Liquid Crystal Polarization Gratings for Photonics Applications
Our objective is to develop a liquid crystal (LC) display element capable of modulating unpolarized light with high contrast, and to ultimately integrate it into a highly efficient portable projection display based on a light-emitting-diode (LED) light engine. A family of theoretically polarization-independent, binary, LC gratings was previously studied [1-3] but was plagued by the presence of domain boundary lines and random disclinations, were Diffraction gratings that operate by periodically modulating the state of polarization of light passing through them are generally classed as "polarization gratings" [6, 7]. These are composed essentially of a periodically patterned optical anisotropy, and offer a unique opportunity to create switchable diffractive optics, particularly since it was observed early that PGs with 100% diffraction efficiency into the first order(s) could be achieved with a thin-grating regardless of input polarization. Several research groups [8, 9] recognized that a continuous LC diffractive grating will have improved diffraction properties (over binary LC gratings), and that holography can be used to greatly simplify fabrication  and achieve smaller grating periods. Further theoretical studies by Zeldovich and coworkers [11, 12] identified compelling characteristics, including the potential to modulate unpolarized light with high contrast. Initial experimental results by Crawford and coworkers [10, 13] were promising, but were plagued by pervasive defects degrading their optical properties. Consequently, the maximum diffraction efficiency and switching contrast ratio was poor, and strong incoherent scattering outside of the diffraction orders was present . We have overcome these deficiencies by carefully balancing the choice of LC and photo-alignment materials with cell geometry to experimentally realize ideal polarization gratings. As far as we are aware, this is the first experimental success at realizing the anticipated properties of LCPGs with high quality. limited to very large grating periods, and did not achieve theoretical diffraction efficiencies (limiting contrast and brightness). Even the more recent improvements [4, 5] with polymer-wall LC gratings still manifest less than ideal efficiencies, diffract noticeably up to the 5th diffraction order, andare challenging to fabricate at periods on the order of 10s of μm.The central limitation in all of these approaches is the binary nature of the gratings.