  Light-engineering of quantum materials via electromagnetic dressing is
considered an on-demand approach for tailoring electronic band dispersions and
even inducing topological phase transitions. For probing such dressed bands,
photoemission spectroscopy is an ideal tool, and we employ here a novel
experiment based on ultrafast photoemission momentum microscopy. Using this
setup, we measure the in-plane momentum-dependent intensity fingerprints of the
electromagnetically-dressed sidebands from a Au(111) surface for s- and
p-polarized infrared driving. We find that at metal surfaces, due to screening
of the driving laser, the contribution from Floquet-Bloch bands is negligible,
and the dressed bands are dominated by the laser-assisted photoelectric effect.
Also, we find that in contrast to general expectations, s-polarized light can
dress free-electron states at large photoelectron momenta. Our results show
that the dielectric response of the material must carefully be taken into
account when using photoemission for the identification of light-engineered
electronic band structures.
