**About the Editor**

**Victor Pacheco-Pe ˜na** received his PhD degree, cum laude, at Public University of Navarra (Spain). He holds a prestigious Newcastle University Research Fellowship, and he works within the School of Mathematics, Statistics and Physics at the same institution in the UK. He has published more than 100 papers in high impact journals and international conferences. He was a visiting researcher at the Imperial College London (UK) and University of Pennsylvania (USA) in 2014 and 2015, respectively. From 2016 to 2018, he was appointed as a Postdoctoral Fellow within the Department of Electrical and Systems Engineering from the University of Pennsylvania (USA). He has been awarded as "Young Scientist" by the URSI GASS 2020 in Rome, Italy, "Young Scientist of the Year" by the Spanish URSI during the XXXI Spanish Conference URSI 2016 and has received a CST University Publication Award for Best International Journal Publication using CST Microwave Studio (R) in 2016. He serves on the Editorial Board of several international journals including Frontiers in Photonics, *Applied Sciences* (MDPI) and *Electronics* (MDPI). His current research is focused on Metamaterials, THz, Plasmonics, Optics and Electromagnetism.

### *Editorial* **Terahertz Technologies and Its Applications**

**Victor Pacheco-Peña**

School of Mathematics, Statistics and Physics, Newcastle University, Newcastle Upon Tyne NE1 7RU, UK; victor.pacheco-pena@newcastle.ac.uk

#### **1. Introduction**

The terahertz frequency range (0.1–10) THz has demonstrated the provision of many opportunities in various fields, such as high-speed communications, biomedicine, sensing, and imaging [1–6]. Historically, this frequency range, lying between the fields of electronics and photonics, was known as the "terahertz gap" because of the lack of sources, detectors and fabrication technologies.

However, considerable effort is now being devoted worldwide to improving this technology. Within this context, great progress has been made to fill the gap in this interesting spectral range, such as multiplexers and tuneable devices [7], among others. The aim of this Special Issue is to provide a platform to highlight the work being conducted within this range of the electromagnetic spectrum.

#### **2. In This Special Issue**

This Special Issue consists of thirteen papers covering a range of applications using THz technologies, including THz sensing and imaging, spectroscopy applications, and nondestructive testing. The contents of these papers are introduced below.

Reference [8] presents the modelling and evaluation of zero-biased Schottky diodes. Two different mounting techniques are considered: wire bonding and flip-chip. The experimental results are supported by numerical simulations demonstrating the validity of the proposed models. The improvement of radar cross-section using THz signals is shown and demonstrated in Reference [9], where the concept of adaptive gates is adopted to reduce the signal-to-noise ratio, thereby improving the accuracy of the measurement. The design of a frequency multiplier source working at 0.335 THz is reported in Reference [10] with two different schemes; experimental validations of the proposed designs are provided.

This Special Issue also includes applications in THz spectroscopy. In Reference [11], the authors propose a mechanism to improve the accuracy of optical delay lines for THz spectroscopy applications by using an optical encoder. In Reference [12], the authors demonstrate a radiation power improvement of almost four times for spiral photoconductive antennas. Analyses of the structures are carried out using THz time-domain spectroscopy. In Reference [13], it is shown how THz spectroscopy can be used for nondestructive testing of the hollowing deterioration of stone relics (Yungang Grottoes in this case). Further applications for non-destructive testing using THz radiation are presented in Reference [14], where an optimal scanning technique for honeycomb sandwich composite panels is proposed. THz spectroscopy is applied in Reference [15] to evaluate the vulcanization and macrodispersion of silica for rubber products using THz absorption measurements.

The design, study, and experimental demonstration of a biased sub-harmonic mixer working at a frequency of 0.67 THz is presented in Reference [16], demonstrating a conversion loss of 18.2 dB in the band between 0.650 THz and 0.690 THz. A synthetic aperture THz imaging technique based on the light field imaging system is proposed in Reference [17]. An on-chip THz detector is presented in Reference [18]; it is designed by using both an onchip inset-feed rectangular patch antenna and a catadioptric lens. Reference [19] presents a nano displacement sensor using hetero-structure waveguides working in the THz frequency range of 0.8–1.1 THz, demonstrating a maximum sensitivity of around 1.2 GHz/μm.

**Citation:** Pacheco-Peña, V. Terahertz Technologies and Its Applications. *Electronics* **2021**, *10* , 268. https:// doi.org/10.3390/electronics10030268

Academic Editor: Hirokazu Kobayashi Received: 23 December 2020 Accepted: 21 January 2021 Published: 23 January 2021

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**Copyright:** © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

A coupled stack oscillator working at 0.350 THz is presented in Reference [20], showing an output power of −0.8 dBm at 0.3532 THz.

**Funding:** V.P-P. is supported by Newcastle University (Newcastle University Research Fellowship).

**Acknowledgments:** I would like to thank all of the researchers who submitted their work to this Special Issue. Their contribution is invaluable, and they have equally contributed to make this Special Issue a success. I would also like to express my gratitude to all of the reviewers who helped in the evaluation process of all of the manuscripts, made important suggestions, and contributed to improving the quality of the accepted manuscripts. I would also like to acknowledge the support from the Editorial Office of Electronics who worked extremely hard to maintain the high standard of the journal when the Special Issue was live and supported me during the whole process for a rigorous and timely peer review of the manuscripts.

**Conflicts of Interest:** The author declares no conflict of interest.

#### **References**


*Article*
