ABSTRACT
Diffractive optical elements (DOE) are optical elements incorporating fine structures to diffract light, as opposed to reflecting or refracting light. In general, diffractive optical elements diffract an incident plane wave into a number of reflected and/or transmitted diffraction orders. The diffraction operation is described by a set of amplitude coefficients, like Fresnel coefficients, for each diffraction order. Since each diffraction order diffracts a different amount of transverse electric (TE) and transverse magnetic (TM) modes, each order has different diattenuation and retardance. Often diffraction orders are quite polarizing. The calculation of these polarization dependent amplitude coefficients is generally performed by the Rigorous Coupled Wave Analysis (RCWA) which solves Maxwell’s equations for periodic boundary conditions.
Diffraction gratings are essential elements in spectrometers and monochromators but are the source of many polarization problems. Reflective diffraction gratings commonly have triangular profiles, where one grating facet receives most of the illumination. Aluminum grating surfaces are common for visible applications while gold surfaces are common for infrared applications. Light is distributed among all the diffraction orders in a complex fashion. In particular abrupt variations of the metal grating’s TM mode’s diffraction efficiency, the Woods anomalies, occur when the diffracted light in one order is diffracting tangential to the grating’s surface, and is transitioning from a real wave into an evanescent wave. The energy of the diffracted light is redistributed to lower diffraction orders, which causes abrupt changes in the diffraction efficiency, with corresponding polarization problems.
Wire grid polarizers are analyzed by RCWA where the effect of the depth of the wires on the polarizer performance is considered. Subwavelength phase gratings have retardance, but little diattenuation. The application of DOEs to retarders is considered but it is shown that the retardance is too small for most applications. Subwavelength gratings have found use as antireflection coatings and a lens is analyzed and the resulting polarization aberrations studied.
