In simultaneous spatial and temporal focusing (SSTF) a wide bandwidth pulse with transverse spatial chirp is focused, resulting in a pulse that is temporally compressed only near the focal plane. The pulse also has a pulse front tilt angle that depends on the amount of initial transverse chirp. Using an improved design of an asymmetric pulse compressor, we can easily vary the amount of output spatial chirp and thus the pulse front tilt at the focus. We direct this beam into a vacuum chamber and focus it onto an argon gas jet to achieve high harmonic generation (HHG). Since the harmonics are created with a tilted pulse, they emerge from the focus with an amount of angular chirp based on the input spatial chirp and harmonic number. We angularly disperse the harmonics in the direction perpendicular to the spatial chirp with a curved reflective grating, which focuses in the spectral direction onto an x-ray CCD camera. We observe that each of the harmonics possesses angular spatial chirp. To our knowledge, this is the first experimental verification of our earlier published theory of spatially chirped high harmonics. These harmonics are in a sense the Fourier complement to harmonics produced with the Lighthouse Effect. In that case, the attosecond pulse train is angularly dispersed while here each harmonic has angular spectral dispersion. This technique could be used for hyperspectral XUV spectroscopy and, when the beam is refocused, would allow for temporal focusing of the attosecond pulse train.
David Dwight Schmidt, Alex Wilhelm, Randy Lemons, and Charles Durfee, "High harmonic generation of spatially chirped pulses with a variable pulse-front tilt compressor (Conference Presentation)," Proc. SPIE 10908, Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XIX, 1090810 (Presented at SPIE LASE: February 04, 2019; Published: 4 March 2019); https://doi.org/10.1117/12.2508623.6008595935001.
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