  Optical imaging with mechanical contrast is critical for material and
biological discovery since it allows contactless light-radiation
force-excitation within the sample, as opposed to traditional mechanical
imaging. Whilst optical microscopy based on stimulated Brillouin scattering
(SBS) enables mechanical imaging of materials and living biological systems
with high spectrospatial resolution, its temporal resolution remains limited.
Here, we develop Brillouin gain microscopy (BGM) with a 200-fold higher
temporal resolution by detecting the Brillouin gain at a mechanically
contrasting frequency in the sample with high sensitivity. Using BGM, we
demonstrate mechanical imaging of materials, living organisms and cells at high
spectro-spatiotemporal resolution.
