Mid-Infrared Sensing and Ultrafast Photoresponse in Silicon-Based Plasmonic Detectors

Mid-infrared photonics enables a large number of applications in chemical sensing, medical diagnostics, environmental monitoring, and optical communications, but widespread adoption of this technology is hindered by the lack of compact, CMOS-compatible photodetectors capable of room-temperature operation. This work presents an Al-Si-Al planar heterostructure acting as a plasmonic photodetector that leverages an electrostatically tunable Schottky barrier to detect sub-bandgap mid-infrared photons (5-7 μm) in silicon with a responsivity ∼0.9 mA/W, <0.4 mA/cm² dark current density, and a broad and uniform spectral response that does not require cooling. Fast internal dynamics with time constants of 3.7 ps and 1.4 ns are observed at 1560 nm, suggesting the potential for high-speed operation. Our monolithic and crystalline Al-Si heterostructure features abrupt interfaces obtained without epitaxy, ensuring CMOS compatibility and scalability. We demonstrate the potential of this device for mid-infrared sensing by detecting and spectrally characterizing water molecule absorption lines. By leveraging plasmonically generated hot-carriers combined with the inherent scalability of silicon-based technology, our work opens new pathways for cost-effective and high-speed infrared photodetectors suitable for next-generation integrated photonic systems.