Low Loss Mid-infrared Plasmonic Waveguides: Extending the Limits of Noble Metals

Abstract

In this work, we surpass the limitations of traditional noble metal-based plasmonics by exploiting the approach of surface-leading using two different highly transparent mid-IR materials leading to different characteristics. In the first part of this work, we introduce, supported by finite element (FEM) simulations, a new concept of semiconductor-loaded SPPS (SLSPPS), resulting in experimental low—loss, ultra- broadband waveguides covering a full octave between 5.6 pm and 11.2 um. This is obtained by depositing thin Ge-slabs on a gold layer supported by a Si substrate, allowing to couple and confine mid-IR photons on the wavelength-scale to the Chip- surface and efficiently guide them for a few millimetres. Germanium combines multiple advantages including broadband transparency, i.e. low loss, characteristics throughout the mid-IR spectral range, well—known interface characteristics and fabrication protocols from its decades of use in micro- and nano—electronics as well as CMOS compatibility. Its refractive index profile allows the realization of Ge/Au SLSPP waveguides, where >95% of the mode are guided in the surrounding medium, making it highly suitable for monolithic chip—scale liquid spectrometers. Moreover, we exploited previous work on combining high-k dielectrics like Hf02, A|203 and Zr02 with Ge for stabilizing its surface-oxides Geox. We demonstrate that 10 nm of atomic layer dielectric deposition protects our Ge/Au SLSPP waveguides from being etched in normal water. This opens the pathway towards bio—sensing applications, where water is the most relevant background matrix.