Chie-Tong Kuo

Electrically controllable liquid crystal random lasers below the Fréedericksz transition threshold

This investigation elucidates for the first time electrically controllable random lasers below the threshold voltage in dye-doped liquid crystal (DDLC) cells with and without adding an azo-dye. Experimental results show that the lasing intensities and the energy thresholds of the random lasers can be decreased and increased, respectively, by increasing the applied voltage below the Fréedericksz transition threshold. The below-threshold-electric-controllability of the random lasers is attributable to the effective decrease of the spatial fluctuation of the orientational order and thus of the dielectric tensor of LCs by increasing the electric-field-aligned order of LCs below the threshold, thereby increasing the diffusion constant and decreasing the scattering strength of the fluorescence photons in their recurrent multiple scattering. This can result in the decrease in the lasing intensity of the random lasers and the increase in their energy thresholds. Furthermore, the addition of an azo-dye in DDLC cell can induce the range of the working voltage below the threshold for the control of the random laser to reduce.

Color cone lasing emission in a dye-doped cholesteric liquid crystal with a single pitch

This work investigates a novel color cone lasing emission (CCLE) based on a one-dimensional photonic crystal-like dye-doped cholesteric liquid crystal (DDCLC) film with a single pitch. The lasing wavelength in the CCLE is distributed continuously at 676.7-595.6 nm, as measured at a continuously increasing oblique angle relative to the helical axis of 0-50 degrees . This work demonstrates that lasing wavelength coincides exactly with the wavelength at the long wavelength edge of the CLC reflection band at oblique angles of 0-50 degrees . Simulation results of dispersion relations at different oblique angles using Berremans 4X4 matrix method agrees closely with experimental results. Some unique and important features of the CCLE are identified and discussed.

Rotatable diffractive gratings based on hybrid-aligned cholesteric liquid crystals.

This work proposes a thermally rotatable grating that is based on hybrid-aligned cholesteric liquid crystals (HBA-cholesteric LCs). Experiments reveal that the HBA-cholesteric texture has a uniformly striped domain, which forms a grating, when the ratio of the cell gap to the helical pitch (d/p) is in the range of 2≤d/p≤3. The stripe direction of the HBA-cholesteric grating is predicted by the proposed vertically aligned LC layer model. The stripe direction of the HBA-cholesteric grating rotates continuously under thermal and electrical effects. Furthermore, the HBA-cholesteric grating has a larger rotational angle under the thermal effect (~101°) than under the electrical effect (~48°). Potential applications of the proposed thermally rotatable cholesteric grating for beam steering devices are emphasized.

Enhancement of Luminescence of Nematic Liquid Crystals Doped with Silver Nanoparticles

The photoluminescence of nematic liquid crystals doped with silver nanoparticles has been investigated. The peak intensity of photoluminescence varies with the concentration of doped silver nanoparticles. The peak intensity of photoluminescence shows the enhancement effect in the range of concentration from 0% ∼ 1% and the quenching effect in the range of concentration from 1% ∼ 4%. The photoluminescence can be enhanced around 55% for silver nanoparticles with 88 nm in diameter.

High performance Cu 2 O/ZnO core-shell nanorod arrays synthesized using a nanoimprint GaN template by the hydrothermal growth technique

Nanoimprint technology was used to synthesize a series of nanostructures with hexagonal holes on a n-GaN baseplate. The hydrothermal growth technique was then used to produce 1.5-μm n-type ZnO nanorods. Radio frequency reactive magnetron sputtering was employed to grow a 50-nm thick layer of cuprous oxide film over the nanorods to form a p-n Cu2O/ZnO core-shell structure. Based on the different imprint widths and intervals obtained, Cu2O/ZnO heterostructure samples A, B, C, and D showed aperture ratios of 0.0627, 0.0392, 0.0832, and 0.0537, respectively. Scanning electron microscopy and atomic force microscopy indicated that a 50-nm Cu2O film coated the ZnO nanorods, forming a core-shell structure. X-ray diffraction and x-ray rocking curve (XRC) analysis showed that the Cu2O lattice structure had polycrystalline characteristics. The lattice planes of Cu2O were (111) and (220), and Sample C exhibited the narrowest XRC half-height full-width value. Therefore, among the samples obtained, Sample C had the optimal material properties. Measurement of the optical properties of the samples demonstrated that their luminous peak did not change with variations in temperature. Sample C also showed optimal optical properties. High-resolution transmission electron microscopy indicated the presence of a midlayer in the Cu2O/ZnO junction that had a direct impact on the Cu2O lattice arrangement on the top, corner, and side faces of the ZnO nanorods. The sample with the largest aperture ratio exhibited the most favorable optical and material properties. The novel structure obtained can potentially be used in solar cell applications.

Diagnosis of Human Bladder Cancer Cells at Different Stages Using Multispectral Imaging Microscopy

Bladder cancer presents a spectrum of different diatheses. A precise assessment for individualized treatment depends on the accuracy of the initial diagnosis. Detection relies on comprehensive and accurate white light cystoscopy. White light cystoscopy has limitations in addition to its invasive nature and the potential risks related to the method. These limitations include difficulties in flat lesion detection, precise tumor delineation to enable complete resection, inflammation and malignancy differentiation, and grade and stage determination. The resolution of these problems depends on the surgeons ability and experience with available technology for visualization and resection. In this study, we used multispectral imaging technology combined with phase contrast microscopy to analyze bladder cancer cells (BCCs) at various stages using a single-cell array chip. We found from the spectral characteristics of single cell that the cell spectra at the different cancer stages demonstrate a change in the cells composition. We cultured 419 normal and diseased bladder cells. We used principal component analysis and a principal component score map to distinguish the different cancer stages. Diagnosis sensitivity and specificity of this method were 85.7% and 90.2% in 119 stage 0 (normal) cells, 84.3% and 90.8% in 79 stage 2 cancer cells, 87.6% and 92.4% in 151 stage 3 BCCs, and 85.3% and 91.2% in 70 stage 4 BCCs, respectively.

The ultrafast dynamics of liquid CBrCl3 studied with optical Kerr effect and Raman scattering

The ultrafast inter- and intramolecular dynamics of liquid CBrCl3 are studied with a femtosecond laser in a pump/probe optical Kerr experiment. The prompt part of the observed Kerr signal is found to comprise not just electronic response but also coherent coupling. As reported in the literature, the weak and delayed part of the signal is attributed to three kinds of intermolecular dynamics: diffusive reorientation, polarizability distortion, and molecular libration with decay times, respectively, of 2 ps, 500 fs, and 190 fs. Included in the delayed Kerr signal are long lasting oscillations which are ascribed to quantum interference involving four normal vibrations of the molecule. By a careful fitting of both the time domain (Kerr) and frequency domain (Raman) data with existing theories the four vibrations are identified as ν2 = 422 cm−1, ν3 = 246 cm−1, ν5 = 295 cm−1, and ν6 = 191 cm−1, with dephasing times, 1.1 ps, 1.7 ps, 1 ps, and 1 ps, respectively.

Hyperpolarizabilities of the m‐substituent phenyl amine based chromophores determined from the hyper‐Rayleigh scattering and two photon absorption induced fluorescence

The technique of hyper‐Rayleigh scattering is used to determine and compare the first hyperpolarizabilities (β) of five structurally similar m‐substituent phenyl‐amine based chromophores using both internal and external standards. Contribution from the two photon absorption induced fluorescence to the observed signal is carefully removed. The chromophores with the m‐substituted phenyl ring, that stabilizes the maximal charge‐transfer state through resonance effect, are found to have larger static β values and redshifted electronic absorption peaks (λmax) compared with the para‐nitroaniline (pNA) chromophore. The electron‐withdrawing m‐substituent of the chromophore is found to have the effect of reducing both the β and λmax values. Its effect on β is less pronounced when the π‐electron reservoir is large. The enhancement of β value and the redshift λmax are observed to be consistent with the extent of the π electron delocalization.

Dynamics of single-layer polymer breath figures

A single-layer of breath figure pattern was explored via the dynamical optical images and the temperature evolution. The pattern was prepared with the solution of carbon disulfide (CS(2)) dissolved 1% weight concentration of polystyrene. The evaporation of CS(2) was considered to be the most important role to the formation of the breath figure pattern. The understanding of the breath figures pattern will promote the technique to fabricating an imprinted template with demanded hexagonal structures.

Polarization-independent and fast tunable microlens array based on blue phase liquid crystals.

This work investigates a polarization-independent and fast response microlens array. This array is composed of a concave polymer microlens array and blue phase liquid crystals (BPLCs). The microlens array can be either positive or negative, depending on the birefringence of the BPLCs. The experimental results show that the microlens array is fast switched between positive and negative focal lengths via controlling the electric fields, and the response time is a few hundred microseconds. Additionally, the focusing efficiency is independent of the polarization of the incident light.