Measurement of laser intensities approaching 1015 W/cm2 with an accuracy of 1%

Accurate knowledge of the intensity of focused ultrashort laser pulses is crucial to the correct interpretation of experimental results in strong-field physics. We have developed a technique to measure laser intensities approaching 10(15) W/cm(2) with an accuracy of 1%. This accuracy is achieved by comparing experimental photoelectron yields from atomic hydrogen with predictions from exact numerical solutions of the three-dimensional time-dependent Schrodinger equation. Our method can be extended to relativistic intensities and to the use of other atomic species.