This work examines the use of higher order multiphoton resonances in higher harmonic generation together with judicious exploitation of coherent interaction properties to achieve efficient harmonic generation. A detailed experimental study on third harmonic generation in two photon resonant coherent interaction and a theoretical study on four photon resonant coherent interaction have been conducted. Two photon resonant coheren propagation in lithium vapor (2S-4S and 2S-3D interaction) has been studied in detail as a function of phase and delay of the interacting pulse sequence. Under coherent lossless propagation of 90 phase shifted pulse pair, third harmonic generation is enhanced. A maximum energy conversion efficiency of 1% was measured experimentally. This experiment shows that phase correlated pulse sequence can be used to control multiphoton coherent resonant effects. A larger two photon resonant enhancement does not result in more efficient harmonic generation, in agreement with the theoretical prediction. An accurate (to at least 0.5 A°) measurement of intensity dependent Stark shift has been done with the newly developed "interferometric wavemeter." Stark shifts as big as several pulse bandwidths (of picosecond pulses) result in a poor tuning of multiphoton resonance and become a limiting factor of resonant harmonic generation. A complete theory has been …

The picosecond optical response of five thin germanium samples was measured following intense optical excitation using two variations of the excitation and probe technique. Seven-picosecond laser pulses of wavelength 1.054 um were used to measure the optical transmission of the samples for a variety of probe delays, excitation fluences, and sample temperatures. These parametric experiments were performed in an effort to determine if carrier cooling, carrier diffusion, or carrier recombination dominates the carrier dynamics immediately following excitation. The studies of a 5.7 um thick sample indicated that Auger recombination does not dominate the carrier dynamics, but that the carriers most likely cool immediately to within a few optical phonons of the lattice temperature. Lattice heating may also occur depending on excitation level. Neither cooling nor diffusion was ruled out as a major contributor to the transient optical response. A numerical analysis indicated that, although diffusion may be minimized in the thinner samples, the importance of surface recombination increases as the sample thickness decreases. The lattice temperature dependence of the optical transmission was found not to be in disagreement with the known temperature dependence of the low-density diffusion coefficient. Finally, new structure was observed in the data which is consistent with …

Absolute K-shell x-ray cross sections and Auger-electron cross sections are measured for carbon for 0.6 to 2.0 MeV proton incident on CH₄, n-C₄H₁₀ (n-Butane), i-C₄H₁₀ (isobutane), C₆H₆ (Benzene), C₂H₂ (Acetylene), CO and CO₂. Carbon K-shell fluorescence yields are calculated from the measurements of x-ray and Auger-electron cross sections. X-ray cross sections are measured using a variable geometry end window proportional counter. An alternate method is described for the measurement of the transmission of the proportional counter window. Auger electrons are detected by using a constant transmission energy Π/4 parallel pi ate electrostatic analyzer. Absolute carbon K-shell x-ray cross sections for CH₄ are compared to the known results of Khan et al. (1965). Auger-electron cross sections for proton impact on CH₄ are compared to the known experimental values of RΦdbro et al. (1979), and to the theoretical predictions of the first Born and ECPSSR. The data is in good agreement with both the first Born and ECPSSR, and within our experimental uncertainties with the measurements of RΦdbro et al. The x-ray cross sections, Auger-electron cross sections and fluorescence yields are plotted as a function of the Pauling charge, and show significant variations. These changes in the x-ray cross sections are compared …

The problem of the quadrupole interaction occurring in a vibrating-rotating C₃v symmetric top molecule has been studied in detail. The quadrupole interaction has been treated as another perturbation term to a general frequency expression accounting for the vibrating-rotating interaction of the molecule so that a complete frequency formula is obtained for both interactions, and from which hyperfine spectral components are predicted and measured. The hyperfine transitions in the ground, and v₈=1 and v₈=2 excited vibrational states of the ¹³C isotopes of methyl cyanide have been investigated in the frequency range 17-72 GHz, primarily in the low J transitions (0≤J≤3). The study of the ground state of isotope i3CH3i3CN, and the v₈=1, v₈=2 excited vibrational states for all the isotopes have been conducted here for the first time. A substantial perturbation has been discovered and discussed at the ΔJ=3→4 transitions within the Kl=1 sets in the v₈=1 mode for isotopes ¹³CH₃CN and CH₃¹³CN. A total of 716 hyperfine transitions have been assigned from measurements, only 7 of which have been measured previously. A total of 84 molecular constants have been reported; 70 of these constants are derived for the first time from microwave data.

The energy states of C₃ᵥ symmetric top polyatomic molecules were studied. Both classical and quantum mechanical methods have been used to introduce the energy states of polyatomic molecules. Also, it is shown that the vibration-rotation spectra of polyatomic molecules in the ground and excited vibrational states can be predicted by group theory. A comprehensive model for predicting rotational frequency components in various v₁₀ vibrational levels of propyne was developed by using perturbation theory and those results were compared with other formulas for C₃ᵥ symmetric top molecules. The v₁₀=1,2,3 and ground rotational spectra of propyne in the frequency range 17-70 GHz have been reassigned by using the derived comprehensive model. The v₁₀=3 and v₁₀=4 rotational spectra of propyne have been investigated in the 70 GHz, and 17 to 52 GHz regions, respectively, and these spectral components assigned using the comprehensive model. Molecular constants for these vibrationally excited states have been determined from more than 100 observed rotational transitions. From these experimentally observed components and a model based upon first principles for C₃ᵥ symmetry molecules, rotational constants have been expressed in a form which enables one to predict rotational components for vibrational levels for propyne up to v₁₀=5. This comprehensive model also …

The problem of a vibrating-rotating polyatomic molecule is treated, with emphasis given to the case of molecules with C_3v symmetry. It is shown that several of the gross features of the rotational spectra of polyatomic molecules in excited vibrational states can be predicted by group theoretical considerations. Expressions for the rotational transition frequencies of molecules of C_3v symmetry in the ground vibrational state, singly excited degenerate vibrational states, and doubly excited degenerate vibrational states are given. The derivation of these expressions by fourth order perturbation theory as given by Amat, Nielsen, and Tarrago is discussed. The ground and V_10=1 rotational spectra of propyne have been investigated in the 17 to 70 GHz, and 17 to 53 GHz regions, respectively, and compared with predictions based on higher frequency measurements. The V_9=1 and V_10=2 rotational spectra of propyne have been investigated and assigned for the first time. A perturbation of the V_9=1 rotational spectra for K=-l has been discovered and discussed.

A 10.6 ym C02 laser operating a power range S P 200 watts was used to pump some select vibrational transitions in the NO2 molecule while monitoring the rotational transitions (91/9—'100/10), (232f 22 ~~"*242,23> ' (402,38 "393,37) in the (0, 0, 0) vibrational level and the (8q,8—*"^1,7) rotational transition in the (0, 1, 0) vibrational level. These rotational transitions were monitored by microwave probing to determine how the population of states in the rotational manifolds were being altered by the laser. Coincidences between some components of the V3-V2 band of N02 and the C02 infrared laser lines in the 10 um region appeared to be responsible for the strong interaction between the continuous laser beams and the molecular states.

A new method is developed for treating atoms and molecules in a magnetic field in a gauge-invariant way using the Rayleigh-Ritz energy variational principle. The energy operator depends on the vector potential which must be chosen in some gauge. In order to adapt the trial wave function to the gauge of the vector potential, the trial wave function can be multiplied by a phase factor which depends on the spatial coordinates. When the energy expectation value is minimized with respect to the phase function, the equation for charge conservation for stationary states is obtained. This equation can be solved for the phase function, and the solution used in the energy expectation value to obtain a gauge-invariant energy. The method is applicable to all quantum mechanical systems for which the variational principle can be applied. It ensures satisfaction of the charge conservation condition, a gauge-invariant energy, and the best upper bound to the ground-state energy which can be obtained for the form of trial wave function chosen.

A pair of nonidentical two-level atoms, separated by a fixed distance R, interact through photon exchange. The system is described by a state vector which is assumed to be a superposition of four "essential states": (1) the first atom is excited, the second one is in the ground state, and no photon is present, (2) the first atom is in its ground state, the second one is excited, and no photon is present, (3) both atoms are in their ground states and a photon is present, and (4) both atoms are excited and a photon is also present. The system is initially in state (1). The probabilities of each atom being excited are calculated for both the minimally-coupled interaction and the multipolar interaction in the electric dipole approximation. For the minimally-coupled interaction Hamiltonian, the second atom has a probability of being instantaneously excited, so the interaction is not retarded. For the multipolar interaction Hamiltonian, the second atom is not excited before the retardation time, which agrees with special relativity. For the minimally-coupled interaction the nonphysical result occurs because the unperturbed Hamiltonian is not the energy operator in the Coulomb gauge. For the multipolar Hamiltonian in the electric dipole approximation the …

This dissertation presents experimental and theoretical studies of transient degenerate four wave mixing (DFWM) in organic dyes. Chapter 1 is an introduction to DFWM. Chapter 2 describes DFWM experiments that were performed in the gain medium of a dye laser. Chapter 3 presents the theory of DFWM of short pulses in three level saturable media. Chapter 4 presents DFWM experiments of femtosecond pulses in the saturable absorber of a passively modelocked ring dye laser. Chapter 5 presents the theory of DFWM of ultrashort pulses in resonant media.

The shape of collision-broadened microwave absorption lines is reviewed, along with a number of other broadening mechanisms. The Anderson and Murphy-Boggs linewidth theories are reviewed in detail. Several published modifications to these theories are reviewed. Computer programs which numerically evaluate linewidths and lineshifts are presented. Approximations are made to reduce the need for extensive use of the modified Bessel functions, thereby reducing computation time. Only dipole-dipole forces are considered.

The work of this dissertation has been to prove that the coherence of multiphoton excitation can be studied by an appropriately phased and time delayed sequence of pulses. An application of this fundamental study of coherence has been made for the enhancement of third harmonic generation. The coherent recovery of the energy lost to the two photon absorption process enalled a larger propagation distance for the fundamental than in an interaction which is incoherent or coherent, but not using a 90 degree phase shifted pulse pair. Phase matching over this longer propagation distance gave an enhancement of third harmonic generation.

We present a comparative theoretical study of the transient grating coherent effects in resonant picosecond excitation-probe experiments. Signals in both the probe and conjugate directions are discussed. The effects of recombination, non-radiative scattering and spatial and orientational diffusion are included. The analysis is applied to both a molecular and to a semiconductor model. Signal contributions from concentration and orientational gratings are distinguished and their temporal natures discussed. The theory is used to explain our recent observations in germanium. Included are discussions of picosecond transient grating self-diffraction measurements that can be understood in terms of an orientational grating produced by anisotropic (in k-space) state-filling. Though there have been predictions and indirect experimental evidence for isotropic state-filling in germanium, this is the first direct experimental indication of anisotropic state-filling in a semiconductor.

An optical detection technique is developed to detect and measure the resonant vibration of the cellular membrane. Biological membranes are active components of living cells and play a complex and dynamic role in life processes. They are believed to have oscillation modes of frequencies in the range of 1 to 1000 GHz. To measure such a high-frequency vibration, a linear laser cavity is designed to produce a train of femtosecond pulses of adjustable repetition rate. The method is then directly applied to liposomes, "artificial membrane", stained with a liphophilic potential sensitive dye. The spectral behavior of a selection of potential sensitive dyes in the membrane is also studied.

We have conducted a detailed experimental and theoretical study of nonlinear absorption in semiconductors. Experimental measurements were made on a variety of materials at wavelengths of 1.06 and 0.53 microns using a picosecond Nd:YAG laser. Both two- and three-photon processes were investigated. Values of nonlinear absorption coefficients extracted from these measurements show excellent agreement with recent theory and scaling rules. Our theoretical investigation has been carried out for two-, three-,and n-photon absorption, for both continuous and pulsed sources. Expressions are obtained for the transmission of the sample in terms of the incident irradiance for each case. The physical interpretation of these results is discussed. We have also considered the effects of the photogenerated carriers on the measurements. Equations are developed that include linear absorption by these carriers. We have observed severe distortions on the transmitted beam, caused by changes in the refractive index of the material, due to the presence of these carriers. We present a model that accurately describes these effects in terms of the photogenerated carrier density. We have developed several novel techniques for monitoring nonlinear absorption. In particular, we have adapted the photoacoustic technique to the measurement of nonlinear absorption in semiconductors. We have also developed a …

The contributions of nonlinear absorption processes to laser-induced damage of three selected groups of transparent dielectrics were investigated. The studied materials were irradiated and non-irradiated fused silica, doped and undoped fluorozirconate glass and cubic zirconia stabilized with yttria. The laser-induced damage thresholds, prebreakdown transmission, and nonlinear absorption processes were studied for several specimens of each group. Experimental measurements were performed at wavelengths of 1064 nm and 532 nm using nanosecond and picosecond Nd:YAG laser pulses. In the irradiated fused silica and fluorozirconate glasses, we found that there is a correlation between the damage thresholds at wavelength λ and the linear absorption of the studied specimens at λ/2. In other words, the laser-induced breakdown is related to the probability of all possible two-photon transitions. The results are found to be in excellent agreement with a proposed two-photon-initiated electron avalanche breakdown model. In this model, the initial "seed" electrons for the formation of an avalanche are produced by two-photon excitations of E' centers and metallic impurity levels which are located within the bandgaps of irradiated Si02 and fluorozirconate glasses, respectively. Once the initial electrons are liberated in the conduction band, a highly absorbing plasma is formed by avalanche impact ionization. The resultant …

Incident 0.5 to 2.5 MeV charged particle beams were used to ionize the inner-shells of selected targets and study their subsequent emission of characteristic x-rays. ⁹Be⁺ ions were used to examine K-shell x-ray production from thin F, Na, Al, Si, P, Cl, and K targets, L-shell x-ray production from thin Cu, An, Ge, Br, Zr and Ag targets, and M-shell x-ray production from thin Pr, Nd, Eu, Dy, Ho, Hf, W, Au, Pb and Bi targets. L-shell x-ray production cross sections were also measured for ²⁷Al⁺ ions incident upon Ni, Cu, Zn, As, Zr, and Pd targets. M-shell x-ray production cross sections were measure for ²⁷Al⁺ and ⁴⁰Ar⁺ ions incident upon Pr, Nd, Gd, Dy, Lu, Hf, Au, Pb, Bi, and U targets. These measurements were performed using the 2.5 MV Van de Graaff accelerator at North Texas State University. The x-rays were detected with a Si(Li) detector whose efficiency was determined by fitting a theoretical photon absorption curve to experimentally measure values. The x-ray yields were normalized to the simultaneously measured Rutherford backscattered (RBS) yields which resulted in an x-ray production cross section per incident ion. The RBS spectrum was obtained using a standard surface barrier detector calibrated for …

A variation of the excite-and-probe technique is used to measure the picosecond evolution of laser-induced transient gratings that are produced in germanium by the direct absorption of 40 psec optical pulses at 1.06-μm. Grating lifetimes are determined for free carrier densities between 10¹⁸ cm⁻³ and 10²¹ cm⁻³ . For carrier densities less than 10¹⁹ cm⁻³ , a linear diffusion-recombination model for the grating provides a good fit to the experimental data and allows the extraction of the diffusion coefficient and an estimation of the linear recombination lifetime. Above carrier densities of approximately 10²⁰ cm⁻³ , the density dependence of the diffusion coefficient and nonlinear recombination processes must be considered. Numerical solutions to the resulting nonlinear partial differential equation are obtained that allow extraction of information concerning the high density diffusion coefficient and the nonlinear recombination rates.

Protons with energies ranging from 0.4 to 2.0 MeV were used to measure K-shell vacancy production cross sections (oVK) for N_2, CH_4, C_2H_4, C_2H_6, and CO_2 gas targets under single collision conditions. An electron-ion time-of-flight coincidence technique was used to determind the ration of the K-K electron capture cross section, OECK, to the K-vacancy production cross section, oVK. These ratios were then combined with the measured values of oVK to extract the K-K electron capture cross sections. Measurements were also made for protons of the same energy range but with regard to L-shell vacancy production and L-K electron capture for Ar targets. In addition, K-K electron capture cross sections were measured for 1.0 to 2.0 Mev 42He^_ ions on CH_4.

The Shubnikov-de Haas magneto-resistance oscillations and photoconductivity were experimentally studied in order to investigate the interaction of CO laser radiation with n-InSb at liquid helium temperatures. The roles of various absorption mechanisms on these effects were considered, particularly near the intrinsic band edge. From these measurements an effective electron temperature Tₑ was defined that increased or decreased under illumination, depending upon the strength of the applied electric field.

The picosecond optical response of germanium is investigated by performing excitation-probe experiments on a thin, intrinsic-germanium wafer maintained at 135 K. The results of three distinct experiments are reported: (1) the transmission of a single pulse is measured as a function of irradiance, (2) the probe transmission is measured at a fixed time after excitation as a function of the excitation energy, and (3) the transmission of a probe pulse is monitored as a function of time after excitation. These experiments employ 10-picosecond laser pulses at 1.06 um and Stokes-shifted pulses at 1.55-um.

Bulk laser-induced breakdown and self-focusing in single samples of fused quartz and NaCl were examined using picosecond optical pulses at 1.0 ym and 0.5 ym. The results of three separate but related experiments are reported. First the nonlinear index of refraction, n2, of each of the test materials is measured near the respective damage thresholds of the samples. The values of 1*2 were determined by detecting beam distortions in the far field, transmitted laser beam profile caused by the irradiance dependent index of refraction. The experimental traces were compared to theoretical beam profiles generated by a nonlinear propagation code and n2 was extracted from the resulting fits.

A system of nonrelativistic charged particles and radiation is canonically quantized in the Coulomb gauge and Maxwell's equations in quantum electrodynamics are derived. By requiring form invariance of the Schrodinger equation under a space and time dependent unitary transformation, operator gauge transformations on the quantized electromagnetic potentials and state vectors are introduced. These gauge transformed potentials have the same form as gauge transformations in non-Abelian gauge field theories. A gauge-invariant method for solving the time-dependent Schrodinger equation in quantum electrodynamics is given. Maxwell's equations are written in a form which holds in all gauges and which has formal similarity to the equations of motion of non-Abelian gauge fields. A gauge-invariant derivation of conservation of energy in quantum electrodynamics is given. An operator gauge transformation is made to the multipolar gauge in which the potentials are expressed in terms of the electromagnetic fields. The multipolar Hamiltonian is shown to be the minimally coupled Hamiltonian with the electromagnetic potentials in the multipolar gauge. The model of a charged harmonic oscillator in a single-mode electromagnetic field is considered as an example. The gauge-invariant procedure for solving the time-dependent Schrodinger equation is used to obtain the gauge-invariant probabilities that the oscillator is in an …

A high resolution photoconductivity investigation of two 13 -3 photon magneto-absorption (TPMA) in n-InSb (n - 9 x 10 cm ) has been performed. This is the first time that two-photon absorption in a semiconductor has been studied with cw lasers only. With a stable cw CC>2 laser and a highly sensitive sampling and magnetic field modulation technique, a minimum of 4 2 transitions in the TPMA photoconductivity spectra can be observed. Most of these transitions are a result of the usual spherical approximation TPMA selections rules (An =0, ±2; As = 0 for e ⊥ B and Δn = 0; Δs = 0 for e || B) . However, some transitions, in particular several near the TPMA band edge, are not explained by these rules. The TPMA spectra have been found to depend upon crystallographic orientation. This has not been previously observed. The temperature variation of the fundamental energy gap Eg between 2 and 100° K is also obtained from TPMA experiments.