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摘要:
作为太赫兹技术中的重要组成部分，太赫兹脉冲焦平面成像一经问世就引起了行业内的广泛关注，人们引入了各种方法去提升此成像技术的测量性能，同时也尝试将此成像技术应用于不同的工业和基础研究领域。本文综述了近年来人们对太赫兹脉冲焦平面成像的技术改良和应用研究，包括提升成像系统的空间分辨率、信噪比、信息获取能力，以及将此成像技术应用于光谱识别检测、超表面器件功能验证、太赫兹特殊光束测量、太赫兹表面波观测等，希望该综述能够推动太赫兹脉冲焦平面成像的进一步技术革新和应用拓展。
Abstract:As an important composition of terahertz (THz) technology, THz pulsed focal-plane imaging has been paid widely attention since it was invented. Until now, researchers have introduced all kinds of methods to enhance the performance of this imaging technique. Simultaneously, this imaging technique has been tried to apply into various industrial and fundamental research fields. In this paper, recent technique improvements and application researches for THz pulsed focal-plane imaging are reviewed, including the spatial resolution enhancement, signal-to-noise ratio improvement, information acquiring ability as well as applications of this imaging technique in spectroscopic identification inspections, function demonstrations of meta-surface devices, measurements of THz special beams, observations of THz surface electromagnetic waves, and so on. The aim of this paper is to push the technique innovation and application exploration of THz pulsed focal-plane imaging.
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Key words:
- terahertz /
- focal-plane imaging /
- resolution /
- meta-surface
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Overview: As a class of novel far-infrared testing technology, terahertz (THz) imaging has been rapidly developed for recent decades due to characteristics of the THz radiation, such as low photon energy, broad bandwidth, and high transmission to non-polar materials. Notably, the THz pulsed focal-plane imaging technique has become an important composition in all kinds of THz imaging methods because of its obvious measurement advantages. When the THz pulsed focal-plane imaging is employed, two-dimensional THz information of a substance can be accurately acquired in a single measurement and the raster scan process in traditional THz imaging is effectively avoided, which leads to the reduction of the experimental time as well as the enhancements of the measurement stability and sampling ratio. In this review, the technique innovations and application explorations of THz pulsed focal-plane imaging are introduced. This THz imaging technique was firstly proposed in 1996 and various means have been applied to improve its performance. With the development of the imaging technique, the super-thin sensor crystal and the quasi-near-field detection are introduced to improve the imaging spatial resolution; the dynamics subtraction and the balanced electro-optic detection are applied to enhance the signal-to-noise ratio of the imaging system. In addition, this imaging system can individually measure different THz polarization components (Ex, Ey, and Ez) by varying the polarization of the probe beam and using the sensor crystals with different crystalline orientations. Currently, it can be said that almost all of THz wave-front information can be obtained by using this imaging technique. With the maturation of the imaging technique, it has been applied into various industrial and fundamental research fields. Utilizing the spectroscopic measurement ability of the imaging system, identification of different chemical and biological samples can be achieved. Utilizing the vectorial measurement ability of the imaging system, the function of THz meta-surface devices, characterizations of THz special beams, and observations of THz surface electromagnetic waves have been demonstrated. Besides, this imaging technique has been also applied to measure transmission modes of THz waveguides, inspections to concealed objects, and so on. Of course, there is still much room for the future improvement of this imaging technique, such as the further enhancement of the signal-to-noise ratio, the enlargement of the imaging region, and the simplification of the optical configuration. Nevertheless, it can be expected that the imaging technique will show its tremendous application potentials in the future.
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图 1 太赫兹脉冲焦平面成像与太赫兹逐点扫描成像的比较[12-13]。(a)太赫兹逐点扫描成像；(b)由太赫兹逐点扫描成像获得的树叶太赫兹强度图像；(c)太赫兹脉冲焦平面成像；(d)由太赫兹脉冲焦平面成像获得的太赫兹光斑分布
Figure 1. Comparison of THz pulsed focal-plane imaging and THz raster scan imaging[12-13]. (a) Scheme of a THz raster scan imaging system and (b) THz images of a leaf by THz raster scan imaging; (c) Scheme of a THz pulsed focal-plane imaging system and (d) THz intensity distribution obtained by THz pulsed focal-plane imaging
图 3 太赫兹脉冲焦平面成像系统的改进[17-19]。(a)改进后的太赫兹脉冲焦平面成像系统；(b)准近场探测技术；(c)差分电光探测技术；(d)偏振探测技术
Figure 3. Improvement of a THz pulsed focal-plane imaging system[17-19]. (a) Improved THz pulsed focal-plane imaging system; (b) Quasi-near-field detection technique; (c) Balanced electro-optic detection technique; (d) Polarization detection technique
图 4 太赫兹脉冲焦平面显微成像[21]。(a)太赫兹脉冲焦平面显微成像系统；(b)双缝金属模板的光学和太赫兹图像；(c)在(b)中虚线位置处提取的太赫兹强度包络曲线
Figure 4. THz pulsed focal-plane microscopy[21]. (a) Scheme of a THz pulsed focal-plane microscopy system; (b) Visible and THz images of a double slit metallic mask; (c) THz intensity profile curve extracted along the dashed line in (b)
图 5 太赫兹脉冲焦平面成像实现对矢量光场测量[22]。(a)太赫兹脉冲焦平面成像系统，探测晶体选用了 < 100>ZnTe；(b)线偏振汇聚太赫兹光场，其Ez分量在传输过程中振幅和相位的演变；(c)左旋圆偏振汇聚太赫兹光场，其Ez分量的复振幅演化过程
Figure 5. Vectorial measurement of a THz field by THz pulsed focal-plane imaging[22]. (a) THz pulsed focal-plane imaging system in which a < 100> ZnTe is selected as the sensor crystal; (b) Evolutions of the Ez amplitude and phase for a converging linearly polarized THz field and (c) evolution of the Ez complex field for a focused left circularly polarized THz field
图 6 太赫兹脉冲焦平面成像的光谱识别检测[24-25]。(a)利用太赫兹脉冲焦平面反射成像系统对化学药品的检测；(b) 2, 4-DNT、可可碱、RDX、谷氨酸和玻璃的光学图像以及太赫兹图像；(c)利用太赫兹脉冲焦线成像系统对生物切片的检测；(d)人类牙齿切片样品的光学图像以及太赫兹图像
Figure 6. Spectroscopic identification by THz pulsed focal-plane imaging[24-25]. (a) Inspection of different chemicals by reflective THz pulsed focal-plane imaging; (b) Visible and THz images of 2, 4-DNT, theopylline, RDX, glutamic acid, and glass samples; (c) Inspection to biological tissues by THz pulsed focal-line imaging; (d) Visible and THz images of a human tooth slice
图 7 利用太赫兹脉冲焦平面成像对超表面器件进行功能表征[29-32]。(a)太赫兹超表面透镜的成像功能表征；(b)太赫兹超表面光子霍尔器件的偏振选择性响应表征；(c)太赫兹超表面环形艾里光束调制器功能表征；(d)太赫兹超表面多波长全息图功能表征
Figure 7. Function characterization of metasurface elements by THz pulsed focal-plane imaging[29-32]. (a) Imaging function characterization of a THz metasurface lens; (b) Polarization-dependent response of a THz metasurface photonic Hall element; (c) Function characterization of a THz metasurface ring Airy beam modulator; (d) Function characterization of a wavelength de-multiplexing THz metasurface hologram
图 8 利用太赫兹脉冲焦平面成像对太赫兹特殊光束进行矢量表征[38-39, 41]。(a)太赫兹涡旋光束的矢量表征；(b)太赫兹贝塞尔光束的矢量表征；(c)太赫兹瓶子光束的矢量表征
Figure 8. Vectorial characterization of THz special beams by THz pulsed focal-plane imaging[38-39, 41]. (a) Vectorial characterization of a THz vortex beam; (b) Vectorial characterization of a THz Bessel beam; (c) Vectorial characterization of a THz bottle beam
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