Loss compensation by amplification in nanoplasmonic waveguides: Possibilities and limitations

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 the rapid advances in integrated photonics over the years, primarily brought about by using silicon/air or quartz interfaces, giving a larger refractive index difference than previously employed [1]. One can show that 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. Thus, any attempts in nanophotonics integration should surpass these values, as measured in proportion to the relevant vacuum wavelength. Thus, to increase spatial integration, it is necessary to find a successor to the current silicon nanowire technology, which, as noted, has to a large extent enabled the recent progress in integration density, see e g [2]. Such successors seem to have to rely on materials with negative epsilon [3] since these offer a possibility for increasing the integration density in photonic lightwave circuits in two ways: (i) By using principles other than total internal reflection (limited as noted above by the refractive index difference), i e plasmonics and (ii) employing e g metamaterials to generate artificially very large refractive indices. Both methods will allow denser lateral packing of waveguides as well as shorter resonators and filters. For planar waveguides, the first method suffices: one can use metamaterials such as a diluted metal, e g silver nanospheres in an embedding dielectric, or use pure metals to generate a negative epsilon, which is then interfaced to a dielectric with positive epsilon of a slightly smaller magnitude to give a near surface plasmon resonance and a tightly confined plasmon mode with the desired high effective index [4]. From this, a channel waveguide can be created by confinement in the orthogonal dimension, employing the high effective index to achie-ve a highly confined field in this orthogonal dimension, as well as offering the possibility of making subwavelength resonators due to the large resulting effective index [5]. However, the tighter the confinement the greater are the problems pertaining to optical losses (determined by the imaginary part of epsilon,ε ″ ), remaining the critical issue impeding the ubiquitously usefulness of nanophotonics light wave circuits (but certainly not pertaining to all applications). Recently, the use of quantum dots (QDs) to reduce or compensate the losses has been analyzed [6]. While the gains required are certainly high, they are not out of reach [7]. The concomitant effects of the gain compensating loss process are, however, increased power dissipation and signal to noise ratio (SNR) degradation.