On‐Chip Spiral Waveguides for Ultrasensitive and Rapid Detection of Nanoscale Objects

Ultrasensitive and rapid detection of nano‐objects is crucial in both fundamental studies and practical applications. Optical sensors using evanescent fields in microcavities, plasmonic resonators, and nanofibers allow label‐free detection down to single molecules, but practical applications are severely hindered by long response time and device reproducibility. Here, an on‐chip dense waveguide sensor to monitor single unlabeled nanoparticles in a strong optical evanescent field is demonstrated. The spiral nanowaveguide design enables two orders of magnitude enhancement in sensing area compared to a straight waveguide, significantly improving the particle capture ability and shortening the target analysis time. In addition, the measurement noise is suppressed to a level of 10^?4 in the transmitted power, pushing the detection limit of single particles down to the size of 100 nm. The waveguide sensor on the silicon‐on‐isolator platform can be fabricated reproducibly by the conventional semiconductor processing and compatible with surface functionalization chemistries and microfluidics, which could lead to widespread use for sensing in environmental monitoring and human health.