A polarization-independent two-dimensional grating with strong process robustness and high diffraction efficiency

To maintain groove profile consistency across large 2D gratings, this paper proposes a gold-based polarization-independent 2D grating that combines high process tolerance and excellent diffraction efficiency, thus enabling efficient fabrication of 2D gratings. By establishing a comprehensive perturbation model incorporating morphological errors, we analyze the coupling effects of material and geometric parameters on the (−1,0)-order diffraction efficiency of metallic 2D gratings for 780 nm transverse electric (TE) or transverse magnetic (TM) polarized light under Littrow incidence. Analysis results indicate that within tolerances of ±35 nm for groove depth, ±0.1 for duty cycle, and ±15° for sidewall angle, the dual-polarization diffraction efficiency remains above 60 %, with theoretical maximum efficiencies reaching 94.67 % for TE polarization and 92.96 % for TM polarization. Measurements on fabricated gold-coated 2D gratings using dual-beam interference lithography show that dual-polarization efficiencies consistently exceed 70 % when the duty cycle is adjusted with sidewall angle variations within ±15°. The optimal sample achieves 84.15 % TE efficiency and 84.32 % TM efficiency with a polarization imbalance of only 0.1 %, demonstrating excellent agreement between experimental results and theoretical predictions. The robust design presented here establishes an important theoretical foundation for fabricating large 2D gratings with high diffraction efficiency.