Low Loss, Small Mode Field Terahertz Waveguide Research

Terahertz?THz?wave is referred to the frequency of electromagnetic wave in the range of 0.1¹ THz,located between the infrared and microwave,in the transition stage from macroelectronics to micro-electronics,which has many unique properties.It has important research value and wide application prospect in the fields of basic physics science,super speed communication,high resolution imaging,safety inspection,nondestructive testing and so on.However,so far the lack of devices with low loss and long propagation distance makes the applications of terahertz very limited.To pave the way of the terahertz waveguide with low loss and small mode field has very important significance to promote the practical application of terahertz technology.Due to the contradiction between low loss and small mode field is to hard realize simultaneously for the existing waveguide,therefore,In order to make a feasible improvement from the aspects of the structure or material,we proposed respectively a metal nanowire placed in a dielectric hole,graphene-coated drop-extended-nanowire,and dielectric-Gap-Metal symmetrical waveguide.Numerical analysis method and COMSOL software based on finite element methods are used to analyze the propagation properties of the above three kinds of waveguides.Firstly,we use COMSOL software to simulate the surface plasmon mode of the metal nanowire placed in the dielectric hole,and the results are matched with the results obtained by numerical analysis.It is proved that the metal wire placed in the hole can keep the mode width as the same as that of the single wire,in the meanwhile,the loss is ultra-low.The main reason is that the introduction of a sufficiently large dielectric hole can change the mode field partly,so that the field energy in the metal wire can be reduced,and the propagation loss can be reduced.When the frequency is 0.3 THz,the wire radius is 100 nm,the loss in theory is very small,0.42 m^-1,and the mode field width?the width when the Poynting vector reduce to 1/e2?is scaled down to only the order of nanometer magnitude?290 nm?.Secondly,the Graphene-coated drop-extended-nanowire propagating THz surface plasmon polaritons is presented.In this paper,we introduce the graphene material with metalloid property in the terahertz frequency range as the coatings on the drop-extended silicon nanowire.COMSOL is used to solve it,it's found that the waveguide can reduce the mode width dramatically.When the working frequency is 3 THz and the bottom radius of curvature R is 1 ?m,there is a light spot at wedge apex of the waveguide,the width of the mode is 34 nm,and the propagation loss is 1.3 mm^-1,that is,the propagation distance is nearly 800 ?m?equivalent to 7 times of the wavelength?.Therefore,under the premise of low propagation loss,the waveguide can realize the nanofocusing.At last,dielectric-Gap-Metal symmetrical waveguides are proposed,and mainly include two types: flat type and cylinder type.They work mainly by inserting dielectric layer to change the characters of the original mode field of metal plates?or metal hollow tube?.We derive the dispersion equations of TE mode,TM mode and HE mode respectively.According to the numerical results,it is found that the mode field is more concentrated in the dielectric layer,and it can realize the ultra-low loss propagation of THz wave on the premise that the mode field width is not increased.When the frequency is 0.5 THz,the space of two metal parallel plates is ?/2,the minimum loss of TE1 mode and TM1 mode,respectively 0.21 m^-1 and 0.17 m^-1;for DGM cylindrical waveguide,when the inner diameter of metal tube is 2?/3,the minimum loss HE11 mode is 0.20 m^-1.Therefore,when compared with the waveguide without dielectric layer,their losses can be decreased by 1 3 orders of magnitude.