  In this work we study the strong confinement effects on the electromagnetic
response of metallic nanoparticles. We calculate the field enhancement factor
for nanospheres of various radii by using optical constants obtained from both
classical and quantum approaches and compare their size-dependent features. To
evaluate the scattered near-field, we solve the electromagnetic wave equation
within a finite element framework. When quantization of electronic states is
considered for the input optical functions, a significant blue-shift in the
resonance of the enhanced field is observed, in contrast to the case in which
functions obtained classically are used. Furthermore, a noticeable
underestimation of the field amplification is found in the calculation based on
a classical dielectric function. Our results are in good agreement with
available experimental reports and provide relevant information on the
cross-over between classical and quantum regime, useful in potentiating
nanoplasmonics applications.
