Abstract
Abstract Long-wave infrared (LWIR, 6–14 µm) processes enormous potential for chem/biosensing as it covers abundant molecular absorption fingerprints. Waveguides provide an attractive chip-scale miniaturization solution for optical sensors. However, the exploration of waveguide sensors in this wavelength range is limited. Here, an LWIR photonic platform for fast and sensitive on-chip gas sensing is developed using suspended silicon (Si) waveguide supported by subwavelength grating (SWG) metamaterial claddings. This platform provides a viable approach to fully exploit the transparency window of Si. The SWG structure provides a promising solution to engineer the mode profile for strong light–analyte interaction. Propagation loss and bending loss are studied in the broad wavelength range of 6.4–6.8 µm. Functional devices including grating couplers, Y-junctions, and directional couplers are also demonstrated with high performance. Sensing demonstration based on our platform is presented using toluene vapor detection as an example. The corresponding limit of detection reaches 75 ppm. The response and recovery time to 75 ppm toluene are about 0.8 and 3.4 s, respectively. This good performance makes our platform a promising candidate for on-site medical and environmental applications.
Highlights
Numerous photonic solutions have been proposed for chem/biosensing in the midinfrared (MIR) regime, for their label-free analysis capability arising from the unique characteristic spectrum of a given molecular species (a.k.a. molecular fingerprint) [1,2,3,4,5,6,7,8,9,10]
The severe absorption caused by the buried oxide (BOX) hinders the use of SOI platform for the long-wave infrared (LWIR) wavelength range above 6 μm, which covers the vibrational fingerprints of plentiful chemical bonds including C–H, O–H, N–H, etc. [27, 28]
With the sensing waveguide purged with mixture analytes and the reference waveguide purged with atmosphere air, in principle, we can decouple the sensing signal from external influences, including environmental absorption and temperature fluctuation
Summary
Numerous photonic solutions have been proposed for chem/biosensing in the midinfrared (MIR) regime, for their label-free analysis capability arising from the unique characteristic spectrum of a given molecular species (a.k.a. molecular fingerprint) [1,2,3,4,5,6,7,8,9,10]. In order to enhance the LoD, several enrich coating layers have been employed for analyte accumulation and sensitivity enhancement of waveguide platforms [38,39,40,41], while they usually suffer from slower response times of several tens of seconds, even several tens of minutes. Another approach that does not compromise the response time is to directly enlarge the interaction area between the optical mode and surrounding analyte through waveguide structure design. Our proposed platform can achieve sensitive and super-fast detection, identification, and quantification of a targeted analyte in a gas mixture without additional adsorptive coating, providing many opportunities for on-site applications in environmental monitoring, and clinical diagnosis
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