Polarization Gratings (PGs) are anisotropic optical elements where the optical anisotropy (birefringence, optical axis, dichroism) is spatially varying. These are fundamentally different from conventional diffraction gratings which are usually amplitude or phase gratings. Although this term broadly describes the functionality of these elements, our research group and others have suggested alternative names (geometric phase holograms, optical axis gratings) to describes these optics.
There are several compelling reasons for studying these elements. These gratings act as highly efficient, thin film, polarizing beam splitters. Plus, the wavelength range over which they can do this can be custom tailored. This is illustrated in the picture below which shows a PG splitting incoming light into two rainbows, one on each side. The central direction (0-order) looks dark (slightly bluish) because this particular grating was designed to split almost all of the white light (450 nm - 650 nm).
Not only can PGs split light efficienctly as shown above, the splitting in the individual directions can be precisely controlled by controlling the polarization of input light. This is shown in the images below where by doing this, light can be switched from the left rainbow to the right rainbow.
Beyond this, the 2D pattern of the PG can be changed to shape the beam appropriately as well (as a lens, etc.) Further PGs can be easily fabricated across large areas at low cost using commercial materials. These key features make PGs extremely important for several applications.
Through my research, I was able to initially develop foundational design rules for these elements, critical for practially realizing high quality PGs. This was accomplished by modeling these elements using both elastic continuum principles (fluid dynamics), and Finite-Difference-Time-Domain (FDTD) concepts. Our group has developed an in-house simulation tool called WOLFSIM for modeling complex anisotropic optics.