3. Plasmonic nanoantennas

            The improvement of emitters with plasmonic nanostructures is of large importance for applications in lighting and quantum optics. Selective modification of only radiative decay channels is critical for light-emitting devices that benefit only from radiative enhancement. We have investigated a novel class of optical antenna designs that are optimized specifically with respect to this radiative outcoupling of plasmons into far-field light.

           We have studied the resonant enhancement of the fluorescence emission from dye molecules close to single optical nanoantennas. Dimer antenna structures were fabricated consisting of two interacting gold nanorods with varying lengths and interparticle separation. Two-dimensional confocal fluorescence lifetime scans are used to visualize the spontaneous emission enhancement of the molecular fluorescence around the antenna clusters. At every position, a fluorescence decay curve is measured as shown in Figure 1.

Figure 1 Normalized fluorescence decay measured at a resonant nanoantenna (diamonds, black) and away from the nanoantenna (open dots, red), with exponential decay fits (lines, black). Inset: SEM image of the resonant gold nanoantenna, consisting of two 90x60x20 nm³ gold nanorods with an antenna gap of 20 nm.

           By fabricating arrays of nanoantennas with controlled length and interparticle gap distance, we have measured the radiative rate enhancement to the antenna parameters, as shown in Figure 2. The radiative rate shows a maximum related to the spectral resonance position of the longitudinal plasmon mode at the fluorescence emission wavelength of 730 nm. For antennas with a narrow gap, an additional increase of the decay rate is observed compared to uncoupled antennas.

           A clear effect of antenna coupling on the spontaneous emission enhancement was found and explained, using a theoretical model, by the strong local field enhancement in the antenna gap (see Figure 3). We conclude that, although enhancement is limited by metal losses, these can be small enough to result in considerable quantum efficiency improvement.

Figure 3 Radiative and nonradiative decay rates g_R, g_NR for a dipole emitter are shown versus antenna arm length L, for the strongly coupled antenna arms. (Top graphs) Calculated near-field intensities around two resonant antennas with spots indicating the location of the emitter in the calculations (scale bar denotes 100 nm).

By designing metal antennas with optimized arm length and gap width, the spontaneous emission of a fluorescent dye was enhanced by more than a factor 5, while the quantum efficiency was improved from 40% to 53%. This holds promise for enhanced light extraction from luminescent devices and for quantum optics using single emitters.

Reference:
O. L. Muskens, V. Giannini, J. A. Sánchez Gil, J. Gómez Rivas, Strong enhancement of the radiative decay rate of emitters by single plasmonic nanoantennas, Nano Lett. 7(9), 2871:75 (2007)