Research On Focusing And Super-Focusing Of THz Wave

Terahertz(THz)is an electromagnetic wave with a frequency range from 0.1 THz to 10 THz.It has many unique properties,such as broadband,coherence,low energy,and perspective,etc.It has important research value in material analysis,transmission imaging,military communication and biological spectrum measurement.In terahertz applications,focusing on terahertz radiation is a basic and important technology.However,traditional focusing lenses can cause phase difference.On the other hand,due to the existence of the diffraction limit,the focus of the terahertz wave can not reach the sub wavelength size,which block the development of terahertz in practical applications.Therefore,in the focus of this paper,a new type of full transparent zone plate structure is proposed.The full transparent zone plate is based on the Fresnel zone plate.In theory,the incident wave,which is shielded in the Fresnel zone plate,can be reused to improve the focusing intensity.By using the COMSOL multiphysics software,the focusing characteristics of all full transparent zone plate and Fresnel zone plate under the 0.14 THz,one of the atmospheric windows,were simulated,compared and analyzed.The results show that the focusing intensity of the full transparent zone plate can be increased by more than twice compared with the Fresnel zone plate.At the same time,the focusing characteristics of the full transparent zone plate when the electromagnetic wave in a certain range is incident to the fixed focal length is also simulated.The results show that a good focus can still be achieved,and the focal length increases with the increase of frequency.In particular,we found that the substrate thickness of the full transparent zone plate will affect the transmission strength,and the Fabry-Perot effect will be formed on the substrate.Finally,the focusing characteristics,FWHM and transmission efficiency of the full transparent zone plate and Fresnel zone plate are studied by experiments.For the focusing intensity,the focusing intensity of the quartz and PE full transparent zone plate is between the 1.8~2.8 and 1.5~2.2 times of the Fresnel zone plate,and the transmission efficiency is higher than that of the Fresnel zone plate by 52%~93% and 33%~91%,respectively.Then,we also studied an electromagnetic collimator to collimate the terahertz wave.Compared with the commonly used collimation lens,the collimating lens can collimate all electromagnetic waves outside the angle of divergence.In the aspect of super-focusing,we have studied a Coated Drop-Shaped Nanowires to conduct the Surface plasmon polaritons,the surface plasmon polaritons of the nanowires at different angles,different radii and different frequencies are calculated by COMSOL simulation.When the top radius increases,the width of the top mode is mainly similar,and the mode transmission length and effective index change are very small.When the bottom radius is larger,the top mode FWHM is almost unchanged,and the mode transmission distance increases,and the effective refractive index decreases.When the top angle increases,the top mode FWHM is almost unchanged.The maximum change of mode transmission length and effective index is 10% and 5% respectively.When the coating thickness varies from 4~10 μm,the mode propagation length decreases and the effective refractive index increases.When the coating thickness is greater than 10μm,the mode energy is almost all in PE,and the transmission length changes very little,and the effective refractive index tends to PE index.When the incident frequency increases gradually in 1~10 THz,the FWHM of the top mode will decrease.The synthesis shows that the top mode width(~10 nm)and the mode transmission length(~2.3 mm)can be balanced by adjusting the top radius and the bottom radius.