Doctor of Philosophy / Department of Physics / Zenghu Chang / The observation and control of dynamics in atomic and molecular targets requires the use of laser pulses with duration less than the characteristic timescale of the process which is to be manipulated. For electron dynamics, this time scale is on the order of attoseconds where 1 attosecond = 10[superscript]-18 seconds. In order to generate pulses on this time scale, different gating methods have been proposed. The idea is to extract or “gate” a single pulse from an attosecond pulse train and switch off all the other pulses. While previous methods have had some success, they are very difficult to implement and so far very few labs have access to these unique light sources. The purpose of this work is to introduce a new method, called double optical gating (DOG), and to demonstrate its effectiveness at generating high contrast single isolated attosecond pulses from multi-cycle lasers. First, the method is described in detail and is investigated in the spectral domain. The resulting attosecond pulses produced are then temporally characterized through attosecond streaking. A second method of gating, called generalized double optical gating (GDOG), is also introduced. This method allows attosecond pulse generation directly from a carrier-envelope phase un-stabilized laser system for the first time. Next the methods of DOG and GDOG are implemented in attosecond applications like high flux pulses and extreme broadband spectrum generation. Finally, the attosecond pulses themselves are used in experiments. First, an attosecond/femtosecond cross correlation is used for characterization of spatial and temporal properties of femtosecond pulses. Then, an attosecond pump, femtosecond probe experiment is conducted to observe and control electron dynamics in helium for the first time.

Doctor of Philosophy / Department of Physics / Brian R. Washburn / Optical frequency combs produced by mode-locked fiber lasers are useful tools for high precision frequency metrology and molecular spectroscopy in a robust and portable format. We have specifically investigated erbium doped fiber mode-locked lasers that use single-walled carbon nanotubes as a saturable absorber. We have, for the first time, developed and phase- stabilized a carbon nanotube fiber laser (CNFL) frequency comb. The carbon nanotube saturable absorber, which was fabricated using an optically driven deposition method, permits a high repetition frequency (>150 MHz) since an optical nonlinearity of fibers is not used for mode-locking. The CNFL comb combined with a parabolic pulse erbium doped fiber amplifier (EDFA) has shown a compact, robust, and cost-effective supercontinuum source. The amplified pulse from the parabolic pulse EDFA was compressed with a hollow-core photonic bandgap fiber, which produced a wave-breaking-free pulse with an all-fiber set-up. The stabilized comb has demonstrated a fractional instability of 1.2 ×10[superscript]-11 at 1 sec averaging time, the reference-limited instability. We have performed optical frequency metrology with the CNFL comb and have measured an optical frequency, P(13) which is a molecular overtone transition of C2H2. The measured frequency has shown a good agreement with the known value within an uncertainty of 10 kHz. In order to extend the application of the CNFL comb such as multi-heterodyne dual comb spectroscopy, we have investigated the noise of the CNFL comb and particularly, the broad carrier envelope offset frequency (f[subscript]0) linewidth of the CNFL comb. The primary noise source is shown to be white amplitude noise on the oscillator pump laser combined with the sensitivity of the mode-locked laser to pump power fluctuations. The control bandwidth of f[subscipt]0 was limited by the response dynamics of the CNFL comb. The significant reduction of comb noise has been observed by implementing a phase-lead compensation to extend control bandwidth of the comb and by reducing the pump relative intensity noise simultaneously. Therefore the f[subscipt]0 linewidth has been narrower from 850 kHz to 220 kHz. The integrated phase noise for the f[subscipt]0 lock is 1.6 radians from 100 Hz to 102 kHz.

Doctor of Philosophy / Department of Physics / Brett D. DePaola / In many experiments involving a magneto-optical trap (MOT) it is of great importance to know the fraction of atoms left in an excited state due to the trapping process. Generally speaking, researchers have had to use overly simplistic and untested models to estimate this fraction. In this work, the excited fraction of 87Rb atoms in a MOT is measured using a model-free approach. A simple model is fit to the fractions which were obtained for a range of MOT parameters. Using the results of this work, the excited fraction of 87Rb atoms trapped in a MOT can be accurately estimated with knowledge of only the trapping laser intensity and detuning. The results are only weakly dependent on other MOT parameters.

Doctor of Philosophy / Department of Physics / Charles L. Cocke / An underlying goal of studying atomic or molecular dynamics with short laser pulses is to reach a time scale short enough to study the evolution of the system in the time domain. In this thesis, the strong field ionization of atoms and molecules has been investigated with the highly resolved technique known as cold target recoil momentum spectroscopy (COLTRIMS). The thesis can be divided into two parts: single and double ionization. In the first part, we studied the momentum vectors of low energy electrons generated by short laser pulses of wavelengths varying from 400 to 800 nm with atomic and molecular targets with intensities in the tunneling region. Most of the structures observed in the momentum spectra of atomic and molecular targets can be explained as due to above-threshold ionization, and Freeman resonances. The most significant structure in our observed spectra is the angular structure in the lowest part of the momentum image, and this is attributed to the diffraction pattern evolved by tunneling electrons. Surprisingly, we observed that the structure produced by the electrons from high Rydberg states is independent of the internal structure of the target atom and molecules. The same work is extended to aligned molecules. The basic idea of this part of the work is to see whether the angular distribution of electrons from aligned molecules resembles the orbital structures of the molecules. The rotational revival structure was used to align the molecules. We observed pronounced energy and angular structures of the momentum images which show a dependence on the alignment of the molecule. The last part of this work mainly focuses on double ionization, i.e. the removal of two electrons from the target atoms sequentially by a short laser pulse. Measuring the complete momentum vector of Ar2+ and Ne2+, we demonstrate that these can be used to extract the angular correlation between two electrons sequentially released in the circularly polarized pulse. We demonstrate how the measurement of full momentum vectors of the doubly ionized argon and neon ions can be used to extract the time gap of the two emissions.

Doctor of Philosophy / Department of Physics / Brett D. DePaola / Many people in the science community are interested in the prospect of cold molecules for such applications as quantum computing and molecular Bose-Einstein condensates. Current methods of production fall short of the requirements for such projects. Photo association is a promising technique for forming cold molecules, but is currently facing significant obstacles. By understanding the photo association process and utilizing higher excited states, it is hoped that cold molecules can be formed from more easily produced cold atoms. Photo associative Ionization (PAI) is presented as a means to study excited state molecular dynamics at large internuclear separation, including photo association. This thesis presents a number of techniques for studying PAI in cold rubidium and a number of results obtained with the techniques. Excitation pathways for the process are explored in both narrow linewidth (MHz) and ultrafast (fs), large bandwidth (20 nm) domains.

Doctor of Philosophy / Department of Physics / Zenghu Chang / The few-cycle femtosecond laser pulse has proved itself to be a powerful tool for controlling the electron dynamics inside atoms and molecules. By applying such few-cycle pulses as a driving field, single isolated attosecond pulses can be produced through the high-order harmonic generation process, which provide a novel tool for capturing the real time electron motion. The first part of the thesis is devoted to the state of the art few-cycle near infrared (NIR) laser pulse development, which includes absolute phase control (carrier-envelope phase stabilization), amplitude control (power stabilization), and relative phase control (pulse compression and shaping). Then the double optical gating (DOG) method for generating single attosecond pulses and the attosecond streaking experiment for characterizing such pulses are presented. Various experimental limitations in the attosecond streaking measurement are illustrated through simulation. Finally by using the single attosecond pulses generated by DOG, an attosecond transient absorption experiment is performed to study the autoionization process of argon. When the delay between a few-cycle NIR pulse and a single attosecond XUV pulse is scanned, the Fano resonance shapes of the argon autoionizing states are modified by the NIR pulse, which shows the direct observation and control of electron-electron correlation in the temporal domain.

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 / We have made significant accomplishments in the development of portable frequency standard inside hollow optical fibers. Such standards will improve portable optical frequency references available to the telecommunications industry. Our approach relies on the development of a stabilized Cr:forsterite laser to generate the frequency comb in the near-IR region. This laser is self referenced and locked to a CW laser which in turn is stabilized to a sub-Doppler feature of a molecular transition. The molecular transition is realized using a hollow core fiber filled with acetylene gas. We finally measured the absolute frequency of these molecular transitions to characterize the references. In this thesis, the major ideas, techniques and experimental results for the development and absolute frequency measurement of the portable frequency references are presented. A prism-based Cr:forsterite frequency comb is stabilized. We have effectively used the prism modulation along with power modulation inside the cavity in order to actively stabilize the frequency comb. We have also studied the carrier-envelope-offset frequency (f0) dynamics of the laser and its effect on laser stabilization. A reduction of f0 linewidth from [similar to]2 MHz to [similar to]20 kHz has also been observed. Both our in-loop and out-of-loop measurements of the comb stability showed that the comb is stable within a part in 10^11 at 1-s gate time and is currently limited by our reference signal. In order to develop this portable frequency standard, saturated absorption spectroscopy is performed on the acetylene v1+v3 band near 1532 nm inside different kinds of hollow optical fibers. The observed linewidths are a factor 2 narrower in the 20 um fiber as compared to 10 um fiber, and vary from 20-40 MHz depending on pressure and power. The 70 um kagome fiber shows a further reduction in linewidth to less than 10 MHz. In order to seal the gas inside the hollow optical fiber, we have also developed a technique of splicing the hollow fiber to solid fiber in a standard commercial arc splicer, rather than the more expensive filament splicer, and achieved comparable splice loss. We locked a CW laser to the saturated absorption feature using a Frequency Modulation technique and then compared to an optical frequency comb. The stabilized frequency comb, providing a dense grid of reference frequencies in near-infrared region is used to characterize and measure the absolute frequency reference based on these hollow optical fibers.

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.