Terahertz Recollisions

高能物理学家通过质子和电子等基本粒子的碰撞来探索物质的结构。在固体中，电流——就像光伏电池在阳光照射后产生的电流——通常由称为“准粒子”的实体携带。当受到力的作用时，准粒子表现得像粒子，但实际上涉及数千到数百万原子的协调运动。如果将固体中的原子按比例放大到一个人的大小，那么准粒子就会看起来像一个巨大的、满座的体育场中的“波”。PI的小组最近发现了一种加速和碰撞准粒子的方法，大约每秒1万亿次（1太赫兹）。准粒子碰撞的实验特征是彩虹状的光谱，它包含了几十个频率或边带，这些频率或边带像梳子上的齿一样均匀间隔。每个边带都携带着准粒子碰撞速度的信息，以及它们在碰撞前被加速穿过的固体的量子力学特性。在这个项目中，PI的团队将仔细分析来自各种电子材料的边带的强度和极化，以阐明控制准粒子运动的定律，并寻找强驱动物质的新阶段。这项拟议研究的可能应用包括更快、更节能的光通信和互联网，改进全球定位系统所必需的光钟，以及快速、可逆地调整材料特性的能力。该项目将支持两名博士生和几名本科生的培训，他们将学习各种对保持美国在高科技领域的竞争力至关重要的技能。本项目解决了21世纪科学的重大挑战之一——当量子物质远离热平衡时，它是如何表现的。该项目的目标是利用强驱动物质研究中的新机会：(1)开发一种测量固体中能带的贝里曲率的方法，这对理解准粒子的动力学至关重要；(2)阐明电子之间相关性强的材料中准粒子的性质，如高tc超导体的母体化合物；(3)在强时间周期场驱动的材料边缘附近寻找新的量子力学相。为了达到这些目标，该项目将使用PI小组最近发现的高阶边带生成（HSG）。每一种感兴趣的材料都将被近红外激光照射，同时被一个强大的太赫兹频率电场驱动，并分析产生的HSG光谱的强度和极化。实验将与理论紧密结合以达到项目目标。

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.