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

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1 2
k1, 1 k2, 2
Figure 2.1: Schematic experimental arrangement for two-wave mixing in PR materials.
drives the whole PR process is the interference pattern created by beams 1 and 2.
The spatial variation of intensity across the slab can be described by the modulus
square of the total electric field created by beams 1 and 2 (Eqs. 2.1)
I = |E1 + E2| 2 = ˜ E 2 10 + ˜ E 2 20 + 2 ˜ E10 ˜ E20 cos(K · r + ϕ1 − ϕ2) (2.2)
where K = k1 − k2 is the grating vector. This relationship between the beam wave
vectors and the grating vector assumes that the Bragg condition is satisfied. The
interference pattern described by Eq. 2.2 has a spatial wavelength (periodicity) given
by
Λ =
λ
2n sin ˜ θ
z
K
E
x
(2.3)
where n is the refractive index of the material and ˜ θ is the internal angle of incidence
for each beam with the respect to surface normal. The physical processes that occur