Coherent control of pulsed X-ray beams

Synchrotrons produce continuous trains of closely spaced X-ray pulses. Appl ication of such sources to the study of atomic-scale motion requires effici ent modulation of these beams on timescales ranging from nanoseconds to fem toseconds. However, ultrafast X-ray modulators are not generally available. Here we report efficient subnanosecond coherent switching of synchrotron b eams by using acoustic pulses in a crystal to modulate the anomalous low-lo ss transmission of X-ray pulses. The acoustic excitation transfers energy b etween two X-ray beams in a time shorter than the synchrotron pulse width o f about 100 ps. Gigahertz modulation of the diffracted X-rays is also obser ved. We report different geometric arrangements, such as a switch based on the collision of two counter-propagating acoustic pulses: this doubles the X-ray modulation frequency, and also provides a means of observing a locali zed transient strain inside an opaque material. We expect that these techni ques could be scaled to produce subpicosecond pulses, through laser-generat ed coherent optical phonon modulation of X-ray diffraction in crystals. Suc h ultrafast capabilities have been demonstrated thus far only in laser-gene rated X-ray sources, or through the use of X-ray streak cameras(1-6).