Detecting single ultra-fine particles with fiber tip sensors

Fiber-optic sensors allow for accurate and remote sensing capabilities of the properties around us while being minimally invasive and insensitive to electromagnetic interference. Previously, a wafer-to-fiber transfer technique developed at the TU/e was utilized to realize novel nanophotonic fiber-tip sensors. The approach is to pattern a sensitive optical structure on a thin membrane of semiconductor wafer, then to transfer it to the fiber facet without using an adhesive. In the case of this study, the patterned sensor elements are based on photonic crystal cavities. Building up on the findings from the previous studies from TU/e, inverse-design optimization algorithms were implemented to achieve tiny a mode volume V_m ≈ 0.38(^λ/_n)³, high quality factor Q ≈ 22000, and a large reflection modulation from the fiber ΔR ≈ 29%, with a goal of reducing the size limit of detection for single nanoparticles. These nanophotonic cavities on fiber-tips can be used for the detection of individual nanoscale objects or even large single molecules. So far, single polystyrene particles in an aqueous environment with diameters of 50 nm were successfully detected.