# The Application of Microplasma in the Terahertz Field: A Review
## 2 Application of Microplasma in THz Functional Devices
### Terahertz Amplifier  
Currently, there are three types of THz amplifiers, including solid-state terahertz amplifiers (such as InP and GaN field-effect transistors) [40], waveguide-based traveling-wave terahertz amplifiers (TWAs) [41], and terahertz amplifiers based on photonics (such as laser-induced microplasma). Solid-state terahertz amplifiers have the disadvantage that the energy efficiency decreases with the increase in the operating frequency of the amplifier. Compared to the solid-state terahertz amplifiers, the microplasma can achieve higher-power amplification and higher operating frequency at higher efficiency.  
Laser-induced plasma is used as an amplifier for terahertz wave propagation. In 2007, [PERSON] et al. reported an amplification effect of THz wave in gases excited by femtosecond laser pulses caused by the four-wave-mixing parametric processes, as shown in Figure 5a [42]. In their experiment, a laser beam with a wavelength of 800 nm and its second harmonic is used to generate a nitrogen plasma. A seed THz wave is sent into the plasma with a length of 5 mm. When the total excitation intensity is 8 \(\times\) 10\({}^{14}\) W/cm\({}^{2}\), the amplification factor is 65%, as shown in Figure 5b.  
In addition to the four-wave-mixing parametric processes, the effect of the negative absolute conductivity of plasma contributes to the amplification of the THz radiation in the plasma channel. [PERSON] et al. analyzed and simulated the electron energy distribution function in the nitrogen, xenon, and argon plasma created by a femtosecond laser energy pulse. It is found that the plasma has to be highly nonequilibrium for THz amplification. Numerical results demonstrated that the amplification of the seed THz wave in the nonequilibrium Xe plasma channel up to several orders of magnitude is possible. The Xe plasma at atmospheric pressure is created by an intense femtosecond KrF laser pulse. The electron density of the Xe plasma is in the range of 10\({}^{13}\)-10\({}^{15}\) cm\({}^{-3}\)[43; 44; 45].  
Interestingly, [PERSON] et al. demonstrated for the first time that the microplasma in the capillary could amplify the THz wave inside a meandering waveguide, which is helpful in terahertz traveling-wave amplifiers. A curved TE01 waveguide is coupled to the microplasma. The microplasma is confined within a silicon capillary with a thin dielectric coating on the inner surface wall. The usual size of the microplasma is in the range of 100-200 \(\upmu\)m. The typical plasma density is 10\({}^{10}\)-10\({}^{14}\) cm\({}^{-3}\). The interaction between the plasma and the terahertz signal is coupled through electric dipoles. When the microplasma  
Figure 5: (**a**) Schematic diagram of laser-induced plasma amplification and detection of terahertz waves; (**b**) relationship curve between peak amplification factor and light excitation intensity [42].  
phase and group velocities match that of the guided terahertz wave, the wave gains energy from the microplasma, and the terahertz signal gets amplified. Terahertz amplifiers based on microplasma instead of electron beam TWA have the following advantages: (1) thermionic emission required in e-beam generation can be replaced with gas ionization; (2) electrostatic lenses and magnetic focusing structures can be eliminated or reduced in complexity since plasmas can be self-focusing; (3) larger acceleration fields can be used by taking advantage of plasmas' space-charge electric fields of \(\sim\)10\({}^{4}\)-10\({}^{6}\) V/cm; (4) higher-power amplification at higher efficiency can be achieved. Their experimental results prove that the maximum amplification of the meandering waveguide with microplasma is about 12 dB at a frequency of 0.9 THz [46].