# The Application of Microplasma in the Terahertz Field: A Review
## 2 Application of Microplasma in THz Functional Devices  
### Terahertz Source  
The methods of terahertz sources development are usually based on electronics and photonics [21]. From the aspect of electronics, vacuum electronic devices and solid-state semiconductors are usually used. Due to the power and operation frequency limitation of the electronic devices, it is usually hard to obtain a powerful THz source with a frequency spectrum up to tens of THz. From the aspect of photonics sources, photoconductive antenna (PCA), optical rectification, and laser-induced microplasma are usually used [22]. The THz radiation intensity based on PCA is limited by the carrier lifetime. The optical rectification is limited by the nonlinear crystal, which has photon absorption and a damage threshold. THz sources based on laser-induced microplasma have a wide frequency spectrum and no need to worry about the damage threshold of the medium. It allows the operation of THz sources with high-intensity laser input. Therefore, THz sources based on laser-induced microplasma have great potential in generating super terahertz radiation with a wide frequency spectrum. The THz source based on laser-induced plasma can be used to produce short ultra-broadband pulses covering a broad frequency spectrum reaching well into the infrared, which is essential to identify the spectral \"fingerprints\" of molecular crystals. The laser-induced plasma has a diameter of 10-100 \(\upmu\)m and an electron density of over 10\({}^{17}\) cm\({}^{-3}\). The radiation terahertz spectrum spans from 0.1 to 10 THz, as shown in Figure 1[23], and the electric field of THz waves is kV/cm\(\sim\)MV/cm [24; 25; 26; 27]. With the contribution of a 10 fs laser pulse, a spectrum of the ultrabroadband pulse from far infrared to 200 THz through a plasma is observed, as shown in Figure 2[28]. In 1993, [PERSON] et al. first reported the THz generation from the ionized helium gas [29]. A 50 mJ near-infrared pulsed laser is focused on a helium gas target. The helium gas is ionized, resulting in the formation of microplasma with an electron density of 2.5 \(\times\) 10\({}^{17}\) cm\({}^{-3}\). The optical-to-THz conversion efficiency is less than 10\({}^{-6}\). In 2000, [PERSON] and [PERSON] improved the infrared-terahertz conversion efficiency, which increases by at least one order of magnitude when an ultrafast laser field composed of the fundamental wave and its second harmonic is used [30]. Classified by the laser wavelength, the two approaches are generally referred to as the \"one-color\" and \"two-color\" methods, as shown in Figure 3. A femtosecond or picosecond infrared laser is used to focus on the gas targets. The gases are ionized by the intense laser electric field, resulting in the formation of microplasma. The electrons in microplasma are accelerated with ultrafast oscillation in the strong electric field, radiating broadband electromagnetic waves (including THz waves). Figure 1: A frequency spectrum covering from 0.1 THz to 35 THz produced by plasma [23]. Figure 3: Schematic diagram of laser-induced plasma radiation terahertz wave. (**a**) single-color method; (**b**) two-color method. Figure 2: A spectrum of the ultrabroadband pulse from far infrared to 200 THz through a plasma [28]. In the \"one-color\" method, the minimum laser pulse energy required to generate measurable THz waveforms is crucial. The laser energy threshold in some previous works is in the range of 30-50 uJ, where the microplasma induced by infrared laser is usually elongated with a radius of tens to hundreds of microns and a length of several millimeters to several centimeters. The estimate scaling law for the THz peak power as a function of the relevant laser source parameters proposed by [PERSON] et al.