Fig. 1: Chiral high-harmonic generation in solids.

a Two-color laser pulses (frequencies marked ω and 2ω) with perpendicular polarization and adjustable phase delay can be converted to circularly-polarized fields with opposite helicity using a dichroic quarter-wave plate with the fast-optical axis set to 45° relative to the laser polarization. The superposition yields a bi-circular field with a threefold rotation symmetry. The ellipticity and the orientation of the field (see angle θ in (b)) depend on the quarter-wave plate rotation and the phase delay between the two spectral field components, respectively. The emitted harmonic radiation is collected with an extreme-ultraviolet spectrometer. LHC left-handed circular, RHC right-handed circular. b Schematic depiction of the symmetry and chirality probing principle using a bi-circular field for HHG in solids. c High-harmonic spectra from quartz(0001) (harmonic orders label with H5–H10), showing suppression of every third harmonic order. The blue and red arrows indicate the helicity of the harmonics. d Spectrogram for quartz(0001) as a function of the threefold field rotation angle θ (see also pictograms), exhibiting a threefold beating.