Generation And Application Of Terahertz Vortex Beam

In recent years,terahertz beam has shown great prospects in security detection,imaging and wireless communication owing to its unique advantages such as rich hyperspectral information and not fully developed communication spectrum resources.As one of the basic properities of electromagnetic waves,orbital angular momentum(OAM)can provide a new degree of freedom for electromagnetic wave modulation and multiplexing,which is independent of ampltitude,phase,polarization and other properities.The combination of terahertz technology and OAM can provide new ideas for short-distance high-speed communications,imaging and other applications.At present,most research of OAM is mainly concentrated in the radio frequency and optical domain.The terahertz OAM technology has just started,and there are many problems in generation,detection and application that need to be solved urgently.This thesis mainly studies the generation and demodulation of terahertz OAM beam and its application in imaging field.In this thesis,three methods of generating terahertz OAM beam are studied,all working at 0.1 THz.Aiming at the problems of high reflectivity,poor beam performance and low efficiency of spiral phase plate,a spiral phase plate antenna with low reflectivity and high transmittance based on transmission line theory is designed.The equivalent relative permittivity of the dielectric is changed by perforating the dielectric structure,and the specific parameters of the unit structure are determined through software simulation.The simulation realized the low reflectivity and high transmittance spiral phase plate with modes l=±1 and l=±2,which effectively improved the energy reflection problem compared to ordinary spiral phase plates.A terahertz OAM multicasting board that can generate modes l=-1,+2 at the same time is designed,which constructs the basis for the proof-of-principle experiment of terahertz OAM demultiplexing.Taking advantage of ability of metasurface to flexibly control electromegnetic waves,using high-resistance silicon with high dielectric constant and low loss in the terahertz band a metasurface based terahertz OAM antenna is designed.The simulation verifies the generation of the terahertz OAM beam.The low-reflectivity spiral phase plate and the multi-mode multicasting plate are produced using 3D printing technology.The near-field scanning system of terahertz OAM antenna is built,and the near-field electric field distribution of the low-reflectivity spiral phase plate and the multicasting plate are measured,which is coincident with the simulated one.Compared with the ordinary spiral phase plate,the reflectivity of the perforated spiral phase plate is reduced by at least 25 dB.A proofof-principle experiment of terahertz OAM multiplexing is carried out using the broadcast board.A bright spot will appear in the center after the demodulation of an oppisite spiral phase plate,which shows an effective demodulation of the expect OAM mode,and can provides an experimental basis for terahertz OAM multiplexing communication.The OAM beam’s phase factor ejφ,also known as the radial Hilbert transform factor,can effectively extract the edge contour of the object,that is,phase contrast imaging.The feasibility of terahertz OAM wave in phase contrast imaging is studied through both the simulation and the experiment.The results of brightfield imaging and phase contrast imaging are compared,and the influence of object size,working frequency,and topological charge are briefly analyzed.In order to reduce the influence of sidelobe noise on vortex phase contrast imaging,the Bessel-like amplitude modulation vortex filtering is also studied.The results show that it can effectively suppress the sidelobe noise and improve the imaging quality.At last a phase constrast imaging system is built to verify the simulated results.All the analysis and experiment results can provide a technical support for the application of terahertz OAM beams in the further imaging field.