The 6.1-Angstrom (A) family of III-V semiconductors---InAs, GaSb, and AlSb and their alloys and heterostructures---has unique properties that may find application in next generation multi-functional semiconductor devices. In this project, we investigate intersubband transitions in 6.1-A quantum wells. The short-term objective is to explore high-photon-energy (or short-wavelength) intersubband transitions that are expected to occur in these systems due to their extremely large conduction-band offsets. The ultimate goal of this project is to construct a solid-state terahertz (THz) emitter based on 6.1-A quantum wells under intersubband pumping. We have established reliable and reproducible sample preparation methods for measuring intersubband transitions using Fourier-transform infrared spectroscopy. In particular, we observed intersubband transitions in 6.1-A quantum wells in a previously unexplored short-wavelength range. We systematically studied intersubband transition energies, intensities, and linewidths as functions of well width and temperature, and compared the results with calculations based on an 8-band k·p theory. Experimental methods, experimental results, and discussion will be presented in detail. Furthermore, transmission electron microscopy (TEM) was used to assess the quality of interfaces in 6.1-A quantum wells. A description is given for the sample preparation procedure together with some TEM pictures.

Pulsed laser irradiation in conjunction with cryogen spray cooling (CSC) can induce selective vascular injury to remove cutaneous hypervascular malformations such as port wine stains (PWS), hemangiomas, and facial veins. In this group of studies, we characterized the cryogen heat removal process and determined the effects of pulsed laser irradiation in conjunction with cryogen spray cooling of human skin. First, we employed an inverse heat conduction algorithm to measure the thermal boundary condition due to CSC in in vitro skin phantoms. Second, we developed a mathematical model of laser irradiation in conjunction with CSC in human skin. We determined tissue damage and temperature profiles due to varying combinations of laser pulse duration, radiant exposure, and CSC application times. Finally, we used ex vivo and in vivo human skin to determine the effects of high radiant exposures and CSC on epidermal and vascular injury. CSC induces a dynamic cooling effect, removing heat from the skin both during and following the spurt application time. Residual cryogen, deposited on the skin surface during the cryogen spurt, remains on the skin surface several times as long as the as cryogen spurt itself. The heat removal rate during the cryogen spurt is greatest; however, the total energy removed following the cryogen spurt is also substantial (approximately half as much as during the spurt application time). CSC was effective in protecting the human skin epidermis in light to moderately pigmented skin. Mathematical modeling, ex vivo, and in vivo studies showed that the epidermal damage threshold could be increased by a factor of approximately two. Increased radiant exposures increased the risk of non-selective vascular injury observed in histology as injury to the epidermis and perivascular collagen; however, proper choice of cryogen cooling durations resulted in the elimination of epidermal injury as well as perivascular tissue injury. In addition, higher radiant exposures induced vascular injury deeper within the dermis. Although cooling alone was not sufficient in protecting the epidermis in dark skin patients, the mathematical model suggests that selecting the proper pulse duration in conjunction with CSC may eliminate epidermal injury in darker skin patients.

This thesis describes a Magneto-Optical Kerr Effect (MOKE) and Magnetic Circular Dichroism (MCD) study of ferromagnetism in III-V magnetic semiconductor heterostructures. This work was initiated due to the much recent interest in spin-related phenomena in semiconductors. The discovery of carrier/light-induced ferromagnetism in InMnAs-based heterostructures has opened up new possibilities for spin-based multifunctional devices that will integrate photonics, magnetics and microelectronics. However, an experimental demonstration of direct ultrafast optical manipulation of spins/magnetic order in this system has not been reported, although it is considered to be a crucial step towards pico- or femto-second operation of information encoded in the spin degree of freedom. In addition, the origin of ferromagnetism is still a subject of debate. To address these issues, we have developed an experimental setup for performing continuous-wave (CW) and time-resolved light-induced MOKE/MCD experiments. Our data clearly demonstrate that magnetic properties, e.g., coercivity and remanent magnetization, can be optically controlled in an ultrafast manner (∼ ps). The experimental results of CW and two-color time-resolved MOKE/MCD measurements are presented. Finally, the mechanism of the ultrafast MOKE response is discussed in terms of transient photo/carrier-induced modifications of exchange interaction and domain wall energy.

This thesis describes magneto-optical processes in ferromagnetic (III,Mn)V semiconductors and their heterostructures induced by femtosecond laser pulses. Two-color ultrafast magneto-optical spectroscopy has been developed, which allows us to observe a rich array of dynamical phenomena that are absent in traditional nonmagnetic semiconductors or metallic/insulating magnets. We isolate several distinct temporal regimes in spin dynamics, interpreting the fast (< 1 ps) dynamics as spin heating through sp-d exchange interaction between photo-carriers and Mn ions while the ∼100 ps component as a manifestation of spin-lattice relaxation. Charge carrier and phonon dynamics were also carefully studied, showing an ultrashort charge lifetime of photoinjected electrons (∼ 2 ps) and propagating coherent acoustic phonon wavepackets with strongly probe energy dependent oscillation period, amplitude and damping.

Propagation of single-cycle terahertz (THz) pulses through a random medium leads to dramatic amplitude and phase variations of the electric field because of multiple scattering. We present the first set of experiments that investigate the propagation of THz pulses through scattering media. The scattering of short pulses is a relevant subject to many communities in science and engineering, because the properties of multiply scattered or diffuse waves provide insights into the characteristics of the random medium. For example, the depolarization of diffuse waves has been used to form images of objects embedded in inhomogeneous media. Most of the previous scattering experiments have used narrowband optical radiation where measurements are limited to time averaged intensities or autocorrelation quantities, which contain no phase information of the pulses. In the experiments presented here, a terahertz time-domain spectrometer (THz-TDS) is used. A THz-TDS propagates single-cycle sub-picosecond pulses with bandwidths of over 1 THz into free space. The THz-TDS is a unique tool to study such phenomena, because it provides access to both the intensity and phase of those pulses through direct measurement of the temporal electric field. Because of the broad bandwidth and linear phase of the pulses, it is possible to simultaneously study Rayleigh scattering and the short wavelength limit in a single measurement. We study the diffusion of broadband single-cycle THz pulses by propagating the pulses through a highly scattering medium. Using the THz-TDS, time-domain measurements provide information on the statistics of both the amplitude and phase of the diffusive waves. We develop a theoretical description, suitable for broadband radiation, which accurately describes the experimental results. We measure the time evolution of the degree of polarization, and directly correlate it with the single-scattering regime in the time domain. Measurements of the evolution of the temporal phase of the radiation demonstrate that the average spectral content depends on the state of polarization. In the case of broadband radiation, this effect distinguishes photons that have been scattered only a few times from those that are propagating diffusively.

The principal objective of this work is the investigation and development of wavefront engineering or optical resolution enhancement techniques using deep-UV lithography. Current technologies used in optical microlithography allow the manufacture of microelectronic circuit features that are larger or about equal to about 2/3 of the wavelength of the exposure tools. In order to extend the capabilities of optical lithography, new optical resolution enhancement techniques are needed and the existing methods have to be analyzed and improved. The study of limitations of tolerances and the overall performance of diverse wavefront engineering techniques is critically important to the microelectronics industry. Techniques such as phase-shift masks, optical proximity correction, and off-axis illumination are being studied in order to obtain a clear understanding of their impact on critical dimension uniformity, resolution and depth of focus. Proximity effects, feature offset and pattern displacement issues, as well as sensitivity to lens aberrations and coherence of the light source are being investigated. Tolerances of these techniques and their performance when used with 248 nm and 193 nm deep-UV lithography are also of concern. In addition, a new resolution enhancement technique is being developed. This technique is based on coherent image superposition which is expected to improve both the resolution and depth of focus of integrated circuit features.

This doctoral thesis documents my research on novel devices and systems for terahertz (THz) spectroscopy and imaging. The research is particularly focused on the manipulation of THz radiation, including subwavelength concentration and low-loss wave guiding. One of the major obstacles for THz imaging is the poor spatial resolution due to the diffraction of the long-wavelength light source. To break this restriction, we build a THz near-field microscopy system by combining apertureless near-field scanning optical microscopy (ANSOM) with terahertz time-domain spectroscopy (THz-TDS). The experimental result indicates a sub-wavelength spatial resolution of about 10 micron. Abnormal frequency response of the ANSOM probe tip is observed, and a dipole antenna model is developed to explain the bandwidth reduction of the detected THz pulses. We also observe and characterize the THz wave propagation on the near-field probe in ANSOM. These studies not only demonstrate the feasibility of ANSOM in the THz frequency range, but also provide fundamental insights into the near-field microscopy in general, such as the broadband compatibility, the propagation effects and the antenna effects. Motivated by our study of the propagation effects in THz ANSOM, we characterize the guided mode of THz pulses on a bare metal wire by directly measuring the spatial profile of electric field of the mode, and find that the wire structure can be used to guide THz waves with outstanding performance. This new broadband THz waveguide exhibits very small dispersion, extremely low attenuation and remarkable structural simplicity. These features make it especially suitable for use in THz sensing and imaging systems. The first THz endoscope is demonstrated based on metal wire waveguides. To improve the input coupling efficiency of such waveguides, we develop a photoconductive antenna with radial symmetry which can generate radially polarized THz radiation matching the waveguide mode. Through THz-TDS measurements and theoretical calculations, we study the dispersion relation of the surface waves on metal wires, which indicates the increasing importance of skin effects for surface waves in the THz frequency range.

<p> Refractive development has been shown to be influenced by optical defocus in the eye and the interpretation of this signal appears to be localized in the retina. Optical defocus is not uniform across the retina and has been suggested as a potential cause of myopia development. Specifically hyperopic focus, i.e. focusing light behind the retina, may signal the eye to elongate, causing myopia. This non-uniform hyperopic signal appears to be due to the retinal shape. Ultimately, these signals are detected by the retina in an as yet undetermined manner. The purpose of this thesis is to examine the retinal profile using a novel method developed at Indiana University and then to examine retinal structural changes across the retina associated with myopia. </p><p> Myopes exhibited more prolate retinas than hyperopes/emmetropes using the SD OCT. Using the SD OCT, this profile difference was detectable starting at 5 degrees from the fovea, which was closer than previously reported in the literature. These results agreed significantly with results found from peripheral refraction and peripheral axial length at 10 degrees. Overall, the total retina was thinner for myopes than hyperopes/emmetropes. It was also statistically significantly thinner for the Outer Nuclear Layer (ONL), Inner Nuclear Layer (INL) and Outer Plexiform Layer (OPL) but not for other retinal layers such as the Ganglion Layer. Thinning generally occurred outside of 5 degrees. </p><p> The SD OCT method provided a nearly 10 fold increase in sensitivity which allowed for detection of profile changes closer to the fovea. The location of the retinal changes may be interesting as the layers that showed significant differences in thickness are also layers that contain cells believed to be associated with refractive development (amacrine, bipolar, and photoreceptor cells.) The reason for the retinal changes cannot be determined with this study, but possible theories include stretch due to axial elongation, neural remodeling due to blur, and/or direct influence on refractive development due to neural cell densities.</p>

<p>GaN-based LEDs have generally made use of ITO transparent contacts as current-spreading layers for uniform current injection. However, the high raw material and processing costs of ITO layers have generated interest in potentially cheaper alternatives. In this work, zinc oxide transparent layers were fabricated by a low-cost, low-temperature aqueous epitaxial growth method at 90&deg;C for use as transparent contacts to GaN LEDs on <i>c</i>-plane sapphire, and on semipolar bulk GaN substrates. Low-voltage operation was achieved for <i>c</i>-plane devices, with voltages below 3.8V for 1mm² broad-area LEDs at a current density of 30A/cm². Blue-green LEDs on 202&macr;1&macr;-plane GaN also showed low voltage operation below 3.5V at 30A/cm². Ohmic contact resistivity of 1:8 &times; 10^&minus;2&Omega;cm² was measured for films on (202&macr;1) <i>p</i>-GaN templates. Ga-doped films had electrical conductivities as high as 660S/cm after annealing at 300&deg;C. Optical characterization revealed optical absorption coefficients in the 50&ndash;200cm^&minus;1 range for visible light, allowing thick films with sheet resistances below 10&Omega;/&square; to be grown while minimizing absorption of the emitted light. Accurate and reproducible etch-free patterning of the ZnO films was achieved using templated growths with SiO_x hard masks. A roughening method is described which was found to increase peak LED efficiencies by 13% on <i>c</i>-plane patterned sapphire (PSS) substrates. In addition, ZnO films were successfully employed as laser-cladding layers for blue (202&macr;1) lasers, with a threshold current density of 8.8kA/cm². </p>

Photonic crystals (PCs) and binary supergratings (BSGs) with large refractive index steps are promising structures for designing new compact optical devices. This thesis presents an inverse design tool in these two important areas of photonics. The tool consists of an optimization module and a simulation engine. Due to the binary nature of PCs and BSGs, Boolean particle swarm optimization (Boolean PSO), a recently proposed binary stochastic optimization algorithm, is used in the optimization module. The simulation engine, on the other hand, is chosen according to the structure to be modeled. The proposed inverse design tool has been used to design a very low F-number photonic crystal lens and compact BSG filters for applications such as wavelength-division multiplexing, tunable lasers and intrachip optical networks. The inverse design tool allows designing optical filters with almost arbitrary wavelength filtering, in addition the proposed filters are more compact than previous demonstrations of BSG. Furthermore, it is found that Boolean PSO outperforms Genetic Algorithm (GA) as an optimization technique for use in the inverse design tool developed in this thesis.