Progress in research on active control materials and devices for terahertz band in China University of Science and Technology

[ China Instrument Network Instrument Development ] Professor Lu Yalin, a professor at the University of Science and Technology of China, has made progress in the research of terahertz active control devices. The team studied the interaction mechanism of terahertz waves with superstructure materials and oxide superlattice thin films, and successfully prepared ultra-fast terahertz modulators, and first achieved a picosecond-order high-modulation-depth terahertz ultrafast switching. At the same time, a multi-functional terahertz device has been prepared to realize multiple functions of electrical switching, optical storage, and ultra-fast modulation in a single device. The related research results have recently been published in the international academic journals "Adv. Optical Mater." and "Opt. Express."

A schematic diagram of a silicon dielectric super surface device and its experimental results on super-fast control of terahertz waves

Composite Super-Surface Device Schematic and Experimental Results of Electrical Switching and Optical Storage Function

Relationship between dielectric constant and excitation power of the superlattice film in the terahertz band

Terahertz wave has unique characteristics such as time-domain pulse, low energy, spectrum fingerprint, and broadband, and it has broad application prospects in fields such as physical chemistry, materials science, biomedicine, environmental science, security inspection, and satellite communications. Among them, one of the key factors that affect the development and application of terahertz technology is that it is difficult to obtain active terahertz control components. Superstructure materials, an artificial material consisting of a sub-wavelength microstructure array of metal or dielectric materials, have unique electromagnetic response characteristics that provide an excellent solution for terahertz-controlled devices. Unfortunately, in the past, terahertz components based on superstructure materials were all made of metal materials. After the processing dimensions were fixed, the device's functions would be difficult to change in practice. Therefore, the development of active regulation of terahertz components has important research significance.

Usually the active regulation is to regulate the polarization, amplitude, and phase of the terahertz wave, and the regulation speed is another indicator. Some practical applications also urgently require ultra-fast control of terahertz waves. Lu Yalin team designed and produced an ultra-fast super-controlled surface based on silicon media. By ion implantation and rapid heat treatment of the silicon thin film, the carrier lifetime of silicon is greatly reduced and free carrier concentration is increased. Then, the silicon thin film is processed into a super surface of a disk array structure capable of resonance in the terahertz wave band through photolithography and etching processes. The use of infrared femtosecond pulse excitation, the first to achieve a picosecond-level high modulation depth terahertz ultrafast switching (open 20ps, off 300ps), and based on the theoretical model of semiconductor carrier dynamics for a reasonable explanation . Research results have recently been published in the journal "Advanced Optical Materials" [Adv. Optical Mater. 2018, DOI: 10.1002/adom.201800143].

In addition, the currently studied terahertz active control devices have a single function, that is, they can only achieve a single function under a single external field. However, a single function is difficult to adapt to the requirements of today's technological development. Therefore, in a single device, the realization of multi-physics control, and the realization of multi-functional control of the terahertz wave, is one of the cutting-edge development of the current terahertz technology, but also the actual needs of practical applications. In view of this, based on VO2's insulation-metal phase transition, the team designed a terahertz wave-based multi-functional tunable composite super surface by combining VO2 with asymmetric metallic split-ring resonators, and utilized the National Synchrotron Radiation Laboratory. The high-quality VO2 thin film provided by associate researcher Zou Chongwen prepared the device through etching, photolithography and other processes. The composite super surface can achieve amplitude modulation of transmitted terahertz waves by heating and applying current. The absolute modulation depth is as high as 54% and the quality factor is as high as 138%. Based on the hysteresis characteristics of VO2 in the phase transition process, the composite super surface can realize the memory storage function of the terahertz wave at room temperature through current triggering. In addition, the use of ultra-fast pulse pumping, this composite super surface can also achieve ultra-fast control of the terahertz wave. As a result, multifunctional control of the terahertz wave is achieved in a single device. Related research results have recently been published in the "Advanced Optical Materials" journal [Adv. Optical Mater. 2018, DOI: 10.1002/adom.201800257].

In addition, the response of many materials in the terahertz band is still unknown, and only by studying the characteristics of the interaction of various materials with the terahertz wave, the design of active terahertz devices can be traced. The team used two sets of self-built terahertz systems to measure and analyze the interaction of quantum functional materials with terahertz waves. Focusing on the terahertz response of superlattice films with different number of cycles, it was found that the pumping of a 532 nm continuous laser has a greater regulation effect on the dielectric constant of the superlattice in the terahertz band and passes Drude-Lorentz. The fitting of the model illustrates the microscopic mechanism of this phenomenon, which opens up a new path for finding new functional materials that can be used in THz active control devices. The relevant research results were published in Opt. Express. 26, 7842 (2018).

The first author of the above thesis was Dr. Cai Honglei, a doctoral student at the Hefei National Laboratory Center for Physical Sciences at the Microscale. Corresponding authors are Huang Qiuping and Lu Yalin. The work was funded by key projects such as the Ministry of Science and Technology, the National Natural Science Foundation of China, the Chinese Academy of Sciences and the Ministry of Education.

(Original Title: China University of Science and Technology has made series of progresses in the study of active control materials and devices in the terahertz band)