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原理 | 太赫兹 (THz) 传感器
利用超表面开发 THz 探测器
光电倍增管 (PMT) 和图像增强器 (II) 均采用基于光电效应原理的光电表面。
然而，与光波不同，THz 波的光子能量较低，很难直接引发光电效应。
因此，我们开发了一种将 THz 波转换为电子的超表面。在此，我们介绍超表面的概念和 THz 波到电子的转换原理。
什么是超表面？
超表面是一种人工设计的具有微结构的表面结构，广泛用于各种应用技术。之所以称之为超表面，是因为这是一种排列着周期性微结构（超原子）的表面。
滨松集团将这种超表面应用于 THz 检测。因此，这里的超表面是指 THz 波到电子的转换元件。
超表面由石英或硅等基板上的规律排列的黄金天线阵列组成，在 THz 波入射时，可根据场致电子发射原理发射电子。这些电子经过放大后输出，用于检测 THz 波。
场致电子发射图像
超表面微结构的放大图像
场致电子发射原理
THz 波到电子的转换利用的是场致电子发射原理。在此，我们对比用于普通光电倍增管的光电变换原理，介绍利用超表面和 THz 波电场进行电子发射的原理。
众所周知，超构表面场致电子发射是一种依赖电场强度的高度非线性现象。
此外，研究发现，涂覆碱性金属可以降低天线金属（黄金）表面的功函数，从而增强对 THz 波电场的灵敏度。滨松集团的超表面正是利用这一特性，在天线表面涂有碱性金属以提高灵敏度。
该天线设计还允许自由设计 THz 波与天线的谐振频带，从而可以支持从亚 THz 区到中红外区的广泛频带。
超表面（THz 光电倍增管）实现的场致电子发射
超表面的能带模型
在使用超表面的电子发射中，THz 波的电场经过天线放大后集中于发光体。
高电场集中于发光体。THz 波产生的高电场使黄金天线表面的真空能级倾斜，导致黄金的费米能级电子隧穿到真空侧。这一现象称为场致电子发射。
场致电子发射所发射的电子按照周围电场的作用在真空中运动。
1. THz 波入射至天线并聚集形成电场。
2. 高电场使黄金天线表面的真空能级倾斜。
3. 黄金的费米能级电子隧穿至真空侧。
碱性光阴极面（普通光电倍增管）的光电效应
光阴极面的能带模型
半导体具有“能隙”（即电子无法存在的禁带宽度）、“电子亲和能”（导带与真空能级间的能量差），以及“功函数”（费米能级与真空能级间的能量差）。
光子入射到光阴极面时，价带中的电子吸收光子能量并受到激发，从而扩散至表面。扩散的电子超过真空能级并以光电子的形式发射到真空中。
1. 光子入射至光阴极面，激发价带中的电子。
2. 激发的光电子扩散至表面。
3. 超过真空能级的光电子被发射到真空中。
场致电子发射的非线性输出
场致电子发射原理遵循 Fowler-Nordheim 关系公式。
其中，a 和 b 为 FN 常数，F 为功函数，β为电场集中因子，E 为 THz 波电场强度。电子发射量取决于 THz 波的电场强度 E。因此，利用该原理的超表面或许能够直接测量 THz 波电场强度。
另外，由于电子发射量与电场强度 E 的关系是高度非线性的，电场强度的微小变化都会引起电子发射量的巨大变化。如图所示，与线性设备相比，（该装置）可以清楚地观测到非线性响应特性。这种非线性响应特性可应用于细微变化的测量。
Fowler-Nordheim 关系公式
响应特性比较
相关论文
| 分类 | 标题 | 作者 | 链接 |
|---|---|---|---|
| 基本原理 | 光波驱动型长波长光电倍增管 | Simon J. Lange 等人 | https://doi.org/10.1002/lpor.202470002 |
| 用途 | IEEE 太赫兹科学与技术学报 | T. O. Buchmann 等人 | https://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=5503871 |
| 用途 | 基于 THz-PMT 电场干涉法的傅里叶变换 THz 光谱 | H. Takahashi 等人 | https://opg.optica.org/oe/fulltext.cfm?uri=oe-32-7-12774&id=548237 |
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