Physics Theses and Dissertations

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INTERFACIAL SOLVATION AND EXCITED STATE PHOTOPHYSICAL PROPERTIES OF 7-AMINOCOUMARINS AT SILICA/LIQUID INTERFACES
(2010) Roy, Debjani; Walker, Robert A; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The properties of solutes adsorbed at interfaces can be very different compared to bulk solution limits. This thesis examines how polar, hydrophilic silica surfaces and different solvents systematically change a solute's equilibrium and dynamic solvation environment at solid/liquid interfaces. The primary tools used in these studies are steady state fluorescence spectroscopy and time correlated single photon counting (TCSPC) -a fluorescence method capable resolving fluorescence emission on the picosecond timescale. To sample adsorbed solutes, TCSPC experiments were carried out in total internal reflection (TIR) geometry. These studies used total of six different 7 aminocoumarin dyes to isolate the effects of molecular and electronic structure on solute photophysical behavior. Fluorescence lifetimes measured in the TIR geometry are compared to the lifetimes of coumarins in bulk solution using different solvents to infer interfacial polarity and excited state solute conformation and dynamics. Steady state emission experiments measuring the behavior of the coumarins adsorbed at silica surfaces from bulk methanol solutions show that all coumarins had a similar affinity &delta G _ads &sim &minus 25-30 kJ/mole. Despite these similar adsorption energetics solute structure had a very pronounced effect on the tendency of solutes to aggregate and form multilayers. Our finding suggests that hydrogen bonding donating properties of the silica surface plays a dominant role in determining the interfacial behavior of these solutes. The silica surface also had pronounced effects on the time dependent emission of some solutes. In particular, the strong hydrogen bond donating properties of the silica surface inhibit formation of a planar, charge transfer state through hydrogen bond donation to the solute's amine group. A consequence of this interaction is that the time dependent emission from solutes adsorbed at the surface appears to be more similar to emission from solutes in nonpolar solvation environments. To test the role of solvent identity on the photophysical properties of adsorbed solutes, additional experiments were carried out with a nonpolar solvent (decane), a moderately polar solvent (n decanol) and a polar aprotic solvent (acetonitrile). The results from these studies demonstrated that interfacial solvation depends sensitively on a balance of competing forces including those between the solute and substrate, the solute and solvent and the surface and adjacent solvent.
Two Experiments with Cold Atoms: I. Application of Bessel Beams for Atom Optics, and II. Spectroscopic Measurements of Rydberg Blockade Effect
(2010) Arakelyan, Ilya; Hill, III, Wendell; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In this dissertation we report the results of two experimental projects with laser-cooled rubidium atoms: I. Application of Bessel beams for atom optics, and II. Spectroscopic measurements of Rydberg blockade effect. The first part of the thesis is devoted to the development of new elements of atom optics based on blue-detuned high-order Bessel beams. Properties of a 4th order Bessel beam as an atomic guide were investigated for various parameters of the hollow beam, such as the detuning from an atomic resonance, size and the order of the Bessel beam. We extended its application to create more complicated interferometer-type structures by demonstrating a tunnel lock, a novel device that can split an atomic cloud, transport it, delay, and switch its propagation direction between two guides. We reported a first-time demonstration of an atomic beam switch based on the combination of two crossed Bessel beams. We achieved the 30% efficiency of the switch limited by the geometrical overlap between the cloud and the intersection volume of the two tunnels, and investigate the heating processes induced by the switch. We also showed other applications of crossed Bessel beams, such as a 3-D optical trap for atoms confined in the intersection volume of two hollow beams and a splitter of the atomic density. The second part of this dissertation is devoted to the spectroscopic measurements of the Rydberg blockade effect, a conditional suppression of Rydberg excitations depending on the state of a control atom. We assembled a narrow-linewidth, tunable, frequency stabilized laser system at 480 nm to excite laser-cooled rubidium atoms to Rydberg states with a high principal quantum number n ~ 50 through a two-photon transition. We applied the laser system to observe the Autler-Townes splitting of the intermediate 5p state and used the broadening of the resonance features to investigate the enhancement of Rydberg-Rydberg interactions in the presence of an external electric field.
Photon Pair Production from a Hot Atomic Ensemble in the Diamond Configuration
(2009) Willis, Richard Thomas; Rolston, Steven; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This thesis discusses four-wave mixing (4WM) in a warm ensemble of rubidium using the diamond configuration level structure. Both classical 4WM and non- classical photon-pair production are investigated. Quantum information science has spawned a great amount of experimental work on the interaction of light with collective modes of excitation in atomic ensem- bles. Plans to build quantum networks and quantum repeaters with atom ensembles take advantage of nonlinear interactions to produce and store non-classical states of light. These technologies will require photon sources that not only generate non- classical light, but also resonant, narrow band light. Here we investigate a system which could be used as such a source. We take advantage of the 4WM interaction in a warm ensemble of Rubidium atoms. Our scheme utilizes the diamond energy level configuration which, in ru- bidium, allows for correlated pairs at telecommunications wavelengths. We start by examining the properties of classical 4WM in the system. We measure the reso- nance structure and see that it can be understood in terms of velocity class selective resonant enhancement and power splitting effects. The efficiency of the process is low and limited by linear absorption of the pumps. Our observations agree with a semi-classical Maxwell-Bloch theoretical treatment. Next we observe pair generation by spontaneous 4WM from the warm ensem- ble. The temporal profile of the cross-correlation function (CCF) for the photons depends on pump-laser power and detuning. This allows us to produce biphotons with controllable spectra. A simple quantum optical theoretical treatment based on linear filtering gives qualitative agreement with the data. We show that the photon pairs are polarization entangled, clearly violating Bell's Inequality. A perturbative quantum optical treatment predicts the polariza- tion state of the pairs and agrees with our measurements. We analyze the photon statistics of the source and find the largest violation of the two beam Cauchy-Schwarz inequality from a warm atomic source yet. We cast the system as a heralded sin- gle photon source at telecommunications wavelengths and see that it is competitive with other systems in terms of spectral brightness.
Studies of atomic properties of francium and rubidium.
(2009) Perez Galvan, Adrian; Orozco, Luis A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
High precision measurements of atomic properties are excellent probes for elec- troweak interaction studies at the lowest possible energy range. The extraction of standard model coupling constants relies on a unique combination of experimen- tal measurements and theoretical atomic structure calculations. It is only through stringent comparison between experimental and theoretical values of atomic prop- erties that a successful experiment can take place. Francium, with its heavy nucleus and alkali structure that makes it amenable to laser cooling and trapping, stands as an ideal test bed for such studies. Our group has successfully created, trapped and cooled several isotopes of francium, the heaviest of the alkalies, and demonstrated that precision studies of atomic properties, such as the measurement of the 8S_1/2 excited state lifetime of 210Fr presented here, are feasible. Further work in our program of electroweak studies requires a better control of the electromagnetic environment observed by the sample of cold atoms as well as a lower background pressure (10-10 torr or better). We have designed and adapted to our previous setup a new &ldquo science &rdquo vacuum chamber that fulfills these requirements and the transport system that will transfer the francium atoms to the new chamber. We use this new experimental setup as well as a rubidium glass cell to perform precision studies of atomic and nuclear properties of rubidium. Spectroscopic studies of the most abundant isotopes of rubidium, 87Rb and 85Rb, are a vital component in our program. Performing measurements in rubidium allows us to do extensive and rigorous searches of systematics that can be later extrapolated to francium. We present a precision lifetime measurement of the 5D_3/2 state of 87Rb and a measurement of hyperfine splittings of the 6S_1/2 level of 87Rb and 85Rb. The quality of the data of the latter allows us to observe a hyperfine anomaly attributed to an isotopic difference of the magnetization distribution in the nucleus i.e. the Bohr-Weisskopf effect. The measurements we present in this work complement each other in exploring the behavior of the valence electron at different distances from the nucleus. In addition, they constitute excellent tests for the predictions of ab initio calculations using many body perturbation theory and bolster our confidence on the reliability of the experimental and theoretical tools needed for our work.
Novel applications of high intensity femtosecond lasers to particle acceleration and terahertz generation
(2008-11-20) York, Andrew Gregory; Milchberg, Howard M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
We have investigated new applications for high intensity femtosecond lasers theoretically and experimentally, including a novel method to accelerate electrons to relativistic energy and a new type of coherent lasing medium for amplification of few-cycle, high energy pulses of terahertz radiation. We report the development of corrugated `slow wave' plasma guiding structures with application to quasi-phase-matched direct laser acceleration of charged particles. These structures support guided propagation at intensities up to 2x10^17 W/cm^2, limited by our current laser energy and side leakage. Hydrogen, nitrogen, and argon plasma waveguides up to 1.5 cm in length with corrugation period as short as 35 microns are generated, with corrugation depth approaching 100%. These structures remove the limitations of diffraction, phase matching, and material damage thresholds and promise to allow high-field acceleration of electrons over many centimeters using relatively small femtosecond lasers. We present simulations that show a laser pulse power of 1.9 TW should allow an acceleration gradient larger than 80 MV/cm. A modest power of only 30 GW would still allow acceleration gradients in excess of 10 MV/ cm. Broadband chirped-pulse amplification (CPA) in Ti:sapphire revolutionized nonlinear optics in the 90's, bringing intense optical pulses out of large government facilities and into the hands of graduate students in small university labs. Intense terahertz pulses (>> 10 microjoules, <5 cycles), however, are still only produced at large accelerator facilities like Brookhaven National Labs. CPA is theoretically possible for terahertz frequencies, but no broadband lasing medium like Ti:sapphire has been demonstrated for terahertz. Dipolar molecular gases such as hydrogen cyanide (HCN), `aligned' by intense optical pulses, are a novel and promising medium for amplification of broadband few-cycle terahertz pulses. We present calculations that show rotationally excited molecules can amplify a few-cycle seed pulse of terahertz radiation: a sequence of short, intense optical pulses aligns a dipolar gas, driving the molecules into a broad superposition of excited rotational states. A broadband seed terahertz pulse following the optical pulses can then be amplified on many pure rotational transitions simultaneously. We also discuss plans and progress towards experimental realization of a few-cycle terahertz amplifier.
New Technologies for Broadband Quantum Key Distribution: Sources, Detectors, and Systems
(2008-11-12) Rogers, Daniel; Goldhar, Julius; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In this thesis I describe three independent projects that advance the development of broadband quantum cryptography. While each project pertains to a different part of the QKD chain, together they provide key developments in implementing QKD at bit rates that are practical for use in the modern telecommunications infrastructure. The first project comprises the bulk of the thesis and involves developing a novel source of correlated photon pairs for use in free-space QKD. This source is based on a birefringent semiconductor optical waveguide as a Kerr medium. We demonstrate the feasibility of using birefringent phase-matched four-wave mixing to generate correlated photon pairs. We further propose that, by reversing the process and pumping with conjugate wavelengths, one can use the same effect to produce entangled photon pairs with the same device. These pairs can then be used for QKD to realize the most secure and efficient quantum cryptographic data links. The second project examines the implications of operating a BB84 QKD protocol at clock rates that are faster than the recovery time of the constituent detectors. We show that operating such systems under conventional protocols results in a security violation that allows an eavesdropper to learn significant information about the key and present a modification to the BB84 protocol that maintains key security at fast transmission rates. This modification to the protocol will become vital to QKD viability as links become faster and clock rates go into the tens of gigahertz. We also demonstrate, rather counterintuitively, that there exists an optimal transmission rate for a QKD system that exceeds the inverse of an individual detector's dead time. The final project describes a new design for a free-space QKD link that centers around faster silicon detectors. These detectors have a peak quantum efficiency in the visible range, requiring that the system operate at a wavelength that is more susceptible to solar interference. To mitigate this effect, the link is designed around a Fraunhofer line in the solar spectrum where the background solar light levels are reduced by up to 90%. By implementing this system, we expect at least a two-fold increase in the secret key rate, coming ever closer to the goal of a 10 Mb/s QKD system compatible with first-generation ethernet technology.
Dynamics and Synchronization of Nonlinear Oscillators with Time Delay: A Study with Fiber Lasers
(2007-06-20) Franz, Anthony Lawrence; Roy, Rajarshi; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The effect of time delay on nonlinear oscillators is an important problem in the study of dynamical systems. Erbium-doped fiber ring lasers have an internal time scale set by the length of the laser's electromagnetic cavity. Long cavities allow thousands of modes to experience gain making it very difficult to model the lasers. We examine the effect of adding external time delays through feedback and coupling. In the first experiment an external time delay is added to a laser by adding a feedback loop to the cavity. These delay times are varied over four orders of magnitude by changing the length of fiber in the feedback loop. The laser intensity dynamics are examined using time series, power spectra, time delay embeddings, and spatiotemporal representations. We apply Karhunen-Loeve (KL) decomposition on the spatiotemporal representations and use the Shannon entropy as calculated from the KL eigenvalue spectra as a measure of the complexity of the dynamics. For long delays we find that the complexity increases as expected, but also that the fluctuation size increases. In the second experiment two lasers are mutually coupled together with a coupling time delay that is varied over four orders of magnitude. The analysis is repeated and we find the surprising result that the dynamical complexity decreases for short coupling delays as compared to the uncoupled lasers. Measurements of the optical spectra indicate a narrowing of the spectra indicating that the simplification in dynamics could be due to the reduction in the number of electromagnetic modes experiencing gain. The fluctuation size increases for all delay times and is largest when the internal and external time delays match. Lag-synchrony is also observed for the mutually coupled lasers. Recent modeling using Ikeda ring oscillators showed that stable isochronal synchrony could be achieved if a third drive laser was unidirectionally coupled with enough strength. We experimentally find that increasing the coupling strength of a third drive laser added to the mutually coupled lasers above quenches the lag-synchrony. The two response lasers become more synchronized to the drive than to each other, however the levels of isochronal synchrony are low.
Stochastic and Deterministic Dynamics in a Semiconductor Laser with Optical Feedback
(2006-08-29) Ray, William Richard; Roy, Rajarshi; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Semiconductor lasers have been widely utilized in optical communications and optical data storage. However, in many important applications a small amount of the output light may be reflected back into the laser cavity resulting in large intensity fluctuations and a broadened laser linewidth. Here we experimentally and numerically characterize the subtle influence that spontaneous emission in the laser cavity has on shaping the instabilities produced by the time-delayed optical feedback from external reflections. In the first experiment, we estimate the relative role played by deterministic and stochastic influences in the semiconductor laser at high injection currents under the influence of reflective feedback over a large range of feedback strengths. An empirical mode decomposition method is utilized to provide a physically significant definition of the Hilbert phase. Hurst exponent measurements of the Hilbert phase fluctuations show a clear transition from regular Brownian motion to fractional Brownian motion as the amplitude of coherent feedback is incremented in the experiment and model equations. At lower injection currents noise is believed to play a much more crucial role in the intensity dropout dynamics witnessed by the system. In a second experiment we adapt a methodology commonly used to evaluated escape phenomena in the theory of large fluctuations to elicit deterministic features shared by many dropouts in an experimental an simulated intensity time series. The optimal path of dropout derived from this analysis demonstrates epochs both before and after the dropout where the system dynamics exhibits a chaotic itinerancy between external cavity lasing modes supported by the system. Finally, we numerically investigate the role of additive noise in the selection of a chaotic instability supported by the semiconductor laser with time-delayed optical feedback for different parameter settings. We find that a single instability is preferred by the system over a larger region of the parameter space as the amplitude of the noise term is increased in the model equations. An experimental characterization of this stability region serves as a sensitive indicator of the amount of Langevin noise relevant in numerically describing stochastic influences present in the evolution of the light dynamics.
Cross-correlations and Entanglement in Cavity QED
(2006-06-30) Terraciano, Matthew Louis; Orozco, Luis A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Every quantum system subjected to measurements is an open quantum system. The cavity QED system is elegant in that it probes the interaction between two quantum systems, the atom and the field, while its loss mechanisms are well understood and can be externally monitored. The study of cross-correlations in cavity QED is important for understanding how entanglement evolves in open quantum systems. As quantum information science grows we need to learn more about entanglement and how it can be quantified and measured. Correlation functions have been used to compare an electromagnetic field (intensity) of one mode with the electromagnetic field (intensity) of the same mode at a later time or different spatial location. In quantum optics, correlation functions have been calculated and measured to probe the nonclassical field that results from the interaction of a single mode of the electromagnetic field and an ensemble of two-level atoms (the canonical cavity QED system). This field can exhibit antibunching, squeezing, and can violate inequalities required for a classical field. Entanglement in the steady state of a cavity QED system cannot be measured directly with traditional correlation functions (Hanbury-Brown and Twiss type experiments). Cross-correlations, however, interrogate directly both modes of the entangled pair, the transmitted (cavity) and the fluorescent (atom) intensities, and can act as an entanglement witness. This thesis presents the implementation of a cross-correlation measurement in a cavity QED system. The work has required the construction of an apparatus that incorporates laser cooling and trapping with quantum optics to carefully control both the external (center of mass motion) and internal (atomic state) degrees of freedom of a collection of atoms that interact with a single mode of a high finesse Fabry-Perot cavity. We examine theoretically and experimentally a new intensity cross-correlation function which probes the evolution of the cavity field conditioned on the detection of a fluorescent photon from an atom in the cavity. The results open the possibility to generalize the dynamics of entanglement as a physical resource necessary for the nascent quantum information science.
Ultra-fast Dynamics of Small Molecules in Strong Fields
(2006-04-24) Zhao, Kun; Hill, Wendell T; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Correlation detection techniques (image labeling, coincidence imaging, and joint variance) are developed with an image spectrometer capable of collecting charges ejected over 4\pi sr and a digital camera synchronized with the laser repetition rate at up to 735 Hz. With these techniques, molecular decay channels ejecting atomic fragments with different momenta (energies) can be isolated; thus the initial molecular configurations (bond lengths and/or bond angles) and orientations as well as their distributions can be extracted. These techniques are applied to study strong-field induced dynamics of diatomic and triatomic molecules. Specific studies included the measurements of the Coulomb explosion energy as a function of bond angle in linear (CO_2) and bent (NO_2) triatomics and the ejection anisotropy relative to the laser polarization axis during Coulomb explosions in both triatomic (CO_2 and NO_2) and diatomic (H_2, N_2 and O_2) systems. The experiments were performed with 100 fs, 800 nm laser pulses focused to 0.1 ~ 5 \times 10^15 W/cm^2. The explosion energy of NO_2 decreases monotonically by more than 25% from the smallest to the largest bond angle. By contrast, the CO_2 explosion energies are nearly independent of bond angle. The enhanced-ionization and static-screening models in two-dimension with three charge centers were developed to simulate the explosion energies as a function of bond angle. The predictions are consistent with the measurements of CO_2 and NO_2. The observed explosion signals as a function of bond angle for both triatomics show large-amplitude vibrations. The ejection angular distributions in triatomic (CO_2 and NO_2) and diatomic (H_2, N_2, and O_2) Coulomb explosions were measured; the contribution made to the ejection anisotropy by dynamic alignment was studied by comparing the images obtained with linearly and circularly polarized fields. Different angular distributions of the molecules are consistent with different ionization stages, induced dipole moments and rotational constants. The dynamic alignment of H_2 is found to be nearly complete. A larger dynamic alignment of CO_2 than that of N_2 or O_2 is consistent with that more electrons have been removed from CO_2 and the precursor molecular ion spends more time in the field prior to the explosion.

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