Terahertz Nanoantennas: Design and Applications

Dear Colleagues,

Antennas operating in the terahertz (THz) frequency regime, also called nano-antennas, are currently attracting exceptional research interest. This is motivated by a plethora of empowered applications, such as: high speed terahertz communications, ultrafast nanodevices, biosensing, biomedical imaging, photo-detection, energy harvesting, and even the inspection of photovoltaics. The first attempts toward nano-antennas elaborated on the idea to extend-transfer microwave antenna technology toward THz. This is mainly based on metallic structures in air or printed-loaded by dielectrics, and ultimately by agile materials such as ferrites, ferroelectrics or multi-ferroics, which enable tunability in the operating frequency or in the radiation pattern. The major challenge in this trend is related to the metal properties, which tend to behave like lossy dielectrics in the THz and optical frequencies, rather than a perfect electric conductor in the microwave regime. In spite of this, numerous useful metallic nano-antennas and arrays are developed exploiting the “surface plasmon polariton, SPP” (known as surface waves in microwaves) phenomena, in conjunction with periodic structures known as frequency selective surfaces or meta-surfaces. The trend toward possibly more appropriate materials supporting current flow with lower losses at THz frequencies and ultimately tunability has already revealed the value of graphene, nematic liquid crystals and vanadium dioxide (VO₂), and the quest continues. The important feature offered by them is the possibility of dynamic control (indications refer to colossal values) through a DC electric field (applying DC voltage), which may be implemented in thin or thick film or even integrated in chip form, similar to transistor topologies, offering vast possibilities in THz applications. An inherent property of ordinary nano-antenna geometries built on metal, graphene, nematic liquid crystals, or VO₂, is their sharp resonances and thus extremely narrow bandwidth. These may be directly exploited by tuning via DC voltage control through the frequency band of interest. The narrowband restriction can be overcome by employing ultra-wideband (UWB) geometries, known in the microwaves regime as “frequency independent antennas-arrays” or self-complementary antennas: logarithmically periodic arrays, biconical, spiral, helical or Vivaldi type. Particular effort is needed to transfer such antenna technology to THz, so as to become UWB nano-antennas. An elegant way to provide for directional antennas beam steering is through “leaky wave antennas”, and particularly through their implementation as periodic structures (possibly tunable) based on graphene or VO₂, nematode liquid crystals, and as metallic ones operating in SPP. Besides the above trends, nano-antennas in the THz and optical regimes can be built on semiconductors or as generally known “silicon photonics technology”. These, similar to nematode liquid crystals, offer the additional possibility of controlling the antenna characteristics through their operating temperature. An exotic feature of silicon nano-antennas is their ultimate control through the DC-magnetic field, when operated at cryogenic temperatures where the semiconductor behaves as solid-state plasma. The latter feature is expected to be implemented in combination with superconducting cryogenic frontends.

All the above are vast possibilities of nano-antenna structures which enable yet unexplored practical applications in the THz and optical regimes. These constitute research challenges for the years and efforts to come. Efforts toward the study of the involved material properties and the nano-antenna implementation-fabrication and measurements-testing are of equal importance and are highly encouraged.

The purpose of this Special Issue is to lay out the state-of-the-art in the above technologies, as well as to offer an insight into the forthcoming evolutions in the THz and optical nano-antennas, arrays or metasurfaces. Particular emphasis is sought toward achieving tenability and broadband operation. Toward these tasks, it welcomes review articles, comprehensive group research activities, and reviews, as well as high quality research papers highlighting the recent evolutions of the subject.

Prof. Dr. George Kyriacou
Prof. Dr. Georgios Trichopoulos
Prof. Dr. Filippos Farmakis
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Electronics is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2400 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.