Wide bandgap photoconductor (SiC:V)-based optically addressed light valve for high fluence operation

Optically addressable light valves based on wide bandgap 4H- and 6H-SiC as photoconductors were designed to withstand higher operational laser fluences than the state-of-the-art bismuth silicon oxide (BSO; Bi₁₂SiO₂₀)-based devices. Vanadium-doped SiC was selected as the photoconductor due to its reasonable photoresponsivity while many-fold improvement in laser-induced damage threshold as compared to BSO. The laser-induced damage threshold values of the materials were measured after exposing ∼200 sites on the samples to increasing levels of fluence of a Gaussian pulsed Nd:YAG laser system (1064 nm) with a 5 Hz repetition rate. The measured damage threshold values for BSO, 4H-, and 6H-SiC were 0.4, 1.75, and 1.8J/cm², respectively. Photoconductive switches based on 4H- and 6H-SiC samples were characterized at wavelengths of 380, 405, and 447 nm. The peak photoresponsivity values of the 4H- and 6H-SiC materials were measured to be under 380 and 405 nm, respectively. The photoconductor was bonded to a BK7 optical window with 5 µm diameter microspheres as spacers. A twisted nematic type E7 liquid crystal (LC) was filled in the 5 µm gap in a vacuum chamber. The desired alignment of the liquid crystal was achieved by mutually orthogonal orientation of LC alignment layers on the two mating faces (SiC and BK7). The fabricated devices were modulated using address beams of wavelengths 380, 405, and 447 nm. Required transmission levels of >90% were achieved for the fabricated OALVs for a sinusoidal voltage waveform that meets the lifetime requirement of the device.