A novel method of detecting single nanoparticles (NPs) in a microfluidic channel directly using a photonic nanojet (PNJ) was investigated. The theoretical model comprised a plane wave-illuminated, liquid-filled hollow-microcylinder (LFHM) and a single Au NP. Relevant studies were implemented and demonstrated with a finite element method (FEM)-based numerical simulation and explained physically through a ray-optics theoretical analysis with the assistance of energy flow line shifts. When depicting the optical-field distribution by gradually altered contour lines for LFHMs with or without a single Au NP, the outward distances of the specific points on the right end of each contour line, for a LFHM with a single Au NP relative to a LFHM without a NP, increased exponentially with decreasing contour levels. By dividing the contour levels into ten levels, the detectable NP of size of a few nanometers can be reflected through the outward distance of the contour points. The key parameters of the PNJ (the maximum light intensity, decay length and lateral beam waist), combined with the electric field distribution and focal point offset, can provide information on NP location. This work showed the PNJ itself to be a powerful and promising tool for the detection and identification of single NPs.
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