Tunable Photonic Bandgap Device and Laser Based on Cholesteric and Blue Phase Liquid Crystals
Jia-De Lin1*, Meng-Hung Hsieh1, Yu-Meng Lin1, Hong-Sheng Wang1, Chia-Rong Lee1,2
1Department of Photonics, National Cheng Kung University, Tainan, Taiwan
2Advanced Optoelectronics Technology Center, National Cheng Kung University, Tainan, Taiwan
* presenting author:Jia-De Lin, email:geman1218@yahoo.com.tw
Self-assembled photonic crystals (PhCs) based on liquid crystals (LCs) with chirality have high potential for the applications (e.g., soft matter-based integrated photonic circuit) because of their simple fabrication process as well as excellently controllable photonic properties. Cholesteric liquid crystal (CLC) and blue phase (BP) liquid crystal are two most representative materials with chiralities. With the addition of the chiral molecules, the LC molecules in CLC and BP can spontaneously self-organize as helical structures. The self-assembled helical structures may induce photonic bandgap (PBG) structure for circularly polarized light in CLC and BP. Lasers based on such soft-matter PhCs can also be developed by simply doping active materials into the CLC and BP systems.
This dissertation, entitled “Tunable photonic bandgap device and laser based on cholesteric and blue phase liquid crystals and their applications”, mainly includes three works, which are briefly described as follows:
(1) The topic of the first work is “Optically tunable/switchable omnidirectionally spherical microlaser based on a dye-doped cholesteric liquid crystal microdroplet with an azo-chiral dopant.” This work presents an optically wavelength-tunable and intensity-switchable dye-doped CLC (DDCLC) spherical microlaser with an azo-chiral dopant. Experimental results present that two functions of optical control — tunability of lasing wavelength and switchability of lasing intensity — can be obtained for this spherical microlaser at low and high intensity regimes of non-polarized UV irradiation, respectively. The 3D DDCLC spherical microlaser is a highly promising controllable 3D micro-light source or microlaser for applications of 3D all-optical integrated photonics, laser displays, and biomedical imaging and therapy, and as a 3D UV microdosagemeter or microsensor.
(2) The topic of the second work is “Photosensitive and all-optically fast-controllable photonic bandgap device and laser in a dye-doped blue phase with a low-concentration azobenzene liquid crystal.” This work demonstrates the feasibility of a novel photosensitive and all-optically fast-controllable PBG device based on a dye-doped blue phase (DDBP), embedded with a low-concentration azobenzene liquid crystal (azo-LC). The PBG of the DDBP can be reversibly fast-tuned off and on with the successive illumination of a weak UV and green beams. The UV irradiation can transform the trans azo-LCs into bend cis isomers, which can easily disturb LCs at the boundary between the double twisting cylinders (DTCs) and the disclinations, and, then, quickly destabilize BPI to become a BPIII-like texture with randomly-oriented DTCs. With the successive illumination of a green beam, the BPI PBG device can be fast-turned on, owing to the fast disappearance of the disturbance of the azo-LCs on the boundary LCs via the green-beam-induced cis→trans back isomerization. The BP PBG device can significantly contribute to efforts to develop a photosensitive and all-optically fast-controlling LC laser.
(3) The topic of the third work is “Spatially tunable photonic bandgap of wide spectral range and lasing emission based on a blue phase wedge cell.” This study demonstrates for the first time a continuously tunable PBG of wide spectral range based on a BP wedge cell. A continuously shifting PBG of the BP wedge cell occurs due to the thickness gradient of the wedge cell at a fixed temperature. The wedge cell provides a gradient of boundary force on the LCs and thus forms a distribution of BP crystal structure with a gradient lattice. Additionally, a spatially tunable lasing emission based on a DDBP wedge cell is also demonstrated. The tunable band of the PBG and lasing emission is about 130 nm and 70 nm, respectively, which tuning spectral ranges are significantly wider than those of CLC and DDCLC wedge cells, respectively. Such a BP device has a significant potential in applications of tunable photonic devices and displays.

Keywords: Liquid Crystals, Lasers, Photonic Crystals, Azobenzene Materials