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Now showing 1 - 6 of 6
  • Publication
    Optical Studies of the Structure and Dynamics of Opaque Colloids
    (1993) Kaplan, Peter D
    Three fundamental experiments on multi-particle interactions in hard-sphere systems are presented in this thesis. In the first experiment, a mixture of two monodisperse colloids with different particle diameters, but the same particle composition, is studied using diffusing-wave spectroscopy. We find that both the particle structure and the hydrodynamic interactions between large and small balls are well modeled by existing theories. These theories, however, do not comment on the observed diffusion of particles in complex solutions over distances larger than a particle diameter. Incidental observations during this experiment led to the second experiment, the study of the phase diagram of binary hard-sphere mixtures of colloidal particles. In the process of mapping this phase diagram, driven entirely by entropy, a novel surface phase was discovered. This surface phase is explained geometrically in terms of the entropy of the small particles in the suspension. In the final experiment, the diffuse model of photon transport is extended to create a new technique called Diffuse Transmission Spectroscopy (DTS) for exploiting the wave vector dependent structure of colloidal suspensions. DTS uses the wavelength dependent transmission of a slab of colloid to probe interparticle structure. The diffuse transmission spectrum is shown to be useful for measuring particle diameter and differences in interparticle potential in dense, undiluted systems, as well as for testing the validity of structural models.
  • Publication
    Nonlinear Optical Spectroscopy of Solid/Solid Interfaces
    (1992-12-01) Yeganeh, Mohsen S
    Three-wave mixing (3WM) spectroscopy is an exciting and relatively unexplored probe of buried solid interfaces. It possesses long penetration depths characteristic of most optical methods and intrinsic interface specificity characteristic of second-order optical processes. In this thesis we present frequency domain measurements of the ZnSe/GaAs(OOl) heterojunction by second-harmonic (SH) and sum-frequency (SF) generation. Our experiments reveal an unusual three-wave mixing resonance that arises as a result of virtual transitions between an interfacial quantum well state and the ZnSe valence band. The interfacial quantum well was brought about by interdiffusion of Zn (Ga) into GaAs (ZnSe) during sample growth. The observation introduces a new class of nonlinear optical phenomena at interfaces that can provide useful information about band profiles, diffusion and defects along the boundary of two semiconductors. We have found that this interfacial SH resonance is sensitive to a variety of structural phenomena. In essence any process that modifies the band profile near the junction will affect the strength of the resonance. We have observed the variation of interface SH spectra with respect to lattice strain relaxation and to surface reconstruction of the buried GaAs. In addition, using a newly developed photomodulationSHG (PSHG) technique, we have exploited this sensitivity to determine the nature and relative density of interface charge traps as a function of substrate surface reconstruction. The PSHG method was also used to study free charge trapping mechanisms at ZnSe/GaAs(OOl) heterointerfaces. Our measurements determined that the interfacial trap-centers are mainly hole-traps with lifetime of 35 sec. In the course of carrying out these experiments we also observed interference in reflected second harmonic generation from two adjoined nonlinear slabs. A theory for the phenomena was presented and was used to understand our experimental results with ZnSe/GaAs(OOl) heterostructures. This interference phenomena was introduced as a new methodology to measure the second-order susceptibility of thin overlayer materials.
  • Publication
    Multi-band Oversampled Noise Shaping Analog to Digital Conversion
    (1996) Aziz, Pervez M
    Oversampled noise shaping analog to digital (A/D) converters, which are commonly known as delta-sigma (ΔΣ) converters, have the ability to convert relatively low bandwidth signals with very high resolution. Such converters achieve their high resolution by oversampling, as well as processing the signal and quantization noise with different transfer functions. The signal transfer function (STF) is typically a delay over the signal band while the noise transfer function (NTF) is designed to attenuate quantization noise in the signal band. A side effect of the NTF is an amplification of the noise outside the signal band. Thus, a digital filter subsequently attenuates the out-of-band quantization noise. The focus of this thesis is the investigation of ΔΣ architectures that increase the bandwidth where high resolution conversion can be achieved. It uses parallel architectures exploiting frequency or time slicing to meet this objective. Frequency slicing involves quantizing different portions of the signal frequency spectrum using several quantizers in parallel and then combining the results of the quantizers to form an overall result. Time slicing involves quantizing various groups of time domain signal samples with different quantizers in parallel and then combining the results of the quantizers to form an overall output. Several interesting observations can be made from this general perspective of frequency and time slicing. Although the representation of a signal are completely equivalent in time or frequency, the thesis shows that this is not the case for known frequency and time sliced A/D architectures. The performance of such systems under ideal conditions are compared for PCM as well as for ΔΣ A/D converters. A multi-band frequency sliced architecture for delta-sigma conversion is proposed and its performance is included in the above comparison. The architecture uses modulators which realize different NTFs for different portions of the signal band. Each band is converted in parallel. A bank of FIR filters attenuates the out of-band noise for each band and achieves perfect reconstruction of the signal component. A design procedure is provided for the design of the filter bank with reduced computational complexity. The use of complex NTFs in the multi-band ΔΣ architecture is also proposed. The peformance of real and complex NTFs is compared. Performance evaluations are made for ideal systems as well as systems suffering from circuit implementation imperfections such as finite opamp gain and mismatched capacitor ratios.
  • Publication
    Multiple Light Scattering from Isotropic and Anosotropic Turbid Media
    (1995) Kao, Ming Hsu
    The Brownian motion of particles in a fluid is a problem with spectacular historical roots [1, 2] and continues to provide us with intriguing new physics. Recently there has been resurging interest in the first steps of these motions [3, 4, 5, 6, 7, 8]. This is largely a result of new experimental probes, such as Diffusing-Wave Spectroscopy (DWS) [7, 9, 10, 11], that enable us to measure particle displacements down to 1Å or even smaller, and thereby offer the possibility to quantitatively test hydrodynamic theories [4, 6] of nondiffusive particle motion, especially in the first random walk step. In the first experiment we present here, we use Diffusing-Wave Interferometry (DWI) [12, 13] to resolve the mean square displacement, (∆r2(τ)), of particles in hard-sphere suspensions during the first twenty nanoseconds of their motion. These are the shortest time scales ever probed in the study of Brownian dynamics.
  • Publication
    Nonlinear Optical Spectroscopy of Gallium Arsenide Interfaces
    (1995) Qi, Jining
    The physics of buried solid/solid interfaces is interesting for both fundamental and technological reasons. In this thesis, second-order nonlinear optical spectroscopy was used to probe the electronic structure of GaAs based interfaces in the near infrared. Second-order nonlinear optical spectroscopy possesses intrinsic interface sensitivity and a long penetration depth. This intrinsic interface sensitivity enables us to suppress spectral contributions from the adjoining bulk media, resulting in more specific assignments of the spectral features. Our observations provide strong evidence for the existence of atomic displacement induced defect states at the buried metal:GaAs interface. In particular, in Au:GaAs n-type systems we have observed two resonance features at 0.715 eV and 0.731 eV for the Ga-rich interface and a single resonance feature at 0.715 eV for the As-rich interface. Similar single resonance features at 0.715 eV were observed in As:GaAs n-type samples, but were not present in oxide:GaAs and metal:GaAs p-type systems. After consideration of various three-step optical processes, we conclude that the transitions originate from the midgap states. These resonances differ substantially from what has been observed by other less interface sensitive technologies probing similar system. The origin of these difference is discussed. In the case of the native oxide GaAs system, we also studied depletion electric field induced effects on second-order nonlinear optical processes. Using a simple three-band model, we calculated the depletion field induced perturbation of the second-order nonlinear susceptibility of GaAs(OOl). The field induced modification of the second-order nonlinear susceptibility has been found to add destructively to the unperturbed existing bulk nonlinear second-order susceptibility χ(2)/xyz. This diminution depends on the square of the near surface depletion electric field. Through studies as a function of dopant type and concentration, we demonstrate that the bulk χ(2)/xyz is systematically reduced as a result of the depletion field, and that this diminution depends on the square of the near surface depletion field. This is consistent with our theoretical calculations. The sensitivity of the near surface bulk second-order susceptibility to the depletion electric field has also been demonstrated by a photomodulation second harmonic generation (PSHG) technique, by which we measured surface minority trap lifetimes and transverse diffusion of carriers confined near GaAs(OOl) surfaces.
  • Publication
    Improving the Persistence of First-Year Undergraduate Women in Computer Science
    (2008-07-31) Powell, Rita M.
    This paper describes a study of undergraduate women's retention in the first-year of the computer science major at the University of Pennsylvania for the purpose of identifying the underlying issues responsible for attrition. The subsequent steps taken by the faculty to improve women's retention is also discussed.