Circularly\nPolarized Attosecond Pulses Enabled by\nan Azimuthal Phase and Polarization Grating

High-harmonic generation (HHG) is an extreme nonlinear\noptical\nprocess that can map the properties of an infrared driving laser beam\nonto short wavelength attosecond pulse trains. However, current techniques\nfor generating circularly polarized high harmonics for probing magnetic\nmaterials and chiral systems have limitations: two-color collinear\ncounter-rotating driving lasers result in a low cutoff photon energy,\nwhile single-color noncollinear counter-rotating schemes suffer from\nlow conversion efficiency. In this work, we generate circularly polarized\nattosecond pulse trains by using a structured laser driver which has\na rotating polarization and phase grating along the azimuthal coordinate.\nOur experimental and numerical results demonstrate the production\nof left and right circularly polarized harmonics, which naturally\nseparate upon propagation. Our approach uses a single laser color\nin a collinear geometry, that can be scaled for high efficiency. Simulations\nshow this scheme can extend into the soft X-ray region when driven\nby mid-infrared driving lasers, while preserving the same high phase-matching\ncutoff photon energy as for linearly polarized high harmonics.