Hyperbolic metamaterials for large-area Surface-Enhanced Raman Scattering (SERS) sensing

Abstract

Typical surface-enhanced Raman scattering (SERS) approaches rely on localized surface plasmon resonances that provide a significant enhancement of the localized electric field. Unfortunately, this technique faces challenges in terms of repeatability, which appears due to the strong dependence of the field enhancement on the surface roughness and the presence of hot-spots in nanostructures; and adequate excitation, as the laser beam must be tuned at a very specific wavelength that corresponds to the resonant frequency of the system. Hyperbolic metamaterials (HMTMs), a type of composite materials whose effective permittivity changes as a function of the electric field polarization, can effectively address these challenges because they support bulk and surface hyperbolic modes able to drastically boost the local fields over a broadband portion of the electromagnetic spectrum. In fact, the frequency response of these artificial materials can be manipulated by adjusting the system composing materials and filling ratios. This work aims to explore the potential of HMTMs to enhance the SERS of molecules located nearby and to address some of the challenges faced by common SERS platforms. To this purpose, we focus on Au/SiO2 HMTMs stacks that exhibit a hyperbolic dispersion for wavelengths larger than ~580 nm. A prototype has been fabricated and characterized using TEM and ellipsometry measurements. Power-dependent SERS measurements were obtained for a monolayer of biphenyl-4,4’-dithiol (BPDT) molecules self-assembled onto the HMTM surface and a gold-based control sample. HMTMs provide repeatable SERS detection with low laser powers <900µW and integration times <97ms (~30X and ~100X lower than control, respectively) over a large surface area, exhibiting a performance like complex TERS systems.