In microelectronics, the notion of a circuit is a powerful concept in which a flow of a certainquantity (e.g., electric current as the “flow” of charges) is related to a potential of another quantity (e.g., electric potential) through the functions of “lumped” elements (e.g., resistor, inductor, capacitor, diode). This “lumpedness” of circuit elements is an important assumption in modeling, allowing simplification and, effectively,modularization of the function of each element. From a systems point of view, in effect what is happening inside the element becomes less relevant to the connectivity and functionality of this modularized element to the rest of the system. This notion has been extensively and successfully used in the radio frequency (RF) and microwave domains and has been proven to be a powerful tool in the design, innovation, and discovery of new functionalities in circuits in those frequency domains. Extending the operating frequency to higher frequency regimes—for example, terahertz, infrared (IR), and visible wavelengths— may in general lead to miniaturization of devices, higher storage capacities, and larger data transfer rates.
Therefore, a natural questionmay be asked:
Can this concept of lumped circuit elements, and the mathematical machinery and tools of circuit theory, be extended and applied to the optical domain? Initially, one may imagine that merely scaling down the sizes of elements from the microwave to optical wavelengths may achieve this goal. However, several obstacles must be overcome before such optical lumped elements can be conceived. The first challenge is the size of such an optical module. Just as circuits in the lower frequency domains (e.g., in RF and microwave domains) indeed involve elements that are much smaller than the wavelength of operation, fabrication techniques can be used to construct nanoparticles with subwavelength dimensions at optical wavelengths. Therefore, the obstacle of size reduction may be overcome. The second, more limiting, hurdle is the response of metals at IR and optical frequencies, which cannot be simply scaled from RF to optics. Metals such as gold, silver, aluminum, and copper are highly conductive materials at RF and microwaves and consequently are commonly used in many circuits in these regimes.
Can this concept of lumped circuit elements, and the mathematical machinery and tools of circuit theory, be extended and applied to the optical domain? Initially, one may imagine that merely scaling down the sizes of elements from the microwave to optical wavelengths may achieve this goal. However, several obstacles must be overcome before such optical lumped elements can be conceived. The first challenge is the size of such an optical module. Just as circuits in the lower frequency domains (e.g., in RF and microwave domains) indeed involve elements that are much smaller than the wavelength of operation, fabrication techniques can be used to construct nanoparticles with subwavelength dimensions at optical wavelengths. Therefore, the obstacle of size reduction may be overcome. The second, more limiting, hurdle is the response of metals at IR and optical frequencies, which cannot be simply scaled from RF to optics. Metals such as gold, silver, aluminum, and copper are highly conductive materials at RF and microwaves and consequently are commonly used in many circuits in these regimes.
However, at optical frequencies, some noble metals behave differently in that they do not exhibit conductivity in the usual sense but instead exhibit plasmonic resonance (i.e., couplingof optical signals with collective oscillation of conduction electrons at thesemetal surfaces) asa result of the negative real part of their permittivities. Therefore, clearly, at optical wavelengths the conduction current may not be the main current flowing in such lumped optical elements Therefore, just scaling down the element size may not provide answers !!!Circuits with Light at Nanoscales: Optical Nanocircuits Inspired by MetamaterialsNader EnghetaScience 21 September 2007: 1698-1702.
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