# The Application of Microplasma in the Terahertz Field: A Review
## 4 Conclusions  
The application of microplasma to THz functional devices from the aspects of terahertz sources, amplifiers, filters, and detectors is reviewed in this work. The conclusions are presented as follows.  
1. THz sources generated by the \"one-color\" and \"two-color\" methods are based on the laser-induced microplasma, where the microplasma has high electron density in the range of \(10^{17}\) cm\({}^{-3}\)-\(10^{19}\) cm\({}^{-3}\). The size of microplasma and the electron density greatly impact the optical-to-THz conversion efficiency. The THz sources have the advantages of a broad frequency spectrum and high power, which is very useful for identifying molecular crystals and biological macromolecules.
2. In addition to the laser-induced microplasma, the microplasma in the capillary generated by gas discharge has proved helpful in amplifying the THz wave. It shows the possibility that the microplasma can be served as a THz amplifier, which has the advantages of high-power amplification at high efficiency. However, different amplification mechanisms based on four-wave-mixing parametric processes, the negative absolute conductivity of microplasma, and electric dipoles are proposed, which are needed to investigate in the future.
3. Both experimental and simulation results demonstrated that microplasma photonic crystals have THz stopbands or passbands in the frequency range of 0.1-0.2 THz, which allows tuning the THz wave propagation by changing the plasma parameters. The THz filters based on the microplasma photonic crystals have advantages of fast response and reconfiguration ability. However, the control of THz wave propagation with frequency up to 1 THz or higher by microplasma photonic crystals needs further exploration.
4. The detection methods of THz radiation, based on the second-harmonic generation, emission of fluorescence, and acoustic signals from laser-induced microplasma, have been developed and well studied. In addition, a simple detection method of THz radiation, based on the interaction between THz wave and the microplasma produced by glow discharge, is introduced. However, the detection mechanism of this method remains unknown at present.  
In general, the microplasma technology successfully applied to the THz field, such as THz sources, amplifiers, filters, and detectors, opens a new area of exploration for both THz technology and plasma. It is beneficial to promote interdisciplinary research between them.  
Conceptualization, S.W. and L.Y.; writing--original draft preparation, Y.G. and X.L.; writing--review and editing, Y.G., S.W. and X.L.; visualization, Y.G.; supervision, S.W.; project administration, C.Z. and S.W.; funding acquisition, C.Z. and S.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Key Research and Development Program of China, Grant Number 2021 YFF0603100, the National Natural Science Foundation of China, Grant Number 51977110, and Research on Evaluation Method of Overall Function and Effectiveness of Cable Fire Protection Products of Jiangsu Electric Power Research Institute of State Grid of China, Grant Number J2021137.