|| The advantages of liquid crystal (LC) diffraction gratings include their low cost and low-power electrical switchability. Liquid crystal is easily controlled by exploiting common electro-optical behaviors. To form a phase grating profile, the LC must have a periodically varying refractive index. Previous works by the authors showed that, because of the intrinsic optical anisotropy of LCs, the optical properties of|
these gratings are highly dependent on the polarization state of incident light. Some polarization-independent gratings use conventional orthogonally aligned nematic LC in one cell. However, even though the diffraction intensity is independent of light polarization, the diffraction beam polarization still depends on the angle between the polarization of the incident beam and the LC alignment.
Blue phases (BPs) have been in chiral liquid crystals between the cholesteric and isotropic phases The BP of a self-assembled three-dimensional cubic structure with lattice periods of several hundred nanometers exhibits not only selective Bragg reflections of light in the visible wavelength, but also optical isotropy owing to its highly symmetric molecular structure. A BPLC is optically isotropic when no voltage is
applied, and phase-only modulation occurs when an electric field is applied parallel to the direction of propagated light. However, the use of in-plane switching (IPS) electrodes makes the grating polarization-dependent and retains background diffraction from the patterned electrode.
This dissertation discusses the control of a polarization-independent and rapidly responding polymer-stabilized blue phase liquid crystal (PSBP) phase grating by exploiting the variation in the voltage-induced birefringence of the LC under a varying phase. The electro-optical properties, including the voltage-dependent diffraction intensity, polarization-independence and response time, are also discussed.
Firstly, the hybrid PSBP liquid crystal phase grating was studied. This grating consists of hybrid PSBPs with different Kerr constants that are determined by varied phase separation conditions. When cured at a high temperature, a loose PSBP network is formed and a large Kerr effect is obtained. Accordingly, the electric-field-induced birefringence of PSBPI is lower than that of PSBPII at a given driving voltage. The
non-patterned electrode and optical isotropy of both PSBPI and PSBPII enable elimination of the diffraction effect when the voltage is off. The diffraction intensity increases with the applied voltage up to a maximum at 150V.
Secondly, a polarization-independent and rapidly responding dye-doped (DD) PSBP liquid crystal phase grating was realized with two different phase states. The non-patterned electrode and the optical isotropy of the PSBP completely eliminate the diffraction effect when the voltage is off. The diffraction intensity can be increased by applying a uniform electrical field, which induces a phase difference in the DDPSBP. The diffraction efficiency exceeds 6%, which is 7-fold higher than those of most hybrid PSBP gratings and the device also supports optical writing, erasing, and rewriting. Additionally, the driving voltage is much smaller than that of hybrid PSBP grating. The phase grating is completely independent of polarized incident light. Finally, switching response time is in the sub-millisecond range.