The phonon Bragg switch: a proposal to generate sub-picosecond X-ray pulses

We discuss a method to modulate an X-ray beam that relies on the scattering of hard X-ray photons by a superlattice of optical phonons generated by ul trafast laser pulses incident on a Bragg crystal. Using commercially availa ble laser systems and well-developed techniques for pulse shaping, the crys tal lattice motion can be controlled so that efficient Bragg diffraction is switched on and off in a time comparable to a phonon oscillation period (t ypically, 0.1-1 ps). A detailed proposal is given for the case of GaAs. (C) 1999 Elsevier Science Ltd. All rights reserved.