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Markus Raschke1

1, University of Colorado Boulder, Boulder, Colorado, United States

Combining tip-enhanced Raman, photoluminescence, and nonlinear spectroscopy with active atomic force tip interaction, I will discuss novel approaches for the study of photo-physical and photo-chemical processes in molecules and nano-solids [1,2,3]. In recent examples, we nano-image the exciton behavior and its correlation with defects, grain boundaries, and local strain in different transition metal di-chalcogenides (TMDs). Based on exciton-plasmon coupling we achieve a 105-fold enhancement of the photo-luminescence yield [4].

Further, we are able to achieve TEPL spectroscopy of the otherwise forbidden radiative emission from excitonic dark states with the optical antenna tip coupling to its out-of-plane transition dipole moment [5]. We achieve room temperature contrast, not possible in conventional approaches, due to the ultrafast radiative dark exciton to the tip-antenna mode with few-nm tip-sample gap localized mode volume induced Purcell factor of > 2x103. With the atomic-force microscope controlled antenna tip we demonstrate correlative nano- opto-mechanical swit- ching and programmable modulation of the dark exciton emission. This hybrid tip- enhanced nano- spectroscopy and -imaging method allows to probe and control neutral-, multi-, localized-, and dark-excitons and their correlation with lattice and electronic structural heterogeneities in 2D materials and molecular systems.

[1] Park, K. D., Muller, E. A., Kravtsov, V., Sass, P. M., Dreyer, J., Atkin, J. M., and Raschke, M. B. Variable-Temperature Tip-Enhanced Raman Spectroscopy of Single-Molecule Fluctuations and Dynamics. Nano Lett.16, 479-487 (2016),
[2] Kravtsov, V., Ulbricht, R., Atkin, J. M., and Raschke, M. B. Plasmonic nanofocused four-wave mixing for femtosecond near-field imaging. Nat. Nanotechnol. 11, 459-464 (2016).
[3] Kravtsov, V., Berweger, S., Atkin, J. M., and Raschke, M. B. Control of Plasmon Emission and Dynamics at the Transition from Classical to Quantum Coupling. Nano Lett. 14, 5270-5275 (2014).
[4] Park, K.-D. et al., “Hybrid tip-enhanced nanospectroscopy and nanoimaging of monolayer WSe2 with local strain control,” Nano Lett., Vol. 16, No. 4, 2621–2627, 2016.
[5] Park, K. D., T. Jiang, G. Clark, X. Xu, and Raschke, M. B. “Radiative control of dark excitons at room temperature by nano-optical antenna-tip Purcell effect”, Nature Nanotechnol. (in press), 2017.

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