Designed achromatic quarter wave plates of subwavelength grating structure

We presented the design of an achromatic quarter wave plates based on subwavelength grating structure and considered the dispersion of material in design. The refractive indices of most material become small with the light wavelength increasing, and the dispersion of effective refractive indices of TE and TM polarized waves in the subwavelength structure increase with the light wavelength. Using this feature, we can design a quarter wave plates whose phase retardation is maintained at π /2 for a larger wavelength range. A design method using the effective medium theory is described, and the transmittance of TE and TM polarized waves are evaluated by numerical calculation with the rigorous couple wave theory. The results showed that phase retardation of the quarter wave plates had maintained retardation errors at ±1° for a ±9.85% change in wavelength, and the transmittance of both TE and TM waves were more than 94%. We also designed an achromatic quarter wave plates applied to visible light wavelength range using the method whose phase retardation maintained at ±2° in the 0.47 μm ~ 0.63 μm wavelength interval, and there also had a transmittance of more than 94% for the TE and TM waves.