Fundamental Limits to the Coupling between Light and 2D Polaritons by Small Scatterers

Abstract

Polaritonic modes in two-dimensional van der Waals materials display short in-plane wavelengths compared with light in free space. As interesting as this may look from both fundamental and applied viewpoints, such large confinement is accompanied by poor in/out optical coupling, which severely limits the application of polaritons in practical devices. Here, we quantify the coupling strength between light and 2D polaritons in both homogeneous and anisotropic films using accurate rigorous analytical methods. In particular, we obtain universal expressions for the cross sections associated with photon-polariton coupling by point and line defects, as well as with polariton extinction and scattering processes. Additionally, we find closed-form constraints that limit the maximum possible values of these cross sections. Specifically, the maximum photon-to-plasmon conversion efficiency in graphene is ∼ 10 −6 and ∼ 10 −4 for point and line scatterers sitting at its surface, respectively, when the plasmon and photon energies are comparable in magnitude. We further show that resonant particles placed at an optimum distance from the film can boost light-to-polariton coupling to order unity. Our results bear fundamental interest for the development of 2D polaritonics and the design of applications based on these excitations. arXiv:1812.03034v2 [cond-mat.mes-hall]

Thermal manipulation of plasmons in atomically thin films

“…In Fig. 2, we show the dispersion relations of three types of plasmonic modes supported by the structures sketched in the insets at three different graphene electron temperatures T e , as calculated using methods discussed in previous studies 37,41,54 . In all cases considered in Fig.…”

Functional Mid‐Infrared Polaritonics in van der Waals Crystals

“…These two properties make vdW materials an effective and compact platform for active modulation of both guided waves and free space light at mid‐infrared frequencies. The extremely confined nature of vdW plasmons, however, makes them difficult to couple strongly to the free waves and thus hinders their efficiency in optoelectronic devices . Recent advances have shown that this wavevector mismatch problem can be overcome by introducing multiscale nanophotonic design, which enables electronically tunable perfect absorption in graphene …”

“…Despite these efforts, strong light modulation using graphene nanoresonators has been elusive due to the inefficient coupling between the graphene plasmons and free space photons. This weak coupling is, in fact, related to the large confinement factors of graphene plasmons, which is—in principle—a desirable property, but also creates a large mismatch between the free space and graphene plasmon optical wavelengths . In other words, scattering cross sections in graphene nanoresonators are limited by the inherent large momentum mismatch.…”

“…The extremely confined nature of vdW plasmons, however, makes them difficult to couple strongly to the free waves and thus hinders their efficiency in optoelectronic devices. [56] Recent advances have shown that this wavevector mismatch problem can be overcome by introducing multiscale nanophotonic design, which enables electronically tunable perfect absorption in graphene. [13] Adv.…”

“…This weak coupling is, in fact, related to the large confinement factors of graphene plasmons, which is-in principle-a desirable property, but also creates a large mismatch between the free space and graphene plasmon optical wavelengths. [56,81] In other words, scattering cross sections in graphene nanoresonators are limited by the inherent large momentum mismatch.…”