Far-field optical superlens

Far-field optical lens resolution is fundamentally limited by diffraction, which typically is about half of the wavelength. This is due to the evanescent waves carrying small scale information from an object that fades away in the far field. A recently proposed superlens theory offers a new approach by surface excitation at the negative index medium. We introduce a far-field optical superlens (FSL) that is capable of imaging beyond the diffraction limit. The FSL significantly enhances the evanescent waves of an object and converts them into propagating waves that are measured in the far field. We show that a FSL can image a subwavelength object consisting of two 50 nm wide lines separated by 70 nm working at 377 nm wavelength. The optical FSL promises new potential for nanoscale imaging and lithography.     

Radiation from a current sheet at the interface between a conventional medium and anisotropic negative refractive medium

In this paper we investigate the radiation from a current sheet at the interface between semiinfinite isotropic positive refractive medium and anisotropic negative refractive medium. The distribution of the electric and magnetic fields in two regions and Poynting vectors associated with propagating and evanescent waves are calculated. The reasons for the singularity of the electric or magnetic field are briefly provided if the waves are evanescent in two media.     

Subwavelength imaging by a graded-index photonic-crystal flat lens in a honeycomb lattice

The left-handed behavior of a photonic-crystal flat lens with a graded index in a honeycomb lattice is proposed and theoretically studied. The performance of the flat superlens imaging of this structure has been demonstrated by finite-difference time-domain simulations. The full width at half-maximum of the image decreases to 62% compared to that of the image of a photonic-crystal slab without a graded index. The evanescent waves can be amplified and propagate to the far-field range. The image is not limited to be near the interface. The canalization effect of this structure is analyzed, and the tolerance of the edge cut of the graded-index structure is pretty good.     

Optical negative refraction by four-wave mixing in thin metallic nanostructures

The law of refraction first derived by Snellius and later introduced as the Huygens-Fermat principle, states that the incidence and refracted angles of a light wave at the interface of two different materials are related to the ratio of the refractive indices in each medium. Whereas all natural materials have a positive refractive index and therefore exhibit refraction in the positive direction, artificially engineered negative index metamaterials have been shown capable of bending light waves negatively. Such a negative refractive index is the key to achieving a perfect lens that is capable of imaging well below the diffraction limit. However, negative index metamaterials are typically lossy, narrow band, and require complicated fabrication processes. Recently, an alternative approach to obtain negative refraction from a very thin nonlinear film has been proposed and experimentally demonstrated in the microwave region. However, such approaches use phase conjugation, which makes optical implementations difficult. Here, we report a simple but different scheme to demonstrate experimentally nonlinear negative refraction at optical frequencies using four-wave mixing in nanostructured metal films. The refractive index can be designed at will by simply tuning the wavelengths of the interacting waves, which could have potential impact on many important applications, such as superlens imaging.     

Perfect lenses made with left-handed materials: Alice's mirror?

In a recent paper, Pendry [Phys. Rev. Lett. 86, 3966 (2000)] mentioned the possibility of making perfect lenses by using a slab of left-handed material with relative permeability and permittivity equal to -1, a property first stated by Veselago [Sov. Phys. Usp. 10, 509 (1968)]. Pendry gave a demonstration of the vital effect of the evanescent waves in this process, arguing that these waves are amplified inside the slab. We present first a very simple theoretical demonstration that a homogeneous material with both relative permittivity and permeability equal to -1 cannot exist, even for a unique frequency. This demonstration shows that the perfect lens proposed by Pendry can be interpreted as a means to move in real space the virtual perfect image of a point source given by a plane mirror. We show that, owing to evanescent waves, the concept of effective medium for heterogeneous materials is questionable, even when the wavelength of the incident light is much larger than the size of the heterogeneities. The effect of heterogeneities is compared with that of absorption. We conclude that a material able to focus the light more efficiently than the current devices (but not perfectly) could exist.     

Development of a total internal reflection ultrafast transient lens method for studying molecular dynamics on an interface

We have developed the total internal reflection ultrafast transient lens (TIR-UTL) method to detect nonradiative chemical processes at interfaces and surfaces with subpicosecond time resolution. In the TIR-UTL measurements, the evanescent field of a pump beam irradiated under the TIR condition generates a refractive index change. The refractive index change is attributed to changes of the molecular electronic state, of density by molecular orientation/structure change, and of temperature by vibrational relaxation processes. The refractive index change is detected as a change of the power intensity of the probe beam adjusted coaxially with the pump beam. At first, we discuss a theoretical principle of a coaxial configuration in the TIR-UTL measurement. This configuration has an advantage of versatility over the established TIR configuration. Then, we evaluate time resolution of TIR-UTL and obtain a value of less than 400 fs. We measure the ultrafast molecular dynamics of the cationic chromophore Auramine O (AuO) at a silica/water interface. Two slow time constants originating from AuO adsorbed on the silica surface are detected by TIR-UTL. These are attributed to AuO, whose twisting motion is strongly hindered by adsorption on a silica surface.     

Silver halide sensitized gelatin process effects in holographic lenses recorded on Slavich PFG-01 plates

In this work we study the feasibility of using silver halide sensitized gelatin based on PFG-01 (Slavich) emulsions to construct uniaxial compound lenses. This processing is able to introduce variations in the thickness and refractive index of the emulsion. We prove that these changes are not sufficient to provide the observed variations in Bragg conditions in the reconstruction and that a shear-type effect must exist to explain the performance of processed emulsions. We study the characteristics of a compound lens, obtaining acceptable image quality, good resolution, and the typical field limitation of volume holographic elements.     

超分辨近场光学成像技术及其产业开发

超分辨近场光学成像技术是当前国内外一个重要的高新技术前沿课题,也将是我国21世纪初应该发展的一项高新技术产业。文中介绍了我国自1991年以来开拓研究的进展,探讨了国际学术界及产业开发中当前存在的主要问题,提出了各类超分辨扫描模式成像公式的乘法表达式,并作了分析比较。为解决消除假像和从有形貌等混合图像中分离纯光学图像两大难题,作者曾于1993年和1996年提出两项发明专利,为发展我国的该产业解决了两大技术关键。     

Lens based on the use of left-handed materials

A new lens that is a modification of the Veselago lens is proposed. It consists of a slab of the left-handed material embedded into a regular material. The materials for the new lens should have the same refractive index, unlike that of the Veselago lens in which the materials should in addition have the same impedance. Therefore the new lens should be easier to manufacture. As the Veselago lens, the new lens might be useful for the three dimensional imaging. In contrast to the Veselago lens the new lens has multiple foci, and it may image an object that is located at any large distance from the slab.     

Design of two-dimensional photonic crystal structure based in all-angle negative refractive effect for application in focusing systems

A design of a polarisation independent focusing system of electromagnetic waves in a two-dimensional photonic crystal (PC) is proposed and numerically demonstrated by using the finite-difference time-domain method. We have shown that by the careful selection of both the refractive index of the high index material and the air cylinder radius, a complete all-angle negative refraction can be obtained for both the transverse electric (TE) and the transverse magnetic (TM) polarisations. We have demonstrated that the proposed PC structures can focus either the TE or the TM or both TE and TM polarised waves with sub-wavelength resolution. In addition, a significant improvement of the image resolution has been presented     

Influence of size parameter and refractive index of the scatterer on polarization-gated optical imaging through turbid media

The influence of incident polarized light, refractive index, and size parameter of the scatterer on achievable resolution and contrast (image quality) of polarization-gated transillumination imaging in turbid media is reported here. Differential polarization detection led to significant improvement of image quality of an object embedded in a medium of small-sized scatterers (diameter Dor=lambda,g>or=0.7), the improvement in image quality was less pronounced using either linear or circular polarization gating when the refractive index of the scatterer was high (ns=1.59), but for a lower value of refractive index (ns=1.37), image quality improved with the differential circular polarization gating. We offer a plausible explanation for these observations.     

Metamaterials with gradient negative index of refraction

We propose a new metamaterial with a gradient negative index of refraction, which can focus a collimated beam of light coming from a distant object. A slab of the negative refractive index metamaterial has a focal length that can be tuned by changing the gradient of the negative refractive index. A thin metal film pierced with holes of appropriate size or spacing between them can be used as a metamaterial with the gradient negative index of refraction. We use finite-difference time-domain calculations to show the focusing of a plane electromagnetic wave passing through a system of equidistantly spaced holes in a metal slab with decreasing diameters toward the edges of the slab.     

Topsy-turvy world of materials

Imagine a medium that reverses every notion you had about the electromagnetic (EM) behavior of materials. The index of refraction is negative so that light, for example, bends the ‘wrong way’ when it enters the medium from a vacuum. A flat plate of the medium could focus light instead of dispersing it. A light source located behind the plate would appear to be in front of it (Fig. 1

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Fig. 1. An electromagnetic source incident on a slab of transparent right-handed material appears closer to an observer on the other side (left). The same source incident on a slab of transparent left-handed material (right) appears to emanate from the opposite side of the slab (same side as the observer).

). Because permittivity and permeability — the main determinants of a material’s response to EM energy — are both negative, EM waves appear to transmit energy one way while undulating the other. And the familiar right-hand rule for EM visualization becomes a left-hand rule.Among other strange consequences, the Doppler effect is reversed: a passing EM source would be characterized by falling then rising frequency rather than the other way around, while light from a source coming towards you would be reddened instead of blue-shifted. The Cerenkov effect is similarly reversed, with charged particles passing though a medium emitting light in a cone behind the particle rather than in front. Evanescent waves, with extremely short wavelengths that decay rapidly and never really escape the surface of a normal material, would actually grow in our topsy-turvy medium. In optics, therefore, these waves — less than the wavelength of light and carrying the finest details of an optical image — could be focused along with the normally propagating waves.     

Planar Waveguides for Fluorescence-Based Biosensing: Optimization and Analysis

Optimization of planar waveguides for fluorescence biosensing is presented in this paper. In particular, the authors show that optical (refractive index) and geometrical parameters have a strong influence on the efficiency of excitation and collection of fluorescent signals. Numerical analyses show that a single-mode slab waveguide, operating at its fundamental transverse magnetic mode and near its cutoff point, results in an efficient fluorescence excitation when employed as evanescent wave biosensor. A high-refractive index contrast is demonstrated to be the key parameter for an efficient fluorescence collection. Other geometries that are an alternative to the classical slab waveguide may result in an improvement of the fluorescence excitation and collection efficiencies.     

Optimisation of distributed feedback laser biosensors

A new integrated optical sensor chip is proposed, based on a modified distributed- feedback (DFB) semiconductor laser. The semiconductor layers of different refractive indices that comprise a laser form the basis of a waveguide sensor, where changes in the refractive index of material at the surface are sensed via changes in the evanescent field of the lasing mode. In DFB lasers, laser oscillation occurs at the Bragg wavelength. Since this is sensitive to the effective refractive index of the optical mode, the emission wavelength is sensitive to the index of a sample on the waveguide surface. Hence, lasers are modelled as planar waveguides and the effective index of the fundamental transverse electric mode is calculated as a function of index and thickness of a thin surface layer using the beam propagation method. We find that an optimised structure has a thin upper cladding layer of ~0.15 mum, which according to this model gives detection limits on test layer index and thickness resolution of 0.1 and 1.57 nm, respectively, a figure which may be further improved using two lasers in an interferometer-type configuration.     

Transmission of dipole radiation through interfaces and the phenomenon of anti-critical angles

Radiation emitted by an electric dipole consists of traveling and evanescent plane waves. Usually, only the traveling waves are observable by a measurement in the far field, since the evanescent waves die out over a length of approximately a wavelength from the source. We show that when the radiation is passed through an interface with a medium with an index of refraction larger than the index of refraction of the embedding medium of the dipole, a portion of the evanescent waves are converted into traveling waves, and they become observable in the far field. The same conclusion holds when the waves pass through a layer of finite thickness. Waves that are transmitted under an angle larger than the so-called anti-critical angle theta (1) ac are shown to originate in evanescent dipole waves. In this fashion, part of the evanescent spectrum of the radiation becomes amenable to observation in the far field. We also show that in many situations the power in the far field coming from evanescent waves greatly exceeds the power originating in traveling waves.     

Broadband microwave Luneburg lens made of gradient index metamaterials

Luneburg lenses are able to form perfect focus that is free of aberration. Because of the varying refractive index throughout the lens, incoming electromagnetic waves can travel in a curved path and be guided to focus at the back of the lens. The implementation of Luneburg lenses is often difficult due to the challenges in creating a medium with varying refractive index using normal materials. This problem can be overcome with the use of gradient index metamaterials. We report a two dimensional Luneburg lens made of gradient index metamaterials. It consists of 17 concentric shells with etched patterns on a printed circuit board working in microwave X band frequency. The broad properties of the Luneburg lens are then discussed.     

Construction of glass waveguide refractive index profiles by the effective-index finite-difference method

A numerical method is applied to construct the refractive index profiles of optical waveguides from the measured effective indices (EI). The method is based on choosing a proper analytical function for the refractive index profile and searching its unknown parameters using the simplex search algorithm. Simultaneously, the finite-difference method (FDM) is used to solve the semi-vectorial Helmholtz equation for the guided modes effective indices. The method is applied successfully to two particular Ag⁺---Na⁺ ion-exchanged glass slab waveguides. The results are as accurate as those obtained from from commonly used IWKB-based method. The EI-FDM in principle can be applied to both slab and channel waveguides and does not require that the index profiles are monotonically decreasing, like most of IWKB-based methods. The relation between the induced refractive index and silver concentration profile, measured by SIMS, is found to be almost linear.     

声子晶体平板组合成像特性分析

用多重散射方法分析了声子晶体单平板和多平板的成像特性。发现：在特定频率范围内,引入等效负折射率后,单板负折射成像可用反Snell定律描述;多层声子晶体平板成像服从依次成像规律,可类比传统几何光学中共轴球面系统的成像,引入实物实像、虚物虚像等概念;平板透镜不改变出射波的出射方向,仅使出射位置产生平移;多平板透镜的累加成像与等厚度的单一平板透镜成像具有相同的效果。平板透镜的这些成像特点,使其在成像质量、加工方法、系统组合等方面比球面透镜更具优势。