Revealing the plasmon coupling in gold nanochains directly from the near field

We studied the near-field properties of localized surface plasmon resonances in finite linear gold nanochains using photoemission electron microscopy (PEEM). The localization of the electromagnetic field in the near-field region was mapped at high spatial resolution. By tuning the excitation laser wavelength, we can obtain the near-field spectra, from which the energy splitting between longitudinal (L) and transverse (T) plasmon modes can be revealed. In particular, the L-mode red shifts and the T-mode blue shifts with increasing chain length. The red shift of the L-mode is highly dependent on the gap distance. In contrast, the T-mode almost remains constant within the range of gap distance we investigated. This energy splitting between the L-mode and the T-mode of metallic chains is in agreement with previous far-field measurements, where it was explained by dipole-dipole near-field coupling. Here, we provide direct proof of this near-field plasmon coupling in nanochains via the above-described near-field measurements using PEEM. In addition, we explore the energy transport along the gold nanochains under excitation at oblique illumination via PEEM measurements together with numerical simulations.