Using graphene as the emission source, researchers at AMO GmbH worked with other European universities to develop a MIR emitter for integrated photonic gas sensors. The researchers integrated the graphene-based emitter with photonic waveguides that couple directly into silicon waveguides operating in the region relevant for gas sensing. The integration of these components at the wafer level could reduce the size and cost, improve the mechanical stability, and potentially enhance the performance of environmental sensors. Applications in environmental monitoring, industrial process control, medical diagnostics, and other areas require compact, reliable gas sensors to monitor air quality in real time. The graphene IR emitters, which could be used for a distributed network of sensors, offer a potential solution for gas sensor systems across various industries. Traditional gas sensing methods are based on the chemical reaction of the targeted gas to the sensor material. The reaction of the gas affects the sensor, which can lead to the need for frequent calibration, drift, performance degradation, and a limited sensor lifetime. A graphene-based IR emitter for integrated photonic gas sensors. Courtesy of AMO GmbH. Absorption spectroscopy, in contrast, works by characterizing the absorption wavelengths of gases, such as greenhouse gases, and providing a spectral fingerprint that can be used to determine the composition of the gas. This method provides high specificity, minimal drift, and long-term stability without chemically altering the sensor. It is, therefore, useful for precise gas detection and robust, real-time air quality monitoring. Photonic integrated circuits (PICs) can be used to shrink spectroscopy equipment to the size of a chip, creating a compact, cost-efficient optical gas sensor system. However, PIC-based gas sensors still require light coupling from external sources, and coupling to detectors in and out of the waveguides. Integrating light sources and detectors directly on the wafer could enable spectroscopic gas sensing in a highly compact format. The researchers chose to use graphene as the active material for their thermal MIR emitter because graphene can reach the temperatures necessary for thermal emission, and its emissivity is comparable to that of other very thin emitters. These properties make it a good source for MIR emission. Monolayers of graphene are so thin that the entire emitting volume can be placed closest to the waveguide, generating ideal near-field coupling of the emission directly into the waveguide mode. Monolayer graphene causes minimal distortion to the waveguide mode, which lessens the mismatch between the mode in the emitter region and outside this region. Researcher Nour Negm worked with colleagues at RWTH Aachen University, KTH Royal Institute of Technology, Senseair AB, and the University of Bundeswehr to integrate the graphene emitters on top of silicon photonic waveguides, enabling direct coupling of the emitters into the waveguide mode. The researchers operated the emitter for approximately one hour under ambient conditions, demonstrating MIR emission into the waveguide and out of a grating coupler. They detected emissions in the spectral range of 3 to 5 μm. Based on thermal simulations, the researchers predict emitter temperatures could reach the range of 500 to 900 Kelvin, which is comparable with other nanoscale emitters. They estimate emission coupling efficiencies into the waveguide of up to 68%, which compares favorably with other nanoscale emitters. Although the PIC used in the experiment was primarily characterized for a 4.2 μm wavelength targeting CO2 detection, the integrated graphene thermal emitters radiate in a broad gas absorption fingerprint wavelength range of 3 to 10 μm. Combined with integrated graphene MIR photodetectors, the graphene MIR emitters could enable fully integrated photonic sensors, where the evanescent fields of the waveguides could interact directly with the gaseous environment, enabling their broad application for gas and environmental sensing. The research was published in ACS Photonics (www.doi.org/10.1021/acsphotonics.3c01892).
