In this study, several designs of photorefractive composites containing liquid crystals are presented, and their photorefractive characteristics are examined by the wave-mixing experiments.
Chapter 3 is descriptive of the photorefractive composite containing liquid crystals, photoconducting polymer, and charge generator molecules. In Chapter 4, various types of layer-structured photorefractive composite containing liquid crystal are introduced and discussed. In Chapter 5, the novel method for the investigation of the holographic grating formation characteristics is introduced.
First, the wave-mixing characteristics in photorefractive polymer-dispersed liquid crystals (PDLC), which contains a photoconducting polymer, low molecular liquid crystal and a photoconductive sensitizer, was examined as a novel type of photorefractive material. Poly(N-vinylcarbazole) (PVK) was used as a photoconducting polymer, 2,4,7-trinitro-9-fluorenone (TNF) as a photoconductive sensitizer, and E7 as a low molecular liquid crystal. The weight ratio of these three components in the tested sample was 10.75:0.2:89.15 (E7:TNF:PVK). Photorefractivity was demonstrated by the two-beam-coupling experiments in an external dc field. In order to test the composite as a photorefractive material, the photoresponse sensitivity was measured and two-beam coupling coefficient was calculated, as variations were given in the grating spacing, sample tilt angle, and applied voltage. This composite material showed comparable photoresponse sensitivity (with no external voltage) with general polymers, and a purely refractive-index grating ($\Delta n \sim 4.65 \times 10^{-5}$) was generated by irradiation of a interference light (laser diode, λ = 650 nm). The gain coefficient was saturated at a relatively low voltage of approximately 1.25 V/㎛, and the value was $7.07 cm^{-1}$ when the grating spacing was 3.54㎛.
In the second section of this thesis, several different designs of layer-structured photorefractive composites containing liquid crystals and their wave-mixing properties are introduced. When we introduced the PVK layer into the liquid crystals doped with a photocharge generator, the more stable asymmetric energy exchange in two beam coupling occurred. Since the PVK layer may play a role of a charge transporter and provide trapping sites, it is expected that stable energy exchange would take place in the sample with a PVK layer. The effects of the location of the charge generator in the layer-structured photorefractive composite were examined with a pair of E7-PVK-$C_{60}$. A much higher gain coefficient was obtained when the charge generator was located in the liquid crystal layer rather than the photoconducting layer. When $C_{60}$ is located in both the liquid crystal layer and photoconducting layer, it showed wave-mixing characters intermediate between those of the sample with $C_{60}$ located in the liquid crystal layer only and those of the sample with $C_{60}$ located in the photoconducting layer only. When $C_{60}$ is located in liquid crystal layer, the gain coefficient showed a maximum at a certain field strength. The change in birefringence of the sample cell as a function of external field was investigated and it was found that the dependency of the birefringence of the liquid crystal on external field was almost same as that of the gain coefficient. The maximum two beam coupling gain coefficient was about $120 cm^{-1}, 110 cm^{-1}$, and $40 cm^{-1}$ for the samples with the $C_{60}$ located in both the liquid crystal and photoconducting layer, in the liquid crystal layer only, and the photoconducting layer only, respectively.
Finally, a novel wave-mixing experiment, in which the photorefractive sample cell is rotated along with a rotating platform during the wave-mixing experiments, was performed. This experiment verified both the amplitude and direction of the external field dependency of the holographic grating simultaneously, and the speed of grating formation could be approximated. The gain and the loss beam were changed at a point where the bisector of the two beams was normal to the grating wavevector, and at that point the direction of the phase shift was also changed. When the rotation speed was increased, the gain coefficient decreased, and at the speed of 3°/sec, it became almost zero.
