Groups | Seminars || Courses | Outreach

Seminars

Seminars will be held held at room S-141 in the Physics and Astronomy Department building on Mondays at 4:00 PM, unless noted otherwise.

Fall 2017

September 11, 2017 Ruaridh Forbes NRC Ottawa/University College London Strong-Field Probes of Ultrafast Molecular Dynamics (Host: Tom Weinacht)

Strong laser-field based methods such as high-harmonic generation and strong-field ionization (SFI) are considered novel probes of ultrafast molecular dynamics. Details of an experimental femtosecond time-resolved SFI study of the excited state dynamics of NO$_{2}$ using channel-resolved above-threshold ionization (CRATI) as the probe technique will be presented. CRATI makes use of PhotoElectron-PhotoIon COincidence (PEPICO) spectroscopy to study correlations in fragmentation dynamics in molecular systems. The use of PEPICO and covariance methods allows us to correlate ATI photoelectrons associated with a particular SFI electron orbital ionization channel. In disentangling the excited state dynamics of NO$_{2}$, the complex roles of one-photon excitation, multiphoton excitation to higher-lying neutral states, non-adiabatic dynamics and several neutral and ionic dissociation channels are examined. The results will likely have implications for all SFI based time-resolved studies in polyatomic molecules.

September 29, 2017 (Fri 2:30pm) Prof. Doerte Blume The University of Oklahoma Probing the helium dimer and trimer with fast, intense lasers (Host: Dominik Schneble)

Helium is the only element that remains liquid under normal pressure down to zero temperature. Below 2.17K, the bosonic isotope helium-4 undergoes a phase transition to a superfluid. Motivated by this intriguing bulk behavior, the properties of finite-sized helium droplets have been studied extensively over the past 25 years or so. A number of properties of liquid helium-4 droplets are, just as those of nuclei, well described by the liquid drop model. The existence of the extremely fragile helium dimer was proven experimentally in 1994 in diffraction grating experiments. Since then, appreciable effort has gone into creating and characterizing trimers, tetramers and larger clusters. The ground state and excited state of the helium trimer are particularly interesting since these systems are candidates for Efimov states. The existence of Efimov states, which are unique due to scale invariance and an associated limit cycle, was predicted in 1971. However, till recently, Efimov states had -- although their existence had been confirmed experimentally -- not been imaged directly. Recently, ingenious experimental advances that utilize femtosecond lasers made it possible to directly image the static quantum mechanical density distribution of helium dimers and trimers. I will review some of these experiments and related theoretical calculations that led to the experimental detection of the excited helium trimer Efimov state. Extensions to the time domain will also be discussed. Intriguing laser-kick induced dynamics of the fragile helium dimer is observed experimentally and analyzed theoretically. These initial results open the door for future studies that probe scattering length dominated few-body systems using fast, intense lasers.

October 23, 2017 Prof. Jonathan Simon University of Chicago An introduction to topological photonics (Host: Hal Metcalf

Charged particles placed in a magnetic field exhibit unique behaviors resulting from the handedness of the Lorentz force. From chiral dynamics in the non-interacting limit to fractional statistics when the particles are allowed to interact with one another, the quantum Hall effect has introduced topology into physics in unexpected and beautiful ways. In this seminar I'll provide an introduction to the quantum Hall effect of non-interacting and strongly interacting electrons, and then discuss the development of a photonic platform to study analogous physics. In particular, I will describe an exploration of a quantum Hall effect of light using a non-planar optical resonator to create a synthetic magnetic field for photons; from here I will describe innovations that have enabled us to employ Rydberg atoms to mediate interactions between photons, and our plans for combining these two exciting new platforms to the entangled, fractional Hall fluids that result.

October 30, 2017 Prof. Jenny Magnes Vassar College Diffraction Time Series of Live C. elegans (Host: Hal Metcalf)

The locomotion of microorganisms is presently understood through video analysis under a microscope. While effective in many aspects, this method is often time-consuming, computationally heavy and omits subtle components of the motion. Time dependent diffraction signals are a complimentary method that speeds up certain aspects of the data collection and analysis while reducing error. We use C. elegans as a sample species since they are easily maintained and have been the focus of many neuroscience studies. C. elegans are free-living soil-dwelling nematodes that navigate in three dimensions. They move both on a solid matrix like soil or agar in a sinusoidal locomotory pattern called crawling and in liquid in a different pattern called swimming. The roles played by sensory information provided by mechanosensory, chemosensory, and thermosensory cells that govern plastic changes in locomotory patterns and switches in patterns are only beginning to be elucidated. We describe an optical approach to measuring nematode locomotion in three dimensions that does not require a microscope and enables us to begin to explore the complexities of nematode locomotion under different conditions. The experimental method presented in this work allows for the characterization of locomotion and the distinction of phenotypes using Fourier analysis. Even more excitingly, principles of complex and chaotic systems indicate that the locomotion of the wildtype as well as a mutant known as ‘Roller’ is chaotic in nature.

November 6, 2017 Dr. Scott A. Diddams NIST Optical frequency combs: From lab-scale to chip-scale (Host: Tom Allison)

In the past decade we have witnessed significant advances associated with the frequency stabilization of the comb present in the output of a mode-locked femtosecond laser. While proving itself to be fantastically successful in its role as the “gears” of optical atomic clocks, the optical frequency comb has further evolved into a valuable tool for a wide range of applications, including ultraviolet and infrared spectroscopy, frequency synthesis, optical and microwave waveform generation, astronomical spectrograph calibration, and attosecond pulse generation, to name a few. In this talk, I will trace our progress on a few of these applications, and highlight the frequency comb advances that have made them possible. In addition, I will attempt to offer a perspective on the challenges and opportunities for frequency combs that might lie ahead. Along these lines, I will describe a new class of parametric frequency combs that are based on monolithic microresonators. Such microcomb devices are compatible with semiconductor processing and can be further integrated with other photonic and electronic components on a silicon chip. In the future, this technology will bring the precision, flexibility, and measurement power of frequency combs to a wide range of new and emerging applications beyond the confines of the metrology laboratory.

November 13, 2017 Dr. Anthony Cirri Stony Brook University Achieving Surface Sensitivity in Ultrafast XUV Spectroscopy: M2,3-Edge Reflection & Absorption of Transition Metal Oxides (Host: Tom Allison)

Solid-state heterogeneous catalysts serve as the industrial workhorse for facilitating large-scale chemical transformations partially due to their ease in post-synthetic recovery, longevity, and robust preparation methods. However, despite their industrial utility, little is known about how the electronic structure of the surface facilitates such chemical transformations. It is critical to understand the role that the surface plays in driving a reaction in the forward direction, as this will lead to the development of more efficient and chemoselective heterogeneous catalysts. Spectroscopically, this necessitates the use of a light source that is element and electronic state (i.e., oxidation state, spin state, coordination environment) specific, has a probe depth commensurate with the states responsible for catalytic activity, and can easily produce short pulses that are well-matched with the timescale of exciton dynamics. To this end, we have begun to develop ultrafast extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy, where high-harmonic generation is utilized to generate ~50 fs pulses of XUV radiation between 35 and 72 eV – a spectral raange resonant with the core-to-valence M2,3-edge of the first-row transition metals. Since RA spectra not only probe the imaginary part of the refractive index (i.e., absorption), but also the real part, it is paramount to develop theoretical methods to interconvert between transmission-absorption and RA spectra. I will present recent advances we have made in the modeling of XUV-RA spectra for a series of first-row transition metal oxides and demonstrate that RA spectroscopy provides valuable insight into the active state of catalytic metal oxides.

November 17, 2017 (Friday) 3:30pm Prof. Kunal Das Kutztown University Synthetic Gauge Structures and Wilson Loops with Internal and External States (Host: Tom Bergeman)

We propose a new cyclic model that can realize both Abelian and non-Abelian synthetic gauge structures within a single configuration in laser-coupled ultracold atoms, and describe feasible implementations within the ground state manifold of alkali atoms. Continuous variation from U(1) to U(2) gauge group can be achieved by varying the  detuning of the laser fields. We will demonstrate how this model can be transcribed to a ring-shaped lattice to create analogues of gauge structures in real physical space with the external states of trapped ultracold atoms. We provide a mechanism to determine the U(2) Wilson loop and the complete Wilson matrix from the population of the states, and then demonstrate the necessity of minimally three Wilson loops to identify truly non-Abelian scenarios.

2017