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

22
Figure 2.3). Due to nonlocality of the grating, one diffracted beam (for example, 1’)
interferes destructively with its companion beam 2, and the other diffracted beam 2’
interferes constructively with beam 1. As a result, the beam 1 is amplified (energy
gain) while the beam 2 experiences energy loss. The direction of energy transfer de-
pends on the sign of the electro-optic coefficient and the sign of the charge carrier.
The experiment involves measurement of the transmitted beam intensities. In the
case of a purely local grating, there is no energy transfer and thus, the intensities of
the transmitted beams will be the same as those of the incident beams (if we neglect
absorption and reflection losses).
As one notices in Figure 2.3, the beams 1 and 2 are incident at some angles
(θ1 and θ2) with respect to a sample surface normal. This choice of experimental
geometry is governed by several factors. First, all the processes in polymers that
are responsible for space charge field build-up are strongly electric field dependent.
Therefore, to assist in charge transport along the grating vector K, a large component
of external electric field Ea in the direction of the vector K is needed. Second, since
the NLO chromophores that are part of the PR polymer composite are aligned in the
direction of applied field, it is necessary to provide a nonzero component of electro-
optic response along the grating vector K.
Ideally, the beams 1 and 2 would be incident symmetrically from opposite sides
of the sample, so that the directions of applied electric field Ea and grating vector K