  Strong coupling between vacuum fields and quantum matter occurs at the
nanoscale and broadens the horizon of light-matter interaction. Nanoscale
Casimir force, as an exhibition of vacuum fields, inevitably experiences the
influence of surface electrons due to their quantum character, which are
ignorable in micron Casimir force. Here, we develop a three-dimensional
conformal map method to tackle typical experimental configurations with surface
electron contributions to Casimir force purposely and delicately included.
Based on this method, we reveal that surface electrons can either enhance or
suppress the nanoscale Casimir force, depending on materials and crystal
facets. The mechanism is demonstrated to be the Casimir force softening, which
results from surface electrons effectively altering the distance seen by the
Casimir interaction. Our findings not only highlight the interaction between
surface electrons and vacuum fields but also provide a recipe for theoretical
and experimental investigation of nanoscale fluctuation-type problems.
