Time-dependent Dynamical Reorientation Induced By CW Laser Beam In A Nematic Liquid Crystal Film

Liquid crystals in their mesophases are attractive materials for nonlinear optical devices because of their huge response to optical fields and their capability of being easily inserted in optically integrated structures. From the macroscopic point of view, we may consider a nematic liquid crystal as a fluid endowed with internal orientational degrees of freedom. These additional degrees of freedom, absent in ordinary fluids, may be accounted for by the director field n(r), a unit vector field attached at each point r in the medium. Unlike in the case of externally applied static fields, where n couples with the external field directly, in the case of optical fields n couples with the light polarization. As a consequence, n is driven by exchange of angular momentum between the light beam and the medium. This yields the appearance of various new nonlinear optical phenomena having no analogue in the static-field case. For instance, the molecular director n can be put into uniform precession about the beam propagation direction by transfer of intrinsic angular momentum (photon spin) from the beam to the liquid crystal. This effect can be observed only with circular polarization of the incident beam, for, in this case only, the deposition of angular momentum in the medium has a constant rate. With an elliptically polarized beam, the situation is far more intriguing. The main difference between circular and elliptical input polarization is that in the former case the molecular reorientation first breaks the azimutal symmetry of the whole system ( radiation + liquid crystal). This yields characteristic different results in the two cases: the elliptically polarized input can lead the system through various dynamic regimes: torsional oscillations, nonuniform precession, nutation of n superimposed on precession, and others.