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电子对强激光脉冲的基本响应，这种脉冲导致发射出仅持续数百阿秒的高频“谐波”光闪烁(1阿秒是“纳纳秒”，即0.000000000000000001秒)。在过去的二十年里，人们已经在气体材料中研究了光发射的机制，现在已经相当清楚了。这种理解使得气相谐波被广泛地应用于电子运动的时间分辨研究和高分辨率成像应用。然而，在固体中，由于低频激光和高频谐波都必须通过晶体，因此可以从实验中隐藏谐波发射的机制。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更高效的基于薄膜材料的谐波光源。这项研究的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器件的发展具有重要意义。参与该项目的学生将学习先进的光学、电子和计算技术，获得从高科技行业开发和推进这些下一代技术所需的经验。2011年，用中红外飞秒激光脉冲照射块状氧化锌晶体，首次观察到固体晶体的高次谐波发射。从那时起，人们提出了几个描述固体高次谐波产生背后的电子动力学的模型。一个非常成功的模型已经被用来解释在氧化锌和其他宽带隙半导体中的实验，它涉及一个类似于气相高次谐波产生的半经典的“广义再碰撞”过程：电荷载流子首先通过在强激光场中的隧穿被激发。然后，电子和空穴在各自的频带内被激光场加速，直到它们重新相遇，重新组合，并发射出高频光子。该小组将：研究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，目的是对固体中产生高次谐波和进行相位匹配的机制进行分类；确定广义再碰撞和其他拟议机制的特征，这些机制将通过使用双色激光场对各种晶体中产生的高次谐波进行空间和光谱分辨测量来进行测试；重点关注高于阈值和低于阈值的谐波，其各自的光子能量大于和小于材料的带隙，因为用于描述阈值以上的谐波产生的重碰撞图预计将被分解为阈值以下的谐波；并通过实验研究和模拟来研究强激光脉冲和产生的谐波在晶体中的传播的作用，目的是通过相位匹配来增强谐波发射。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.