In Situ Sub-50-Attosecond Active Stabilization of the Delay Between Infrared and Extreme-Ultraviolet Light Pulses

The blooming of attosecond science (1 as = $10^{-18}$ s) has raised the need to exquisitely control the delay between two ultrashort light pulses, one of them being intense and in the visible spectral range, while the second is weak and in the Extreme Ultra-Violet spectral range. Here we introduce a robust technique, named LIZARD (Laser-dressed IoniZation for the Adjustment of the pump-pRobe Delay), allowing an active stabilization of this pump-probe delay. The originality of the method lies in an error signal calculated from a two-photon photoelectron signal obtained by photoionizing a gas target in an electronic spectrometer with the two superimposed beams. The modulation of sidebands in phase quadrature allows us to perform an \textit{in situ} measurement of the pump-probe phase, and to compensate for fluctuations with an uniform noise sensitivity over a large range of delays. Despite an interferometer length of several meters, we achieved a long term stability of 28 as RMS over hours. This method could be applied to the stabilization of other types of two-color interferometers, provided that one of the propagating beams is capable of photoionizing a target.