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Gary Beane, Brendan S. Brown, Paul Johns, Tuphan Devkota, and Gregory V Hartland “Strong Exciton-Plasmon Coupling in Silver Nanowire Nanocavities”, J. Phys. Chem. Lett., Just Accepted Manuscript, DOI: 10.1021/acs.jpclett.8b00313

Abstract: The interaction between plasmonic and excitonic systems and the formation of hybridized states is an area of intense interest, due to the potential to create exotic light-matter states. We report herein coupling between the leaky surface plasmon polariton (SPP) modes of single Ag nanowires and excitons of a cyanine dye (TDBC) in an open nanocavity. Silver nanowires were spin-cast onto glass coverslips, and the wavevector of the leaky SPP mode was measured by back focal plane (BFP) microscopy. Performing these measurements at different wavelengths allows the generation of dispersion curves, which show avoided crossings after deposition of a concentrated TDBC-PVA film. The Rabi splitting frequencies (Ω) determined from the dispersion curves vary between nanowires, with a maximum value of Ω = 390 ± 80 meV. The experiments also show an increase in attenuation of the SPP mode in the avoided crossing region. The ability to measure attenuation for the hybrid exciton-SPP states is a powerful aspect of these single nanowire experiments, as this quantity not readily available from ensemble experiments.

Gary Beane, Kuai Yu, Tuphan Devkota, Paul Johns, Brendan Brown, Guo Ping Wang, and Gregory Hartland, “Surface Plasmon Polariton Interferences in Gold Nanoplates,” J. Phys. Chem. Letters 2017, 8, 4935–4941.

Abstract: Transient absorption microscopy (TAM) measurements have been used to study the optical properties of surface plasmon polariton (SPP) modes in gold nanoplates on a glass substrate. For thin gold nanoplates the TAM images show an oscillation in the signal across the plate due to interference between the ‘bound’ and ‘leaky’ SPP modes. The wavelength of the interference pattern is given by where is the difference between the wavevectors for the bound and leaky modes, and is sensitive to the dielectric constant of the material above the gold nanoplate. Back focal plane imaging was also used to measure the wave-vector of the leaky mode which, in combination with the information from the TAM images, enabled the bound mode wavevector to be determined. These experiments represent the first far-field optical measurement of the wavevector for the bound mode in metal nanostructures.

Zhongming Li, Kyle Aleshire, Masaru Kuno , and Gregory V. Hartland, “Super-Resolution Far-Field Infrared Imaging by Photothermal Heterodyne Imaging,” J. Phys. Chem. B, 2017, 121, 8838–8846

Abstract: Infrared (IR) imaging provides chemical-specific information without the need for exogenous labels. Conventional far-field IR imaging techniques are diffraction limited, which means an effective spatial resolution of >5 μm with currently available optics. In this article, we present a novel far-field IR imaging technique based on photothermal heterodyne imaging (IR-PHI). In our version of IR-PHI, an IR pump laser excites the sample, causing a small temperature rise that is detected by a counterpropagating visible probe beam. Images and spectra of several different types of soft matter systems (polystyrene beads, thin polymer films, and single Escherichia coli bacterial cells) are presented to demonstrate the sensitivity and versatility of the technique. Importantly, the spatial resolution in the IR-PHI measurements is determined by the visible probe beam: a spatial resolution of 0.3 μm was achieved with a 0.53 μm probe wavelength and a high numerical aperture focusing objective. This is the highest spatial resolution reported to date for far-field IR imaging. Analysis of the experiments shows that for polymer beads in a dry environment, the magnitude of the IR-PHI signal is determined by the scattering cross section of the nano-object at the probe wavelength. This is in contrast to conventional PHI experiments in a heat-transfer medium, where the signal scales as the absorption cross section. This different scaling can be understood through the optical theorem. Our analysis also shows that both thermal expansion and changes in the refractive index of the material are important and that these two effects, in general, counteract each other.