Broadband cross-slots fractal nano-antenna and its extraordinary optical transmission characteristics
Liu Juefu, Chen Jiao, Li Kangkang, Liu Yuanyuan, Zhu Lu     
School of Information Engineering, East China Jiaotong University, Nanchang, Jiangxi 330013, China

Overview: The nano-antenna structure can break through the optical diffraction limit and achieve efficient transmission of light. For nano-antennas with specific wavelengths of radiation, transmission characteristics are an important characteristic of effective light transmission. Ebbesen et al. found optical extraordinary transmission phenomena when analyzing the transmission characteristics of metal film sub-wavelength aperture arrays. When light is incident on a sub-wavelength aperture array, the light transmission is higher than the ratio of the aperture area to the total area of the film at a particular wavelength, and the transmission is 1 to 2 orders of magnitude higher than the classical aperture transmission theory. Study has shown that the generation of extraordinary optical transmission is generally attributed to the mutual coupling of light waves with free electron oscillations at the surface of metal holes or slots structures, and localized surface plasmons at the edges of apertures or slots also have a non-negligible effect on extraordinary transmission. The coupling efficiency of the plasmon polarization of the upper and lower surfaces of the structure can be improved, thereby enhancing the transmission of light. In order to achieve wide-band extraordinary transmission and the purposes of controllable and adjustable, we introduce fractal theory, and utilize the properties of self-similarity and fractal dimension to propose an extraordinary transmitted cross-slots fractal nano-antenna. Furthermore, the finite-time-difference method is used to calculate the extraordinary transmission characteristics and surface electric field distribution of the cross-slots fractal nano-antenna structure, and the transmission characteristics mechanism is systematically analyzed and compared in detail. The results show that the cross-slots fractal structure is smaller in size, wider in the full width at half maximum (FWHM), and higher in transmittance, up to 99.51%. At 851.536 nm, the light transmittance is much higher than that of the uniform cross-slots structure. The ratio of the hole area to the Ag material area realizes the extraordinary optical transmission. By adjusting the physical parameters, the transmission spectrum exhibits a red-shift or blue-shift characteristic, and achieves the controllability of the transmission spectrum. Meanwhile, when h=50 nm, the FWHM is about 356 nm, the transmittance is still as high as 95.66%, which is generally higher than the traditional structures; At a large incident angle (70 degrees), the peak transmittance is still greater than 74%. In short, the cross-slots fractal nano-antenna has the characteristics of wide frequency, controllable and adjustable, and more miniaturized structure compared with other nano-antenna structures, and realizes the extraordinary transmission of light and full 2π phase transmission control. In addition, the nano-antenna produces a significant resonance in the short-band, which further enhances the transmission of light.

Supported by Jiangxi Outstanding Youth Talent Funding Scheme (20171BCB23062), Jiangxi Natural Science Foundation (20171BAB204022), and Jiangxi Provincial Department of Education Science and Technology Research Key Project (GJJ170360)