Ultrafast Transient Absorption Microscopy Studies of Carrier Dynamics in Epitaxial Graphene

Huang, Libai; Hartland, Gregory V.; Chu, Li‐Qiang; Luxmi,; Feenstra, R. M.; Lian, Chuanxin; Tahy, Kristof; Xing, Huili Grace

doi:10.1021/nl904106t

Cited by 170 publications

(197 citation statements)

References 32 publications

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“…The selected time traces at different probe energies undergo a biexponential decay, with a first time constant t 1 ' 150-170 fs, and a second longer one t 2 41 ps. As in previous studies, we assign the first decay to the cooling of the hot-electron distribution via interaction with optical phonons 16,17 , and the longer one to relaxation of the thermalized electron and phonon distributions by anharmonic decay of hot phonons 34 . By varying the excitation intensity we observe a linear dependence of the PB peak on pump fluence, while its dynamics is nearly fluenceindependent.…”

Section: Resultsmentioning

confidence: 60%

“…Pump-probe spectroscopy has been extensively employed to investigate relaxation processes in carbon-based materials: a variety of different samples have been studied, including thin graphite 13 , few- [14][15][16] and multi-layer 17 graphene, but very few did experiments on single-layer graphene (SLG) [18][19][20][21] . Furthermore, the temporal resolution reported in earlier literature, either in degenerate (i.e., pump and probe with same photon energy) or two-colour pump-probe, was in most cases Z100 fs [14][15][16][17][18]21 . This prevented the direct observation of the intrinsically fast e-e scattering processes.…”

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confidence: 99%

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Ultrafast collinear scattering and carrier multiplication in graphene

et al. 2013

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Graphene is emerging as a viable alternative to conventional optoelectronic, plasmonic and nanophotonic materials. The interaction of light with charge carriers creates an out-ofequilibrium distribution, which relaxes on an ultrafast timescale to a hot Fermi-Dirac distribution, that subsequently cools emitting phonons. Although the slower relaxation mechanisms have been extensively investigated, the initial stages still pose a challenge. Experimentally, they defy the resolution of most pump-probe setups, due to the extremely fast sub-100 fs carrier dynamics. Theoretically, massless Dirac fermions represent a novel many-body problem, fundamentally different from Schrödinger fermions. Here we combine pump-probe spectroscopy with a microscopic theory to investigate electron-electron interactions during the early stages of relaxation. We identify the mechanisms controlling the ultrafast dynamics, in particular the role of collinear scattering. This gives rise to Auger processes, including charge multiplication, which is key in photovoltage generation and photodetectors.

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Section: Resultsmentioning

confidence: 60%

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confidence: 99%

Ultrafast collinear scattering and carrier multiplication in graphene

et al. 2013

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“…Generally, photoexcited charge-carriers in graphene can very efficiently relax by the emission of optical phonons with a subsequent interband recombination or via plasmon emission on a sub-picosecond timescale 15,16,[30][31][32] . Our detection scheme is only sensitive to coherent electromagnetic radiation with constant phase with respect to each pulsed laser excitation.…”

Section: Discussionmentioning

confidence: 99%

Time-resolved ultrafast photocurrents and terahertz generation in freely suspended graphene

et al. 2012

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Graphene, a two-dimensional layer of carbon atoms, is a promising building block for a wide range of optoelectronic devices owing to its extraordinary electrical and optical properties, including the ability to absorb ~2% of incident light over a broad wavelength range. While the RC-limited bandwidth of graphene-based photodetectors can be estimated to be as large as 640 GHz, conventional electronic measurement techniques lack for analysing photocurrents at such frequencies. Here we report on time-resolved picosecond photocurrents in freely suspended graphene contacted by metal electrodes. At the graphene-metal interface, we demonstrate that built-in electric fields give rise to a photocurrent with a full-width-halfmaximum of ~4 ps and that a photothermoelectric effect generates a current with a decay time of ~130 ps. Furthermore, we show that, in optically pumped graphene, electromagnetic radiation up to 1 THz is generated. our results may prove essential to build graphene-based ultrafast photodetectors, photovoltaic cells and terahertz sources.

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“…In addition, the third order nonlinear optical responses associated with saturable absorption of graphene [42][43][44], few-layer graphite films [10,[43][44][45][46][47][48][49][50][51][52][53][54][55][56], and suspensions of graphene flakes [57] have been measured and timeresolved using a variety of pump-probe configurations. Typically, in these experiments, the interband absorption of the pump pulse produces nonequilibrium electron and hole populations, which subsequently relax by carrier-carrier scattering, phonon emission, diffusion and recombination.…”

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confidence: 99%

Third harmonic generation in graphene and few-layer graphite films

et al. 2013

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We observe optical third harmonic generation from graphene and few-layer graphite flakes produced by exfoliation. The emission scales with the cube of the intensity of the incident near-infrared femtosecond pulses and has a wavelength that is one-third of the incident wavelength, both consistent with third harmonic generation. We extract an effective third-order susceptibility for graphene that is on the order of 10 −16 m 2 /V 2 , which is comparable to that for materials that are resonantly excited, but larger than for materials that are transparent at the fundamental and third harmonic wavelengths. By measuring a set of flakes with different numbers of atomic layers, we find that the emission scales with the square of the number of atomic layers, which suggests that the susceptibility of graphene is independent of layer number, at least for a few layers.Graphene is a monolayer of carbon atoms arranged in a hexagonal two-dimensional lattice. It has a linear dispersion relationship between energy, E, and wavenumber, k, E(k) = ±hv F k, where the Fermi velocity v F ≈ 10 6 m/s [see Fig. 1 -3]. Since the material became readily available less than a decade ago [4], it has been the subject of extensive investigations [e.g., see Refs. 5 and 6 and references therein]. Graphene exhibits a number of unusual and remarkable transport properties that make it attractive for nano-electronic applications [7][8][9], including high mobilities [4,5,9, 10] and nearly-ballistic transport at room temperature [5,6]; however, it is the optical properties of graphene that are of primary interest here.The linear absorbance of graphene is flat and approximately 2.3% across the entire visible spectrum [11,12]. Thus, graphene can be considered to be both highly absorbing and/or highly transmitting, depending upon one's point of view or application. Since a negligible fraction (< 0.1%) is reflected [12], 97.7% of the incident light is transmitted. In this sense the sample is certainly transparent. On the other hand, if one were to assign an effective absorption coefficient, α, to a monolayer of thickness 0.33 nm (even though it may be questionable to use macroscopic parameters, such as α, for such thin samples), it would be very large (α ≈ 7 × 10 5 /cm). As suggested by this large effective absorption coefficient, graphene interacts strongly with light. The strong broadband nature of the interaction of light with graphene is consistent with its linear bandstructure, where interband transitions of roughly equal strength are available throughout the visible. The combination of broadband transparency and the high mobilities mentioned earlier makes graphene a promising candidate for use as a transparent conductor [13][14][15][16] in photovoltaic devices [17,18] and touch screens [14]. The high broadband absorption (i.e., optical conductivity) and high carrier mobility suggest that graphene may find applications in a range of optically-controlled transport devices, such as broadband and ultrafast photodetectors [19][20][21][22][23][24][25] The s...

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Ultrafast Transient Absorption Microscopy Studies of Carrier Dynamics in Epitaxial Graphene

Cited by 170 publications

References 32 publications

Ultrafast collinear scattering and carrier multiplication in graphene

Ultrafast collinear scattering and carrier multiplication in graphene

Time-resolved ultrafast photocurrents and terahertz generation in freely suspended graphene

Third harmonic generation in graphene and few-layer graphite films

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