Published: March 28, 2011 r2011 American Chemical Society 1540 dx.doi.org/10.1021/nl104303c | Nano Lett. 2011, 11, 1540–1545 LETTER pubs.acs.org/NanoLett Broadband Light-Induced Absorbance Change in Multilayer Graphene Petr A. Obraztsov,* ,†,‡ Maxim G. Rybin, ‡ Anastasia V. Tyurnina, § Sergey V. Garnov, ‡ Elena D. Obraztsova, ‡ Alexander N. Obraztsov, †,§ and Yuri P. Svirko † † Department of Physics and Mathematics, University of Eastern Finland, Joensuu, Finland ‡ A.M. Prokhorov General Physics Institute, RAS, Moscow, Russia § Physics Department, M.V. Lomonosov Moscow State University, Moscow, Russia O utstanding optical and electronic properties of graphene predetermined by its nonparabolic gapless band structure are of great interest for both fundamental research and for a wide range of applications. Because of Dirac-like behavior of massless charge carriers in the one-atom thick honeycomb lattice, 1 graphene constitutes an ideal playground for study of quantum phenomena. The ability of electrons in graphene to cover submicrometer distances at room temperature without scattering together with a small Fermi velocity makes graphene an attractive material for optoelectronics. 2 The efficient photogeneration of electronÀhole pairs in graphene could also lead to highly sensitive photodetectors 3,4 and enhanced efficiency photovoltaics. 5 In particular, the small Fermi velocity in graphene suggests that the high-energy carriers should efficiently generate multiple electronÀhole pairs via impact excitation process opening a way to overcome the standard thermodynamic limit of the solar cells efficiency. 6 The subpicosecond relaxation times of photo- excited carriers mediated by carrierÀcarrier and carrier Àphonon coupling makes graphene a promising candidate to replace semi- conductor saturable absorbers in modelocked lasers. 7À9 However, despite the extensive study of the photoexcited carriers dynamics in thin graphite films since 1990 10 and later in graphene, 11À14 the underlying physical mechanisms of the ultrafast nonlinear response in graphene still remain unclear. Temporal evolution of the photoexcited carriers in graphene is usually considered to be a four stage process. 11À14 In the first stage, a femtosecond light pulse creates nonequilibrium ensembles of elec- trons and holes centered at a half of the excitation photon energy (E 0 = pω pump /2) with respect to the Dirac point. During the second stage, lasting for several tens of femtoseconds, the ensembles arrive at quasi-equilibrium via carrierÀcarrier scattering governed by a strong Coulomb interaction. In the third stage that takes place in the time scale of hundreds of femtoseconds, the photoexcited carriers are cooling down by transferring energy to the lattice via optical phonons and electronÀhole recombination, which is accompanied by photon or plasmon 15 emission. The electronÀhole recombination gives rise to a broadband photoluminescence. 16À18 Finally (stage 4), the system arrives at an equilibrium within several picoseconds via interaction of carriers with acoustic phonons. The scenario outlined above resembles that taking place in conventional semiconductors in which carrierÀphonon scattering dominates while the Auger-type processes play no signi ficant role in the relaxation dynamics. How- ever, such a conventional scenario does not necessarily take place in graphene in which the Auger-type processes may prevail especially at high excitation densities. 19 In this Letter, we report a time-resolved broadband pumpÀprobe (probe range, 900À1700 nm; pump range, 1000À1700 nm) study Received: December 9, 2010 Revised: February 14, 2011 ABSTRACT: We report the ultrafast light-induced absorbance change in CVD-grown multilayer graphene. Using femtosecond pumpÀprobe measure- ments in 1100À1800 nm spectral range, we revealed broadband absorbance change when the probe photon energy was higher than that of the pump photon. The observed phenomenon is interpreted in terms of the Auger recombination and impact ionization playing a significant role in the dynamics of photoexcited carriers in graphene. KEYWORDS: Graphene, pumpÀprobe, ultrafast optics, carrier dynamics, Auger scattering, saturable absorption