Time-resolved transmissivity and reflectivity of exfoliated graphene and thin graphite films on a 295 K SiO(2)/Si substrate are measured at 1300 nm following excitation by 150 fs, 800 nm pump pulses. From the extracted transient optical conductivity we identify a fast recovery time constant which increases from approximately 200 to 300 fs and a longer one which increases from 2.5 to 5 ps as the number of atomic layers increases from 1 to approximately 260. We attribute the temporal recovery to carrier cooling and recombination with the layer dependence related to substrate coupling. Results are compared with related measurements for epitaxial, multilayer graphene.

Optical second harmonic generation ͑SHG͒ of 800 nm, 150 fs fundamental pulses is observed from exfoliated graphene and multilayer graphitic films mounted on an oxidized silicon ͑001͒ substrate. The SHG anisotropy is observed as a sample is rotated about the surface normal. For p-polarized fundamental and SHG light, the isotropic SHG from a graphene layer only slightly interferes with the fourfold symmetric response of the underlying substrate, while other samples show a threefold symmetry characteristic of significant SHG in the multilayer graphitic films. The dominance of the threefold anisotropy is maintained from bilayer graphene to bulk graphite.

We have measured second-harmonic generation ͑SHG͒ from graphene and other graphitic films, from two layers to bulk graphite, at room temperature; all samples are mounted on a 300 nm oxide layer of a Si͑001͒ substrate. With 800 nm, 150 fs fundamental pulses, the anisotropic response was recorded for combinations of p-, sand nd diagonally polarized fundamental and second-harmonic beams as the samples were rotated about their normal. Graphene samples display SHG signatures only slightly different from that of the bare substrate which shows SHG with fourfold rotational symmetry. All other layered systems show threefold symmetry, although the ratio of isotropic to anisotropic response varies with the number of layers. A model based on linear light propagation in layered media with interface dipole and bulk quadrupole SHG sources is presented for the analysis. We show that data from all layered samples can be understood in terms of well-known linear optical properties, the SHG response of the bare substrate and four independent, complex nonlinear dipole susceptibility tensor elements of the graphene/air interface.

No abstract

Optical second harmonic generation (SHG) is used to probe surface strain in 150 and 190-nm thin films of MnAs grown epitaxially on GaAs(001). The p-polarized SHG signal produced by p-polarized 775-nm, 200-fs pulses is theoretically and experimentally shown to be sensitive to the normal component of surface strain from −20 to 70 • C, which includes the ferromagnetic/paramagnetic striped coexistence phase region that exists from ∼10 to 40 • C. We use this dependence to time-resolve the surface strain dynamics in MnAs following pumping with 200-fs pulses of 1.0 or 2.0 mJ cm −2 that raise the surface temperature by tens of degrees. For a film at −20 • C the strain reaches a minimum value in ∼10 ps, indicative of electron-lattice thermalization, before recovering on a 500-ps time scale consistent with a one-dimensional heat diffusion model. For a film at 20 • C the minimum strain is reached only after ∼200 ps and attains a value higher than predicted by the heat diffusion model; recovery, however, still occurs in ∼500 ps. The long strain fall time possibly reflects the influence of latent heat and stripe dynamics in the coexistence phase. The larger calculated drop in surface strain may be due to deficiencies in the one-dimensional heat diffusion model. The nonequilibrium surface strain also may not be determined by the local temperature alone but by the constraints throughout the film/substrate system, which are certainly known to govern the strain and stripe characteristics under equilibrium conditions.