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

Abstract : The flourishing of attosecond science (1 as = 10−18s) has created a need to control exquisitely the delay between two ultrashort light pulses, where one of them is intense and in the visible spectral range, while the other is weak and in the extreme-ultraviolet spectral range. Here we introduce a robust technique, named LIZARD (Laser-dressed IoniZation for the Adjustment of the pump-pRobe Delay), allowing active stabilization of this pump-probe delay. The important feature of the method is the use of an error signal calculated from a two-photon photoelectron signal obtained by photoionizing a gas target in an electron spectrometer with the two superimposed beams. The modulation of sidebands in phase quadrature allows us to perform an in situ measurement of the pump-probe phase and to compensate for fluctuations with a uniform noise sensitivity over a large range of delay. Despite an interferometer length of several meters, we achieve a long-term stability of 28 as rms over hours. This method could be applied to the stabilization of other types of two-color interferometer, provided that the required phase-shifted photoionization signals can be obtained.