The second harmonic light generated in crystals with a random distribution of nonlinear domains is usually emitted in a
broad range of directions. When the fundamental light has good coherence, the intensity of the second harmonic shows a
speckle pattern even when the crystal is transparent. We explain that with the interference at the detection point of the
second harmonic generated by the different domains. Using a phase-only spatial light modulator in the fundamental
beam, it is possible to concentrate the second harmonic intensity in one direction at the same time that the intensity is
reduced in the other directions. In our experiments we measured enhancements in the selected direction up of 700 times
over the average intensity in other directions.
The nonlinear optical response of gold films of different thicknesses is investigated by two-beam second-harmonic
generation to address the role of surface and bulk effects in their second-order nonlinear optical response. Preliminary
results suggest that both types of effects contribute to the measured second-harmonic signals. Furthermore, both effects
are found to be enhanced for the thin film with a higher level of nanoscale surface roughness.
We show that hydrogen bonding between azo molecules and polymer host enhances the photoinduced optical
anisotropy in azo-containing polymers without sacrificing the ease of processing of conventional guest-host
systems. The primary mechanism behind the enhancement is the possibility to use high dye doping levels
compared to conventional guest-host systems due to reduced aggregation tendency of the dyes. For Disperse
Red 1, the saturated birefringence is enhanced by a factor of 8 due to hydrogen bonding at 30 wt % loading.
Moreover, hydrogen bonding reduces the mobility of the guest molecules in the polymer host which improves
the temporal stability of the induced birefringence to a level comparable to side-chain polymers.
The all-optical poling technique allows writing non-centrosymmetric gratings that are automatically phase-matched for
second-harmonic generation by purely optical means. One drawback of all-optical poling in organic materials is the poor
stability of the recorded gratings due to thermal and/or photo-induced molecular randomization. Using a two beam
technique, we have compared the all-optical poling process in different kinds of polymers with Disperse Red 1 dye:
guest-hosts with hydrogen-bond interactions between the guest dye and the host polymer and a side-chain polymer in
which the dyes are attached through covalent bonds. We show that in the investigated polymer systems, hydrogen-bonded
guest-hosts are capable of surpassing the stability of side-chain polymers.