Molecular Nanopolaritonics

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Daniel Neuhauser of UCLA is supported by an award from the Theoretical and Computational Chemistry program for work to develop a theoretical methodology to understand molecular nanopolaritonics. The project is intended to unify the treatment of radiation and matter in such a way as to efficiently and accurately describe systems of arbitrary physical geometry and electronic structure. Prior to this reseach, combined plasmon-matter studies typically utilized multipole mode expansions on the plasmon-carrying structure, which although simple, are unable to accurately capture arbitrary geometries or field singularities. Using TDDFT for the whole system, on the other hand, is prohibitively expensive computationally. The PI is, thus, extending previous simulations, both in terms of method used and applications chosen for testing those methods. The research starts with FDTD-type algorithms (to be modified for near-field applications), and continues on to discrete-dipole studies, and finally implements embedding formalisms using Hydrodynamic Tensor DFT. The molecular part is being described by a real-time TDDFT algorithm, and treated non-linearly as to capture the multiharmonic and frequency mixing characteristics of the system.The drive towards ever smaller scales for radiation features has led to the new field of plasmonics in which light transport along metal nanoparticle arrays and waveguides is studied at distances as small as a few nanometers (nm). At the same time, electronic structure calculations have also reached the nm size scale, so that the distinction between radiation and matter scale is being blurred out. This has led to a variety of studies where dipolar emission coupling of a few plasmons and excitons are considered, with interesting resonance, field- and spatial-dependence and more. Matter-radiation on the nanoscale (nanopolaritonics in short) is now ripe for a realistic description of both near-field radiation and molecules. The PI and his group are merging Maxwell's near-field description with modern studies of electronic dynamics, to simulate combined matter-radiation (plasmon-exciton, i.e., polariton) systems on the nanoscale. Some applications being considered are: gating of radiation transfer, specifically in large scale plasmonic systems with birefringence effects, including questions about whether molecules can control these systems, an application with possible use in imaging; nonlinear selective microscopy on the nanoscale -- a field with potentially huge impact for sensing applications from engineering to medicine; conversion of electromagnetic near-field energy to physical motion on the nanoscale, which will have practical importance in any field requiring motion control; photovoltaics where plasmons are hoped to reduce absorber sizes; matching of near and far fields, which could conceivably be affected by molecular motion; and the development of plasmon logic circuits .