Low Loss Mid-Infrared Plasmonic Waveguides—Extending the Limits of Noble Metals

Surface plasmon polaritons (SPPs) combine the high-speed capabilities of photonic circuits with the ability of plasmonic confinement below the diffraction limit. In particular, modes supported at (noble) metal/dielectric interfaces in various configurations have gained great attention owing to their prospects for attractive applications. However, considering such plasmonics as an established technology in the visible (VIS) and near-IR spectral range, plasmonic concepts in the mid-IR spectral range are still in their infancy. This significantly hampers addressing emerging mid-IR applications by chip-scale devices, e.g. in real-time liquid sensing experiments or optical free-space communication. Thus, realizing novel device concepts and introducing new materials aims at mimicking VIS/near-IR properties such as wavelength-scale mode confinement and guiding. Succeeding will enable a novel class of photonic integrated circuits (PICs) with breakthrough performance characteristics like compact chip-scale Mach-Zehnder interferometers, monolithic heterodyne detectors or on-chip logic networks. They all strongly benefit from miniaturization and the use of mid-IR wavelengths.<br/>In this work, we surpass the limitations of traditional noble metal-based plasmonics by exploiting the approach of surface-loading 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 µm and 11.2 µm. 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 &gt;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 HfO₂, Al₂O₃ and ZrO₂ with Ge for stabilizing its surface-oxides GeO_x. 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.<br/><br/>In the second part, we show a novel fabrication technique for spin-coating high-quality polyethylene (PE) films as well as their processing into micrometre-thick ridges to efficiently confine and guide mid-IR plasmons along the chip surface. In this way, we find a new path to extend the concept of so-called dielectric-loaded SPP (DLSPP) structures to the mid-IR and beyond, based on the broad transparency window (2-200 μm) of this material. Therefore, PE can play a similarly important enabling role in the mid-IR as PMMA does at shorter wavelengths.<br/><br/>We show the design (including FEM-simulations), fabrication, and optical characterization of such PE-based DLSPP waveguides. This includes the first experimental demonstration of SPPs in PE-based mid-IR plasmonic waveguides. They show capabilities in directing the mode along the chip surface in low-loss straight waveguides and very importantly also adiabatic S-shaped bends (50 µm bending offset along 120 µm waveguide length) with extracted bending losses of &lt;2.5 dB per bend.<br/>Overall, we show for both types of presented waveguides (SLSPP & DLSPP) total losses of significantly below 20 dB/mm together with their broadband mid-IR plasmonic operation.