Kei-Hsiung Yang

Generation of Far‐Infrared Radiation by Picosecond Light Pulses in LiNbO3

We have observed far‐infrared radiation generated by picosecond pulses in LiNbO3 with several different phase‐matching conditions. The output spectra, analyzed by a far‐infrared Michelson interferometer and by a Fabry‐Perot interferometer, agree well with theoretical calculations. The laser pulsewidth deduced from these measurements was about 2 psec in comparison with 5 psec obtained from two‐photon fluorescence measurements.

VUV fluorescence of Nd3+‐, Er3+, and Tm3+‐doped trifluorides and tunable coherent sources from 1650 to 2600 Å

Broad‐band vacuum ultraviolet (VUV) fluorescence has been observed in Nd3+‐, Er3+‐, and Tm3+‐doped LaF3, YF3, LiYF4, and LuF3 by electron‐beam and VUV excitation. The VUV excitation spectra indicate a large Stokes shift (≳5000 cm−1) and a high fluorescence quantum yield of 0.8. The threshold power for laser action in these crystals by optical pumping has been estimated. Using a molecular H2 laser as a pumping source, a laser system which is tunable from 1650 to 2600 A could be constructed.

Determination of the anisotropic potential at the nematic liquid crystal‐to‐wall interface

The integrated birefringence of a surfactant (C16H33NH2)‐aligned methoxybenzylidene butylaniline cell as a function of applied magnetic field (up to 100 kG) has been measured. The Oseen and Frank elastic continuum theory and a generalized torque balance equation at the interface have been utilized for the calculation to fit the measured data. A very good fit to the data was obtained using an anisotropic liquid crystal‐to‐wall interfacial potential of the form C cos2 θ+C4 cos4 θ for the calculation. It was found that C4/C=−0.38.

Depolarization field and ionic effects on the bistability of surface‐stabilized ferroelectric liquid‐crystal devices

Based on a collective switching model coupled to a simplified continuity equation for impurity ions, the internal electric field and light transmission of a surface‐stabilized (SS), layer‐tilted ferroelectric liquid‐crystal (FLC) cell with insulating alignment layers have been calculated as a function of time, under bipolar voltage pulses. The polarization and ionic charges, accumulated on the interfaces between the FLC medium and alignment layers, tend to partially screen the external on‐field and generate a depolarization field to reverse the spontaneous polarization when the external field is off. When the thickness of the alignment layer is greater than 1000 A, it is difficult to achieve bistability using FLC mixtures with a spontaneous polarization larger than approximately 20 nC/cm2. For bistability to occur in a SSFLC cell with high spontaneous polarization, thin insulating or conducting alignment layers are preferred. We also found that the existence of impurity ions in a SSFLC cell is always detr...

Structural and electrical charaterization of crystallographic defects in silicon ribbons

Abstract Silicon ribbons grown as wide as 50 mm by the Capillary Action Shaping Technique (CAST) are classified according to perfection and electrical performance. The classification is obtained through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and minority carrier lifetime and solar cell efficiency measurements. First the electrical activity of the defects is measured through EBIC contrast in the SEM. Subsequently the defects are analyzed in the TEM. The defect state in the crystal thus obtained is correlated with minority carrier lifetime and related to solar cell efficiency. Four crystal quality groups are established for CAST ribbons. In these groups solar cell efficiency decrease from 5–8% in class I to less that 1% in class IV. Such degradation in solar cell efficiency is caused by dislocations (I) high order twin boundaries (II), low angle grain boundaries (III), and, silicon carbide dendrites (IV). The electrical activity of such defects varies from low activity to strong activity. Solar cell efficiency depends on the number of “electrically active defects” and not on the total defect content of the ribbon.

Phase‐matched far‐infrared generation by optical mixing of dye laser beams

We report the use of a dual‐frequency dye laser system to generate continuously tunable far‐infrared radiation over the frequency range from 20 to 190 cm−1. We have investigated both collinear (forward and backward) and noncollinear phase matching in LiNbO3 over most of this frequency range and forward collinear phase matching in ZnO, ZnS, CdS, and CdSe at selected frequencies.

uv fluorescence of cerium‐doped lutetium and lanthanum trifluorides, potential tunable coherent sources from 2760 to 3220 Å

Broad‐band uv flurescence is reported for LaF2 : Ce3+ (2760–3120 A) and LuF3 : Ce3+ (2880–3220 A). The five‐peak excitation spectrum expected for the 5d state of Ce3+ in a noncubic crystal field is observed. The fluorescence quantum yields for LaF3 : 1% Ce3+ and LuF3 : 0.1% Ce3+ are 0.9 and 0.82, respectively. Estimates of the threshold power for lasing action suggest that a laser system tunable from 2760 to 3220 A is feasible with noble‐gas–halide lasers as pumping sources.

Directionally solidified solar-grade silicon using carbon crucibles

Abstract Directional solidification of silicon, in carbon crucibles, was achieved by two variations of the Bridgman method. One is a static technique, wherein liquid silicon in a 5 cm diameter × 5 cm high carbon crucible was positioned in a temperature gradient of about 35°C/cm, with the temperature at the crucible top being hottest. Solidification was achieved by lowering the system temperature at a rate of 4–5°C/min. The second technique entailed lowering a 5 cm × 5 cm × 12 cm high carbon crucible, loaded with silicon, through a fixed RF-coil at a rate of 5.5 mm/min. Crack-free silicon was produced by both methods. The equilibrium grain structure was initiated by nucleation at the crucible walls, with surviving grains tending to grow in alignment with the temperature gradient to produce an axially columnar grain structure, of mainly 〈110〉 orientation. This behavior dominated attempts to seed the growth at the crucible bottom. The average grain diameter was 1.08 mm and the typical length was 7 mm. Transmission electron microscopy was used to assess crystal perfection. A solar cell efficiency of 11.5% (AM1) was achieved using this material.

Measurements of empty cell gap for liquid‐crystal displays using interferometric methods

In order to determine the empty cell gap of a liquid‐crystal (LC) cell to better than 1% accuracy, the empty LC cell has been modeled as an optical medium consisting of seven layers with an air gap representing the true cell gap in the middle layer. The given index of refraction and thickness for each layer, optical reflections as a function of wavelength ranging from 500 to 650 nm have been calculated. To simulate the experimental measurement, the apparent cell gaps were deduced from reflection peaks and compared with the true cell gaps in the region of 5–10 μm. It was found that the difference between the apparent and true cell gaps was larger than 0.66 μm using a 0.1‐μm polyimide as an aligning layer coated on top of the transparent indium‐tin‐oxide electrode of 0.04 μm in thickness. This modeling is applicable to three interferometric methods commonly used for the measurement of an empty cell gap.

Contrast properties of reflective liquid crystal light valves in projection displays

Projectors that use reflective light valves must employ beam splitters or analogous components to separate bright-state light from dark-state light, since both states must propagate in the space above the light valve. Polarization ray tracing shows that such beam splitters will not usually achieve high rejection of dark-state light when the beam has the typical angular divergence of about ±10°. At such propagation angles, different rays in the beam will have appreciably different planes of incidence at tilted optical coatings in the system (because of the compound angles involved). If the light valve is mirrorlike in dark state, we show that to correct the depolarization resulting from compound incidence angles, it is necessary that the optics introduce no rotation in the illuminating polarization. To a reasonable approximation, such a rotation in polarization will double in the return pass through the optics. To the same approximation, induced ellipticity in the illuminating polarization will cancel in double pass, and pure rotation can be converted to pure ellipticity with a quarterwave retarder. An important qualification, however, is that a light valve can only be exactly mirrorlike in restricted cases [i.e., if linearly polarized input light remains exactly linearly polarized (though possibly rotated) at all wavelengths when it reaches the mirror backplane of the light valve, independent of small manufacturing errors]. We calculate contrast loss in the more realistic case of a reflective twisted nematic liquid crystal (TNLC) light valve interacting with tilted coatings in the projection optics over finite numerical aperture (NA), and discuss the impact on LC thickness tolerances and spectral bandwidth Δλ. We extend our results to apply to more general light valves and more general projection optics configurations. Dark-state background is found to scale as NA2 (or in some cases as ∼NA2Δλ2). Because of this interaction, the complete system almost always shows a lower contrast than the light valve alone.