Dual subwavelength imaging based on two-dimensional photonic crystals
Niu Jinke, Liang Binming, Zhuang Songlin, Chen Jiabi     
School of Optical-Electrical and Computer Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China

Overview:In recent years, negative refractive index materials (NIMs) have attracted more attention. There are lots of studies on the special characteristics of NIMs such as negative refraction and subwavelength imaging. As a NIM, photonic crystal (PC) can greatly amplify the evanescent waves and break the diffraction limit, the subwavelength resolution can be achieved.

In this paper, The edge length of PC is L=50a. The point source is placed at 0.3 μm below the edge of PC, and its horizontal coordinate is -10 μm. The light path simulated by Rsoft software. The gratings on both sides of the PC increases the transmission of light, eliminating the influence of the reflected light on dual imaging. As the clear two images are achieved, the positional relationship between two images and the point source is obtained. Based on the results, a confocal system with a triangular PC is proposed. Unlike the conventional confocal system, the PC confocal system has a simple structure, and it achieves imaging by negative refraction.

Dual sub-wavelength imaging is achieved clearly by adjusting the grating gap on PC, it eliminates the effects of reflected light. Through varying the wavelength of the point source, a broad spectrum which can achieve sub-wavelength imaging is found. Then adjust the lateral coordinates of the light source points to obtain the positional relationship between the two image points and the light source points. Based on the above results, the photonic crystal confocal system was designed and verified by simulation. The normalized peak value of image1 is increased from 1.104 to 1.326 and the half-width is decreased from 0.461λ to 0.433λ by adjusting the size of the grating air slit; meanwhile, the side spot at image2 is eliminated when the grating air slit is w=0.76a and distance between gratings and air hole is dg=0.1a. The minimum half-width of images is obtained when the incident wavelength is 3.216a, and the wide-spectrum dual subwavelength imaging is achieved when the incident wavelength varies from 3.19a to 3.26 a, which the minimum half-width is less than 0.44λ. In addition, the position formulas of the images and point source are demonstrated, that provides a reference for the precise location of two images. Based on the results, we propose a confocal system that can achieve subwavelength imaging. Compared with the traditional confocal microscope, this structure does not need objective lens. As its focusing and imaging through the negative refraction of PC, the structure is more simple. Furthermore, dual subwavelength imaging can also be used in other aspects.

Supported by National Natural Science Foundation of China (61177043)