Kuang Yao Lo

Electrooptical Responses of Carbon Nanotube-Doped Liquid Crystal Devices

The electrooptical characteristics of carbon nanotube-doped liquid crystal (LC) devices were investigated. Two complementary operation modes of the liquid crystal cells were fabricated. The measured results reveal that anisotropic carbon nanosolids modify the dielectric anisotropy and the viscosity of the liquid crystal carbon nanotube mixture, hence significantly modifying the threshold voltage and the switching behavior of a liquid crystal device. Doping a small amount of carbon nanotubes into the liquid crystal mixture is effective in improving the electrooptical characteristics of an LC device when the employed LC mixture is viscous.

Bistable transflective cholesteric light shutters

We have presented a bistable transflective cholesteric light shutter. The shutter contains dual-frequency cholesteric liquid crystals that incorporate a photocurable monomer. The electro-optical properties and optical microscope images of the shutter were examined. Good correlations between the cholesteric textures and optical properties of the shutters were obtained. The shutter was switched into a wide-band homogeneous transmission mode by a low frequency voltage pulse, and into a selective reflection mode using a high-frequency voltage pulse. The concentration of the monomer apparently affected the electro-optical characteristics of the shutter.

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.

Silica-nanoparticle-doped nematic display with multistable and dynamic modes

We demonstrate a silica-nanoparticle-doped hybrid-aligned nematic device, which can be operated in the dynamic mode and the multistable mode. Under ac voltage excitation, the interfacial polarization effect between the homogeneously dispersed nanoparticles and the liquid crystal (LC) host reduces the dielectric relaxation time of the LC-silica dispersion, giving the LC cell a fast response time for the dynamic mode. Under dc voltage excitation, the silica nanoparticles accumulated on the planar side of the cell, creating agglomerates to stabilize the homeotropically aligned LCs, which is retained after switching off the voltage, giving the cell bistable/multistable switching characteristics.

Irreversible redshift of transmission spectrum of gold nanoparticles doped in liquid crystals

The absorption peak of gold (Au) nanoparticles doped in liquid crystal (ANDLC) is controllable by applying an external applied voltage before aggregation occurs. Au nanoparticles aggregate when the dielectrophoresis force, which is induced by the gradient of the square of the electric filed close to the sides of the host liquid crystal cell, overcomes the viscosity between Au nanoparticles and the LCs. This result leads to the irreversible redshift. A controllable filter with unpolarized light enables an ANDLC cell under the threshold voltage to prevent the aggregation effect.

Linear polarization rotators based on dye-doped liquid crystal cells

Linear polarization rotator is fabricated by a single-side homogenously aligned dye-doped liquid crystal cell and linear variable neutral density filter (LVNDF). When a pump beam passing through the transmittance-linear region of LVNDF irradiates on the untreated surface, the surface LC director in the irradiation region is photoaligned into a continuous twist from 0° to 90°. Consequently, the bulk director gradually transits from a homogeneous to twist orientation. This device is capable of rotating the polarization of an input linearly polarized light depending on the beam position, exhibiting a large continuous twist region (5.6 mm) and high contrast ratio (∼1000:1).

Unipolar resistive switching behavior of Pt/LixZn1−xO/Pt resistive random access memory devices controlled by various defect types

The unipolar resistive switching behavior of Pt/LixZn1−xO/Pt structures fabricated via radio-frequency magnetron sputtering is investigated. Various Li doping concentrations influence the defect types (i.e., VO, Lii, and LiZn) in LixZn1−xO films for adjustable resistance ratio. The resistance ratio reaches 108 due to the minimized leakage current in the high-resistance state (HRS) at 6 at. % Li dopants. The dominant conduction mechanisms are explained in terms of Ohmic behavior and Poole-Frenkel (PF) emission. The coefficient r of the PF emission in the HRS is evaluated to confirm that the total number of defects in LixZn1−xO films decreases with increasing Li content.

Reflective second harmonic generation from ZnO thin films: A study on the Zn-O bonding

The structures of the Zn–O bonding in ZnO (0002) thin films prepared by metal organic chemical vapor deposition have been studied by reflective second harmonic generation (RSHG). The polar Zn–O bond on the top layer is not canceled out and presents 3mm symmetrical structures on the well-grown ZnO (0002) surface. The average polar strength of the Zn–O bond is correlated with the quality of the ZnO (0002) thin film. The mirror symmetry is caused by the nonvanished polar of twin boundary due to the mismatch between the ZnO film and sapphire substrate and analyzed using s-polarized RSHG with s-polarized fundamental light irradiation. These results demonstrate that the Zn–O heteropolar bonds on the smooth ZnO surface contribute to the SHG intensity.

Optical second harmonic generation from the twin boundary of ZnO thin films grown on silicon

The symmetry of the twin boundaries of ZnO epitaxial film was detected with reflective second harmonic generation (RSHG). The twin boundaries exhibit mirror symmetry with a polar configuration across the boundary plane and yield a nonvanishing polar contribution to RSHG. The nonvanishing second-order susceptibility supports the notion that the measured RSHG originates from the planar defect, which depends on the residual stress in the thin film. We analyzed our RSHG result by correlating the macroscopic data from optic probe with the microscopic data from tunneling electron microscope.

Dynamics of photoalignment in azo-dye-doped liquid crystals

A transmitted light without analyzer was led into the pump-probe twist nematic experiment in azo-dye-doped liquid crystals to observe the light scattering from the dye-adsorbed surface during photoalignment. The morphology of the dye-absorbed surface monitored by this transmitted light is correlated with the formation of the ripple structure, which is reflected by the anchoring of the dye-adsorbed layer, which is, in turn, revealed by measuring the surface director reorientation angle throughout the process. As the regular ripple structure is gradually formed, the Rayleigh scattering is transferred to the Mie scattering since the scale of the ripple structure is at the wavelength.