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 approximate to 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.