Strong coupling between Surface Plasmon Polaritons and Excitons for Silver Nanowires

When dye molecules or quantum dots are placed in photonic or plasmonic cavities, new hybrid states can be formed that are mixtures of the excitonic and the plasmonic/photonic states. These states have unusual properties, and have been the subject of a number of recent studies. A particularly useful way to investigate coupled plasmonic-excitonic systems is to measure a dispersion curve (frequency versus wavevector). These curves show an avoided crossing in the presence of strong coupling. Analysis of the avoided crossing yields the coupling strength or “Rabi Frequency” for the system. In this talk, recent measurements of dispersion curves for the “leaky” surface plasmon polariton (SPP) modes of Ag nanowires will be discussed. In these experiments the SPP modes are launched by focusing a laser at the end of the nanowire with a high numerical aperture objective. Scattered light from the leaky mode is collected by the same objective and sent to a camera. The wavevector for the leaky mode was measured by imaging the back focal plane of the objective onto the camera. Performing these measurements for a series of laser wavelengths generates a dispersion curve. The experiments were conducted on bare nanowires, and on nanowires coated by a thin dye-doped polymer film. The dye-coated nanowires show clear evidence of an avoided crossing. The measured dispersion curves are compared to finite element simulations, which provide a more accurate description of the system compared to the conventional coupled oscillator model for plasmon-exciton coupling.