Impacts of Refraction Index Mismatch on Performance of Target Detection and Imaging by Using Flat LHM Lens

Refraction index mismatch between flat left-handed metamaterial (LHM) lens and its surrounding medium generally destroys the focusing of flat LHM lens and degrades the performance of near-field target detection by using flat LHM lens. For LHM lens of refraction index mismatch within ±30%, numerical simulations demonstrate that lenses with large refraction index may suffer less resolution degradation than lenses with small refraction index, and the enhancement of refocused microwave backscattered from target can be subsided by up to approximately 5.5dB. The refraction index mismatch will also shift the target position in the reconstructed image so that theoretical prediction of target position needs to be modified.     

Influence on the resolution of target detection with defective NR-PC flat lens incorporating Al,Cu by dynamic scanning scheme

Based on the defective NR-PC flat lens incorporating Al or Cu, the effects on the resolution for target detection and imaging are studied with dynamic scanning scheme. We use the finite-difference-time-domain method to do this research. Firstly, by using the NR-PC flat lens may improve the refocusing resolution by four times, if compared to the directly backscattered lightwave without using the NR-PC lens. Then, incorporating Al or Cu into the NR-PC flat lens, we find that the dope modes with very high quality factors will occur in the photonic forbidden band. Also the stimulated radiation is enhanced for the character of energy localization of defective photonic crystal. Further studies show that different kinds of metal and structures will result in different dopped modes in the photonic forbidden band, which will lead to the change of the corresponding resolution. Due to the displacement field redistribution, the photonic crystal containing metal ingredients may take on the typical qualities of all-directional reflection, and the resonant tunneling effect may effectively reduce the loss, which provides greatly improvement on the refocusing resolution when using the defective NR-PC flat lens. In conclusion, our investigation provided the basis for converting an idealized LHM lens into a physically realizable NR-PC flat lens. Meanwhile, by impregnating metallic defect into the NR-PC lens, a new method is found out for the improvement of the refocusing resolution, which further optimizes the performance of a detecting and imaging system.     

On the cylinder air holes of NR-PC double flats lens group with active impurities for lightwave target detection and imaging

In the paper, the influence of the cylinder air holes of negative-refraction photonic crystal (NR-PC) double flats lens group with active impurities on the performance of lightwave target detection and imaging are studied with finite-difference time-domain (FDTD) method. Firstly, we conclude the focusing imaging circuit diagram of NR-PC flat lens with the Snell extension law and geometrical optics principle. Then we use NR-PC flat lens to detect target with dynamic scanning system according to that. Numerical simulations indicate that significant enhancement of the scattering signal can be obtained due to the use of the NR-PC flat lens. We further research the influence of active impurities on target detecting by using NR-PC double flats lens group with cylinder air holes. We use NR-PC flat with active impurities instead of the perfect ones. By using dynamic scanning scheme, we find that it could improve the lateral resolution of target scanning through introducing appropriate active impurities. In conclusion, our investigation optimized the performance of the small target detection and imaging system, and provided the basis for converting an idealized LHM lens into a physically realizable NR-PC double flats lens group.     

Propagation of Gaussian beams family through a Kerr-type left-handed metamaterial

In this paper the propagation of elegant Hermite-cosh-Gaussian, elegant Laguerre Gaussian, and Bessel Gaussian beams through a Kerr left-handed metamaterial(LHM) slab have been studied. A split-step Fourier method is used to investigate the propagation of laser beams through this media. Numerical simulation shows that Gaussian beams have different focusing behaviors in a Kerr LHM slab with positive or negative nonlinearity. Beam focusing happens in slabs with positive nonlinearity and not in slabs with negative nonlinearity; however, negative nonlinearity is required for a Kerr LHM slab to act like a lens. Additionally, the focusing properties of beams can be controlled by controlling the thickness of the slab or the input power of the incident beam. A multilayer structure is also proposed to have beam focusing by thinner slabs and passing longer distances.     

Fourth order correction to a Gaussian beam focused with a parabolic index lens

We formulate the fourth order correction to a paraxial Gaussian beam propagated along the axis of symmetry of a parabolic index lens. First we examine the evolution of a complex-source-point spherical wave (equivalent paraxially to a Gaussian beam) through the lens in a two-dimensional xz plane. Taking into account the terms of up to fourth order in aperture variables, we find a ray-optical solution to the exit beam that is represented in terms of aberration function. We also analyze the effect of the lens aberration exerted on the degradation in the quality of a Gaussian beam. The fourth order-corrected wave function derived here may be used to evaluate the quality of a Gaussian beam focused with a parabolic index lens. Further it may be applied to the case of an orthogonal system in which the index variations are different in the xz and yz planes.     

Veselago’s lens consisting of left-handed materials with arbitrary index of refraction

We study Veselago’s lens with arbitrary index of refraction and characteristic impedance. Using a full wave optics calculation, we show that this lens can be considered as an imaging system and we derive the appropriate lens formula. The lens with arbitrary index and impedance retains some of the properties of the matched lens, such as the invariance of its optical axis, three-dimensional imaging and easy manufacturing, but it loses the property of sub-wavelength resolution. We also show that identical results can be obtained for the impedance matched lens in the framework of paraxial geometrical optics, from which it can be inferred that optical systems containing such a lens can be studied and designed using traditional ray-tracing tools.     

Multiple reflection effect inside a hemispherical solid immersion lens

The reflection inside a hemispherical solid immersion lens (h-SIL) is analyzed by using the film-interference optics and the new unique characteristic of the h-SIL is presented. The image theory of SIL microscopy is developed. Numerical results show that the reflection inside the h-SIL, especially for large mismatch in refractive indices between the SIL and the medium, has an important influence on the resolution of SIL surface microscope and the collection efficiency from the emitter. Theoretical and experimental results are compared through three examples, the reflection-mode microscope, the transmission-mode microscope, and the collection of light from the emitters. The present theoretical values are quite consistent with the experimental values in SIL microscopy. It is of interest to note that owing to the interference in the SIL, the resolution and collection efficiency periodically oscillate with the radius of an h-SIL, which mean that the radius of an h-SIL has to be well-controlled to achieve enough high resolution and collection efficiency.     

On the defective NR-PC flat lens for lightwave target detection and imaging

Influence of the defective cylinder air holes of negative-refraction photonic crystal (NR-PC) lens on the performance of lightwave target detection and imaging are studied with finite-difference time-domain method in the paper. Numerical simulations indicate that significant enhancement of the scattering signal can be obtained using the NR-PC flat lens; consequently, great improvement of the refocusing gain as well as the imaging resolution will be provided. We further study the effect on target detection and imaging using the defective NR-PC flat lens. Using dynamic scanning scheme, it is found that its focusing resolution is better than the non-defective lens. We can also get better resolution by appropriately decreasing the radius of the defective cylinder (R = 0.3a). In conclusion, appropriate radius of the defective cylinder (R = 0.3a) will provide better image 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.     

左手性介质平板透镜系统的赛德尔像差特性

根据初级像差理论广泛研究了由左手性介质构造的平板透镜系统的赛德尔像差特性.由初级像差加和定理推导了适合于这种系统的赛德尔和数公式,并指出了寻找消赛德尔像差的平板透镜系统的方法.作为两个特例,分别讨论了消赛德尔像差的左手性介质单透镜和双胶合透镜系统.     

The filtering characteristics of simple grating optical low-pass filters

We studied the characteristics of a two-dimensional grating optical low-pass filter (GOLF) theoretically and experimentally. The modulation transfer function (MTF) of an optical system that consists of a lens and a GOLF is theoretically derived by taking all orders of diffracted beams into consideration. The MTFs of a two-phase chess-board-type GOLF and a three-phase GOLF were calculated for various phase differences and compared with that of a birefringent low-pass filter (BLF). The three-phase GOLF with nine center beams of equal strength removes most moiré fringes, but the resolution degradation is severe compared to the BLF. The two-phase GOLF with a phase difference of 180°, which is similar to the BLF in term of beam distribution, has a medium characteristic somewhere between those of the three-phase GOLF and the BLF. Samples of two GOLFs are made and experimented on by attaching them to a digital camera. The experimental result coincides with the theoretical development. Received: 31 October 2001 / Revised version: 4 March 2002 / Published online: 2 May 2002     

Advances in X-ray excitation of Kossel patterns by a focusing polycapillary lens

In this paper, the polychromatic X-ray excitation of Kossel patterns by an X-ray tube and a focusing polycapillary lens will be presented. Additionally, it will be shown that the lateral resolution of the Kossel technique under X-ray tube excitation can be improved and the exposure times can be strongly reduced by using a polycapillary lens. The advantageous combination of X-ray fluorescence and crystal structure analysis by means of Kossel microdiffraction will be demonstrated.     

散斑自相关分辨率测量和修正

散斑相关是许多基于散斑的光学测量和成像技术的基础,决定了光学系统的分辨率。当前散斑尺寸（颗粒度或分辨率）的理论描述不够精确,也缺乏实验验证。探究了散斑图样自相关尺寸的影响因素,与相同数值孔径物镜聚焦进行比对,揭示薄散射介质的“散射透镜”性质。通过散斑自相关和透镜聚焦尺寸的多组测量,结果表明,截趾函数会影响其分辨率,需要根据具体光路对阿贝判据做修正。对基于散斑的测量和成像技术具有一定的参考价值。     

Enhanced twinkling in left-handed media

A slab of left-handed material (LHM) with refractive index -1 forms a perfect lens that retains subwavelength information about a source or object. Such lenses are highly susceptible to perturbations affecting their performance. It is shown that illuminating a roughened interface between air and an LHM produces a regime for enhanced focusing of light close to the boundary. This generates caustics that are brighter, fluctuate more, and cause Gaussian speckle at distances closer to the interface than in right-handed matter. These effects present fresh challenges for perfecting the perfect lens.     

ABCD matrix formalism for propagation of Gaussian beam through left-handed material slab system

We apply a simple ABCD matrix formalism to investigate beam propagation in left-handed material (LHM) slab systems. Firstly, we derive the expressions for the matrix elements for a single LHM slab. Based on the ABCD matrix, we obtain the field distributions both inside and outside the LHM slab, and the conditions for focusing and phase compensation of the Gaussian beam. Secondly, the derived ABCD law is applied to analyze the propagation properties of Gaussian beams through various cascaded LHM slab systems. The corresponding focusing and phase compensation conditions of the Gaussian beam are also obtained. Our theoretic formalism may be applied as a compact and effective tool for the research of light beam propagation in complex LHM slab systems, since it avoids the complicated integral operation.     

Holographic phase conjugation through a sub-wavelength hole

Holographic phase conjugation is analyzed as a method to create a photo-refractive lens with high numerical aperture. For this purpose a sub-wavelength hole is drilled into a metal surface directly on top of an iron-doped lithium niobate crystal. An interference pattern generated by the light coming from this point source and a plane reference wave is recorded. By using the phase-conjugated reference wave for read-out, a light wave being focused onto the former point source is reconstructed. In principle, a focusing system close to the theoretical diffraction limit could be implemented by this method. The performance of this arrangement is mainly determined by properties of the lithium niobate crystal, especially the crystal symmetry. Experimentally, the tight holographic focusing is demonstrated. The focus width of the reconstructed wave is shown to be below 1.2 μm, which is our spatial resolution. The diffraction efficiency obtained, however, is just 3×10⁻⁵ compared to 3×10⁻² in the plane-wave case. This can be explained by experimental reasons, the inhomogeneous light intensity and limitations originating from the crystal symmetry. We estimate that the diffraction efficiency for phase conjugation through a sub-wavelength hole can be improved by three to four orders of magnitude by addressing the above-mentioned issues.     

Prediction of negative index material lenses based on metallo‐dielectric nanotubes

We propose to assemble negative index materials (NIMs) from dielectric nanotubes with inner and outer surfaces covered by thin metallic films. The focusing properties of flat and concave lenses assembled from metallized titania nanotubes are compared with those of lenses made from nanorods with the refractive index n = –1 by performing numerical calculations using a multiple‐scattering approach. Focusing is proved for both types of lenses, however, the focusing properties of concave lenses are better. The lenses are shown to be tolerant to the introduction of disorder in the arrangement of nanotubes. Moreover, the disorder proves to improve the quality of the focal spot. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A Non-Energy-Concentrating Lens Using Left-Handed Material

In a planar lens made of left-handed material (LHM), the rays converge to a focal point in the material and once again in free space. The problems exhibited by the planar lens are its great thickness, the long optical path length, and energy concentration in the LHM. We examine a bowl-shaped LHM lens with an index n = −1. In this lens, the back is a concave hemispherical surface and the rays never converge in the LHM, but in free space only. We also designed a quantized bowl-shaped LHM lens composed of multiple layers of thin LHM plates of uniform thickness. A bowl-shaped LHM lens quantized with more than 4 bits has a lens function similar to that of a smooth curved lens. The bowl-shaped lens is characterized by its thinness, its short optical path length, and the absence of energy concentration in the LHM. Furthermore, the quantized bowl-shaped LHM lens can be fabricated. Wave propagation through these lenses is numerically simulated using the finite difference time domain (FDTD) method.     

Focusing and phase compensation of paraxial beams by a left-handed material slab

On the basis of angular spectrum representation, a formalism describing paraxial beams propagating through an isotropic left-handed material (LHM) slab is presented. The treatment allows us to introduce the ideas of beam focusing and phase compensation by LHM slab. Because of the negative refractive index of LHM slab, the inverse Gouy phase shift and the negative Rayleigh length of paraxial Gaussian beam are proposed. It is shown that the phase difference caused by the Gouy phase shift in right-handed material (RHM) can be compensated by that caused by the inverse Gouy phase shift in LHM. If certain matching conditions are satisfied, the intensity and phase distributions at object plane can be completely reconstructed at the image plane.     

Coherent imaging of extended objects

When used with coherent light, optical imaging systems are inherently unable to reproduce both the amplitude and the phase of a two-dimensional field distribution. This is because their impulse response function varies slowly from point to point, a property known as non-isoplanatism. For sufficiently small objects, this usually results in a phase distortion and has no impact on the measured intensity. Here, we show that the intensity distribution can be dramatically distorted when extended objects are imaged. We illustrate the problem using two simple examples: the pinhole camera and the thin lens. The effects predicted by our theoretical analysis are confirmed by experimental observations.