Abstract

Bragg gratings have been recorded holographically in photopolymers doped with nematic liquid crystals. Gratings are enhanced with liquid crystal doping, as evidenced by increased diffraction efficiency. Microscopy studies support the theory that microscopic phases of liquid crystals are formed during the curing process. In situ monitoring of diffraction efficiency during photocuring allows a study of the grating formation kinetics as a function of laser power. At 0.58 mW/cm², 90% of maximum diffraction efficiency has been achieved in 2.2 s. Measurable diffraction occurs in 250 ms. Post-irradiation curing has also been observed. Diffraction efficiency at 633 nm of fully cured samples is as high as 60% for a Bragg angle of 57.5° (2700 lines/mm grating). The diffraction efficiency can be controlled by the application of ac voltages. The Bragg diffraction peak has been observed to completely vanish at approximately 300 V rms. This indicates an electro-optic control of the refractive index modulation formed by the microscopic liquid crystal phases.

© 1992 Optical Society of America