With an index of refraction approaching 9 at microwave frequencies, water is a high-index dielectric material suitable for studying morphology-dependent resonance (MDR) analogs of metallic plasmonic hotspots. We use experimental, analytical, and computational approaches to study MDRs in aqueous dimers, paying particular attention to formation of evanescent axial hotspots. We use hydrogel beads and thermal imaging, along with FEM simulations to explore polarization, orientation, and size-dependence in MDR hybridization. A novel analytical approach involving vectorial addition of spherical harmonics of monomer MDRs provides geometric insight into which modes most strongly interact to yield strong subwavelength electromagnetic hotspots in aqueous bispheres.
Despite its low index of refraction at visible wavelengths, water exhibits a large complex index of refraction at 2.5 GHz. As such, cm-scale aqueous objects like grapes and hydrogel beads are resonant in microwave radiation, showing the expected sequence of spherical Mie scattering modes. We expand our study to aqueous spheroids, where analytical prediction of resonant sizes and shapes are considerably more difficult to make. We show that 3D standing-wave solutions accurately predict certain fundamental microwave Mie resonances in both oblate and prolate spheroids, and we compare this conceptually and mathematically-simplified approach with results obtained using FEM simulations.
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