We study the breakdown of the bulk plasmonic model of semiconductors as we reduce their size. For metallic nanostructures in the few-nm range it is well known that the usual description breaks down since nonlocal-response and spill-out effects start to play a role, both of which can be described by the semiclassical hydrodynamic Drude model or by more microscopic theories. On the other hand, the smallest semiconductor structures such as quantum wells or dots are described by particle-in-a-box quantum mechanics, while larger semiconductors get a bulk plasmonics description.
Recently we proposed that an intermediate size regime for semiconductor nanostructures should exist where a semiclassical hydrodynamic Drude description replaces the classical Drude-like response; deviations from classical plasmonics will occur for larger-sized structures than for metals and nonlocal effects will be even more pronounced than for metals [1]. For example, we predicted the existence of hydrodynamic standing bulk plasmons which have no classical analogue. Interestingly, their experimental observation has recently been reported [2].
Further novel hydrodynamic plasmonic effects can be expected in semiconductors where two or more species of charge carriers co-exist, for example in intrinsic or p-doped semiconductors. While classically the effect of two or more such 'fluids' is rather trivial, in the hydrodynamic theory, the collective resonances of the two plasma's can lead to novel resonances including an acoustic resonance at low frequencies which has no analogue in classical plasmonics [3].
To facilitate the observation of two-fluid hydrodynamic effects, we currently investigate plasmonic nanostructures for which the nonlocal responses are most pronounced [4]. To that end we have numerically implemented the two-fluid hydrodynamic model and found fine agreement with Mie calculations in a benchmark problem (Fig1). We report nonlocal effects in rectangular waveguide structures and study the two-fluid acoustic modes in plasmonic dimers (Fig2). Our analytical and numerical study paves the way for experimental confirmation of rich hydrodynamic plasmonics in various semiconductor nanostructures.
[1] J.R. Maack et al., EPL 119, 17003 (2017).
[2] D. de Ceglia et al., Sci. Rep. 8, 9335 (2018).
[3] J.R. Maack et al., PRB 97, 115415 (2018).
[4] T. Golestanizadeh et al., in preparation (2018).
Photonic & plasmonic nanomaterials , Optical properties of nanostructures
