Holograms may be recorded in photorefractive LiNbO3:Fe and LiNbO3:Cu w
ith infrared light pulses (wavelength lambda = 1064 nm, pulse duration
t(p) = 20 ns), if the crystals are previously or simultaneously illum
inated with green light pulses (lambda = 532 nm, t(p) = 15 ns). Refrac
tive index changes and time constants are studied for crystals with di
fferent iron and copper concentrations after thermal reduction and oxi
dation treatments. A model explaining this effect is discussed.