Phase stabilization of an attosecond beamline combining two IR colors.

We present a phase-stabilized attosecond pump-probe beamline involving two separate infrared wavelengths for high-harmonic generation (HHG) and pump or probe. The output of a Ti:sapphire laser is partly used to generate attosecond pulses via HHG and partly to pump an optical parametric amplifier (OPA) that converts the primary Ti:sapphire radiation to a longer wavelength. The attosecond pulse and down-converted infrared are recombined after a more than 20-m-long Mach-Zehnder interferometer that spans across two laboratories and separate optical tables. We demonstrate a technique for active stabilization of the relative phase of the pump and probe to within 450 as rms, without the need for an auxiliary continuous wave (cw) laser. The long-term stability of our system is demonstrated with an attosecond photoelectron streaking experiment. While the technique has been shown for one specific OPA output wavelength (1560 nm), it should also be applicable to other OPA output wavelengths. Our setup design permits tuning of the OPA wavelength independently from the attosecond pulse generation. This approach yields new possibilities for studying the wavelength-dependence of field-driven attosecond electron dynamics in various systems.