Infrared antennas and resonant structures take advantage of the successful designs in the radioelectric and microwave spectra. A quite simple and naïve approach would be to consider the geometry and shape of those designs and transfer them into the optical regime with simple wavelength structure size scaling. However, this translation misses the specific behavior of metals and dielectrics at optical frequencies, and how these characteristics can strongly change the geometries and arrangements of a working antenna in the infrared.
Fabrication of optical antennas and resonant structures has been possible only since technology has provided tools and processes able to produce smooth metallic surfaces with sizes ranging in the subwavelength scale for the optical domain. This means that resonant elements are nanophotonic devices and systems. This tiny size also needs an appropriate approach for the design of working elements. The first constraint is related to the physical substrate that the antennas are placed on or embedded in. Most of the antenna-coupled detectors and resonant structures are written on a dielectric substrate or on a dielectric stand-off layer deposited on top of a metallic surface that typically has been evaporated on a dielectric substrate. The thicknesses of these layers are a fraction of a wavelength, and the substrates are dielectric wafers, or plastic flexible substrates. When the device is illuminated from the substrate side, the substrate has to be transparent enough for the given wavelength of operation. In any case, the optical properties of every material involved in the proposed design have to be properly included in the design and modeling of the device. But not only optical properties are important. When thermal effects are at play, thermal conductivity and electrothermal coefficients are also of interest and need to be considered.
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