Writing beam intensity

Figure 3.39. Holographic setup for photorefractive molecular glasses. The sample is tilted toward the grating, allowing an applied external field to support the motion of the mobile charges. The phase shift of the refractive index grating can be determined by measuring the transmitted writing beam intensities (two-beam coupling).

FIG. 11.13 Dependence of the initial growth of the photoinduced susceptibility in function of the writing beams intensities (in arbitrary units). The sample is a film of the DRI -MMA 35/65 copolymer, 0,1 pm-thick. 

Before any slit operation check, write down, or save the old motor positions Operation of slits can be useful to change the beam intensity (instead of operating absorbers). Imperfect thermal stabilization of mirrors and monochromators can be compensated by proper slit operation. Before such operation is undertaken, it should be made sure that the instrument is close to thermal equilibrium. In particular after opening the main beam shutter for the first time, it may be indicated to wait for several hours. Otherwise the operator will have to follow the thermal expansion continuously. This bears the risk to destroy the adjustment or even the detector. 

Figure 3.36. Different types of holographic gratings, (a) Both writing beams have the same polarization intensity grating. (b) The writing beams are left and right circularly polarized linear polarization grating, (c) The writing beams are s and p polarized mixed-polarization grating.

Figure 8 The phase-shift of PR material of compound 1 as a function of the applied field of steady-state refractive index grating with respect to the writing hght intensity grating. The inset is a typical intensity of the two beams when the grating is moved about 14 im in the direction of the grating vector. The solid line is the theoretical fitting using Eq. (20), with Eq = 99.9 V/p.m. (Reprinted from Ref. 27. Copyright 2000 The American Physical Society.)...

Uniform preillumination is discussed in the literature in two different time regimes. Fast diffraction response is obtained from materials which are illuminated uniformly and intensely for a few seconds before a second writing beam is used to pattern the intensity in the material and begin hologram formation. This maximizes the mobile hole density in the material at the time of hologram formation and helps to reach the charge generation limit of Eq. (1). 

After several tests, it is possible to estimate approximately the optimal relative intensities needed to obtain large photoinduced nonlinearities within relatively short preparation periods. The dependence of the generated SH signal is a function of the phase difference AO between the writing beams at CD and 2co frequencies. The relative phase difference AO can be varied by tilting a BK7 plate of known thickness and refractive index dispersion. ... 

FIG. t I.IO Intensity of the generated SH signal (in arbitrary units) after 20 minutes of seeding time, in function of the relative phase difference A between the writing beams at frequencies co and 2(o/The reference (i.e. A = 0) is arbitrarily taken at normal incidence of the writing beams onto the BK7 plate.The solid line represents a theoretical fit to Equation 11.6. The curve shown in the insert shows... 

FIG. 11.15 Intensity of the SH generated signal at saturation in function of the reluive intensities of the writing beams. Samples are spin-coated films of the DR I-MMA 35/65 copolymer with various thicknesses. Each experimental point is measured for an optimized value of the relative phase difference AO between the seeding beams. 

The photoinduced susceptibility shown in Equation 11.14a is the sum of two terms one with exp(-2Dt) (relaxation of the first-order parameter A ) decay and the second with exp(-12D ) (relaxation of the third-order pammeter A3) decay. Hence, the first very rapid decay may contain the fast exp(-12D ) contribution. However, as can be seen from Figure 11.14, the relative magnitude of this initial very fast decay does not depend on the optimization of the intensity ratio between the writing beams. So, this first rapid decay may not be due to the decay of the third-order parameter A3. In addition, because the hyperpolarizability P of DRl is different in the ds and in the trans state, the first very rapid decay also contains a contribution connected with the hferime of the metastable ds form, which is due to molecules coming back to the trans form without any net orientation. A better model would have to account for a distribution of diffusion constants for molecules embedded with various free volumes, which may explain the multiexponential behavior of the decay. 

The arrangement employed for the VPC experiment is described in Reference 4. A cw argon-ion laser at 488 nm was used in a standard DFWM geometry. The s-polarized output beam was first split by a beam-splitter to provide the pump and the probe beams. The transmitted beam from the beam-splitter was then divided into the two s-polarized pump beams each with a power of approximately 0.35 mW. The reflected beam from the beamsplitter was used as the probe beam, whose intensity was about 7% of the total intensity in both pump beams. The forward pump beam and the probe, which constituted writing beams, were overlapped at the sample. Their optical path length difference was much smaller than the laser coherence length, so that they were coherent at the sample. The backward pump beam was... 

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