Detection of guided-wave plasmon polariton modes in a high-index dielectric MIM structure

Abstract

Surface plasmon polaritons (SPPs) are surface charge density oscillations localized to a metal-dielectric interface. In addition to being considered as promising candidates for a variety of applications, structures that support SPPs, including metal-insulator-metal (MIM) multilayers, are of fundamental interest because of the variety of collective plasmonic modes they support. Previously, a particular class of “forbidden” plasmon polariton modes (PPMs) was proposed that includes plasmon polariton modes confined to a region of dispersion space not typically accessible to surface-constructed collective excitations. Specifically, for these modes, known as Guided Wave PPMs (GW-PPMs), due to the dielectric asymmetry of the central layer, the solution to the wave equation in the center insulator layer is oscillatory while remaining surface bound both to the supporting substrate and the exposed surface. These modes are supported by a simple physical structure that results from a minor symmetry modification of the traditional MIM structure, specifically the use of a central insulator layer with a higher refractive index than the supporting substrate. However, they display fundamental properties that are distinctly different from those of standard SPPs and from recently reported hybrid plasmonic modes. While GW-PPMs have been explored theoretically, they have not yet been realized experimentally. In this article, we present the first experimental demonstration of GW-PPMs. Specifically, we excite and detect GW-PPMs at visible frequencies and match model predictions to experimental results with remarkable accuracy using minimal parameter fitting. In addition to the experimental detection, we calculate and report on other interesting and relevant features of the detected modes, including the associated electric field profiles, confinement values, and propagation lengths, and discuss in terms of the applications-relevance of GW-PPMs.

Excitation of “forbidden” guided-wave plasmon polariton modes via direct reflectance using a low refractive index polymer coupling layer

“…We anticipate that surface roughness could possibly have an outsized effect and therefore explain this discrepancy. We note that in our previous work [ 45 ], in order to get good fits between experimental and theoretical ATR scans, the plasma damping was similarly always at least 50% larger than the literature value. For more discussion justifying our choice of fitting parameters, see the Supplementary Material.…”

“…The propagation lengths were computed as 2/Im( k x ), where k x is the complex wave vector of the mode whose frequency matches that of the minimum of the ATR scan. To compute both the ATR and dispersion relations, we solved the electromagnetic wave equation in each layer by assuming that the solution can be written as a sum of counter-propagating plane waves perpendicular to the interfaces, subject to the condition that the displacement field in each region be divergenceless, as described previously [ 45 ]. We then matched the solutions in consecutive layers by ensuring that the electromagnetic boundary conditions are satisfied across each interface.…”

“…Careful details of the calculations and of the dielectric function used to model the gold can be found in Ref. [ 45 ].…”

“…In an effort unrelated to the development of LRSPPs, our group recently provided the first experimental demonstration of so-called “forbidden” or guided-wave plasmon polariton modes (GW-PPMs), also referred to as high-index dielectric PPMs [ 44 , 45 ]. GW-PPMs are supported by a metal-insulator-metal (MIM) structure (having dimensions smaller than the excitation wavelength) with a high refractive index insulator layer (in this case TiO 2 ) in place of the standard SiO 2 .…”

Engineering terahertz surface magnon-polaritons in hyperbolic antiferromagnets

“…Metamaterials, for example, which were initially engineered to display negative refraction [1], have recently received significant attention in the context of unusual surface polaritons [2]. This has led to the design of new types of metamaterials which not only support but also guide such waves [3][4][5][6]. For example, van der Waals heterostructures [7][8][9][10] that have hyperbolic dispersion [6,11] display interesting features in their surface polaritons.…”