Revealing nanoscale motion under photon limited coherent x-ray diffraction

Time-resolved coherent x-ray diffraction provides a powerful probe of nanoscale dynamics. Yet its extension beyond static imaging is constrained by photon exposure, background scattering, and instability of phase retrieval when individual diffraction frames are weak. Here, we demonstrate an in situ coherent x-ray diffraction approach that detects nanoscale motion even when frame-by-frame image reconstruction becomes unreliable. The method combines a microelectromechanical systems (MEMS) tensile platform, providing periodic mechanical actuation and time-resolved displacement readout, with coherent hard x-ray diffraction at a synchrotron nanoprobe. Using a self-assembled gold nanoparticle superlattice, we perform ptychography to obtain an experimentally calibrated complex probe and a high-resolution object. These serve as a reference for a probe-informed forward model that reproduces realistic diffraction movies under exposure-limited conditions, including photon-shot noise. We show that conventional frame-by-frame coherent diffraction imaging reconstructions are then dominated by reconstruction variability and scattering from nearby device features. To overcome these limitations, we use the measured MEMS displacement signal as an independent temporal reference and apply lock-in demodulation to the diffraction time series. This analysis isolates the periodic scattering response synchronized with the applied mechanical drive, while suppressing the large mean diffraction signal, slow drift, and broadband noise. As a result, nanoscale motion remains detectable as lock-in phase signatures even when individual diffraction frames cannot support stable retrieval of the whole lattice as an image. Our results establish this technique as a robust strategy for accessing periodic, drive-synchronized dynamics in exposure-limited coherent x-ray diffraction experiments, and guide future in situ studies of device-integrated nanomaterials.