propagation and transmission. The optical transfer functions are computed. Comparisons are made between the results of lenses with flat and periodic/random rough surfaces. We also study the problem of practical imaging system geometry using laser as an illumination source. Results show that the lens with periodic or random roughness can reduce the field interference effects, and provide improved focus on the transmission field and the Poynting flux. We illustrate that the subwavelength roughness in a plasmon lens can enhance the image resolution over a flat lens for both matched and unmatched permittivity conditions. The enhancement of resolution occurs because the introduced subwavelength roughness can amplify the evanescent wave components and suppress the surface plasmon resonance peaks.