Sensing Chemicals in the mid-Infrared Using Chalcogenide Glass Waveguides and PbTe Detectors Monolithically Integrated On-chip

Chemical sensors are important for many applications, from sensing explosive residues for homeland security and defense to sensing contaminants in air and water for environmental monitoring. However, the sensors currently used for these purposes are either bulky, not very sensitive, or not able to identify a chemical specifically. Integrated photonic sensors, which include a light source, photonic sensing element, and photonic detector integrated directly on-chip, that can operate in the mid-infrared (MIR) chemical fingerprint region, promise to be small, sensitive, and specific chemical sensors. They achieve this by confining light within waveguides packed into a small area and using the evanescent field that exists outside the waveguides to sense the presence of a chemical through absorption spectroscopy, identifying chemicals by their unique absorption spectra. This work focuses on designing and fabricating the first ever MIR integrated sensing element combined with a detector, operating at room temperature.

A spiral waveguide design was chosen for the sensing element due to its long interaction length, which improves sensitivity, while still maintaining a small area footprint. Fabrication was done using a double layer electron beam lithography and liftoff technique to reduce the waveguide sidewall roughness, and therefore loss, of the thermally evaporated chalcogenide glass waveguides. The thermally evaporated polycrystalline PbTe detector was deposited directly underneath the waveguide using photolithography and liftoff. This direct integration of the detector with the waveguide improves coupling of light into the detector while also reducing the size, and therefore noise, level of the detector, allowing it to function at room temperature when most MIR detectors need cooling. We show the spiral sensing element and waveguide integrated PbTe detector. The results from sensing methane gas using 3.3 µm light are shown, demonstrating that this integrated sensing element and detector can effectively sense the presence of chemicals using their MIR absorption spectra.