Plasmonic nanoparticles (NPs) exhibit localized surface plasmon resonance (LSPR), arising from collective excitation of conduction electrons upon interaction with light. The LSPR peak wavelength is tunable via particle size, shape,material and dielectric environment¹˒².
LSPR excitation enhances and confines the electromagnetic field near the NP surface due to coherent electron oscillation³. This drives energy dissipation through inelastic electron–electron collisions (10–100 fs), followed by electron–phonon coupling (100 fs–1 ps), causing lattice heating, and phonon–phonon coupling (1 ps–1 ns), which transfers heat to the environment⁴.
These processes can lead to morphological changes. Gold nanorods (GNRs), for instance, may reshape into spherical forms upon pulsed laser irradiation⁵. Since these transformations are absorption-driven, their efficiency depends on the absorption cross-section (σabs), which varies with shape, size, and dielectric context⁶-7 as well as on the heat transfer processes and the power of the irradiation source.
GNRs exhibit two plasmon modes, with their spectral positions determined by the aspect ratio (AR). Here, we have performed a selective enrichment of GNR with a specific AR corresponding to the peak position of the experimental longitudinal LSPR mode, via laser irradiation at wavelengths slightly larger and shorter than the experimental LSPR peak. Spectral and thermal responses were analyzed via computational simulations. Morphological changes were studied using ANSYS Lumerical (FDTD), and thermal effects modeled in COMSOL Multiphysics (Heat Transfer in Solids – Time-Dependent). Post-irradiation, a marked decrease in FWHM of the longitudinal LSPR band was observed. Simulations indicated that this spectral narrowing results from selective reshaping of resonant GNRs, enriching populations with constrained AR. COMSOL temperature profiles pointed to localized melting as the dominant mechanism—over surface diffusion or bulk melting—affecting only resonant NPs.
References
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