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

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Ionization Potential
Sensitizer (N )
A
h
s
Shallow Traps (M )
T1
DA
Vacuum Energy Level
Electric Field E
Deep Traps (M )
T2
T
T
Transport Sites
Spatial Coordinate x
Figure 2.5: Schematic representation of the modified model for photorefractive polymers.
Symbols are described in the text.
modified model presented here differs from Schildkraut and Buettner’s by introducing
two kinds of traps - shallow and deep. Here, by the term “deep,” we mean that
the rate of thermal detrapping for these traps is at least an order of magnitude
lower than that of shallow traps, but still has a nonzero probability for detrapping.
The processes taken into account in here are depicted in Figure 2.5. A sensitizer
(acceptor) with density NA is excited and subsequently ionized by light of frequency
ω with cross-section s. A free hole is injected into the transport manifold and hops
between transport sites until it either becomes trapped or recombines with ionized
acceptors with rate γ. Although, generally, the energy spectrum of trapping sites has
a continuous distribution [14], we consider only two kinds of traps with well-defined
Energy