Study on broadband achromatic quarter-wave plate by subwavelength gratings

Based on rigorous coupled-wave theory, the relationship between diffraction property of rectangular phase gratings in resonant region and grating period, duty cycle, thickness is analyzed. An achromatic quarter-wave grating for 400-800 nm is designed. In the broadband, the grating has smooth efficiency and phase retardation. Specially, the phase retardation could maintain about 90°C, and the maximum retardation deviation is less than 9%. Its performance is even better than some normal binary achromatic wave-plates. With the analysis of fabrication error, it is noted that thickness error and duty cycle error affect the grating's performance greatly.