Leandro M. Malard

Observation of intense second harmonic generation from MoS2atomic crystals

The nonlinear optical properties of few-layer MoS

Observation of intra- and inter-band transitions in the transient optical response of graphene

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Erratum: Symmetry-Dependent Exciton-Phonon Coupling in 2D and Bulk MoS_{2} Observed by Resonance Raman Scattering [Phys. Rev. Lett. 114, 136403 (2015)].

two-dimensional crystals are studied using femtosecond laser pulses. We observed highly efficient second-harmonic generation from the odd-layer crystals, which shows a polarization intensity dependence that directly reveals the underlying symmetry and orientation of the crystal. Additionally, the measured second-order susceptibility spectra provide information about the electronic structure of the material. Our results open up opportunities for studying the nonlinear optical properties in these two-dimensional crystals.

Measuring the electronic properties of single-walled carbon nanotubes with adsorbed porphyrins using optical transitions

The transient optical conductivity of freely suspended graphene was examined by femtosecond time-resolved spectroscopy using pump excitation at 400 nm and probe radiation at 800 nm. The optical conductivity (or, equivalently, absorption) changes abruptly upon excitation and subsequently relaxes to its initial value on the time scale of 1 ps. The form of the induced change in the optical conductivity varies strongly with excitation conditions, exhibiting a crossover from enhanced to decreased optical conductivity with increasing pump fluence. We describe the graphene response in terms of transient heating of the electrons, with the characteristic relaxation time of the transient conductivity reflecting the cooling of the electron system and the strongly coupled optical phonons through emission of lower energy phonons. The change in the optical conductivity is attributed to a combination of induced absorption from intra-band transitions of the photo-generated carriers and bleaching of the inter-band transitions by Pauli blocking. The former effect, which corresponds to the high-frequency wing of the Drude response, dominates at low pump fluence. In this regime of a limited rise in the electron temperature, an increase in the optical conductivity is observed. At high pump fluence, elevated electron temperatures are achieved. The decrease in the inter-band bleaching then dominates the transient response, the intra-band contribution being overwhelmed despite an increase in the Drude scattering rate with temperature. The temporal evolution of the optical conductivity in all the regimes can be described within a model including the intra- and inter-band contributions with a time-varying electronic temperature. An increased Drude scattering rate is inferred for high electron temperature and mechanisms for this enhancement are considered. The calculated scattering rate for interactions of the carriers with zone-center and zone-edge optical phonons agrees well with the rates obtained from experiment.

Defect-Induced Supercollision Cooling of Photoexcited Carriers in Graphene

This corrects the article DOI: 10.1103/PhysRevLett.114.136403.

Anomalous Nonlinear Optical Response of Graphene Near Phonon Resonances

The dielectric constants of diverse media surrounding single-walled carbon nanotubes (SWCNTs) were probed using photoluminescence (PL) excitation maps of porphyrin/SWCNT aqueous suspensions. The excitation and emission maxima of the nanotubes in these maps were used to probe the dielectric constant variation and doping originated from the porphyrin molecules. The net dielectric constant was calculated for the surrounding medium for each nanotube index and porphyrin isomer. The spread of the dielectric constant values calculated from the data for each (n, m) nanotube chiral index is interpreted on the basis of selective adsorption by each (n, m) nanotube, for each porphyrin isomer. Ultraviolet (UV) Raman spectroscopy corroborates the doping process through the shift of a G band around 1608 cm-1

Supercollision cooling effects on the hot photoluminescence emission of graphene.

Defects play a fundamental role in the energy relaxation of hot photoexcited carriers in graphene, thus a complete understanding of these processes are vital for improving the development of graphene devices. Recently, it has been theoretically predicted and experimentally demonstrated that defect-assisted acoustic phonon supercollision, the collision between a carrier and both an acoustic phonon and a defect, is an important energy relaxation process for carriers with excess energy below the optical phonon emission. Here, we studied samples with defects optically generated in a controlled manner to experimentally probe the supercollision model as a function of the defect density. We present pump and probe transient absorption measurements showing that the decay time decreases as the density of defect increases as predicted by the supercollision model.

Observation of Layer-Breathing Mode Vibrations in Few-Layer Graphene through Combination Raman Scattering

In this work we probe the third-order nonlinear optical property of graphene and hexagonal boron nitride and their heterostructure by the use of coherent anti-Stokes Raman spectroscopy. When the energy difference of the two input fields matches the phonon energy, the anti-Stokes emission intensity is enhanced in h-BN, as usually expected, while for graphene an anomalous decrease is observed. This behavior can be understood in terms of a coupling between the electronic continuum and a discrete phonon state. We have also measured a graphene/h-BN heterostructure and demonstrate that the anomalous effect in graphene dominates the heterostructure nonlinear optical response.

Comparative Study of Raman Spectroscopy in Graphene and MoS2-type Transition Metal Dichalcogenides

We report on hot photoluminescence measurements that show the effects of acoustic phonon supercollision processes in the intensity of graphene light emission. We use a simple optical method to induce defects on single layer graphene in a controlled manner to study in detail the light emission dependence on the sample defect density. It is now well accepted that the graphene photoluminescence is due to black-body thermal emission from the quasi-equilibrium electrons at a temperature well above the lattice temperature. Our results show that as the sample defect density is increased the electrons relax energy more efficiently via acoustic phonon supercollision processes leading to lower electron temperatures and thus lower emission intensities. The calculated intensity decrease due to supercollision energy relaxation agrees well with the experimental data.