Review Provides Blueprint for Optical Nanocircuits
Increasing the clock speed of an electronic device can affect its interaction with light and other electromagnetic waves that pass through it. Controlling light in this way may enable the creation of superior optical microscopes that can reveal the smallest molecules as well as of superior light-based solutions for data storage, processing and exchange.
To control light, promising structures are nanometer-scale metamaterials, which can be optical circuits. A few metamaterials have been created that operate in either the terahertz, infrared or visible ranges.
This image shows a simulated electric field for a ring of four negative-epsilon nanoparticles when excited by a magnetic field at optical frequencies. Reprinted with permission of Science.
However, making metamaterials and optical circuits at the nanoscale remains challenging because nanoscale fabrication methods must be used and because nanometer-size electronic devices behave differently from classical large electronic devices. In a review of metamaterial-inspired optical nanocircuits, Nader Engheta of the University of Pennsylvania in Philadelphia has elucidated approaches to solving these challenges by making analogies between metamaterials and classical RLC circuits.
In RLC circuits, insulating dielectric materials prevent current and energy losses that affect the clock speed. Engheta points out that optical nanocircuit elements can be insulated with layers of materials with relative epsilon parameters near zero, where ε is the symbol used to represent permittivity in Maxwell’s equations. On the other hand, materials with large relative ε parameters can be used as conduits of optical displacement because current wires are used in classical circuits. Moreover, nanoparticles with positive or negative ε may represent, respectively, nanometer-size capacitor or inductor elements at optical frequencies. When the permittivity and permeability of metamaterials are negative, Maxwell’s equations may provide solutions for waves with a negative refractive index. Indeed, negative-permittivity and -permeability metamaterials have been created that reverse the refractive index of incident light from positive to negative.
Engheta and his group have applied the principles of metamaterials to developing the concept of nanoscale optical lumped circuits, which can be applied toward developing lenses with superior resolution and nanometer-size antennas. The main feature of conventional radio-frequency antennas of the Yagi-Uda variety is their resonant dipole and several accompanying parasitic elements, so his team has suggested placing dipole-oriented nanoparticles around an optical source and has analyzed theoretically the Yagi-Uda phenomenon on the nanoscale.
Because the wavelength of light employed in optical microscopy limits the resolution, Engheta’s team and others have designed metamaterials that manipulate the propagation of light and have predicted that these metamaterials can serve as magnifying lenses that resolve smaller objects than conventional optical microscopy. By linking the classical and nanoscale worlds, Engheta has provided in the Sept. 21 issue of Science a blueprint for optical nanocircuits.
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