Achromatic phase retarder by slanted illumination of a dielectric grating with period comparable with the wavelength.

We discuss some properties of dielectric gratings with period comparable with the illuminating wavelength for slanted illumination (this illumination geometry is often referred to as concical mounting). We demonstrate the usefulness of such an illuminating geometry. We show that the threshold period (under which only the zeroth transmission and reflection orders are nonevanescent) can be significantly higher, thereby easing fabrication constraints, and that this illumination setup makes it possible to design achromatic phase retarders. Such a design, for an achromatic quarter-wave plate with lambda/60 uniformity of the retardation phase in the 0.47-0.63-mum wavelength interval, is demonstrated.