Near-field scanning optical microscopy and spontaneous emission engineering with plasmonic metamaterials
In this work we have studied the effects of aperture-sample interactions on nearfield optical imaging and its application to plasmonic metamaterials. Specifically, we have extensively studied a system of paired gold nanorods using near-field scanning optical microscopy (NSOM). When illuminated using the small aperture of a metal-coated fiber probe, vertically paired nanorods act like nanoantennae in the NSOM; they show reverse contrast in reflection- and transmission-mode images in the visible wavelength range. Coupling between the aperture and the nanorod array makes this contrast higher depending on polarization, and a near-field analog of extraordinary optical transmittance has been observed. In addition, the average near-field transmission exhibits an opposite sign of anisotropy relative to the far-field case. These results demonstrate that the broad angular wave-vectors of small-aperture sources play a crucial role in near-field interactions with nanorod arrays. We have also studied this probe-nanorod system using finitedifference time-domain (FDTD) simulations. By varying the NSOM tip geometry and the wavelength, we have determined and tuned the resonance wavelength of the probe-sample system at which the near-field interaction is enhanced. The near-field maps of the electric and magnetic fields in the metamaterial structure were also obtained. This knowledge allows us to optimize the design of plasmonic nanostructures, which are the building blocks which enable applications such as directional nanoantennae, optical metamaterials with artificial magnetism, and highly sensitive biosensors.
In addition, the spontaneous emission processes has been studied for organic dyes near hyperbolic metamaterials (HMMs), which have different signs of the real parts of the dielectric tensor components. HMMs enhance the photonic density of states dramatically, and thus enable increased spontaneous emission, opening new possibilities for photonic density of states engineering. In this regard, we directly demonstrated that the radiative decay rate of the emitter has been increased due to coupling with the HMM states when compared with measurements from control samples such as thin and thick gold substrates, as predicted by theoretical calculations.