Abstract:

Our microscopically designed epitaxial structures for Vertical External Cavity Semiconductor Lasers (VECSELs) have led to significant experimental progress in advancing high-power CW single frequency and ultrafast mode-locked VECSELs. Specific experiments have focused on offset-free GHz mid-IR frequency comb generation and novel GHz sources for multi-contrast biological imaging. Mode-locked VECSELs fill an important gap between MHz fiber and hundreds of GHz micro ring  resonators. I will discuss our success in implementing these sources in the lab including a demonstration of gas sensing and 2D biological imaging. Recent joint theoretical work with Marburg on THz driven extreme NLO in semiconductors and nonequilibrium dynamics in Transition Metal Dichalgonides (TMDCs) will also be reviewed.

Abstract:

The elementary CuO₂ plane sustaining cuprate high temperature superconductivity occurs typically at the base of a periodic array of edge-sharing CuO₅ pyramids. Virtual transitions of electrons between adjacent planar Cu and O atoms, occurring at a rate t/ℏ and across the charge-transfer energy gap E,  generate ‘superexchange’ spin-spin interactions of energy J≈4t4E3 in an antiferromagnetic correlated-insulator state. However, hole doping this CuO₂ plane converts this into a very high temperature superconducting state whose electron-pairing is exceptional. A leading proposal for the mechanism of this intense electron-pairing is that, while hole doping destroys magnetic order it preserves pair-forming superexchange interactions governed by the charge-transfer energy scale E. To explore this hypothesis directly at atomic-scale, we combine single-electron and electron-pair (Josephson) scanning tunneling microscopy to visualize the interplay of E and the electron-pair density n_P in Bi₂Sr₂CaCu₂O_8+x.  The responses of both E and n_P to alterations in the distance δ between planar Cu and apical O atoms are then determined. These data reveal the empirical crux of strongly correlated  superconductivity in CuO₂, the response of the electron-pair condensate to varying the charge transfer energy. Concurrence of predictions from strong-correlation theory for hole-doped charge-transfer insulators with these observations, indicates that charge-transfer superexchange is the electron-pairing mechanism of superconductive Bi₂Sr₂CaCu₂O_8+x.

Abstract:

Recent years have seen tremendous experimental progress in the realm of optically-trapped neutral atoms. In this talk, three theoretical proposals for quantum-state engineering in this type of systems will be presented. It will first be demonstrated that a deterministic conversion of a three-qubit W state into its Greenberger-Horne-Zeilinger counterpart can efficiently be carried out in the Rydberg-blockade regime of neutral-atom systems using a dynamical-symmetry-based approach. It will then be shown that a W-type entanglement can be engineered in arrays of neutral atoms with Rydberg-dressed resonant dipole-dipole interaction. Finally, a time-efficient control scheme for coherent single-atom transport in moving optical lattices (optical conveyor belts and double-well lattices) -- based on the enhanced shortcuts-to-adiabaticity approach -- will be described. 

Abstract: 

Heat transfer is a limiting factor in reliability and performance of next-generation batteries, electronic devices, electric vehicles, and energy generation and conversion systems. Mobile platforms with limited heat dissipation pathways are becoming ubiquitous while requiring integration of dissimilar materials and components with a high density of interfaces and placing additional constraints on device performance. Open challenges exist in optimizing and tuning the thermal transport within these heterogeneous systems, while meeting constraints on mechanical properties and device performance. Ultimately, efficient, thermally informed engineering is needed to translate research into technology and requires integrated modeling, experiments, and materials development. This talk will describe several recent examples from my group of engineering materials from the nano- and micro-structural level to achieve targeted performance objectives. First, it will highlight our work on thermal engineering of materials and design of novel metrology techniques to understand thermal aspects of lithium ion batteries from the granular electrode level to the cell level. Second, we evaluate tuning of thermal transport with mechanical strain or compression at both the nano- and macro- scale to achieve variable thermal transport properties.

Abstract:

In addition to harboring a supermassive black hole at its very core, the Galactic center is one of the most physically extreme environments in the Galaxy. Dense and massive molecular clouds on non-circular orbits are abundant in this region, yet star formation is not as active and frequent as expected. In addition, radio observations have revealed a population of synchrotron-emitting filaments that provide insight on the magnetic field strength and configuration in this unique region of the Galaxy. I will review observational results from several recent studies undertaken by my research group: we have examined the properties and kinematics of a group of unusual molecular clouds that appear to be part of an orbital “stream” of material around the Galactic center. In addition, we have been studying the detailed structure of the synchrotron-emitting radio filaments and their connection to larger-scale energetic outflows from the Galactic center. Our relative proximity to the Galactic center provides an unprecedented view of a galactic core and studies of this region can be used as an astrophysical analog to understanding the nuclei of more distant galaxies.

Abstract:

Observations of human-made satellites using arrays of radio telescopes can provide the ultra-precise determination of their speed and their position on the celestial sphere. The Planetary Radio Interferometry and Doppler Experiment (PRIDE) is a technique that connects ground-based radio astronomy and space science to deliver the sharpest view of spacecraft in our solar system. PRIDE's precise determination of the lateral position of spacecraft can be used for a variety of scientific applications, including improvement of ephemerides, ultra-precise celestial mechanics of planetary systems, gravimetry, spacecraft orbit determination, and fundamental physics. Furthermore, observations of the radio signal transmitted by a spacecraft are an important source of information on interplanetary plasma and solar wind. In this talk, I will present novel results of observing ESA and NASA spacecraft with ground-based radio telescopes and demonstrate the capabilities of such a technique.

Seminar Speaker: 

Abstract: 

Today’s connected world is only possible due to an underpinning technology that has been implemented over the last 30-40 years: optical fibres. Understanding their principles and physical limitations are critical to future proof our communications networks for the next 30-40 years. This includes the need for technologies that enable high-capacity throughput utilising current optical fibres already installed on the ground, borrowing techniques developed for wireless systems, but also exploring new directions with novel optical fibres. In this talk, we will discuss a few fundamental technology leaps that our group has been working on: from maximising the optical fibre bandwidth utilisation with concepts such as Coherent WDM, to opening a new telecommunication window at the 2mm waveband.