Sideband modulation by subcycle interference

We experimentally and theoretically show that the electron energy spectra strongly depend on the relative helicity in highly intense, circularly polarized two-color laser fields, which is an unexpected finding. The employed counter-rotating two-color (CRTC) fields and the corotating two-color (CoRTC) fields are both a superposition of circularly polarized laser pulses at a central wavelength of 390 and 780 nm (intensity ratio ${I}_{390}/{I}_{780}\ensuremath{\approx}250)$. For the CRTC field, the measured electron energy spectrum is dominated by peaks that are spaced by 3.18 eV (corresponds to the photon energy of light at a wavelength of 390 nm). For the CoRTC field, we observe additional energy peaks (sidebands). Using our semiclassical, trajectory-based models, we conclude that the sideband intensity is modulated by a subcycle interference, which sensitively depends on the relative helicity in circularly polarized two-color fields.