Control of spectral chirality in high-harmonic generation using two-color bicircular laser fields

We have extended the two-color bicircular approach for generating chiral XUV radiation via high-harmonic generation by combining femtosecond driving laser fields with 1:3 frequency ratio of wavelengths 1200 and 400 nm. With optimized driving-field intensities, we produced highly chiral spectra in which the vast majority of harmonics sharing the same helicity constitute the vast majority of spectral content. We support these experimental finding with a numerically comprehensive analytical model that describes the propensity (selection) rules for emission of circularly polarized high harmonics. We identify and confirm the rules responsible for the contrast between the allowed odd harmonics of orders 4 q + 1 and 4 q − 1 ( q an integer), which posses opposite helicities, as well as the complete prohibition of even-order harmonics in the high-harmonic spectra. We further apply these rules to explain the spectral shape and ellipticity of the emitted odd harmonics as functions of both the intensity and the ellipticity of the driving-field components. Using XUV polarimetry, we retrieved full Stokes vector, confirming achieved ellipticities higher than 0.9.