Far-field super-resolution scanning imaging based on fractal resonator

Abstract

The resolution of traditional far-field imaging system is generally restricted by half of the wavelength of incident light due to the diffraction limit. The more specific reason is that evanescent waves carrying sub-wavelength information cannot propagate in the far field and make no contribution to the imaging. However, higher imaging resolution is required in practical applications. To realize the far-field super-resolution imaging, the imaging system should be able to collect both propagating waves and evanescent waves. Many designs have been proposed to solve this issue. In 2007, a far-field superlens was proposed by Liu et al. (Liu Z W, Durant S, Lee H, Pikus Y, Fang N, Xiong Y, Sun C, Zhang X 2007 <i>Nano Lett</i>. <b>7</b> 403) to realize far-field super-resolution in optical range, which consisted of a silver film and a nanoscale grating coupler. The silver film was used to amplify the evanescent waves, which were then converted into propagating waves by the sub-wavelength gratings. However, the special material properties limit the freedom of design. In microwave band, the incident components can be converted into Bloch modes by the resonant metalens, which consists of subwavelength resonators, and then be radiated to far field. Nevertheless, Green function between antenna and target is necessary, which is difficult to obtain due to the complex and even time-dependent imaging environment in practical applications, especially for super-resolution imaging system. It has been demonstrated in recent research that frequency information can be associated with spatial information of imaging target by localization resonant modes. Therefore, super-resolution imaging can be realized based on frequency information, without using Green function. Thus, a novel microstructure array is proposed to realize the far-field super-resolution scanning imaging based on a fractal resonator. The fractal resonator can work at several frequencies because of the self-similarity, which provides higher selectivity according to practical conditions. Several working statuses can be obtained for the resonator by adding photoconductive semiconductor switches, which are controlled by laser. On account of localization mode resonance, the array can realize the conversion between evanescent waves and propagating waves. Then with the help of antennas in the far-field to receive the frequency information, the location of imaging source can be confirmed according to the spectrum. Then by using the magnitude of resonant peak, sub-wavelength image can be reconstructed without using Green function. To verify the super-resolution scanning imaging characteristics of the array, an imaging simulation of “laugh face”-shaped target is performed. The image is reconstructed very well and the resolution determined by the period of the array is 20 mm, corresponding to <i>λ</i>/10. In view of the particularity of proposed fractal resonator, a novel scanning method is proposed. By combining the first and the third resonance, the imaging efficiency can be well improved compared with by the traditional point-by-point scanning method.