Waveguide Integrated Gradient Nanoantenna for Broadband Surface-enhanced Infrared Spectroscopy

The integration of waveguide and surface enhanced nanoantenna provides a new method for the efficient utilization of incident light by combining μm-magnitude evanescent field depth on the surface of waveguide and the field enhancement maintained at distances of up to ~100 nm. The waveguide integrated nanoantenna device avoids the need for large free-space optics and, together with on-chip lasers and detectors, can constitute an extremely small, fully integrated on-chip optical sensing system. In this work, we demonstrate waveguide integrated gradient nanoantenna for mid-infrared broadband sensing. The design of the silicon waveguide with large-core rib allows low loss transmission below 7 μm. In addition, in order to improve the sensitivity and bandwidth of waveguide integrated nanoantenna, the concept of loss engineering to optimize the parameters of surface enhanced nanoantenna and designed the gradient antenna arrangement are respectively introduced. Theoretical calculation and simulation results show that the average and maximum electric field between 5–7 μm of the designed device is increased by a factor of 42 and 55 respectively relative to evanescent field amplitude of the bare waveguide.