Femtosecond carrier dynamics and saturable absorption in graphene suspensions

Abstract

Nonlinear optical properties and carrier relaxation dynamics in graphene, suspended in three different solvents, are investigated using femtosecond (80 fs pulses) Z-scan and degenerate pumpprobe spectroscopy at 790 nm. The results demonstrate saturable absorption property of graphene with a nonlinear absorption coefficient, β, of ~2 to 9x10 -8 cm/W. Two distinct time scales associated with the relaxation of photoexcited carriers, a fast one in the range of 130-330 fs (related to carriercarrier scattering) followed by a slower one in 3.5-4.9 ps range (associated with carrier-phonon scattering) are observed.Graphene is a two-dimensional carbon nanomaterial which has received tremendous interest in recent years owing to its various remarkable properties and applications in modern electronics and photonics [1,2]. Ultrafast degenerate and nondegenerate pump-probe measurements on single and multilayer epitaxial [3,4] or exfoliated [5] graphenes deposited on a substrate have shown two types of dynamics of the carriers: a fast component of the order of ~100 fs attributed to the intraband carrier-carrier scattering and a slower component ~2 ps associated with carrierphonon scattering. In the degenerate pump-probe studies on single and multilayer graphene films grown on a SiC substrate using 85 fs laser pulses centered at 790 nm, a positive change in the transient differential transmission of the probe was observed with two relaxation times, the faster one in the range of 70-120 fs and a slower one between 0.4-1.7 ps [3]. Similar carrier relaxation dynamics was obtained in the nondegenerate pump-probe experiments on exfoliated graphene films on SiO 2 /Si substrate [5]. On a few layer thick graphene film on SiC, nondegenerate pump-probe experiments [4] using 800 nm pump showed that the sign of the differential transmission signal is positive over the entire probe spectral range of 1.1 to 2.6 µm but becomes negative after 2 ps if the probe wavelength falls between 1.78 and 2.35 µm. The initial positive part of the signal within 150 fs has been described in terms of thermalization and emission of high energy-phonons followed by a slow decay of the order of a few ps determined by electron-acoustic phonon scattering. Nonlinear optical properties of graphene have been reported recently in the nanosecond (ns) and picosecond (ps) regimes [6,7]. Using 35 ps laser pulses centered at 532 nm, it has been shown that the nonlinear response of graphene oxide suspensions changes from saturable absorption at low intensity (2.1 GW/cm 2 ) to reverse saturable absorption or optical limiting at higher intensities (>4.5 GW/cm 2 ) [6]. In comparison, in the ns regime suspensions of graphene oxide and functionalized graphene in dimethylformamide showed optical limiting property at all values of intensities above 0.6 GW/cm 2 [6,7].We have carried out femtosecond (80 fs) Z-scan and degenerate pump-probe experiments at 790 nm to study the nonlinear optical response and carrier dynamics in colloidal suspensions of graphene which have not been invest...

Monolayer graphene as a saturable absorber in a mode-locked laser

“…The carrier relaxation time comprises a fast time constant (τ 1 ) in the 100-150 fs range, resulting from carrier-carrier intraband scattering processes, and a slower time constant (τ 2 ) in the 405-570 fs range, corresponding to carrier-phonon intraband scattering and electron-hole recombination (Auger scattering). The initial fast relaxation time τ 1 of monolayer graphene is around 100 fs, which agrees with previous reports for graphene [3,[22][23][24]. However, we should note that the faster time τ 1 is of the order of the pulse width and is, therefore, not accurately resolved.…”

Revealing the ultrafast process behind the photoreduction of graphene oxide

“…Although previous ultrafast optical studies [41][42][43][44] have focused on studying the dynamics of graphene, GO and rGO under various conditions (showing the evolution of valence band electrons after their excitation), it is very important for the reader to notice that what we showed here is the dynamics of a non-reversible chemical reaction (GO-rGO) which is a fundamentally different and so far unobserved process. This work demonstrates that the photoinduced transformation of GO to rGO is an indirect process.…”

Cooling of photoexcited carriers in graphene by internal and substrate phonons

“…Upon fast excitation of graphene carriers with light or other means, the dynamics of the resulting non-equilibrium carrier distribution evolve on a fast time scale and has been extensively studied both experimentally [1][2][3][4][5][6][7][8][9][10][11][12][13] and theoretically [14][15][16] . The relaxation involves an initial fast evolution towards quasi-thermal distribution on a femtosecond timescale via electron-electron collisions 14,15,17,18 , followed by energy transfer to phonons on a longer picosecond timescale.…”