Terahertz Plasmonics for Linear and Nonlinear Spectroscopy and Sensing

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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.