Cascaded frequency conversion of highly charged femtosecond spatiotemporal optical vortices

TL;DR

This paper demonstrates cascaded harmonic generation of spatiotemporal optical vortices with very high topological charges, enabling tunable wavelength and broad applications in advanced light manipulation.

Contribution

It introduces the first experimental realization of cascaded second and third harmonic generation of STOVs achieving topological charges up to 40, expanding the control over complex light fields.

Findings

Achieved unprecedented topological charge l=40 in STOVs.

Demonstrated tunable wavelength by phase singularity control.

Extended conservation principles to nonlinear optical processes.

Abstract

The degrees of freedom inherent in spatiotemporal optical vortices (STOV's) afford intriguing opportunties to manipulate complex light fields for broad applications such as optical communication, light-matter interactions, particle manipulation, quantum optics, and electron acceleration in the relativistic regime. Unlike previous studies examining the second harmonic generation (SHG) of STOV's having an input topological charge (TC) l^{\left(\omega\right)}=1, here we experimentally demonstrate cascaded second and third harmonic generation of STOV's to achieve more diverse wavelength selectivity via sum frequency generation (SFG), achieving unprecedented TC values up to l^{\left(\omega\right)}=40. The large TC values are attributed to second (third) harmonic generation of an incident beam satisfying l^{\left(2\omega\right)}=2l^{\left(\omega\right)} (l^{\left(3\omega\right)}=3l^{\left(\omega\right)}). What is more, the wavelength of the generated STOV field was found to be tunable by controlling the position of the phase singularity in the frequency domain, and optimizing the nonlinear phase matching condition. Our experimental measurements extend the principle of conservation of the spatiotemporal topological charge to general nonlinear optical parametric processes, suggesting a fundamental approach to the production of STOV fields of arbitrarily large TC and at arbitrary visible wavelengths, and beyond (e.g., the ultraviolet range).