A major concern of High Harmonic Generation (HHG) is the small conversion efficiency at the single emitter level. Thus, ensuring that the emission at different locations are emitted in phase is crucial. At high pump intensities, it is impossible to phase match the radiation without reverting to ordered modulations of either the medium or the pump field itself, a technique known as Quasi-Phase-Matching (QPM). To date, demonstrated QPM techniques of HHG were usually complicated and/or lacked tunability. Here we demonstrate experimentally a relatively simple, highly and easily tunable all-optical QPM technique in which different spatial modes are superposed together to create a pump beam in which either the intensity or the polarization is periodically modulated. With these we demonstrate on-the-fly, tunable QPM of harmonic orders 25 to 41 with up to 40 fold enhancement of the emission at pressures ranging from 15 to 100 Torr.
lilya Lobachinsky, Liran Hareli, Georgiy Shoulga, Yaniv Eliezer, Linor Michaeli, and Alon Bahabad, "On the fly control of high harmonic generation using a structured pump beam (Conference Presentation)," Proc. SPIE 10684, Nonlinear Optics and its Applications 2018, 106840N (Presented at SPIE Photonics Europe: April 24, 2018; Published: 23 May 2018); https://doi.org/10.1117/12.2302804.5788853104001.
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