Achromatic Polarization Gratings
We design, model, and implement achromatic polarization gratings (PGs) in forms of thin polymer films as well as active liquid crystal cells for electro-optical switching. We investigate a number of different configurations of PGs for the best performance in terms of high efficiency and bandwidth. We also study the fundamental diffraction limits (i.e., grating regime, angular performance, polarization sensitivity, etc) of achromatic PG designs using analytical and numerical techniques. Various fabrication techniques are considered to develop an effective way to realize achromatic PGs.
Motivation for Approach
Diffraction gratings are often used in spectrometers, optical data processing devices, optical communication devices, and many other optical instruments because of their ability to separate light into different wavelengths (dispersion). However, wavelength sensitivity of diffraction efficiencies has been a problem of diffraction gratings in applications where a broadband light source is used. We have found that unique and effective ways to realize achromatic diffraction using polarization gratings, which are anisotropic diffractive elements composed of a spatially varying linear birefringence pattern.
One of our approaches for achromatic PGs is to use the self-compensation structure. As shown in figure, we stack two anti-symmetric PG layers with opposite twists. The compensation can be done by balancing chromatic dispersions due to linear birefringence and induced optical activity (or circular birefringence) by twist. A similar effect is well known in super-twisted liquid crystal displays. When optimized, nearly 100% efficiency can be achieved from a single diffraction grating over a broad range of spectrum (i.e., whole visible wavelengths). A reflective version of the achromatic PG is also possible by using the asymmetric nature of twist on reflection. We develop fabrication techniques to create both transmissive and reflective achromatic PGs using UV holography and commercial liquid crystal materials.
Achromatic polarization gratings are the first diffraction grating in a practical sense that shows achromatic diffraction at ~100% efficiency over a wide range of spectrum. Achromatic PGs (transmissive and reflective) allow unique opportunities to new applications or extension of existing technologies dealing with broadband light source (e.g., optical films/devices for displays, spatial light modulators, spectrometers, and etc.). Especially, these thin gratings benefit spectrometers for visible and IR light. Since achromatic PGs show same diffraction properties over a wide range of wavelengths, instruments can detect or measure the spectral information without mechanical or electrical switching. In addition, achromatic PGs can be used as polarizing beamsplitters which are thin, light-weight, broadband polarization optics and can be easily adapted to existing optical systems. We also note that these anisotropic gratings can be used in both visible and IR wavelength ranges. In fact, there is no fundamental limit in its operation range except material restrictions (i.e., absorptions).
Polarization Gratings (PGs)
PG Fabrications and Applications
Liquid Crystal Materials and Related Technologies