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Exploring the control and characterization of structured high-order harmonic beams driven by ultrafast laser pulses
Schmidt, David Dwight
Schmidt, David Dwight
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2023
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2025-06-24
Abstract
High-order harmonic generation (HHG) is a nonlinear optical process that converts ultrashort laser pulses from the visible or near-infrared to high laser frequencies that are multiples of the driving frequency. HHG has been a focal point of research not only for its capacity to generate tabletop Extreme Ultraviolet (EUV) light but for the capability to form attosecond pulses.
Our primary objective is to enhance our understanding of the HHG process by demonstrating how the control and structuring of the fundamental laser beam can be leveraged to manipulate and regulate the output harmonics. Unlike conventional perturbative harmonic generation, the HHG process produces a spatial phase on the harmonic beams that depends on the intensity of the fundamental, providing an added complexity along with an extra degree of freedom to manipulate. Given the challenges of creating optics for EUV light, controlling the driving laser provides a beneficial way to influence the output harmonic light. Deterministic control (rather than algorithmic or machine-learning-based optimization) requires a solid understanding of how the phase and amplitude of the input fundamental maps to the harmonics.
In this study, an experimental setup was constructed from the ground up to conduct experiments on the harmonic beams driven by structured fundamental beams. The driving laser and the resulting harmonics were characterized using a scanning ptychography setup, along with direct measurement methods to supplement and validate the system. We utilized this setup to investigate harmonics produced from a variety of structured beams, including vortex beams carrying orbital angular momentum, polarization-structured vector beams, and the generation of high contrast, highly stable interference from Hermite Gaussian beams. We also developed a passive approach for non-collinear mixing of two beams of opposite circular polarization, which can produce sets of circular harmonics that share a common vortex order, enabling true vortex attosecond pulses.
The robust HHG setup established in this work is a significant advancement of the research capabilities at CSM. The research establishes the means to characterize and control properties of high harmonic beams, paving the way for further exploration in the thrilling field of ultrafast lasers and attosecond science.
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