Ultrashort-pulse optical manipulation of anisotropic nanoparticles via photonic nanojets and hooks in aqueous environments.

We present a computational study on the optical manipulation of anisotropic gold nanoparticles, rods, and disks in aqueous environments using ultrashort pulsed photonic nanojets and photonic hooks. Using three-dimensional finite-difference time-domain simulations and analytical models of anisotropic polarizability, we analyze how shape, orientation, and surrounding medium influence the optical force landscape. Our results show that deviations from spherical particle geometry introduce strong orientation dependence, with optical forces varying by up to a factor of 5-10 across particle orientations and structured-field configurations. In particular, force minima emerge at intermediate tilt angles due to destructive coupling between polarizability axes. We also demonstrate that the surrounding medium significantly alters field confinement and force magnitude, with water enhancing field localization and modifying the dominant particle geometry. These findings provide design guidelines for structured-light-based nanoparticle manipulation, particularly for applications in nanomedicine, sensing, and fabrication, where real-world particles are non-spherical and operate in aqueous conditions.