Monolayer graphene provides an ideal material to explore one of the fundamental light-field driven interference effects: Landau-Zener-Stückelberg interference. However, direct observation of the resulting interference patterns in momentum space has not proven possible, with Landau-Zener-Stückelberg interference observed only indirectly through optically induced residual currents. Here we show that the transient electron momentum density (EMD), an object that can easily be obtained in experiment, provides an excellent description of momentum resolved charge excitation. We employ state-of-the-art time-dependent density function theory calculations, demonstrating by direct comparison of EMD with conduction band occupancy, obtained from projecting the time propagated wavefunction onto the ground state, that the two quantities are in excellent agreement. For even the most intense laser pulses we find that the electron dynamics to be almost completely dominated by the $π$-band, with transitions to other bands strongly suppressed. Simple model based tight-binding approaches can thus be expected to provide an excellent description for the laser induced electron dynamics in graphene.
Graphene is an ideal material to study fundamental Coulomb- and phonon-induced carrier scattering processes. Its remarkable gapless and linear band structure opens up new carrier relaxation channels. In particular, Auger scattering bridging the valence and the conduction band changes the number of charge carriers and gives rise to a significant carrier multiplication - an ultrafast many-particle phenomenon that is promising for the design of highly efficient photodetectors. Furthermore, the vanishing density of states at the Dirac point combined with ultrafast phonon-induced intraband scatteri…
Fast carrier cooling is important for high power graphene based devices. Strongly Coupled Optical Phonons (SCOPs) play a major role in the relaxation of photoexcited carriers in graphene. Heterostructures of graphene and hexagonal boron nitride (hBN) have shown exceptional mobility and high saturation current, which makes them ideal for applications, but the effect of the hBN substrate on carrier cooling mechanisms is not understood. We track the cooling of hot photo-excited carriers in graphene-hBN heterostructures using ultrafast pump-probe spectroscopy. We find that the carriers cool down f…
We study coherent nonlinear dynamics of carriers under ultrafast interband excitation of an intrinsic graphene. The Rabi oscillations of response appear with increasing of pumping intensity. The photoexcited distribution is calculated versus time and energy taking into account the effects of energy relaxation and dephasing. Spectral and temporal dependencies of the response on a probe radiation (transmission and reflection coefficients) are considered for different pumping intensities and the Rabi oscillations versus time and intensity are analyzed.
Using ultrafast optical pump-probe spectroscopy, we have measured carrier relaxation times in epitaxial graphene layers grown on SiC wafers. We find two distinct time scales associated with the relaxation of nonequilibrium photogenerated carriers. An initial fast relaxation transient in the 70-120 fs range is followed by a slower relaxation process in the 0.4-1.7 ps range. The slower relaxation time is found to be inversely proportional to the degree of crystalline disorder in the graphene layers as measured by Raman spectroscopy. We relate the measured fast and slow time constants to carrier-…
The ultrafast relaxation and recombination dynamics of photogenerated electrons and holes in epitaxial graphene are studied using optical-pump Terahertz-probe spectroscopy. The conductivity in graphene at Terahertz frequencies depends on the carrier concentration as well as the carrier distribution in energy. Time-resolved studies of the conductivity can therefore be used to probe the dynamics associated with carrier intraband relaxation and interband recombination. We report the electron-hole recombination times in epitaxial graphene for the first time. Our results show that carrier cooling o…
We theoretically investigate the ultrafast terahertz(THz) properties of monolayer graphene. The analytical formulations of the photon carrier, electric polarization and optical current are obtained by solving the Bloch-equations in present of the ultrafast THz Gaussian pulse. Graphene shows a large nonlinear and ultrafast optical response at THz frequencies due to the gapless and relativistic Dirac particles with nearly linear energy dispersion. It is found that the photon carrier density, electric polarization and optical current density increase with increasing the frequency of the THz pulse…
The transient evolution of carriers in an intrinsic graphene under ultrafast excitation, which is caused by the collisionless interband transitions, is studied theoretically. The energy relaxation due to the quasielastic acoustic phonon scattering and the interband generation-recombination transitions due to thermal radiation are analyzed. The distributions of carriers are obtained for the limiting cases when carrier-carrier scattering is negligible and when the intercarrier scattering imposes the quasiequilibrium distribution. The transient optical response (differential reflectivity and tran…
We propose an ultrafast all-optical anomalous Hall effect in two-dimensional (2D) semiconductors of hexagonal symmetry such as gapped graphene (GG), transition metal dichalcogenides (TMDCs), and hexagonal boron nitride (h-BN). To induce such an effect, the material is subjected to a sequence of two strong-field single-optical-cycle pulses: a chiral pump pulse followed within a few femtoseconds by a probe pulse linearly polarized in the armchair direction of the 2D lattice. Due to the effect of topological resonance, the first (pump) pulse induces a large chirality (valley polarization) in the …
We perform a femtosecond-THz optical pump-probe spectroscopy to investigate the cooling dynamics of hot carriers in quasi-free standing bilayer epitaxial graphene. We observed longer decay time constants, in the range of 2.6 to 6.4 ps, compared to previous studies on monolayer graphene, which increase nonlinearly with excitation intensity. The increased relaxation times are due to the decoupling of the graphene layer from the SiC substrate after hydrogen intercalation which increases the distance between graphene and substrate. Furthermore, our measurements do not show that the supercollision …
Electronic states in 2D materials can exhibit pseudospin degrees of freedom, which allow for unique carrier-field interaction scenarios. Here, we investigate ultrafast sublattice pseudospin relaxation in graphene by means of polarization-resolved photoluminescence spectroscopy. Comparison with microscopic Boltzmann simulations allows to determine a lifetime of the optically aligned pseudospin distribution of $12\pm 2\,\text{fs}$. This experimental approach extends the toolbox of graphene pseudospintronics, providing novel means to investigate pseudospin dynamics in active devices or under exte…
Understanding the ultrafast carrier dynamics of graphene on a substrate is a fundamental step in the development of graphene based opto-electronic devices. Here, we present ultrafast pump-probe measurements of supported graphene on quartz for a range of pump fluences that enable us to observe both decreased and enhanced probe transmission regimes on a femtosecond timescale. Unexpectedly, at an intermediate pump fluence, an order of magnitude decrease in the relaxation time constant of the differential transmission is observed. By employing a number of different models to interpret our experime…
Graphene is emerging as a viable alternative to conventional optoelectronic, plasmonic, and nanophotonic materials. The interaction of light with carriers creates an out-of-equilibrium distribution, which relaxes on an ultrafast timescale to a hot Fermi-Dirac distribution, that subsequently cools via phonon emission. Here we combine pump-probe spectroscopy, featuring extreme temporal resolution and broad spectral coverage, with a microscopic theory based on the quantum Boltzmann equation, to investigate electron-electron collisions in graphene during the very early stages of relaxation. We ide…
We theoretically study the Dirac fermion dynamics in a graphene monolayer in the presence of an applied ultrafast laser pulse. The pulse has the duration of a few femtoseconds and the amplitude of ~ 0.1 - 0.5 $\mathrm{V/Å}$. The waveform of the pulse is described by Hermit Gaussian polynomials with varying carrier-envelope phase. We show that the ultrafast dynamics of Dirac fermions strongly depends on the carrier-envelope phase and the frequency of the applied pulse. The ultrafast pulse generates an electric current which results in a finite transferred charge. The ultrafast field-driven curr…
We present a joint theory-experiment study on ultrafast photoluminescence from photoexcited graphene. Based on a microscopic theory, we reveal two distinct mechanisms behind the occurring photoluminescence: Besides the well-known incoherent contribution driven by non-equilibrium carrier occupations, we found a coherent part that spectrally shifts with the excitation energy. In our experiments, we demonstrate for the first time the predicted appearance and spectral shift of the coherent photoluminescence.
We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast charge transfer in an epitaxial heterostructure made of monolayer WS$_2$ and graphene. This heterostructure combines the benefits of a direct gap semiconductor with strong spin-orbit coupling and strong light-matter interaction with those of a semimetal hosting massless carriers with extremely high mobility and long spin lifetimes. We find that, after photoexcitation at resonance to the A-exciton in WS$_2$, the photoexcited holes rapidly transfer into the graphene layer while the photoexcited electro…
Van der Waals heterostructures consisting of graphene and transition metal dichalcogenides (TMDCs) have recently shown great promise for high-performance optoelectronic applications. However, an in-depth understanding of the critical processes for device operation, namely interfacial charge transfer (CT) and recombination, has so far remained elusive. Here, we investigate these processes in graphene-WS$_2$ heterostructures, by complementarily probing the ultrafast terahertz photoconductivity in graphene and the transient absorption dynamics in WS$_2$ following photoexcitation. We find that CT …
We propose a novel ultrafast electronic switching device based on dual-graphene electron waveguides, in analogy to the optical dual-channel waveguide device. The design utilizes the principle of coherent quantum mechanical tunneling of Rabi oscillations between the two graphene electron waveguides. Based on a modified coupled mode theory, we construct a theoretical model to analyse the device characteristics, and predict that the swtiching speed is faster than 1 ps. Due to the long mean free path of electrons in graphene at room temperature, the proposed design avoids the limitation of low tem…
The electronic properties of graphene are unique and are attracting increased attention to this novel 2-dimensional system. Its photonic properties are not less impressive. For example, this single atomic layer absorbs through direct interband transitions a considerable fraction of the light (~2.3%) over a very a broad wavelength range. However, while applications in electronics are vigorously being pursued, photonic applications have not attracted as much attention. Here, we report on ultrafast photocurrent response measurements in graphene (single and few-layers) field-effect-transistors (FE…
Terahertz (THz) Faraday rotation measurements were performed to investigate carrier dynamics in p-type CVD graphene. We used static and time-resolved polarization-sensitive THz transmission measurements in a magnetic field to probe free carriers in GaAs, InP and Graphene. Static measurements probe the equilibrium carrier density and momentum scattering rate. Time-resolved (optical pump/THz probe) measurements probe the change in these quantities following photoexcitation. In a typical CVD graphene sample we found that 0.5ps following photoexcitation with 1*10^13 photons/cm2 pulses at 800nm the…
Electron-hole generation and recombination rates for plasmon emission and absorption in Graphene are presented. The recombination times of carriers due to plasmon emission have been found to be in the tens of femtoseconds to hundreds of picoseconds range. The recombination times depend sensitively on the carrier energy, carrier density, temperature, and the plasmon dispersion. Carriers near the Dirac point are found to have much longer lifetimes compared to carriers at higher energies. Plasmons in a Graphene layer on a polar substrate hybridize with the surface optical phonons and this hybridi…
The fast decay of carrier inversion in photoexcited graphene has been attributed to optical phonon emission and Auger recombination. Plasmon emission provides another pathway that, as we show here, drives the carrier relaxation dynamics on ultrafast timescales. In studying the nonequilibrium relaxation dynamics we find that plasmon emission effectively converts inversion into hot carriers, whose energy is then extracted by optical phonon emission. This mechanism not only explains the observed fs-lifetime of inversion but also offers the prospect for atomically thin ultrafast plasmon emitters.
We study theoretically the strong-field absorption of an ultrafast optical pulse by a gapped graphene monolayer. At low field amplitudes, the absorbance in the pristine graphene is equal to the universal value of $2.3$ percent. Although the ultrafast optical absorption for low field amplitudes is independent of the polarization, linear or circular, of the applied optical pulse, for high field amplitudes, the absorption strongly depends on the pulse polarization. For a linearly polarized pulse, the optical absorbance is saturated at the value of $\approx 1.4$ percent for the pulse's amplitude o…
Polarization characteristic of ultrafast carrier dynamics in multi-layer CVD-grown graphene is probed with tilted beams (with respected to the graphene plane). The graphene ultrafast carrier dynamics measurement greatly depends on both polarization (i.e., orientation of linear polarization) and wave vector of probe beam. The differential reflectivity ΔR=R signal of picosecond dynamics could be continuously altered from positive to negative by changing the probe polarization from P to S when the dynamics is probed by a total internal reflected beam. The polarization dependent ΔR=R signal around…
Oscillator-strength sum rule in light-induced transitions is one general form of quantum-mechanical identities. Although this sum rule is well established in equilibrium photo-physics, an experimental corroboration for the validation of the sum rule in a nonequilibrium regime has been a long-standing unexplored question. The simple band structure of graphene is an ideal system for investigating this question due to the linear Dirac-like energy dispersion. Here, we employed both ultrafast terahertz and optical spectroscopy to directly monitor the transient oscillator-strength balancing between …
Exploring low-loss two-dimensional plasmon modes is considered central for achieving light manipulation at the nanoscale and applications in plasmonic science and technology. In this context, pump-probe spectroscopy is a powerful tool for investigating these collective modes and the corresponding energy transfer processes. Here, I present a first-principles study on non-equilibrium Dirac plasmon in graphene, wherein damping channels under ultrafast conditions are still not fully explored. The laser-induced blueshift of plasmon energy is explained in terms of thermal increase of the electron-ho…
Nonradiative transfer processes are often regarded as loss channels for an optical emitter1, since they are inherently difficult to be experimentally accessed. Recently, it has been shown that emitters, such as fluorophores and nitrogen vacancy centers in diamond, can exhibit a strong nonradiative energy transfer to graphene. So far, the energy of the transferred electronic excitations has been considered to be lost within the electron bath of the graphene. Here, we demonstrate that the trans-ferred excitations can be read-out by detecting corresponding currents with picosecond time resolution…
Ultrafast optical pump-probe spectroscopy measurement on monolayer graphene observes significant optical nonlinearities. We show that strongly photoexcited graphene monolayers with 35 fs pulses quasi-instantaneously build up a broadband, inverted Dirac fermion population. Optical gain emerges and directly manifests itself via a negative conductivity at the near-infrared region for the first 200fs, where stimulated emission completely compensates absorption loss in the graphene layer. To quantitatively investigate this transient, extremely dense photoexcited Dirac-fermion state, we construct a …
Using ultrafast optical pump-probe spectroscopy, we study the relaxation dynamics of hot optical phonons in few-layer and multi-layer graphene films grown by epitaxy on silicon carbide substrates and by chemical vapor deposition on nickel substrates. In the first few hundred femtoseconds after photoexcitation, the hot carriers lose most of their energy to the generation of hot optical phonons which then present the main bottleneck to subsequent carrier cooling. Optical phonon cooling on short time scales is found to be independent of the graphene growth technique, the number of layers, and the…
Ultrafast carrier dynamics of pristine bilayer graphene (BLG) and bilayer graphene intercalated with FeCl3 (FeCl3-G), were studied using time-resolved transient differential reflection (delta R/R). Compared to BLG, the FeCl3-G data showed an opposite sign of delta R/R, a slower rise time, and a single (instead of double) exponential relaxation. We attribute these differences in dynamics to the down-shifting of the Fermi level in FeCl3-G, as well as the formation of numerous horizontal bands arising from the d-orbitals of Fe. Our work shows that intercalation can dramatically change the electro…
We present an ultrafast graphene-based detector, working in the THz range at room temperature. A logarithmic-periodic antenna is coupled to a graphene flake that is produced by exfoliation on SiO2. The detector was characterized with the free-electron laser FELBE for wavelengths from 8 um to 220 um. The detector rise time is 50 ps in the wavelength range from 30 um to 220 um. Autocorrelation measurements exploiting the nonlinear photocurrent response at high intensities reveal an intrinsic response time below 10 ps. This detector has a high potential for characterizing temporal overlaps, e. g.…
Dirac Fermion, which is the low energy collective excitation near the Dirac cone in monolayer graphene, have gained great attention by low energy Terahertz probe. In the case of undoped graphene, it has been generally understood that the ultrafast terahertz thermal relaxation is mostly driven by the electron-phonon coupling (EOP), which can be prolonged to tens and hundreds of picoseconds. However, for the high doped graphene, which manifests the negative photoinduced terahertz conductivity, there is still no consensus on the dominant aspects of the cooling process on a time scale of a few pic…
For most optoelectronic applications of graphene a thorough understanding of the processes that govern energy relaxation of photoexcited carriers is essential. The ultrafast energy relaxation in graphene occurs through two competing pathways: carrier-carrier scattering -- creating an elevated carrier temperature -- and optical phonon emission. At present, it is not clear what determines the dominating relaxation pathway. Here we reach a unifying picture of the ultrafast energy relaxation by investigating the terahertz photoconductivity, while varying the Fermi energy, photon energy, and fluenc…
We theoretically show that buckled two-dimensional graphene-like materials (silicene and germanene) subjected to a femtosecond strong optical pulse can be controlled by the optical field component normal to their plane. In such strong fields, these materials are predicted to exhibit non-reciprocal reflection, optical rectification and generation of electric currents both parallel and normal to the in-plane field direction. Reversibility of the conduction band population is also field- and carrier-envelope phase controllable. There is a net charge transfer along the material plane that is also …
Interband optical transitions in graphene are subject to pseudospin selection rules. Impulsive excitation with linearly polarized light generates an anisotropic photocarrier occupation in momentum space that evolves at timescales shorter than 100fs. Here, we investigate the evolution of non-equilibrium charges towards an isotropic distribution by means of fluence-dependent ultrafast spectroscopy and develop an analytical model able to quantify the isotropization process. In contrast to conventional semiconductors, the isotropization is governed by optical phonon emission, rather than electron-…
Graphene-based laminates, with ultralong and tortuous nanocapillaries formed by simply stacking graphene flakes together, have great promises in filtration and separation. However, the information on liquid water trans-membrane permeation is lacking, which is the most fundamental problem and of crucial importance in solution-based mass transport. Here, based on isotope labelling, we investigate the liquid water transportation through graphene-based nanocapillaries under no external hydrostatic pressures. Liquid water can afford an unimpeded permeation through graphene-based nanochannels with a…
A foundation of the modern technology that uses single-crystal silicon has been the growth of high-quality single-crystal Si ingots with diameters up to 12 inches or larger. For many applications of graphene, large-area high-quality (ideally of single-crystal) material will be enabling. Since the first growth on copper foil a decade ago, inch-sized single-crystal graphene has been achieved. We present here the growth, in 20 minutes, of a graphene film of 5 x 50 cm2 dimension with > 99% ultra-highly oriented grains. This growth was achieved by: (i) synthesis of sub-metre-sized single-crystal Cu…
Using the ultrafast pump-probe transient absorption spectroscopy, the femtosecond-resolved plasmon-exciton interaction of graphene-Ag nanowire hybrids is experimentally investigated, in the VIS-NIR region. The plasmonic lifetime of Ag nanowire is about 150 femtosecond (fs). For a single layer of graphene, the fast dynamic process at 275 fs is due to the excitation of graphene excitons, and the slow process at 1.4 picosecond (ps) is due to the plasmonic hot electron interaction with phonons of graphene. For the graphene-Ag nanowire hybrids, the time scale of the plasmon-induced hot electron tra…
In the model of gapped graphene, we have shown how the recently predicted topological resonances are solely related to the presence of an energy band gap at the $K$ and $K^\prime$ points of the Brillouin zone. In the field of a strong single-oscillation chiral (circularly-polarized) optical pulse, the topological resonance causes the valley-selective population of the conduction band. This population distribution represents a chiral texture in the reciprocal space that is structured with respect to the pulse separatrix as has earlier been predicted for transition metal dichalcogenides. As the …
We investigate waveguide-integrated plasmonic graphene photodetector operating based on the hot carrier photo-bolometric effect, which is characterized simultaneously by high responsivity on the scale of hundreds of AW-1 and high speed on the scale of 100s of GHz. We develop a theory of bolometric effect originating from the band nonparabolicity of graphene and estimate responsivity due to bolometric effect is shown to significantly surpass the responsivity of co-existing photo-conductive effect thus convincingly demonstrating the dominance of bolometric effect. Based on the theory we propose …
We study theoretically the interaction of ultrashort optical pulses with gapped graphene. Such strong pulse results in finite conduction band population and corresponding electric current both during and after the pulse. Since gapped graphene has broken inversion symmetry, it has an axial symmetry about the $y$-axis but not about the $x$-axis. We show that, in this case, if the linear pulse is polarized along the $x$-axis, the rectified electric current is generated in the $y$ direction. At the same time, the conduction band population distribution in the reciprocal space is symmetric about th…
The unusual electrical and optical properties of graphene make it a promising candidate for optoelectronic applications. An important, but as yet unexplored aspect is the role of photo-excited hot carriers in charge and energy transport at graphene interfaces. Here, we perform time-resolved (~250 fs) scanning photocurrent microscopy on a tunable graphene pn junction. The ultrafast pump-probe measurements yield a photocurrent response time of ~1.5 ps at room temperature increasing to ~4 ps at 20 K. Combined with the negligible dependence of photocurrent amplitude on environmental temperature th…
We have measured the ultrafast anisotropic optical response of highly doped graphene to an intense single cycle terahertz pulse. The time profile of the terahertz-induced anisotropy signal at 800 nm has minima and maxima repeating those of the pump terahertz electric field modulus. It grows with increasing carrier density and demonstrates a specific nonlinear dependence on the electric field strength. To describe the signal, we have developed a theoretical model that is based on the energy and momentum balance equations and takes into account optical phonons of graphene and substrate. Accordin…
Ultrafast fiber lasers with short pulses and broad bandwidth are in great demand for a variety of applications, such as spectroscopy, biomedical diagnosis and optical communications. In particular sub-200fs pulses are required for ultrafast spectroscopy with high temporal resolution. Graphene is an ideal ultra-wide-band saturable absorber. We report the generation of 174fs pulses from a graphene-based fiber laser
Since graphene has no band gap, photoluminescence is not expected from relaxed charge carriers. We have, however, observed significant light emission from graphene under excitation by ultrashort (30-fs) laser pulses. Light emission was found to occur across the visible spectral range (1.7 - 3.5 eV), with emitted photon energies exceeding that of the excitation laser (1.5 eV). The emission exhibits a nonlinear dependence on the laser fluence. In two-pulse correlation measurements of the time-domain response, a dominant relaxation time of tens of femtoseconds is observed. A two-temperature model…
Bilayer graphene is a highly promising material for electronic and optoelectronic applications since it is supporting massive Dirac fermions with a tuneable band gap. However, no consistent picture of the gap's effect on the optical and transport behavior has emerged so far, and it has been proposed that the insulating nature of the gap could be compromised by unavoidable structural defects, by topological in-gap states, or that the electronic structure could be altogether changed by many-body effects. Here we directly follow the excited carriers in bilayer graphene on a femtosecond time scale…
We use a quantum-dynamical model to investigate the optical response of graphene under low excitation power. Ultrafast carrier relaxation processes, which play an important role for understanding the optical response of graphene, are included phenomenologically into the model. We obtain analytical solutions for the linear and third-order nonlinear optical response of graphene, and four-wave mixing in particular. This theory shows agreement with recently reported experimental data on linear complex optical conductivity and four-wave mixing, providing evidence for ultrafast quantum-dephasing tim…
Layers of twisted bilayer graphene exhibit varieties of exotic quantum phenomena1-5. Today, the twist angle Θ has become an important degree of freedom for exploring novel states of matters, i.e. two-dimensional superconductivity ( Θ = 1.1°)6, 7 and a two-dimensional quasicrystal (Θ = 30°)8, 9. We report herein experimental observation on the photo-induced ultrafast dynamics of Dirac fermions in the quasicrystalline 30° twisted bilayer graphene (QCTBG). We discover that hot carriers are asymmetrically distributed between the two graphene layers, followed by the opposing femtosecond relaxations…
Pauli blockade occurs when the excited electrons fill up the states near the conduction bands and block subsequent absorption in semiconductors, and has been widely applied in mode-locking for passively-pulsed-laser systems. In this letter, we report the first direct observation that the Pauli blockade is broken by ultrafast cooling of hot electrons in optically-pumped graphene. With femtosecond spectroscopy, we demonstrate that the time scale to excite an electron (~100 fs) is of the same order as that of the electron decay via electron-electron scattering, which allows the electron excitatio…
For future on-chip communication schemes, it is essential to integrate nanoscale materials with an ultrafast optoelectronic functionality into high-frequency circuits. The atomically thin graphene has been widely demonstrated to be suitable for photovoltaic and optoelectronic devices because of its broadband optical absorption and its high electron mobility. Moreover, the ultrafast relaxation of photogenerated charge carriers has been verified in graphene. Here, we show that dual-gated graphene junctions can be functional parts of THz-circuits. As the underlying optoelectronic process, we expl…
Hot electron effects in graphene are significant because of graphene's small electronic heat capacity and weak electron-phonon coupling, yet the dynamics and cooling mechanisms of hot electrons in graphene are not completely understood. We describe a novel photocurrent spectroscopy method that uses the mixing of continuous-wave lasers in a graphene photothermal detector to measure the frequency dependence and nonlinearity of hot-electron cooling in graphene as a function of the carrier concentration and temperature. The method offers unparalleled sensitivity to the nonlinearity, and probes the…
The equilibrium optical phonons of graphene are well characterized in terms of anharmonicity and electron-phonon interactions, however their non-equilibrium properties in the presence of hot charge carriers are still not fully explored. Here we study the Raman spectrum of graphene under ultrafast laser excitation with 3ps pulses, which trade off between impulsive stimulation and spectral resolution. We localize energy into hot carriers, generating non-equilibrium temperatures in the ~1700-3100K range, far exceeding that of the phonon bath, while simultaneously detecting the Raman response. The…
