  We present a systematic comparison between guided modes supported by slab
waveguides and Bloch Surface Waves (BSWs) propagating at the surface of
truncated periodic multilayers. We show that, contrary to common belief, the
best surface field enhancement achievable for guided modes in a slab waveguide
is comparable to that observed for BSWs. At the same time, we demonstrate that,
if one is interested in maximizing the electromagnetic energy density at a
generic point of a dielectric planar structure, BSWs are often preferable to
modes in which light is confined uniquely by total internal reflection. Since
these results are wavelength independent and have been obtained by considering
a very wide range of refractive indices of the structure constituent materials,
we believe they can prove helpful in the design of future structures for the
control and the enhancement of the light-matter interaction.
