Optical wave-mixing and photorefractivity in liquid crystals
Open Access
- Author:
- Ding, Jianwu
- Graduate Program:
- Electrical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 09, 2004
- Committee Members:
- Iam Choon Khoo, Committee Chair/Co-Chair
Kultegin Aydin, Committee Member
Ruyan Guo, Committee Member
Thomas Nelson Jackson, Committee Member
Mark Maroncelli, Committee Member - Keywords:
- nonlinear optics
photorefractivity
liquid crystals
stimulted scattering - Abstract:
- Optical wave-mixing has a lot of applications such as optical amplification, adaptive optics, hologram and optical image processing, harmonic wave generation, and multi-photon microscopy. Photorefractivity has been utilized in many important applications including high density optical data storage, image processing (correlation, pattern recognition), spatial light modulation, phase conjugation, optical limiting, simulations of neural networks and associative memories, and programmable optical interconnections. They have been investigated, theoretically and experimentally, in this thesis, in the contexts of stimulated orientational scattering and photorefractivity in liquid crystals. Stimulated orientational scattering (SOS) is a unique optical wave-mixing phenomenon that a scattered noise beam, generated by liquid crystal director oscillation, gets amplified through its interaction with the transmitted beam and liquid crystal director reorientation grating under an intense laser illumination. In this research work, I developed the first steady-state SOS theory without the compromise of non-pump-depletion approximation and it showed good agreement with previous experimental results. Based on the prediction of the developed theory, I successfully implemented polarization conversion in the infrared wavelength regime (1.55 ƒÝm) and the experimental observations agree well with the numerical simulation. The control of external low frequency AC electric field to the stimulated orientational scattering was experimentally investigated as well. Furthermore, I investigated the dynamics of this liquid crystal director reorientational process by analyzing the Fourier spectrum of the stimulated signal under various input laser intensity. The influence of the modulating AC field to the stimulated signal was also studied. The oscillating frequency of the liquid crystal director grating agreed with theoretically calculated characteristic frequency when the input laser intensity exceeds the threshold value. In addition, I have theoretically and experimentally studied the photorefractivity in nematic liquid crystals. Based on the transport band model, the generated space charge field was developed and showed the similar spatial distribution as the illuminating optical field. With the help of an externally applied DC field, the space charge field is able to induce liquid crystal director reorientation and analytical solutions have been derived for both the director reorientational angle and the resulting refractive index grating. The relationship between the first order diffraction efficiency and various material parameters has been discussed in details and shown to be in good agreement with experimental observations. Single-wall carbon nanotube doped nematic liquid crystal films showed a big increase in the effective optical nonlinearity compared with previous reported result in C60 doped photorefractive liquid crystals. The generation of the diffractive grating in the pure and Methyl-red doped liquid crystal has also been theoretically studied and the absence of the two-beam amplification has been explained as well. In the last, this investigation in the optical wave-mixing and photorefractivity in liquid crystal is concluded and the future works are proposed.