Biphoton-induced refractive index change in 4-amino-4'-nitroazobenzene polycarbonate

Transient biphotonic optical storage in the guest-host system 4-amino-4'-nitroazobenzene/polycarbonate is accomplished by excitation of the azo dye molecules at 488 nm with an argon ion laser and subsequent writing at 633 nm with two interfering HeNe laser beams. The mechanism of the observed refractive index change is investigated by detailed kinetic studies. It is based on the formation of cis isomers of the azo dye by the argon ion laser. These are subsequently transformed back to the trans isomers by fast photoinduced cis-trans isomerization where the interference of the writing beams is constructive and by slow thermal cis-trans isomerization where the interference of the HeNe beams is destructive, respectively. The cis concentration produced by the exciting Light is thus modulated with the HeNe beams, resulting in a modulation of the refractive index.