title: Exciton-exciton annihilation and biexciton stimulated emission in graphene nanoribbons.
abstract: 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 ≈ 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.
authors.affiliation: Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo Da Vinci 32, Milano 20133, Italy.
authors.forename: Giancarlo
authors.initials: G
authors.lastname: Soavi
authors.affiliation: Istituto di Fotonica e Nanotecnologie, CNR, Piazza Leonardo Da Vinci 32, Milano 20133, Italy.
authors.forename: Stefano
authors.initials: S
authors.lastname: Dal Conte
authors.affiliation: Istituto di Fotonica e Nanotecnologie, CNR, Piazza Leonardo Da Vinci 32, Milano 20133, Italy.
authors.forename: Cristian
authors.initials: C
authors.lastname: Manzoni
authors.affiliation: Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo Da Vinci 32, Milano 20133, Italy.
authors.forename: Daniele
authors.initials: D
authors.lastname: Viola
authors.affiliation: Max Planck Institute for Polymer Research, Ackermannweg 10, Mainz 55128, Germany.
authors.forename: Akimitsu
authors.initials: A
authors.lastname: Narita
authors.affiliation: Max Planck Institute for Polymer Research, Ackermannweg 10, Mainz 55128, Germany.
authors.forename: Yunbin
authors.initials: Y
authors.lastname: Hu
authors.affiliation: Max Planck Institute for Polymer Research, Ackermannweg 10, Mainz 55128, Germany.
authors.forename: Xinliang
authors.initials: X
authors.lastname: Feng
authors.affiliation: Institute of Physics, University of Graz, Universitätsplatz 5, Graz 8010, Austria.
authors.forename: Ulrich
authors.initials: U
authors.lastname: Hohenester
authors.affiliation: Dipartimento di Scienze Fisiche, Informatiche e Matematiche, Università di Modena e Reggio Emilia, Modena 41125, Italy.|Istituto Nanoscienze, CNR, via G. Campi 213/a, Modena 41125, Italy.
authors.forename: Elisa
authors.initials: E
authors.lastname: Molinari
authors.affiliation: Istituto Nanoscienze, CNR, via G. Campi 213/a, Modena 41125, Italy.
authors.forename: Deborah
authors.initials: D
authors.lastname: Prezzi
authors.affiliation: Max Planck Institute for Polymer Research, Ackermannweg 10, Mainz 55128, Germany.
authors.forename: Klaus
authors.initials: K
authors.lastname: Müllen
authors.affiliation: Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo Da Vinci 32, Milano 20133, Italy.|Istituto di Fotonica e Nanotecnologie, CNR, Piazza Leonardo Da Vinci 32, Milano 20133, Italy.
authors.forename: Giulio
authors.initials: G
authors.lastname: Cerullo