Doctor of Philosophy / Department of Physics / Zenghu Chang / In this research, the carrier-envelope phase (CE phase) evolution of the pulse train from a Kerr-lens mode-locked chirped-mirror dispersion compensated Ti:Sapphire laser oscillator was stabilized. The offset frequency corresponding to the rate of change of the CE phase was obtained by spectrally broadening the oscillator pulses in a photonic crystal fiber and interfering the f and 2f components. An offset frequency linewidth of 100 mHz was obtained and could be locked over several hours. The effect of path length drift in the interferometer used for CE phase stabilization of the laser oscillator was investigated. By stabilizing the path length drift, the interferometer noise was reduced by several orders of magnitude. The CE phase drift through a grating-based chirped-pulse multi-pass amplifier was investigated. Varying the grating separation by 1μm in the stretcher was found to cause a shift of 3.7 +/- 1.2 rad of the CE phase. The CE phase could be stabilized to within 160 mrad rms error by feedback controlling the grating separation. By locking the path length in the f-to-2f interferometer used to stabilize the CE phase of the oscillator pulses, the fast (>3 Hz) CE phase drift of the amplified laser pulses was reduced from 79 to 48 mrad. It was also found that the CE phase could be shifted and set to any value within a 2π range by changing the grating separation. Also, the CE phase could be continuously modulated within a 2π range while maintaining a relative phase error of 171 mrad. The CE phase shift of a grating-based compressor was found to be stabilized to 230 mrad rms. The effect of laser power fluctuation on the CE phase measurement was also investigated. It was found that a 1% fluctuation of the laser energy caused a 160 mrad error in the CE phase measurement. A two-step model is proposed to explain the phase-energy coupling in the CE phase measurement. The model explains the experimentally observed dependence of the group delay between the f and 2f pulses on the laser energy. Few-cycle pulses were CE phase stabilized to 134 mrad rms and were used to perform above-threshold ionization and high harmonic generation.

Doctor of Philosophy / Department of Physics / Kristan L. Corwin / Current portable near-infrared optical frequency references offer modest accuracy and instability compared to laboratory references. Low pressure reference cells are necessary to realize features narrower than the Doppler broadened overtone transitions, and most setups to date have occurred in free-space. Hollow-core photonic crystal fibers offer a potential alternative to free-space setups through their small cores (~10’s of µm) and low-loss guidance. Furthermore, HC-PCF can be made into fiber cells that could be directly integrated into existing telecommunications networks. Efforts were made to fabricate these fiber cells with a low pressure of molecules trapped inside, but this has proven to be quite challenging. Therefore, investigation of these fibers is conducted by placing the ends of the fiber inside vacuum chambers loaded with acetylene (12C2H2). The linewidths of several P branch transitions (near 1.5 µm) are investigated as a function of acetylene pressure and optical pump power in three different HC-PCFs. Frequency modulation spectroscopy is then implemented on the acetylene-filled HC-PCF to generate sub-Doppler dispersion features that are useful for frequency stabilization using standard servo electronics. Instability and accuracy of this near-IR optical reference were then determined by analysis of heterodyne experiments conducted with frequency combs referenced to a GPS-disciplined rubidium oscillator. The instability and accuracy of this HC-PCF reference are within an order of magnitude of free-space experiments, as expected based on the ratio of linewidths observed in the two experiments. Therefore, HC-PCF has been shown to be suitable for potential frequency references. Further work is necessary to fabricate gas fiber cells with high optical transmission and low molecular contamination.

Doctor of Philosophy / Department of Physics / Itzhak Ben-Itzhak / Present laser technology allows for the production of ultra short (&7 fs) intense (.1016 W/cm2)pulses, which are comparable in duration and interaction strength to the vibrational period and the interaction that binds the electron in molecules, respectively. In this intense-field ultra short-pulse regime one can both measure and manipulate dynamics on the femtosecond timescale. To probe the dynamics of laser-matter interactions in this regime, we have chosen to start from the simplest possible molecule - H+ 2 , which can either dissociate into H + p or ionize into p + p + e. We have designed and employ a coincidence three-dimensional momentum imaging technique which allows us to measure ionization and dissociation of a molecular ion beam target simultaneously, while completely separating the two channels from each other. By varying the laser intensity and the pulse duration, we measure the intensity and pulse length dependent momentum distributions for laser induced fragmentation of H+ 2 at 790 nm. These dissociation measurements are in agreement with the phenomena predicted using the adiabatic Floquet picture, e.g. bond softening, in addition to more sophisticated calculations done by solving the time-dependent Schrodinger equation in the Born-Oppenheimer representation. Furthermore, the structure seen in ionization in our measurements and soon after by others is explained via a unified diabatic Floquet picture, which includes both ionization and dissociation in a single intensity and wavelength dependent picture that includes nuclear motion. Additionally, we use the same experimental techniques and apparatus to probe the laser-induced dynamics of multi-electron diatomic molecules, e.g. O+2, N+2, and ND+. The most probable dissociation and ionization pathways producing the features seen in these measurements are discerned using the angular and kinetic-energy-release distributions in conjunction with the diabatic Floquet picture. Finally, we extend these experimental techniques and interpretive models to the simplest polyatomic molecule - H+ 3 , whose fragmentation presents challenges both in our first-of-their-kind experiments and in physical interpretation.

Doctor of Philosophy / Department of Physics / Brett D. Esry / The Efimov effect plays a central role in few-body systems at ultracold temperature and has thus accelerated a lot of studies on its manifestation in the collisional stability of the quantum degenerate gases. Near broad Feshbach resonances, Efimov physics has been studied both theoretically and experimentally through the zero-energy scattering observables. We have extended the theoretical studies of Efimov physics to a much broader extent. In particular, we have investigated the three-body Efimov physics near narrow Feshbach resonances and have also identified the Efimov features beyond the zero temperature limit. We have found, near a narrow Feshbach resonance, the non-trivial contribution from both of the resonance width and the short-range physics to the three-body recombination and vibrational dimer relaxation. Remarkably, the collisional stability of the Feshbach molecules are found to be opposite to that near the broad resonances: an increased stability for molecules made by bosons and a decreased stability for those made by fermions. The universal physics observed near the narrow Feshbach resonances is further found not to be limited to the zero temperature observables. We have found that the general features of Efimov physics and those pertaining to a narrow resonance are manifested in different energy ranges above zero temperature. This opens the opportunity to observe Efimov physics by changing the collisional energy while keeping the atomic interaction fixed. The landscape of the universal Efimov physics is thus delineated in both of the interaction and the energy domain. We have also investigated Efimov physics in heteronuclear four-body systems where the complexity can be reduced by approximations. In particular, we have proposed ways for controllable production of the Efimov tri-atomic molecules by three-body or four-body recombinations involving four atoms. We have also confirmed the existence of four-body Efimov effect in a system of three heavy particles and one light particle, which has resolved a decade-long controversy on this topic. Finally, we have studied the collisional properties of four identical bosons in 1D, which is important to the experiments on the quantum gases confined in the 1D optical lattices.

Doctor of Philosophy / Department of Physics / B.D. DePaola / In recent years, coherent excitation techniques have focused on the ability to efficiently prepare atomic or molecular systems into a selected state. Such population control plays a key role in cutting-edge research taking place today, such as in the areas of quantum information and laser-controlled chemical reactions. Stimulated Raman adiabatic passage (STIRAP) is a widely-used coherent excitation technique that provides a relatively robust control mechanism for efficiently exciting a target population into a desired state. While the technique is well proven, current experimental techniques yield little information on the population dynamics taking place throughout the excitation process, and experimentalists rely solely on final excited-state measurements to determine the efficiency of population transfer. This dissertation presents a unique diagnostic tool to measure multilevel coherent population transfer on a short (nanosecond) timescale. The technique described here uses magneto-optical trap recoil ion momentum spectroscopy (MOTRIMS) as a noninvasive probe of a coherently-controlled system. It provides extremely detailed information about the excitation process, and highlights some important characteristics seen in excited populations that would otherwise be misleading or completely overlooked if one were to use more traditional diagnostic techniques. This dissertation discusses both the theoretical and experimental results applied to three-level coherently excited target populations of Rb-87.

Doctor of Philosophy / Department of Physics / Uwe Thumm / The time-dependent Schrödinger equation is integrated on a numerical lattice for up to three-dimensional problems. The wave packet propagation technique has been applied to ion – atom collisions in a strong laser field, the vibrational nuclear motion in small homonuclear diatomic molecular ions, and for the scattering of an ion in front of a metallic surface. For laser-assisted proton – hydrogen collisions it is shown, that strong circularly polarized radiation significantly alters the capture and ionization probabilities and results in a dichroism with respect to the helicity. In a pump – control – probe scheme, “stroboscopic” exposure of a nuclear wave packet of the deuterium molecular ion by a single or a series of short and intense laser control pulses may be used to produce an almost stationary distribution of a single vibrational level, where the nodal structure can be tested using the Coulomb explosion imaging technique. Using a pump – probe setup with variable probe delays it is proposed to use Fourier analysis of the time dependence of the Coulomb explosion kinetic energy release spectrum to reveal insight into the initial vibrational state distribution for small diatomic molecules. A last application demonstrates, that resonant charge transfer for scattering of a negative hydrogen anion on a metal surface depends crucially on the position of surface and image states relative to the conduction and valence band, thereby implying different reaction mechanisms for different surface cuts of a metal.

Doctor of Philosophy / Department of Physics / Brett D. Esry / The nuclear rotational period of the simplest molecule H[subscript]2[superscript]+ is about 550 fs, which is more than 35 times longer than its vibrational period of 15 fs. The rotational time scale is also much longer than widely available ultra short laser pulses which have 10 fs or less duration. The large difference in rotational period and ultra short laser pulse duration raises questions about the importance of nuclear rotation in theoretical studies of H[subscript]2[superscript]+ dissociation by these pulses. In most studies, reduced-dimensionality calculations are performed by freezing the molecular axis in one direction, referred to as the aligned model. We have systematically compared the aligned model with our full-dimensionality results for total dissociation probability and field-free dynamics of the dissociating fragments. The agreement between the two is only qualitative even for ultra short 10 fs pulses. Post-pulse dynamics of the bound wave function show rotational revivals. Significant alignment of H[subscript]2[superscript]+ occurs at these revivals. Our theoretical formulation to solve the time-dependent Schrodinger equation is an important step forward to make quantitative comparison between theory and experiment. We accurately calculate observables such as kinetic energy, angular, and momentum distributions. Reduced-dimensionality calculations cannot predict momentum distributions. Our theoretical approach presents the first momentum distribution of H[subscript]2[superscript]+ dissociation by few cycle laser pulses. These observables can be directly compared to the experiment. After taking into account averaging steps over the experimental conditions, we find remarkable agreement between the theory and experiment. Thus, our theoretical formulation can make predictions. In H[subscript]2[superscript]+ dissociation by pulses less than 10 fs, an asymmetry in the momentum distribution occurs by the interference of different pathways contributing to the same energy. The asymmetry, however, becomes negligible after averaging over experimental conditions. In a proposed pump-probe scheme, we predict an order of magnitude enhancement in the asymmetry and are optimistic that it can be observed.

Doctor of Philosophy / Department of Physics / Zenghu Chang / The first part of this thesis describes our novel design, test, and application of our X-ray streak camera to the pulse duration measurement of soft X-rays. We demonstrated a significant improvement in the resolution of the x-ray streak camera by reducing the electron beam size in the deflection plates. This was accomplished by adding a slit in front of the focusing lens and the deflection plates. The temporal resolution reached 280 fs when the slit width was 5 μm. The camera was operated in an accumulative mode and tested by using a 25 fs laser with 2 kHz repetition rate and 1-2% RMS pulse energy stability. We conclude that deflection aberrations, which limit the resolution of the camera, can be appreciably reduced by eliminating the wide-angle electrons. We also employed the same streak camera to demonstrate that it is capable of measuring the pulse duration of X-rays. We measured the pulse duration of X-rays emitted from Ni-like Ag and Cd grazing-incidence laser to be ~5ps. The measured value agrees with the prediction made by the model and the measurement made by changing the delay as a function of the pulse duration. The streak camera was also tested with various sources of X-ray such as high harmonics generation of soft x-rays from an argon atom using a high power Ti:sapphire laser source of KLS. The result of the measurement manifests its capability for serving as a detector in the study of ultrafast dynamics in the field of physics, chemistry, biology and medical sciences. The second part of this thesis describes our design of a spectrometer to study the effect of the Carrier envelope (CE) phase on polarization gated extreme-ultraviolet (XUV) super-continuum generation. Because the challenge of making single shot experiment possible is to generate a sufficient number of photons, our setup has been built to allow generation of high order harmonics at the maximum phase matched pressure. This is the first time to our knowledge that phase matching in the polarization gating process has been studied so far. We measured the maximum phase matching pressure to be ~ 55 Torr which is the pressure above which quadratic increase in intensity of the high harmonics spectrum ceases to appear. At this pressure the number of photons per laser shot was 104 which is sufficient for measuring the single shot XUV spectrum in the range 34 to 45 eV. The spectral profile was a super-continuum for some shots and discrete high harmonics for other shots. It is believed that the shot to shot variation of the spectra is due to the changes of the carrier envelope phase of the few-cycle laser pulses used for the polarization gating. An improved CE phase stabilization system in KLS further eliminated the statistical noise in our observation by allowing us to integrate data over several laser cycles for each CE phase value. The effect of CE phase on a polarization gated XUV spectrum was tested by changing the CE phase with two different methods. In the first method, the CE phase was changed by changing the thickness of fused silica plates on the beam path, and the result shows the shift in the spectral peak of the XUV when the gate width approached less than one optical cycle. As gate width was made less than half the optical cycle, the spectrum was observed with continuum harmonics separated by π radians. We believe that the presence of continuum and discrete harmonics spectra in the observation is due to single and double attosecond pulses generated in the polarization gating. In the second method the carrier-envelope phase of pulses from a grating-based chirped pulse amplification laser was varied smoothly to cover a 2π range by controlling the grating separation. The phase is measured simultaneously by an f-to-2f setup and by the variation of XUV spectra from polarization gated high harmonic generation. A very good similarity between the effect of single and double slits in Yong’s experiment and that of CE phase on the XUV spectrum in the polarization gating experiment has been found, giving better agreement with the theory. The effect of optical properties such as the Gouy phase shift on the polarization gated spectrum has also been studied in the course of investigating the best experimental optimizations to generate the most CE phase sensitive XUV spectrum with less statistical noise. This is the first time to our knowledge experimental study of the effect of the Gouy phase shift on a polarization gated XUV spectrum has been made.