现代物理  >> Vol. 10 No. 3 (May 2020)

基于Smith-Purcell辐射的太赫兹辐射源研究进展
Research Progress of Terahertz Radiation Source Based on Smith-Purcell Radiation

DOI: 10.12677/MP.2020.103004, PDF, HTML, XML, 下载: 343  浏览: 698 

作者: 武明静, 亓丽梅, 杨 君, 陶 翔, 刘紫玉:北京邮电大学电子工程学院,北京

关键词: Smith-Purcell辐射太赫兹辐射超表面Smith-Purcell Radiation Terahertz Radiation Source Metasurface

摘要: 基于Smith-Purcell辐射的太赫兹辐射源因具有转换效率高、功率大、稳定性好、使用方便等优点,从而在科学领域中被广泛研究。本文综述了一维和二维太赫兹辐射源结构的研究现状,并探讨了超表面结构的Smith-Purcell辐射在太赫兹辐射源方面的重要应用前景。
Abstract: The terahertz radiation source based on Smith-Purcell radiation has been widely studied in the scientific field because of its advantages such as high conversion efficiency, high power, good stability, and convenient use. In this article, the research progress of one-dimensional and two-dimensional terahertz radiation source structures is given, and the development trend of Smith-Purcell radiation using metasurfaces is discussed for terahertz radiation sources.

文章引用: 武明静, 亓丽梅, 杨君, 陶翔, 刘紫玉. 基于Smith-Purcell辐射的太赫兹辐射源研究进展[J]. 现代物理, 2020, 10(3): 23-34. https://doi.org/10.12677/MP.2020.103004

1. 引言

太赫兹技术在科学领域具有广泛应用而备受瞩目,对于这些应用而言,设计制作高效且可控的太赫兹辐射源至关重要 [1]。太赫兹波的产生方法一般是使用行波管、返波管、速调管、回旋管、奥罗管等真空器件。基于注波互作用原理,将互作用频率提升到太赫兹频段,从而产生太赫兹波,但由于制作工艺的限制,多数太赫兹辐射源的谱宽较窄,难以覆盖整个太赫兹波段,且单脉冲功率较低 [2] - [11]。基于真空电子学的太赫兹源因其转换效率高、功率大成为科学家研究的热点,例如自由电子激光器、切伦科夫辐射器件以及Smith-Purcell 辐射器件等 [12] [13] [14] [15]。

其中,基于Smith-Purcell辐射的太赫兹源装置具有转换效率高、稳定性好、功率大等优点是提供具有预期特性的太赫兹辐射的新方法 [16]。通常,当运动电子通过周期金属光栅表面时,会产生Smith-Purcell辐射,这是S. J. Smith和E. M. Purcell首次发现的 [17]。随着科学技术的发展,使用自由电子束团(Free Electron Beam)与超表面或亚波长孔阵列相互作用激发Smith-Purcell辐射成为产生太赫兹波的有效方法 [18] [19]。选择毫米(mm)或微米(um)量级的周期结构尺寸刚好可以产生太赫兹频段的辐射。当电子束被调制时,通过改变电子束团的周期实现太赫兹辐射源的可调性,不需要外部腔体就可以在很宽的频率范围内连续调谐 [20]。此类太赫兹辐射源极具吸引力的主要原因在于频率的可调性(0.1 THz至10 Hz)和高功率 [21]。Smith-Purcell辐射机制不会造成束流损失,这使得太赫兹辐射源装置可以利用电子束的能量回收来提高整体效率。因此,基于Smith-Purcell辐射的太赫兹辐射源研究,具有重要的理论价值和应用前景。

2. 太赫兹辐射源

2.1. 一维结构的太赫兹辐射源研究现状

基于Smith-Purcell辐射的太赫兹辐射源研究最早可以追溯到1953年,当时S. J. Smith和E. M. Purcell [17] 两位科学家首先发现了当运动电子通过周期金属光栅表面时,会产生“光”,如图1所示。这种“光”是一种经典的非相干自发电磁波辐射。电磁辐射波长可由惠更斯构造定律说明:,其中,λ是电磁辐射波的波长,L是光栅的周期,n是辐射阶数,β = v/c是光束的相对论因子,θ是电子注运动方向与电磁波向外辐射方向的夹角。

1998年,美国Dartmouth大学J. Urata等 [22] 首次观察到了超辐射现象的产生。结构图如图2所示,实验中采用周期为173 μm的金属光栅作为互作用结构,电子注的能量为20 keV~40 keV,器件的工作频率为0.3 THz~1 THz。结构主要是利用注波互作用的原理,在光栅表面运动的电子注与表面波产生相互作用,这种互作用的结果使得原来的直流电子注群聚,从而产生周期性电子束团。当群聚好的周期电子束团经过光栅表面时就会产生一种辐射,这种辐射便是Smith-Purcell超辐射,这是一种相干辐射。产生Smith超辐射的条件是电子注的电流超过某一临界电流,称为起振电流,当电流达到这一临界电流时,注波互作用才能导致电子注群聚,从而Smith-Purcell超辐射功率才能保障。基于Smith-Purcell超辐射的电子束驱动装置有望发展成为可调、紧凑、功率大的太赫兹辐射源。

Figure 1. Spectrogram of the light emitted from the grating surface at θ = 20˚. Central images are vertically aligned on the left. First-order spectra appear on the right

图1. 在θ = 20˚处从光栅表面发出“光”的频谱图。中心图像在左侧垂直对齐。一阶光谱显示在右侧

Figure 2. Superradiation structure diagram and power output diagram of Dartmouth University

图2. Dartmouth大学的超辐射结构图以及功率输出图

2007年,S. Taga等 [23] 使用二维有限差分时域方法对光学范围中的Smith-Purcell辐射进行了理论研究。采用周期为200nm的介质加载金属光栅,金属的介电常数用Drude模型表示,如图3所示。在模拟过程中,当电子束通过金属光栅上方时,观察到三种类型的电磁辐射,这三种类型的电磁辐射分别是基本Smith-Purcell辐射、原始表面等离子体激元辐射和拟态表面等离子体激元辐射,都是由周期性光栅结构所引起的。

Figure 3. (a) Geometry used for simulations; (b) Terahertz radiation spectrum

图3. (a) 用于仿真的光栅结构;(b) 太赫兹辐射光谱

2017年,P. Zhang等 [24] 提出在金属光栅的鳍片上钻一个方形孔作为有效的电子通道,当电子束团通过时,会激发Smith-Purcell辐射,如图4所示。通过增加移动的振荡偶极子数量来增强辐射强度,偶极子由移动电荷及其在金属光栅中的虚电荷形成。结果表明产生的辐射强度是电子束团靠近光栅表面通过的情况的10倍,极大提高了设备的辐射效率。

Figure 4. (a) The schematic of hole-grating; a hole is drilled in the fins of the grating. (b) The contour map of Ez fields. (c) The time domain of Ez field for grating and hole-grating structure. (d) The corresponding spectrums

图4. (a) 孔栅示意图,在光栅的鳍片上钻了一个孔,(b) Ez场的等高线分布图,(c) 光栅和孔栅结构的Ez场的时域信号,(d) 对应的频谱

2014年,T. Zhan等 [25] 从理论上表明使用低能量电子束激励周期介质光栅上平铺单层石墨烯的结构,可以产生太赫兹辐射,如图5所示。太赫兹波是由移动电子有效激发石墨烯等离子体激元所产生的,由于石墨烯等离子体激元的低损耗和高浓度,其局部场增强,可以大大提高辐射强度。并且可以通过改变石墨烯的化学势调节等离子体激元的频率,为开发可调和微型自由电子太赫兹辐射源提供了可能。

Figure 5. (a) Three-dimensional (3D) view and side view, (b) Frequency at different chemical potential

图5. (a) 三维视图和侧视图,(b) 不同化学势下的频率

尽管可以在太赫兹状态下调节石墨烯结构中的表面等离子体激元共振,但尚未实现将此类表面等离子体转换为相干的太赫兹辐射。S. Liu等 [26] 提出强度明显增强的基于石墨烯的相干太赫兹辐射源,产生的辐射强度比使用传统电介质或半导体表面强400倍,如图6所示,并且可以与最先进的量子级联激光器产生的辐射强度相比。

Figure 6. (a) The schematic of grapheme sheet with a micrometer dielectric slits array substrate. (b) The spectrum of the radiation field (Ez) for the dielectric slits array with graphene and without graphene

图6. (a) 石墨烯电介质光栅结构示意图,(b) 电介质光栅有石墨烯和无石墨烯时的辐射Ez频谱

2019年,H. Zhang等 [27] 对由一系列微束产生的太赫兹相干Smith-Purcell辐射进行了理论与仿真的对比研究。如图7所示,在研究中使用CST软件中的PIC求解器,检测近场和远场区域的辐射频谱。研究得到如果Smith-Purcell辐射被单电子束激发,则辐射的高次谐波振幅呈指数衰减。最初“长”光束的微束化会导致相干Smith-Purcell辐射光谱出现峰值。辐射的最大光谱峰值与微束频率的基波重合,从而可以在不改变光栅的情况下,对高频率的辐射进行有效选择激发。结果表明通过微束间距的变化,辐射源具有潜在的广泛可调谐性,并提供了一种实现可调谐高功率太赫兹辐射源的方法。将仿真结果与理论计算进行了比较,证明了结果之间具有良好的一致性。

(a) (b) (c) (d)

Figure 7. (a) The schematic of the CST simulation. (b) Snapshot of the distribution of the magnetic field By component. (c) Comparison of the theoretical predictions and the Fourier transform. (d) Comparison of the frequency dependence of |F(ν)|2

图7. (a) CST模拟示意图,(b) 电磁辐射分量磁场等高分布图,(c) 理论预测与傅里叶变换的比较,(d) |F(ν)|2的频率依赖性比较

2.2. 二维结构的太赫兹辐射源研究现状

2017年,L. Liu等 [28] 研究了电子束靠近并平行于由金属C孔谐振器组成的Babinet表面运动,会在谐振频率处产生强电磁辐射,如图8所示。通过调整周期,超表面的共振频率可以在GHz至THz和红外范围内变化。并且,对于低于10 GHz的工作频率,吸收损耗与输入功率之比约为3.7%。尽管损耗率随工作频率的增加而增加,但在10 THz的工作频率下损耗仍低至11%。由于存在损耗,谐振频率与结构周期的倒数之间存在非线性关系,如图9所示。因此,Babinet超表面是制造高效、紧凑的太赫兹自由电子光源的理想选择。

2019年,P. Zhang等 [29] 利用二维井阵列超表面呈现出密集的垂直取向的Smith-Purcell辐射,如图10所示。移动的电子可以在井阵列超表面上激励出三个感应表面电流,它们之间会发生强耦合,改善了辐射场强度并调整了辐射角的分布。井阵列超表面上产生的Smith-Purcell辐射的场强比具有90度定向辐射的传统光栅结构中的场强大3倍。

Figure 8. (a) Schematic of the Smith-Purcell emission from a Babinetmeta surface. (b) The simulated reflection (R), transmission (T), and absorption (A) spectra of a Babinet at normal incidence of propagating waves

图8. (a) 从Babinet超表面产生Smith-Purcell辐射的示意图,(b) 在垂直入射下,Babinet超表面的反射(R),透射(T)和吸收(A)光谱

Figure 9. (a) The energy flux densities of Sin, Sup, Sdown, and Sab. (b) The corresponding emission efficiency (Sup/Sin and Sdown/Sin) and absorption efficiency (Sab/Sin)

图9. (a) Sin,Sup,Sdown和Sab的能量密度。(b) 相应的发射效率(Sup/Sin和Sdown/Sin)和吸收效率(Sab/Sin)

(a) (b) (c) (d)

Figure 10. (a) The schematic of well-array metasurface structure. (b) Radiation spectrum of Ez for well-array metasurface and grating structure. (c) Transient Ez field distribution in the cross-section for the well-array metasurface and (d) grating structure

图10. (a) 井阵列超表面结构示意图,(b) 井阵列超表面和光栅结构的Ez辐射光谱,(c) 井阵列超表面的横截面电场分布图和(d) 光栅结构

2019年,Y. Lan等 [30] 设计了在介质–金属–介质基板上带有Yagi-Uda纳米天线阵列的Smith-Purcell辐射,用于产生具有远场模式的光束,如图11所示。通过分别设计光栅周期和天线结构,操纵Smith-Purcell长轴的发射和垂直于电子运动方向的发射。如图12所示,通过使用不同的基板上的纳米天线阵列的不同组,可以发射包含单个波长和多个波长且具有远场角的多光束。

Figure 11. (a) Simulation structure for generation of SPR from a YUNA array deposited on an IMI substrate. (b) Top view. (c) Simulation structure for incident to a single feed element deposited on an IMI substrate. (d) Simulation structure for manipulated SPR on an IMI substrate. (e) Simulation structure for feed element of a single YUNA on an IMI substrate. (f) Generating two beams, using two groups of YUNA arrays, and (g) For three beams, using three groups of YUNA arrays

图11. (a) 由沉积在IMI基板上的YUNA阵列生成SPR的仿真结构,(b) 俯视图,(c) 入射到IMI基板上沉积单个馈电元件的仿真结构,(d) 在IMI基板上操纵SPR的仿真结构,(e) 位于IMI基板上单个YUNA馈电元件的仿真结构,(f) 使用两组YUNA阵列生成两个光束,(g) 以及使用三组YUNA阵列生成三个光束

Figure 12. (a)~(c) Snapshots of electric energy density contours in x-z plane for P = 136, 176 and 240 nm, respectively. (d) The electric energy density contour in the y-z plane

图12. (a)~(c) 分别在P = 136、176和240 nm的情况下,x-z平面中的电场能量密度分布图,(d) 在y-z平面上的电场能量密度分布图

2019年,Z. Su等 [31] 通过合理设计石墨烯超表面实现对Smith-Purcell辐射的按需控制,如图13所示。通过调节石墨烯超表面的结构和费米能级来完全控制辐射波的幅度,相位和极化状态。通过设计超表面每个晶胞的几何参数,可以将每个晶胞的辐射波强度从零更改为最大值,石墨烯带的宽度会影响诱导的石墨烯等离子体激元共振的强度,改变石墨烯带的宽度来控制辐射幅度。同时,通过调节图案化石墨烯结构的位移,在超表面结构任何位置处,辐射波的相位可以在2π的范围内变化。利用这两个特性,可以控制Smith-Purcell辐射的方向,并用双焦点聚焦辐射波。此外,通过引入交叉极化来实现具有任意相位的圆极化波。这为设计电子束感应光源以及具有高效率和紧凑空间的粒子探测器提供了一种新方法。

(a) (b) (c) (d)

Figure 13. (a) Schematic of Smith-Purcell radiation mediated by graphene metasurface; (b) Dependence of the reflectance and phase on the width of the grapheme ribbons. Electric field distribution of graphene metasurface at a frequency of (c) 2 THz and (d) 1.83 THz

图13. (a) 石墨烯超表面的Smith-Purcell辐射结构示意图,(b) 反射率和相位对石墨烯带宽度的依赖性,(c) 石墨烯超表面在频率2 THz处的电场分布图,(d) 1.83 THz

3. 总结与展望

金属光栅结构上的Smith-Purcell辐射是一种经典的电磁辐射,当运动电子束团通过周期金属光栅表面时,会产生电磁波辐射。毫米或微米量级结构产生的Smith-Purcell辐射恰好是太赫兹频段的电磁波,在开发紧凑、可调和高功率太赫兹辐射源方面具有重要的研究价值 [32] [33] [34] [35]。根据上述太赫兹辐射源的研究现状,目前人们利用有限差分时域分析、CST软件等方法对由运动电子束产生的太赫兹频段的Smith-Purcell辐射进行了大量的研究,例如通过在光栅的鳍片上钻一个孔作为有效的电子通道,增加移动的振荡偶极子数量来增强辐射强度;通过改变周期电子束团的频率,实现辐射源的广泛可调谐性;引入二维材料石墨烯,激发石墨烯的等离子体激元共振,通过改变石墨烯层的外部参数可以在很宽的光谱范围内调节辐射频率。这为开发可调谐和微型自由电子太赫兹辐射源提供了可能 [36] [37] [38] [39]。

尽管人们对光栅结构产生Smith-Purcell辐射的物理机理进行了较深入的研究,但是,当一般电子束在靠近金属光栅表面移动时,它将向各个方向发射电磁辐射,且辐射的幅度较弱,在实际应用中不方便控制,且结构大多采用刻有周期槽的全金属平板光栅,色散较强。使用二维超表面结构的太赫兹辐射源不仅可以极大地提高Smith-Purcell辐射的效率,还可以使电磁波更集中的定向辐射。若引入石墨烯,可以通过调节石墨烯超表面的结构和费米能级来完全控制辐射波的幅度,相位和极化状态,为开发更高强度和更集中定向辐射的电子束驱动的太赫兹源提供了有效方法。

随着电子束团驱动装置的工作频率逐渐接近太赫兹频段,结构尺寸不断减小,定向产生辐射和性能优化成为了关键挑战。为满足需求,设计二维超表面太赫兹辐射源结构以产生更高强度和更集中定向的辐射是未来发展的主要方向。随着微纳米技术的不断发展,使制造更精细的微纳米超材料成为可能,为进一步提高Smith-Purcell辐射效率和定向性提供了必要的前提条件。

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