Methods for the measurement of the duration of high-harmonic pulses

We present two methods for measuring the duration of ultrashort high-harmonic pulses produced by an intense laser pulse propagating in an atomic gas. Both methods rely on the nonlinearity of ionization of atomic gases by optical laser pulses. In the first method, the same medium is used for the dual purpose of generating and measuring the high harmonics. The second method uses atomic photoionization in an intense low-frequency laser field to produce the optical analog of a ``streak camera'' capable of subfemtosecond resolution.