Min-Jun Gim

Liquid‐Crystalline Blue Phase Laser with Widely Tunable Wavelength

A lasing peak shift of more than 100 nm is realized due to the large shift of a photonic bandgap of a liquid-crystalline blue phase.

Investigation for correlation between elastic constant and thermal stability of liquid crystalline blue phase I

We investigated the relationship between the elastic constants of host nematic liquid crystals (NLCs) and the thermal stability of their liquid crystalline blue phase I (BPI). By adding bent-core molecules to a conventional NLC, we could gradually increase (decrease) the elastic constant K11 (K33). We confirmed experimentally that the thermal stability of BPI strongly depends on the ratio of the elastic constants K11 and K33 of the host NLCs; the stability increases with decreasing K33/K11 particularly when it is less than unity, being consistent with the theoretical prediction.

Liquid-Crystalline Blue Phase II System Comprising a Bent-Core Molecule with a Wide Stable Temperature Range

A thermodynamically stable blue phase II (BPII) has been prepared, and its electrooptical (EO) performance has been evaluated in a host system of a conventional rodlike nematogen mixed with a bent-core molecule. For the mixed system presented, the widest temperature range of BPII stability, during cooling/heating, was >6 °C. This range is much wider than those of conventional nematogens blended with chiral dopants. EO observations show that the BPII produced exhibited stable EO performance based on the EO Kerr effect. The temperature dependence of the Kerr effect was found to be in approximate agreement with the Landau-de Gennes theory. Furthermore, this material demonstrated very fast, sub-millisecond-scale, response times, thus showing potential for use in high-speed EO devices.

Transition between widened BPs by light irradiation using photo-active bent-core liquid crystal with chiral dopant

Upon UV irradiation of a bent-core liquid crystal (LC) bearing an azo linkage doped with chiral molecules, a photo-induced transition takes place from BPI to BPIII. BPIII is stabilised over 20 °C, while the widening of the BPI range is not so remarkable. The mechanism of photo-induced BPI–BPIII transition is also discussed.

Switchable Photonic Crystals Using One-Dimensional Confined Liquid Crystals for Photonic Device Application

Photonic crystals (PCs) have recently attracted considerable attention, with much effort devoted to photonic bandgap (PBG) control for varying the reflected color. Here, fabrication of a modulated one-dimensional (1D) anodic aluminum oxide (AAO) PC with a periodic porous structure is reported. The PBG of the fabricated PC can be reversibly changed by switching the ultraviolet (UV) light on/off. The AAO nanopores contain a mixture of photoresponsive liquid crystals (LCs) with irradiation-activated cis/trans photoisomerizable azobenzene. The resultant mixture of LCs in the porous AAO film exhibits a reversible PBG, depending on the cis/trans configuration of azobenzene molecules. The PBG switching is reliable over many cycles, suggesting that the fabricated device can be used in optical and photonic applications such as light modulators, smart windows, and sensors.

Fabrication of Alignment Layer Coated Indium-Tin-Oxide Prepared by Ultraviolet Nano-Imprinting Lithography

Nanoscale grating patterns coated indium-tin-oxide (ITO) layers were fabricated using the ultraviolet (UV) nano-imprinting technique. The nanoscale groove coated ITO that was prepared in this experiment could play a role in alignment layers and conducting electrodes. Performance for LC alignment was characterized by evaluating order parameter. The LC order parameter evaluated on the fabricated layer was comparable to that of the conventional rubbed polyimide layer. We also confirmed that the 90° twisted nematic liquid crystal (TNLC) cell with our nanopattern coated ITO layer exhibited stable electro-optical performance.

Homeotropic alignment of multiple bent-core liquid crystal phases using a polydimethylsiloxane alignment layer

The control of the molecular orientation of liquid crystals (LCs) is important in both understanding phase properties and the continuing development of new LC technologies including displays, organic transistors, and electro-optic devices. Many techniques have been developed for successfully inducing alignment of calamitic LCs, though these techniques typically do not translate to the alignment of bent-core liquid crystals (BCLCs). Some techniques have been utilized to align various phases of BCLCs, but these techniques are often unsuccessful for general alignment of multiple materials and/or multiple phases. Here, we demonstrate that glass cells treated with polydimethylsiloxane (PDMS) thin films induce high quality homeotropic alignment of multiple mesophases of four BCLCs. On cooling to the lowest temperature phase the homeotropic alignment is lost, and spherulitic growth is seen in crystal and crystal-like phases including the dark conglomerate (DC) and helical nanofilament (HNF) phases. Evidence of homeotropic alignment is observed using polarized optical microscopy. We speculate that the methyl groups on the surface of the PDMS films strongly interact with the aliphatic tails of each mesogens, resulting in homeotropic alignment.