Effect of the orientation of non-spherical metal nanoparticle with respect to light polarization on its transient optical response

Nanoparticles with non-spherical shapes are now being widely used for various photonic applications. We observe experimentally that the magnitude as well as the time dependence of the transient absorption of a colloid of silver nanoplatelets depends on the relative polarization of the pump and probe pulses. There have been a few reports about the dependence of the transient signal magnitude on polarization, but little information is available on its temporal dependence. Using a theoretical model, we show that this observed behavior arises from the fact that the energy absorption by a non-spherical nanoparticle depends on, among other factors, the nanoparticle orientation with respect to the pump and probe polarization directions. It is essential to consider this when estimating nanoparticle characteristics such as carrier thermalization time, carrier-phonon scattering time, and complex polarizability from transient absorption measurements.