Integrated photonics in the future: Silicon, plasmonics or something else?

摘要

Nanophotonics in general and plasmonics in particular have received much attention in recent years, fuelled by a general interest in nanotechnology but also by rapid advances in integrated photonics over the years, primarily brought about by using silicon/air or quartz interfaces, giving larger refractive index contrast than previously employed [1,2]. The minimum lateral spatial field width for a planar silicon waveguide in air is ∼300 nm, with a wavelength in the medium of ∼500 nm, at a vacuum wavelength of 1550 nm. Photonics integration density has thus shown a steady exponential growth since the 80s, but to pursue this state of affairs, it is necessary to find a successor/complement to the silicon technology. Such new (meta) materials seem to have to rely on a negative ε (such as in metals) somewhere in the system [3,4] since this offers a possibility for increasing the integration density in photonic lightwave circuits in two ways: (i) By using principles other than total internal reflection, i.e. surface polaritons and (ii) Employing e.g. metamaterials to generate artificially very large effective media refractive indices. Both methods could allow denser lateral packing of waveguides as well as shorter resonators and filters. In general, operating the metal/dielectric structures close to resonance [4] gives the highest confinement but also the highest spatial losses, since in essence (i) the field is forced into the lossy metal and (ii) in general the group velocity is significantly reduced. Thus, a main problem for many (albeit not all) applications is the optical loss associated with these high confinement metal-based metamaterials. In anticipation of a possible future breakthrough in developing metamaterials with at least a factor of 10 lower losses [5, 6] we analyze in this paper different resonant and nonresonant structures and the achievable performance. One example is waveguides made from arrays of near resonantly operated near-fi--eld coupled metal nanoparticles in the shape of e.g. spheres, which have attracted some attention [3,7,8]. Nanoarray waveguides, based on silver nanoparticles are, however, very lossy, see e.g. [9].