The Physics Graduate Student Association (PGSA) invites all members of the Physics Department to our annual ice cream social event, to introduce our new graduate students and undergraduate majors and welcome everyone back who has recently joined us or returned for the new academic year.

Dale L Smith,
Department of Physics,
University of Connecticut

Velocity Map Imaging of the Single Ionization of
Molecular Iodine

Single ionization in molecules is a critical first step in many higher order process such as high harmonic generation. However, single ionization remains poorly understood even in simple diatomic systems. We have found that molecular iodine, I2, has a complicated ionization processes which does not originate from valance orbitals, as would be expected, but from more tightly bound inner orbitals comprised of 5s electrons. I will discuss two experiments which use ultrafast lasers to investigate the ionization of diatomic I2. Ultrafast laser pulses have a duration in the tens of femtoseconds (fs). Since the vibrational period of I2 is ≈180 fs, we are able to ionize the molecule before it dissociates. Velocity map imaging allows us to observe the kinetic energy release (KER) of charged fragments from dissociation after single ionization. In the first experiment, we have performed a wavelength study from 800 to 400 nm. We have found in the I + I+ dissociation channel (which we denote as the (1,0)) the KER of the fragments was mostly inconsistent with ionization to the X-, A-, or B-states of I2+ which implies ionization through deeper orbitals. A pump-probe Fourier technique also showed that modulation in the cation products only occurs below 0.2 eV, which is consistent with dissociation through the B-state and ionization of high-lying molecular orbitals. This rules out a two-step process through the X- and A-states for much of the observed KER. Employing intensity-, polarization-, and wavelength-dependent experiments we were able to eliminate bond softening, electron rescattering, and photon mediation through the X- or A-states. In the second experiment, we performed a detailed wavelength analysis of I2 around its one-photon B-state resonance at 530 nm where we have identified a strong enhancement in the (1,0) channel. The resonance enhancement is found in both ionization from outer orbitals through the B-state as well as ionization from inner orbitals. Interestingly, the branching ratio for inner orbital ionization reaches over 98% at 519 nm and the peaks in the branching ratios of the two channels occur at slightly different wavelengths. For double ionization into an excited state of I2+ we find that the branching ratio closely follows the branching ratio of the single ionization of deeply bound inner orbitals. This implies that excitation of molecules comes about through inner orbital ionization. The finding of these experiments are inconsistent with current tunneling ionization theory which postulates that ionization arises through the least bound, outer orbital electrons.

Udaya Raj Dahal,
Department of Physics,
University of Connecticut

Polyethylene Oxide Hydration in Solutions, Nanoconfinement and Nanostructures


Amphiphilic water-soluble polymers are actively used in designing novel nanomaterials and have been the subject of extensive experimental and simulation studies. Polyethylene oxide (PEO) is a water soluble, biocompatible, non-toxic synthetic polymer capable of preventing protein adsorption which is widely used in industry and biomedicine for protein crystallization, control of particle aggregation and drug delivery. Most of the applications of PEO and PEO-based nanoparticles are utilized in an aqueous environment as PEO is highly soluble in water due to its ability to form hydrogen bonds with water and therefore understanding the role of water on the conformation and dynamics of PEO and PEO-based nanomaterials is essential in improving and designing new nanomaterials for a variety of applications.

Using all-atom molecular dynamics simulations, we studied PEO in bulk solutions, under nanoconfinement in carbon nanotubes and in nanostructures (PEO brushes grafted to a planar surface and nanoparticles). In the bulk solution, we find that PEO forms a globule like structure in hexane, coil-like structure in water or benzene and an extended rod-like or helical structure is isobutyric acid. The conformation and mobility of PEO in water and in isobutyric acid is dictated by the hydrogen bonding with PEO. As part of our confinement studies, we found that PEO is spontaneously encapsulated from aqueous solution into carbon nanotubes and forms rod-like, helical and wrapped chain conformation depending on the size of the carbon nanotube. The stable helix inside the carbon-nanotube is a consequence of the stable water arrangement around PEO.

As part of our studies of PEO brushes we investigated PEO grafted to gold nanoparticles and planar gold surfaces at varying grafting densities. We found that PEO hydration in the brush depends on grafting density and for gold nanoparticles also varies with the radial distance and nanoparticle radius of curvature. Our simulation results agree well with the classical scaling theories and we were able to explain the curvature dependent hydration based on bulk-solution PEO behavior.

Di Shu, Department of Physics,
University of Connecticut


Threshold Resonance Effects and the Phase-Amplitude Approach


A fundamental aspect of scattering is the appearance of resonances, which are rather ubiquitous. Although their effect is often lost due to averaging at room temperatures or higher, they can become dominant features at low or ultralow temperatures, where only a few partial waves contribute to the scattering process. Our ability to form and manipulate ultracold molecules provides the seed to study in a precise and controlled fashion the role of single partial waves, and state-to-state processes in chemical systems. We will explore resonances occurring due to the existence of a quasi-bound state in the entrance channel of a scattering system and explain the energy scaling due to these near threshold resonances (NTR) based on the properties of the Jost functions. We will also investigate the threshold resonance effects in the Efimov systems which have been studied in a variety of context, such as three-body Coulomb systems and nuclear three-body systems. Numerical schemes based on Milne's phase-amplitude approach are invented to analyze NTR effects. We will present a simple and practical approach to solve the long-standing problem of finding the non-oscillatory solution to Milne's equation. Our numerical approach also gives an integral representation of scattering phase shift and allows us to compute ultra-narrow shape resonances.

Sahan Handunkanda,
Department of Physics, University of Connecticut

Lattice dynamics studies of negative thermal expansion due to two low-temperature lattice instabilities


A material's tendency to shrink upon heating is known as negative thermal expansion (NTE). A class of materials in which the NTE phenomenon arises from the lattice degrees of freedom has gained much attention over the past couple decades. Simple cubic ScF3 is a prominent candidate of this class of materials as it displays strong, isotropic NTE over a 1000 K temperature range. In addition, no structural phase transition has been reported above 0.4K and it retains the simple cubic structure up to its high melting point of 1800 K, which is unusual compared with other transition metal trifluorides.

Here I present a combined inelastic x-ray scattering (IXS), x-ray diffraction (XRD) and thermal diffuse scattering (TDS) study of ScF3, revealing some exciting features of this material such as clean-limit structural quantum phase transition (SQPT), evidence of essential dimensional reduction, disorder phase diagram and nanoscale correlation of NTE modes. Further investigation of the zone center (ZC) dynamics of ScF3 by using infrared (IR) reflectivity measurement reveals evidence of multi-phonon absorption involving low energy NTE modes resides at Brillouin zone boundary. The IR result motivates us to consider a new approach to study NTE dynamics by populating corresponding modes using optical pump. I then address whether the anomalously strong and thermally persistent NTE behavior of ScF3 is a consequence of the SQPT? We carried out an IXS study of a second system Hg2I2 also tuned near SQPT while retaining stoichiometric composition and high crystallinity. We find similar behavior and significant NTE below 100 K for dimensions along the body-centered tetragonal c axis, bolstering the connection between NTE and zero-temperature structural transitions.

Anees Ahmed, Department of Physics, University of Connecticut


Resurgence and Large N: the Gross-Witten-Wadia model


I present a detailed study of parametric resurgence in the Gross-Witten-Wadia unitary model. I show how trans-series expansions in different sectors transmute into each other as they pass through the large N phase transition. This transition is well-studied in the immediate vicinity of the transition point. Here I present a complementary analysis of the transition at all coupling and all finite N, in terms of a differential equation, using the explicit Tracy-Widom mapping of the Gross-Witten-Wadia partition function to a solution of a Painleve III equation. This mapping provides a simple method to generate trans-series expansions in all parameter regimes. A surprising result is uncovered: the strong coupling expansion is convergent and yet there is a non-perturbative trans-series completion. I also define a uniform large N strong-coupling expansion (a non-linear analogue of uniform WKB), which is much more precise than the conventional large N expansion through the transition region, and apply it to the evaluation of Wilson loops and the Beta function.

Scott Galica, Department of Physics, University of Connecticut


Polychromatic Optical Forces in Diatomic Systems


This dissertation discusses an experiment to demonstrate the application of the stimulated bichromatic optical force (BCF) on the diatomic molecule calcium monofluoride (CaF). The research demonstrates the deflection of a supersonic beam of CaF through the application of BCF under a variety of conditions, highlighting distinctive characteristics of the force. Results show that the measured deflection of the molecular beam is consistent with the behavior of BCF under the tested conditions. In particular, we have measured a predicted reversal in the direction of the force as well as an applied force which exceeds the radiation pressure force. Detailed modeling of the BCF in multilevel systems corroborates our findings. We discuss the development and application of these models with respect to non-ideal conditions present in the experiment. The construction of the molecular beam source and BCF laser systems are discussed in detail. A brief discussion of further modeled extensions of the BCF through the addition of higher order sidebands is presented as well. Overall, our results demonstrate that the BCF is a potentially useful tool for the manipulation of cold molecules.

Fridah Mokaya, Department of Physics, University of Connecticut

High Statistics Analysis of All-Neutral Decays of Vector Mesons with the Radphi Experiment


The differential cross section and spin density matrix elements for ω(782) meson photoproduction in the reaction 9Be(γp, pω)8Li have been measured in the energy range 4.4 - 5.4 GeV using the Radphi detector. Radphi used a 9Be target, and identified the final state by triggering on the recoil proton plus neutrals. At the kinematics of this experiment, the observed nuclear cross section is consistent with the γp cross section measured by prior experiments when multiplied by 4 for the number of protons in the nucleus, in agreement with a naive spectator model. However, the spin observables show some deviation from a spectator model, particularly approaching the low-t limit of the experimental acceptance. Besides the cross sections, spin density matrix elements in Gottfried Jackson and Helicity frame have been measured. The data selection is discussed and the acceptance is examined and quantified. The results are compared to a model for vector photoproduction.

Yiteng Tian, University of Connecticut

Stochastic Tomography: Characterizing Small-scale Heterogeneity in Earth Using Coherence Functions


This thesis work presents a thorough study of stochastic tomography technique, from theory to numerical validation and application in characterizing small-scale heterogeneity in Earth's mantle using a statistical approach. Fluctuations in amplitude and travel time of teleseismic P waves, measured by amplitude and phase coherence beneath elements of EarthScope seismic array, are used to invert for the heterogeneity spectrum of P velocity in a 1000 km thick region of the upper mantle beneath the array. Best fits to joint transverse coherence functions require a depth-dependent heterogeneity spectrum, with peaks in narrow depth ranges that agree well with the predictions for a temperature derivative of velocity that includes the effects of chemical and phase variations expected for standard models of the silicate mineral assemblage of the upper mantle. The results confirm the existence of significant chemical as well as thermal contributions to observed upper mantle heterogeneity at spatial scales between 50 km to 300 km.

Ekaterina Sergan,
Physics Department.
University of Connecticut


Enhanced Harmonic Generation with Ultra-short Laser Pulses


Table top short-pulsed UV lasers are coveted for their usefulness in time-resolved studies of molecular dynamics. However, current wave-mixing and harmonic generation techniques using crystals fail to preserve the short pulse unless the crystal is made thin, thus limiting the conversion efficiency. The alternative is to focus the fundamental beam into a gas, but the phase shift accumulated by a Gaussian beam as it propagates through the focus leads to destructive interference of the generated harmonics. This work presents two methods to alleviate this issue through use of a semi-infinite geometry for third harmonic generation. The first method involves placing a metal septum at the waist such that the laser drills a small pinhole, which in turn disrupts the beam after the waist. The second method uses a very thin septum as a separator for two gases: one with a large third order susceptibility (before the focus), and the other with a small susceptibility (after the focus). Both methods inhibit harmonic generation immediately after the beam waist. Experiments with third harmonic generation lead to increased conversion efficiency and better mode quality, and had the appropriate perturbative behavior. Simulations supported the experimental results and were used to explore limitations on generation. The techniques were extended to fifth harmonic generation. Although an improved spectrum was observed, increased conversion efficiency was not observed in the experiment. Moreover, simulations indicated that fifth harmonic light production is due to wave-mixing, not generation. Finally, simulations with Bessel-like beams are described as an alternative method for future experiments.

Jonathan Hudson,
Physics Department.
University of Connecticut


D-terms in Bosonic and Fermionic Systems


The D-term is a fundamental particle property which is defined through the matrix elements of the energy-momentum tensor and as such in principle on equal footing with mass and spin. Yet the D-term is experimentally not known for any particle. In this thesis the D-terms of bosons and fermions are studied. It is shown that the D-term of a spin-0 boson is D = -1in free theory. This value is modified by even infinitesimally small interactions in Φ^4 theory. This shows how sensitive the D-term is to the dynamics of the theory. It is shown that the property D = -1 is preserved when the boson is not point-like but given a finite size. The situation is fundamentally different for a spin 1/2 fermion, where the D-term vanishes in the free field case. On the example of the bag model it is shown how a fermionic D-term can be generated by interactions.

Xiang Zhang,
Department of Physics,
University of Connecticut

Study of Supercontinuum and High Repetition Rate Short Pulse Generation



The rapidly increasing data traffic nowadays has benefited a lot from the dramatic progress of optical telecommunications such as dense wavelength division multiplexing (DWDM) and time-division multiplexing (TDM), which employ broadband light source and high-repetition ultrashort optical pulse respectively to carry information in optical fibers. High speed all optical data processing and switching components will also be important in future fiber-optic communication systems since conventional electro-optical parts have reached their bottleneck both speed-wise and efficiency-wise. In this PhD thesis work, I propose a dispersion varying scheme realized by non-uniformly tapering fibers or waveguides to enhance supercontinuum generation. The physical mechanism to generate broadband continuum such as dispersion, nonlinearity, and soliton dynamics have been explained. Numerical demonstration has been done in both lead silicate microstructured optical fibers and chalcogenide planar waveguides. A fiber ring laser system with rational harmonic mode-locking and nonlinear polarization rotation of a highly nonlinear photonic crystal fiber has been designed and experimentally demonstrated to generate stable ultrashort optical pulse train at a repetition rate of 30 GHz with low signal noise ratio. All-optical encryption operating at 250 Gb/s using optical Boolean logical gates based on the two-photon absorption (TPA) in bulk semiconductor optical amplifiers (SOAs) has been demonstrated. The effects of TPA on the performance of optical logical gates based on quantum-dot SOAs has also been explored. TPA can improve the operating speed up to 320 Gb/s.