Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...

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Luminescence properties of defects in GaN

第12次 아시아ㆍ太平洋 矯政局長會議 參加記

The review describes the status of the studies and the recent achievements in the field of photoalignment of liquid crystals. An update classification of photoaligning materials and exposure schemes, and analyzes of the relationship between the molecular structure of the materials and characteristics of LC alignment are provided. In addition, bulk mediated photoalignment and combination of photoalignment with other alignment methods are discussed. Along with traditional, recently proposed applications of the photoalignment technique are considered.

Photoalignment of liquid crystals: basics and current trends

5.3.2. Traps and Grating Dark Decay 3288 5.3.3. Other Concerns 3288 6. New Photorefractive Materials 3289 6.1. Polymer Composites 3289 6.2. Organic Amorphous Glasses 3292 6.3. Fully Functionalized Polymers 3294 6.4. Polymer-Dispersed Liquid Crystals 3295 6.5. Other Liquid Crystal-Containing Materials 3296 6.6. Near-Infrared-Sensitive Materials 3297 6.7. Other Materials Directions 3298 6.7.1. Hybrid Organic−Inorganic Composites and Glasses 3298

Organic photorefractives: mechanisms, materials, and applications.

1 What is a liquid crystal 2 1.1 Nematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2 Cholesterics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3 Smectics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.4 Columnar phases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.5 Lyotropic liquid crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Introduction to Liquid Crystals

The response times of rotational molecular orientation of neat and carbon-nanotube-doped nematic liquid crystals were acquired through the measurement of transient current induced by a direct-current step voltage and from optical transmission data. A model based on the dielectric displacement current was developed to describe the electric field dependence of the response and to yield a rotational viscosity that would decrease with increasing concentration of carbon nanotubes. Optical dynamic response also showed a reduced rotational viscosity, with the lightly doped cells exhibiting a faster relaxation process than that of the neat cell.

Faster electro-optical response characteristics of a carbon-nanotube-nematic suspension

A fast-switching bistable optical intensity modulator is demonstrated. Using a dual-frequency cholesteric liquid crystal, the direct switching is achieved from the scattering focal conic state to the transparent long-pitch planar state. In comparison with the bistable cholesteric devices proposed previously, our device, characterized by its capability of direct two-way transitions between the two bistable states, possesses a very short transition time from the focal conic state to the planar state as short as 10 ms. No voltage has to be applied to sustain the optical states, making the device low energy consuming. Potential applications of this device are addressed.

Fast-switching bistable cholesteric intensity modulator

Diffraction gratings were studied in cells of the homogeneously aligned liquid-crystal E7 doped with multiwall carbon nanotubes. These phase gratings were induced by interference modulation of two coherent optical beams, in conjunction with an applied dc field that was perpendicular to the unperturbed director axis. Self-diffraction was observed at all angles of incidence of the writing beams, including normal incidence. A superior nonlinear-index coefficient of 5×10-2 cm2/W was obtained after passage of a 44‐mW/cm2 beam through a film with a grating constant of 18 μm under an external voltage of 15  V. The observed phenomenon depends strongly on the applied dc field, and the memory effect in a nematic film depends strongly on the grating constant.

Observation of self-diffraction by gratings in nematic liquid crystals doped with carbon nanotubes.

This study focuses on the electrical characteristics of nematic liquid crystals doped with anatase titania nanoparticles (NPs). Low-frequency dielectric spectroscopy between 10−2 and 103 Hz is used to investigate the space-charge polarization. The results indicate that the doped samples have lower ionic concentration, diffusion constant and activation energy due to the effect of the dopant on ion transport. Temperature-dependent data provide an insight into the feasibility of using these NPs as a dopant to promote the device performance in a real environment. Verified by the voltage holding ratio, it is clear that doping anatase NPs into liquid crystals is a potentially cost-effective and easy way to improve the device performance.

https://iopscience.iop.org/article/10.1088/0022-3727/44/35/355102/pdf

Electrical properties of nematic liquid crystals doped with anatase TiO 2 nanoparticles

An electrically tunable and polarizer-free photonic device is developed using a one-dimensional photonic crystal (PC) infiltrated with dual-frequency cholesteric liquid crystal (DFCLC) as a defect layer. The PC/DFCLC hybrid cell allows the employment of various frequency-modulated voltage pulses to regulate defect modes and switch between stable states. This device possesses many alluring features such as rapid bistable switching, intensity tunability, and wavelength tunability in the defect modes, and it requires no polarizers. It can be used as a filter, fast-speed shutter, or light-intensity modulator.

Wei Lee

Papers

Faster electro-optical response characteristics of a carbon-nanotube-nematic suspension

Fast-switching bistable cholesteric intensity modulator

Observation of self-diffraction by gratings in nematic liquid crystals doped with carbon nanotubes.

Electrical properties of nematic liquid crystals doped with anatase TiO 2 nanoparticles

Electro-optical device based on photonic structure with a dual-frequency cholesteric liquid crystal