Nonlinear Optical Probes and Processes in Polymers and Liquid ...

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modify the refractive index of a material in response to an optical beam. However,
these mechanisms are local and lack the nonlocal property of the PR effect arising
from the physical motion of charges in the material. This charge transport leads
to a spatial phase shift between the incident light intensity interference pattern and
refractive index modulation. In local mechanisms, the spatial phase shift is equal to
zero. A consequence of the nonzero phase shift in PR materials is energy transfer, so
called asymmetric two-beam coupling (2BC), between two light beams interfering in
a PR medium. If the coupling is sufficiently strong, so that the 2BC gain exceeds the
absorption and reflection losses, then optical amplification occurs. This cannot hap-
pen in local materials, and thus the presence of energy transfer is a distinct feature
of the PR effect in a material. So, when a new material is tested for photorefractive
properties, 2BC is the first experiment to perform to ensure the presence of energy
transfer due to the PR effect. Also, based on 2BC gain, many applications have been
proposed such as coherent image amplification, novelty filtering, self-pumped phase
conjugation, beam fanning optical limiters and others [5].
Given such a variety of proposed applications for PR materials, questions of opti-
mization, low cost and easy fabrication have arisen. This is how organic PR materials
and in particular polymeric PR materials have entered the scene. One motivation for
pursuing the development of organic PR materials is the possibility of achieving a
high figure-of-merit that compares the refractive index change possible in different