Fabry-Pérot Based Temporal Standard at 8.5 µm for Electro-Optic Delay Tracking

In optical experiments involving variably-delayed ultrashort pulses, the precise calibration of the delay axis is of paramount importance. This is particularly challenging if interferometric delay tracking (with an auxiliary, monochromatic laser [1] ) cannot readily be implemented, such as in the case when the pulses originate from separate modelocked oscillators with detuned repetition frequencies. In the new scheme of electro-optic delay tracking [2] (i) an optical-phase-stable mid-infrared (MIR) waveform is obtained via intrapulse difference frequency generation (IPDFG) driven by an ultrashort near-infrared pulse and (ii) electro-optically sampled (EOS) using a second (independent) near-infrared gate pulse. IPDFG grants (sub-)attosecond lock of the intensity envelope of the driving near-infrared pulse to resulting MIR waveform [3] , while both IPDFG and EOS exhibit (multi-)percent quantum efficiencies [4] being 2 nd -order nonlinear processes. Here, we demonstrate the elongation of a MIR pulse obtained by IPDFG via a narrowband, Fabry-Perot type multilayer optical filter, providing a monochromatic MIR waveform for electro-optic delay tracking which is robust against variations of the initial MIR pulse. The zero crossings of the resulting MIR waveform are reproducible down to a few tens of attoseconds over a time window of 8 ps, which is sufficient for most condensed-phase measurements.