Jui-Hsiang Liu

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.

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.

A depropargylation-triggered fluorescence “turn-on” probe for the detection of Pd2+ based on a bispropargylamine–rhodamine conjugate

A bis-propargyl-appended rhodamine B-based receptor BPRB has been synthesised that exhibits pronounced fluorescence enhancement in the presence of Pd2+ ions. The addition of Pd2+ enhanced the fluorescence intensity of BPRB by 113-fold (Φf = 0.37) and BPRB was found to exhibit high selectivity towards Pd2+ compared to a range of other metal ions. The enhancement of fluorescence was triggered by spirolactam ring opening followed by depropargylation of BPRB in the presence of Pd2+, as evidenced by FTIR and NMR analyses. BPRB was able to detect Pd0 without the addition of a reducing agent, and the emission intensity of BPRB–Pd0 was almost identical to that of BPRB–Pd2+; however, a rapid fluorescence response was observed in the presence of PPh3. To explore the efficiency of the rhodamine unit, a bispropargyl derivative of cyclohexane (BPCH) was synthesised and the fluorescence response towards Pd2+ was examined and compared with BPRB, revealing that the rhodamine unit enhanced the fluorescence intensity by 500-fold. The fluorescence images of BPRB and BPRB–Pd2+ samples indicate that BPRB could be useful for imaging Pd2+ in living cells.

Photochemical tuning capability of cholesteric liquid crystal cells containing chiral dopants end capped with menthyl groups

In order to investigate the photochemical tuning capability of chiral monomers and polymers containing end‐capped menthyl groups, a new series of chiral dopants was synthesized and added to commercially available nematic liquid crystals to induce cholesteric liquid crystal (LC) phases. The addition of chiral dopants with azo structure led to phototunability of the reflection colour of the LC cells. Photochromic variation of the LC cells due to photoisomerization of the azo compound was investigated. After photopolymerization of the monomers inside the cholesteric LC cells, the centre wavelength of the reflected band of the incident light was found to be fixed and the reflected bandwidth was broadened, resulting in a red shift. A schematic representation of both the photoisomerization of the azo dopants and its effect on variation of twisting pitches is proposed. Real image recording was performed using 365 nm UV through a mask with text. The top and side views of the morphological network structures of a fabricated cholesteric LC cell were investigated using scanning electron microscopy. The results of this investigation demonstrated that RGB reflected colours of LC cells can easily be achieved through the addition of the menthyl‐containing synthesized chiral compounds to nematic LCs. The addition of synthesized AzoM helped further in recording the patterns onto cholesteric LC films using 365 nm UV exposure.

Optically-tunable beam steering grating based n azobenzene doped cholesteric liquid crystal.

This work proposes an optically controllable beam-steering device, fabricated using cholesteric liquid crystals (CLCs) that are doped with azobenzene. The trans-cis photoisomerization of azobenzene changes the pitch of the CLC fingerprint structure and shifts the diffraction angle. The diffraction angle increases when the cell is irradiated with UV light, and restored when it is irradiated with green light. Combining the photoisomerization effect with electrical effect, the CLC beam-steering device provides a steering angle of approximately 19 degrees. The tuning is continuous and could be completed within a few seconds.

Direction switching and beam steering of cholesteric liquid crystal gratings

This work proposes two mechanisms for switching the direction of stripes in cholesteric liquid crystal (CLC) gratings. The stripe direction depends on the ratio of cell gap to the natural pitch length (d/P0) of a CLC sample. Electrical switching is based on the different pitches at the planar and the transient planar states. Optical switching, however, changes pitch by using the photo-isomerization effect of the azobenzene doped in a CLC sample. Using the two mechanisms, we can switch the stripe directions in two orthogonal directions. Furthermore, the beam-steering capability of CLC gratings also remains effective after switching directions.

Bichromatic optical switch of diffractive light from a BCT photonic crystal based on an azo component-doped HPDLC

This study demonstrates all optical switches between the four diffractive light levels of a body-centered tetragonal photonic crystal. The sample is based on holographic polymer-dispersed liquid crystals that are fabricated using a two-beam interference with multiple exposures. The switching mechanism bases on the effective index modulation of the PC that contains a liquid crystal/azo-dye mixture could be controlled by two pumping laser beams. The switching time between the blue-laser-pumped and the blue-and-green-laser-pumped levels is fast. The study also discusses the switching time of the various levels.

Photoerasable and photorewritable spatially-tunable laser based on a dye-doped cholesteric liquid crystal with a photoisomerizable chiral dopant

This study investigates, for the first time, a photoerasable and photorewritable spatially-tunable laser using a dye-doped cholesteric liquid crystal (DDCLC) with a photoisomerizable chiral dopant (AzoM). UV illumination via a photomask with a transmittance-gradient can create a pitch gradient in the cell such that the lasing wavelength can be spatially tuned over a wide band of 134nm. The pitch gradient is generated by the UV-irradiation-induced gradient of the cis-AzoM concentration and therefore the induced gradient of the cell HTP value, resulting in the spatial tunability of the laser. Furthermore, the laser has advantages of photoerasability and photorewritability. The spatial tunability of the laser can undergo more than 100 cycles of photoerasing and photorewriting processes without decay or damage.

Preparation and characterization of gradient refractive index polymer optical rods

Gradient refractive index (GRIN) polymer optical rods were fabricated by the swollen-gel polymerization technique. The mechanism and preparation conditions of the swollen-gel polymerization were investigated. The effects of initiator concentration, swelling temperature, polymerization temperature and monomer feed ratios on the optical properties of the GRIN polymer rods were studied. To reduce the swelling time, two different kinds of initiator were compared. The gradient refractive index profiles of the plastic rods were estimated. The image observed through the GRIN polymer rod was also examined.