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Title: Harnessing Quantum Light Science for Tabletop X-Ray Lasers, with Applications in Nanoscience and Nanotechnology
Abstract: Ever since the invention of the laser over 50 years ago, scientists have been striving to create an X-ray version of the laser. The X-ray sources we currently use in medicine, security screening, and science are in essence the same X-ray light bulb source that Röntgen used in 1895. In the same way that visible lasers can concentrate light energy far better than a light bulb, a directed beam of X-rays would have many useful applications in science and technology. The problem was that until recently, we needed ridiculously high power levels to make an x-ray laser. To make a practical, tabletop-scale, X-ray laser source required taking a very different approach that involves transforming a beam of light from a visible femtosecond laser into a beam of directed X-rays. The story behind how this happened is surprising and beautiful, highlighting how powerful our ability is to manipulate nature at a quantum level. Along the way, we also learned to generate the shortest strobe light in existence - fast enough to capture the fastest attosecond electron dynamics in materials. We also learned how to achieve sub-wavelength spatial resolution at soft X-ray wavelengths for the first time. These new capabilities are already impacting nano and materials science, as well as showing promise for next-generation electronics, data and energy storage devices.
Reference: J. Miao, T. Ishikawa, I. K. Robinson, and M. M. Murnane, “Beyond Crystallography: Diffractive Imaging with Coherent X-ray Sources,” Science 348, 530 (2015).
Title: High harmonic generation in solids and gases
Abstract: High harmonic generation (HHG) in transparent crystals has attracted much attention recently given its potential as a bright, compact, and easily controllable source of extreme utraviolet (XUV) radiation. Much effort has been devoted to the conceptual understanding of the HHG process in solids, and, interestingly, the dominant generation mechanism has been shown to be substantially different from that of HHG in gases.
In this talk, I will give an overview of the similarities and differences between HHG in solids and gases, in terms of experimental and theoretical findings from the last several years. In particular, I will discuss how HHG in solids can be described as a three-step process in momentum space that involves tunneling from the valence to the conduction band(s), acceleration on one or multiple conduction bands, and radiation via coherence with the valence band. I will also show that for any given crystal this dynamics can be described using a multi-level system that originates as the Gamma-point band structure of that crystal. I will discuss how this points to a close connection between the band structure and a number of properties of the harmonic radiation such as: (i) the cutoff energy and yield of the often multiple plateaus that can be observed in the harmonic spectrum], (ii) the dependence of the harmonic yield on the relative orientation of the crystal and the laser polarization, and (iii) the sub-cycle time structure of the microscopic and macroscopic harmonic radiation. When possible, I will discuss the comparison of our predictions to recent experimental results.
Reference: G. Ndabashimiye, S. Ghimire, M. Wu, D. A. Browne, K. J. Schafer, M. B. Gaarde, and D. Reis, “Solid-state harmonics beyond the atomic limit,” Nature 534, 520 (2016).
Title: Quantum entanglement and quantum computing in the optical frequency comb
Abstract: An ultrafast laser emits over a vastly multimode gain spectral bandwidth — an optical frequency comb, or OFC — but the emission happens but one photon at a time, albeit in a stimulated manner. When one changes the gain medium from linear (one-photon) to nonlinear (two-photon), the laser becomes a two-photon laser (if the pump excites the gain medium) or an optical parametric oscillator (if it doesn’t) and two-photon emission leads to massive multipartite entanglement of the OFC modes, which has been demonstrated experimentally. I will explain how this entanglement can be harnessed and leveraged toward building a universal quantum computer.
Reference: M. Chen, N. C. Menicucci, and O. Pfister, “Experimental realization of multipartite entanglement of 60 modes of a quantum optical frequency comb,” Physical Review Letters 112, 120505 (2014).
Title: Attosecond studies of electronic concerted motion: an ab initio perspective
Abstract: Autoionizing states are a pervasive aspect of atomic and molecular ionization and a distinct signature of electronic correlation. Only with the advent of attosecond sources, however, did it become possible to resolve the evolution of autoionizing states in real time, and to study how they are affected, and can be controlled, by light. Ab initio studies are key to intepret many current attosecond pump-probe experiments. Time-dependent close coupling (TDCC) has affirmed itself as the leading tool to simulate the ionization of atoms under the influence of ultrashort pulses, beyond the single-active-electron approximation. In this talk I will illustrate through examples how TDCC can reproduce the decay dynamics of rare-gas atoms excited by sequences of sub-femtosecond light pulses in realistic experimental conditions. These studies shed light on the concerted motion of electrons in transiently bound states, as well as on the elusive delay with which photoelectrons eventually part from their confined dancing companions.
Reference: C. Ott et al., “Reconstruction and control of a time-dependent two-electron wave packet,” Nature 516, 374 (2014).