| 1. |
- Aeschlimann, S., et al.
(författare)
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Ultrafast momentum imaging of pseudospin-flip excitations in graphene
- 2017
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 96:2
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Tidskriftsartikel (refereegranskat)abstract
- The pseudospin of Dirac electrons in graphene manifests itself in a peculiar momentum anisotropy for photoexcited electron-hole pairs. These interband excitations are in fact forbidden along the direction of the light polarization and are maximum perpendicular to it. Here, we use time-and angle-resolved photoemission spectroscopy to investigate the resulting unconventional hot carrier dynamics, sampling carrier distributions as a function of energy, and in-plane momentum. We first show that the rapidly-established quasithermal electron distribution initially exhibits an azimuth-dependent temperature, consistent with relaxation through collinear electron-electron scattering. Azimuthal thermalization is found to occur only at longer time delays, at a rate that depends on the substrate and the static doping level. Further, we observe pronounced differences in the electron and hole dynamics in n-doped samples. By simulating the Coulomb-and phonon-mediated carrier dynamics we are able to disentangle the influence of excitation fluence, screening, and doping, and develop a microscopic picture of the carrier dynamics in photoexcited graphene. Our results clarify new aspects of hot carrier dynamics that are unique to Dirac materials, with relevance for photocontrol experiments and optoelectronic device applications.
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| 2. |
- Jago, Roland, 1990, et al.
(författare)
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Current enhancement due to field-induced dark carrier multiplication in graphene
- 2017
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Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 4:2
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Tidskriftsartikel (refereegranskat)abstract
- We present a microscopic study on current generation in graphene in response to an electric field. While scattering is generally considered to reduce the current, we reveal that in graphene Auger processes give rise to a current enhancement via a phenomenon we denote dark carrier multiplication. Based on a microscopic approach, we show that, if other scattering channels are absent, this prevents the carrier distribution to reach a stationary value. Taking into account scattering with phonons a finite current is restored, however its value exceeds the stationary current without scattering.
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| 3. |
- Jago, Roland, 1990
(författare)
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Electron dynamics in graphene in the presence of an electrical field
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Graphene as atomically thin two-dimensional material exhibits remarkableoptical and electronic properties that suggest its technological applicationin novel optoelectronic devices, such as graphene-based lasers and photodetectors.The linear electronic bandstructure and the vanishing band gap atthe Dirac point open up new relaxation channels, such as Auger scattering.Here, optically excited carriers can eciently bridge the valence and theconduction band, which might result in an increase of the number of chargecarriers (electrons and holes), i.e. by absorbing a single photon one can createmultiple electron-hole pairs through internal scattering. This many-particleprocess is called carrier multiplication (CM) and has a large technologicalpotential. In the presence of an electric eld, carriers become accellerated inthe momentum space depleting the region around the Dirac point and providingoptimal conditions for Auger scattering and CM. To investigate ultrafastphenomena characterizing the carrier dynamics in graphene, we develop amicroscopic approach based on the density matrix formalism and the semiconductorBloch equations, which provides microscopic access to the timeandmomentum resolved carrier dynamics in the presence of an electric eld.The aim of the thesis is to investigate the many-particle processes behind theultrafast electron dynamics in graphene. The focus lies on understanding thedynamics in the presence of an electrical eld and in particular providing amicroscopic foundation for the photoconduction eect, which is crucial forthe application of graphene as an ultrafast photodetector. The highlightof the thesis is the proposal of a very ecient dark carrier multiplicationin the presence of an electrical eld. While scattering processes are generallyconsidered to reduce the eld-induced current, we have revealed thatin graphene Auger processes give rise to a signicant current enhancementvia dark CM. Furthermore, we have investigated the interplay of optical excitation,many-particle scattering and eld-induced acceleration of carriersresulting in asymmetric scattering processes and generation of photocurrents.
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| 4. |
- Jago, Roland, 1990, et al.
(författare)
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Graphene as gain medium for broadband lasers
- 2015
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 92:8
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Tidskriftsartikel (refereegranskat)abstract
- Efficient nonradiative carrier recombination strongly counteracts the appearance of optical gain in graphene. Based on a microscopic and fully quantum-mechanical study of the coupled carrier, phonon, and photon dynamics in graphene, we present a strategy to obtain a long-lived gain: Integrating graphene into a high quality photonic crystal nanocavity and applying a high-dielectric substrate suppresses the nonradiative recombination channels and gives rise to pronounced coherent light emission. This suggests the design of graphene-based laser devices covering a broad spectral range.
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| 5. |
- Jago, Roland, 1990, et al.
(författare)
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Microscopic origin of the bolometric effect in graphene
- 2019
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 99:3
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Tidskriftsartikel (refereegranskat)abstract
- While the thermoelectric and photoconduction effects are crucial in pristine and low-doped graphene, the bolometric effect is known to dominate the photoresponse in biased graphene. Here, we present a detailed microscopic investigation of the photoresponse due to the bolometric effect in graphene. Based on the semiconductor Bloch equations, we investigate the time- and momentum-resolved carrier dynamics in graphene in the presence of a constant electric field under optical excitation. The magnitude of the bolometric effect is determined by the optically induced increase of temperature times the conductivity change. Investigating both factors independently, we reveal that the importance of the bolometric effect in the high-doping regime can be mostly ascribed to the latter showing a parabolic dependence on the doping.
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| 6. |
- Jago, Roland, 1990, et al.
(författare)
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Microscopic understanding of the photoconduction effect in graphene
- 2017
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 96:8, s. Article no 085431 -
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the photoresponse of intrinsic graphene in an in-plane electric field. Toward that end, we employ a microscopic approach that allows us to determine the time-and momentum-resolved charge-carrier distributions as a result of the interplay between the field-induced acceleration of optically excited carriers and Coulomb- and phonon-driven carrier scattering. Calculating the generated photocurrent that is determined by the asymmetry of the carrier distribution, we reveal the microscopic foundation of the photoconduction effect in graphene. In particular, we discuss the possibility of tuning the photocurrent via externally accessible knobs, such as electric field, temperature, and substrate. Furthermore, we study the impact of Auger-induced carrier multiplication on the photocurrent in graphene.
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| 7. |
- Jago, Roland, 1990, et al.
(författare)
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Recombination channels in optically excited graphene
- 2015
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Ingår i: Physica Status Solidi (B): Basic Research. - : Wiley. - 1521-3951 .- 0370-1972. ; 252:11, s. 2456-2460
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Tidskriftsartikel (refereegranskat)abstract
- We present a theoretical study on the efficiency of non-radiative recombination channels in optically excited graphene on a substrate. We find that depending on the strength of the excitation pulse and the dielectric constant of the applied substrate, either Auger or phonon-induced recombination prevails. The favorable conditions for Auger recombination are (i) strong excitation regime providing a large number of scattering partners and (ii) low-dielectric substrates, which only weakly screen the Coulomb interaction. The gained insights are important for achieving a population inversion in graphene that is temporally limited by the presented recombination channels.
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| 8. |
- Jago, Roland, 1990, et al.
(författare)
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Spatio-temporal dynamics in graphene
- 2019
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 11:20, s. 10017-10022
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Tidskriftsartikel (refereegranskat)abstract
- Temporally and spectrally resolved dynamics of optically excited carriers in graphene has been intensively studied theoretically and experimentally, whereas carrier diffusion in space has attracted much less attention. Understanding the spatio-temporal carrier dynamics is of key importance for optoelectronic applications, where carrier transport phenomena play an important role. In this work, we provide a microscopic access to the time-, momentum-, and space-resolved dynamics of carriers in graphene. We determine the diffusion coefficient to be D ≈ 360 cm 2 s -1 and reveal the impact of carrier-phonon and carrier-carrier scattering on the diffusion process. In particular, we show that phonon-induced scattering across the Dirac cone gives rise to back-diffusion counteracting the spatial broadening of the carrier distribution.
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| 9. |
- Jago, Roland, 1990
(författare)
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Spatiotemporal carrier dynamics in graphene
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Graphene as an atomically thin material exhibits remarkable optical and electronic properties that suggest its technological application in novel optoelectronic devices, such as graphene-based photodetectors and lasers. To understand the properties of such devices on a microscopic level, we study the interplay of optical excitation, carrier-carrier, carrier-phonon, and carrierphoton scattering as well as diffusion processes. We apply a microscopic model based on the density matrix formalism with spatiotemporal graphene Bloch equations in its core. This approach provides microscopic access to the temporally, spectrally and spatially resolved carrier dynamics both in the presence and absence of an electric field, allowing the study of manyparticle mechanisms behind photodetection and gain in graphene. The focus of this thesis lies in modelling optics, dynamics and transport phenomena on consistent microscopic footing. We predict the possibility to achieve a stable population inversion in graphene, which is the crucial prerequisite for using graphene as an active material in a nanolaser. Further, we provide microscopic insights into the impact of an electric field on the carrier dynamics revealing the appearance of an efficient dark carrier multiplication that can even enhance the field-induced current. We also provide a microscopic foundation for the photoconduction and the bolometric effect as important mechanisms in a graphene based photodetector. Furthermore, we provide insights into the spatiotemporal dynamics of optically excited carriers, which create density and temperature gradients resulting in a diffusion of carriers. The gained insights can be used to study the thermoelectric effect and dynamics at interfaces of spatial inhomogeneities.
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| 10. |
- Malic, Ermin, 1980, et al.
(författare)
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Carrier Dynamics in Graphene: Ultrafast Many-Particle Phenomena
- 2017
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Ingår i: Annalen der Physik. - : Wiley. - 0003-3804 .- 1521-3889. ; 529:11
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Tidskriftsartikel (refereegranskat)abstract
- 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 scattering results in an accumulation of carriers and a population inversion suggesting the design of graphene-based terahertz lasers. Here, we review our work on the ultrafast carrier dynamics in graphene and Landau-quantized graphene is presented providing a microscopic view on the appearance of carrier multiplication and population inversion.
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