Modal analysis and numerical design of wideband two-layer grating with low polarization dependence

• Different modes are excited by the incident wave for the two-layer grating.
• The mechanism of high efficiency can be explained by the modes interference.
• Accurate depths are optimized around the depths given by modal method.

A two-layer grating with low polarization dependence is described to obtain high efficiency with the wideband property. Modal method is applied to analyze the propagation process of the incident wave in the grating region. The phase difference is accumulated in both two layers, which determines the efficiency based on the two-beam interference. The grating depths are estimated according to the phase difference for both TE and TM polarizations. The accurate depths are optimized by using rigorous coupled-wave analysis around the depths given by the modal method. With the grating parameters optimized, efficiencies of 98.23% and 97.44% can be exhibited for TE and TM polarizations, respectively. Furthermore, efficiencies more than 95% can be achieved for the given fabrication tolerance, incident wavelength range, and angular bandwidth. The fabrication tolerance should be important for the practical manufacture of the wideband two-layer grating. The modal analysis is significant to explain the physical mechanism and design the two-layer grating.