PHOTOREFRACTIVE NONLINEARITY IN PURE AND DOPED LIQUID CRYSTALS

Open Access
Author:
Chen, Kan
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 22, 2006
Committee Members:
  • Iam Choon Khoo, Committee Chair
  • Thomas Nelson Jackson, Committee Member
  • Timothy Joseph Kane, Committee Member
  • Shizhuo Yin, Committee Member
  • Mark Maroncelli, Committee Member
Keywords:
  • liquid crystals
  • photorefractivity
  • holographic gratings
  • space charge field
Abstract:
A detailed analysis of the photorefractive (PR) effect in liquid crystals (LC) is presented. The photorefractive nonlinearity arises from the formation of spatially modulated space charge field. Two main mechanisms contribute to the formation of space charge field in liquid crystals. One comes from the formation and the subsequent dissociation of charge transfer complexes between the liquid crystal and the dopant, which produces free charge carriers. The drift and diffusion of carriers under the combination action of external dc voltage and inhomogeneous illumination forms the space charge field. Second source comes from the so-called Carr-Helfrich effect, which states that transverse space charge field can be induced by liquid crystal dielectric and conductivity anisotropy under external applied field. Assisted by the space charge field, large nonlinearity of liquid crystal can be obtained. The doping effect, especially for single walled nanotubes (SWNT) doped liquid crystal, is studied. SWNT doping can significantly enhance the photorefractive nonlinearity of liquid crystals. We believe strong photorefractive response of SWNT-doped LC is mainly due to the high conductivity anisotropy of SWNT. The photorefractive property of SWNT doped LC as a function of doping concentration is also investigated. Dynamic scattering is found to be further inhibited in LC samples with higher doping concentration. SWNT dopants seem to form loosely connected network and encumbrances ion flow induced dynamic scattering, which makes it possible to obtain supra photorefractive nonlinearity using overdrive and undershoot voltage effect. The photorefractive effect in pure LC samples is also investigated, with our focus on the surface induced effect. Theoretical models were derived to identify the photorefractive threshold voltage. The PR threshold voltage can be substantially lowered with the increase of incident light intensity, which is mainly due to inhomogeneous interface ions distribution induced surface torque. The thickness dependence of threshold voltage lowing effect was also measured, which allows us to estimate the penetration depth of surface torque. Our result shows surface effect strongly affects photorefractive properties of LC. Based on our preliminary work. several potential further researches are proposed, which are all related to photorefractive effect, yet fall into three categories . First is to further study doping effect: increase SWNT doping concentration using functionalized nanotubes and try other novel doping materials such as nanometals. Second is to continue the investigation of surface effect in doped LC samples. We anticipate more interesting surface effect exists in doping system. The last one is to study PR effect in new LC composites, such as PSLC (polymer stablilized liquid crystal) and PDLC (polymer dispersed liquid crystal).