Laser harmonic generation with tuneable orbital angular momentum and frequency steps using a structured plasma target

High harmonic generation using a laser pulse with circular polarisation (CP) onto a flat or isotropic target is not generally deemed possible. However, when a target with a structured surface is used, a rich spectrum of harmonic frequencies and/or orbital angular momentum (OAM) levels can be unlocked. Building on earlier work [1], we show via numerical simulations how a single CP laser pulse, interacting with both reflective and transmissive structured targets, can produce a harmonic progression with tuneable steps in both frequency and OAM level. We introduce a novel way to analyse the frequency, spin and OAM content of the harmonic radiation which provides enhanced insight into this process. Finally, we demonstrate how a controlled frequency comb can be obtained via the preferential selection of a subset of harmonic modes with a specific OAM value.

[1] R. Trines et al., arXiv:2305.11659 (2023).