Tunable photonic nanojets and optical chirality enabled by perfect vortex beams

The photonic nanojet (PNJ) generation schemes based on passive dielectric structures typically lack dynamic tunability, and the application of traditional vortex beams is further restricted by the inherent dependence of the ring radius on topological charge. In this work, the PNJ generated by perfect vortex (PV) beams illuminating dielectric microlenses was systematically investigated, with quantitative characterization of the optical chirality density, spin angular momentum density, and vortex dichroism in the near field. It is shown that the spiral phase and annular intensity profile of the PV beam modulate the diffraction and focusing process through the microlens, thereby flexibly tuning the PNJ properties; the spin-orbit interaction under strong focusing accounts for the generation and tunability of chiral fields and vortex dichroism in the PNJ region. By adjusting the incident PV beam parameters, the peak intensity, full width at half maximum (FWHM), and jet length of the PNJ can be flexibly tuned; a photonic hook can be formed at large topological charges; and significant chiral field enhancement with continuously tunable vortex dichroism can be achieved. This work clarifies the modulation rules of PV beams for near-field focusing, and provides a flexible strategy for designing dynamically tunable PNJ-based photonic devices.