George H. Watson

Measurements of Raman intensities and pressure dependence of phonon frequencies in sapphire

Absolute Raman cross sections and mode Gruneisen parameters of the Raman‐active optical vibrational modes of sapphire (a = Al2O3) were measured. Intensity measurements show that magnitudes of the Raman tensor components agree very well with group theoretical expectations. Pressure dependence of the frequencies has been measured over a range from 1 to 10 kbars and mode Gruneisen parameters have been calculated.

CARBON INCORPORATION IN SI1-YCY ALLOYS GROWN BY MOLECULAR BEAM EPITAXY USING A SINGLE SILICON-GRAPHITE SOURCE

Pseudomorphic Si1−yCy alloys on silicon (100) were grown by molecular beam epitaxy using a single effusion source of silicon contained in a graphite crucible, producing carbon concentrations of y=0.008. The behavior of carbon incorporation using this source was studied as a function of growth temperature using x-ray diffraction and infrared spectroscopy, and was compared to previous studies, where Si1−yCy was grown from separate silicon and graphite sources. An increased energy barrier for the surface diffusion of carbon was observed using the single silicon–graphite source. An infrared absorption mode near 725 cm−1, observed for growth temperatures up to 700 °C, was attributed to a transitional phase between the loss of substitutional carbon and the formation of silicon carbide precipitates.

Interferometric technique for the measurement of photonic band structure in colloidal crystals.

Under suitable conditions polystyrene microsphere colloids form photonic crystals capable of diffracting visible light, analogous to x-ray diffraction from atomic crystal planes. The lattice spacings of these crystals can be tailored to satisfy the Bragg condition along a certain direction for a particular desired wavelength. A modified Mach-Zehnder interferometer has been developed for accurately measuring relative phase shifts of light propagating in photonic crystals to determine the dispersion resulting from photonic band structure near the band edges.

Nonlinear optical spectroscopy in one-dimensional photonic crystals

We have conducted a spectroscopic investigation of the enhancement of nonlinear optical effects around a defect in a one-dimensional photonic crystal. Degenerate four-wave mixing studies were performed on a dielectric stack that contained a polymer thin-film defect layer doped with a nonlinear organic dye. This sample exhibited a large nonlinear response at a resonant defect frequency. Nonlinear spectroscopy was performed around the defect resonance and at frequencies well away from resonance. We have shown that the four-wave mixing signal exhibits extremely high-quality resonance, consistent with the expected cubic dependence on the calculated intensity within the defect layer.

Defect modes in coaxial photonic crystals

One-dimensional (1D) photonic crystals have been constructed by connecting segments of coaxial cable of differing characteristic impedance. Impurities have been introduced into these crystals by inserting cable segments to break the crystal symmetry. This system provides a simple way to study 1D photonic band structure effects with complete control over impurities in the lattice. We have studied the effects of the size, number, and location of defects in the lattice. We have also measured directly the concentration of energy in the steady-state electromagnetic fields within doped crystals, and observed the influence of the defects on the phase (dispersion). A modified dielectric stack model was developed to describe this system, with the results in excellent agreement with our measurements. Our findings compare favorably to previously published measurements of transmission and phase change in three-dimensional photonic crystals.

Role of surface reflectivity in coherent backscattering measurements

Surface reflectivity effects on coherent backscattering have been studied by using variable-reflectivity mirrors at the sample surface. Photon-transport parameters obtained from coherent backscattering were found to be significantly improved by using current theoretical models for samples with high surface reflectivity.

Transmission line exercises for the introductory physics laboratory

Several exercises are reviewed for introducing the essential elements of transmission lines to students of introductory electricity and magnetism. A simple circuit is presented for generating ∼100 ns pulses suitable for demonstrating reflection and superposition of pulses on long coax cable using an inexpensive oscilloscope.

Injection-Seeded Optical Parametric Oscillator

Injection seeding of an AgGaSe2 mid infrared optical parametric oscillator using a continuous wave HeNe laser operating at 3.39 µm has been demonstrated. A measurement of the resulting spectral bandwidth, 0.001 µm, was resolution limited. Pulse evolution time intervals were found to be significantly shorter when injection seeding was in effect while the efficiency was found to be significantly higher.

Optical Measurements of Photonic Band Structure in Colloidal Crystals

Polystyrene colloidal crystals form three-dimensional periodic dielectric structures which can be used for photonic band structure measurements in the visible regime. Kossel lines obtained from these crystals reveal the underlying photonic band structure of the lattice in a qualitative way. Also, Kossel lines are useful for locating symmetry points of the lattice for exact orientation of the crystals. Prom transmission measurements the photonic band structure of an fcc crystal has been obtained along the directions between the L-point and the W-point. A modified Mach-Zehnder interferometer has also been developed for accurately measuring relative phase shifts of light propagating in photonic crystals to determine the dispersion resulting from photonic band structure near the band edges.

Effect of defect states on photonic dispersion in colloidal crystals

Summary form only given. A colloidal crystal of submicron-sized polystyrene spheres in water may serve as a photonic crystal owing to the dielectric contrast between the constituent materials. Such photonic crystals exhibit strong attenuation in regions of the transmission spectrum (stop bands) arising from Bragg diffraction of light by the crystal planes. Stop bands in polystyrene colloidal crystals have been observed and studied extensively for both FCC and BCC crystal phases. Photonic crystals also modify the photonic dispersion relation near the stop band. In addition, defect states can be introduced into the stop band of a colloidal photonic crystal by substitutional doping with microspheres of different size or dielectric strength. We have significantly improved the Mach-Zehnder interferometer used in a previous dispersion study. We report the application of the improved instrument to the study of dispersion in a colloidal crystal with defect states present.