Generation and Characterization of Phase-Stable Sub-Single-Cycle Pulses at 3000 cm $^{-1}$

This paper summarizes the generation of sub-single-cycle infrared (IR) pulses through a laser filament and the detailed properties of the generated pulses. The fundamental (ω
₁
) and second harmonic (ω
₂
) of 30-fs Ti:sapphire amplifier output were focused into nitrogen gas and produced phase-stable broadband IR pulses (ω
₀
) by four-wave difference frequency generation (ω
₁
+ ω
₁
- ω
₂
→ ω
₀
) through filamentation. The spectrum spread from 200 to 5500 cm
^-1
, corresponding to ~3.5 octaves. The waveform of the sub-single-cycle pulse was completely characterized with the frequency-resolved optical gating capable of a carrier-envelope phase determination. The pulse duration was measured as 6.9 fs, which was 0.63 times the period of the carrier frequency, 3000 cm
^-1
. The carrier-envelope phase of the IR pulse was passively stabilized, and the instability of the phase was estimated as 154-mrad rms over 2.5 h. We also summarize a particular behavior of the spectral phase of the sub-single-cycle pulse; the phase of high-frequency components of the generated pulses changes continuously and linearly with the relative phase between the two-color input pulses, whereas the phase of the low-frequency components takes only two discrete values. The phase behavior is a unique feature of single-cycle pulses generated with a passive carrier-envelope phase stabilization scheme.