Tsung-Hsien Lin

Cholesteric liquid crystal laser with wide tuning capability

This letter examines a planar cholesteric cell (CLC) doped with two collocated laser dyes as a one-dimensional photonic crystal. Adding phototunable chiral material (AzoB) allows the CLC photonic crystal to be lased at the band edges of the photonic band gap with a tuning range of over 100nm. Tuning is performed by irradiating the chiral AzoB material with UV light, causing the material to undergo trans-cis isomerization in the CLC film. The tuning range is the visible region from 563to667nm. Moreover, the tuning is reversible.

Red, Green and Blue Reflections Enabled in an Optically Tunable Self‐Organized 3D Cubic Nanostructured Thin Film

A new light-driven chiral molecular switch doped in a stable blue phase (BP) liquid crystal allows wide optical tunability of three-dimensional cubic nanostructures with a selective reflection wavelength that is reversibly tuned through the visible region. Moreover, unprecedented reversible light-directed red, green, and blue reflections of the self-organized three-dimensional cubic nanostructure in a single film are demonstrated for the first time. Additionally, unusual isothermal photo-stimulated less ordered BP II to more ordered BP I phase transition was observed in the system.

Highly efficient and polarization-independent Fresnel lens based on dye-doped liquid crystal

We demonstrated a highly efficient, polarization-independent and electrically tunable Fresnel lens based on dye-doped liquid crystal using double-side photoalignment technique. The maximum diffraction efficiency reaches 37%, which approaches the theoretical limit ~41%. Such a lens functions as a half-wave plate, and this feature could be well preserved under the applied voltage. In addition, the device is simple to fabricate, and has fast switching responses between focusing and defocusing state.

Electrically controllable laser based on cholesteric liquid crystal with negative dielectric anisotropy

This work examines a planar cholesteric liquid crystal (CLC) cell with a negative dielectric anisotropy, doped with laser dye, as an electrically tunable one-dimensional photonic crystal laser device. The lasing wavelength is demonstrated to be tunable by applying a voltage. Additionally, lasing can be switched on and off changing the frequency of the applied voltage. Wavelength tuning caused by the shift of the reflection band of CLC is attributed to the electrohydrodynamical effect in the negative dielectric cell.

Rollable multicolor display using electrically induced blueshift of a cholesteric reactive mesogen mixture

Electrically controllable blueshift of the reflection band in a planar cholesteric reactive mesogen cell is observed. The responsible mechanism is electric-field-induced Helfrich deformation [J. Chem. Phys. 55, 839 (1971)]. The modified director configuration can be solidified by photopolymerizing the reactive mesogens when a voltage is applied. The correlation between the director configuration and optical properties is validated by the scanning electron microscope photos and the transmission spectra of a planar and an undulated cholesteric film. With masked curing at different voltages, a rollable multicolor display is demonstrated.

Lasing in chiral photonic liquid crystals and associated frequency tuning

This letter addresses a dye-doped planar cholesteric cell as a one-dimensional photonic crystal, which can be lased at the band edges of the photonic band gap. The effect of the composition of the material and the thickness of a cholesteric cell (CLC) on the lasing action, and the photo-control of the lasing frequency, are experimentally investigated. Adding a tunable chiral monomer (TCM) allows the CLCs reflection band to be tuned by varying the intensity and/or exposure time of the UV curing light, enabling the lasing frequency of the CLC sample to be tuned.

Optically tuneable blue phase photonic band gaps

This study investigates an optically switchable band gap of photonic crystal that is based on an azobenzene-doped liquid crystal blue phase. The trans-cis photoisomerization of azobenzene deforms the cubic unit cell of the blue phase and shifts the photonic band gap. The fast back-isomerization of azobenzene was induced by irradiation with different wavelengths light. The crystal structure is verified using Kossel diffraction diagram. An optically addressable blue phase display, based on Bragg reflection from the photonic band gap, is also demonstrated. The tunable ranges are around red, green, and blue wavelengths and exhibit a bright saturated color.

Random lasing in blue phase liquid crystals

Random lasing actions have been observed in optically isotropic pure blue-phase and polymer-stabilized blue-phase liquid crystals containing laser dyes. Scattering, interferences and recurrent multiple scatterings arising from disordered platelet texture as well as index mismatch between polymer and mesogen in these materials provide the optical feedbacks for lasing action. In polymer stabilized blue-phase liquid crystals, coherent random lasing could occur in the ordered blue phase with an extended temperature interval as well as in the isotropic liquid state. The dependence of lasing wavelength range, mode characteristics, excitation threshold and other pertinent properties on temperature and detailed make-up of the crystals platelets were obtained. Specifically, lasing wavelengths and mode-stability were found to be determined by platelet size, which can be set by controlling the cooling rate; lasing thresholds and emission spectrum are highly dependent on, and therefore can be tuned by temperature.

A stable and switchable uniform lying helix structure in cholesteric liquid crystals

This investigation demonstrates an electrically switchable uniformly lying helix (ULH) stable state in cholesteric liquid crystal. A stable ULH state can be achieved by applying the low-frequency (30 Hz) pulse electrical field via an electro-hydrodynamatic effect. The ULH state can be stably maintained with a helical pitch in the visible range (450 nm–630 nm) and exhibit a tunable uniaxial crystal wave plate property under 1 kHz electrical field. The study examines the electro-optical property of ULH state and driving scheme for switching among the three stable states. A multi-stable and electrically switchable cholesteric liquid crystal can provide various optical properties and has extensive potential applications.

Blue-phase liquid crystal cored optical fiber array with photonic bandgaps and nonlinear transmission properties

Blue-phase liquid crystal (BPLC) is introduced into the pores of capillary arrays to fabricate fiber arrays. Owing to the photonic-crystals like properties of BPLC, these fiber arrays exhibit temperature dependent photonic bandgaps in the visible spectrum. With the cores maintained in isotropic as well as the Blue phases, the fiber arrays allow high quality image transmission when inserted in the focal plane of a 1x telescope. Nonlinear transmission and optical limiting action on a cw white-light continuum laser is also observed and is attributed to laser induced self-defocusing and propagation modes changing effects caused by some finite absorption of the broadband laser at the short wavelength regime. These nonlinear and other known electro-optical properties of BPLC, in conjunction with their fabrication ease make these fiber arrays highly promising for imaging, electro-optical or all-optical modulation, switching and passive optical limiting applications.