Two-beam coupling

TBC in a PVK-based system with 4-piperidinobenzylidene-malononitrile (PDCST) as NLO chromophore an out-of-phase oscillation of detected intensities is observed when the interference pattern is translated quickly. On the basis of the dependence of t on the grating spacing and the appHed electric field, a trap density of 1.5 x 10 cm was estimated in this system [74]. 

On the basis of the dependence of F on various parameters such as magnitude of electric field, intensity of the light beams, and the interbeam angles, one can deduce information about the processes and governing physical parameters at the microscopic level. A TBC effect has been observed without an applied electric field in high Tg-poled polymers and sol-gels [63, 76]. 

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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).

To date most of the polymeric materials have demonstrated potential rather than acmal commercial applications. The two-beam coupling behaviour has obvious applications in real-time optical processing and holographic data storage. 

Figure 5. Photorefractive two-beam coupling gain coefficient of undoped and doped BaTi03 crystals. Curves are theoretical fits to the data. Experimental conditions X = 515 nm, I = 3-5 W/cm2, beam ratio > 200, s-polarization, grating wave vector parallel to c-axis Concentrations refer to dopant atoms per BaTi03 formula unit in the melt.

The two-beam coupling gain coefficient can be derived from Eqs. (5) for Beam 2 (the gain beam) ... 

The photorefractive effect is usually probed by two beam-coupling experiments, in which one beam gains intensity at the expense of the other. This coupling is a characteristic property of the photorefractive effect. Such an asymmetric coupling requires an asymmetric shifting of the refractive index grating... 

Figure 9.10 Absolute value of the two beam coupling gain T versus the scattering angle 0S for T = 20 °C (circles), T = 45 °C (squares) and T = 65 °C (crosses). The solid lines are fits according to Equation (9.5).

Figure 25. A) Optical switching directing telephone calls from callers 1 -4 to possible receivers 5-8. B) The five individual switches utilize the two-beam coupling effect in photo-refractive polymers, possibly in a waveguide geometry. By applying a gating input the signal can be redirected to an alternative output route.

Fig. 15. a Geometry for four-wave mixing experiments, b Geometry for two-beam coupling experiments... 

Fig. 17. Schematic of the tilted configuration for four-wave mixing and two-beam coupling experiments in photorefractive polymers...

If bifimctional molecules do not form glasses, they can be incorporated into polymer binders. Examples of bifunctional molecules are shown in Fig. 24. DTNBl (molecule (b) in Fig. 24) was doped in PMMA and Cgg was used as a sensitizer. Diffraction efficiencies of 7%, sub-second grating growth times, and net two-beam coupling gain coefficients of 34 cm" were observed in such samples [81]. DPANST (molecule (c) in Fig. 24) was doped into PBMA [84]. Recently, ms response times were reported in photorefractive glasses based on the bifunctional chromophore DRDCTA (molecule (d) in Fig. 24) doped with the plasticizer DOP (molecule (b) in Fig. 23) and Cgg as a sensitizer [89, 90]. 

Fig. 27. Field dependence of the two-beam coupling gain measured in the photorefractive polymer composite PDCST PVK BBP C5o squares single sample (140 pm-thick) circles a two-layer stack triangles a three-layer stack... 

FIgurs 3. The dependence of diffraction efficiency on applied dc electric field for a PN/KtCgg DEANST composite film (A f, g=645nm, A, jj=632.8nm). The inset shows the beam intensity changes in an asymmetric two-beam coupling process (A=645nm). The arrows denote the moments of switching the electric field "on" and "off". (After Ref. 29). 

A series of photorefractive poly(methacrylate)s, containing a Disperse Red-type chromophore and carbazole as the charge transport agent with various spacer lengths, was synthesized and characterized. The photorefractive effect of these materials was studied by four-wave mixing and two beam coupling at 780 nm after sensitization with 1% (2,4,7-trinitroflu-... 

Fig. 4.3 Two-beam coupling experiments yielding evidence for the occurrence of the PR effect in a film consisting of polymer VI (MHB Br ). The intensity of beam 1 was monitored as beam 2 was switched on at t=0 and switched off at t = 90 s, and the...

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