Design Of Terahertz Beam Splitters Based On The Self-supporting Sub-wavelength Binary Grating

Terahertz functional device is indispensable in the application of terahertz technology, and for the moment pertinent researches mainly focus on achieving the terahertz beam splitter, filter, and polarization control by materials such as, photonic crystals, surface plasmons, metamaterials grating and grapheme, which have a relative complex structure resulting in difficulties in fabrication, high cost and deviations from the ideal effect. Therefore the study of terahertz functional devices with a simple structure and an excellent function is of high scientific significance. Based on our laboratory’s long-term studies of binary optical devices, this paper has proposed a novel method to achieve terahertz functional devices with a structure of self-supporting silicon grating of simple period by coordinating the rigorous coupled wave theory and genetic algorithm. Based on the simple structure, this paper has designed several even beam splitters with high efficiency and zero-order elimination. The application of simple structure reduces the difficulties and cost of production greatly, which is conducive to the large-scale application of terahertz technology. The study on achieving even terahertz beam splitters with self-supporting, simple periodic binary grating hasn’t been reported nationally and internationally. The main contents are summarized as follows:(1)Firstly, establishing the mapping relations between every order’s diffraction efficiency and the self-supporting grating’s structure parameters(grating period d, grating duty ratio f, the grating depth h, and the grating material’s refractive rate n) based on the rigorous coupled-wave theory. Designing an evaluation function consisting of the diffraction efficiency of high orders and zero order and uniformity for genetic algorithm. The next step is to adjusting the weighting factors of the evaluation function according to the optimization results until finding the appropriate selfsupporting grating’s structure parameters.(2) Combining the rigorous coupled wave theory with genetic algorithm, this paper has designed two-beam splitter, four-beam splitter and six-beam splitter based on the self-supporting binary silicon grating. The best splitting effect is: Diffraction efficiency of high-order reaches 99.14%, and the zero-order is suppressed to 0.188%, at the same time the uniformity error is less than 6.16 × 10-16.(3) The impacts of fabrication errors and the incident light’s parameter deviation on the self-supporting even sub-wavelength grating beam splitter’s performance are analyzed, and the influencing rule is given which has some significance for the beam splitter’s fabrication and testing. A gearing analysis of the four-beam splitter’s structure parameters is made to get the optimum parameters to fabricate. This work can guide the actual fabrication of the terahertz beam splitter.(4) The four-beam silicon terahertz splitter is fabricated and the operating wavelength is 118.83μm. In addition, the causes of the practical fabrication error are analyzed and the corresponding solutions are given.This paper has proposed and designed the terahertz even beam splitter based on the self-supporting grating. Combining vector analysis theory with genetic optimization algorithm, this paper has designed two-beam splitter, four-beam splitter and six-beam splitter and broken the traditional limitation of scalar method, achieving a good performance of zero-order elimination and high diffraction efficiency. The splitters designed in the paper have a simple structure, good function expansion and easy to be fabricated. They are expected to find application in THz communication and imaging system in the future.