Generation and detection of coherent optical phonons in germanium

Pfeifer, T.; Kütt, W.; Kurz, H.; Scholz, Reinhard

doi:10.1103/physrevlett.69.3248

Cited by 104 publications

(78 citation statements)

References 18 publications

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“…Because of the band anisotropy of the electronic structure, the electrons excited from the to the band preferentially weaken the c-axis bond, and cause the observed c-axis dynamics, similar to what is observed in germanium using Raman spectroscopy and pump-probe reflectivity measurements [18,19]. Thus, it is reasonable to expect that in the quasi-2D-structure, depending on the carrier density (fluence) in the layers, the force changes between them and hence the rate.…”

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confidence: 66%

“…However, its position is relatively insensitive to the fluence, contrary to the contraction behavior, suggesting that following expansion the strongly coupled optical phonons have a characteristic time scale ('5-7 ps) for transferring the energy into other modes by anharmonic couplings [18,19]. The most striking feature in Fig.…”

mentioning

confidence: 70%

“…In order to gain more insight into the nature of the expansion, we obtained the derivative [18] the Bragg peak shift (inflection point). However, its position is relatively insensitive to the fluence, contrary to the contraction behavior, suggesting that following expansion the strongly coupled optical phonons have a characteristic time scale ('5-7 ps) for transferring the energy into other modes by anharmonic couplings [18,19].…”

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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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Structural Preablation Dynamics of Graphite Observed by Ultrafast Electron Crystallography

Carbone

Baum

Rudolf³

et al. 2008

Phys. Rev. Lett.

141

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“…While the behavior of the dielectric constant under laser-induced phase transition has been extensively studied 13 , the mechanism of the reflectivity modulation by the small nuclear motion in the vicinity of the equilibrium remains largely unexplored in spite of its extensive application. Among the few experimental studies Kurz and coworkers measured the transient reflectivity response of GaAs and Ge for a few probe polarization angles 6,14,15 . On one hand, they found the reflectivity modulation amplitude due to the coherent LO phonon of undoped GaAs to be probe-polarization-angle dependent, which they attributed to the linear electro-optic (LEO) effect 6 .…”

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Allowed and forbidden Raman scattering mechanisms for detection of coherent LO phonon and plasmon-coupled modes in GaAs

2011

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Detection mechanisms of coherent phonons in variously doped GaAs are investigated by transient reflectivity method with photoexcitation near the E0 gap and probe near either the E0 or E1 gaps. By varying the probe light polarization angle, the coherent amplitudes of both the LO phonon and the LO phonon-plasmon coupled (LOPC) modes show evidence for an interference between their anisotropic and isotropic dielectric response components. We attribute the anisotropic and isotropic components to the reflectivity modulation by lattice and electronic polarizations via dipole-allowed and dipole-forbidden Raman scattering processes. The forbidden processes are resonantly enhanced at the E0 and E1 critical points due to the relaxation of momentum conservation and the strong built-in electric field near the surface. The relative contribution of the anisotropic and isotropic components depends on the modes (LO or LOPC) as well as the probe wavelength (E0 or E1), because of the different Raman scattering mechanisms involved. Reflectivity measurements with the near ultraviolet light, which is used to probe the E1 gap, also enable highly surface sensitive detection of ultrafast LO phonon-plasmon dynamics, which is strongly depth-dependent in the depletion layer of n-doped GaAs.

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“…They have been used to study many phenomena of fundamental and applied interest such as optical orientation of spin, 1 ultrafast demagnetization, 2 and ultrafast excitation of coherent phonons, 3 and magnons. 4 Their potential for materials characterization is illustrated by measurements of the electronphonon coupling constant in high T c superconductors 5 and metals, 6 hot electron linear and angular momentum relaxation times 7 and nonlinear susceptibility tensor components 8 in metals, and the spin wave mode spectrum of nanomagnets.…”

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confidence: 99%

Spectroscopic study of optically induced ultrafast electron dynamics in gold

et al. 2007

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Using a supercontinuum pulse as a probe, we have measured the transient reflectivity spectra of a thin film of gold for different values of the pump-probe time delay. The wavelength x at which the measured transient reflectivity changes sign has been found to depend upon the time delay, leading to bipolar time resolved signals. The time dependence of x has been shown to be consistent with calculations that take into account the full dependence of the reflectivity upon the electron occupation number, and to contradict qualitatively a model in which the signal is assumed to be directly proportional to the occupation number. The shift of x has been found to persist at time delays that are much longer than the time required for the electrons to thermalize. Therefore the bipolar reflectivity signals do not necessarily contain a contribution from nonthermalized electrons, as has been previously assumed.

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Generation and detection of coherent optical phonons in germanium

Cited by 104 publications

References 18 publications

Structural Preablation Dynamics of Graphite Observed by Ultrafast Electron Crystallography

Structural Preablation Dynamics of Graphite Observed by Ultrafast Electron Crystallography

Allowed and forbidden Raman scattering mechanisms for detection of coherent LO phonon and plasmon-coupled modes in GaAs

Spectroscopic study of optically induced ultrafast electron dynamics in gold

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