Extraordinary evanescent field confinement waveguide sensor for mid-infrared trace gas spectroscopy

Abstract

Nanophotonic waveguides are at the core of a great variety of optical sensors. These structures confine light along defined paths on photonic chips and provide light–matter interaction via an evanescent field. However, waveguides still lag behind free-space optics for sensitivity-critical applications such as trace gas detection. Short optical pathlengths, low interaction strengths, and spurious etalon fringes in spectral transmission are among the main reasons why on-chip gas sensing is still in its infancy. In this work, we report on a mid-infrared integrated waveguide sensor that successfully addresses these drawbacks. This sensor operates with a 107% evanescent field confinement factor in air, which not only matches but also outperforms free-space beams in terms of the per-length optical interaction. Furthermore, negligible facet reflections result in a flat spectral background and record-low absorbance noise that can finally compete with free-space spectroscopy. The sensor performance was validated at 2.566 μm, which showed a 7 ppm detection limit for acetylene with only a 2 cm long waveguide.

Designing Mid-Infrared Gold-Based Plasmonic Slot Waveguides for CO2-Sensing Applications

“…We note that the packaging of the final device is planned such that the lower region (below the membrane) does not contain the analyte (also for mechanical stability). For integrated waveguides, another parameter that should be taken into account is the group velocity which denotes the waveguide dispersion [35]. The group velocity (V g ), which describes the speed at which the energy flows through a given cross-section of the waveguide, is explained in more detail in [36].…”

“…where n g is the group index (n g = c/V g , c is the speed of light in free space), and n c is the refractive index of the cladding. Combining both, the effects of field delocalization and dispersion can cause Γ to exceed unity, resulting in the facilitation of stronger absorption than obtained with a free-space beam [35,37].…”

Sol-Gel Thin Film Processing for Integrated Waveguide Sensors

“…Efforts to tackle these limitations are underway, with the two main strategies being optimization of waveguide cross-sectional design and fabrication, and the use of a cover medium, so-called top-coating or cladding. The former implies sophisticated waveguide designs to increase the evanescent field confinement and the careful selection of low absorbance materials and processes to guarantee smooth surfaces and low inhomogeneity (Ranacher et al, 2018;Ottonello-Briano et al, 2020;Vlk et al, 2021). The use of top-cladding both increases the evanescent field fraction and reduces the scattering by decreasing the refractive index contrast at the waveguide-analyte boundary.…”

Single Mode, Air-Cladded Optical Waveguides Supported by a Nano-Fin Fabricated with Direct Laser Writing

“…To address these shortcomings, a desired evanescent waveguide sensor would be photonic chip-based-for the ability to integrate interferometric circuitry, along with the robustness of the chip form factor-and conveniently coupled to an optical fiber, and yet have its entire waveguide cladding composed of the environment to be sensed. This can be achieved with complicated fabrication and etching processes to yield free-standing or nearly free-standing waveguides [11][12][13]. Microscale, free-standing polymer waveguides are realized using single-step digital light processing or nearly free-standing waveguides [11][12][13].…”

“…This can be achieved with complicated fabrication and etching processes to yield free-standing or nearly free-standing waveguides [11][12][13]. Microscale, free-standing polymer waveguides are realized using single-step digital light processing or nearly free-standing waveguides [11][12][13]. Microscale, free-standing polymer waveguides are realized using single-step digital light processing (DLP) technology [14].…”