Numerical research on properties of optical-wave target detection and imaging from NR-PC flat lens

The characteristics of optical-wave target detection and imaging from a negative-refraction photonic crystal (NR-PC) flat lens are studied with a two-dimensional (2D) finite-difference time-domain (FDTD) method in the paper. The results show that there exists a transmission peak, with a value far greater than unit, resulting from the influence of the mini-forbidden bands and resonance excitation effect at resonance frequency. Also, the refocusing of backscattered optical wave from the target leads to its image with a sub-wavelength lateral resolution and great amplitude as long as the target is placed at the focus of the NR-PC flat lens. Therefore, the NR-PC flat lens will find many implications in optical area, especially in the detection and imaging of small targets.     

Two-square target detection and imaging using NR-PC flat lens

The characteristics of two-target detection and imaging from negative-refraction photonic crystal (NR-PC) flat lens are studied by using the two-dimensional finite-difference time-domain method. It is demonstrated that due to the influence of the mini-forbidden band and resonance excitation effect, high transmissivity will appear at the normalized resonance frequency of 0.3068 when the lightwave goes through the NR-PC lens. Meanwhile, the use of the NR-PC lens may introduce at least fourfold improvement of the refocusing resolution, if compared to the directly backscattered lightwave without using the NR-PC lens. In addition, by giving a defination for two-target minimum distinguishable distance, we further investigate the performance of two-target detection and imaging system using the proposed focus-scanning scheme. The results show that the smaller the size of the target pair is, the weeker the interference between the two targets will be, which results in a better performance in the minimum distinguishable distance.     

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 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.     

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.     

On the use of NR-PC flat lens for square target detection and imaging by lens-combined scanning scheme

In the paper, target detection and imaging from refraction photonic crystal (NR-PC) slab lens is studied by the finite-difference time-domain (FDTD) method. Numerical simulation shows that, there is a transmission peak with a value far greater than unit, resulting from the mini-forbidden bands and resonance excitation at the resonance frequency of 0.3068 (a/λ). When a target is placed at the focus point (F₂) of the NR-PC lens, its image and great amplitude could be formed in the vicinity of the point source through the focusing of backscattered optical wave from the target. Further investigation demonstrates that the lens-combined scanning scheme provides higher refocusing resolution than lens-fixed and non-dynamic scanning scheme for square target detection and imaging.     

Effect of the defect on the focusing in a two-dimensional photonic-crystal-based flat lens

We have investigated in detail the influence of defect on the focusing of electromagnetic waves in a two-dimensional photonic-crystal flat lens by using the finite-difference time-domain method. The result shows that many focusings can be observed at the symmetrical positions when a defect is introduced into the lens. Furthermore, the wave-guides in the lens can confine the transmission wave effectively and improve the quality of the focusing.     

Analysis of the features and reconstruction of a high resolution infrared image based on a multi-aperture imaging system

Infrared images of good quality are strictly important for such applications as targets detection, tracking and identifying. Traditional single aperture infrared imaging system brings in some defects for its imaging scheme. Multi-aperture imaging system shows promising characteristic of improving image quality and reducing size of optical instruments. We reconstruct a high resolution infrared image from the low resolution sub-images collected by the compact multi-aperture imaging system. A novel reconstruction method called pixels closely arrange (PCA) is proposed based on analyzing the compound eye imaging process, and this method is verified in a simulated 3D infrared scene to capture sub-images. An evaluation of the reconstructed image quality is presented to discuss the significant factors that affect the final result. Experimental results show that the PCA method can be efficiently applied to the multi-aperture infrared imaging system as long as the structure of the micro-lens array is specifically designed to be adaptive to the infrared focal plane array (IFPA).     

强背景下模糊闪烁成像目标探测与跟踪

 强背景下空中运动目标主要表现为边缘残缺、模糊,甚至出现闪烁、间断等复杂成像特性,这成为制约目标探测和成像跟踪的瓶颈问题。结合跟踪过程反馈信息来确定目标点扩展函数,利用一种新的Ben-Ezra图像复原模型,解决了目标运动模糊的图像复原问题;采用基于特征伴随值的相似性判断方法,构造联结机制的联想网络模型,来解决目标闪烁和残缺特征的联想问题。对提出的方法进行了图像序列的仿真,取得了较好的实验结果。     

光子晶体的光学特性与近场光学测量表征

光子晶体具有独特而优异的光学特性及广泛的应用前景。介绍了光子晶体的概念、应用、发展,着重讨论了光子晶体的光子禁带、光子局域以及其它光学特性。从近场光学的原理和实验技术出发阐述了近场光学测量表征光子晶体的方法,对于光子晶体的研究具有重要的意义。     

用于成像侦察的CCD探测器成像分辨力的探讨

通过对CCD探测器成像过程的模拟,分析了CCD成像在像元几何尺寸及成像灰度方面的数字离散化特征对CCD成像分辨力的影响,给出了CCD成像分辨力与CCD像元分辨力的关系,同时也给出了在相应情况下的调制度传递函数(MTF)的数值。     

Near-field imaging of a square-lattice metallic photonic-crystal slab at the second band

Imaging properties of a two-dimensional photonic crystal slab lens are investigated through the finite-difference time-domain method. In this paper, we consider the photonic crystal slab consisting of a square lattice of square metallic rods immersed in a dielectric background. Through the analysis of the equifrequency-surface contours and the field patterns of a point source placed in the vicinity of the photonic crystal slab, we find that a good-quality image can form at the frequencies in the second TM-polarized photonic band. Comparing the images formed at different frequencies, we can clearly see that an excellent-quality image is formed by the mechanisms of simultaneous action of the self-collimation effect and the negative-refraction effect.     

半导体量子阱材料微加工光子晶体的光学特性

利用聚焦离子束刻蚀方法和电子束制版结合干法刻蚀方法制备了二维近红外波段光子晶体，发现两种方法都可以制备出均匀的二维光子晶体，聚焦离子束方法操作简单，电子束制版结合干法刻蚀方法操作步骤复杂.光谱测试表明，利用聚焦离子束方法在有源材料上刻蚀的光子晶体不发光，而电子束制版结合干法刻蚀方法制备的小晶格常数光子晶体即使有些无序，其出光效率也提高到没有光子晶体时的两倍.对两种方法所加工的光子晶体不发光和提高出光效率的机理进行了分析. 关键词： 聚焦离子束 电子束制版 光子晶体 出光效率     

有限方位扫描的光声断层成像分辨率研究

通过有限元仿真实验,定量地研究了有限方位扫描对反投影光声断层成像分辨率的影响,并且给出了有限方位扫描角与分辨率之间关系的经验公式.本文的研究结果对光声成像扫描轨迹设计、成像效果评估等具有参考价值. 关键词： 光声成像 有限方位扫描 分辨率 断层成像     

Diverse imaging of photonic crystal by the effects of channeling and partial band gap

Diverse imaging of point source by a two-dimensional photonic crystal slab with square lattice is studied by the finite-difference time-domain method. Given proper frequencies, there seems to be some special directions like channels guiding the light propagation inside the photonic crystal, along which some images will be created. It is suggest that except negative refraction there should be other mechanisms making point image by two dimensional photonic crystal slab.     

Controlling the focusing properties of a triangular-lattice metallic photonic-crystal slab：

This paper studies the focusing properties of a two-dimensional photonic crystal (PC) slab consisting of a triangular lattice of metallic cylinders immersed in a dielectric background. Through the analysis of the equifrequency-surface contours and the field patterns of a point source placed in the vicinity of the PC slab, it finds that both the image distance and image quality can be controlled by simply adjusting the refractive index of the background material.     

Enhancing the resolution of the forward second harmonic imaging using the two-photon laser scanning microscope

The effect of increasing the numerical aperture and changing the immersion imaging medium of the objective in the forward second harmonic imaging of the grana and intergranal parts of chloroplasts in the leaves of shade moss Plagiomnium affine plant were demonstrated. Also the enhancement of the forward second harmonic generation images and simultaneously records the forward and backward second harmonic signals were demonstrated. The second harmonic signals in the forward and backward directions are generated by a tightly focused linearly polarized femtosecond pulses laser.

Conclusions

Forward second harmonic signals were successfully enhanced by capturing higher order of diffraction rays from the samples using high numerical aperture water immersion objective.     

Improving the imaging intensity of a photonic crystal flat lens by an antireflection method

In this paper, a two-dimensional photonic crystal (PC) consisting of a triangular lattice of silicon rods in air is studied theoretically. Equal-frequency contours (EFCs) analysis shows that this PC structure exhibits an effective refractive index n_eff = −1 at a normalized frequency ω=0.30×2πc/a$\omega =0.30\times 2\pi c/a$. A superlens effect is demonstrated using this PC structure by the finite-difference time-domain (FDTD) simulation. An antireflection layer (ARL) is introduced into the PC slab surface in order to achieve a high-intensity image. Two point sources imaging process is also simulated both in the case of a perfect PC structure and in the case of a PC structure with an ARL. The results show that the method of introducing the ARL into the PC surface is effectual for improving both the intensity of the image and the capability of the super-resolution of two sources, which are in accord with the coupled-mode theory analysis. Theoretical study implies that the surface defect modes play an important role in enhancing the transmitted intensity of energy flow.     

二维蜂窝格子光子晶体的远场成像特性及界面对成像质量的影响

利用有限时域差分法研究了一种二维蜂窝格子光子晶体的远场亚波长成像特性，发现和折射率为-1时的负折射成像规律符合得很好.但是由于晶格结构本身的特性，入射光以大角度入射时，这种结构的耦合效率很低，影响了像的强度和质量.通过适当改变界面上介质柱的半径的方法可以提高耦合效率，进而有效提高成像的质量和强度.     

Engineering the near-field imaging of a rectangular-lattice photonic-crystal slab in the second band

Imaging properties of a two-dimensional rectangular-lattice photonic crystal (PC) slab consisting of air holes immersed in a dielectric are studied in this work. The field patterns of electromagnetic waves radiated from a point source through the PC slab are calculated with the finite-difference time-domain method. Comparing the field patterns with the corresponding equifrequency-surface contours simulated by the plane-wave expansion method, we find that an excellent-quality near-field image may be formed through the PC slab by the mechanisms of the simultaneous action of the self-collimation effect and the negative-refraction effect. Near-field imaging may be obtained within two different frequency regions in two vertical directions of the PC slab. Supported by the Research Foundation of the State Ethnic Affairs Commission of China (Grant No. 07ZY15), the National Key Basic Research Special Foundation of China (Grant Nos. 2004CB719804 and 2006CB921702), the National Natural Science Foundation of China (Grant Nos. 10674185 and 10705056), and the Youth Foundation of Central University of Nationalities (Grant No. CUN0207)