Four-wave phase conjugation in liquid crystals with photoisomerizable dopants

The stable and high quality wave front conjugation (WFC) of short laser pulses in ate-dye doped LC was obtained. The effect is based on the conformational nonlinearity and implies that the refractive index changes arise due to the dependence of the LC order parameter on the concentration of photoinduced dye-isomers. It was found that increasing of the pulse energy density in the range (7.5...13.5) x 10(-2) J/cm(2) leads to the saturation of the conjugation coefficient. This effect was explained as a consequence of the light-induced changes of the absorption coefficient due to the saturation of the tl trans-cis transition. A phenomenological description of the nonlinear absorption was made on the base of the rate equations for the number of isomers. The nonlinear constant, which defines the WFC reflection coefficient, was estimated. It equals 0.25 cm(3)/J, that is ten times higher then the corresponding constant of the thermal nonlinearity. (C) 2000 Published by Elsevier Science B.V.