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Nanoscale materials that contain metallic components can be designed to have excellent light-harvesting capabilities, and can also be used to direct the flow of energy from incident photons into small molecules at or near the surface of metal nanoparticles. One promising route for energy flow is through so-called hot charge carriers, which are optically excited on metal nanoparticles and subsequently transferred to molecules/materials that share an interface with the metal. This article provides an overview of the fundamentals of hot-carrier generation and transfer, discusses both theoretical and experimental means for interrogating these processes, and discusses several potential societally important applications of hot-carrier-driven chemistry to solar fuels and sustainable chemistry. 

Abstract The phase matching between the propagating fundamental and nonlinearly generated waves plays an important role in the efficiency of the nonlinear frequency conversion in macroscopic crystals. However, in nanoscale samples, such as nanoplasmonic structures, the phase-matching condition is often ignored due to the sub-wavelength nature of the materials. Here, we first show that the phase matching of the lattice plasmon modes at the fundamental and second-harmonic frequencies in a plasmonic nanoantenna array can effectively enhance the surface-enhanced second-harmonic generation. Additionally, a significant enhancement of the second-harmonic generation is demonstrated using stationary band-edge lattice plasmon modes with zero phase. 

Active nanostructured optical components show promise as potential building blocks for novel light‐based computing and data processing architectures. However, nanoscale all‐optical switches that have low activation powers and high‐contrast ultrafast switching have been elusive so far. Here, pump–probe measurements performed on amorphous‐Ge‐based micro‐resonator metasurfaces that exhibit strong resonant modes in the mid‐infrared are reported. Relative change is observed in transmittance of ΔT/T ≈ 1 with picosecond (down to τ ≈ 0.5 ps) free carrier relaxation rates, obtained with very low pump fluences of 50 μJ cm⁻². These observations are attributed to efficient free carrier promotion, affecting light transmittance via high quality‐factor optical resonances, followed by an increased electron–phonon scattering of free carriers due to the amorphous crystal structure of Ge. Full‐wave simulations based on a permittivity model that describes free‐carrier damping through crystal structure disorder find excellent agreement with the experimental data. These findings offer an efficient and robust platform for all‐optical switching at the nanoscale.