Tailoring coupled plasmonic structures is an alternative way to obtain new optical properties of plasmonic materials. Recently, film-coupled nanoparticle systems with high stability and controllability have been used to probe the ultimate limits of field enhancement/confinement and near-field interaction with other quantum emitters. When the gap between particles and films is below a few nanometers, induced high order (HO) gap modes become significant. In this report, we investigate these HO modes associated with the system of a gold nanosphere positioned above a gold film, separated by a nanometer scale spacer layer.The shift in far-field scattering profile under different excitation conditions and collection wavelengths indicates the influence of HO modes and the results are compared to the corresponding simulations. In addition, the far-field scattering spectra/patterns by multipole expansion and the near-field distributions by FEM, both calculated with dielectric function of the low damping factor are utilized to resolve the individual HO modes. These findings not only identify the HO gap modes but also clarify their excitation conditions and far-field/near-field scattered field distribution. The effect of HO modes should be taken into account when the interaction between the gap field and quantum emitters nearby is investigated for active plasmonic devices.

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We observe the stabilization of a single-(double-) charge optical vortex propagating in a self-focusing medium. The optical vortex, which carries a phase singularity at its center, usually breaks up in a self-focusing medium due to the so-called azimuthal instability. However, by adding a small rotating azimuthally-periodic intensity modulation on the vortex light beam, which propagates in a noninstantaneous self-focusing medium, we successfully suppress the azimuthal instability. This observation is confirmed by both numerical simulation and perturbational analysis.

Strong coupling between light and matter is the foundation of promising quantum photonic devices such as deterministic single photon sources, single atom lasers and photonic quantum gates, which consist of an atom and a photonic cavity. Unlike atom-based systems, a strong coupling unit based on an emitter-plasmonic nanocavity system has the potential to bring these devices to the microchip scale at ambient conditions. However, efficiently and precisely positioning a single or a few emitters into a plasmonic nanocavity is challenging. In addition, placing a strong coupling unit on a designated substrate location is a demanding task. Here, fluorophore-modified DNA strands are utilized to drive the formation of particle-onfilm plasmonic nanocavities and simultaneously integrate the fluorophores into the high field region of the nanocavities. High cavity yield and fluorophore coupling yield are demonstrated. This method is then combined with e-beam lithography to position the strong coupling units on designated locations of a substrate. Furthermore, the high correlation between electronic transition of the fluorophore and the cavity resonance is observed, implying more vibrational modes may be involved. Our system makes strong coupling units more practical on the microchip scale and at ambient conditions and provides a stable platform for investigating fluorophore-plasmonic nanocavity interaction.

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