By combining the best of both worlds that photons and electrons have to offer, polaritons hold much promise for a variety of applications in optoelectronics and nanophotonics such as miniatiruzed circuits for improved information or energy transfer. Polaritons are hybrid or quasi particles that are made up of photons strongly coupled to an electric dipole. There are different kinds of polaritons such an electron-hole pair that form an exciton polariton, which is present in semiconductors, or electrons at a metal surface that create surface plasmon polaritons (SPPs). Exciton polaritons that are stable at ambient conditions are an active area of research interest. A particular group of semiconductor chalcogenide materials was recently identified to have the existence of polaritons under ambient conditions. However, these materials were previously investigated using far-field methods. These materials are important for their potential applications in information technology, bio-sensing and metamaterials.

In this work, a team of researchers led by Prof. Xu of University of Washington use a Nanonics MV 4000 operating in reflection NSOM to study waveguide polaritons in thin <300nm flakes of WSe2 at ambient conditions. Using this setup, they could directly excite and probe polariton modes by imaging their interference fringes in a method termed “scanning polariton interferometry” at different wavelengths to map out the entire polariton dispersion both above and below the excitation energy. In this study, the polaritons were observed to have a wavelength down to 300nm in WSe2 and propagate many microns below the excitation energy. The near-field illumination allowed for the first time direct excitation and real imaging of the exciton polariton without the need for complicated cavity fabrication. Furthermore, by tuning the excitation laser energy it was possible to map the entire polarity dispersion.


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