Colloidal metal nanoparticles are renowned for their ability to strongly scatter and absorb light due to size- and environment-dependent plasmon modes. Active areas of research focus on using both single and collections of nanoparticles to control the shape of electromagnetic fields on the nanoscale. The excitation of plasmon modes in the nanoparticles confines the energy from incident fields to sub-wavelength scales with distributions controlled by the morphology of the particles, and multiple particles arranged in the near-field can extend the excitation into a collective mode. The excitation of plasmon modes can create enhancements of the field intensity, which have been leveraged for enhancing radiative rates of light-emitting particles and molecules and increasing molecular sensing signals. However, many of these applications rely on electric field enhancements in the near field and using static nanoparticle arrangements. We present extensions to this paradigm, first by exploring the excitation of collective plasmon modes in optically-patterned linear nanoparticle arrays with separations on the order of the wavelength of light, demonstrating new mechanisms for coupling beyond the well-known near-field interactions. The collective excitation over the intermediate-scale arrays is also shown to redirect the scattered light perpendicular to the expected forward scattering. Next, we demonstrate that self-arranged optically bound linear arrays act as optical cavities for co-trapped single-photon emitters, modifying the local density of electromagnetic states in the vicinty of the nanoparticle system. Finally, we probe optically 'dark' modes in a core-satellite nanostructure by exciting magnetic responses separately from electric modes with structured excitation light.




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