Ultrafast Nonlinear Plasmon Decay Processes in Silver Nanoclusters

Tunable plasmonic properties of metallic nanostructures play a significant role in enhancing various photo-optical phenomena including solar energy conversion, nonlinear optics, photoluminescence and photocatalysis. Understanding the fast plasmon decay mechanisms is essential for developing practical applications utilizing these light-matter interaction processes, but has been a challenge both experimentally and computationally. Among theoretical simulation methods, real-Time density functional theory (RT-TDDFT) is a valuable tool to monitor the electron dynamics of molecules subjected to an electric field. Herein, we use the RT-TDDFT method to identify the possible plasmon decay mechanisms of the bare tetrahedral Ag8 nanocluster. We excite the strong linear plasmonic states and examine dipole response and the electron dynamics in the system. Variation of density matrix elements related to occupied and virtual orbital pairs reveals that the one-photon allowed transitions, which have been excited due to the incident electric field, experience ultrafast decay into high energy transitions, specifically to two-photon allowed transitions. The tetrahedral symmetry representations of these transitions confirm that some of these high energy transitions are only allowed via two-photon absorption whereas others can be activated via both one-and two-photon absorption. Moreover, this work suggests that the collective excitations present in the system play an important role in accumulating an enormous amount of energy to enhance nonlinear processes. Overall, this work provides insights into a possible plasmon decay mechanism of nanoclusters which is activation of nonlinear processes such as two-photon absorption.