Terahertz Plasmonics for Linear and Nonlinear Spectroscopy and Sensing

太赫兹科学技术已成为近年来最令人兴奋的研究前沿之一。由于许多可能的研究和技术应用，这一长期被忽视的电磁频谱部分(频率从0.1到5太赫兹，对应于大约0.06到3毫米的波长)引起了极大的关注。太赫兹传感是一个重要的领域，它利用材料独特的光谱特征进行检测和识别。最近的研究表明，太赫兹范围内光谱丰富，许多不同的物质都具有强烈而独特的吸收指纹。因此，敏感的传感技术在食品监测和化学武器检测等领域将有广泛的应用。然而，太赫兹系统相对较差的灵敏度阻碍了这一领域的发展。相比之下，最先进的中红外吸收光谱可以达到万亿分之一的检测水平，而可见光或近红外的表面增强拉曼散射可以用来检测单个分子的光谱特征。这项研究计划的目的是展示使用太赫兹脉冲(包括具有非常高峰值电场的脉冲)进行传感和光谱分析的新技术。这种方法是以最近的工作为基础的，这些工作探索了在小型金属结构的局部环境中可能发生的场的巨大增强。这些想法将与新开发的高能太赫兹脉冲产生技术相结合。这种结合将能够产生极高的局域场强，为太赫兹范围内的极端光-物质相互作用开辟了一个新的领域，并为太赫兹传感系统的灵敏度的大幅提高指明了方向。这项为期三年的综合研究计划有几个重要目标。它将开创基于强太赫兹场诱导的非线性相互作用的时间分辨光谱研究的灵敏新技术的发展。通过产生迄今报道的最高太赫兹场并研究它们与材料的相互作用，这项工作将建立利用高阶非线性相互作用的可能性，例如拉曼散射，而太赫兹文献基本上没有这种相互作用。它还将展示基于这些非线性相互作用的传感新技术，这些技术具有重要的技术意义。该项目的范围由两个广泛的研究项目确定。第一项研究将研究等离子体激元装置对太赫兹波的感应和操纵带来的新的可能性。这将包括对超表面的研究，包括那些在可电切换的有源基板上制造的超表面。它还将涉及使用原子力显微镜进行尖端增强型近场光谱，包括近场发射光谱。第二个推力将通过将这些等离子体设备与高强度太赫兹入射场结合起来，探索由这些等离子体局部场增强引起的非线性相互作用所提供的可能性。广义地说，这项工作将改变对太赫兹科学的普遍看法，人们通常认为太赫兹科学局限于低场或微扰极限。通过扩展最近在太赫兹高场产生方面的突破性工作，该项目将建立极端太赫兹光-物质相互作用的新学科。

Terahertz science and technology has become one of the most exciting research frontiers in recent years. This long-neglected portion of the electromagnetic spectrum (with frequencies ranging from about 0.1 to 5 THz, which corresponds to wavelengths from about 0.06 to 3 mm) has attracted a great deal of attention, because of the many possible research and technology applications. One important area is terahertz sensing, which exploits the unique spectroscopic signatures of materials for detection and identification. Recent research has shown that the terahertz range is spectroscopically rich, with many different substances possessing strong and unique absorption fingerprints. Sensitive sensing technologies would therefore have wide-ranging applications, in areas as diverse as food monitoring and chemical weapons detection. However, the field has been hampered by the relatively poor sensitivity of terahertz systems. By contrast, state-of-the-art mid-infrared absorption spectroscopy achieves parts-per-trillion detection levels, and surface-enhanced Raman scattering in the visible or near- infrared can be used to detect the spectroscopic signature of a single molecule. The aim of this research program is to demonstrate new techniques for sensing and spectroscopy using terahertz pulses, including pulses with very high peak electric fields. The approach is to build on recent work which explored the giant enhancements of fields that can occur in the local environment of small metallic structures. These ideas will be combined with newly developed techniques for high-energy terahertz pulse generation. This combination will enable the generation of extremely high local field strengths, opening up a new realm for extreme light-matter interactions in the terahertz range, and pointing the way towards vast improvements in the sensitivity of terahertz sensing systems.This comprehensive three-year research program has several important goals. It will pioneer the development of sensitive new techniques for time-resolved spectroscopic studies based on nonlinear interactions induced by strong terahertz fields. By generating the highest terahertz fields yet reported and studying their interaction with materials, this work will establish the possibility of exploiting higher-order nonlinear interactions such as Raman scattering, which have been essentially absent from the THz literature. It will also demonstrate new techniques for sensing based on these nonlinear interactions, which are of great technological importance. The scope of the project is defined by two broad research thrusts. The first will investigate the new possibilities enabled by plasmonic devices for sensing and manipulation of terahertz waves. This will include the study of metasurfaces, including those fabricated on active substrates which can be switched electrically. It will also involve the use of atomic force microscopy for tip-enhanced near-field spectroscopies including near-field emission spectroscopy. The second thrust will explore the possibilities offered by nonlinear interactions induced by these plasmonic local field enhancements, by integrating these plasmonic devices together with high-intensity terahertz incident fields. Broadly speaking, this work will change the prevailing view of terahertz science, which is generally thought to be confined to the low- field or perturbative limit. By extending the recent breakthrough work on terahertz high-field generation, this project will establish a new discipline of extreme terahertz light-matter interactions.