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...

We report a simple in situ synthesis procedure for Au:Ag nanocomposite polymer (NCP) films using polyvinyl alcohol as the reducing agent. Optical measurements show absorption bands of varying strengths around 530 and 410 nm. The presence of nanoparticles is confirmed from Transmission Electron Microscopy (TEM). Nonlinear optical response is studied using 7 ns laser pulses, for near-resonant and off-resonant excitation wavelengths (532 and 1064 nm, respectively). Samples exhibit saturable as well as induced absorption. These materials have the potential to be used as saturable absorbers and optical limiters.

Saturable and reverse saturable absorptions are well-known phenomena, originating from the imaginary component of the third order nonlinear optical susceptibility. We note that structures with an axially asymmetric nonlinear absorption can be easily realized from saturable and reverse saturable absorption materials arranged in tandem. In this paper, the basic transmission behavior of such a structure is worked out. Detailed numerical simulations demonstrate passive all-optical diode behavior, and the results are verified experimentally. The principle will work for all light polarizations, has no phase-matching restrictions, and can be extended to a large number of available nonlinear media for possible applications.

The authors present the results of optical limiting measurements of ∼10nm wide bismuth nanorods suspended in chloroform. Their Z-scan measurements reveal that optical limiting under 532nm excitation stems from a strong nonlinear scattering (NLS) subsequent to nonlinear absorption (NLA) by suspension. On the other hand, the optical limiting is entirely due to NLA when excited with 1064nm excitation in the nanosecond regime. The occurrence of NLS at one wavelength and absence at another is unusual, especially when compared to the behavior of carbon nanotubes under similar conditions, in which NLS is dominant at both wavelengths.

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