All-optical diode of subwavelength single slit with multi-pair groove structure based on SPPs-CDEW hybrid model
Authors: Qi Yun-Ping  , Nan Xiang-Hong  , Bai Yu-Long  , Wang Xiang-Xian  ,   College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China;,   School of Science, Lanzhou University of Technology, Lanzhou 730050, China
Published: December 2020

All-optical diode is the most basic photonic device in integrated optical circuits. It is of great significance to develop a modulated optical diode for preparing complex optical circuits in the near future. However, there are few studies on constructing all-optical diodes in subwavelength metal micro-nano structured devices based on the hybrid model of surface plasmon polaritons (SPPs) and composite diffracted evanescent wave (CDEW). In fact, most of the researches have been focusing on how to effectively enhance the unidirectional nonreciprocal transmission of the optical diode and improve the extinction ratio. According to SPPs-CDEW hybrid states, in this paper we put forward a novel method of designing an optical diode and its structure. The structure consists of a subwavelength single micro-nano slit surrounded by symmetric multi-pair grooves on a silver film. First of all, on the basis of the single slit structure of the silver film, the pairs of the groove structures are etched on both sides of the silver film: the positions and quantities of the grooves on the top and bottom surfaces are asymmetric. Then combining with an effect similar to Fabry-Perot resonance effect inside the micro-nano slit, the function of beam unidirectional transmission is achieved by controlling SPPs through changing the geometric parameters of the structure. Furthermore, in order to realize unidirectional nonreciprocal transmission, by means of theoretical derivation and the finite element method (FEM), in this paper we analyze the transmission enhancement phenomenon of single slit-symmetric pair of groove micro-nano structure, discuss the physical mechanisms of transmission enhancement and weakening, and also give the far field transmission spectrum of the normalized transmission changing with the distance between slit and pair grooves. The results obtained from the rigorous theoretical formula are in excellent agreement with the numerical results obtained by using FEM. Finally, as the position and number of the pair grooves are precisely determined by this transmission spectrum, the optimized all-optical diode structure, of which the unidirectional transmission is effectively enhanced and the extinction ratio of the optical diode is improved, is achieved with five pairs of enhanced transmission grooves formed on the top surface of the Ag film and six pairs of weakened transmission grooves formed on the bottom surface. The maximum extinction ratio reaches 38.3 dB, which means that the forward transmittance is 6761 times the reverse transmittance, i.e., it increases 14.6 dB over the result from previous theoretical work. And there appears a 70 nm wavelength band width (20 dB) in the operating wavelength 850 nm. The proposed optical diode has the advantages of simple structure, wide working bandwidth, easy integration, and high coupling efficiency. The research of the optical diode is valuable for the potential applications in optical signal transmission, optical integrated optical circuit, super-resolution lithography and other related fields.