Design of a broadband high-efficiency two-port beam subwavelength grating splitter by modal method

Abstract

On the basis of the comparison of diffraction efficiencies calculated by the simplified modal method to exact results predicted from rigorous coupled-wave analysis, the accuracy of modal method for subwavelength triangular-groove gratings is evaluated. It is revealed that a larger error exists in smaller grating period and lower groove depth. To enhance the validity of simplified modal method, we consider the reflection loss of propagating grating modes by using the optical thin film theory. Then, we design a highly efficient transmission two-port beam splitter grating with subwavelength triangular-grooves by using the enhanced simplified modal method. The physical mechanism of the designed grating splitter with a high transmitted performance over a broad spectrum and a wide angular spectrum can be explained by using the modal method with consideration of accumulated average phase difference of two excited propagating grating modes. Obviously, the designed splitter with triangular-grooves has higher transmitted diffraction efficiency and more broadband property than that with general rectangular ones. The broadband highly efficient transmission characteristic can be attributed to gradient effective index from air-grating interface to grating-substrate interface.