Axicon-based Terahertz Diffraction-free Beams And Their Ptychography Characteristics

A non-diffracting beam is a type of structured beam whose lateral distribution does not change due to propagation.Although a diffraction-free beam generated in practice is not strictly diffraction-free,it varies slowlier in the process of transmission than conventional beams such as a Gaussian beam.As a well-discussed research topic in recent decades,terahertz imaging technology has remarkable potential in the detection of samples such as large-scale biological tissues.However,given the long wavelength of terahertz waves,terahertz beams are easily broadened during propagation(for a terahertz beam with 1 mm scale wavelength,when the beam radius reaches 10 mm scale,the Rayleigh distance is only 100 mm scale),which creates difficulties for terahertz imaging.To address the problem,the promising technology of diffraction-free beam is introduced into the field of terahertz imaging.Conventionally,the most used diffraction-free beam generation method is to use an ordinary axicon to generate a Bessel beam.Since such method can only achieve a diffraction-free distance of about 200 mm in terahertz band which is not sufficient in length for practical application,it is of great necessity to conduct further research on generating non-diffracted beams.The present study is an effort to explore the generation method of the diffraction-free beam.Based on the ordinary axicon method,the research proposes two new methods to generate the diffraction-free beam with a long diffraction-free distance by changing the geometric shape of the ordinary axicon or adding devices in the system.The generated diffraction-free beams are further used to achieve terahertz ptychography.In addition,the spatial spectrum analysis of the beam’s diffraction-free performance and ptychography characteristics is carried out.The main contents and innovations of the dissertationt is summarized as follows:(1)Two diffraction-free beam generation methods are proposed based on the ordinary axicons: the proposal of the elliptical axicon method via the change of the geometry of an ordinary axicon;and the proposal of a cascaded lens-axicon system method via the extension of the ordinary axicon system.The experimental results at 0.1 THz frequency show that,compared with the ordinary axicon method,the two new methods extend the diffraction-free distance from 181 mm to 263 mm and 1099 mm respectively,the latter being the first diffraction-free distance up to 1m scale in terahertz band.(2)The ptychography at 0.1 THz is achieved via the use of generated non-diffraction beam to replace the conventional pinhole diffracted beam as illumination.The lateral resolution based on the elliptical axicon method and the lens-axicon method reach respectively 5.06 mm(1.69 times the wavelength)and 4.58 mm(1.52 times the wavelength),while the longitudinal resolution reaches 0.75 mm(0.25 times the wavelength).(3)The analysis of the beam in spatial frequency domain suggests that the spatial spectra of these two beams consist of a series of rings.The two innermost ring components play an important role in the extension of the diffraction-free distance.In addition,such two ring conpoments enables ptychography.At the low frequency THz band of near 0.1 THz,replacing the conventional pinhole diffractive beam with these two beams can solve the problem of its excessive size.