Biomedical Engineering Reference
In-Depth Information
transport across the HMM slab also results in the directional
emission of the energy by the HMM surface, which can be used for
the design of directional light sources [62].
Likewise, the high DOS within the metamaterial in the type
II hyperbolic regime results in emission from the dipole source
that is predominantly into the material. However, the powerflow
driven by the emitted photons is markedly different from the
type I HMM. In contrast, the optical powerflow through the type
II HMM features significant lateral spread, as shown in Fig. 8.8.
The local topology of the electromagnetic interference field in
the metamaterial slab plotted in Fig. 8.8b and 8.8c reveals the
mechanism driving the lateral energy spread. Once again, the
formation of multiple coupled counter-rotating nanoscale optical
vortices drives the global powerflow by recirculating the energy
around local circulation points. The arrangement of the vortices in
this case is different from that in the type I HMM, resulting in the
markedly different overall powerflow pattern.
Finally,Fig.8.9illustratesthedramaticchangesinthepowerflow
through the metamaterial in the photon energy range where
the material in not in the hyperbolic regime. In this case, the
metamaterial DOS is significantly lower than that in either type
of HMM. This reduces the dipole radiative rate resulting in the
low-energy density within the material as shown in Fig. 8.9a.
The time-averaged Poynting vector field within the material still
features areas of circulating powerflow as shown in Fig. 8.9b
and 8.9c. However, in this case, the vortices once again are
spatiallyrearrangedandformaglobalenergyrecirculationnetwork
that inhibits powerflow across the metamaterial slab. Overall, we
can conclude that the re-arrangement of the local field topology
features is behind the global topological transitions in hyperbolic
metamaterials.
8.5 Conclusion and Outlook
We have demonstrated that the formation of optical vortices—
localized areas of circulating optical powerflow—is a hidden mech-
anism behind many unique characteristics of SPP modes on metal-
