Dynamic gratings in a chiral nematic liquid crystal

摘要

Extremely strong dispersion of light is characteristic of the frequency domain of photonic band gap (PBG) boundary that in case of a chiral nematic liquid crystal (CLC) results in a strong frequency dependence of the optical activity of a CLC layer.rnIn the present work the nonlinear behaviour of the rotation of light polarisation plane is investigated in the spectral region of the selective reflection (SR) band when the light-induced modulation of the helical pitch is achieved by means of the either temperature pitch dependence or photo-stimulated changes of the molecular conformation of the chiral dopant. Nonlinear optical modulation of the optical activity allows for recording of dynamic diffraction gratings with peculiar polarisation states of the diffracting orders. At the edge of the SR band the change of phase retardation between circular eigen waves of differing handedness well exceeds pi/2 even for a few-micron-thick LC layer. Given the proper choice of the experimental parameters the effective parameter of cubic nonlinearity in such a system could surmount by orders of magnitude that characteristic of the "giant" optical nonlinearity of liquid crystals.rnChiral nematic LC with light-controlled helical pitch were investigated from the standpoint of diffraction grating recording. The experimental data have shown a strong potential of these media in different aspects of optical signal processing, all-optical switching and photonics. Theoretical modelling has provided an excellent agreement with the experiment, the role of elastic constrains on the grating formation has been also taken into consideration.