Strongly Coupled Optical Phonons in the Ultrafast Dynamics of the Electronic Energy and Current Relaxation in Graphite

Kampfrath, Tobias; Perfetti, L.; Schapper, F.; Frischkorn, Christian; Wolf, Martin

doi:10.1103/physrevlett.95.187403

Cited by 337 publications

(379 citation statements)

References 31 publications

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“…This amount of absorbed energy, however, would imply substantially higher temperatures than observed for either samples due to the small electronic specific heat of graphene (see the Supporting Information). A similar discrepancy between applied pump fluence and actual observed electronic temperature has been reported in a THz spectroscopy study on graphite 35 and is also observed in another recent TR-ARPES study. 21 Several reasons can be envisaged to account for the observed lower values.…”

Section: (C) This Is In Agreement With Our Previous Study On P-dopedsupporting

confidence: 86%

Tunable Carrier Multiplication and Cooling in Graphene

et al. 2014

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Time-and angle-resolved photoemission measurements on two doped graphene samples displaying different doping levels reveal remarkable differences in the ultrafast dynamics of the hot carriers in the Dirac cone. In the more strongly (n-)doped graphene, we observe larger carrier multiplication factors (> 3) and a significantly faster phonon-mediated cooling of the carriers back to equilibrium compared to in the less (p-)doped graphene. These results suggest that a careful tuning of the doping level allows for an effective manipulation of graphene's dynamical response to a photoexcitation.

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Section: (C) This Is In Agreement With Our Previous Study On P-dopedsupporting

confidence: 86%

Tunable Carrier Multiplication and Cooling in Graphene

et al. 2014

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“…Because of electron-electron scattering the initial fs carrier excitation acquires a broadening in the energy distribution [10,20], and this distribution relaxes by electron-phonon coupling, leading to SCOP in the case of graphite. The time scale has been estimated to be on the order of 500 fs, for the electronphonon-coupling process, and 5 to 7 ps for the decay of SCOP into other modes [20,21]. These two characteristic time constants were obtained from time-resolved photoemission and THz spectroscopy experiments [20,21].…”

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confidence: 99%

“…With impulsive laser excitation the electronic bands are populated anisotropically, and, because energy and momentum have to be conserved, these carriers can excite only large momentum phonons. Such selective coupling leads to an efficient excitation of a subset of vibrations [20,21], socalled strongly-coupled-optical-phonons (SCOP), which ultimately decay to yield other vibrations. Because of electron-electron scattering the initial fs carrier excitation acquires a broadening in the energy distribution [10,20], and this distribution relaxes by electron-phonon coupling, leading to SCOP in the case of graphite.…”

mentioning

confidence: 99%

“…Such selective coupling leads to an efficient excitation of a subset of vibrations [20,21], socalled strongly-coupled-optical-phonons (SCOP), which ultimately decay to yield other vibrations. Because of electron-electron scattering the initial fs carrier excitation acquires a broadening in the energy distribution [10,20], and this distribution relaxes by electron-phonon coupling, leading to SCOP in the case of graphite. The time scale has been estimated to be on the order of 500 fs, for the electronphonon-coupling process, and 5 to 7 ps for the decay of SCOP into other modes [20,21].…”

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confidence: 99%

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Structural Preablation Dynamics of Graphite Observed by Ultrafast Electron Crystallography

Carbone

Baum

Rudolf³

et al. 2008

Phys. Rev. Lett.

141

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By means of time-resolved electron crystallography, we report direct observation of the structural dynamics of graphite, providing new insights into the processes involving coherent lattice motions and ultrafast graphene ablation. When graphite is excited by an ultrashort laser pulse, the excited carriers reach their equilibrium in less then one picosecond by transferring heat to a subset of strongly coupled optical phonons. The time-resolved diffraction data show that on such a time scale the crystal undergoes a contraction whose velocity depends on the excitation fluence. The contraction is followed by a large expansion which, at sufficiently high fluence, leads to the ablation of entire graphene layers, as recently predicted theoretically. DOI: 10.1103/PhysRevLett.100.035501 PACS numbers: 61.82.ÿd, 07.78.+s, 63.20.Kÿ, 78.47.ÿp The unique semimetal physical properties of graphite and its chemical inertness make possible numerous applications [1][2][3] including the use in nuclear reactors [4,5]. When subjected to a shock, the structure distorts, and the ablation to form graphene [6] may result from its instability. It is of fundamental importance to visualize the associated dynamics with atomic-scale spatial and temporal resolutions. Here, by means of ultrafast electron crystallography, we report direct observation of the structural dynamics of graphite following an impulsive near infrared excitation. The diffraction shows that the structure of graphite first contracts perpendicular to the layer planes on the time scale of 0.5 to 3 ps, depending on fluence, and then nonthermally expands in 7 ps. The excitation fluence dependence of both processes indicates a distinct behavior. The highly anisotropic transfer of carrier energy to a subset of phonons of the quasi-2D-structure alters the forces between layers which control the time scale and amplitude of structural change. These atomic motions on different time scales for compression, expansion and restructuring determine the degree of the instability, which at sufficiently high fluences reaches that of graphene ablation [7,8], as predicted theoretically [9].Previous optical studies have focused on the carrier dynamics induced by both high [10,11] and low fluence [12] optical excitation, revealing features of the unique phonon excitations in quasi-two-dimensional graphite and the coherent modes involved. In order to resolve details of the structure, diffraction is our method of choice. In ultrafast electron crystallography, the probing wavelength is 0.07 Å at 30 keV electron acceleration energy, and we use femtosecond (fs) near infrared pulses (800 nm) for the optical excitation of the system of graphite. The overall resolution is determined by group velocity mismatch [13], but as shown here, by tilting the optical wave front, see the inset in Fig. 4, we are able to resolve subpicosecond coherent dynamics. Highly oriented and single crystal graphite were studied, and indeed the static diffraction patterns of both display the characteristic structural features...

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“…In time-resolved experiments 23 two relaxation time scales are typically seen. A faster one, ∼ 100f s, usually associated with carrier-carrier intraband collisions and phonon emission, and a slower one, on a ps scale, corresponding to electron interband relaxation and cooling of hot phonons 24,25 .…”

Section: Saturable Absorptionmentioning

confidence: 99%

Graphene photonics and optoelectronics

et al. 2010

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The richness of optical and electronic properties of graphene attracts enormous interest. Graphene has high mobility and optical transparency, in addition to flexibility, robustness and environmental stability. So far, the main focus has been on fundamental physics and electronic devices. However, we believe its true potential to be in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, even in the absence of a bandgap, and the linear dispersion of the Dirac electrons enables ultra-wide-band tunability. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light emitting devices, to touch screens, photodetectors and ultrafast lasers. Here we review the state of the art in this emerging field.

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Strongly Coupled Optical Phonons in the Ultrafast Dynamics of the Electronic Energy and Current Relaxation in Graphite

Cited by 337 publications

References 31 publications

Tunable Carrier Multiplication and Cooling in Graphene

Tunable Carrier Multiplication and Cooling in Graphene

Structural Preablation Dynamics of Graphite Observed by Ultrafast Electron Crystallography

Graphene photonics and optoelectronics

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