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摘要:
太赫兹(THz)成像技术，因其具有能量低、透射率高、波谱范围宽等独特的分析能力，已经在生物医学、安全检查、航空航天等领域展现出巨大的优势及潜在的应用价值，但是较低的空间分辨率制约了太赫兹成像技术的进一步应用。太赫兹波通过具有适当折射率的介质结构产生的“太喷射”效应调控亚波长尺寸太赫兹光场，突破衍射极限对显微系统空间分辨率的限制，同时不损失光场能量和光谱信息，实现高通量、超宽谱的远场太赫兹高分辨成像。本文首先介绍基于纳米喷射的微球透镜显微技术，接着介绍基于太喷射的太赫兹显微技术，最后对基于喷射效应的太赫兹高分辨成像技术的前景做了展望。
Abstract:Terahertz (THz) imaging technology has shown great advantages and potential applications in the fields of biomedicine, security, and aerospace, due to its low energy, high transmittance, wide bandwidth, and unique analysis abilities; while low spatial resolution restricts its further applications. Recently, a high-resolution, high-throughput, and broad-bandwidth THz imaging method has been proposed based on the terajet effect produced by dielectric structures with appropriate refractive index. The terajet beam can break through the restriction of the diffraction limit on the spatial resolution of the microscopic system without losing the energy and spectral bandwidth of the THz field. In this paper, firstly, a white-light nanoscopy based on photonic nanojet produced by microspheres is introduced, then the THz microscopy based on terajet effect produced by mesoscopic dielectric structures is reviewed. Finally, the prospect of THz high resolution imaging technology based on terajet effect is prospected.
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Key words:
- terahertz imaging /
- terajet /
- spatial resolution
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Overview: In the past decades, great advancements have been made to achieve super-resolution imaging, including near-field THz microscopy, metamaterial superlens, fluorescence microscopy and so on, pushing the resolution to ñm or nm scale. Unfortunately, applications of these methods have been limited in part due to their complication in access and operation, loss of energy and spectral bandwidth, difficulty in information extraction or limited choices of samples. Thus, it is highly desired to develop innovative super-resolution THz imaging modality that is easily accessible and low-cost. Fortunately, at visible frequencies, a unique easy-access super-resolution imaging where dielectric microsphere with appropriate refractive index has been presented and delivered a remarkable 50 nm resolution with white lights in 2010. Furthermore, super-resolution imaging was also presented with a large field-of-view using large polystyrene microspheres (above 30 μm). A strong continuing interest in the technique has led to numerous progress in visible light. In these works, the super-resolution capability of microspheres is determined by the "photonic nanojet" and coupling with evanescent waves. More recently, a straightforward THz imaging method based on terajet effect, analogous to microsphere optical nanoscope, is proposed and developed with spatial resolution beyond the diffraction limit by using either continuous or pulsed THz wave. The terajet beam can break through the restriction of the diffraction limit on the spatial resolution of the microscopic system without losing the energy and spectral bandwidth of the THz field, i.e., a high-resolution, high-throughput and broad-bandwidth THz imaging method. Also, with the extensively longer wavelength of the THz wave, the size of the dielectric spheres is much larger (on the order of millimeters), the spheres are easier to fabricate, simple to manipulate, and capable of handling energy and bandwidth losses. In addition, as unique spectroscopic technique, THz imaging reveals much richer subwavelength structural information, including frequency-dependent amplitude and phase, as well as time-dependent delay and thickness. In this review, firstly, a white-light nanoscopy based on photonic nanojet produced by microspheres is introduced, then the terahertz microscopy based on terajet effect produced by mesoscopic dielectric structures is reviewed. Finally, the prospect of terahertz high resolution imaging technology based on terajet effect is prospected.
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图 1 (a) 微球透镜与传统光学显微镜结合显微镜示意图；(b), (c)电镜图(左)与透射模式下微球超透镜的显微成像图(右)对比：(b)宽度为360 nm、间隔为130 nm的线条；(c)渔网状的镀金AAO模板；(d), (e)电镜图(左)与反射模式下微球超透镜的显微成像图(右)对比：(d)商业蓝光DVD光盘；(e)星状GeSbTe薄膜[27]
Figure 1. (a) Experimental configuration of a white-light microsphere nanoscope; microsphere superlens imaging in transmission mode; (b) 360 nm wide lines spaced 130 nm apart; (c) A gold-coated fishnet AAO sample; and microsphere superlens imaging in reflection mode; (d) A commercial Blue-ray DVD disk; (e) A star structure made on GeSbTe thin film[27]
图 2 (a) 微球纳米喷射示意图[39]；(b)单层同质微球和双层异质微球的纳米喷射对比图[40]；(c)产生纳米喷射的无衍射贝塞尔光束[41]
Figure 2. (a) Schematic diagram of photonic nanojet produced by microsphere[39]; (b) Comparison between single-layer homogeneous microspheres and double-layer heterogeneous microspheres[40]; (c) Microsphere optical nano-jet related to non-diffractive Bessel beams[41]
图 3 (a) 不同折射率介质长方体产生的太喷射波束示意图[28]；(b)反射模式下介质长方体耦合THz成像系统及其对铝板样品的振幅和相位图像[45]
Figure 3. (a) Schematic diagram of terajet beam generated by dielectric cuboids with different refractive index[28]; (b) Dielectric cube coupled THz imaging system in reflection mode and its amplitude and phase images for aluminum plate samples[45]
图 4 (a) 长方体阵列的太喷射波导侧面示意图及其太喷射波束在不同空隙情况下的光场分布[46]；(b)耦合金属掩膜的介质立方体的示意图及其太喷射波束的光场分布[48]
Figure 4. (a) Schematic diagram of all-dielectric period terajet waveguide using an array of couple cuboids and its electric field distribution[46]; (b) Schematic diagram of the pupil-masked 3D dielectric cuboid and its electric field distribution[48]
图 5 不同几何形状介质结构产生的喷射波束的光场分布。(a)金属棒阵列填充长方体[49]；(b)球体[51]；(c)三角椎体[51]；(d)梯形椎体[51]；(e)扇形椎体[51]
Figure 5. Distribution of electric energy generated by different geometric structures. (a) Metal rod array filled rectangular condyle[49], (b) sphere[51], (c) triangular condyle[51], (d) trapezoidal condyle[51], and (e) fan-shaped condyle[51]
图 9 (a) 3 mm直径介质球在不同频率下x-z平面上的空间能量密度分布；(b) 3 mm直径介质球在1.5 THz频率下x-z平面上的空间能量密度分布随折射率的演变[29]
Figure 9. (a) The spatial energy distribution of 3 mm sphere in z-x plane at different frequencies; (b) Evolution of a jet-like energy distribution as the refractive index of 3 mm dielectric sphere increases from 1.4 to 2.0[29]
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