Chemistry & Biochemistry Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2752
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Item Towards Low-frequency Squeezed Light and Its Applications with Four-wave Mixing in Rubidium Vapor(2020) Wu, Meng-Chang; Lett, Paul D.; Rolston, Steven L.; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We study a variety of mechanisms that introduce noise into squeezed light generated by a four-wave mixing (4WM) process in Rb vapor. The noise from the seeding beam itself is a general noise that appears in any squeezed light generated from a seeding process. This noise dominates in the squeezed light at acoustic and lower measurement frequencies. A second excess noise source is observed in the twin beams pumped by either a diode laser system or a Ti:sapphire laser system. This excess noise is much stronger in the diode laser systems. It is present in the twin beams at measurement frequencies when the 4WM gain is reduced toward unity. Most of this excess noise can be removed with a dual-seeded 4WM scheme. A third noise source we examine is from a two-beam coupling that degrades the squeezing of the dual-seeded 4WM process at low frequencies of the order of the atomic transition linewidth. This noise can be avoided by seeding skew rays in the 4WM gain region. This gives us an insight to solve this "cross talk" problem by imaging the source in the 4WM gain region. In addition to studying noise sources, we propose a gain-independent calibration scheme that relies on higher order correlation function for the absolute calibration of photodiodes. Having low frequency squeezing is really important if we record quantum images with a CCD camera, which has a slow shutter speed. Also, it's been very difficult for people to get low-frequency squeezing. We obtain a record level of low-frequency squeezing using a simple dual-seeding technique. With this study of noise sources we are closer to having a portable quantum light source using diode lasers.Item HIGH REPETITION RATE LASER-DRIVEN ELECTRON ACCELERATION TO MEGA-ELECTRON-VOLT ENERGIES(2019) Salehi, Fatholah; Milchberg, Howard M; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Laser-driven particle accelerators have the potential to be a compact and cost-effective replacement for conventional accelerators. Despite the significant achievements of laser wakefield acceleration in the last two decades, more work is required to improve the beam parameters such as the energy spread, emittance, repetition rate, and maximum achievable energy delivered by these advanced accelerators to values comparable to what conventional accelerators offer for various applications. The goal of this dissertation is to lower the threshold of the laser pulse energy required for driving a wakefield and in turn enable higher repetition rate particle acceleration with current laser technology. In the first set of electron acceleration experiments presented in this dissertation, we show that the use of a thin gas jet target with near critical plasma density lowers the critical power for relativistic self-focusing and leads to electron acceleration to the ~MeV range with ~1pC charge per shot, using only ~1mJ energy drive laser pulses delivered by a 1kHz repetition rate laser system. These electron beams accelerated in the self-modulated laser wakefield acceleration (SM-LWFA) regime have a thermal energy distribution and a rather large divergence angle (>150mrad). In order to improve the energy spread and the divergence, in the second set of the experiments, we employ a few-cycle laser pulse with a ~7fs duration and ~2.5mJ energy as the driver, to perform wakefield acceleration in the bubble regime using near critical plasma density targets. The results show a significant improvement in the energy spread and divergence of the beam. The electron bunches from this experiment have a quasi-monoenergetic peak at ~5MeV with an energy spread of ΔE/E≃0.4 and divergence angle of ~15mrad. These results bring us closer to the use of tabletop advanced accelerators for various scientific, medical, and industrial applications.Item Optical and quantum interferences in strong field ionization and optimal control(2017) Foote, David B.; Hill, Wendell T.; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)For decades, ultrafast laser pulses have been used to probe and control strong-field molecular dynamics, including in optimal control experiments. While these experiments successfully recover the optimal control pulses (OCPs), they have a limitation -- it is generally unknown how the OCP guides the target system to its final state. This thesis is concerned with "unpacking" OCPs to explain how they achieve their control goals. The OCPs that inspired this work consisted of pulse trains; a twin-peaked pulse (TPP) is the simplest example. Consequently, TPPs with variable interpeak delay and relative phase were employed in this work to study ionization, the first step in many control experiments. Two types of interference influence ionization from a TPP: optical interference (OI) between the electric fields of the two peaks, and quantum interference (QuI) between the electron wavepackets produced by the two peaks. Two sets of experiments were performed to determine what roles OI and QuI play in controlling ionization from a TPP and how they in turn influence subsequent molecular dynamics. The first set of experiments measured the total ionization yield induced by the TPPs. It was found that OI was principally responsible for changing the ion yield; QuI-induced oscillations were not observed. Small imperfections in the shape of the TPP (i.e., pedestals and subordinate peaks) were found to have a surprisingly large influence in the OI, highlighting the need for researchers in molecular control experiments to characterize the temporal profile of their pulses accurately. A time-dependent perturbation theory simulation showed that the signatures of QuI in the ionic continuum vanish when measuring {\it total} electron yield, but appear in {\it energy-resolved} electron yields. The second set of experiments measured photoelectron energy distributions from a TPP with a velocity map imager to search for QuI. The experiments were performed at high intensities (~10^14 W/cm^2) where the ponderomotive energy tends to wash out the fine energy structures of QuI. The thesis ends by proposing a modified, low-intensity experiment that will allow for the first unambiguous observation of QuI in non-resonant, multiphoton ionization.Item Dissipation in a superfluid atom circuit(2017) Lee, Jeffrey Garver; Hill, Wendell T; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Bose-Einstein condensates of weakly interacting dilute atomic gases provide a unique system with which to study phenomena associated with superfluidity. The simplicity of these systems allows us to study the fundamental physics of superfluidity without having to consider the strong interactions present in other superfluid systems such as superconductors and liquid helium. While condensate-based studies have been around for 20 years, our novel approach to confining ultracold atoms has opened a completely new range of parameter space to investigate. Armed with an ability for straightforward creation of arbitrary, time-dependent potential landscapes in which to study superfluid interactions, we were able to take a closer look at predictions of superfluid behavior that are decades old, but until now have never been tested directly. The purpose of this research was to draw direct analogies between superfluid BEC systems, which we term superfluid atom circuits, and existing superconducting circuits, thus allowing us to take advantage of much of the existing knowledge that has come from this well-studied field. Specifically, existing circuits and devices that have been created with superconductors give us insight into what might be possible someday with atom-circuit devices and inspiration to create them. In these experiments, we employed two different atom circuits; one classical (thermal ideal gas) and one quantum (ultracold superfluid). Our results show that each system is equivalent to an electronic circuit consisting of a capacitor being discharged through an inductor in series with some dissipative element. In the thermal system, dissipation can be described in terms of simple resistive flow with the resistance equivalent to ballistic, Sharvin resistance seen in electronic circuits. The superfluid measurements show that the dissipation is best described as a resistance-shunted Josephson junction, which is an analogue to similar devices in superconducting circuits. Additionally, the specific geometry of the atom circuit we used in our superfluid system allowed us to investigate directly a predicted mechanism responsible for the dissipation in superfluids caused by the generation of collective excitations, namely vortices. Direct observation of this mechanism has not previously been possible in superfluid helium and superconducting systems.Item PHYSICAL MECHANISM OF TERAHERTZ GENERATION IN TWO-COLOR PHOTOIONIZATION(2014) You, Yong Sing; Kim, Ki-Yong; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two-color photoionization has been widely used as a versatile tool for intense, broadband terahertz (THz) radiation generation. In this scheme, an ultrashort laser's fundamental and its second harmonic pulses are co-focused into a gas of atoms or molecules, transforming them into plasma by photoionization. This process produces an intense THz pulse emitted in the forward direction. The main focus of this dissertation is to provide a physical understanding of such THz generation and investigate its generation mechanism at both microscopic and macroscopic levels. First, we examine the generation process by measuring the relative phase between two-color (fundamental and second harmonic) laser fields and the resulting THz field simultaneously. We discover that a relative phase of π/2 yields maximal THz outputs, consistent with a semi-classical plasma current model. We find that this optimal relative phase is independent of laser intensities, gas species, and two-color laser amplitude ratios. We also measure concurrent near-field photocurrents. All these measurements verify laser-produced plasma currents as a microscopic source for THz generation. We also investigate THz radiation from an ensemble of aligned air molecules in two-color laser fields. Our experiments show that THz radiation is strongly affected by molecular (nitrogen and oxygen) alignment. We explain this phenomenon in the context of the plasma current model combined with alignment-dependent ionization. Phase-matching is essential to achieve high-efficiency nonlinear frequency conversion. We discover THz generation by two-color photoionization in elongated air plasmas (filamentation) is naturally phase-matched in the off-axis direction, resulting in donut-shaped radiation profiles in the far field. Because of this off-axis phase-matching, THz yields increase almost linearly with the filament length, scalable for further THz energy enhancement. Lastly, we study the polarization of emitted THz radiation. In the case of in-line focusing geometry, we observe the polarization evolves from linear to elliptical with increasing plasma length. This ellipticity arises from two combined effects--successive polarization rotation of local THz plasma sources, caused by laser phase and polarization modulations, and the velocity mismatch between laser and THz, which produces an elliptical THz pulse from a series of time-delayed, polarization-rotating local THz fields.Item Sum Frequency Generation in Laser Safety and Quantum Telecommunications Applications(2011) Houston, Jemellie; Clark, Charles W; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis describes the implications of sum-frequency generation in both laser safety and quantum telecommunications applications. Green laser pointer technology uses frequency doubling of invisible 1064 nm infrared radiation to visible 532 nm green radiation. An inexpensive green laser pointer was found to emit infrared leakage primarily due to the lack of an infrared-blocking filter. An experimental setup using common household materials was presented to detect unwanted infrared radiation from such devices. Also reported, is the design and characterization of a high-speed versatile 780 nm pump source up to 1.25 GHz through second harmonic generation from a wavelength of 1560 nm. The 780 nm source is currently being used for the production of correlated photon pairs, one of which is at 656 nm, the hydrogen Balmer alpha line. The final goal will be to generate a high-speed entanglement source after some adjustments in the correlated pair source assembly. This will improve an operational quantum key distribution system.Item WAVELENGTH DEPENDENCE IN OPTICAL TWEEZERS(2010) Hester, Brooke Cranswick; Losert, Wolfgang; Rolston, Steven; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Optical trapping forces are dependent upon the difference between the trap wavelength and the extinction (scattering plus absorption) resonances of a trapped particle. This leads to a wavelength-dependent trapping force, which should allow for the optimization of optical tweezers systems, simply by choosing the best trapping wavelength for a given experiment. Although optical forces due to a near-resonant laser beam have been extensively studied for atoms, the situation for larger particles has not been explored experimentally. The ability to selectively trap certain particles with a given extinction peak may have many practical applications. Here, resonance-based trapping is investigated using nanoshells, particles with a dielectric core and metallic coating that exhibit tunable plasmon resonances, and with silica and polystyrene beads. A measure of the trap strength was realized for single particles trapped in three dimensions, and near-resonant trapping was investigated by measuring the trap strength as a function of trap wavelength. Since the resulting trapping is highly temperature dependent, this necessitated temperature measurements of single optically trapped particles. To make these measurements a new optical tweezer apparatus was designed and constructed; the apparatus has wavelength tunability and was used to study these resonance effects. Optical trap stiffness, which is analogous to the spring constant of a stable trap, is measured for trapped particles that exhibit either single or multiple extinction resonances. The applications of this apparatus are not limited to force spectroscopy. Other measurement systems and techniques could be easily implemented into the custom-built apparatus, allowing for the measurement of various properties of single optically trapped particles as a function of wavelength.