Video-rate volumetric chemical imaging by photothermal relaxation diffraction tomography

Three-dimensional molecular imaging of living organisms and cells plays a significant role in modern biology. However, current volumetric imaging modalities largely require fluorescent labelling and lack chemical information. Mid-infrared photothermal microscopy offers infrared spectroscopic information with sub-micrometer spatial resolution but lacks imaging speed for living systems. Here, we present a volumetric mid-infrared photothermal phase tomography that senses the 3D refractive index changes induced by mid-infrared photothermal effects. This enables us to achieve 5D (3D space, 1D wavelength, 1D time) label-free imaging at a submicron spatial resolution. Dynamical activities of protein contents and lipid droplets in ovarian cancer cells are visualized.