Broadband Surface Plasmon Resonance Tuning in Ballistic Aggregates of Metal Nanoparticles for Photonic Applications

Ballistic aggregates (BAs) have long been used in astrophysics to model interstellar dust, particularly for analyzing their light scattering and absorption characteristics. In this work, we extend the application of BA structures into the plasmonic domain by investigating their optical response at the nanoscale. Using the Discrete Dipole Approximation (DDA), we numerically simulate the extinction and absorption behavior of 50 nm radius metal nanoparticles (Al, Ag, Au) arranged in BA, BAM1, and BAM2 configurations. Our results confirm that the optical properties of these clusters are strongly influenced by porosity, number of monomers, and the refractive index of the embedding medium. Notably, we identify a transition wavelength, below which extinction increases with porosity, while above it, the trend reverses. We also observe a redshift in surface plasmon resonance (SPR) with decreasing porosity and increasing cluster size, offering broad tunability of the extinction spectrum. The near-field analysis further reveals significant field enhancement due to inter-particle coupling. These findings suggest that nanoscale BA-type clusters could serve as viable candidates for plasmon-enhanced light absorption in photovoltaic applications.