Electro-Optic Holograms
Electro-optic (E-O) holograms can be electrically switched on-and-off through
E-O effect. These holograms are formed by recording volume phase hologram in
liquid crystal-monomer mixture. Typically, electro-optic holograms are recorded
by illuminating the liquid crystal-polymer mixture with two mutually coherent
laser beams, which interfere to form the desired grating structure. During the
recording process, the monomers polymerize and the liquid crystal-polymer
mixture undergoes a phase separation, creating regions densely populated by
liquid crystals, interspersed with regions of clear polymer. The alternating liquid
crystal and polymer regions form the fringe planes of the grating. The resulting
volume phase hologram can exhibit very high diffraction efficiency, which can be
controlled by the magnitude of the electric field applied. When an electric field is
applied to the hologram via electrodes, the natural orientation of the LC droplets
is changed to cause the refractive index modulation of the fringes.  In other
words, the diffraction efficiency of electro-optic holograms can be adjusted, by
means of the applied voltage, over a continuous range from essentially zero to
near 100% at very fast switching speed.