Electron spin-polarized helium 2$\sp3$S$\sb1$ metastable atoms are a valuable probe of spin-dependent phenomena. Helium metastable atoms can be polarized by illumination with circularly-polarized 1.083 $\mu$m 2$\sp3$S$\sb1 \leftrightarrow$ 2$\sp3$P$\sb{0,1,2}$ resonance radiation in a process known as optical pumping. Metastable atom polarizations obtained in this manner have been limited by the low intensity of optical pumping radiation available from helium discharge lamps, until now the only practical source of 1.083 $\mu$m light. However, recent advances in the field of solid-state near-infrared tuneable lasers have resulted in the development of several materials that exhibit laser action at 1.083 $\mu$m. The most promising of these is a neodymium-doped lanthanum magnesium hexaluminate, known as LNA. During the course of this work several LNA lasers have been designed and fabricated for specialized applications in helium optical pumping. Unstabilized single- and multi-mode lasers have been used to optically pump helium metastable atoms in a flowing helium afterglow, which serves as the basis for a highly competitive polarized electron source that provides electron polarizations approaching 90%. A frequency-stabilized single-mode LNA laser has been developed to optically pump a beam of metastable helium atoms for use in surface studies, and metastable polarizations resulting from its use exceed 96%. Although not designed specifically for other applications, the stabilized laser is ideally suited for application in laser cooling of helium metastable atoms and for helium spectroscopic applications. For example, laser absorption spectroscopy has been used to observe the decay of helium 2$\sp3$S$\sb1$ atoms as they are converted into He$\sb2$(a $\sp3\Sigma\sbsp{\rm u}{+}$) molecules in three-body reactions in a high-pressure helium afterglow, revealing interesting phenomena possibly related to quantum mechanical tunneling. The present work shows that LNA lasers have an important future in helium optical pumping.

The spectral, temporal, and spatial characteristics of plasma-induced self-phase and cross-phase modulation in rare gases have been investigated using a femtosecond KrF excimer laser focused to peak intensities of 10$\sp{14}$-10$\sp{15}$ W cm$\sp{-2}.$ The quiver energy of a free electron under these conditions is less than the ionization potential of all rare gases, ensuring that ionization occurs only by optical field-induced processes. Spectral blueshifts of up to 2 nm have been observed, and the blueshifted spectra show an oscillatory structure. The blueshifted spectra are shown to be the result of plasma-induced self-phase modulation and can be modeled by assuming tunneling ionization and one dimensional pulse propagation. The newly discovered oscillatory structure in the spectra is related to that observed in earlier experiments on self-phase modulation in optical fibers. To investigate the temporal behavior of the field ionization process, pump-probe experiments have been performed with a 100 fs probe pulse at 497 nm and a 400 fs pump pulse at 248 nm. Under conditions of weak ionization (Z $\ll$ 1), pump-probe experiments and theoretical calculations show that the ionization rate of the field ionized gas is maximum at the peak of the laser pulse and that the degree of ionization changes over a time equal to about half of the pump pulse width. By observing changes in the transmission of the probe pulse caused by plasma absorption, the electron temperature of a field ionized rare gas is determined to be on the order of 1 eV. The time varying electron density in the pump-probe experiments also causes plasma-induced cross-phase modulation, or spectral blueshifting of the probe pulse spectrum of up to 15 nm. The pump-probe experiments show that plasma defocusing causes the spectral blueshifting to be spatially dependent. Experimental results and a two dimensional pulse propagation model indicate that the most defocused beam components also show the maximum spectral blueshift. Plasma-induced cross-phase modulation has also been used to characterize the amplitude and phase of a 1 ps chirped pulse at 497 nm and the pulse width of a 400 fs pulse at 147 nm generated by four wave frequency mixing in xenon.

Focusing effects on phase front curvature are important in determining the band-width generated in the near vacuum ultraviolet by sum frequency mixing of ultrashort pulses. Noncollinear sum frequency mixing is utilized to produce 6.0 eV fourth harmonic radiation of a passively mode-locked titanium doped sapphire laser operating at a high repetition rate. The spectrum of the generated fourth harmonic radiation is broader than would be predicted if focusing is not taken into account. A method of calculating the bandwidth for sum frequency mixing of short pulses is developed and is experimentally supported.

This work focuses on the development of a new high power, ultrashort pulse dye laser system based on solid state Nd:YAG pump sources for a synchronously pumped dye laser and a series of dye amplifiers. This system will be applied to the study of picosecond and sub-picosecond laser-produced plasmas. For comparison, experiments are presented in which the propagation velocity of laser-produced plasmas pumped by 10 ns pulses is studied. Results of the propagation velocity as a function of ambient gas pressure and input pulse energy are given and are in good agreement with theoretical models. In order to produce ultrashort pulse pumped plasmas, it is necessary to focus the input beam to a small spot size without any temporal broadening of the pulse. A novel solution to the pulse front distortion problem using a combination of a lens and Fresnel type zone plate as a focusing device is presented.

We describe what are to our knowledge the first measurements of the propagation of coherent, single-cycle pulses of terahertz radiation in a scattering medium. We measure the propagation constants for pulses in a dense collection of spherical scatterers, and compare to the predictions of the quasi-crystalline approximation. Even though the fractional volume in our measurements exceeds the limit of validity of this model, we find that it still predicts certain features of the propagation with reasonable accuracy. By measuring the transmission as a function of the length L of the medium, we extract the scattering mean free path lambdasc (o) over a broad bandwidth. We observe variations in lambda sc ranging over nearly two orders of magnitude, and covering the entire thin sample regime from L/lambda sc << 1 to L/lambdasc ∼10.

In this dissertation work we have developed an ultracompact optically-detected terahertz resonance (ODTR) spectroscopy system, using quantum cascade lasers (QCLs). This system can be used for investigating various terahertz (THz) resonances in semiconductors. We use a THz QCL, of an appropriate frequency, operating in the close vicinity of the sample. Due to the small size of the QCLs, we can mount them on the sample holder of the magnet, which allows focusing of the THz beam onto the sample with minimal loss. This precludes the need of any external source of radiation. We use a single fiber to send the pump laser beam to the sample and collect the PL signal. The use of the fiber makes our system versatile enough to be used in a variety of situations, especially inside our magnet. We have used this setup to obtain some preliminary ODTR data on an InGaAs/AlGaAs multiple-quantum well sample.

This thesis describes the study of two-dimensional photonic crystals slabs with terahertz time domain spectroscopy. In our study we first demonstrate the realization of planar photonic components to manipulate terahertz waves, and then characterize photonic crystals using terahertz pulses. Photonic crystal slabs at the scale of micrometers are first designed and fabricated free of defects. Terahertz time domain spectrometer generates and detects the electric fields of single-cycle terahertz pulses. By putting photonic crystals into waveguide geometry, we successfully demonstrate planar photonic components such as transmission filters, reflection frequency-selective filters, defects modes as well as superprisms. In the characterization study of out-of-plane properties of photonic crystal slabs, we observe very strong dispersion at low frequencies, guided resonance modes at middle frequencies, and a group velocity anomaly at high frequencies. We employ Finite Element Method and Finite-Difference Time-Domain method to simulate the photonic crystals, and excellent agreement is achieved between simulation results and experimental results.

Metal nanoshells consist of a spherical dielectric core surrounded by a metallic shell. The fabrication of silver nanoshells is experimentally described and quantified using Mie scattering theory. These particles are used as surfaced enhanced Raman scattering (SERS) substrates at an excitation wavelength of 1.06 □m. This represents the first time silver particles in solution have been used as SERS substrates at this wavelength. Enhancement factors on the order of 1 x 106 are observed. It is also demonstrated that silver colloidal aggregates deposited on a large silica particle are sufficient to move the surface plasmon to the infrared for surface enhanced Raman spectroscopy. The measured enhancement factors were on the order of 4 x 105. The use of SnCl2 in functionalizing a silica surface as a precursor for metal nanoshell growth is explored along with theoretical calculations of the optical extinctions for nanoshells using different metals, such as copper, nickel, or platinum.

Several novel compact architectures of diode laser based absorption gas sensors have been developed, characterized and applied to real world applications. The motivation for this research has been the need to develop highly sensitive, selective and rapid response gas sensors that operate reliably in a non-laboratory environment. The gas sensors utilize rare earth doped fiber amplified near infrared diode lasers and are difference frequency mixed in periodically poled LiNbO3 to generate narrow linewidth muW to mW-level mid infrared light in the molecular fingerprint region from 3 to 5 mum. In particular, the spectroscopic performance of an automated widely tunable (3.3--4.4 mum) multi-species and a high-power single species (3.5 mum) gas detection sensor are discussed. Sensitive, selective and real-time detection of over 10 gas species including CH4, H2CO, CO2, CH 3OH, NO2, N2O; SO2, HCl, C6H 6, and H2O using extractive gas sampling in a multi-pass cell was demonstrated. The gas sensors were used for an evaluation of a trace contaminant catalyst system at TDA Research, Wheat Ridge, Colorado, and successfully applied to the detection of volcanic gases at Masaya volcano, Nicaragua.

In metallic nanostructures, the interaction of excited electrons with the nanostructure surface may result in electron relaxation dynamics that are significantly different than those predicted by electron-lattice coupling. These ultrafast electron dynamics were monitored by pump-probe measurements of the time-resolved change in transmission. Using femtosecond pulses from a cavity-dumped titanium-doped sapphire laser, two types of nanoparticles with a core-shell geometry were studied. Nanoshells are nanoparticles with a dielectric core surrounded by a continuous thin metal shell. For nanoshells, the plasmon resonance wavelength is tunable by changing the core and shell dimensions. For nanoshells with a gold sulfide core and a gold shell, two conditions were observed under which electron relaxation was different than predicted by electron-phonon coupling. First, electron relaxation occurred more rapidly for gold-gold sulfide nanoshells embedded in polymer films than for nanoshells dispersed in water, with lifetimes of 1.6 ps and 3 to 5 ps, respectively. Second, for nanoshells dispersed in water, the electron relaxation lifetime decreased with adsorption of p-aminobenzoic acid (to 1.7 ps) or aniline (to 1.9 ps) on the nanoshells. With adsorbed n-propylamine or p-mercaptobenzoic acid, electron relaxation transpired in 2.8 ps or 2.4 ps, respectively. Density functional theory calculations indicated that the molecules leading to the fastest electron relaxation possessed the largest induced dipole moments near a metal surface. Semicontinuous gold films grown around a silica nanoparticle core exhibited spectral and dynamical optical signatures of the percolation threshold. Compared to continuous shells, the electron dynamics in the semicontinuous shell layer were dramatically different as additional induced bleaching was observed in the first 500 fs. The observed dynamics are consistent with a rate equation model in which the electrons are initially excited in localized surface plasmons or "hot spots" and subsequently achieve an equilibrium with electrons throughout the film on a timescale faster than electron-phonon thermalization.