ACHROMATIC QUARTER-WAVE PLATES USING THE DISPERSION OF FORM BIREFRINGENCE

We propose achromatic quarter-wave plates of a subwavelength grating s tructure. When the period of the grating structure is smaller than the wavelengths of the incident light, the structure is considered to be an optically anisotropic medium. The effective refractive indices stro ngly depend on the wavelengths, especially when the period is close to the wavelength. Using this feature, we can design a grating quarter-w ave plate whose phase retardation is maintained at pi/2 for a wide wav elength range. A design method using the effective medium theory is de scribed, and the wave plates designed were evaluated by numerical calc ulation with a rigorous electromagnetic grating theory. The calculatio n results led to the possibility of an achromatic quarter-wave plate w hose retardation errors are smaller than 3 degrees for a +/-10% change in wavelength. (C) 1997 Optical Society of America.