# The Application of Microplasma in the Terahertz Field: A Review
## 2 Application of Microplasma in THz Functional Devices
### Terahertz Detector  
The methods of terahertz wave detection are classified as incoherent detection and coherent detection. Incoherent detection is usually based on the thermal effect of detection materials on terahertz waves, which can be used to directly measure the intensity and average power of terahertz waves, such as bolometer, pyroelectric, and Golay cell [60]. These detectors can be applied to the power measurement of continuous terahertz wave and terahertz imaging. For the measurement of pulsed terahertz waves, the phase information of the pulse terahertz wave will be lost. On the other hand, coherent detection is usually applied to the measurement of pulsed terahertz waves, such as photoconductivity (PC) sampling and electro-optical sampling (FS-EOS) detectors [61]. The speed of PC sampling is limited by the resonant characteristic of the antenna structure. For FS-EOS detectors, the balance between the detection sensitivity and the frequency response is largely affected by the type and thickness of crystal materials. The thicker the electro-optic crystal is, the higher the detection sensitivity will be, but the detection bandwidth will be reduced. In addition to the above THz detectors, microplasma can also be applied to THz detection, which has an ultrawide detection bandwidth and high sensitivity compared to other types of THz detectors.  
Laser-induced plasma in the air is an important approach to detect broadband THz waves. [PERSON] et al. firstly demonstrated the coherent detection of broadband THz waves with ambient air. As shown in Figure 9, it is achieved by the E-field-induced second-harmonic generation from the third-order nonlinear optical process with femtosecond laser pulses, which can be applied to the temporal measurement of pulsed THz pulses. The coherent detection is limited to the cases in which the tunnel ionization process dominates in gases [62]. [PERSON] et al. reported a detection method of THz pulses by the THz-enhanced emission of fluorescence from air plasma. The air plasma induced by a femtosecond laser has an electron density of 10\({}^{14}\)-10\({}^{15}\) cm\({}^{-3}\). The THz-enhanced fluorescence emission is caused by the electron heating and electron-impact excitation of air molecules or ions under the THz field [63]. [PERSON] et al. observed a 10% acoustic enhancement from a laser-induced plasma under single-cycle THz radiation, providing a promising method for coherent THz wave remote detection. It is probably caused by the THz plasma heating through energy transfer from THz wave into translational motion of the gas molecules [64; 65].  
Due to the high complexity and high cost of the femtosecond laser-induced plasma system, a simple method of THz wave detection is required. [PERSON] et al. firstly proposed a weakly ionized plasma detector for THz continuous wave. The dc neon plasma is generated in the electrode gap of 0.15 mm. As the discharge current increases, the responsivity of the THz detector increases firstly, reaches the peak of 194.4 V/W at 1.4 mA, and finally decreases [66]. Furthermore, a microplasma array is fabricated to detect the THz wave [67].  
Figure 8: (**a**) Three-dimensional plasma/metal/dielectric photonic crystal band-stop filter; (**b**) The narrowing of the 138 GHz resonance line shape with the introduction of plasma to the crystal [59].