Terahertz Electron Hole Recollisions

非技术摘要：在高能物理学中，通过加速和碰撞电子和质子等基本粒子来探索物质的结构。在凝聚态物理中，基本激发称为准粒子。最常见的准粒子是半导体中的电子和空穴，它们可以通过波长足够短的光在太阳能光伏电池中产生。在这个项目中，电子和空穴将由波长略高于人眼可见波长的弱近红外激光产生，并将在一个每秒振荡近1万亿次（1太赫兹）的强大电场的作用下加速，然后相互碰撞。通过分析发射的近红外光的光谱（哪些波长存在），将研究回收过程。除了产生电子-空穴对的近红外激光波长外，该光谱已被证明包含多达18个独立的近红外波长或边带。这项研究将阐明有多少准粒子可以被加速而不受其宿主材料中的缺陷或原子运动的干扰。这项提议的研究可能会带来更快、更节能的光通信和互联网，并改进全球定位系统中所必需的光学时钟。该项目将支持两名博士生的培训，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。技术摘要：PI课题组最近发现了光与物质相互作用中的一种新现象——高阶边带生成。将频率~350太赫兹的较弱连续近红外（NIR）激光和频率~0.5太赫兹的强激光入射到半导体薄膜上。发射出等距边带梳状结构，边带频率=近红外频率+ 2n太赫兹频率处有清晰的线条，其中n为整数。已观察到在近红外频率以上具有多达14个边带（数量级高达2*14=28）的梳子。高阶边带可以用半经典模型来理解，类似于最初用来解释高阶谐波产生的模型，这是一种发生在强激光场中的原子中的类似现象。在高阶边带生成（HSG）中，近红外激光产生激子，束缚电子-空穴对。强太赫兹场电离激子，并加速产生的电子和空穴进入大幅度振荡。当电子和空穴碰撞时，多余的动能在近红外频率以上的边带中被带走。该项目将探索高阶边带产生的开始，是否存在可观测边带数量的基本限制，边带频谱的形状是否可以控制，以及在圆极化太赫兹场的情况下，近红外辐射的偏振是否被旋转。通过探索HSG的局限性，提出的研究将阐明HSG在从光通信到光时钟等电光技术中的潜在应用。该项目将支持两名博士生的培训，他们将学习各种技能，包括近红外和太赫兹光学、低温学、电子学、计算机编程、机械和光机械设计。

Non-technical abstract:In high-energy physics, the structure of matter is explored by accelerating and colliding elementary particles like electrons and protons. In condensed matter physics, the fundamental excitations are called quasi-particles. The most familiar quasi-particles are electrons and holes in semiconductors, which can be created for example in a solar photovoltaic cell - by light with a sufficiently short wavelength. In this project, electrons and holes will be created by a weak near-infrared laser with a wavelength slightly longer than is visible to the human eye, and will be made to accelerate and then recollide with one another by a very strong electric field oscillating nearly 1 trillion times per second (1 Terahertz). The recollision process will be studied by analyzing the spectrum (which wavelengths are present) in the transmitted near-infrared light. This spectrum has been shown to contain up to 18 separate nearinfrared wavelengths, or sidebands, in addition to the wavelength of the near-infrared laser that creates electron-hole pairs. This research will elucidate how much quasiparticles can be accelerated without being disturbed by defects or the motion of atoms in their host material. The proposed research may lead to faster and more energy efficient optical communications and internet, and improved optical clocks that are necessary in the global positioning system. This project will support the training of two Ph. D. students, who will learn a variety of skills that are critical to preserving U. S. competitiveness in the high-technology sector.Technical abstract:High-order sideband generation, a new phenomenon in the interaction of light with matter, was recently discovered in the PI's research group. A relatively weak, continuous-wave near-infrared (NIR) laser at frequency ~350 THz, and an intense laser at frequency ~0.5 THz are incident on a thin film of semiconductor. A comb of equally-spaced sidebands is emitted, with sharp lines at sideband frequency = NIR frequency + 2n THz frequency, where n is an integer. Combs with up to 14 sidebands (order up to 2*14=28) above NIR frequency have been observed. The high-order sidebands can be understood in terms of a semiclassical model similar to one that was first introduced to explain high-order harmonic generation, an analogous phenomenon that occurs for atoms in intense laser fields. In high-order-sideband generation (HSG), the NIR laser creates excitons, bound electron-hole pairs. The strong THz field ionizes the excitons, and accelerates the resulting electron and hole into a large-amplitude oscillation. When the electron and hole recollide, the excess kinetic energy is carried off in sidebands above the NIR frequency. This project will explore the onset of high-order sideband generation, whether there is a fundamental limit on the number of observable sidebands, whether the shape of the sideband spectrum can be controlled, and whether, in the case of a circularly-polarized terahertz field, the polarization of the near-ir radiation is rotated. By exploring the limits of HSG, the proposed research will elucidate potential applications of HSG to electro-optic technologies ranging from optical communications to optical clocks. This project will support the training of two Ph.D. students, who will learn a variety of skills including near-ir and terahertz optics, cryogenics, electronics, computer programming, and mechanical and optomechanical design.