Theoretical And Experimental Study Of Terahertz Structured Beams

Recently,terahertz(THz)science and technology attract more and more attentions due to their intruiging properties that may enable a lot of potential applications over many fields,such as safety screening,nondestructive testing,high-volume wireless communications and so on.Since the THz band locates between the microwave band and the far infrared band,many THz researchs are still immature.Considering the THz band fundamentally lacks functional elements,such as high-power THz sources and high-efficiency THz wave modulators,many complicated technologies and applications can not be directly scaled into the THz band from the optics.In optical domain,structured beams gradually become one of the research highlights,for example,vortex beams,Airy beams and Bessel beams.Various researches indicate that the vortex beams can serve as a new degree of freedom to greatly increase the bandwidth and spectrum efficiency of the communication systems,and the Airy beams and Bessel beams exhibit non-diffracting property,which can be utilized to enlarge the focal depth of the imaging systems.However,the amount of researchs related to these structured beams in the THz domain is far fewer than that in the optical domain.In this dissertation,we present an efficient method based on the 3D printing technology to statically manipulate the phase of the continuous THz wave.With such a method,THz vortex beams,Airy beams and Bessel beams are generated,hence they could be applied in the THz imagings and THz wireless communication links.The main work of the dissertation can be concluded as below:Firstly,we introduce the optical coordinates transformation method into the THz domain to efficiently demultiplex the OAM states of the THz vortex beams.Considering the fact that the analytical expressions of the optical coordinates transformation method do not work well in the low frequency domain,we modify the method via the angular spectrum theory.At the frequency of 0.3 THz,we design the required elements with modified optical coordinates transformation method,and fabricate the corresponding phase plates with the 3D printing technology.Assisted by the 3D printed phase plates,the OAM states of the THz vortex beams are efficiently separated and measured,which could benefit the OAM-based wireless THz communication links.Secondly,we theoretically study the principle for generating the accelerating Airy beams.Based on the angular spectrum theory,THz accelerating Airy beams are numerically researched,with different parameters.By eliminating the Fourier transform lens of the general generation system,the total length of the generation system can be reduced.Assisted by the 3D printed diffractive phase plates,we first demonstrate the THz accelerating Airy beams with a 0.3-THz continuous wave.The non-diffracting,self-accelerating and self-healing properties of the generated THz accelerating Airy beams are experimentally observed,and the observed results are in good accordance with the simulated ones.The THz accelerating Airy beams may enable applications of depth-extented imaging systems and robust wireless communication links.Thirdly,the Airy function is applied into the cylindrical coordinate system for generating the THz circular Airy beams.Based on the angular spectrum theory,the THz circular Airy beams(CABs)are numerically researched with different parameters.Furthermore,the spiral phase term is added into the THz CABs,and the THz circular Airy vortex beams(CAVBs)are numerically generated.With the 3D printing technology,the diffractive phase plates are fabricated.Assisted by the 3D printed phase plates,we first demonstrate the THz CAVBs with a 0.3-THz continuous wave.The experimental results exhibit that the THz CAVBs are capable of carrying individual OAM states and multiplexed OAM states,providing an optional candidate for the OAM-based wireless THz communication links.Moreover,the zero-order THz CAVB possesses a non-diffracting property,which could be utilized to extend the focal depth of the imaging system.Fourthly,by wrapping the height of typical axicons to produce diffractive axicons,we generate practical THz Bessel beams with a 0.3-THz continuous wave.Bessel beams are fomous with the non-diffracting property that could be used to extend the focal depth of the THz imaging systems.Here we apply such beams into the THz computed tomography(THz-CT)imaging system to measure typical sample,and the results show that the inner structure can be clearly distinguished.