Recent advances in terahertz digital holography
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摘要:随着太赫兹成像技术的不断成熟，其空间分辨率和系统信噪比逐渐提高，成像速度逐渐加快，光学信息获取能力逐渐变强，人们对太赫兹成像在基础研究和工业应用的开发也逐渐深入。本文综述了近年来科研人员利用太赫兹数字全息成像系统进行的部分研究工作，包括对平板太赫兹元件的性能表征、对光控太赫兹元件的功能验证、对衍射太赫兹场中的纵向分量进行观测、以及对金属亚波长器件的太赫兹表面波进行分析。这些工作的完成对于太赫兹集成系统的研究和太赫兹成像技术的应用都具有积极的推动作用。Abstract:With the maturation of terahertz (THz) imaging technology, the spatial resolution, signal-to-noise ratio, imaging speed and ability acquiring information of the imaging system are gradually enhanced. Researchers have paid more attention to THz imaging applications in fundamental researches and industrial exploitation. In this paper, several recent studies of THz digital holography are reviewed, including performance demonstration of THz planar elements, function validation of optical tunable THz elements, observation of the longitudinal component in diffraction THz fields and analysis of THz surface waves on the metallic sub-wavelength devices. These research works are very valuable for the development of THz integration systems and THz imaging technology.
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Key words:
- terahertz/
- digital holography/
- optical tunable elements/
- surface wave
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图 2太赫兹涡旋相位板及成像结果。(a) V型天线结构单元设计图；(b)8种不同参数V型天线，相邻天线的相位调制相差π/4；(c)太赫兹涡旋相位板样品图；(d)拓扑荷数l=1的太赫兹涡旋光束振幅分布；(e)相应太赫兹场相位分布；(f)和(g)为拓扑荷数l=2和l=3的太赫兹场相位分布；(h)测量太赫兹涡旋光束聚焦过程的实验系统；(i)和(j)为实验测得的距离焦点位置-10 mm、0 mm、10 mm处太赫兹光束振幅和相位分布；(k)为利用Laguerre-Gaussian模型所模拟的太赫兹场相位分布
Figure 2.THz vortex phase plate and imaging results. (a) Schematics of a V-shaped antenna phase modulation unit. (b) Eight kinds of V-shaped antenna structures corresponding to phase shifts with a π/4 interval. (c) Photography of the designed vortex phase plate. (d) Measured amplitude distribution of the generated THz vortex beam withl=1. (e) Corresponding phase distribution. (f) and (g) are the measured phase distributions of the THz vortex fields withl=2 andl=3. (h) Experimental setup for observing the focusing process of the THz vortex beam. (i) and (j) are the amplitude and phase maps of the measured THz vortex beam withZ=-10 mm, 0 mm, and 10 mm. (k) The simulated phase distributions by using the Laguerre-Gaussian module
图 3基于超表面太赫兹平板透镜的偏振选择性聚焦与成像。(a)太赫兹平板透镜实物图，插图展示了棒形天线的结构单元；(b)偏振选择性聚焦示意图；(c)~(e)对应右旋圆偏振(RCP)、左旋圆偏振(LCP)和水平线偏振太赫兹入射场，在焦平面上出射0.75 THz太赫兹光场强度分布；(f)~(h)相应的出射太赫兹光场在X-Z平面上的聚焦效果；(i)成像测试样品；(j)~(l)对应RCP、LCP和水平线偏振太赫兹入射场的0.75 THz成像结果
Figure 3.Polarization-selected focusing and imaging based on a metasurface THz planar lens. (a) Photograph of the THz planar lens. The inset shows the schematics of a bar antenna unit. (b) Procedure of spin-selected focusing. (c)-(e) Intensity images of the 0.75 THz component on the focal plane for a right circularly polarized (RCP), left circularly polarized (LCP), and horizontally linearly polarized THz incident fields. (f)-(h) Corresponding longitudinal focusing processes of transmitted THz fields on theX-Zplane. (i) Imaging test sample. (j)-(l) Imaging results of 0.75 THz components for RCP, LCP and horizontally linearly polarized THz incident fields
图 4全光可控虚拟太赫兹波带片。(a)光控太赫兹数字全息成像系统，插图展示了波带片实物图；(b)随太赫兹波与控制光之间时间延迟而变化的太赫兹峰值信号曲线；(c)和(d)为在-10 ps和10 ps时间延迟位置的1 THz强度图像；(e)为在波带片图像放大率为R=1.12、1.00、0.91时，0.8 THz、1.0 THz和1.2 THz分量的强度图像；(f)成像测试样品实物图；(g)相应的太赫兹成像结果
Figure 4.All-optical steerable virtual THz Fresnel zone plate (FZP). (a) Optical tunable THz digital holographic imaging system. The inset shows the photograph of the FZP. (b) Variation of the THz peak signal with the time delay between the THz and control beams. (c) and (d) present the 1.0 THz intensity images with the time delays of -10 ps and 10 ps. (e) THz intensity images of 0.8 THz, 1.0 THz and 1.2 THz components for three FZP patterns with the amplification ratiosR=1.12, 1.00, 0.91. (f) Photographs of imaging samples, (g) Corresponding imaging results
图 5空间太赫兹调制器。(a)空间太赫兹调制器概念图；(b)实验系统图；(c)“C”、“N”和“U”的离轴太赫兹计算全息图；(d)1 THz一级衍射分量强度图；(e)利用空间太赫兹调制器生成的拓扑荷数l=1、l=2和l=3太赫兹涡旋光束强度图；(f)太赫兹涡旋光束相位分布图
Figure 5.Spatial THz Modulator (STM). (a) Prototype of the STM. (b) Experimental configuration of the STM. (c) Off-axis THz computer-generated holograms for letters "C", "N", and "U", respectively. (d) Corresponding intensity distributions of first-order diffraction components at 1 THz. (e) Intensity patterns of THz vortex beams with topological numbersl=1,l=2, andl=3 generated by using the STM. (f) Corresponding the phase patterns of the THz vortex beams
图 6光控太赫兹矢量光束的生成与表征。(a)光控太赫兹矢量光束产生原理图；(b)太赫兹矢量光束表征系统；(c)和(d)为径向偏振太赫兹光束在x、y和r分量的振幅和相位模拟结果；(e)和(f)为径向偏振太赫兹光束在x、y和r分量振幅和相位的实验测量结果
Figure 6.Generation and characterization of optical steerable THz vector beams. (a) Schematic for generating an optical-tunable THz vector beam. (b) Characterization system of THz vector beams. Simulated (c) amplitude and (d) phase distributions ofx,y, andrcomponents of a radially polarized THz beam. Measured (e) amplitude and (f) phase patterns ofx,y, andrcomponents of a radially polarized THz beam
图 7太赫兹衍射场纵向分量重建。(a)实验系统图；(b)线偏振太赫兹光束聚焦，其纵向分量的振幅和相位分布；(c)利用Richards-Wolf公式得到的模拟结果；(d)圆偏振太赫兹光束聚焦，其纵向分量的振幅和相位分布；(e)相应的模拟结果
Figure 7.Reconstruction of the Ez component of a THz diffraction field. (a) Experimental system. (b) Amplitude and wrapped phase distributions of theEzcomponent for a converging THz beam with a linear polarization. (c) Simulation results obtained by using the Richards-Wolf equation. (d) Amplitude and wrapped phase maps of theEzcomponent for a focused THz beam with a circular polarization. (e) Corresponding simulation result
图 8金属等离子体器件的太赫兹表面波再现。(a)实验系统图，插图展示了测试样品；(b)太赫兹表面波时域峰值图像；(c)在x=0 mm方向上y=-1 mm、-0.5 mm、0 mm、0.5 mm、1 mm位置处的太赫兹时域信号；(d)0.73 THz太赫兹表面波的振幅分布图；(e)利用FDTD算法得到的模拟结果；(f)和(g)展示了归一化横向和纵向振幅轮廓曲线；(h)0.73 THz表面太赫兹波的相位分布图；(i)相应的模拟结果；(j)不同光谱成分太赫兹表面波的纵向相位轮廓曲线；(k)相应的模拟结果
Figure 8.Reconstruction of THz surface waves on metallic plasmon devices. (a) Experimental setup. The inset shows the test sample. (b) The temporal peak image of the THz surface wave. (c) The temporal THz signals measured aty=-1 mm, -0.5 mm, 0 mm, 0.5 mm, and 1 mm along thex=0 mm direction. (d) Amplitude distribution of the THz surface wave at 0.73 THz. (e) Simulation result by using the FDTD algorithm. (f) and (g) show the normalized transverse and longitudinal amplitude profile curves. (h) Phase distribution of the THz surface wave at 0.73 THz. (i) Corresponding simulation result. (j) Longitudinal phase profile curves for THz surface waves with difference spectral components. (k) Corresponding simulation result
图 9太赫兹表面波偏振选择性聚焦。(a)太赫兹表面波成像系统，插图为样品示意图；(b)和(c)为LCP和RCP入射光激发的太赫兹表面波聚焦过程振幅分布图；(d)和(e)为图(b)和(c)中截取的x=0 mm处振幅分布曲线；(f)和(g)为LCP和RCP入射光激发的太赫兹表面波聚焦过程相位分布图；(h)为图(f)和(g)中截取的x=-2 mm处相位分布曲线；(i)相位相减分布曲线；(j)y偏振入射光激发的太赫兹表面波振幅分布图；(k)相应的数值模拟结果；(l)和(m)为在图(j)和(k)中截取的x=0 mm处振幅和相位分布曲线
Figure 9.Polarization-controlled focusing of THz surface waves. (a) THz surface waves imaging system. The inset shows the schematic of the sample. (b) and (c) present the amplitude distributions of converging THz surface waves excited by LCP and RCP THz incident fields. (d) and (e) show the amplitude profile curves alongx=0 mm taken from (b) and (c). (f) and (g) present the phase patterns of focused THz surface waves excited by LCP and RCP THz incident fields. (h) shows the phase profile curves alongx=-2 mm extracted from (f) and (g). (i) Curve of the subtraction phase. (j) Amplitude map of the THz surface wave excited by a linearlyy-polarized THz incident field. (k) Corresponding simulation result. (l) and (m) present the amplitude and phase curves alongx=0 mm taken from (j) and (k)
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