Essay: Nanoscience Review of Optical Antennas and their Various Applications

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In recent years, science and technology fields have mainly concentrated on nanoscale devices. As such, optical antennas, also known as antenna-coupled optical detectors, are being developed at a fast pace. They are being introduced as alternative detection devices with nanoscale features for the infrared, millimeter, and visible spectra. Optical antenna functionality relies on understanding the relationship between the geometric parameters and the resulting near-field antenna modes. Optical and infrared antennas couple electromagnetic radiation in the visible and infrared wavelengths. The size of an optical antenna is in the range of the detected wavelength and they involve fabrication techniques with nanoscale spatial resolution. Optical antennas can also be considered as directionally sensitive elements and point detectors. So far, these detectors show positive results in the mid-infrared and visible regions. Optical antennas are a critical component in nanophotonics research and have been used to enhance nonlinear and Raman cross-sections and to make nanoscale optical probes. The disruption of the coherent current oscillation by introducing a gap gives rise to an effective multipolar mode for the two near-field coupled segments. Using antenna theory and numerical electrodynamics simulations, two distinct coupling regimes are considered. Both regimes scale with gap width or reactive near-field decay length, respectively. The results emphasize the distinct antenna behavior at certain optical frequencies, compared to impedance matched radio frequency (RF) antennas, and provide experimental confirmation of theoretically predicted scaling laws at those optical frequencies. Furthermore, we can derive a fundamental limit on the antenna emittance and theoretically support that these structures are nearly ideal black-body antennas.
In optical science and engineering, light is commonly controlled by redirecting the wave fronts of propagating radiation by means of lenses, mirrors, and diffractive elements. This type of manipulation depends on the wave nature of electromagnetic field and is therefore not open to controlling fields on the sub-wavelength scale. By expanding the concept of geometric optics, optical antennas provide a means of focusing radiant visible as well as infrared (IR) light down to nanometer length scales. However, when addressing up to several orders of magnitude, dimensional mismatch between the emitter or receiver in the form of molecules, quantum dots, or waveguides on one side and the associated wavelengths of the radiation on the other, has remained a major challenge. By using optical antennas, this challenge typically needs to be met by through-space near-field coupling and not by a feed line from the receiver or emitter as in the radio frequency (RF) case. Recent work in nano-optics and plasmonics has generated huge interest in the field of optical antennas and several studies are currently focused on how to translate established radio wave and microwave antenna theories into the optical frequency regime. Due to the small scale of optical antennas in technological applications, they are not mainly considered. The fabrication of optical antenna structures is an emerging opportunity for novel optoelectronic devices. The diffraction of optical waves limits the confinement of propagating radiation to about half a wavelength. The length scales over, such that …

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