Determination of the phase of terahertz few-cycle laser pulses.

We propose an approach to determine the carrier-envelope phase (CEP) of a terahertz few-cycle pulse by observing the field-free molecular orientation. We find that the degree of orientation sensitively depends on the CEP, providing a new route for measuring the CEP without phase ambiguity. By taking advantage of the field-free molecular orientation, an important effect of the CEP drift caused by the dephasing of the generating medium on the accurate measurement of the CEP value is eliminated.