Study on Theory of Propagation Properties of Terahertz Waves in Plasmas

The basic theoretical framework of interaction of electromagnetic （EM） wave with p lasma h as b een established, and w idely u sed in n umerical s imulation a nd experiments o f m icrowave i n p lasma. Meanwhile, m ore an d more t heoretical an d experimental investigations on interaction of laser with plasma have been presented. The study of terahertz （THz） region of the EM spectrum, wedged between the laser and microwave end, has been a challenge in the past due to the lack of powerful THz sources and detectors. For this reason, THz region of EM spectrum was called the THz-Gap. Recent major technical advances in developing intensive THz sources and detectors ha ve pr ovided us w ith ne w o pportunities both i n the i nvestigation a nd application of THz waves. One of the important investigations of THz waves is the interaction of THz waves with plasmas, which is rich in physical phenomena and will be great i n potential a pplications. However, t o da te, t here a re onl y a f ew experimental works in the THz regime have been presented. These previous works mainly f ocused o n e xperimental i nvestigation an d s imple t heoretical an alysis; however, the detailed EM propagation characteristics of the THz wave in the plasma and the effects of the plasma parameters on the characteristics have yet to be studied. For this reason, in my research works, we have done an analytical study of THz wave propagation in cold plasma, hot plasma and microplasma models.Aiming at applications, several models of THz wave propagation in plasmas are established and some numerical methods are introduced. One of three layered plasma structure m odels ha s be en us ed t o s tudy t he effects of the plasma t hickness an d plasma sheath on t he propagation of EM wave in it, and the scaling laws on w ave frequencies to optimize the plasma parameters for attenuation of EM waves are given. The details are stated as follows.Firtsly, t he pr opagation properties of THz waves in co ld p lasmas have b een investigated. The cold plasma model is used to simulate the absorption of THz waves in plasmas, and the simulated results are in good agreement with experiment results published by other research groups. The numerical results show that the effects of plasma p arameters o n t he T Hz w ave p ropagation ar e remarkable, and s trong absorption occurs when the frequency of THz wave and plasma parameters match well. One can find that the THz waves can transmit the plasma with a high electron density of 10²² m^-3. As a comparison, the numerical results of THz wave propagation in a magnetized plasma show that the reasons of absorption of THz waves are the joint effects of collisional absorption and electron cyclotron resonance. Secondly, the propagation properties of THz waves in hot plasmas have been investigated. The dispersion relations of a THz wave in a hot plasma is introduced to study the propagation of THz waves in non-magnetized plasmas with FDTD methods. The r esults i ndicate t hat t he f actor o f t emperature can c ause t he ch ange o f THz characterization of hot plasmas. Moreover, the zero absorption and negative power absorption are observed when the THz waves propagate in hot magnetized plasmas.Thirdly, the propagation properties of THz waves in microplasmas have been investigated. From the values of microplasma parameters, one can find that they are related to THz waves. The modified Drude model is used to describe the dispersion relationships of microplasmas. The effects of microplasma size and the permittivity of outer materials on the propagation of THz waves through plasmas are studied. The research r esults o f T Hz w aves i n p eriodic m icroplasmas s how that a p eriodic microplasma can m odulate the T Hz w ave t o make it on a n a pproximate pe riodic variation.In c onclusion, THz characterizations o f several m ain kinds of plasmas a re studied in this work and the effects of plasma parameters on the propagation of THz waves ar e an alyzed. T he r esults s how that t he T Hz w aves can t ransmit a h ot magnetized plasma with high electron density. It is demonstrated that THz wave can be used as an effective diagnostic tool to measure the high density plasma properties and means in communication in high electron density plasmas. Moreover, the plasma can be as a novel device to control the propagation of THz waves