Graphene nanoribbons display extraordinary optical properties due to one-dimensional quantum-confinement, such as width-dependent bandgap and strong electron-hole interactions, responsible for the formation of excitons with extremely high binding energies. Here we use femtosecond transient absorption spectroscopy to explore the ultrafast optical properties of ultranarrow, structurally well-defined graphene nanoribbons as a function of the excitation fluence, and the impact of enhanced Coulomb interaction on their excited states dynamics. We show that in the high-excitation regime biexcitons are formed by nonlinear exciton-exciton annihilation, and that they radiatively recombine via stimulated emission. We obtain a biexciton binding energy of â ‰250 meV, in very good agreement with theoretical results from quantum Monte Carlo simulations. These observations pave the way for the application of graphene nanoribbons in photonics and optoelectronics.

Exciton-exciton annihilation and biexciton stimulated emission in graphene nanoribbons / Soavi, Giancarlo; Dal Conte, Stefano; Manzoni, Cristian; Viola, Daniele; Narita, Akimitsu; Hu, Yunbin; Feng, Xinliang; Hohenester, Ulrich; Molinari, Elisa; Prezzi, Deborah; Müllen, Klaus; Cerullo, Giulio. - In: NATURE COMMUNICATIONS. - ISSN 2041-1723. - 7(2016), pp. 11010-11010.

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