Octave-spanning low-loss mid-IR waveguides based on semiconductor-loaded plasmonics

Abstract

Plasmonic waveguides are crucial building blocks for integrated on-chip mid-infrared (mid-IR) sensors, which have recently attracted great interest as a sensing platform to target enhanced molecular sensing. However, while hosting a wide range of applications from spectroscopy to telecommunication, the mid-IR lacks suitable broadband solutions that provide monolithic integration with III-V materials. This work reports a novel concept based on hybrid semiconductor-metal surface plasmon polariton waveguides, which result in experimentally demonstrated low loss and broadband devices. Composed of a thin germanium slab on top of a gold layer, the waveguiding properties can be directly controlled by changing the geometrical parameters. The measured losses of our devices are as low as 6.73 dB/mm at 9.12 µm and remain <15 dB/mm in the mid-IR range of 5.6–11.2 µm. The octave-spanning capability of the waveguides makes them ideal candidates for combination with broadband mid-IR quantum cascade laser frequency combs and integrated spectroscopic sensors.

A mid-infrared lab-on-a-chip for dynamic reaction monitoring

“…The obtained values at both wavelengths, i.e., 1597 cm −1 and 1620 cm −1 , for the propagation length L p (1/e-decay distance in μm), effective mode index n e f f and losses (in dB mm −1 and dB per 48 μm = plasmonic section between QCL and QCD) are shown in Table 1 . The propagation length is 7% lower at shorter wavelength, a consequence of the slightly more suitable plasmonic waveguide geometry for longer wavelengths 28 . Still, L p is ≥1.7 mm, which corresponds to losses below 0.13 dB for a 48 μm waveguide section, confirming the low-loss characteristics of our DLSPP waveguides in air.…”

“…In this work, we present a fully monolithic integrated mid-IR sensor, that combines all of the above features into a single, miniaturized device. Through the combination of the laser, interaction region, and detector on one chip, and avoiding typical diffraction limitations of conventional chip-scale photonic systems 6 , 25 by exploiting plasmonic waveguides 26 – 28 , we realize a fingertip sized (<5 × 5 mm 2 ) next-generation rapid liquid sensor. Simulation results confirm the preservation of plasmonic capabilities in a liquid environment and enable the use of spectrally optimized QCLDs, i.e., devices that emit and detect similar-wavelength photons 29 .…”

Structure and mid-infrared optical properties of spin-coated polyethylene films developed for integrated photonics applications

“…Despite providing broadband and low-loss transmission throughout the entire mid-IR spectrum (2-14 µm), similar limitations emerge for semiconductor-loaded SPP (SLSPP) waveguides based on, e.g., previously developed Ge/Au architectures 44 . Due to the broad transparency of Germanium, it is possible to increase the Neff by appropriately tailoring the ridge cross-section.…”

“…Due to the broad transparency of Germanium, it is possible to increase the Neff by appropriately tailoring the ridge cross-section. However, this comes at the expense of a significantly lower propagation length 44 . Therefore, new materials are needed for the LWIR spectral range, providing high transparency and low refractive index (RI), thus enabling similar performances as for their near-IR counterparts.…”

Beyond Karl Fischer titration: a monolithic quantum cascade sensor for monitoring residual water concentration in solvents

“…40,41 Further important milestones towards integrated sensors were the (photovoltaic) detector operation of unbiased QCL structures [42][43][44] and the combination of both devices into one single optimized QC active region (AR) for same-wavelength emission and detection (QCLD). 45 For improved sensor operation surface-sensitive mid-IR plasmonic waveguides can additionally be employed, 46,47 paving the way for miniaturized, on-chip devices, 46,48 which suppress beam distortions from standard ridge geometries. 49 The LOD can be adjusted to the investigated analyte by appropriate design of the waveguide length between the laser and detector.…”