title: Ultrafast zero balance of the oscillator-strength sum rule in graphene.
abstract: Oscillator-strength sum rule in light-induced transitions is one general form of quantum-mechanical identities. Although this sum rule is well established in equilibrium photo-physics, an experimental corroboration for the validation of the sum rule in a nonequilibrium regime has been a long-standing unexplored question. The simple band structure of graphene is an ideal system for investigating this question due to the linear Dirac-like energy dispersion. Here, we employed both ultrafast terahertz and optical spectroscopy to directly monitor the transient oscillator-strength balancing between quasi-free low-energy oscillators and high-energy Fermi-edge ones. Upon photo-excitation of hot Dirac fermions, we observed that the ultrafast depletion of high-energy oscillators precisely complements the increased terahertz absorption oscillators. Our results may provide an experimental priori to understand, for example, the intrinsic free-carrier dynamics to the high-energy photo-excitation, responsible for optoelectronic operation such as graphene-based phototransistor or solar-energy harvesting devices.
authors.affiliation: 1] School of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, Korea [2].
authors.forename: Jaeseok
authors.initials: J
authors.lastname: Kim
authors.forename: Seong Chu
authors.initials: SC
authors.lastname: Lim
authors.forename: Seung Jin
authors.initials: SJ
authors.lastname: Chae
authors.forename: Inhee
authors.initials: I
authors.lastname: Maeng
authors.forename: Younghwan
authors.initials: Y
authors.lastname: Choi
authors.forename: Soonyoung
authors.initials: S
authors.lastname: Cha
authors.forename: Young Hee
authors.initials: YH
authors.lastname: Lee
authors.forename: Hyunyong
authors.initials: H
authors.lastname: Choi
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