Mr. Alex Wilhelm Profile

Alex Wilhelm

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Proceedings Article | 13 March 2019 Presentation

Simultaneous spatial and temporal focusing of vortex and vector beams (Conference Presentation)

Alex Wilhelm, David Schmidt, Chloe Keefer, Randy Lemons, Charles Durfee

Proceedings Volume 10904, 109041J (2019) https://doi.org/10.1117/12.2510745

KEYWORDS: Superposition, Beam analyzers, Beam propagation method, Gaussian beams, Ultrafast phenomena, Spiral phase plates, Polarization

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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. In this work, we explore computationally and experimentally the properties of SSTF vortex and vector beams. To analyze the beam propagation, we build on the concept that a spatially chirped beam is a superposition of Gaussian beams with a position or angle that depends on frequency. We extend this to superpositions of Hermite-Gauss high-order modes to describe the singular beams. At focus, the beams of the ultrashort pulses are tilted versions of the familiar doughnut beams. Away from focus, however, where the spectral components do not fully overlap, we find that vortex and vector beams result in strikingly different mapping of the singularity mapping in the spatio-temporal domain. The use of higher-order modes increases the focal spot size without reducing the already short SSTF depth of focus. Experimentally, we use spiral phase plates to produce vortex beams and a linear to radial polarization converter for the vector beams. The vector beam is not distorted by the polarization-insensitive transmission gratings. The spatial chirp compressor is improved over our previous work to vary the chirp positive and negative. The sensitivity of the singular beam focus to grating misalignment can actually be used to optimize the compressor alignment.

Proceedings Article | 4 March 2019 Presentation

High harmonic generation of spatially chirped pulses with a variable pulse-front tilt compressor (Conference Presentation)

David Dwight Schmidt, Alex Wilhelm, Randy Lemons, Charles Durfee

Proceedings Volume 10908, 1090810 (2019) https://doi.org/10.1117/12.2508623

KEYWORDS: High harmonic generation, Argon, Reflectivity, X-rays, CCD cameras, Extreme ultraviolet, Spectroscopy

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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.

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