title: Population inversion in monolayer and bilayer graphene.
abstract: The recent demonstration of saturable absorption and negative optical conductivity in the Terahertz range in graphene has opened up new opportunities for optoelectronic applications based on this and other low dimensional materials. Recently, population inversion across the Dirac point has been observed directly by time- and angle-resolved photoemission spectroscopy (tr-ARPES), revealing a relaxation time of only ∼130 femtoseconds. This severely limits the applicability of single layer graphene to, for example, Terahertz light amplification. Here we use tr-ARPES to demonstrate long-lived population inversion in bilayer graphene. The effect is attributed to the small band gap found in this compound. We propose a microscopic model for these observations and speculate that an enhancement of both the pump photon energy and the pump fluence may further increase this lifetime.
authors.affiliation: Max Planck Institute for the Structure and Dynamics of Matter, Luruper Chaussee 149 22761, Hamburg, Germany.
authors.forename: Isabella
authors.initials: I
authors.lastname: Gierz
authors.forename: Matteo
authors.initials: M
authors.lastname: Mitrano
authors.forename: Jesse C
authors.initials: JC
authors.lastname: Petersen
authors.forename: Cephise
authors.initials: C
authors.lastname: Cacho
authors.forename: I C Edmond
authors.initials: IC
authors.lastname: Turcu
authors.forename: Emma
authors.initials: E
authors.lastname: Springate
authors.forename: Alexander
authors.initials: A
authors.lastname: Stöhr
authors.forename: Axel
authors.initials: A
authors.lastname: Köhler
authors.forename: Ulrich
authors.initials: U
authors.lastname: Starke
authors.forename: Andrea
authors.initials: A
authors.lastname: Cavalleri