Terahertz (THz) waves have shown great application in several fields such as THz imaging, spectroscopy analysis and optical communications. However, it is still a great challenge to develop THz sources with high power density and nanoscale size, which seriously hinders the applications of THz waves in nonlinear optics and nanoscale devices.

Recently, researchers from the Ultrafast Optics Group in WNLO developed a graphene multilayer (multiple graphene monolayers separated by dielectric) taper to enhance the plasmonic nanofocusing of THz waves. Due to interlayer coupling between SPPs, the nanofocusing effect in multilayer graphene taper has been significantly enhanced. For a 3-layer graphene taper, the THz field amplitude can be enhanced by 620 folds at λ=50 µm. The field amplitude enhancement can be further increase to 1800 by optimizing the incident wavelength and graphene chemical potential. In addition, the optical field can be compressed to a scale of 1/2800 of the THz wavelength. This study provides a unique approach to largely enhance and compress the THz field, which will have considerable potential for THz nonlinear optics and nanophotonics.

This research is supported by the 973 Program (No. 2014CB921301), the National Natural Science Foundation of China (No. 11304108), and the Natural Science Foundation of Hubei Province (No. 2015CFA040). (https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-13-14765)

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Fig. (a) Schematic for plasmonic nanofocusing of THz waves by graphene multilayer taper. (b) Field distribution in a 3-layer graphene taper. (c) Field amplitude enhancement at the taper tip as a function of graphene layer number. (d) Mapping of the field amplitude enhancement as functions of incident wavelengths and graphene chemical potential.