Ultra-CEP-stable single-cycle pulses at 2.2  µm

Single-cycle optical pulses with controllable carrier-envelope phase (CEP) form the basis to manipulate the nonlinear polarization of matter on a sub-femtosecond time scale. Moreover, nonlinear light–matter energy exchange and frequency conversion processes benefit from longer, infrared wavelengths. We report a highly stable source of 6.9-fs, single-cycle pulses at 2.2 µm, based on a directly diode-pumped Cr:ZnS oscillator with 22.9-MHz repetition rate. Extreme spectral broadening of the oscillator output to a super-octave bandwidth (1.1–3.1 µm) is achieved in a single rutile (TiO 2 ) plate. Excellent agreement with simulations provides a precise understanding of the underlying nonlinear pulse propagation. A comprehensive investigation of alternative broadening materials and additional simulations single out the exceptional broadening in TiO 2 due to the favorable interplay of self-focusing, and plasma formation accompanied by self-phase modulation and self-compression. Unprecedented reproducibility of the single-cycle waveforms is ensured by a unique combination of active CEP stabilization with a residual CEP jitter of only 5.9 mrad (0.1 Hz to 11.45 MHz) and a relative intensity noise of 0.036% (0.1 Hz to 1 MHz). The new single-cycle source permits efficient downconversion to the mid-infrared by cascaded intra-pulse difference frequency generation, giving access to sub-femtosecond manipulation of electric currents in low-bandgap materials with an unprecedented degree of control.