  Rapid evolutions of microscopic fields govern the majority of elementary
excitations in condensed matter and drive microelectronic currents at
increasing frequencies. Beyond nominal "radio frequencies", however, access to
local electric waveforms remains a challenge. Several imaging schemes resolve
sub-wavelength fields up to multi-Terahertz (THz) frequencies - including
scanning-probe techniques, electro-optic sampling or recent ultrafast electron
microscopy. Yet, various constraints on sample geometries, acquisition speed
and maximum fields limit applications. Here, we introduce ubiquitous far-field
microscopy of ultrafast local electric fields based on drop-cast quantum-dot
probes. Our approach, termed Quantum-probe Field Microscopy (QFIM), combines
fluorescence imaging of visible photons with phase-resolved sampling of
electric fields deeply in the sub-wavelength regime. We capture stroboscopic
movies of localized and propagating ultrafast Terahertz excitations with
sub-picosecond temporal resolution. The scheme employs field-driven modulations
of optical absorption in colloidal quantum-dots via the quantum-confined
Stark-effect, accessible via far-field luminescence. The QFIM approach is
compatible with strong-field sample excitation and sub-micrometer resolution -
introducing a route towards ultrafast field imaging in active nanostructures
during operation.
