High-order nonlinear optical response of a twisted bilayer graphene

Focusing on the twist angle for the minimal commensurate structure, we perform nonperturbative calculations of electron dynamics in the twisted bilayer graphene (TBG) under intense laser fields. We show that the TBG exhibits enriched high-harmonic generation that cannot occur in monolayer or conventional bilayers. We elucidate the mechanism of these nonlinear responses by analyzing dynamical symmetries, momentum-resolved dynamics, and roles of interlayer coupling. Our results imply nonlinear "optotwistronics," or controlling optical properties of layered materials by artificial twists.Comment: 5 pages, 4 figures + Supplemental Materia

Extremely high-intensity laser interactions with fundamental quantum systems

The field of laser-matter interaction traditionally deals with the response of atoms, molecules and plasmas to an external light wave. However, the recent sustained technological progress is opening up the possibility of employing intense laser radiation to trigger or substantially influence physical processes beyond atomic-physics energy scales. Available optical laser intensities exceeding 10^{22}\;\text{W/cm^2} can push the fundamental light-electron interaction to the extreme limit where radiation-reaction effects dominate the electron dynamics, can shed light on the structure of the quantum vacuum, and can trigger the creation of particles like electrons, muons and pions and their corresponding antiparticles. Also, novel sources of intense coherent high-energy photons and laser-based particle colliders can pave the way to nuclear quantum optics and may even allow for potential discovery of new particles beyond the Standard Model. These are the main topics of the present article, which is devoted to a review of recent investigations on high-energy processes within the realm of relativistic quantum dynamics, quantum electrodynamics, nuclear and particle physics, occurring in extremely intense laser fields.Comment: 58 pages, 26 figures, version accepted by Reviews of Modern Physic

Detecting nonlinear and many-body dynamics in nuclear quantum optics

In this thesis, different measurement and data evaluation approaches for the detection and characterization of collective nuclear level schemes arising in the low-excitation regime of thin-film x-ray cavities are discussed. The first approach uses Fourier transforms to analyze time- and frequency-resolved spectra recorded using nuclear reference absorbers. This allows for the extraction of the phase-resolved nuclear resonant response of the sample under investigation. Next, to study the dynamics of nuclear ensembles upon suitably-shaped x-ray light, a density matrix perturbation theory is presented that allows for the study of multi-level and many-body dynamics in the low-excitation regime of the x-ray-nuclei interaction. This method is used to interpret numerical data simulating several experimental scenarios: First, it is used to derive an equivalence between coherently and incoherently scattered x-ray intensity detectable in nuclear resonant scattering experiments, which serves as a criterion for nonlinear excitation of nuclear ensembles at coherent x-ray sources. Second, signatures of couplings between collective excited nuclear states in thin-film cavities upon differently-shaped x-ray pulses are proposed and identified in time-frequency-spectra. Finally, the feasibility of a specific coherent double pulse spectroscopic method under low-excitation conditions is discussed and numerically simulated spectra upon different double pulse sequences are compared

Ab initio quantum models for thin-film x-ray cavity QED

We develop two ab initio quantum approaches to thin-film x-ray cavity quantum electrodynamics with spectrally narrow x-ray resonances, such as those provided by M\"ossbauer nuclei. The first method is based on a few-mode description of the cavity, and promotes and extends existing phenomenological few-mode models to an ab initio theory. The second approach uses analytically-known Green's functions to model the system. The two approaches not only enable one to ab initio derive the effective few-level scheme representing the cavity and the nuclei in the low-excitation regime, but also provide a direct avenue for studies at higher excitation, involving non-linear or quantum phenomena. The ab initio character of our approaches further enables direct optimizations of the cavity structure and thus of the photonic environment of the nuclei, to tailor the effective quantum optical level scheme towards particular applications. To illustrate the power of the ab initio approaches, we extend the established quantum optical modeling to resonant cavity layers of arbitrary thickness, which is essential to achieve quantitative agreement for cavities used in recent experiments. Further, we consider multi-layer cavities featuring electromagnetically induced transparency, derive their quantum optical few-level systems ab initio, and identify the origin of discrepancies in the modeling found previously using phenomenological approaches as arising from cavity field gradients across the resonant layers.Comment: 41 pages, 20 figures, added clarifications and minor correction

Doctor of Philosophy

dissertationThis work presents the results of various investigations using various techniques of hyperpolarizing the nuclei of atoms. Hyperpolarization implies magnetic order in excess of the thermal order obtained naturally as described by Curie's law. The main portion of this work presents the results of a detailed experimental exploration of predictions arising from a new model of transverse nuclear spin relaxation in quantum systems, based on possible manifestations of microscopic chaos in quantum systems. Experiments have been carried out on a number of hyperpolarized xenon samples, each di ering in its relative percentage of xenon isotopes in order to vary the homonuclear and heteronuclear dipole couplings in the spin system. The experiments were performed under a variety of conditions in an attempt to observe the behaviors predicted by the model. Additionally, much more extensive measurements were made on a number of samples of solid CaF2 in both single crystal and powder forms. These samples, although thermally polarized, were observed with superior signal to noise ratios than even the hyperpolarized xenon solids, allowing for more precise measurements for comparison to the theory. This work thus contains the rst experimental evidence for the majority of the model's predictions. Additionally, this work contains the rst precise measurements of the frequency-shift enhancement parameters for 129Xe and krypton in the presence of spin-polarized Rb. The determination of these important numbers will be useful to many groups who utilize spinexchange optical pumping in their labs. This work built on the prior knowledge of a precise number for the frequency-shift enhancement parameter of 3He in Rb vapor. Finally, I detail work using NMR to detect nuclear-spin polarization enhancement in silicon phosphorus by a novel, photo-induced hyperpolarization technique developed by the Boehme research group at the University of Utah. Signiif cant nuclear polarization enhancements were observed by the Boehme group due to electron-photon interactions in semiconductor soilds; these enhancements were observed by their e ffects on the ambient electrons and measured with electron spin resonance techniques. The work described here details experiments to observe the enhanced nuclear polarization by directly measuring the intensity increase in an NMR measurement