Terahertz Near-field Study Based On The Photoconductive-antenna

Recently,terahertz radiation has been a branch of cutting-edge science and technology involving many fields such as public security,military defense and national economy.In the past,far-field measurements were widely carried out based on terahertz time-domain spectroscopy.But the spatial resolution is limited by far-field diffraction effect.In order to break diffraction limits and gain sub-wavelength spatial resolution in terahertz frequency region,a series of near-field detection methods came into being.The photoconductive-antenna tip delivers the possibility of near-field measurements of terahertz waves.In this paper,recent progress of near-field scanning terahertz microscopy system with photoconductive-antenna tip was introduced in detail.The work is mainly in the following four aspects:1.After introducing the basic concept and important nature and applications of terahertz,we analyzed and summarized the near-field detection methods in these years.2.Using the femtosecond laser whose center wavelength is 800 nm and the photoconductive-antenna tip detector coupled with fiber,we constructed fiber near-field scanning terahertz spectroscopy(NSTM).The frequency bandwidth was from 0.2 THz to 1.5 THz and the terahertz spot was circular and uniform after performance test.Also the amplitude and phase of the terahertz field were recorded simultaneously.It has the ability of three-dimension scan in various experiment conditions conveniently.3.NSTM was used on doing research at propagation features of terahertz SPs.By theoretical analysis and experimental verification,it noted that the metal structure of rectangle arrays which is sensitive to the polarization can be used to make various functional terahertz SPs devises.4.NSTM was used to scan spatial electric distribution in three dimensions and test the spectral properties in terahertz range.By theoretical analysis and experimental verification,it noted that terahertz lens array which is comprised of C-shape resonant rings had good focusing effect and could be used to inspect wavefront distortion due to its wavefront sensing properties.Also it was used on doing research at far-field optical properties of natural and artificial materials.Thus,this paper shows that NSTM is useful in applications in metamaterials,graphene,surface plasmons,waveguide transmission,near-field imaging,biological test and chip inspection.