High-efficiency polarization-independent gold-coated crossed grating under normal incidence

A gold-coated crossed grating (GCCG) is designed and fabricated to improve the signal contrast and signal-noise-ratio (SNR) of the grating interferometer (GI) with the high-efficiency polarization-independent property. The proposed grating based on two dimensional (2D) periodic gold nanocylinder structure can diffract both TE and TM polarizations into four first orders with measured diffraction efficiency (DE) of more than 21.8% and total DE of more than 88.8% at 780 nm incident wavelength under normal incidence, which shows the highest DE and brings much benefits to the SNR and compactness in practical applications. Moreover, the optimized GCCG also shows great tolerances of duty cycle, groove depth, and sidewall angle for grating fabrication. The measured DEs from the experimental results substantiate themselves with the numerical calculations. Additionally, the measured DE of the (±1, 0) order and (0, ±1) order all exceed 18% at the wavelength range from 750 nm to 810 nm and present great polarization independence at the polarization angle varying from 0° to 90°. The experimental results show the excellent performance of GCCG for improving heterodyne grating interferometer and indicate great potential for applications in microscopy techniques, semiconductor manufacturing, and material processing.