Terahertz Recollisions

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Nontechnical AbstractHigh-energy physicists explore the structure of matter by colliding elementary particles like protons and electrons. In solids, currents--like those generated in a photovoltaic cell after illumination by sunlight--are usually carried by entities called "quasi-particles." When acted on by a force, quasi-particles behave like particles, but actually involve the co-ordinated motions of thousands to millions of atoms. If an atom in a solid were scaled up to be the size of a person, then a quasi-particle would look something like "the wave" in a large, full stadium. The PI's group has recently discovered a method to accelerate and collide quasi-particles about 1 trillion times per second (1 Terahertz). The experimental signature of quasi-particle collisions is a rainbow-like spectrum of light that contains dozens of frequencies, or sidebands, that are equally spaced like the teeth on a comb. Each sideband, carries information about the speed with which the quasiparticles have collided, and the quantum-mechanical properties of the solid through which they have been accelerated before colliding. In this project, the PI's group will carefully analyze both the intensities and polarizations of the sidebands from a variety of electronic materials in order to elucidate the laws that govern the motion of quasiparticles and to search for new phases of strongly-driven matter. Possible applications of the proposed research include faster and more energy efficient optical communications and internet, improved optical clocks that are necessary in the global positioning system, and the ability to rapidly and reversibly tune the properties of materials. This project will support the training of two Ph. D. students and several undergraduates, who will learn a variety of skills that are critical to preserving U. S. competitiveness in the high-technology sector.Technical AbstractThis project addresses one of the grand challenges of 21st century science--how does quantum matter behave when it is driven very far from thermal equilibrium. The goals of this project are to take advantage of new opportunities in the study of strongly-driven matter to (1) develop a method of measuring the Berry curvature of bands in solids, which is critical to understanding the dynamics of quasiparticles; (2) elucidate the nature of quasiparticles in materials in which correlations between electrons are strong, like the parent compounds of high-Tc superconductors; and (3) search for new quantum-mechanical phases near the edges of materials that are driven by strong, time-periodic fields. In order to reach these goals, this project will use the recent discovery of high-order sideband generation (HSG) by the PI's group. Each of the materials of interest will be illuminated by a NIR laser while it is being driven by a strong THz-frequency electric field, and the intensities and polarizations of the resulting HSG spectra will be analyzed. Experiments will be closely coupled with theory to reach the project goals.