Ultrafast sublattice pseudospin relaxation in graphene probed by polarization-resolved photoluminescence

Danz, Thomas; Neff, Andreas; Gaida, John H.; Bormann, Reiner; Ropers, Claus; Schäfer, Sascha

doi:10.1103/physrevb.95.241412

Cited by 12 publications

(13 citation statements)

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“…The time frame for the decay of the pump-induced momentum anisotropy as implied by our data matches well findings for graphene: Results of calculations show that for pump fluences in the 1 mJ/cm 2 range an isotropic momentum distribution is formed well within the first 50 fs after excitation [35]. Polarizationdependent photoluminescence measurements performed at excitation fluences of ≈ 100 µJ/cm 2 hint to a charac- teristic timescale for this process of ≈ 12 fs [36].…”

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confidence: 84%

Ultrafast Formation of a Fermi-Dirac Distributed Electron Gas

et al. 2018

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Time-and angle-resolved photoelectron spectroscopy with 13 fs temporal resolution is used to follow the different stages in the formation of a Fermi-Dirac distributed electron gas in graphite after absorption of an intense 7 fs laser pulse. Within the first 50 fs after excitation a sequence of time frames is resolved which are characterized by different energy and momentum exchange processes among the involved photonic, electronic, and phononic degrees of freedom. The results reveal experimentally the complexity of the transition from a nascent non-thermal towards a thermal electron distribution due to the different timescales associated with the involved interaction processes. 63.20.kd, 81.05.ue, 81.05.uf The extraordinary nonlinearities and optical response times of graphitic materials suggest useful applications in photonics and electronics including light harvesting [1, 2], ultrafast photodetection [3,4], THz lasing [5,6], and saturable absorption [7,8]. Both characteristics are closely linked to the ultrafast dynamics of photoexcited carriers which for this material class is governed by weakly screened carrier-carrier scattering and carrier-phonon interaction. Fundamental aspects related to these processes were addressed in different time-domain studies in the past [9][10][11][12][13]. Because of limitations in the time resolution, most of these studies were restricted, however, to the characteristic timescales of electron-lattice equilibration, i.e., timescales ranging from ≈100 fs to ≈10 ps. The primary processes directly after photoexcitation are, in contrast, still largely unexplored and were investigated experimentally only in a few studies so far [14][15][16]. The dynamics in this strongly non-thermal regime is determined by phenomena such as transient population inversion, carrier multiplication, Auger recombination, but also phonon-mediated carrier redistribution [17][18][19][20]. The challenge is to decode the relative importance and temporal sequence of these processes that drive the electronic system from a nascent non-thermal distribution as generated by photoexcitation towards a Fermi-Dirac (FD) distribution within only ≈ 50 fs [14,15]. It is obvious that such investigations rely on experiments capable of sampling this time window at an adequate time resolution of the order of 10 fs, as well as high energy and momentum resolution. This letter reports on the non-thermal carrier dynamics in highly-oriented pyrolytic graphite (HOPG) as probed in a time-and angle-resolved photoemission spectroscopy (trARPES) experiment that is operated near the transform limit at a resolution of 13 fs (FWHM of the pumpprobe cross correlation) [21]. Over the first 100 fs, we monitor the different stages in the temporal evolution of an initially non-thermal carrier distribution generated by the absorption of a 7 fs near-infrared pulse. We are able to dissect the non-thermal to thermal transition into FIG. 1. (a) WL-pump/XUV-probe cross correlation signal of the experiment. For details see Refs. [21] and [24]. (b) ...

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confidence: 84%

Ultrafast Formation of a Fermi-Dirac Distributed Electron Gas

et al. 2018

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“…An anisotropic population of photogenerated carriers was predicted around the K point as a manifestation of the sublattice pseudospin [9,13]. The anisotropy and ultrafast carrier relaxation was measured in polarization-resolved pump-probe experiments, which showed good agreement with the microscopic calculations [14][15][16][17][18][19]. Furthermore, it might explain a dependence of the photocurrent in graphene-based photodetectors on the light polarization [20].…”

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confidence: 59%

Microscopic theory of optical absorption in graphene enhanced by lattices of plasmonic nanoparticles

Mueller

Reich

2018

Phys. Rev. B

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We present a microscopic description of plasmon-enhanced optical absorption in graphene, which is based on perturbation theory. We consider the interaction of graphene with a lattice of plasmonic nanoparticles, as was previously realized experimentally. By using tight-binding wave functions for the electronic states of graphene and the dipole approximation for the plasmon, we obtain analytic expressions for the coupling matrix element and enhanced optical absorption. The plasmonic nanostructure induces nonvertical optical transitions in the band structure of graphene with selection rules for the momentum transfer that depend on the periodicity of the plasmonic lattice. The plasmon-mediated optical absorption leads to an anisotropic carrier population around the K point in phase space, which depends on the polarization pattern of the plasmonic near field in the graphene plane. Using Fourier optics, we draw a connection to a macroscopic approach, which is independent from graphene-specific parameters. Each Fourier component of the plasmonic near field corresponds to the momentum transfer of an optical transition. Both approaches lead to the same expression for the integrated optical absorption enhancement, which is relevant for the photocurrent enhancement in graphene-based optoelectronic devices.

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“…x . Unlike studies with linearly polarized optical pump 37,[44][45][46][47][48] , where momentum distribution of the photoexcited electrons and holes is highly anisotropic (∼ sin 2 ϕ) from the very beginning in spite of the zero total momentum, in our case the anisotropy arises as the electron Fermi sphere is displaced from zero momentum by the strong THz field. In both cases the distribution functions acquire nonzero second angular harmonics (∼ cos 2ϕ) that is necessary for the anisotropy of the optical response.…”

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confidence: 61%

“…We demonstrate that the anisotropic optical response measured with subcycle temporal resolution contains information on the ultrafast dynamics of the electron gas, its heating, isotropization and concomitant nonlinearities. Our work links the areas of nonlinear THz electrodynamics of graphene 3 , ultrafast pseudospin dynamics of Dirac electrons 37,[44][45][46][47][48] , and strong-current graphene physics 30,50 , thereby providing an alternative tool for studying high-field phenomena in graphene in the far-IR and THz range.…”

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confidence: 93%

Anisotropic ultrafast optical response of terahertz pumped graphene

Melnikov

Sokolik

Frolov

et al. 2019

Applied Physics Letters

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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. According to the theory, the anisotropic response is caused by the displacement of the electronic momentum distribution from zero momentum induced by the pump electric field in combination with polarization dependence of the matrix elements of interband optical transitions.Due to the peculiar electronic band structure of graphene 1,2 the field-induced motion of electrons was predicted to be strongly nonlinear in this material 3 . High nonlinearity together with the unique electronic and optical properties make graphene a prospective material for photonic and optoelectronic applications. In the light of this perspective, nonlinear optical phenomena in graphene are actively studied 4-6 . Among them are harmonic generation 7-16 , saturable absorption 17-19 , self-phase modulation 20 , and fourwave mixing 21,22 . In the optical range the frequency of light is higher than the electron-electron scattering rate, so the resulting "coherent" electronic response is determined by the properties of single-electron band structure 12-14 . In the THz range another limiting case is realized -the characteristic time of electron-electron scattering processes is shorter than the period of the light wave. The energy imparted by the electric field to electrons is quickly redistributed heating the electron gas, while the electron-phonon collisions cool and decelerate the gas. The concept of "incoherent" nonlinearity that appears due to the change of electron gas conductivity upon heating 23,24 was employed recently to explain highly effective generation of THz harmonics in graphene 16 .The routine technique used to evaluate the optical nonlinearity of graphene is the spectral analysis of light transmitted through the sample in search of harmonics of the pump frequency radiated by the nonlinear current. In the present work we employ an alternative approach by using an optical probe to detect the transient THz field-induced shift of electron momentum distribution (note that such shift can induce the optical 2-nd harmonic generation, as was recently observed 25 ). In graphene, due to the specific polarization dependence of matrix elements of interband transitions in combination with Pauli blocking, an anisotropy of electronic distribution implies an anisotropy of infrared optical conductivity, which can be measured by detecting depolarization of probe light reflected from the sample. We measure the ultrafast anisotropic optical response of graphene to inten...

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Ultrafast sublattice pseudospin relaxation in graphene probed by polarization-resolved photoluminescence

Cited by 12 publications

References 73 publications

Ultrafast Formation of a Fermi-Dirac Distributed Electron Gas

Ultrafast Formation of a Fermi-Dirac Distributed Electron Gas

Microscopic theory of optical absorption in graphene enhanced by lattices of plasmonic nanoparticles

Anisotropic ultrafast optical response of terahertz pumped graphene

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