Lights, nano, action! New plasmonic materials and methods to probe nanoscale phenomena

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oduction In 1833, Faraday combined silver and sulfur and discovered the first material with a negative temperature coefficient of resistance, silver sulfide. At the time, the word semiconductor did not exist. Yet we now know that this first semiconducting material laid the foundation for an entirely new and extremely important class of electronic materials. Today, a similar revolution is unfolding for optical materials. Textbook conceptions of light-matter interactions, such as the notions of exclusively positive refractive indices1–5 and reciprocal light propagation,6–12 are being redefined by new optical materials. These materials allow light to be controlled in ways previously thought impossible, providing techniques to circumvent the diffraction limit of light and tune both electric and magnetic light-matter interactions. At Stanford University, my research team is developing such new optical materials and using them to directly visualize, probe, and control nanoscale systems and phenomena—particularly

those relevant to energy and biology. We aim to address questions such as: Can optical microscopy achieve a resolution comparable to electron microscopy to study nanoscale systems in situ and in real time? Can catalytic processes be probed on the single particle or molecule level, to understand and improve catalytic reactions? Can proteins or small molecules be optically trapped and manipulated in vivo to directly probe molecular mechanics and interactions in cells? Though seemingly diverse, these questions all require precise control of light-matter interactions across wavelength and sub-wavelength scales, as enabled by new optical and plasmonic materials. To tailor light-matter interactions, noble metal nanoparticles provide a particularly versatile platform. In this article, first we explore the impact of quantum effects on the plasmonic properties of small (