Nonlinear optical phenomena in both semiconductor microcavities and sodium vapor are investigated. Systems displaying atomic and excitonic coupling are studied in detail in order to unravel underlying physical principles. Possible applications for these systems are evaluated where appropriate. Normal-mode coupling (NMC) in a semiconductor microcavity is achieved when a narrow-linewidth quantum well exciton resonance is coincident with the cavity mode of a high-finesse microcavity. This interaction leads to a double-peaked spectrum in either transmission, reflection, or photoluminescence (PL). The high-quality microcavities studied here at liquid-Helium temperatures reveal intensity dependent behavior that has not previously been observed. Nonlinear saturation of the exciton leads to a spectral broadening of the excitonic absorption without a significant loss in oscillator strength. This results in a reduction of the two transmission peaks with almost no change in the splitting between the peaks. Such behavior is easily explained using phenomenological nonlinear dispersion theory. In this nonlinear regime, the luminescence from the microcavity shows a gradual transition from the nonperturbative regime of NMC to lasing with increasing excitation. The observed behavior is explained by density-dependent changes in both the transmission of the microcavity and the bare-exciton emission, rather than a boson-condensation of excitons which has been previously proposed. An intermediate-finesse microcavity also modifies the emission distribution from a bulk-semiconductor at room temperature. Angularly resolved emission spectra and quantum efficiency measurements show the PL is strongly dependent on the reflectivity of the microcavity. Unfortunately, the enhancement in the decay rate of excitons due to high-reflectivity mirrors seen previously by our group does not result in an increased quantum efficiency. High-gain optical-wavefront amplification in atomic sodium vapor is demonstrated via both the three-photon effect and stimulated Raman scattering (SRS). In both cases, single-pass weak-field gain of nearly 400 is achieved with only 800 mW of pump power near 589 nm. In the case of SRS, phase preservation of the amplified wavefront, which is necessary in adaptive imaging applications, is also demonstrated.

The coherent response of a semiconductor three-state system to one or two intense light pulses is investigated experimentally on a 100 fs time scale. Three experiments constitute this dissertation: observation of excitonic Rabi oscillations, measurement of two-exciton coupled Stark shifting, and an attempt to observe dark states. Basic concepts of time-resolved ultrafast semiconductor spectroscopy are explained, followed by an analysis of semiconductor two- and three-state systems. Pure two- and three-state dynamics are derived from first principles, followed by the development of the appropriate semiconductor Bloch equations (SBE). Two-color pump-probe, two-color pump-pump, and pump-pump-probe techniques are explained in the context of three-state semiconductor experiments. The experimental setup is explained in detail. Resonant two-color pump-probe measurements resulted in the first observation of multiple excitonic Rabi oscillations. The common conduction band shared by the light-hole and heavy-hole excitons of an InGaAs multiple quantum well allowed us to measure hh-exciton density (Rabi) oscillations by probing lh-exciton absorption. By studying the intensity dependence of the Rabi frequency, we showed the important role of many-body effects in renormalizing the dipole energy. The shared conduction band also causes the lh-exciton resonance to Stark shift when the hh exciton is Stark shifted. We measured a transient Stark shifting of both resonances due to virtual hh-exciton transitions. We observed that the ratio of the hh-exciton shift to lh-exciton shift was 2:1 at large pump-exciton detuning, as predicted from a simple three-state dressed exciton picture. For smaller detunings we saw an increase in the ratio and a redshift of the non-dipole coupled exciton state. Both of these observations are consistent with the most recent theories and experiments on excitonic Stark shifting. For strong near-resonant pumping of both lh- and hh-exciton transitions, an intervalence-band Raman-type coherence follows from the SBE that results in a transparent eigenstate (dark state) when both pumps are equally detuned from resonance. The existence of this coherence is well-known in atomic-optical systems, but has been elusive in semiconductors. Our inconclusive experimental result is presented along with an evaluation of experimental shortcomings. In brief, the expected change is absorption was too small to see.

Semiconductor microcavities have emerged to present abundant opportunities for both device applications and basic quantum optics studies. Here we investigate several aspects of the cw and ultrafast optical response of semiconductor quantum well microcavities. The interaction of a high-finesse semiconductor microcavity mode with a quantum well (QW) exciton leads to normal mode coupling (NMC), where a periodic energy exchange develops between exciton and photon states, appearing as a double peak in the cavity transmission spectrum and a beating in the time resolved signal. The nonlinear saturation of the excitonic NMC leads to a reduction of the modulation depth of the NMC oscillations and corresponding transmission peaks with little change in oscillation period or NMC splitting. This behavior arises from excitonic broadening due to carrier-carrier and polarization scattering without reduction of the oscillator strength. The nonlinear NMC microcavity luminescence exhibits three excitation regimes, from reversible normal mode coupling, through an intermediate double-peaked emission regime, to lasing. The nonlinear PL spectrum is governed by density-dependent changes in both the bare QW emission and in the microcavity transmission. The temporal evolution of the microcavity emission is analogous to the density-dependent behavior, and can be attributed to a time-dependent carrier density which results from a combination of carrier cooling and photon emission. A strong magnetic field applied perpendicular to the plane of a QW confines electrons and holes to Landau orbits in the QW plane, transforming the QW into a quantum dot (QD) whose radius shrinks with increasing magnetic field strength. This strong magnetic confinement enhances the normal mode coupling strength in the microcavity via an increase in exciton oscillator strength. The time-resolved stimulated emission of a QW microcavity which has been transformed to a QD laser by magnetic confinement reveals a fast relaxation which is uninhibited by the magnetic field, indicating the absence of a phonon bottleneck. As a novel manifestation of cavity-modified emission, we demonstrate synchronization of the stimulated emission of a microcavity laser to the electron spin precession in a magnetic field, achieved by modulating the optical gain for the circularly polarized emission via the Larmor precession. The oscillating laser emission is locked to the completely internal electron spin precession clock, and the GHz oscillation frequencies depend only on the magnetic field strength and the QW material parameters.

This dissertation presents an investigation of the charge-carrier dynamics in highly excited III-V semiconductor compounds. More precisely, after femtosecond excitation the photoluminescence of the GaN and related materials based Nichia NLPB 500 blue-light emitting diode (LED) is temporally and spectrally resolved using streak-camera techniques. Emission spectra are gathered both at 20K and room temperature. In addition, the emission of a pure GaN film grown by metal organic chemical vapor deposition is studied and compared to the more complicated structure of the diode. In either case, two spectrally distinct emission bands are found. Both samples show a large emission close to the band edge of the material. For the LED, amplified spontaneous emission is found in this band under very high excitation. The time resolved data shows simple, almost exponential, decays independent of pumping power or lattice temperature except for the amplified spontaneous emission in LED. The second emission band is impurity related and very different for the two samples. In the LED, impurities are deliberately doped into the active region of the device to provide luminescence centers whereas the pure GaN film is not intentionally doped. The LED emission shows a two component decay that changes its time constant with pump power which is well explained with in a three-level rate-equation model with saturable intermediate state. In the case of the GaN film an exponential decay independent of excitation density is observed up to a certain pump power where a fast initial component appears. Furthermore, spectral-hole burning experiments are performed on GaAs/AlGaAs multiple quantum wells. A new measurement technique for precise investigations of temporal gain evolution is given and subpicosecond gain in type II multiple quantum wells is demonstrated. Additionally, the experimental evidence for phonon sidebands of spectral holes in cool electron-hole plasmas is presented and theoretically investigated based on the Boltzmann equation coupled to the semiconductor Bloch equations. The carrier-dephasing time is studied as a function of plasma temperature, and indication for a strong dependence of carrier-carrier scattering rates on the temperature of the plasma is given. The findings are explained in a simple picture of blocked scattering channels for cold plasmas.

A gallium doped extrinsic silicon (Si:Ga) photoconductive detector was tested for sensitivity and quickness of response. The developmental goal for this detector material was high speed operation without compromised detectivity (D*). The high speed, p-type infrared photoconductor, with photoconductive gain less than unity, was tested at 10.5 μm to determine an experimental value for the detectivity-bandwidth product of D*f* = 3.8 x 10¹⁸ cm-Hz³/²/W. Subsequently a theoretical model taking into account the optical absorption profile and majority carrier transport processes within the detector was developed which agreed with the experimental data.

<p>Extensive ⁷⁵As nuclear magnetic resonance (NMR) studies were conducted on a variety of 122 iron-based superconductors. NMR frequency swept spectra and the spin-lattice relaxation rate (<i>T</i>₁^-1) were measured in CaFe₂As₂ as a function of temperature. The temperature dependence of the internal hyperfine field was extracted from the spectra, and <i>T</i>₁^-1 exhibits an anomalous peak attributed to the glassy freezing of domain walls associated with filamentary superconductivity. The field dependence of <i>T</i>₁^-1 and subsequent bulk resistivity and magnetization measurements also show signatures of filamentary superconductivity nucleated at antiphase domain walls. Systematic doping-dependent NMR studies were also carried out on Ni- and Co-doped BaFe₂As₂. In the Ni-doped variant, local magnetic inhomogeneities were observed via field swept NMR spectral analyses, and the doping dependence of the N&eacute;el temperature T_N was confirmed by fits to (<i>T</i>₁T)^-1(T). Spectral wipeout and stretched exponential relaxation behavior in the Co-doped variant reveal inhomogeneous behavior and the emergence of a cluster spin glass state. The NMR measurements bring into question the details of the phase transition from coexisting antiferromagnetism and superconductivity to pure superconductivity.

<p> To determine the mechanical and optical response of diamond crystals at high stresses and to evaluate anisotropy effects, single crystals (Type IIa) were shock compressed along the [100], [110], and [111] orientations to ~120 GPa peak elastic stresses. Particle velocity histories and shock velocities, measured using laser interferometry, were used to examine nonlinear elasticity, refractive indices, and Hugoniot elastic limits of shocked diamond. Time-resolved Raman spectroscopy was used to measure the shock compression induced frequency shifts of the triply degenerate 1332.5 cm^-1 Raman line. </p><p> Longitudinal stress-density states for elastic compression along different orientations were determined from the measured particle velocity histories and elastic shock wave velocities. The complete set of third-order elastic constants was determined from the stress-density states and published acoustic data. Several of these constants differed significantly from those calculated using theoretical models. </p><p> The refractive index of diamond shocked along [100] and [111] was determined from changes in the optical path length along the direction of uniaxial strain. Linear photoelasticity theory predicted the measured refractive index along [111]. In contrast, the refractive index along [100] was nonlinear. The refractive indices for [110] compression were not determined, but the data showed evidence of birefringence. </p><p> The splitting and frequency shifts of the diamond Raman line were measured for shock compression along [111] and were in good agreement with predictions from prior shock work. Frequency shifts were also measured along [100] and [110] up to ~60 GPa, extending previous measurements. The anharmonic force constants determined from all shock compression measurements agree with the previous shock compression determinations. </p><p> Hugoniot elastic limits for diamond shock compressed along different orientations were determined from the measured wave profiles. The elastic limits for the three orientations were highest at ~90 GPa peak elastic stress, but decreased at the higher peak elastic stress. Shear strengths were determined from the measured elastic limits: shocked diamond was strongest for compression along [110] and weakest for compression along [111]. The shear strength dependence on shock propagation direction was correlated with the stress magnitude normal to the slip plane, which appeared to inhibit the onset of inelastic deformation. </p>

This thesis presents numerical studies of phase behavior for both thermotropic and lyotropic liquid crystals. The nature of the orientational transition in the Lebwohl-Lasher model for the nematic-isotropic phase transition is found to be weak first-order with the stability limits of the nematic and isotropic phases being extremely close to the equilibrium transition temperature. It is also found that the director fluctuations in the nematic phase correspond to fractional Brownian motion whereas the fluctuations in the isotropic phase follow ordinary Brownian motion. The Pink model is extended to give an accurate description of the main phase transition in lipid bilayers by introducing hydrophobic mismatch interactions between acyl chains and also direct inter-monolayer attractive interactions. The lateral density fluctuations and the resulting dynamic bilayer heterogeneity are studied. Lipid-protein interactions are further included to describe the phase separation of lipid-protein mixtures, gramicidin channel formation and the effects of protein on the lipid bilayers. A model is also proposed for phase transitions involving hydrogen bonding between the polar heads in lipid bilayers.

<p> This thesis describes experimental studies of magnetization dynamics in both spin valves (SVs) and magnetic tunnel junctions (MTJs) subject to spin-polarized currents. A spin-polarized electrical current can transfer its angular momentum to a ferromagnet through a spin-transfer torque (STT), resulting in intriguing magnetization dynamics such as the reversal of the magnetization direction, precession and relaxation. </p><p> The ferromagnetic systems investigated were nanopillars, tens to hundreds of nanometers in cross section and a few nanometers in thickness, which were further integrated into SV or MTJ structures. </p><p> The magnetization switching and relaxation studies were performed on all-perpendicularly magnetized SVs. The switching probabilities were investigated for different pulse conditions at room temperature, where thermal fluctuations can play an important role. The pulse duration was varied over 10 orders of magnitude, from the fundamental timescales of magnetization precessional dynamics, 50 ps, to 1 s. Three switching regimes were found at different timescales. In the short-time regime, the switching probability was mainly determined by the angular momentum transfer between the current and the magnetization. In the long-time regime, the switching becomes thermal activation over an effective energy barrier modified by the STT. In the crossover regime, both spin-transfer and thermal effects are important. </p><p> The magnetization relaxation was studied by a two-pulse correlation method, where the relaxation time is measured by the interval between the two pulses. The thermal effects were shown to be important even at nanosecond time scales. </p><p> The switching and precession of magnetization were also studied in structures where a perpendicular spin polarizing layer is employed with an in-plane magnetized MTJ. When subject to pulses, the initial STT from the polarizer to the free layer is perpendicular to the free layer plane. For a large enough STT, this tilts the free layer magnetization out of the plane to create a large demagnetization field, typically at tens or hundreds of millitesla. This demagnetization field then becomes the dominant magnetic field acting on the free layer, leading to the precession of its magnetization. This magnetization precession was observed through real-time device resistance measurements, where precessions with hundreds of picoseconds are found from single current pulse stimuli. </p>

<p> Gyromagnetic phenomena have been of interest since the dawn of modern electromagnetic theory. While rotation-induced magnetization in electronic systems has been known for over 100 years, the phenomenon remains largely unexplored in nuclear degrees of freedom. This thesis explores the influence of external angular momentum on nuclear polarization, utilizing optical fields endowed with orbital angular momentum (OAM). To that end, I employ novel holographic methods to project light fields with programmable OAM content into fluid samples. To quantify the OAM in such fields, I introduce new techniques of holographic video microscopy to characterize optical forces. These optical manipulation and detection schemes are combined with standard NMR spectroscopy to reveal the effects of optical forces on the nuclear hyperpolatization of both absorbing and non-absorbing samples. These experiments provide evidence of a non-resonant coupling between the orbital angular momentum of light and nuclear spins.</p>