Highly enhanced broadband infrared absorption of germanium by multi-layer plasmonic nano-antenna

High-sensitivity and broad bandwidth photo-detector devices are important for both fundamental studies and high-technology applications. Here, by using three-dimensional (3D) finite-difference time-domain simulation, we design an optimized 3D multi-layer gold nano-antenna to enhance the near-infrared (NIR) absorption of germanium nanoparticles. The key ingredient is the simultaneous presence of multiple plasmonic resonance modes with strong light-harvesting effect that encompass a broad bandwidth of germanium absorption band. The simulation results show more than two orders of magnitude enhanced absorption efficiency of germanium around 1550 nm. The design opens up a promising way to build high-sensitivity and broad bandwidth NIR photo-detectors.