Holographic PDLCs for optical beam modulation, deflection, and dynamic filter applications

A new type of electro-optical device, a switchable hologram: has emerged from the combination of two technologies, namely, photopolymer holographic materials and polymer-dispersed liquid crystals (PDLCs). Starting from a simple homogeneous mixture containing a polymerizable monomer, a liquid crystal (LC), and a photoinitiator dye, this system cures under holographic illumination to form well defined channels of PDLCs interspersed between dense, LC-free polymer channels. These periodic PDLC planes produce diffraction of light in the Bragg regime with surprisingly good optical quality. LC droplets within the PDLC channels can be remarkably small, ranging in size from 20 nm to 200 nm. Even within this small domain, the molecules can be reoriented by an applied field, leading to electro-optical modulation of the diffraction efficiency. The polarization properties and sharp threshold switching of the diffracted Light may be partially explained by the unique shape and consequent nematic ordering of the LC droplets. Complex holograms can be written in this material, and electro-optical switching at <5 V/mu m with response and relaxation times in the 20-40 mu s regime have been observed. With these attractive properties and the promise of further improvements, many applications are under active consideration. Spatial light modulators which modulate the intensity can make use of these switchable holograms. Some of the potential applications are in fiber optic switches, programmable optical interconnects, digital zoom lenses, optically assisted true-time-delay phased array radar, flat panel displays, and dynamic filters.