L.D. Hess

Birefringence measurements of liquid crystals

A convenient and accurate technique for measuring the birefringence of liquid crystals at discrete wavelengths or as a continuous function of wavelength in the ultraviolet, visible, or infrared spectral regions is described. The method is based on determination of the phase differences which occur when monochromatic polarized light propagates through a medium with an anisotropic refractive index. Birefringence measurements at 0.6328 μm for two liquid crystal materials, BDH-E7 and ZLI-1132, and a continuous birefringence spectrum of ZLI-1132 from 2 to 16 μm are reported. Additionally, a liquid crystal based phase retardation plate which can be voltage tuned and calibrated to provide any degree of phase shift from 0 to 2π over a wide wavelength range (0.4–16 μm) is discussed.

Direct observation of laser‐induced solid‐phase epitaxial crystallization by time‐resolved optical reflectivity

Direct measurement of solid phase epitaxial crystallization during cw laser annealing of ion‐implanted silicon is reported. The measurement technique utilizes optical interference effects between reflected light from the sample surface and from the epitaxial growth plane to time‐resolve the growth process with high spatial resolution. Laser‐induced solid phase epitaxial growth was monitored for two values of incident laser power; corresponding epitaxial growth rates and calculated surface temperatures are given.

Infrared birefringence of liquid crystals

The first measurements of infrared birefringence of liquid crystals are reported. Continuous birefringence spectral data were calculated from measurements of phase differences which occur when monochromatic polarized light propagates through a medium with an anisotropic refractive index. It was found that molecular absorption bands can provide significant resonant enhancement of the refractive indices of liquid crystals, and this effect causes the birefringence of these materials to be relatively large throughout the spectral region from 2 to 16 μm. The birefringence and absorption spectra of two particular liquid crystal mixtures indicate that liquid crystals will be useful for electro‐optic applications in the infrared region.

Kinetics of laser‐induced solid phase epitaxy in amorphous silicon films

The kinetics of laser and furnace‐induced solid phase epitaxial crystallization of As‐implanted amorphous layers (∼0.16 μm) on Si (100) substrates was studied using a time‐resolved optical reflectivity technique. Epitaxial rates from 10−10 to 0.2 cm/sec were measured over a temperature range from 750 to 1550 °K at two different As concentrations, ∼2×1019 cm−3 and ∼4×1019 cm−3. Temperatures achieved during cw‐laser heating were calculated using a three‐dimensional steady‐state thermal analysis. The crystal growth rates are accurately described by the Arrhenius equation v = v0e−Ea/kT , with Ea dependent on the dopant concentration; Ea = 2.62 and 2.52 eV for the lower and the higher concentrations, respectively. Additionally, the rate measurements at high temperatures imply that the melt temperature of amorphous Si is greater than 1550 °K.

Laser-Induced Solid Phase Crystallization in Amorphous Silicon Films

We review recent work on the kinetics of laser-induced solid phase epitaxial crystallization of silicon as determined from time-resolved reflectivity measurements. Specific topics which are addressed include: the intrinsic kinetics of solid phase epitaxy (SPE) in ion-implanted and UHV-deposited films; SPE rate enhancement by implanted dopant atoms and the effects of electrical compensation on the SPE rate; and the temperature dependence of SPE and competing processes in samples containing impurity atoms at concentrations exceeding the solid solubility limit. The high temperature kinetics results are compared with predictions from transition state theory and are discussed with respect to a proposed depression in the amorphous Si melting temperature.

Reversible Photodissociative Laser System

Laser action at 2.7 μ from atomic bromine was produced by flash photolysis of gaseous iodine monobromide. The output consists of a series of sharp pulses having peak powers up to 50 W and a total duration of 5 μsec. Optical gain was estimated to be ∼1 dB/m. Chemical reversibility of the system is very good; regeneration of the starting material occurs spontaneously and requires about 3 msec. The fact that excited bromine rather than excited iodine atoms are formed leads to clarification of previous conflicting spectroscopic assignments.

Photoacoustic measurements of ion‐implanted and laser‐annealed GaAs

Photoacoustic techniques were developed for use as a rapid and sensitive diagnostic of crystalline disorder produced by ion implantation in gallium arsenide. With a 150‐keV silicon implant, doses as low as 1012 cm−2 were detectable. Recrystallization of ion‐implanted gallium arsenide by laser annealing is readily detected by the photoacoustic method.

Liquid-Crystal-Based Visible-To-Infrared Dynamic Image Converter

The development of a liquid-crystal-based visible-to-IR dynamic image converter (VIDIC) is described. Liquid crystal studies in the IR as well as the structure, operation, and preliminary performance results of a 10.6 um VIDIC are reported.

Optical rotatory power of 90° twisted nematic liquid crystals

A technique was developed and used to measure the rotatory power of 90° twisted nematic liquid crystals in the visible and infrared spectral regions. Experimental results for two liquid crystal mixtures, BDH‐E7 and ZLI‐1132, are in very good agreement with theoretical calculations for a wide spectral range and for different liquid crystal thicknesses. A quantitative relationship between liquid crystal birefringence, wavelength, and cell thickness was derived for use in the design of IR liquid crystal light valves with high contrast ratios and minimum response times.

Chain‐Reaction Chemical Laser Using H2–F2–He Mixtures

Laser action was observed from HF produced in a chain reaction between H2 and F2 initiated by flash photolysis of H2–F2–He mixtures. Conditions were found for mixing the reagents at room temperature without the occurrence of spontaneous ignition.