Browsing by Author "Zallen, Richard H."
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- Detection limits of CO₂in fluid inclusions using microthermometry and Raman spectroscopy and the spectroscopic characterization of CO₂Rosso, Kevin M. (Virginia Tech, 1994)In many geologic environments, dominantly aqueous solutions contain low concentrations of CO₂. At ambient temperature, in fluid inclusions which trap these solutions, the typical phase assemblage consists of a CO₂-rich vapor (where PCO₂ ≈ PinternaI) and an aqueous phase containing dissolved salts and CO₂. In this study, the CO₂ minimum detection limits (MDLs) using microthermometry and laser Raman spectroscopy are established in terms of PCO₂ using synthetic H₂O-CO₂ inclusions. The purpose of the microthermometric experiments was to examine the diagnostic CO₂ phase changes and determine the quantity of CO₂ necessary to result in observable solid CO₂ melting. The results of these experiments show that an observable solid CO₂ melting event requires PCO₂ ≥ 45 bar at 25°C. The Raman spectroscopic detection limits were investigated using a multichannel Raman spectrometer. Because the Raman spectroscopic MDLs are a function of counts, the CO₂ MDLs were determined by collecting signal-to-noise ratios for both the upper and lower v₁-2v₂ bands as a function of CO₂ pressure (5-60 bars) and over a range of integration times and incident laser power to predict the optimal instrument settings. The resulting CO₂ MDLs are on the order of 1 bar for our instrument. The band splitting of the v₁-2v₂ diad as a function of CO₂ pressure was measured up to 500 bar at ambient temperature. The CO₂ pressures were converted to ρCO₂ and the results are given in terms of the frequency separation between the upper and lower bands. These results are compared to those of previous studies. An analysis of the estimated errors indicates that the technique can be used to determine CO₂ densities in fluid inclusions containing a homogenous, free CO₂ phase to a precision of approximately ± 0.02 g/cm³. The temperature dependence of the intensity ratio of the hot bands to the v₁-2v₂ diad was measured from 270 to 315 K. The close agreement between the calculated and observed results indicate that laser induced sample heating is not significant. The intensity ratio can be used to estimate the CO₂ temperature and, combined with the Raman density determination, allows calculation of the CO₂ pressure.
- The diagramatical solution of the two-impurity Kondo problemZhou, Chen (Virginia Polytechnic Institute and State University, 1988)The problem of the two-impurity Kondo problem is studied via the perturbative diagrammatical method. The high-temperature magnetic susceptibility is calculated to fourth order in the coupling constant J for different regimes. The integral equations for the ground state energy are established and solved numerically. The two-stage Kondo effect and corresponding energy scales are found which agree with the scaling results.
- Effect of pressure on a defect-related band-resonant vibrational mode in implanation-disordered GaAsSauncy, T.; Holtz, M.; Zallen, Richard H. (American Physical Society, 1994-10)We have used hydrostatic pressure as a means for studying a resonant Raman mode observed at 47 cm(-1) in highly disordered, ion implanted, unannealed GaAs. The mode shifts weakly (-0.07 +/- 0.15 cm(-1)/GPa), supporting an identification of this band-resonant vibration as stemming from the breathing mode of the gallium vacancies, which are expected to be in high concentration. We measure a pressure coefficient of the longitudinal-optic phonon in these (5.5 nm) nanocrystals of GaAs to be 3.6 +/- 0.1 cm(-1)/GPa. The good agreement between our value and the pressure shift of this phonon in bulk GaAs implies that the bulk modulus is independent of size at least down to this size crystallite.
- Effects of ion processing and substrate variables on electrical characteristics of GaAsSen, Sidhartha (Virginia Tech, 1991)The main objective of this study was to determine fundamental information related to ionbeam-induced damage to gallium arsenide (GaAs). The study covers experimental results concerning defect creation in GaAs versus parameters such as implantation energy, nature of GaAs substrate, crystalline orientation, and annealing. Transport and deep level transient spectroscopy (DLTS) results are presented for 50 keV Si-implanted and RTA (rapid thermal annealing) GaAs with (100) and (211) substrate orientations. Several electron traps are identified and their possible origins discussed. It is observed that (211) GaAs, after Si-implantation and RTA, has higher residual damage than (100) oriented GaAs. The electrical properties of active GaAs on Cr-doped and undoped GaAs substrates are compared. The DLTS response of active layers on Cr-doped GaAs is significantly different from those on undoped GaAs. A viable explanation that accounts for this difference is presented. The effects of furnace annealing on electrical properties of 50 keV, 4 x 10¹³ cm⁻² Si-implanted GaAs are addressed. A correlation between the structural recovery and electrical activation is established. The effects of 2 and 6 MeV Si implantation followed by RTA on the electrical characteristics of GaAs are investigated in detail. MeV Si-implantation and RTA generates active buried layers in GaAs. The buried layer quality is found to be at least comparable to a similarly processed keV Si-implanted active GaAs layer. The deep traps in MeV-implanted GaAs are identified and explained in terms of their probable origins. The deep level behavior of MeV Si-implanted and RTA GaAs is distinctly different from keV Si-implanted and RTA GaAs. This difference is largely due to the dynamic annealing occurring during MeV implantation. MESFETs formed on MBE-grown Al.35Ga.65As and low temperature MBE-grown GaAs buffer layers have shown peculiar characteristics (improved transconductance, sharper carrier profile, variability in threshold voltage, significant backgating, etc.). The effects of Al.35Ga.65As buffers and low temperature GaAs buffers on the electrical properties of the overlying active GaAs are investigated. Transport, DLTS, and SIMS (Secondary Ion Mass Spectroscopy) measurements are employed to explain the abnormalities in buffered MESFETs. Deep states and impurities are identified in buffers; they appear to migrate toward the channel-buffer interface during processing. The defects originating from the buffer are correlated to the performance of MESFETs formed on them. The effects of ion processing parameters, substrate chemistry, buffer layers, and annealing on the electrical characteristics of active GaAs layers are identified. An understanding of these effects is extremely critical to obtain reproducible devices with desirable characteristics.
- Electrical conduction transport mechanisms of barium titanate- based multilayer ceramic capacitorsZhang, Tong (Virginia Tech, 1988-08-15)The major objectives of this study were to examine electrical conduction properties of BaTiO3-based multilayer ceramic ( MLC ) capacitors in order to gain a better understanding of the conduction transport mechanisms inside the devices. The experiments involved mainly leakage current versus time measurements under both low temperature-low voltage stress and high temperature-high voltage stress. It was established that leakage current conduction in a MLC capacitor under temperature-voltage stress can be divided into three different conduction regions due to different mechanisms. Those regions are polarization current, DC conduction current and degradation current. The polarization current decreases with time as a power law relation, i.e. Ic(t) t-m where the exponent value m is strongly dependent on the type of capacitor and temperature, but is only weakly dependent on the applied voltage. It has been proposed that two degradation models ( a charge carrier concentration model and a reduction of grain boundary barrier height model ) can explain the degradation behavior for the Z5U devices tested. Degradation measurements indicate that the lifetime for Z5U capacitors can be described by Minford's expression. However, these models account only partly for X7R degradation. X7R behavior, is characterized by an early power law time dependence, followed by exponential voltage dependence. The most probable conduction transport mechanism in X7R capacitors is small polaron hopping, while grain boundary transmission may be the predominant conduction transport mechanism in Z5U capacitors.
- Electrical studies on ion-etched n-GaAs(100) surfacesSen, Sidhartha (Virginia Tech, 1987-12-15)The major objective of this thesis was to evaluate electrically the damage caused by a low energy (< 4keV) Ar+ bombardment on n-GaAs(100) surfaces. Electrical measurements were performed on Schottlky diodes formed on the virgin and the ion-etched surfaces. The l-V measurements show deterioration of diode parameters by ion etching. The ion etched diodes have a strong component of surface leakage current. The high frequency capacitance of ion-etched diodes is less than that of the virgin diodes. The low frequency capacitance of ion-etched diodes was found to be frequency dispersive. The extent of frequency dispersion diminishes at low temperatures and at low reverse biases. Virgin diode capacitance, on the other hand, was found to be independent of frequency. The electrical characteristics of ion-etched diodes are explained by means of an amorphous layer and a donor-like damaged layer formed as a result of ion etching. The depth of the top amorphous layer increases with etch energy. The damaged layer containing the ion induced traps superimposes over the amorphous layer and extends deep into the bulk semi-conductor. The density of such traps is very bias sensitive and also temperature dependent. A possible equivalent circuit model for the ion-etched material is proposed. Low temperature isochronal annealing (< 450°C, 10mins.) was not found effective in causing complete recovery of the ion-damaged surface.
- Ferroelastic phase in SrBi2Ta2O9 and study of the ferroelectric phase-transition dynamicsKamba, S.; Pokorny, J.; Porokhonskyy, V.; Petzelt, J.; Moret, M. P.; Garg, A.; Barber, Z. H.; Zallen, Richard H. (AIP Publishing, 2002-08)Polarized microscope observation of ferroelastic domains in a SrBi2Ta2O9 (SBT) single crystal reveals the presence of domains up to T(c1)similar or equal to770 K, which supports the ferroelasticity and Amam symmetry of the intermediate phase between ferroelectric and paraelectric phases. Far-infrared spectra of SBT ceramics, single crystal and thin films show a well underdamped optical soft mode at 28 cm(-1), which partially softens to 21 cm(-1) near the ferroelectric transition temperature (T(c2)congruent to600 K). This softening does not explain the entire anomaly of low-frequency permittivity observed near T-c2. On the basis of high-frequency measurements, which do not show a significant dispersion, central-mode-type dispersion in the 10-100 GHz range is proposed as an explanation. So, the phase transition at T-c2 apparently shows a crossover behavior between the displacive and the order-disorder type. (C) 2002 American Institute of Physics.
- Impulse electrical breakdown of high-purity waterGehman, Victor H. (Virginia Tech, 1995-05-05)Experiments have been conducted on the electrical breakdown of high-purity water and water mixtures. The electrical regime of interest has been carefully defined and documented to consist of electrical impulses with approximately microsecond rise time and fall time greater than 65 microseconds, on approximately 81-square-centimeter-area planar electrodes with a dielectric gap of approximately one centimeter. The results of over 25,000 shots by a Marx generator have been distilled into database form in an Excel spreadsheet and analysis performed to try to find patterns or indirect evidence into the nature of the breakdown-initiation process. An extensive review of all the experiments, which had been conducted over eight years by the Naval Surface Warfare Center and which had been designed to find the largest water-breakdown fields, was conducted with the intention of delineating the physical factors that led to breakdown. A variety of theoretical models of breakdown initiation were compared to the data, until it became clear that many of the breakdowns were dominated by impurities of various sorts. An extensive study of old and new experiments led to a more detailed understanding of the phenomenology of impurity-dominated water breakdown (such as the process of "conditioning" the electrodes and hysteresis) and the proposal of a number of new experiments to further characterize the intrinsic role of electrode materials on determining high-electric-field dielectric breakdown in water.
- Infrared activity in elemental crystalsZallen, Richard H.; Martin, R. M.; Natoli, V. (American Physical Society, 1994-03)In a previous paper, Zallen [Phys. Rev. 173, 824 (1968)] reported a group-theoretical analysis of the competition between unit-cell complexity and crystal symmetry in determining the presence or absence of infrared-active phonons in an elemental crystal. Here we correct an error in that paper's treatment of certain hexagonal space groups. Our results modify the minimum-complexity condition for infrared activity: For 228 of the 230 space groups, a necessary and sufficient condition for the existence of symmetry-allowed infrared-active modes in an elemental crystal is the presence of three or more atoms in the primitive unit cell. The two exceptional space groups are P6/mmm (D6h1) and P6(3)/mmm (D6h4); for each of these symmetries, there exists one structure with four atoms per cell and no infrared modes. The P6(3)/mmc structure includes, as special cases, Lonsdaleite (or ''wurtzite silicon'') as well as a c-axis-aligned hcp arrangement of diatomic molecules which is relevant to models of solid molecular hydrogen at high pressure.
- Infrared activity in the Aurivillius layered ferroelectric SrBi2Ta2O9Moret, M. P.; Zallen, Richard H.; Newnham, R. E.; Joshi, Pooran C.; Desu, Seshu B. (American Physical Society, 1998-03-01)Experimental studies were carried out on infrared-active phonons in the Aurivillius ferroelectric SrBi2Ta2O9 (SBT), using reflectivity measurements (down to 200 cm(-1)) and transmission measurements (down to 100 cm(-1)) on crystals, pellets, and thin films. An analysis was done of the contrasting consequences of the competing orthorhombic and pseudotetragonal symmetries in SBT. Reflectivity results for light polarized in the ab plane show that the anisotropy in this plane is small in the frequency range from 300 to 1200 cm(-1),, indicating the influence of the approximate tetragonal symmetry. Dielectric dispersion properties were derived for this polarization (E perpendicular to c) in this frequency range. The transverse-optical (TO) and longitudinal-optical (LO) frequencies corresponding to the dominant E perpendicular to c band are 613 and 773 cm(-1), respectively. This strong, high-frequency band arises from a mode dominated by the motion of the oxygen sublattice; its TO and LO frequencies yield an oxygen-ion Szigeti effective charge of -1.5e. Frequency estimates for the TO (LO) pairs of other strong bands are 188(330) and 334(451) cm(-1) for E perpendicular to c, and 610(675) and 780(815) cm(-1) for E parallel to c. In addition to the main infrared bands, the main Raman bands of SBT are also reported.
- Infrared reflectivity and lattice fundamentals in anatase TiO2Gonzalez, R. J.; Zallen, Richard H.; Berger, H. (American Physical Society, 1997-03-15)Polarization-dependent far-infrared reflectivity measurements were carried out on single crystals of anatase TiO2. The results were analyzed to yield the dielectric dispersion properties of anatase in the lattice fundamentals regime. The frequencies (in cm(-1)) of the transverse optical (TO) and longitudinal optical (LO) zone-center phonons were determined to be 367(755) for the TO (LO) of the A(2u) mode, 262(366) and 435(876) for the E(u) modes. The large TO-LO splittings were used to estimate effective charges.
- Infrared studies of hole-plasmon excitations in heavily-doped p-type MBE-grown GaAs : CSongprakob, W.; Zallen, Richard H.; Liu, W. K.; Bacher, K. L. (American Physical Society, 2000-08-15)Infrared reflectivity measurements (200-5000 cm(-1)) and transmittance measurements (500-5000 cm(-1)) have been carried out on heavily-doped GaAs:C films grown by molecular-beam epitaxy. With increasing carbon concentration, a broad reflectivity minimum develops in the 1000-3000 cm(-1) region and the one-phonon band near 270 cm(-1) rides on a progressively increasing high-reflectivity background, An effective; plasmon/one-phonon dielectric function with only two free parameters (plasma frequency omega(p) and damping constant gamma) gives a good description of the main features of both the reflectivity and transmittance spectra. The dependence of omega(p)(2) on hole concentration p is linear; at p = 1.4 x 10(20) cm(-3), omega(p) is 2150 cm(-1). At each doping, the damping constant gamma is large and corresponds to an infrared hole mobility that is about half the Hall mobility. Secondary-ion mass spectroscopy and localized-vibrational-mode measurements indicate that the Hall-derived p is close to the carbon concentration and that the Hall factor is dose to unity, so that the Hall mobility provides a good estimate of actual de mobility. The observed dichotomy between the de and infrared mobilities is real, not a statistical-averaging artifact. The explanation of the small infrared mobility resides in the influence of intervalence-band absorption on the effective-plasmon damping, which operationally determines that mobility. This is revealed by a comparison of the infrared absorption results to Braunstein's low-p p-GaAs spectra and to a k.p calculation extending Kane's theory to our high dopings. For n-GaAs, which lacks infrared interband absorption, the de and infrared mobilities do not differ.
- Intervalenceband and plasmon optical absorption in heavily doped GaAs:CSongprakob, Wantana; Zallen, Richard H.; Tsu, D. V.; Liu, W. K. (American Institute of Physics, 2002-01-01)By using direct numerical-solution techniques for the reflectance (R) and transmittance (T) equations of a multilayer structure, we have analyzed infrared R and T measurements on heavily doped p-type GaAs:C films grown by molecular beam epitaxy. The optical properties, for films with hole concentrations up to 1.4x10(20) cm(-3), were determined for photon energies from 0.07 to 0.6 eV, in which region plasmon (intraband) and intervalenceband contributions are in competition. Our results for the optical absorption coefficient resolve two separate peaks located (at high doping) at about 0.1 and 0.2 eV. By carrying out calculations of the intervalenceband (IVB) absorption processes for our dopings, we identify the peak near 0.2 eV with light-hole to heavy-hole IVB transitions, and we attribute the lower-energy peak to hole-plasmon excitations. Our experimental absorption spectra are very well described by a model combining the IVB contribution to the dielectric function with a plasmon contribution. The hole-plasmon parameters we obtain for highly doped p-GaAs yield an infrared mobility which (unlike the too-small IVB-entangled infrared mobility implied by the use of the usual effective-plasmon model) is in substantial agreement with the dc mobility. (C) 2002 American Institute of Physics.
- Investigation of the optical properties of Bi₂Sr₂Can-1CunOy (n=1,2) by transmission electron energy loss spectroscopyWang, Yun-Yu (Virginia Tech, 1990)A high energy resolution transmission electron energy loss spectrometer was reassembled for this research project. The vacuum system, electron optical lenses, electronic control elements, and high voltage system were reconditioned. A CAMAC interface system was installed into the spectrometer, and a data collecting software package was developed which included a direct convolution method for removing the contributions of multiple scattering from the data. The spectrometer is running very well. Samples can be changed routinely without disturbing the performance of the spectrometer. The research conducted for this thesis was an investigation of the optical properties of the high temperature superconductors of Bi₂Sr₂Can - 1CunO₃(n = 1,2) by transmission electron energy loss spectroscopy. A thin film of Bi₂Sr₂CaCu₂O₈ was prepared by the flux method. A single crystal of Bi₂Sr₂CuO₆ also was grown from which a self-supporting thin film was prepared. The energy loss spectra of Bi₂Sr₂CaCu₂O₈ and Bi₂Sr₂CuO₆ materials were investigated, and the dielectric functions of these materials were derived by Kramers-Kronig analysis. A broad excitation centered at 2.7eV was identified as associated with the Cu — O₂ layer by comparing the spectrum of Bi₂Sr₂CaCu₂O₈ with that of Bi₂Sr₂CuO₆. A pseudo gap of 1.2eV in the spectrum of Bi₂Sr₂CuO₆ suggests that the one electron approximation for states derived from the hybridization of O 2pσ and Cu3dx² - y² orbital might not be valid. Two excitations at 3.6eV and 4.6eV were observed in both spectra. Comparing the spectrum of Bi₂Sr₂CaCu₂O₈ with that of Bi₂Sr₂CuO₆ suggests that the 3.6eV excitation is associated with the Bi — O layer. It was concluded that the 3.6eV and 4.6eV excitation are a spin-orbit doublet derived from the atomic bismuth 6p level. This identification is based on a comparison of Bi core level excitations from electron energy loss spectroscopy with X-ray photoemission measurements. A simplified atomic energy level picture of Bi in Bi₂Sr₂Can - 1CunO₃(n = 1, 2) is presented. A 1.0eV excitation in the energy loss spectrum of Bi₂Sr₂CaCu₂O₈ was observed, and its dispersion relationship with the momentum transfer q is presented. A Drude model was used to describe this controversial excitation.
- Iterative image processing using a cavity with a phase-conjugate mirror: possibilities and limitationsLo, Kanwai Peter (Virginia Tech, 1991-09-05)An optical image feedback system utilizing a cavity with a phase-conjugate mirror (PCM) has been studied. A new theory, based on operators, is developed to describe the steady-state output of the cavity. The use of operators allows one to describe the various optical operations and transformations needed in the optical implementation of iterative algorithms. The characteristics of the cavity are discussed using an expansion of the cavity fields in the cavity eigenfunctions. Several image processing applications using a PCM cavity are proposed and are studied using computer simulations. These theoretical studies indicate that a PC11 cavity can be useful in many applications. Optical phase conjugation was realized using a single crystal of photorefractive BaTi0₃ in a degenerated four-wave mixing geometry. The reflectivity gain from the PCM was optimized experimentally by the geometrical parameters and by the beamintensity ratios. The ability of the PCM to remove phase distortion as predicted theoretically, was demonstrated experimentally. The output of a PCM cavity can be substantially influenced by self-oscillations of the cavity above threshold. This was experimentally studied by observing the time evolution of the input. To avoid the influence of self-oscillation, the cavity must be operated below threshold. It is found that the cavity decay time constant diverges at and about threshold. This can be used as an indicator to show whether the cavity has crossed the threshold or to measure how close to threshold the cavity operates. To verify that a PCM cavity can be used in iterative image processing, an experiment was set up to implement an image restoration scheme based on the Gerchberg algorithm. It is shown that an optical implementation of the Gerchberg algorithm is feasible for objects made of few pixels. The experiment confirmed .that image iteration in a PCm cavity is possible. The limitations of the cavity and the technical difficulties are discussed.
- Monte Carlo simulation of aqueous dilute solutions of polyhydric alcoholsLilly, Arnys Clifton (Virginia Polytechnic Institute and State University, 1989)In order to investigate the details of hydrogen bonding and solution molecular conformation of complex alcohols in water, isobaric-isothermal Monte Carlo simulations were carried out on several systems. The solutes investigated were ethanol, ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol and glycerol. In addition, propane, which does not hydrogen bond but does form water hydrates, was simulated in aqueous solution. The complex alcohol-water systems are very nonideal in their behavior as a function of solute concentration down to very dilute solutions. The water model employed was TIP4P water¹ and the intermolecular potentials employed are of the Jorgensen type² in which the interactions between the molecules are represented by interaction sites usually located on nuclei. The interactions are represented by a sum of Coulomb and Lennard-Jones terms between all intermolecular pairs of sites. Intramolecular rotations in the solute are modeled by torsional potential energy functions taken from ethanol, 1-propanol and 2-propanol for C-O and C-C bond rotations. Quasi-component pair correlation functions were used to analyze the hydrogen bonding. Hydrogen bonds were classified as proton acceptor and proton donor bonds by analyzing the nearest neighbor pair correlation function between hydroxyl oxygen and hydrogen and between solvent-water hydrogen and oxygen. The results obtained for partial molar heats of solution are more negative than the experimental values by 3.0 to 14 kcal/mol. In solution, all solutes reached a contracted molecular geometry with the OH groups generally on one side of the molecule. There is a tendency for the solute OH groups to hydrogen bond with water, with more proton acceptor bonds than proton donor bonds. The water-solute binding energies correlate with experimental measurements of the water-binding properties of the solute. 1. Jorgensen, W.L. et al, J. Chem. Phys., 79, 926 (1983). 2. Jorgensen, W.L., J. Phys Chem., 87, 5304 (1983).
- A new surface resistance measurement method with ultrahigh sensitivityLiang, Changnian (Virginia Tech, 1993-05-05)A superconducting niobium triaxial cavity has been designed and fabricated to study residual surface resistance of planar superconducting materials. Unlike many other structures where the test samples are placed in strong magnetic field positions, the edge of a 25.4 mm or larger diameter sample in the triaxial cavity is located outside the strong field region. Therefore, the edge effects and possible losses between the thin film and the substrate have been minimized in this design, ensuring that the induced RF losses are intrinsic to the test material. The fundamental resonant frequency of the cavity is 1.5 GHz, the same as the working frequency of CEBAF cavities. The cavity has a compact size compared to its TE₀₁₁ counterpart, which makes it more sensitive to the sample's loss.
- Nonlinear optical studies of dye-doped nematic liquid crystalsKlysubun, Prapong (Virginia Tech, 2002-03-28)Nematic liquid crystals possess large optical nonlinearities owing to their large refractive index anisotropy coupled with the collective molecular reorientation. Doping absorbing dyes into liquid crystals increases their optical responses significantly due to increased absorption in the visible region, absorption-induced intermolecular torque, cis-trans photoisomerization, and other guest-host effects. The guest-host mixtures can be employed in display applications, optical storage devices, and others. In this dissertation, nonlinear optical studies were carried out on dye-doped nematic liquid crystal cells. The main objectives of the studies were to distinguish and characterize the several processes that can lead to the formation of dynamic gratings of different types in the samples, and to study the photorefractive and the orientational responses of these samples. Furthermore, we tried to explain and model the dynamical behaviors of the observed grating formations. The experimental techniques employed in this study include asymmetric two-beam coupling, forced light scattering, and polarization holographic method. The asymmetric two-beam coupling experiments revealed that the induced grating was a photorefractive phase grating created by the nematic director reorientation within the plane of incidence. The dynamics of the beam coupling showed that two different mechanisms with different temporal responses were involved. The grating translation technique identified both gratings as pure photorefractive index gratings with phase shifts of ~ p/2 between the grating and the interference pattern. In addition, the dynamical behavior of the grating formation, obtained from forced light scattering experiments, also exhibited a two-time constant response. The dynamical behaviors of the build-up and decay of the photocurrent were investigated. The two dynamics exhibited both a two-time constant behavior, suggesting that the origin of the two-time constant dynamics observed in the two-beam coupling and the forced light scattering experiments resides in the process of photo-charge generation. The photorefractive gain coefficients were found to be in the range of 100 – 400 cm-1. The values of the nonlinear optical Kerr index (~ 0.08 cm2/W) measured in samples with certain dye/liquid crystal combinations are higher than what has been observed in other dye-doped nematics and other liquid crystal/polymer systems. All the samples showed a threshold behavior with respect to the magnitude of the applied electric field. This threshold behavior was observed both in forced light scattering experiments and polarization holographic experiments. We believe that the origin of this threshold lies in the process of photogeneration, which was found to exhibit the same threshold behavior at the same value of the applied voltage. An asymmetry of the photorefractive gain with respect to the direction of the applied electric field was observed in samples with high dye concentration. This was attributed to the beam fanning effect, which has also been observed in other high-gain photorefractive materials. Polarization holographic measurements showed that the dye enhancement effect is primarily due to the intermolecular interaction between the dye molecules and the liquid crystal host, and that the trans-cis photoisomerization plays a lesser role. The photoinduced orientational response was also studied using polarization holographic experiments. A number of observations confirmed that the birefringent grating is due to the nematic director reorientation within the plane of incidence, under the combined effect of the applied electric field and the optical field. The diffraction efficiency was found to depend linearly on the writing beam power, while the dependence of the self-diffraction efficiency on the writing beam power roughly assumes a cubic relationship. The dynamical behavior of the birefringent grating formation was investigated. The build-up dynamics was found to be best modeled as a double-time constant response, while the decay is best fitted by a single exponential. The response of the samples to an oscillating electric field was studied as a function of the modulation frequency. Very interesting and reproducible dynamics was observed, revealing the complex dynamical response of the liquid crystal director to the magnitude and rate of change of an applied electric field. The small signal response was also measured, but did not reveal any sign of a resonance behavior. The conductivity and the photoconductivity of the samples were measured. The relationship between the measured current and the applied voltage was found to be cubic at low applied voltage, and to become linear at higher applied voltage. We could explain this behavior using a double-charge-injection-in-a-weak-electrolyte model, but this is only one of the possible mechanisms that could explain this behavior. The photocurrent was found to increase linearly with the illumination power, which indicates that the charge carrier recombination rate is proportional to the carrier density. The measured electrical conductivity was found to be proportional to the square root of the dye concentration, confirming the validity of the proposed charge-injection model.
- Optical characterization of processed gallium arsenideSiochi, Ramon Alfredo Carvalho (Virginia Tech, 1990)Raman scattering and ultraviolet-visible reflectivity have been used to characterize the structural and electronic changes that occur in GaAs during ion implantation and subsequent annealing. In this work, the damaged structure is modelled as an amorphous GaAs matrix embedded with GaAs microcrystals. The longitudinal-optic (LO) Raman-line characteristics were monitored to determine the amorphous volume fraction, the average microcrystal diameter and, for the annealed samples, the carrier concentration. An oscillator analysis of the reflectivity spectra, along with the effective medium approximation, was carried out to determine the linewidths of the interband peaks and the amorphous volume fraction in the damage layer. To determine damage depth profiles, spectra were taken as a function of the amount of material removed via chemical etch. A new method of interpreting reflectivity spectra was developed to deal with the etchant-induced roughness. This roughness reduced the reflectivity by a constant factor in the region between 4.5 and 5 eV. The ratio between reflectivities at 4.55 and 4.75 eV was monitored to determine qualitatively the amount of damage. The annealing studies show that structural recovery occurs at a lower temperature than that for which electrical activation occurs. The depth profile of a sample annealed at 400°C reveals that nucleation takes place not only at the boundary between the damaged and undamaged layers (i.e., "epitaxial regrowth") but also at the microcrystal/amorphous interfaces within the damage layer. The oscillator analysis of the dielectric properties was further developed, and a connection was established between the Strengths, positions, and linewidths of the interband oscillators and the shift in position of the LO Raman line. The results indicate that the static dielectric constant is independent of microcrystal size. A comparison between (211) and (100) oriented Si-implanted GaAs was done as well, showing greater near surface damage and a shallower total damage layer for the (211) samples. Finally, a method for characterizing damage, based on the observed shifts of the two-phonon ("2LO") Raman peak as the incident photon energy is varied around the E₁ interband energy (2.9 eV), has been developed. The results suggest that the total mass of the electron-hole pair involved in the scattering process increases even for large (>400 Å) microcrystals. The 525°C annealed sample had little damage, and was studied with this technique.
- Optical Limiting and Degenerate Four-Wave Mixing in Novel FullerenesMarciu, Daniela (Virginia Tech, 1999-02-09)Two experimental methods, optical limiting and degenerate four-wave mixing, are employed to study the nonlinear optical properties of various novel fullerenes structures. Optical limiting refers to decreased transmittance of a material with increased incident light intensity. Detailed measurements of the wavelength-dependence of fullerene optical limiters have illustrated several key features of reverse saturable absorption. Most important among these is the requirement of weak but non-negligible ground state absorption. We have shown that the optical limiting performance of C₆₀ can be extended into the near infrared range by appropriate modifications of the structure such as higher cage fullerenes or derivatization of the basic C₆₀ molecule. The higher cage fullerene C₇₆ shows improved optical limiting behavior compared to C₆₀, for wavelengths higher than 650 nm, but becomes a weak limiter in the 800 nm range. C₈₄, even at high concentrations in [alpha]-chloronaphthalene, does not reach the good performance of C₆₀, but instead shows weak optical limiting in the 800 nm range. We also demonstrate that by attaching various groups to the C₆₀ molecule, we can extend the optical limiting performance in the near infrared regime. The C₆₀ derivatives studied, (C₆₀ cyclic ketone, C₆₀ secondary amine, C₆₀CHC₆H₄CO₂H, and C₆₀C₄H₄(CH₃)CH₂O₂C(CH₂)CO₂H), have a similar characteristic: the attached groups cause a symmetry-breaking of the C₆₀ sphere and, therefore, there are new allowed transitions that appear as absorption features up to 750 nm. The optical limiting measurements show that these materials, even for low input energies, have an exceptionally strong optical limiting response in the 640 to 750 nm spectral region. For wavelengths higher than 800 nm, however, they become transparent and no optical limiting is observed. Excited state absorption cross-sections obtained from analysis of the optical limiting data reveal that the C₆₀ derivatives have a maximum triplet-triplet absorption cross-section at 700 nm, which is shifted from the 750 nm value for the C₆₀ molecule. For the first time, optical limiting measurements are performed on five separate C₈₄ isomers. These intriguing results show that the optical limiting behavior is strongly dependent on the cage symmetry. It is also found that the most abundant isomer does not have the strongest optical limiting performance, but is in fact one of the weaker optical limiters of the isomers isolated so far. The endohedral metallofullerenes are a unique class of fullerene materials and consist of one or more metal atoms encapsulated inside the buckyball cage. An important characteristic of these materials is the charge-transfer from the dopant atoms to the fullerene cage, which has a high electron affinity. The charge-transfer is similar to the optical excitation in a material, but although the electrons are placed in the lowest unoccupied molecular orbital (LUMO), there are no holes produced in the highest occupied molecular orbital (HOMO). This is an important analogy, since it has been previously shown that optical excitation enhances the nonlinear optical properties of a material. The nonresonant degenerate four-wave mixing experiments performed on the endohedral metallofullerene Er₂@C₈₂, at 1064 nm, show that the third order nonlinear susceptibility value is increased by orders of magnitude relative to the empty cage fullerenes, thus, confirming the charge-transfer process from the encapsulated atoms to the fullerene cage. We obtain a value [gamma]xyyx(3)( [omega]; [omega], [omega], [omega])= 8.65 × 10⁻³² esu for the molecular second order hyperpolarizability, which is almost three orders of magnitude larger than the values reported in literature for an empty cage fullerene.
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