OP: Mechanisms and Phase Matching of Below-Threshold High-Order Harmonic Generation in Solids

该项目的目的是研究固体晶体中的电子对强激光脉冲的基本响应，这导致发射仅持续数百阿秒的高频“谐波”闪光（1阿秒是“纳秒”，或0.00000000000001秒）。在过去的二十年里，人们已经在气态材料中研究了光发射的机制，现在已经相当好地理解了。这种理解使气相谐波广泛采用的时间分辨的电子运动和高分辨率成像应用的研究。然而，在固体中，谐波发射的机制可以从实验中隐藏起来，因为低频激光和高频谐波都必须穿过晶体。在这个项目中，该小组将使用创新的激光技术从发射光的测量中提取固体晶体中的超快电子运动，并利用我们获得的知识开发基于薄膜材料的更高效的谐波光源。研究结果将推进强激光与材料相互作用的科学知识，并将对未来基于激光驱动电流的高速电子设备的发展至关重要。参与该项目的学生将学习先进的光学，电子和计算技术，获得必要的经验，以开发和推进这些来自高科技行业的下一代技术。2011年，通过用中红外飞秒激光脉冲照射块状氧化锌（ZnO）晶体，首次观察到固体晶体的高次谐波发射。从那时起，已经提出了几个模型来描述固态高次谐波产生的电子动力学基础。一个非常成功的模型，已被用来解释实验中的ZnO和其他宽带隙半导体涉及一个半经典的“广义反激”过程类似于气相高次谐波产生：电荷载流子首先通过隧道激发在强激光场。然后，电子和空穴在各自的带中被激光场加速，直到它们重新相遇，重新组合并发射高频光子。该小组将：研究由中红外激光脉冲驱动的宽带隙半导体中高次谐波产生的物理学，目的是对固体中高次谐波产生和相位匹配的机制进行分类;确定广义扩散和其他拟议机制的特征，这些机制将通过高分辨率空间和光谱测量进行测试，使用双色激光场在各种晶体中产生的阶谐波;将注意力集中在阈值以上和阈值以下的谐波上，相应的光子能量大于和小于材料的带隙，因为已经用于描述阈值以上谐波产生的分解图预期对于阈值以下谐波分解;并通过实验研究和模拟，研究强激光脉冲和产生的谐波在晶体中传播的作用，该奖项反映了NSF的法定使命，并通过评估被认为值得支持使用基金会的知识价值和更广泛的影响审查标准。

The purpose of this project is to study the fundamental response of electrons in solid crystals to intense laser pulses, which results in the emission of high-frequency "harmonic" light flashes lasting only a few hundreds of attoseconds (1 attosecond is a "nano-nanosecond", or 0.000000000000000001 seconds). The mechanisms of the light emission have been studied in gaseous materials over the previous two decades and are now fairly well understood. This understanding has enabled widespread adoption of gas-phase harmonics to time-resolved studies of electron motion and high-resolution imaging applications. In solids, however, the mechanisms of harmonic emission can be hidden from experiments, since both the low-frequency laser and the high-frequency harmonics have to travel through the crystal. In this project, the group will use innovative laser techniques to extract the ultrafast electron motion in solid crystals from measurements of the emitted light and use the knowledge we obtain to develop more efficient harmonic light sources based on thin film materials. The results of the research will advance scientific knowledge of intense laser interactions with materials and will be important to future development of high-speed electronic devices based on laser-driven currents. Students involved in the project will learn advanced optical, electronics, and computational techniques, gaining the experience necessary to develop and advance these next-generation technologies from high-tech industries. High-order harmonic emission from solid crystals was first observed in 2011 by irradiating bulk zinc oxide (ZnO) crystals with mid-infrared femtosecond laser pulses. Since then, several models describing the electron dynamics underlying solid-state high-order harmonic generation have been proposed. One highly successful model which has been used to interpret experiments in ZnO and other wide-band gap semiconductors involves a semiclassical "generalized recollision" process similar to gas-phase high-order harmonic generation: charge carriers are first excited via tunneling in the strong laser field. The electrons and holes are then accelerated in their respective bands by the laser field, until they re-encounter one another, recombine, and emit a high-frequency photon. The group will: investigate the physics of high-order harmonic generation in wide bandgap semiconductors driven by mid-infrared laser pulses, with the goals of classifying the mechanisms through which high-order harmonics are generated and phase matched in solids; identify signatures of the generalized recollision and other proposed mechanisms which will be tested through spatially- and spectrally-resolved measurements of the high-order harmonics generated in a variety of crystals using a two-color laser field; focus attention to both the above- and below-threshold harmonics, with respective photon energies larger and smaller than the material's band gap, since the recollision picture which has been used to describe above-threshold harmonic generation is expected to break down for below-threshold harmonics; and investigate the role of propagation of the intense laser pulses and generated harmonics in the crystal, through experimental studies and simulations, with the goal of enhancing the harmonic emission through phase matching.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.