Photorefractive composites of liquid crystals and photoconducting polymers for holographic optical devices

Abstract

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 photorefr...