The Optical and Near-Field Properties of Plasmonic Janus Nanoparticles: An FDTD Study

Janus nanoparticles (JPs), consisting of hemispherical metal–dielectric interfaces, exhibit structural asymmetry that enables polarization-sensitive localized surface plasmon resonances (LSPRs). Unlike isotropic nanoparticles, JPs enable spatially confined near-field enhancement at the material interface, where one hemisphere concentrates the field and the other can be selectively functionalized for targeted sensing applications. We used finite-difference time-domain (FDTD) simulations to study the optical and near-field responses of Au–SiO₂ JPs across diameters ranging from 10 to 100 nm. Four distinct illumination configurations were modeled, each defined by a specific incident polarization relative to the Janus interface, covering both longitudinal and transverse field orientations. We calculated absorption, scattering, and extinction spectra, and evaluated electric field enhancement (|E/E₀|²) spectrally and spatially within the cross-sectional plane normal to the propagation direction. Simulations reveal that when the electric field is aligned parallel to the interface, the resulting near-field is localized at the metal–dielectric boundary and exhibits approximately a 3.4x stronger E-field enhancement compared to the perpendicular configuration. Across all polarization setups, we consistently observe that the maximum near-field intensity occurs at the particle size where absorption and scattering cross-sections intersect—a resonance crossover point that defines an optimal condition for field enhancement. Notably, the parallel polarization configurations produce up to a 18.7x greater E-field enhancement than the perpendicular mode at their respective optimal sizes. Compared to spherical Au nanoparticles, Au-SiO₂ JPs exhibit redshifted LSPR peaks, increased E-field enhancement, and polarization-dependent field enhancement at the metal–dielectric interface. This study demonstrates that JPs enable interface-localized field confinement, with polarization and size serving as tunable parameters for optimizing hotspot activation and magnitude. These findings establish a framework for designing asymmetric plasmonic nanostructures which enable spatial and spectral control of near-field enhancement properties—paving the way for high-performance, targeted nanosensors.