# The Application of Microplasma in the Terahertz Field: A Review
## 2 Application of Microplasma in THz Functional Devices
### Terahertz Filter  
The terahertz filter is one of the essential components in THz spectroscopy, imaging, and technology. It is used to selectively filter or efficiently extract terahertz signals for the improvement of the signal-to-noise ratio. Photonic crystals have photonic bandgaps, within which the propagation of THz waves is not permitted. It is usually used to confine and guide the THz wave propagation. To enhance the tunability of photonic bandgaps for THz wave modulation, plasma is introduced into the photonic crystals, forming an artificial periodic structured composition of plasma and dielectrics, known as a plasma photonic crystal (PPC). Compared to other methods based on the variety of external electric field, temperature, and pressure, the advantages of PPCs are wide tunability of the relative permittivity from negative to positive and rapid reconfiguration ability [12; 18; 47; 48; 49; 50]. The PPCs used to modulate the THz wave propagation have been investigated recently. In terms of THz PPC simulation, [PERSON] et al. investigated the characteristics of a microplasma photonic crystal for THz modulation [51]. Numerical results show that the characteristics of the PPC bandgaps depend on the incident angle of the terahertz wave, the electron density, and the thickness of the plasma layer. As the thickness of the plasma layer or plasma density increases, both the center frequency of the bandgap and the forbidden bandwidth decrease. Although these results show the possibility of microplasma photonic crystals for active terahertz filtering, the effects of collision frequency, discharge transient process, and the interaction between adjacent microplasma cells are ignored. In 2017, [PERSON] et al. considered the interaction between microplasma cells and simulated an open low-pressure surface microplasma array to modulate subterahertz wave propagation. It reveals that the diffusion of metastable atoms or ions between microplasma cells significantly impacts the subterahertz wave transmittance and the discharge characteristics of the microplasma [52]. In 2020, with the transfer matrix method, [PERSON] et al. studied the effects of the collision frequency on the terahertz bandgap structure and transmission characteristics of a microplasma photonic crystal, as shown in Figure 6a. The increase in gas pressure leads to a decrease in the center frequency of the THz bandgap and the intensity of THz radiation. In Figure 6b, it is interesting to find that if the electron density is less than 10\({}^{15}\) cm\({}^{-3}\), the center frequency and bandwidth of the first THz bandgap are almost unchanged. As the electron density further increases up to 10\({}^{16}\) cm\({}^{-3}\), both the center frequency and bandwidth increase significantly [53]. What is more, [PERSON] et al. investigated the effects of plasma frequency, collision frequency, plasma column radius, lattice constant, and background dielectric permittivity on the subteratr wave propagation with the plane wave expansion method. It is found that the radius of the microplasma cylinder and the background dielectric are the most effective control variables for the THz bandgap and its center frequency, respectively [54]. Previous works focused on the THz bandstop PPCs. If a defect layer is appropriately applied to the bandstop PPC, a THz passband PPC will occur. [PERSON] et al. proposed a one-dimensional magnetized microplasma photonic crystal with a (AC)N(CBC)M(AC)N structure. A is the plasma layer with a 0.1-0.2 mm diameter, B is the dielectric defect layer, and C is the ordinary dielectric layer. N and M are the periods of the plasma layer and the dielectric defect layer, respectively. It is found that the width of each layer of A, B, and C will affect the center frequency and transmission coefficient of the passband, but the width of the C layer has little effect on the transmission coefficient [55]. A flat passband is beneficial to the amplitude-frequency characteristic of the THz bandpass filter.