# The Application of Microplasma in the Terahertz Field: A Review
## 2 Application of Microplasma in THz Functional Devices
### Terahertz Source
is presented as follows:  
\[P_{\text{THz}}\propto\frac{\left(\text{W}\times\text{NA}\times\lambda\right) ^{2}}{\tau^{4}} \tag{1}\]  
where W is the laser pulse energy, NA is the numerical aperture of the focusing laser cone, \(\lambda\) is the laser wavelength, and \(\tau\) is the pulse duration. The equation implies that the laser energy threshold can be reduced by increasing NA and the laser intensity at the focal plane. Inspired by this, [PERSON] et al. reported an ambient air laser-induced microplasma as a THz emitter, as shown in Figure 4. The laser beam is focused by a high-NA microscope objective lens into ambient air, generating a microplasma with dimensions of less than 40 u. [31; 32]. The electron density reaches as high as 10\({}^{18}\) cm\({}^{-3}\). The required laser pulse energy is as low as less than 1 uJ. In 2018, they further demonstrated that the microplasma approach could reduce the necessary optical pulse energy by five orders of magnitude while still obtaining a comparable signal-to-noise ratio for THz time-domain spectroscopy [33]. Herein, the electron density of the microplasma should reach up to 10\({}^{19}\) cm\({}^{-3}\) or more. The THz radiation is usually a forward propagating cone. As the size of the plasma decreases, the divergence angle increases. For microplasma of fewer than 40 u, the THz radiation direction is orthogonal to the optical path. In addition, it should be mentioned that the THz generation process is tightly reliant on the characteristics of microplasma, including the size of microplasma, the electron density, the gas pressure, and the gas composition [34; 35]. For example, [PERSON] et al. found that the gas length can tune the shape of the THz waveform and while the amplitude of the THz radiation is constant, which provides a new pathway for terahertz radiation control. In addition, their simulated results show that the THz radiation can be controlled by changing the strength and direction of an external dc magnetic field with a few teslas, and the maximum energy conversion efficiency is doubled in the magnetized plasmas [36]. In the \"two-color\" method, the femtosecond laser-induced microplasma has a greater degree of freedom for THz radiation regulation than the \"one-color\" method. [PERSON] et al. Figure 4: Schematic diagram of laser-induced plasma generation and detection of terahertz waves [30]. simulated the formation of terahertz radiation from microplasma induced by a two-color femtosecond laser [37]. A microplasma with a length of about 10 \(\upmu\)m and a thickness of about 1 \(\upmu\)m is generated. The argon gas is completely ionized at the focal plane, and the peak electron density reaches 3 \(\times\) 10\({}^{19}\) cm\({}^{-3}\). The decrease in the thickness of the microplasma layer is beneficial to improve the optical-to-THz conversion efficiency. The efficiency is above 10\({}^{-4}\) for 10 \(\upmu\)J laser pulse energy. However, due to the defocusing effect of the plasma and the absorption of terahertz waves, as the laser intensity reaches a specific value, the intensity of THz radiation saturates [38]. In 2018, [PERSON] et al. demonstrated that the terahertz emission spectrum could be broadened due to plasmonic resonance controlled by the polarization of the elliptically shaped driving laser pulse [39].