Yan Zhang

Guest Editorial TTA Special Section on Terahertz Functional Devices

Yan Zhang

Terahertz （THz） radiation has been extensively investigated in recent years due to its potential applications in communication， homeland security， safety inspection，sensing， and imaging. For a common THz system， three parts are quite important: THz sources， THz detectors， and THz functional devices. Functional devices can effectively improve the signal to noise ratio of a system， increase the transfer efficiency and even the sensitivity of the sensor. However， functional devices are also facing the “THz gap”，just similar with THz sources and detectors.1

In this special section， several progresses in the THz spectroscopy are reported， including the development of terahertz time domain spectroscopy （THz TDS） system，application of THz spectroscopy for investigation of explosive， and using emission spectrum for THz photoconductive antennas improvement.

In the first paper， a group from the University of Shanghai for Science and Technology reports a tunable THz filter based on parallel-plate waveguides （PPWG） with a single cavity and double cavities. The air gap can be tunable by mechanical control or electrical adjustment to effectively select band number. An on-to-off control of the electromagnetic induced transparency （EIT） resonance is also achieved by mechanically tuning the spacing. It may inspire interest in developing mechanically tunable waveguides based EIT， resulting in a wide range of novel compact THz devices， such as slow light components，sensitive sensors， and electromagnetically induced absorbers.

In the second paper， a joint research group from Tianjing University and Shanghai Institute of Applied Physics reports a THz metamaterial sensor based on the EIT peak shift. The metamaterial sensor， which consists of an EIT element unit with a cut-wire metallic resonator and two split-ring metallic resonators fabricated on a 490-μm thick silicon substrate， operates in a transmission geometry. The EIT peak red-shifts and decreases with the increase of the water volume. A maximum red-shift about 54 GHz of the EIT peak is detected between the 1，4-dioxane and water. The linear behavior and high sensitivity of the EIT peak depending on the water concentration provides a novel avenue for THz sensors.

The third paper presents an electric-controlled THz modulator which can be used to realize amplitude modulation of THz waves with slight photo-doping. A team from the Capital Normal University develops the monolayer silicon-based device for electrically modulating THz wave transmission. The modulation depth can arrive almost 100% when the applied voltage is only 7 V at an external laser intensity of 0.6 W/cm2. The saturation voltage reduces with the increase of the photo-excited intensity. A significant fall in both THz transmission and reflection is also observed with the increase of the applied voltage. This reduction in THz transmission and reflection is induced by absorption for electron injection. The results demonstrate that a high-efficiency and high modulation depth broadband electric-controlled terahertz modulator in pure Si structure has been realized.

As the guest editor of this issue， I would like to express my sincere thanks to all the authors for their generous contributions to this special section， they have shared their newest works and exciting results with us. I also appreciate the assistance of the editorial staff， their great efforts make this special section to be published in time.

Yan Zhang， Guest Editor

Capital Normal University

Beijing， China

Yan Zhang received his M.S. and Ph.D. degrees from the Harbin Institute of Technology and Institute of Physics， Chinese Academy of Science in 1996 and 1999，respectively. He is now a full professor with the Department of Physics， Capital Normal University. He has published more than 150 journal papers. His research interests include optical information processing， surface plasmonic optics， and THz spectroscopy and imaging. He was the recipient of the New Century Excellent Talents Program Award from the Ministry of Education of China.

Digital Object Identifier: 10.3969/j.issn.1674-862X.2015.02.004