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Plasmonic Nanoantennas and Their Applications
University of Ottawa1
Nanometallic structures, such as resonant plasmonic nanoantennas, are useful for the conversion of light to surface plasmon-polaritons (SPPs) localized to ultra-small volumes . Such structures can provide highly enhanced fields, strong confinement, high surface sensitivity, and can double as a device electrode for applying voltages or passing currents to active regions in optoelectronic devices. Here we review some of their many applications, in the areas of plasmon-enhanced nonlinear optics [2-5] and optoelectronics [6-9].
In nonlinear optics, plasmon enhancement can be applied to the pump , or to the nonlinear emission [3-5], through spectral alignment. An advantage of the former is that all nonlinear processes involving the pump are enhanced, but a disadvantage is that the pump, typically of high intensity, can damage the nanostructures. Nonetheless, such enhancement was exploited in high-harmonic generation via re-collision radiation in Si , where a 10x enhancement was observed for all harmonics emitted up to the 9th. Alternatively, exploiting the latter is advantageous because the structures are resonant with the nonlinear emission which is typically weak, and non-resonant with the pump so are less susceptible to damage. Such enhancement was exploited in Raman scattering experiments [3-5], where the nanoantennas were spectrally-aligned with the Stokes wavelengths of graphene. Under this scenario, single-antenna scattering cross-section enhancements of over 500× were observed .
In optoelectronics, plasmon enhancement can be exploited to improve the performance of, e.g., electro-optic modulators , photodetectors [7,8], and beam-steering devices . A key requirement is for the plasmonic enhancement to overlap strongly with the active region of the device, which requires that the nanostructure also operate as a device electrode. Such a structure consists of an array of Au nanoantennas contacted perpendicularly by electrical contact lines. The contact lines may be aligned on the nanoantennas such that they are minimally invasive optically. Structures such as these are very promising for beam steering, as each row can be routed to an individual contact such that a 1D phase gradient is applied over the array . Nanostructures that consist of holes in a metal film are also of significant interest , especially arrangements of nanoholes (e.g., heptamers), which can be fabricated via focussed ion beam milling because they exhibit interesting and useful Fano resonances .
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