Effect of pump depletion on transient absorption signal in metal nanoparticles

Electron-lattice thermalization time in metal nanoparticles or in bulk metal are usually estimated by measuring the decay time of the change in transmission following optical excitation. Such measurements can be performed using a femtosecond laser in transient absorption geometry. We find that the decay time of the change in absorption of silver nanoplatelets in water depends on the volume fraction of silver in water for a given intensity. By estimating the volume fraction dependence of third-order nonlinearity in the same samples, we attribute the variation in the measured decay time of change in absorption to pump depletion effects.