Determination of the carrier-envelope phase of few-cycle laser pulses with terahertz-emission spectroscopy

The availability of few-cycle optical pulses opens a window to physical phenomena occurring on the attosecond timescale. To take full advantage of such pulses, it is crucial to measure and stabilize their carrier-envelope (CE) phase, that is, the phase difference between the carrier wave and the envelope function. We introduce an approach to determine the CE phase by down-conversion of the laser light to the terahertz (THz) frequency range by means of plasma generation in ambient air, an isotropic medium where optical rectification (down-conversion) in the forward direction is only possible if the inversion symmetry is broken by electrical or optical means. We show that few-cycle pulses directly produce a spatial charge asymmetry in the plasma. The asymmetry, associated with THz emission, depends on the CE phase, which allows determination of the phase by measurement of the amplitude and polarity of the THz pulse.