Liquid Crystal Photonic Applications Combined With Grating Structures

This thesis introduces the design, fabrication and characterization of several liquid crystal (LC) photonic devices combined with grating structures. We utilize the electro-tunability of LCs and the unique properties of grating structures with different size and shape to design some photonic devices at different working wavelengths. Through theoretical analysis and simulation, optical design and finally various micro/nano fabrication method, we have developed a fast response optical switch with polarization independence, a bilayer metallic grating as the brightness enhancement film in an LCD panel and three Terahertz (THz) LC devices including self-polarizing phase shifter, tunable quarter wave plate and tunable filter.1. We introduce the principle and advantages of photo-alignment technology. Based on such technology, we propose a design for polarization independent optical switch by employing orthogonal homogenously-aligned LC cell structure. The polarization independency is analyzed theoretically and then checked experimentally. However, the electro-optical properties of such structure are not good enough with low extinction ratio and slow response. Therefore we modify the structure design and propose an improved LC cell structure with orthogonal hybrid alignments. By introducing special dual-frequency LC materials into such structure, the optical switch based on which could be fast response while keeping the property of polarization independency. Trough simple optimization on structural parameters, we realize by experiment a polarization independent optical switch with over20dB extinction ratio as well as sub-millisecond response time. Further improvements for the performance are also discussed.2. We introduce the significance and principle of brightness enhancement (BE) in liquid crystal displays (LCDs). Based on the polarization selectivity of subwavelength metallic gratings, we propose a new type of bilayer subwavelength wire grids to work as a BE film and take place the conventional absorptive polarizer in LCD panels. Through simulation, we optimize grating structures and theoretically obtain the best optical performance we can reach. Then we calculate the BE efficiency by employing a power recycling system, and the result is that our bilayer subwavelength wire grids could bring80%more light intensity compared to commercial absorptive polarizer. Afterwards we try to realize such optimized grating structure experimentally by nanoimprint lithography, however, due to some technical problem, we have not reached satisfied performance yet, which remains to be further improved.3. We introduce the unique characteristics and great application potentials of THz waves as well as the importance of developing LC tunable components for THz regions. Trough the combination with gratings, we design and fabricate several liquid crystal photonic components for THz regime. First, taking account into the lack suitable materials for transparent conductive fims in THz region, we introduce subwavelength gratings to act as both transparent electrodes and polarizers. Based on this design, we fabricate a self-polarizing LC phase shifter in THz range. During the effective frequency range of measurement (0.2-1.8THz), we realize a maximal phase shift of66°, and the phase shift values could be tuned by applying external electric fields to the LC cell.Secondly, we follow the design for metallic grating as electrodes and propose a quarter wave plate for THz regions. When keeping the grating structure on one substrate to be still subwavelength scale so that it can work as a polarizer for the component, we enlarge the period of grating on the other substrate to wavelength scale, in order to prevent the polarization selectivity of this grating (which means we need a polarizer but not an analyzer) so that the function of quarter wave plate would not be affected. In this work we employ photo-alignment technology to precisely control the alignment directions on both substrates to be strictly parallel to each other. We further introduce large birefringent LC materials into this work. Compared to common nematic LC materials, the birefringence of such material is more than2times larger, so that the required cell gap could be evidently reduced when realizing the same modulation. We realize experimentally precise matching of required phase retardation values at different frequencies by applying different voltages, so that for the region of0.7-2.0THz, quarter wave plate function can be realized for every frequency.Finally, we introduce the principle of Fabry-Perot-like behavior in narrow dielectric slits in thick metallic screen. Based on this principle, we propose a metallic grating structure for THz filter and fill LC into the slits so that the resonant peaks could be shifted by tuning the effective refractive index of LCs. We optimize the structural parameters of this grating theoretically and preliminary experimental results are obtained. Further improvements on this work are now on going.