Ultrafast dynamics in ZnO thin films irradiated by femtosecond lasers

The damage morphologies, threshold fluences in ZnO films were studied with femtosecond laser pulses. Time-resolved reflectivity and transmissivity have been measured by the pump-probe technique at different pump fluences and wavelengths. The results indicate that two-phase transition is the dominant damage mechanism, which is similar to that in narrow band gap semiconductors. The estimated energy loss rate of conduction electrons is 1.5 eV/ps. (c) 2005 Elsevier Ltd. All rights reserved.

Complex behavior and perception in Drosophila emerges from iterative feedback-regulated reflexes

For a hungry fruit fly, locating and landing on a fermenting fruit where it can feed, find mates, and lay eggs, is an essential and difficult task requiring the integration of both olfactory and visual cues. Understanding how flies accomplish this will help provide a comprehensive ethological context for the expanding knowledge of their neural circuits involved in processing olfaction and vision, as well as inspire novel engineering solutions for control and estimation in computationally limited robotic applications. In this thesis, I use novel high throughput methods to develop a detailed overview of how flies track odor plumes, land, and regulate flight speed. Finally, I provide an example of how these insights can be applied to robotic applications to simplify complicated estimation problems. To localize an odor source, flies exhibit three iterative, reflex-driven behaviors. Upon encountering an attractive plume, flies increase their flight speed and turn upwind using visual cues. After losing the plume, flies begin zigzagging crosswind, again using visual cues to control their heading. After sensing an attractive odor, flies become more attracted to small visual features, which increases their chances of finding the plume source. Their changes in heading are largely controlled by open-loop maneuvers called saccades, which they direct towards and away from visual features. If a fly decides to land on an object, it begins to decelerate so as to maintain a stereotypical ratio of expansion to retinal size. Once they reach a stereotypical distance from the target, flies extend their legs in preparation for touchdown. Although it is unclear what cues they use to trigger this behavior, previous studies have indicated that it is likely under visual control. In Chapter 3, I use a nonlinear control theoretic analysis and robotic testbed to propose a novel and putative mechanism for how a fly might visually estimate distance by actively decelerating according to a visual control law. Throughout these behaviors, a common theme is the visual control of flight speed. Using genetic tools I show that the neuromodulator octopamine plays an important role in regulating flight speed, and propose a neural circuit for how this controller might be implemented in the flies brain. Two general biological and engineering principles are evident across my experiments: (1) complex behaviors, such as foraging, can emerge from the interactions of simple independent sensory-motor modules; (2) flies control their behavior in such a way that simplifies complex estimation problems.

Application of microwave diagnostics to copper chloride and carbon dioxide lasers

Part A

A problem restricting the development of the CuCl laser has been the decrease in output power with increases of tube temperature above 400°C. At that temperature the CuCl vapor pressure is about .1 torr. This is a small fraction of the buffer gas pressure (He at 10 torr).

The aim of the project was to measure the peak radiation temperature (assumed related to the mean energy of electrons) in the laser discharge as a function of the tube temperature. A 24 gHz gated microwave radiometer was used.

It was found that at the tube temperatures at which the output power began to deteriorate, the electron radiation temperature showed a sharp increase (compared with radiation temperature in pure buffer).

Using the above result, we have postulated that this sudden increase is a result of Penning ionization of the Cu atoms. As a consequence of this process the number of Cu atoms available for lasing decrease.

PART B

The aim of the project was to study the dissociation of CO₂ in the glow discharge of flowing CO₂ lasers.

A TM₀₁₁ microwave (3 gHz) cavity was used to measure the radially averaged electron density n_e and the electron-neutral collision frequency in the laser discharge. An estimate of the electric field is made from these two measurements. A gas chromatograph was used to measure the chemical composition of the gases after going through the discharge. This instrument was checked against a mass spectrometer for accuracy and sensitivity.

Several typical laser mixtures were .used: CO₂-N₂-He (1,3,16), (1,3,0), (1,0,16), (1,2,10), (1,2,0), (1,0,10), (2,3,15), (2,3,0), (2,0,15), (1,3,16)+ H₂O and pure CO₂. Results show that for the conditions studied the dissociation as a function of the electron density is uniquely determined by the STP partial flow rate of CO₂, regardless of the amount of N₂ and/or He present. The presence of water vapor in the discharge decreased the degree of dissociation.

A simple theoretical model was developed using thermodynamic equilibrium. The electrons were replaced in the calculations by a distributed heat source.

The results are analyzed with a simple kinetic model.

Effect of cerium oxide on the precipitation of silver nanoparticles in femtosecond laser irradiated silicate glass

We investigated the effect of cerium oxide on the precipitation of Ag nanoparticles in silicate glass via a femtosecond laser irradiation and successive annealing. Absorption spectra show that Ce3+ ions may absorb part of the laser energy via multiphoton absorption and release free electrons, resulting in an increase of the concentration of Ag atoms and a decrease of the concentration of hole-trapped color centers, which influence precipitation of the Ag nanoparticles. In addition, we found that the formed Ag-0 may reduce Ce4+ ions to Ce3+ ions during the annealing process, which inhibits the growth of the Ag nanoparticles.

Optimal feedback control of two-photon fluorescence based on genetic algorithm

An optimal feedback control of two-photon fluorescence in the ethanol solution of 4-dicyanomethylene-2-methyl-6-p-dimethyl-amiiiostryryl-4H-pyran (DCM) using pulse-shaping technique based on genetic algorithm is demonstrated experimentally. The two-photon fluorescence of the DCM ethanol solution is enhanced in intensity of about 23%. The second harmonic generation frequency-resolved optical gating (SHG-FROG) trace indicates that the effective population transfer arises from the positively chirped pulse. The experimental results appear the potential applications of coherent control to the complicated molecular system.

Optimal feedback control of two-photon fluorescence in Coumarin 515 based on genetic algorithm

An optimal feedback control of two-photon fluorescence in the Coumarin 515 ethanol solution excited by shaping femtosecond laser pulses based on genetic algorithm is demonstrated experimentally. The two-photon fluorescence intensity can be enhanced by similar to 20%. Second harmonic generation frequency-resolved optical gating traces indicate that the optimal laser pulses are positive chirp, which are in favor of the effective population transfer of two-photon transitions. The dependence of the two-photon fluorescence signal on the laser pulse chirp is investigated to validate the theoretical model for the effective population transfer of two-photon transitions. The experimental results appear the potential applications in nonlinear spectroscopy and molecular physics. (c) 2005 Elsevier B.V. All rights reserved.

Investigations of charge-carrier dynamics at semiconductor/liquid interfaces

Theoretical and experimental investigations of charge-carrier dynamics at semiconductor/liquid interfaces, specifically with respect to interfacial electron transfer and surface recombination, are presented.

Fermi's golden rule has been used to formulate rate expressions for charge transfer of delocalized carriers in a nondegenerately doped semiconducting electrode to localized, outer-sphere redox acceptors in an electrolyte phase. The treatment allows comparison between charge-transfer kinetic data at metallic, semimetallic, and semiconducting electrodes in terms of parameters such as the electronic coupling to the electrode, the attenuation of coupling with distance into the electrolyte, and the reorganization energy of the charge-transfer event. Within this framework, rate constant values expected at representative semiconducting electrodes have been determined from experimental data for charge transfer at metallic electrodes. The maximum rate constant (i.e., at optimal exoergicity) for outer-sphere processes at semiconducting electrodes is computed to be in the range 10^-17-10^-16 cm⁴ s^-1, which is in excellent agreement with prior theoretical models and experimental results for charge-transfer kinetics at semiconductor/liquid interfaces.

Double-layer corrections have been evaluated for semiconductor electrodes in both depletion and accumulation conditions. In conjuction with the Gouy-Chapman-Stern model, a finite difference approach has been used to calculate potential drops at a representative solid/liquid interface. Under all conditions that were simulated, the correction to the driving force used to evaluate the interfacial rate constant was determined to be less than 2% of the uncorrected interfacial rate constant.

Photoconductivity decay lifetimes have been obtained for Si(111) in contact with solutions of CH₃OH or tetrahydrofuran containing one-electron oxidants. Silicon surfaces in contact with electrolyte solutions having Nernstian redox potentials > 0 V vs. SCE exhibited low effective surface recombination velocities regardless of the different surface chemistries. The formation of an inversion layer, and not a reduced density of electrical trap sites on the surface, is shown to be responsible for the long charge-carrier lifetimes observed for these systems. In addition, a method for preparing an air-stable, low surface recombination velocity Si surface through a two-step, chlorination/alkylation reaction is described.

Wave interactions in periodic structures and periodic dielectric waveguides

This work is concerned with a general analysis of wave interactions in periodic structures and particularly periodic thin film dielectric waveguides.

The electromagnetic wave propagation in an asymmetric dielectric waveguide with a periodically perturbed surface is analyzed in terms of a Floquet mode solution. First order approximate analytical expressions for the space harmonics are obtained. The solution is used to analyze various applications: (1) phase matched second harmonic generation in periodically perturbed optical waveguides; (2) grating couplers and thin film filters; (3) Bragg reflection devices; (4) the calculation of the traveling wave interaction impedance for solid state and vacuum tube optical traveling wave amplifiers which utilize periodic dielectric waveguides. Some of these applications are of interest in the field of integrated optics.

A special emphasis is put on the analysis of traveling wave interaction between electrons and electromagnetic waves in various operation regimes. Interactions with a finite temperature electron beam at the collision-dominated, collisionless, and quantum regimes are analyzed in detail assuming a one-dimensional model and longitudinal coupling.

The analysis is used to examine the possibility of solid state traveling wave devices (amplifiers, modulators), and some monolithic structures of these devices are suggested, designed to operate at the submillimeter-far infrared frequency regime. The estimates of attainable traveling wave interaction gain are quite low (on the order of a few inverse centimeters). However, the possibility of attaining net gain with different materials, structures and operation condition is not ruled out.

The developed model is used to discuss the possibility and the theoretical limitations of high frequency (optical) operation of vacuum electron beam tube; and the relation to other electron-electromagnetic wave interaction effects (Smith-Purcell and Cerenkov radiation and the free electron laser) are pointed out. Finally, the case where the periodic structure is the natural crystal lattice is briefly discussed. The longitudinal component of optical space harmonics in the crystal is calculated and found to be of the order of magnitude of the macroscopic wave, and some comments are made on the possibility of coherent bremsstrahlung and distributed feedback lasers in single crystals.

Investigations of the conductor-semiconductor interface

I. Schottky barriers produced by polymeric sulfur nitride, (SN)_x, on nine common III-V and II-VI compound semiconductors are compared to barriers formed by Au. The conductor (SN)_x produces significantly higher barriers to n-type semiconductors and lower barriers to p-type semiconductors than Au, the most electronegative elemental metal. The barrier height improvement, defined as ɸ(SN)_x - ɸ(Au), is smaller on covalent semiconductors than on ionic semiconductors; (SN)x barriers follow the ionic-covalent transition. Details of (SN)_x film deposition, samples preparation, and barrier height measurements are described.

II. The rate of dissolution of amorphous Si into solid Al is measured. The rate of movement of the amorphous Si/Al interface is found to be much faster than predicted by a simple model of the transport of Si through Al. This result is related to defects in the growth of epitaxial Si using the solid phase epitaxy process.

Stimulated Raman adiabatic passage from atomic to molecular Bose-Einstein condensates: Feedback laser-detuning control and suppression of dynamical instability

We present a feedback control scheme that designs time-dependent laser-detuning frequency to suppress possible dynamical instability in coupled free-quasibound-bound atom-molecule condensate systems. The proposed adaptive frequency chirp with feedback is shown to be highly robust and very efficient in the passage from an atomic to a stable molecular Bose-Einstein condensate.

The origin of the transverse relaxation time in optically excited semiconductor quantum wells

The origin of the transverse relaxation time in optically excited semiconductor quantum wells is investigated based on the vector property of the interband transition matrix elements. The dephasing rate due to carrier-carrier (CC) scattering is found to be equal to half of the common momentum relaxation rate. The analytical expression of the polarization dephasing due to CC scattering in two-dimension is established and the dependence of the dephasing rate Gamma(cc) on the carrier density N is determined to be Gamma(cc) = constant (.) N-1/2, which is used to explain the experimental results and provides a promising physical picture. (C) 2004 Elsevier B.V. All rights reserved.