# The Application of Microplasma in the Terahertz Field: A Review
## 1 Introduction  
The field of terahertz (THz) spectroscopy, imaging, and technology has grown dramatically over the last three decades due to its promising THz applications in explosives detection [1, 2], biology/medicine [3], wireless communication [4], and radio astronomy [5]. The THz frequency range generally refers to the 0.1-10 THz range, which is between microwaves and infrared light in the frequency band [4, 6]. With photon energies in the meV range, THz radiation interacts strongly with matters that have characteristic lifetimes in the picosecond range and energetic transitions in the meV range, such as absorption resonances of molecules, chemical reactions, dielectric relaxation and vibrational spectroscopy of liquids, weak collective excitations in solids, and biomolecular collective motions [7]. These characteristics make THz radiation a unique tool for numerous applications. For example, THz radiation does not ionize molecules because of its low energy, presenting a significant advantage in medical imaging over X-rays [8]. THz spectroscopy can act as spectroscopic fingerprints for material identification because molecular crystals possess vibrational absorption bands in the THz range [9]. Compared with microwave frequencies, the THz spectral region offers a higher available bandwidth, which could meet the ever-growing demand for higher data transfer rates in wireless communications [10]. THz images also have the advantage of superior spatial resolution due to the shorter wavelength [11]. Compared with infrared frequencies, many common materials are relatively transparent for THz radiation to penetrate, including common packaging materials such as paper, plastics, and composites [1]. The THz region has been considered the last remaining scientific gap in the electromagnetic spectrum, which is underdeveloped but ripe for exploitation. One of the fundamental reasons is the lack of many functional devices in the THz frequency range.  
Terahertz sources, amplifiers, filters, and detectors are necessary functional devices in THz spectroscopy, imaging, and technology. Proof-of-concept experiments and simulations have shown that microplasma interacts strongly with the THz radiation, making it possible to be part of THz functional devices. Microplasma sources have been applied to light sources, material processing, plasma medicine, and electromagnetic wave control [12]. Microplasma usually refers to a plasma with a feature size of less than 1 mm. Atmospheric pressure microplasma is of high electron density and low gas temperature. Due to itsplasma frequency in the terahertz range and featured size, microplasma is well suited to interact with THz radiation in principle [13]. The relative plasma permittivity is dependent on the plasma frequency, the frequency of incident electromagnetic waves, and the collision frequency between electrons and molecules or atoms, which has been described by the well-known Drude model [14]. The relative plasma permittivity is easily tuned by applied voltages or input electric power, which can be positive or negative. From the perspective of dielectrics, microplasma has a nonlinear refractive index and behaves like a metamaterial for THz radiation modulation [15]. It has advantages of fast dynamic response, reconfiguration, and adjustability compared to other methods based on the temperature [16], magnetic field [17], and pressure [18]. At present, extensive works have shown that microplasma plays an important role in THz functional devices, such as THz sources, amplifiers, filters, and detectors. However, although several reviews on the development of THz spectroscopy and imaging [11], THz clinical application [8], THz communication [19], and extreme THz science [20] have been published, there is no overview of microplasma applications in the THz field. Therefore, this work focuses on a few of the recent developments and the implications for the future of microplasma applications in the THz field.