(Invited) Ultrafast Optical Spectroscopy of Plasmonic and Energy Conversion Materials

Abstract

Ultrafast photoprocesses can play a leading role in determining the efficiency of optically induced catalysis and energy conversion. As a result, a rigorous understanding of the ultrafast radiative and nonradiative processes in relevant materials, such as plasmonic and semiconductor materials, and learning how to amplify the magnitude of the favorable photoprocesses, is important for ultimately improving their performance. For example, in nanostructured plasmonic materials the creation of energetic electrons before thermalization has been proposed for a wide number of applications in optical energy conversion and ultrafast nanophotonics. However, the use of "nonthermal" electrons is primarily limited by both a low generation efficiency and their ultrafast decay. We report experimental and theoretical results on the use of broadband plasmonic nanopatch metasurfaces comprising a gold substrate coupled to silver nanocubes that produce large concentrations of hot electrons, which we measure using transient absorption spectroscopy. The metasurfaces bear resemblance to perfect absorber type structures. We further find evidence for three subpopulations of nonthermal carriers, which we propose are due to localization of energetic carriers near the three band crossings of the Fermi surface. The bimetallic character of the metasurface strongly impacts the physics, with dissipation occurring primarily in the gold whereas the quantum process of hot electron generation takes place in both components. Our calculations show that the choice of geometry and materials is crucial for producing strong ultrafast nonthermal electron components and may help to guide future efforts in this area. Additional ultrafast studies of light harvesting and energy conversion materials are described, particularly in using assemblies of chromophore-functionalized biomolecules and also in perovskite materials. This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, and supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.