All-dielectric Meta-lens Designed For Terahertz Photoconductive Antennas

Terahertz spectroscopy is used in studying and analyzing the properties of various materials in the terahertz band,which has significant applications in material science,analytical chemistry and other fields.Terahertz time domain spectroscopy(THz-TDS)system occupies the core position in terahertz spectroscopy technology.When detecting the electromagnetic properties of materials,a THz-TDS system is expected to have high SNR,broadband response and high stability.Terahertz photoconductive antenna,which is widely used as emitter and detector in THz-TDS system,is pivotal as its properties determine the actual performance of a THz-TDS system.A photoconductive antenna emitter needs a collimating device to colimate the terahertz waves into the space,and a detector needs a converging device to focus the terahertz waves on the antenna gap to be detected,silicon lenses are commonly used convergent and collimation devices that can be used both in the emitter and the detector.Silicon lenses have the advantages of low dispersion,low absorption and isotropy,but the collimated beam diverges at cone angle of ~ 30°(aplanatic hyperhemisphere),and has spherical aberration(collimating hemisphere),which needs to be converged again in the latter optical path by parabolic mirrors or terahertz lenses,which aggravate the complexity of the systems.All-dielectric metasurfaces are of high transmittance,isotropy,and high degree of design freedom in wavefront reconstruction,therefore can be designed into various functional devices such as convergent lens,absorber,polarization controller and magnetic mirror,etc..In this article,we present a highly transmissive all-dielectric metasurface lens specifically replace traditional silicon lens to collimate the waves radiating from a terahertz photocontuctive antanna.In this paper,we apply a commercial available software,the CST Microwave Studio,to carry out numerical simulations and find out the optimized structure in collimation performance..Besides,we discuss the focusing performance of the meta-lens when being used in detection module,the influence of position misalignment and the response at different frequencies.Meta-lens samples were processed using deep reactive ion etching(DRIE)method.Then we design and build the scanning THz-TDS system to conduct the experimental measurement.In the emitting module,the distributions of electric field amplitude show that the designed meta-lens performs excellent in narrow-band collimation,which are consistent with simulation results.In the detecting module,experimental results are also consistent with simulation results as the detected terahertz field amplitude is significantly increased,verifying the focusing performance of the meta-lens.The meta-lens proposed in this paper is light and thin,not only has excellent collimation performance,but also owns high transmittance at operation frequency,which greatly reduces the system complexity,and therefore,can contribute to the integration of terahertz systems.This paper provides a new idea to improve the performance of terahertz collimation device,and makes an useful exploration for the promotion and development of terahertz technology.