  Using X-ray scattering and the technique of multiple diffreactions, we
revisit the dynamical transition of charge density waves (CDWs) in K0.3MoO3
under applied voltages. In addition to the usual transport and half width (of
Bragg peaks) measurements, we also measure the triplet phase by three-wave
diffraction, which provides, for the first time, the {\em direct} evidence for
the spatial distortions of CDWs. This novel and sensitive technique developed
here can be applied to general periodic media, including stripes in high
temperature superconductors, and provide a new perspective into interesting
phenomena in these materials.
