“p–n Junction”: Plasmonics Enables Photonic Access to the Nanoworld

Since the development of the light microscope in the 16th century, optical device size and performance have been limited by diffraction. Optoelectronic devices of today are much bigger than the smallest electronic devices for this reason. Achieving control of light–material interactions for photonic device applications at the nanoscale requires structures that guide electromagnetic energy with subwavelength-scale mode confinement. By converting the optical mode into nonradiating surface plasmons, electromagnetic energy can be guided in structures with lateral dimensions of less than 10% of the free-space wavelength. A variety of methods—including electron-beam lithography and self-assembly—have been used to construct both particle and planar plasmon waveguides. Recent experimental studies have confirmed the strongly coupled collective plasmonic modes of metallic nanostructures. In plasmon waveguides consisting of closely spaced silver rods, electromagnetic energy transport over distances of 0.5 m has been observed. Moreover, numerical simulations suggest the possibility of multi-centimeter plasmon propagation in thin metallic stripes.Thus, there appears to be no fundamental scaling limit to the size and density of photonic devices, and ongoing work is aimed at identifying important device performance criteria in the subwavelength size regime. Ultimately, it may be possible to design an entire class of subwavelengthscale optoelectronic components (waveguides, sources, detectors, modulators) that could form the building blocks of an optical device technology—a technology scalable to molecular dimensions, with potential imaging, spectroscopy, and interconnection applications in computing, communications, and chemical/biological detection.