Modulation of super-diffraction-limited chain hotspots via standing wave interference of coaxial photonic nanojets

This work presents a novel method, to our best knowledge, for generating and modulating super-diffraction-limited chain hotspots through standing wave interference of coaxial photonic nanojets (PNJs). The proposed photonic nanojet-controlled optical chain (PNJ-chain) is formed by the constructive interference of a pair of rod-like PNJs (r-PNJs), generated by illuminating cylindrical particles with radially polarized light. The PNJ-chain exhibits uniformly distributed hotspots with comparable transverse and longitudinal dimensions, and the Gouy phase shift effectively confines the transverse size of the hotspots. By adjusting the phase difference and the distance between the two counter-propagating r-PNJs, convenient modulation of the number, position, and intensity distribution of the hotspots is achieved. The discrete dipole approximation method is employed to analyze and verify the superior optical force properties of the generated PNJ-chain. Furthermore, the feasibility and simplicity of generating the PNJ-chain using a single r-PNJ with the assistance of an interface reflection scheme are demonstrated. This research significantly expands the application potential of PNJs in the field of optical tweezers, with practical implications in microparticle trapping, microfluidics, and other related areas.