  Proximity of the metal nanoparticles enhance the plasmonic coupling and
shifts the resonance. This article presents a numerical study of the
photothermal effect in aggregates of small gold nanorods considering the
ordered as well as random aggregates. In the ordered aggregates, there is
lateral coupling which causes blueshifts in the plasmonic resonance, while in
the random aggregates there are redshifts in the plasmonic resonance. The
plasmon response of latter could be tailored up to the second infrared
biological therapeutic window. It has been observed that the aggregates show
higher absorption power and therefore, higher temperature rise compared to the
single gold nanorod or monodispersive nanorods. The absorption resonance peak
position of the random aggregate depends on the incident and polarization
angles of the incident light. The aggregation of the nanoparticles often
inherently occurs in the biological medium which affects the photothermal
process. This study helps to understand the photothermal heating of
nanoparticle aggregates and the use of the optimal light source concerning the
absorption peak of the aggregates suspension for therapeutic uses.
