Spatially switched near field distribution using plasmonic random nanoislands

We investigate switching near-field distribution on metal random nanoislands by changing the direction and the angle of light incidence in 14 channel modes. Distribution of the near-fields induced by different channel modes was calculated by finite difference time domain method. The size of near-fields under oblique channel modes ranges 48 - 77 nm in contrast to 127 - 145 nm with normal incidence. Quantitative analysis of near-field position was performed relative to nanoislands. Near-field position was largely well aligned with the direction of incident channel modes. Switching near fields was experimentally confirmed in two ways, first by measurement of fluorescence intensity and by NSOM. Fluorescence experiment was conducted by using bare glass substrate and gold nanoislands in seven channel modes. Fluorescence intensity on bare glass substrate shows symmetric intensity changes with channel modes. However, fluorescence intensity on gold nanoislands was found to be asymmetric. For quantitative analysis, mean-squared error (MSE) was calculated by defining fluorescence intensity as a 7D vector. Distribution of MSE in case of gold nanoislands was broader than on bare glass substrate. In other words, near fields induced on gold nanoislands were switched more strongly than bare glass substrate. Also, near fields induced on nanoislands were measured directly using NSOM in two channel modes. It was confirmed that spatial positions of near-fields depend on channel modes. The results of this study suggest that the near fields can be controlled by adjusting channel modes, which opens possibilities of highly sensitive and super-resolved detection and imaging.