The phase-resolved imaging of confined light fields
by homodyne detection is a cornerstone of metrology in nanooptics
and photonics, but its application in electron microscopy has
been limited so far. Here, we report the mapping of optical modes
in a waveguide structure by illumination with femtosecond light
pulses in a continuous-beam transmission electron microscope.
Multiphoton photoemission results in a remanent charging pattern
which we image by Lorentz microscopy. The resulting image
contrast is linked to the intensity distribution of the standing light
wave and is quantitatively described within an analytical model.
The robustness of the approach is showcased in a wider parameter
range and more complex sample geometries including micro- and
nanostructures. We discuss further applications of light-interference-
based charging for electron microscopy with in situ optical excitation, laying the foundation for advanced measurement
schemes for the phase-resolved imaging of propagating light fields.