PlasmonicsÐA Route to Nanoscale Optical Devices

In recent years, there has been tremendous progress in the miniaturization of optical devices. Planar waveguides and photonic crystals are currently key technologies enabling a revolution in integrated optical components. The size and density of optical devices employing these technologies is nonetheless limited by the diffraction limit of light, which imposes a lower size limit on the guided light mode of about k/2n (a few 100 nm). Another limitation is the typical guiding geometry. Whereas photonic crystals allow for guiding geometries such as 90 corners, planar waveguides are limited in their geometry because of radiation leakage at sharp bends. Scaling optical devices down to the ultimate limits for the fabrication of highly integrated nanophotonic devices and circuits requires electromagnetic energy to be guided on a scale below the diffraction limit and that information can be guided around 90 corners (bending radius << wavelength of light). Recently, a new method for the guiding of electromagnetic energy has been proposed that allows for a further reduction of the device size to below the diffraction limit in a variety of geometries. It was shown that electromagnetic energy can be guided in a coherent fashion via arrays of closely spaced metal nanoparticles due to near-field coupling. After this initial paper, a number of publications discussing similar structures appeared. In this paper, we will describe the findings of our theoretical and experimental work on metal nanoparticle waveguides, and their macroscopic microwave frequency analogues. In Section 2, we report on theoretical modeling of nanoparticle waveguides and show dispersion relations of propagating modes in such waveguides. In Section 3, we present experimental verification of the guiding properties of periodic metal structures in a macroscopic analogue of nanoparticle waveguides, operating in the microwave regime. Our results confirm that energy is coherently transported in these subwavelength guiding structures and that energy propagation through corners and tee structures is possible and that an all-optical switch can be designed. Additionally, the electric