Research Of Beam Shaping With Terahertz Transmissive All-dielectric Metasurface

Owing to the properties such as nonionizing photon energy and abundant spectral information,terahertz(THz)wave has broad application prospects.In some applications,terahertz light field is required to meet the specific distribution,so the research on terahertz beam shaping is of great s ignificance.Metasurface is a planar device composed of the substrate and two-dimensional ultra-thin resonator array.It can flexibly modulate the amplitude,phase and polarization state of the beam on the sub-wavelength scale,which provides a new and compact method for beam shaping technology.Compared with the reflective metasurface,the transmissive metasurface is more in line with the actual needs;compared with the transmissive single-layer plasmonic metasurface,the transmissive all-dielectric metasurface has a higher transmission efficiency.At present,there are few reports on the generation of terahertz special beams using transmissive all-dielectric metasurface.Therefore,in this thesis,a series of THz transmissive all-dielectric metasurfaces based on the waveguide effect are designed to shape the normally incident terahertz beams into the following special beams:Quasi-plane-wave(peak-to-peak amplitude ripple less than 1 d B and phase ripple less than 10°)used for testing antennas.In order to solve the problem of low energy utilization rate of the binary amplitude grating shaping method,s ilicon cylinders with different dimensions are arranged on the poly(4-methyl-1-pentene)(TPX)substrate to design two kinds of all-dielectric metasurfaces,which can shape the incident fundamental mode Gaussian beam into the quasi-plane-waves that can fully cover the specific area in the in-line and off-axis directions,respectively.Then,combined with the cubic spline interpolation method,the influences of the silicon cylinder diameter fabrication error on the shaping result s are studied.Radially polarized narrow-width annular beam used for trapping particles in three dimensions.The shaping method of using a narrow-width annular aperture to modulate the intensity of the radially polarized beam will induce huge energy loss.Therefore,combined with the vector diffraction iterative algorithm,an all-dielectric metasurface composed of the TPX substrate and rotating rectangular silicon pillars with different dimensions is designed,to shape the incident collimated horizontally polarized Gaussian beam into the radially polarized narrow-width annular beam.Furthermore,an all-silicon metasurfaces is designed,which is more favorable in fabrication,to obtain the radially polarized narrow-width annular beam with different parameters,and the influences of various errors on the shaping results of the all-silicon metasurface are analyzed.Sub-diffraction-limit focus used for particle manipulation.The metalenses and meta-axicons,which are composed of two kinds of materials and can realize sub-diffraction-limit focusing,increase the complexity of the fabrication process.Thus,according to the hyperboloidal and conical phase modulation functions,the cylindrically polarized all-silicon metalens and meta-axicon are designed,respectively;the purpose of converting the incident horizontally(or vertically)polarized plane wave into the radially(or azimuthally)polarized beam and focusing simultaneously to obtain the sub-diffraction-limit circular-spot-shaped(or doughnut-shaped)focus with the full width at half maximum(FWHM)less than half-wavelength,is achieved.Next,the focusing results of the cylindrically polarized all-silicon metalenses and meta-axicons with different numerical apertures(NAs)are compared and analyzed.Abruptly autofocusing beam used for biomedicine.The transmissive single-layer plasmonic metasurface which can generate abruptly autofocusing ring-Airy beam at a single frequency has a low transmission efficiency.As a result,four all-silicon metasurfaces with higher transmission efficiencies are designed;by changing the rotation angles of the rectangular silicon pillars with the same dimension arranged on the silicon substrate,in a wide frequency range,the incident horizontally polarized plane wave is shaped into the vertically polarized and abruptly autofocusing ring-Airy beam and three kinds of primary-ring-blocked ring-Airy beams with different exponential decay factors,respectively.In addition,the propagation characteristics of them are compared and analyzed.