We present methods to visualize light-induced chemical transformations with deeply subwavelength (~2nm) resolution. Our goal is to help unravel how photons control the local chemistry within and near plasmonic and high-index-dielectric resonators, and use that knowledge for improved photocatalysts, nonreciprocal nanophotonics, and dynamic, single-molecule circular dichroism spectroscopy. First, we study the photocatalytic dehydrogenation of individual antenna-reactor systems with sub-2nm spatial resolution, using an environmental transmission electron microscope combined with light excitation. We find that plasmons control the photochemistry in three distinct ways: 1) Plasmons modify the rate of distinct reaction steps differently, increasing the overall rate more than ten-fold; 2) Plasmons open a new reaction pathway that is not observed without illumination; and 3) Reaction nucleation occurs more often at electromagnetic hot-spots. Next, we study the dynamic response of chiral nanostructures and molecules in plasmonic cavities using an environmental atomic force microscope coupled with plasmonic tweezers. Illumination of the plasmonic tweezers with left- and right- circularly polarized light (CPL) results in distinct forces on a chiral nanostructure. In particular, the transverse optical forces on a chiral nanostructure are attractive with left-CPL, but repulsive with right-CPL. We use the AFM tip to map such chiral optical forces over the plasmonic tweezers with 2 nm lateral spatial resolution, and show how the technique can be applied to measure the dynamic conformational changes of DNA and proteins in real time.  Finally, we show how power-independent nanoscale nonreciprocity be can achieved by controlling photon-phonon interactions. By pumping a 4-fold rotationally symmetric metasurface with circularly polarized light, detuned from a high Q resonance, stimulated Raman scattering is shown to occur only when a circularly polarized probe beam is incident from one direction. Subsequently, a nonreciprocal electromagnetic-induced transparency dip is observed in the resonant transmission spectrum.  Unlike previous demonstrations of phonon-mediated optical biasing, here we exploit atomic scale vibrations and so this mechanism has, in principle, no lower bound on the size of a device. Highly subwavelength nonreciprocity should therefore be possible.

Nanophotonic approaches to observe and control atomic and molecular processes

256th National Meeting (2018)

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