Photorefractive performances in polymeric and molecular glass composites for optical memories

Abstract

In this paper, photorefractive performances in polymeric and molecular glass composites are presented for optical memories. Oxidation potential equivalent to ionization potential of each component is sensitive to the photorefractive performances. The combination of poly(diphenylamino)styrene (PDAS) as host photoconductive matrix and aminostyrene derivative of 4-azacycloheptylbenzylidene-malononitrile (7-DCST) as a NLO dye gave the better and faster photorefractive responses compared to that of poly(N-vinylcarbazole) (PVCz) and 7-DCST. This is ascribed to the fact that oxidation potential of PDAS and 7-DCST are close to each other, whereas 7-DCST with lower oxidation potential works as hole trap in the composites of PVCz with higher oxidation potential. Grating geometry is also important. Two types of grating geometry of transmission and reflection was employed for photorefractive performances of composites of molecular glasses endcapped with carbazole moieties. Reflection grating can be used for weak absorption film or no absorption film. Large net optical gain was obtained in reflection grating geometry due to the introduction of 7-DCST as an effective trap sites. Another interesting results were large asymmetric energy transfer and optical diffraction without applying electric field. Using the same type of composites, large optical gain up to 224 cm -1 and diffraction efficiency of ca. 90 % were measured. Glass transition temperature of composite was sensitive to the diffraction efficiency and grating buildup speed. Long lived TNF anion radical and carbazole or triphenylamine cation radicals were responsible for the photorefractive performances in non-electric field condition. Thermally diffused cation radical is trapped in the dark region and leads to the asymmetric energy transfer and diffraction grating.