1. Department of Physics and FOCUS Center, University of Michigan, Ann Arbor, Michigan 48109, USA
2. Advanced Photon Source, Argonne National Labs, Argonne, Illinois 60439, USA
3. Colby College, Waterville, Maine 04901, USA

Correspondence to: M. F. DeCamp¹ Correspondence and requests for materials should be addressed to M.F.D. (e-mail: Email: mattydee@umich.edu).

Synchrotrons produce continuous trains of closely spaced X-ray pulses. Application of such sources to the study of atomic-scale motion requires efficient modulation of these beams on timescales ranging from nanoseconds to femtoseconds. However, ultrafast X-ray modulators are not generally available. Here we report efficient subnanosecond coherent switching of synchrotron beams by using acoustic pulses in a crystal to modulate the anomalous low-loss transmission of X-ray pulses. The acoustic excitation transfers energy between two X-ray beams in a time shorter than the synchrotron pulse width of about 100 ps. Gigahertz modulation of the diffracted X-rays is also observed. 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 localized transient strain inside an opaque material. We expect that these techniques could be scaled to produce subpicosecond pulses, through laser-generated coherent optical phonon modulation of X-ray diffraction in crystals. Such ultrafast capabilities have been demonstrated thus far only in laser-generated X-ray sources, or through the use of X-ray streak cameras^{1, 2, 3, 4, 5, 6}.