  Nonlinear optical propagation in cholesteric liquid crystals (CLC) with a
spatially periodic helical molecular structure is studied experimentally and
modeled numerically. This periodic structure can be seen as a Bragg grating
with a propagation stopband for circularly polarized light. The CLC
nonlinearity can be strengthened by adding absorption dye, thus reducing the
nonlinear intensity threshold and the necessary propagation length. As the
input power increases, a blue shift of the stopband is induced by the
self-defocusing nonlinearity, leading to a substantial enhancement of the
transmission and spreading of the beam. With further increase of the input
power, the self-defocusing nonlinearity saturates, and the beam propagates as
in the linear-diffraction regime. A system of nonlinear couple-mode equations
is used to describe the propagation of the beam. Numerical results agree well
with the experiment findings, suggesting that modulation of intensity and
spatial profile of the beam can be achieved simultaneously under low input
intensities in a compact CLC-based micro-device.
