The electromagnetic radiation emitted by an ultrarelativistic accelerated electron is extremely sensitive to the precise shape of the field driving the electron. We show that the angular distribution of the photons emitted by an electron via multiphoton Compton scattering off an intense ($I>{10}^{20}\mathrm{W}/{\mathrm{cm}}^2$), few-cycle laser pulse provides a direct way of determining the carrier-envelope phase of the driving laser field. Our calculations take into account exactly the laser field, include relativistic and quantum effects and are in principle applicable to presently available and future foreseen ultrastrong laser facilities.

• Received 20 January 2010

DOI:https://doi.org/10.1103/PhysRevLett.105.063903