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

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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.