Dr. Thomas Allison,
Stony Brook University


Ultrafast extreme ultraviolet photoemission without space charge


Time- and Angle-resolved photoelectron spectroscopy from surfaces can be used to record the dynamics of electrons and holes in condensed matter on ultrafast time scales. However, ultrafast photoemission experiments using extreme-ultraviolet (XUV) light have previously been limited by either space-charge effects, low photon flux, or limited tuning range. In this article, we describe space-charge-free XUV photoelectron spectroscopy experiments with up to 5 nA of average sample current using a tunable cavity-enhanced high-harmonic source operating at 88 MHz repetition rate. The source delivers \(> 10^{11}\) photons/s in isolated harmonics to the sample over a broad photon energy range from 18 to 37 eV with a spot size of \(58 \times 100 \mu m^2\). From photoelectron spectroscopy data, we place conservative upper limits on the XUV pulse duration and photon energy bandwidth of 93 fs and 65 meV, respectively. The high photocurrent, lack of space charge distortions of the photoelectron spectra, and excellent isolation of individual harmonic orders allow us to observe laser-induced changes to the photoelectron spectrum at the \(10^{-4}\) level in minutes of integration time, enabling time-resolved XUV photoemission experiments in a qualitatively new regime. I will discuss demonstration experiments on Au (111) and progress towards electron dynamics in molecular films.

Dr. Thomas Allison
Stony Brook University


Cavity-enhanced ultrafast spectroscopy: ultrafast meets ultrasensitive


Ultrafast optical spectroscopy methods, such as transient absorption spectroscopy and 2D spectroscopy, are widely used across many disciplines. However, these techniques are typically restricted to optically thick samples, such as solids and liquid solutions. Using a frequency comb laser and optical cavities, we present a technique for performing ultrafast optical spectroscopy with high sensitivity, enabling work in dilute gas-phase molecular beams. Resonantly enhancing the probe pulses, we demonstrate transient absorption measurements with a detection limit of \(\Delta OD = 2 \times 10^{−10}\) (\(1 \times 10^{-9} / \sqrt{Hz}\)). Resonantly enhancing the pump pulses allows us to produce a high excitation fraction at a high repetition rate, so that signals can be recorded from samples with optical densities as low as \(OD ≈ 10^{−8}\), or column densities \( < 10^{10}\) molecules∕cm\(^2\). In this talk, I will discuss initial demonstration experiments, methods for cavity-enhancing multidimensional spectroscopy signals using multiple frequency combs, and progress towards widely tunable cavity-enhanced ultrafast spectrometers operating from the ultraviolet to the infrared.

Jonathan Kwolek,
Department of Physics,
University of Connecticut

Studying Charge Exchange in a Hybrid Ion-neutral Trap


Interactions between neutral and ionized atoms are medium-range, and the spiraling nature of these collisions can lead to long interaction times and large cross sections. The field of ion-neutral interactions is bolstered by methods from atomic physics, which allow us to accurately control and determine the quantum-states of the reactants. Atomic physics opens the door to studying these interactions over a large range of energies, in order to observe interesting effects from the ultracold to hot temperature regimes.

The hybrid-trap enables the concentric trapping of a sample of cold atoms and ions. The dissertation discusses our exploration into the controlled reactions in the Na+Ca^+ system, including measurements of sympathetic cooling and a charge-exchange rate which changes as a function of energy and atomic state. The quantum-state distributions of Na in our magneto-optical trap is also explored, as well as its agreement and subsequent deviation based on a predictive model. The measurements of reaction rates indicate a reaction-energy threshold in two different reaction channels, which can be corroborated by theoretical investigation.

Dr. Daniel Angles-Alcazar,
Center for Computational Astrophysics, Flatiron Institute

The dual role of stellar feedback regulating galaxy evolution and the growth of supermassive black holes


Feedback from massive stars, including supernovae, stellar winds, and radiation, is believed to play a central role in galaxy evolution. In this talk, I will present recent progress in understanding the implications of stellar feedback using cosmological hydrodynamic simulations from the Feedback In Realistic Environments (FIRE) project. In these simulations, stellar feedback regulates star formation locally in galaxies while driving large-scale winds that connect galaxies with their surrounding circumgalactic medium. I will discuss the efficiency of winds evacuating gas from galaxies, the importance of wind re-accretion to galaxy mass assembly, and the surprising contribution of intergalactic transfer of material between galaxies via winds. Zooming into galactic nuclei, I will show that stellar feedback regulates the early growth of the central black hole in low mass galaxies by continuously evacuating the nuclear gas reservoir, which has important implications for the co-evolution of supermassive black holes and galaxies and the frequency and mass scale of massive black hole mergers.

Dr. Nora Kling
Department of Physics,
University of Connecticut

Ultrafast Nuclear Motion: Resolving Hydrogen Migration(s) in Time

Intramolecular hydrogen migration is a natural process that plays a crucial role in many biological, chemical, and physical phenomena, and is the object of current research efforts in fuel cells, proteomics, and combustion chemistry. Using ultrafast, intense lasers and sophisticated pump-probe ion imaging techniques, we can explore the rich dynamics of hydrogen migration reactions, including what timescale they occur on. In my talk, I will present two studies. The first study is on acetylene, a small hydrocarbon, where a hydrogen can migrate across the carbon atoms to form its vinylidene isomer [1,2]. The second study is on ethanol, a complex molecule where both single and double hydrogen migrations occur, and the timescale for each is individually determined [3]. These experimental results, in combination with state-of-the art quantum chemical trajectory calculations, give us new insight into the complex nuclear motions in complex polyatomic molecules.



[1] C. Burger et al. Faraday Discussions 194, 495 (2016).

[2] M. Kübel et al. Phys. Rev. Lett. 116, 193001 (2016).

[3] N. G. Kling et al. submitted for publication

Dr. David Murphy, MIT

Matrix Elements for Neutrinoless Double Beta Decay from Lattice QCD

While neutrino oscillation experiments have demonstrated that neutrinos have small, nonzero masses, much remains unknown about their properties and decay modes. One potential decay mode --- neutrinoless double beta decay (\(0 \nu \beta \beta\)) --- is a particularly interesting target of experimental searches, since its observation would imply both the violation of lepton number conservation in nature as well as the existence of at least one Majorana neutrino, in addition to giving further constraints on the neutrino masses and mixing angles. Relating experimental constraints on \(0 \nu \beta \beta\) decay rates to the neutrino masses, however, requires theoretical input in the form of non-perturbative nuclear matrix elements which remain difficult to calculate reliably. In this talk we will discuss progress towards first-principles calculations of relevant nuclear matrix elements using lattice QCD and effective field theory techniques, assuming neutrinoless double beta decay mediated by a light Majorana neutrino.

Professor Nadia Fomin, University of Tennessee

Life and Death of the Free Neutron

Modern neutron sources provide extraordinary opportunities to study a wide variety of physics topics, including the physical system of the neutron itself. One of the processes under the microscope, neutron beta decay, is an archetype for all semi-leptonic charged-current weak processes. Precise measurements of the correlation parameters in neutron beta decay as well as the neutron lifetime itself are required for tests of the Standard Model and for searches of new physics. The state of the field will be presented and a program of current and future experiments and potential impacts explored.

Dr. Turgut Yilmaz, Department of Physics, University of Connecticut

Topological insulators and their interaction with magnetic impurities and superconductors

Topological properties of materials have recently attracted much attention in the condensed matter physics community due, in part to their possible applications in quantum electronic devices. In particular, topological insulators (TIs) that possess gapless surface states that are topologically protected and can lead to dissipationless quantum electronics. TI's surface state exhibit relativistic momentum dispersion with the Dirac point (DP) located at the time reversal invariant momenta point while the bulk bands are inverted and gapped [1]. Currently, two majority experimental efforts have concentrated on inducing the magnetic and superconducting properties in TI, which would lead to the experimental realizations of quantum anomalous Hall effect (QAHE) and Majorana fermions, respectively [2-3].

Magnetism can be induced in TIs by incorporating impurities into the bulk or on the surface of TIs. If there is a net out-of-plane magnetic moment, an energy gap opens at the DP due to broken time reversal symmetry, which would give rise to QAHE. On the other hand, the superconductivity in can be induced in TI through the proximity effect by growing TIs on the surface of a superconductor. Both, the gapped DP and the superconductivity can be probed by angle-resolve photoemission spectroscopy (ARPES). In this talk, I will present our photoemission studies on the search of broken time reversal symmetry in magnetic TIs and proximity induced superconductivity in TI/superconductor heterostructures. Also, the experimental setup to grow and study TIs at the Physics Department of the University of Connecticut will be presented.

[1] L. Fu, C. L. Kane, and E. J. Mele, Phys. Rev. Lett. 98, 106803 (2007).

[2] X.-L. Qi and S.-C. Zhang, Rev. Mod. Phys. 83, 1057 (2011).

[3] T. Yilmaz, et al., Appl. Surf. Sci. 407, 371 (2017).