Designer bulk plasmon polariton modes in hyperbolic metamaterials for sensing applications

Optical metamaterials are composed of sub-wavelength structures and exhibit unusual electromagnetic properties [1]. These materials can enable applications with negative refraction, sub-wavelength imaging and nanoscale light... [ view full abstract ]

Optical metamaterials are composed of sub-wavelength structures and exhibit unusual electromagnetic properties [1]. These materials can enable applications with negative refraction, sub-wavelength imaging and nanoscale light confinement. One class of them, hyperbolic metamaterials (HMM), are highly anisotropic media composed of alternating dielectric and metal layers. As such, the motion of free electrons is confined in one or two spatial dimensions, and the dispersion relation of HMMs is hyperbolic [1]. In addition to the usual surface plasmon polaritons (SPPs) on metal surfaces, HMMs can support bulk Bloch plasmon polaritons (BPPs) localized inside the multilayer structure, which can be used for nanoscale spectroscopic sensing due to the highly confined fields associated with these modes [2].

For this application we demonstrate theoretically and experimentally the excitation of BPPs through a grating coupling technique based on the excitation of SPPs. The structures consist of metallic diffraction gratings and an artificial HMM made of alternating layers of gold and alumina (Fig. 1), which can be called a hypergrating. We fabricated the multilayers with e-beam evaporation and atomic layer deposition, and produced the gratings with electron beam lithography.

The angle-dependent plasmon polariton modes can be seen in reflection spectra: in Fig. 2 the SPP is visible at 600-800 nm wavelengths and several BPP modes are seen in the near-infrared (1000-1800 nm). These measurements are well reproduced by simulations with different numerical methods. Our theory predicts configurations with almost zero reflection for BPP modes, as well as very narrow spectral features in the visible-near-infrared range. As the modes are sensitive to the dielectric medium in the immediate vicinity of the plasmon polariton, these systems are useful for ultrasensitive sensor applications, both for single-molecule detection and sub-wavelength optical imaging [1].

[1] Poddubny et al., Nature Photonics 7, 948–957 (2013).

[2] Sreekanth et al., Nature Materials 15, 621–627 (2016).