Efficient generation of intense spatial and spatiotemporal vortex harmonics using plasma mirrors

Intense spatial or spatiotemporal vortex pulses from the extreme ultraviolet to soft X-ray spectral windows are expected to provide new degrees of freedom for a variety of key applications since they carry longitudinal or transverse orbital angular momentum (OAM), respectively. Plasma-based high harmonic generation driven by a near-infrared spatial or spatiotemporal optical vortex offers a promising route to such novel light sources. However, the energy conversion efficiency from the incident vortex beam to the vortex harmonics is rather low because of the limited driving intensities available in practice. Here, we propose and demonstrate through simulations that by adding a readily available relativistic Gaussian pump beam as a source of energy, the energy conversion efficiency can be increased by several orders of magnitude. In addition, the proposed scheme allows independent control over the frequency and OAM of the vortex harmonics. Furthermore, by tuning the overlapping duration between the Gaussian pump and the vortex seed, an isolated high-intensity attosecond vortex pulse with tunable carrier-envelope-phase can be obtained even though the incident optical vortex has a relatively long duration.