Terahertz Electron Hole Recollisions

非技术摘要：在高能物理中，物质的结构是通过加速和碰撞基本粒子，如电子和质子来探索的。在凝聚态物理中，基本的激发被称为准粒子。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长于人眼可见的微弱近红外激光产生，并将通过每秒振荡近1万亿次(1太赫兹)的非常强的电场来加速，然后相互碰撞。再碰撞过程将通过分析传输的近红外光中的光谱(存在哪些波长)来研究。除了产生电子-空穴对的近红外激光的波长外，这一光谱还被证明包含多达18个独立的近红外波长或边带。这项研究将阐明在不受缺陷或主体材料中原子运动干扰的情况下，有多少准粒子可以被加速。拟议的研究可能会导致更快、更节能的光通信和互联网，并改进全球定位系统所需的光时钟。该项目将支持两名博士生的培训，他们将学习对保持美国在高科技领域的竞争力至关重要的各种技能。技术摘要：高阶边带产生是光与物质相互作用中的一种新现象，最近在PI的研究小组中发现。将频率为~350 THz的较弱的连续近红外(NIR)激光和频率为~0.5 THz的强激光分别入射到半导体薄膜上。发射等间隔的边带的梳子，在边带频率=NIR频率2n太赫兹频率处具有尖锐的线条，其中n是整数。在近红外频率上观察到了高达14个边带(阶数高达2*14=28)的梳状结构。高阶边带可以用一个半经典模型来理解，这个模型最初是为了解释高次谐波的产生而引入的，这种现象类似于原子在强激光场中发生的现象。在高阶边带产生(HSG)中，近红外激光产生激子、束缚电子-空穴对。强烈的太赫兹场电离激子，加速产生的电子和空穴进入大幅度振荡。当电子和空穴重新碰撞时，多余的动能在高于近红外频率的边带中带走。该项目将探索高阶边带产生的开始，可观察到的边带数量是否有基本限制，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发生旋转。通过探索HSG的局限性，这项拟议的研究将阐明HSG在从光通信到光时钟的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、电子学、计算机编程以及机械和光学机械设计。

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.