Elucidating and Exploiting Hot-Carrier Dynamics in Active and Nonlinear Plasmonic Metamaterials

Hybrid plasmonic metamaterials consisting of nanostructured metals and electron-accepting materials serve as an enormously fruitful arena for the investigation and utilization of optically excited energetic carriers. The coherent coupling of electromagnetic radiations to plasmons creates a nonequilibrium distribution of electrons at an elevated temperature, known as hot electrons. The dynamics of such hot carriers has been wide employed for various applications, ranging from the detection of sub-bandgap photons in semiconductors to the production of hydrogen gas via photocatalytic processes.<br/><br/>The generation, transfer, and relaxation of hot carriers also provide a novel route to nonlinear optical effects. For example, the optical Kerr nonlinearity of plasmonic metals provides enticing prospects for developing reconfigurable and ultracompact all-optical modulators. Although enhanced nonlinear responses of metals are known to enable the optical control of light, the intrinsically slow relaxation dynamics of photoexcited carriers, primarily governed by electron-phonon interactions, impedes sub-picosecond modulation speeds. In this talk, we present femtosecond all-optical modulation in plasmonic systems via the activation of relaxation pathways for hot-electrons at the interface of metals and electron acceptor materials. We show that the relaxation kinetics and the optical nonlinearity can be tuned by leveraging the spectral response of the plasmonic system in the linear regime. Our findings introduce a generic scheme for achieving sub-picosecond modulation speeds in plasmonic systems, suitable for the ultrafast control of the intensity, polarization, and phase of light upon exchange of energetic hot carriers.<br/><br/>The dynamics of hot carriers in plasmonic system is further exploited for the creation of optically induced second-order nonlinear materials. Second-order optical processes are pivotal to the active modulation and wave mixing of light waves. The inversion symmetry in most materials, however, prevents achieving a bulk chi-2 effect, thereby limiting the portfolio of second-order nonlinear materials. We propose and demonstrate ultrafast conversion of a statically-passive dielectric to a transient second-order nonlinear medium upon the generation and transfer of plasmonically induced hot electrons. Triggered by an optical switching signal, the amorphous dielectric with vanishing intrinsic chi-2 develops dynamically tunable second-order nonlinear responses, which can be leveraged to address the critical need for all-optical control of second-order nonlinearities in nanophotonic systems.<br/><br/>Therefore, the consolidation of the relaxation dynamics of hot electrons with externally induced optical nonlinearity enables a unique opportunity to elucidate the generation, transport, and decay of hot carriers in hybrid plasmonic systems. Conversely, leveraging the ultrafast dynamics of plasmonically induced hot electrons allows us to achieve ultrafast all-optical control of light and realize externally triggered second-order optical nonlinearity in hybrid plasmonic metamaterials.