Research

Manipulation of Structural Colors in Liquid-Crystal Helical Structures Deformed by Surface Controls, C. Lim, S. Bae, S. M. Jeong, N. Y. Ha*, ACS Applied Materials & Interfaces, Vol. 10, Issue 14, 120690-120695, (April 11, 2018).
We show the simultaneous manipulation of spectral position and width of its structural color in a single cholesteric liquid crystal (CLC) mixture condition by employing an amorphous perfluoropolymer as an alignment layer. In addition, electrical tunings of structural colors from the deformed CLCs occur at very low operating voltages, compared with those of conventional CLC structures.

Sol–Gel Substrates with Engineered Multiple Photonic Bandgaps for Full-Color Photonic Devices, S. H. Lee and N. Y. Ha^*, Small, Vol. 7, Issue 19, 2704 - 2708 (October 4, 2011) (Frontispiece).
We fabricated an inorganic glass film with engineered multiple photonic bandgaps (MPBGs) by sol–gel processing. This sol–gel film consists of TiO₂, SiO₂, and mixed SiO₂–TiO₂ films and has controllable characteristics of MPBGs in the overall visible region. By using the sol–gel film as a resonator, a red–green–blue pixelated liquid-crystal laser is also demonstrated.

Color- and reflectance-tunable multiple reflectors assembled from three polymer films, N. Y. Ha^*, S. M. Jeong, S. Nishimura, and H. Takezoe^*, Advanced Materials, Vol. 22, Issue 14, 1617 - 1621 (April 12, 2010).
We demonstrate tunings of reflectance and colors of multiple photonic bandgaps in the whole visible region by changing the polarization state of incident light. Two tunable systems are assembled from the same three materials of an anisotropic polymeric nematic liquid crystal film and two kinds of isotropic polymer film. The only difference between the two systems is the stacking sequence.

Perfluoropolymer Surface for Shock-free Homeotropic Alignment of Smectic Liquid Crystals, S. M. Jeong, J. K. Kim, Y. Shimbo, F. Araoka, S. Dhara, N. Y. Ha, K. Ishikawa, H. Takezoe^*, Advanced Materials, Vol. 22, Issue 1, 34 - 38 (Jan 5, 2010) (Frontispiece).
We demonstrate shock-free homeotropic alignment of smectic liquid crystals onto the amorphous perfluoropolymer. The hydrophobic surface of perfluoropolymer as an alignment layer enables spontaneous recovery to the original vertical orientations from deformed smectic layers caused by mechanical shock, such as pressing with tweezers or crystallization.

Simultaneous red, green, and blue lasing emissions in a single-pitched cholesteric liquid-crystal system, N. Y. Ha, S. M. Jeong, S. Nishimura, G. Suzaki, K. Ishikawa, and H. Takezoe^*, Advanced Materials, Vol. 20, Issue 13, 2503-2507 (July 2 2008).
Simultaneous red, green, and blue lasing emissions are demonstrated in a single resonator with a single gain medium by a single optical pumping (see figure). The resonator is fabricated using cholesteric liquid crystal systems with multiple photonic band gaps, and the energy transfer between two fluoresence dyes is employed in gain medium.

Fabrication of a simultaneous red-green-blue reflector using single-pitched cholesteric liquid crystals, N. Y. Ha, Y. Ohtsuka, S. M. Jeong, S. Nishimura, G. Suzaki, Y. Takanishi, K. Ishikawa, and H. Takezoe^*, Nature Materials, Vol. 7, Issue 1, 43-47 (January 2008).
Cholesteric liquid crystal (CLC) exhibit selective reflection at the wavelength corresponding to the optical pitch of CLCs. However, this is not always the case. We found that sophisticated CLC structures can exhibit simultaneous RGB reflection, even if we use only a single-pitched CLC.

Self-assembled silica photonic crystal as a liquid crystal alignment layer and its electro-optic application in Fabry-Perot cavity structure, N. Y. Ha, Y. K. Woo, B. Park, H. Takezoe and J. W. Wu^*, Advanced Materials, Vol. 16, No. 19, 1725-1729 (4 October 2004) (Inside Cover Picture).
Microgrooves on self-assembled photonic crystal (PC) films align liquid crystals (LCs) without any additional rubbing or surface treatment. Nematic or twisted-nemtaic LCs can be formed, depending on the relative orientaion of the microgrooved films. A LC Fabry-Perot (FP) cavity was fabricated using these PC films, and electro-optic tuning of FP cavity modes is demonstrated.