Taking sub-cycle THz nanoscopy to the limits

We combine ultrafast field-resolved terahertz spectroscopy with scattering-type near-field scanning optical microscopy to access sub-optical-cycle dynamics on the nanoscale. We trace the oscillating electric near field from a (10 nm)
³
volume on the surface of an indium arsenide nanowire with 10 fs temporal resolution, revealing the formation of a depletion layer on the nanowire surface. Furthermore, we apply ultrafast terahertz nanoscopy to strained vanadium dioxide nanobeams and observe a previously unseen correlation between the local steady-state phase-switching susceptibility and the ultrafast response to below threshold photoexcitation. Our findings point towards a nanoscale periodic strain profile that is frozen into the VO
₂
insulating state.