Optical field imaging of elongated rectangular nanovoids in gold thin film

We studied optical properties of single elongated rectangular nanovoids opened on a thin gold film by scanning near-field optical microscopy. We found that the luminescence is induced via two-photon absorption of near-infrared pulses when the sample is locally photoexcited inside the void as well as on the film. The luminescence spectrum of the void shows spectral features very similar to those observed for gold nanorods and is attributable to that from gold. On the basis of the near-field two-photon excitation imaging by detecting the luminescence, we visualized the optical fields near the voids. The optical image shows a characteristic spatial feature depending on the dimension of the void. From the close inspection of the topography and the optical images, we found that excitation probability becomes high at the metal-void boundary as well as inside the void. We found an oscillating feature along the long axis of the void. The observed spatial features were qualitatively reproduced by photonic local density-of-states based on electromagnetic calculations, and we concluded that the oscillation observed inside the void is attributed to the electromagnetic modes in the void coupled with the plasmon excitations.