Reduction of laser-intensity-correlated noise in high-harmonic generation

We present a scheme for correcting the spectral fluctuations of high-harmonic radiation. We show that the fluctuations of the extreme-ultraviolet (XUV) spectral power density can be predicted solely by monitoring the generating laser pulses; this method is in contrast with traditional balanced detection used in optical spectroscopy, where a replica of the signal is monitored. Such possibility emerges from a detailed investigation of high-harmonic generation (HHG) noise. We find that in a wide parameter range of the HHG process, the XUV fluctuations are dominated by a spectral blueshift, which is correlated to the near-infrared (NIR) driving laser intensity variation. Numerical simulations support our findings and suggest that non-adiabatic blueshift is the main source of XUV fluctuations. A straightforward post-processing of the XUV spectra allows for noise reduction and improved precision of attosecond transient absorption experiments. The technique is readily transferable to attosecond transient reflectivity and potentially to attosecond photoelectron spectroscopy.