基于超表面的太赫兹远场超衍射聚焦方法及实验研究

Aiming to address the physical issue of the diffraction limited focusing of the optical components in conventional terahertz imaging systems, this project is proposed to achieve terahertz sub-diffraction focusing based on metasurfaces. It is well known that the limited aperture of conventional terahertz lens determines its limited capacity of spectrum transferring, which means that the focal spot size cannot break through the diffraction limit, and thus makes the conventional terahertz far-field imaging system unable to resolve the object whose spatial size is smaller than the diffraction limit. Super-oscillation phenomenon, known as the fact that band-limited functions are able locally to oscillate faster than their highest Fourier components, is demonstrated to achieve local light diffraction shrinkage in the far field. In addition, the appearance of metasurface technology provides an effective way to control the terahertz wave arbitrarily. Accordingly, the combination of metasurface and super-oscillation is expected to beat the diffraction limit of terahertz far-field focusing, and provide a promising technology for achieving terahertz far-field super-resolution imaging. In this project, the physical model of the sub-diffraction focusing device is analytically established based on the mata-surface and super-oscillation theories. The theoretical design and experimental fabrication of the related devices are also studied. Subsequently, an experimental platform using continuous-wave terahertz source will be constructed to validate the characteristic performance of the fabricated device and achieve terahertz super-resolution imaging.

针对传统太赫兹成像系统中光学元件聚焦能力衍射受限的物理问题，提出基于超表面的太赫兹远场超衍射聚焦方法及实验研究。传统太赫兹透镜有限通光口径决定了其自身有限带通频谱传递能力，这意味其焦斑尺寸无法突破衍射极限，导致传统太赫兹远场成像系统无法分辨空间尺寸小于衍射极限的物体。超振荡现象，频率带限函数在局部区域的振荡比带限函数包含的最高频率分量更快，已被证实可用于实现远场局部光场的衍射压缩。另外，超表面技术的出现为任意调控太赫兹光场提供了有效的办法。超表面与超振荡的结合，有望获得突破衍射极限的太赫兹远场聚焦光斑，从而为实现太赫兹远场超分辨成像奠定关键技术基础。本项目从产生太赫兹超振荡的角度着手，构建基于超表面的太赫兹超衍射聚焦器件的数理模型，开展相关器件的理论设计和实验制备研究；搭建连续太赫兹波实验平台，完成太赫兹超衍射聚焦器件的性能表征和太赫兹超分辨成像实验。

针对传统太赫兹成像系统中光学元件聚焦能力衍射受限的物理问题，提出基于超表面的太赫兹远场超衍射聚焦方法及实验研究。传统太赫兹透镜有限通光口径决定了其自身有限带通频谱传递能力，这意味其焦斑尺寸无法突破衍射极限，导致传统太赫兹远场成像系统无法分辨空间尺寸小于衍射极限的物体。超振荡现象，频率带限函数在局部区域的振荡比带限函数包含的最高频率分量更快，已被证实可用于实现远场局部光场的衍射压缩。另外，超表面技术的出现为任意调控太赫兹光场提供了有效的办法。超表面与超振荡的结合，有望获得突破衍射极限的太赫兹远场聚焦光斑，从而为实现太赫兹远场超分辨成像奠定关键技术基础。本项目从产生太赫兹超振荡的角度着手，构建基于超表面的太赫兹超衍射聚焦器件的数理模型，开展相关器件的理论设计和实验制备研究；我们通过实验研究了太赫兹超振荡透镜（TSOL）的超聚焦性能。在焦平面产生太赫兹超振荡光场的TSOL由太赫兹非球面透镜和优化的二元0-π相位器件组成。 对于经过验证的 TSOL（数值孔径 NA = 0.4614）@工作波长 λ=119μm，超振荡光斑~0.80λ 的实验 FWHM 约为艾里光斑的 0.70 倍。 进一步进行理论仿真验证了TSOL的共焦扫描成像分辨率增强，TSOL的成像分辨率约为Rayleigh准则的0.85倍。