High-Order Harmonic Generation in Zinc Oxide Driven by a 1.5-Cycle Mid-Infrared Vortex Beam

Abstract The interaction of twisted light with matter opens up new regimes of light–matter physics. Here, we demonstrate the generation of few-cycle (1.5-cycle) mid-infrared vortex beams with a pulse duration of ≈ 16 fs, characterized in both the temporal and spectral domains. By compensating the temporal dispersion introduced by the spiral phase plate, we preserve the characteristic doughnut-shaped spatial structure of the vortex beam while achieving near-transform-limited pulse durations. We use these ultrashort vortex beams to drive high-order harmonic generation in zinc oxide and investigate whether the orbital angular momentum is preserved in the generated harmonics. We show that harmonic vortices with a well-defined topological charge can still be produced despite the large spectral bandwidth of the driving field, following the linear OAM upscaling rule ℓ q = qℓ 0. However, when the inversion symmetry of the crystal is broken, the combination of the few-cycle driving field and the reduced spectral separation between even-and odd-order harmonics leads to OAM mode overlap and contamination. These results establish practical limits on the generation of pure harmonic vortices with few-cycle driving fields and demonstrate that the interplay between the pulse duration and the crystal symmetry can be exploited to engineer the spatial structure and topological charge of high-harmonic radiation, including the generation of fractional vortex beams.