Surface Plasmon Generation And Field Intensification By Sub-wavelength Ring Structure Of Plasmas

As a special material composed of a large number of freely moving charged particles,the electromagnetic properties of plasma are different from those of ordinary dielectric media in a wide range frequency band.In the laboratory,by varying the parameters of discharge power and gas pressure,the density and collision frequency of the plasma can be adjusted quantitatively,in consequence the electromagnetic characteristics of the plasma changed.Surface plasmon resonance(SPR)can be excited when the electromagnetic parameters of the media,the geometric dimensions of the plasma structure and the frequency of electromagnetic wave meet certain relationship.Surface plasmon(SP),also known as surface wave(SW),is essentially a collective oscillation produced by the coupling of free electrons in a metal or plasma with an external electromagnetic field.This effect can confine the energy of the external electromagnetic field on the surface of the plasma or metal structure at the sub-wavelength scale,therefore applications like high efficiency of light transmission,light induction and absorption can be achieved.Due to the properties of surface plasmons in many aspects,they are widely used in the fields of surface enhanced Raman scattering,transmission enhancement of sub-wavelength aperture,photonic integrated circuits,nano lithography,high-density optical storage and nano waveguides.The surface plasmon excited by the plasma material share the optical properties with the visible light on nano metal particles.Therefore,the sub-wavelength plasma structure has great potential application value in the modulation and enhancement of microwave.At present,the research mainly focuses on the near-field modulation of electromagnetic radiation signals by the spherical or cylindrical shell sub-wavelength plasma.On the experiment side,the signal gain of transmitting antenna reaches 10 d B in the frequency band from0.6GHz to 1.0GHz,while on the theory side an explanation of this enhancement phenomenon is still lacking.Previous simulations and experiments mainly focus on the radiation enhancement of the transmitting antenna,but the investigation on the received signal is inadequate;in addition,only the closed plasma structures,such as spherical and circular shells,are discussed,other novel plasma structures are without discussion.Aiming at the above problems,in this dissertation we study the receiving gain of various ring structures of sub-wavelength plasma,and explains the local field enhancement effect of ring plasma structures by using the theory of surface plasmon resonant hybridization of nano optics and the theory ofmetal-insulator-metal(MIM)cavity.The main contents are as follows:The local field enhancement effect of the sub-wavelength plasma cylinder and ring structure is studied.Based on the theory of plasmon hybridization,the energy level of hybrid surface plasmon resonance in the plasma ring structure is derived.The local field enhancement effect of the plasma ring structure is explained as a result of the resonance of hybrid surface plasmon.It is found that both the high-frequency and low-frequency modes of the hybrid plasmon resonance have dipole and multipole components,and the field enhancement effect of the dipole field is far stronger than that of the multipole one.For higher frequency bands,sub wavelength condition is no longer met.The surface plasmon resonance of the high frequency band is studied by the plane plasmon dispersion relation and the whispering-gallery modes theory.The plasma double-ring structure is designed.Our simulation shows that the internal and external plasma rings and the air layer between them constitute the MIM cavity structure.By exciting the Fabry Perot resonance(FPR),the incident electromagnetic wave energy is guided into the plasma ring,which overcomes the scattering effect owned by the traditional closed spherical shell and single ring structure.It can effectively enhance the surface plasmon resonance and realize the further enhancement of the electromagnetic wave in GHz band.The results show that the FPR in the slit interferes with the FPR in the air layer.When either of the two resonances occur alone,the enhancement effect of FPR in the air layer is more significant.The dispersion relation of surface plasmon polaritons based on MIM cavity theory is used to verify the simulation results.It is shown that the FPR resonance at the air layer is the main factor for the local field enhancement of the double-ring structure.The plasma cavity ring structure is designed,which realizes the local field enhancement up to 20 d B.The results show that the phase of FPR in the slit determines the coupling relationship between it and SPR in the ring.When the resonance mode of FPR is a half integer,the two resonances are coupled with each other,and the local field enhancement effect is significant.It is found that appropriately choosing the thickness of the plasma plate can effectively increase the energy collection efficiency of the MIM cavity,and the highest energy collection efficiency is obtained when the resonance mode of FPR is 1.5.The dispersion relation of SPP in the slit is verified by the MIM cavity theory.It is found that when the thickness of the plasma plate is much smaller than the skin depth of the plasma,the simulation results show a blue shift compared with the theoretical values.Furthermore,the SPP dispersion relation derived from theIMIMI cavity theory is compared with the simulation results and they are basically consistent with each other.The blue shift phenomenon is explained,and the principle underlying the local field enhancement of the plasma slit ring structure is analyzed.