Studies On Localized Resonant Modes In Subwavelength Plasma Structures And Their Applications

Plasma is a state of matter that contains a large number of unbound charged particles.There are usually different kinds of heterogeneities in plasmas.If the scale of plasma non-uniformity is much smaller than the wavelength of the eigenmode,the plasma is of a typical sub-wavelength scale,localized resonant modes with large amplitude will then be formed in a small region,so that the system energy is obviously concentrated.This phenomenon can give rise to many micro effects such as local field enhancement and surface plasmon resonance,which are often accompanied by important macro processes such as system energy focusing,conversion,and transfer.Therefore,localized modes in plasma show unique advantages and great application prospects in manipulating electromagnetic processes,such as the detection and control of high-speed vehicles by electromagnetic methods.In this thesis,localized resonant modes in the following three non-uniform plasma structures are studied.First,we focus on the characteristics and mechanism of the localized eigenmodes in "discrete" non-uniform dust plasma chain with alien particles.Second,localized surface plasmon resonance(LSPR)in the "truncated" non-uniform plasma and its effect on the far-field radiation of the antenna is investigated.Third,resonance mode conversion in "gradient" non-uniform plasma and its influence on electromagnetic wave energy absorption is then studied.Focusing on the localized eigenmodes in the "discrete" non-uniform dust plasma chain with alien particles,we employ both theoretical analysis and numerical simulation to study the properties and mechanism of the localized modes.First,we theoretically give the characteristic equations for the linear small-amplitude oscillation of particles in the dusty chain,and the frequencies of the localized modes are obtained by solving the equation that coefficient determinant of the characteristic equation equals to zero.Results show that the localized modes are essentially the eigenstates of the characteristic equation in dusty chain.Then we numerically simulate the stochastic "thermal noise" of the particles oscillations without any external drive,and perform a two-dimensional Fourier transform on the spatial and temporal information of the random oscillations of the particles to obtain its dispersion relationship.It is found that the discrete frequency peaks in the spectrum exactly represent the localized modes in dusty chain.Thus we find a new method to rapidly detect the localized modes with discrete frequencies.Further analysis shows that these discrete eigenmodes are bound states in the non-uniform plasma structure composed of dust and impurity particles,and can be excited only if the oscillation frequency satisfies the resonant condition.As for LSPR in the "truncated" non-uniform plasma,this thesis studies the properties of LSPR and its effect on the enhancement of the antenna's far-field radiation.First,we derive the characteristic equations of electrostatic potential in the plasma structure,and obtain a series of eigenmodes with discrete spectra localized near the plasma,namely localized surface plasmon resonance.It shows that the LSPR is essentially the eigenstate of the characteristic equation of plasma oscillation.Further analysis reveals the relationship between LSPR and radiation enhancement of antenna by plasma.Results show that the field of LSPR around plasma causes a sharp change of the antenna impedance,leading to resonant circuit between the antenna and the feed cable.Then far-field radiation of antenna is significantly intensified.This work reveals the physical mechanism of the enhancement of antenna radiation by plasma,which has confused researchers for decades.Resonant mode conversion in "gradient" non-uniform plasma is another focus of this thesis.We study the properties of resonant mode conversion and its effects on the electromagnetic wave absorption in plasma under different parameters.First,we give the linear equations of the electromagnetic fields in the non-uniform cold plasma,from which the dispersion relation of the electromagnetic waves in the non-uniform plasma is obtained.The results show that the wave vector of electromagnetic waves rises sharply near the resonance layer,showing that a resonant phenomenon has occurred.When the collision frequency increases,the resonant peak of wave vector becomes smooth,indicating that strong collision weakens the resonant mode conversion obviously.Simulation results show that strong electrostatic field is excited in resonant layer,so that the incident electromagnetic waves are converted into electrostatic waves.However,the electrostatic peak is significantly weakened under strong collision,indicating that the influence of collision on resonant mode conversion is significant.In addition,we also calculate the resonant absorption under different parameters such as plasma density,thickness,incident angle and polarization of electromagnetic wave,and give the excitation conditions of the resonant mode conversion.