XUV Supercontinuum Generated by Incommensurate Two-Color Mid-IR Optical Parametric Amplifier

High-order harmonic generation is nowadays a topic of crucial interest since it allows the production of isolated attosecond pulses. Several efforts have been made so far for extending the generation of attosecond bursts beyond the 100 eV spectral region, in order to access to deeper electronic states in atoms and molecules. Recently, mid-infrared laser sources have demonstrated to be ideal candidates for pushing the high harmonic emission up to the soft X-rays. Here we show that it is possible to obtain continuous harmonic spectra, that are the signature of single attosecond pulse generation, by properly mixing two laser pulses, coming from optical parametric amplifiers, with wavelengths of 1.35 and 1.75μm respectively, with either parallel or perpendicular polarizations. Our results demonstrate the generation of isolated attosecond burst driven by mid-infrared laser pulses of a duration larger than 30 fs.