Determining the absolute carrier phase of a few-cycle laser pulse

In a strong laser field, electrons tunnel from an atom at a rate determined by the instantaneous field. If the pulse is only a few cycles in duration, the highly nonlinear nature of tunnel ionization ensures that the resultan t electron wave packet is primarily formed in less than one period. Measuri ng the direction of above-threshold-ionization electrons produced by circul arly polarized light provides a direct method of measuring the absolute car rier phase of a single pulse. The method is robust, surviving spatial and t emporal integration as well as intensity fluctuations. (C) 2000 Optical Soc iety of America.