Plasmonic Engineering of Superconducting Nanowire Single-Photon Detectors

Superconducting Nanowire Single-Photon Detectors (SNSPDs) are the most prominent and versatile technology for single-photon counting at near-infrared (NIR) wavelengths. Through the introduction of specialized superconducting materials, SNSPDs have also been demonstrated at mid-infrared (MIR) wavelengths. However, reaching the same performances at this range remains an elusive goal. For example, traditional methods for enhancing optical absorption, such as optical cavities, are practically not suitable at longer wavelengths due to the large thickness required. This thesis proposes a novel detector architecture that combines the resonant plasmonic perfect absorber effect with a fractal metasurface to achieve high-efficiency, broadband, and polarization-insensitive single photon absorption and detection in the MIR. This is achieved by using a superconducting niobium nanowire as both the photon-detecting element and the active absorber component, taking advantage of its plasmonic properties. Furthermore, we use a space-filling Gosper curve to excite multiple plasmonic resonances at different wavelengths, thereby exploiting the multiple scales inherent in fractal geometry. The fractal absorber is integrated with a reflective backplane and a thin dielectric spacer, forming a compact cavity structure optimized for MIR operation. The optical simulations of the designed structure show an average absorption of approximately 85% across the 3–7 μm range. The fabrication process of the proposed detector was carefully designed. The most challenging step, electron-beam lithography, was successfully executed and optimized, enabling device fabrication and the completion of preliminary optical and electrical tests that validate its performance. The proposed design enables compact, scalable detectors with potential impact in thermal imaging, quantum sensing, and mid-infrared molecular spectroscopy.