Electromagnetic field enhancement in Bloch surface waves
D. Aurelio and M. Liscidini
University of Pavia, department of Physics, via A. Bassi 6, Pavia (PV), 27100, Italy
In the last decade we have witnessed a growing interest in Bloch surface waves (BSWs), which are electromagnetic modes that propagate at the interface between a truncated periodic multilayer and a dielectric external medium. Light confinement in BSWs occurs near the multilayer surface and is caused by TIR from the homogeneous layer and by the presence of a photonic band gap (PBG) from the multilayer. Although these modes have been known since the late seventies , this renewed interest is due to the improvement of those fabrication and growing techniques that today make high-quality multilayers available for a vast class of materials, from semiconductors to oxides and organic compounds. BSWs have been used in all those situations that require light to be confined near the surface of a device, e.g. optical surface sensors  and control of light emission [3, 4].
Many works take for granted that BSWs have a strategic advantage in terms of surface field enhancement over simpler solutions such as guided modes in dielectric slab waveguides . Similarly, guided modes in dielectric slabs are often assumed to have smaller modal length than BSWs, thus making dielectric slabs the first choice to achieve the largest field enhancement in planar structures. Here we show that both these assumptions are incorrect by comparing BSWs supported by truncated periodic multilayers with the fundamental mode of asymmetric slab waveguides in terms of electric field at the surface and modal length.
We show that – somewhat surprisingly – in most of the cases, the largest field enhancement (i.e., the smallest modal length) is obtained for a BSW supported by a truncated periodic multilayer. Since we consider any combination of refractive indices spanning from typical values of polymers and oxides to those characterizing semiconductors, we believe that these results give a complete and definite answer in determining the best strategy for light confinement in planar dielectric structures.
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