| 1. |
- Brem, Samuel, 1991, et al.
(författare)
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Electrically pumped graphene-based Landau-level laser
- 2018
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Ingår i: Physical Review Materials. - 2475-9953. ; 2:3
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Tidskriftsartikel (refereegranskat)abstract
- Graphene exhibits a nonequidistant Landau quantization with tunable Landau-level (LL) transitions in the technologically desired terahertz spectral range. Here, we present a strategy for an electrically driven terahertz laser based on Landau-quantized graphene as the gain medium. Performing microscopic modeling of the coupled electron, phonon, and photon dynamics in such a laser, we reveal that an inter-LL population inversion can be achieved resulting in the emission of coherent terahertz radiation. The presented paper provides a concrete recipe for the experimental realization of tunable graphene-based terahertz laser systems.
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| 2. |
- Brem, Samuel, 1991, et al.
(författare)
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Microscopic modeling of tunable graphene-based terahertz Landau-level lasers
- 2017
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 96:4
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Tidskriftsartikel (refereegranskat)abstract
- In the presence of strong magnetic fields the electronic band structure of graphene drastically changes. The Dirac cone collapses into discrete nonequidistant Landau levels, which can be externally tuned by changing the magnetic field. In contrast to conventional materials, specific Landau levels are selectively addressable using circularly polarized light. Exploiting these unique properties, we propose the design of a tunable laser operating in the technologically promising terahertz spectral range. To uncover the many-particle physics behind the emission of light, we perform a fully quantum mechanical investigation of the nonequilibrium dynamics of electrons, phonons, and photons in optically pumped Landau-quantized graphene embedded in a high-quality optical cavity. The microscopic insights gained allow us to predict optimal experimental conditions to realize a technologically promising terahertz laser.
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| 3. |
- Funk, H., et al.
(författare)
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Microscopic view on Landau level broadening mechanisms in graphene
- 2015
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 92:20
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Tidskriftsartikel (refereegranskat)abstract
- Placing a two-dimensional sheet of graphene in an external magnetic field the continuous electronic band structure is discretized due to Landau quantization. The resulting optical transitions are subject to a broadening, which can lead to a significant overlap of Landau levels. We investigate the possible microscopic processes that could cause a broadening of the corresponding peaks in the absorption spectrum of Landau-quantized graphene: (i) radiative decay, (ii) Coulomb interaction, (iii) optical phonons, (iv) acoustic phonons, and (v) impurities. Since recent experiments have shown that independent of the magnetic field the resolvable number of Landau levels is constant, we put a special focus on the dependence of the broadening on the external magnetic field B and the Landau level index n. Our calculations reveal the impurities to be the crucial broadening mechanism, where different regimes of well separated and densely spaced Landau levels need to be taken into account. Furthermore, carrier-carrier and carrier-phonon scattering give rise to a very specific dependence on the Landau level index n that has not been observed yet.
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| 4. |
- 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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| 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. |
- 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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| 8. |
- Malic, Ermin, 1980, et al.
(författare)
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Dark excitons in transition metal dichalcogenides
- 2018
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Ingår i: Physical Review Materials. - 2475-9953. ; 2:1
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Tidskriftsartikel (refereegranskat)abstract
- Monolayer transition metal dichalcogenides (TMDs) exhibit a remarkably strong Coulomb interaction that manifests in tightly bound excitons. Due to the complex electronic band structure exhibiting several spin-split valleys in the conduction and valence band, dark excitonic states can be formed. They are inaccessibly by light due to the required spin-flip and/or momentum transfer. The relative position of these dark states with respect to the optically accessible bright excitons has a crucial impact on the emission efficiency of these materials and thus on their technological potential. Based on the solution of the Wannier equation, we present the excitonic landscape of the most studied TMD materials including the spectral position of momentum- and spin-forbidden excitonic states. We show that the knowledge of the electronic dispersion does not allow to conclude about the nature of the material's band gap since excitonic effects can give rise to significant changes. Furthermore, we reveal that an exponentially reduced photoluminescence yield does not necessarily reflect a transition from a direct to a nondirect gap material, but can be ascribed in most cases to a change of the relative spectral distance between bright and dark excitonic states.
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| 9. |
- Malic, Ermin, 1980, et al.
(författare)
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Review on carrier multiplication in graphene
- 2016
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Ingår i: Physica Status Solidi (B): Basic Research. - : Wiley. - 1521-3951 .- 0370-1972. ; 253:12, s. 2303-2310
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Tidskriftsartikel (refereegranskat)abstract
- The remarkable gapless and linear band structure of graphene opens up new carrier relaxation channels bridging the valence and the conduction band. These Auger scattering processes change the number of charge carriers and can give rise to a significant multiplication of optically excited carriers in graphene. This is an ultrafast many-particle phenomenon that is of great interest both for fundamental many-particle physics as well as technological applications. Here, we review the research on carrier multiplication in graphene and Landau-quantized graphene including theoretical modeling and experimental demonstration. Illustration of the electronic band structure of graphene including Auger scattering channels that can lead to a carrier multiplication (CM). Figure adapted from Ref. [2].
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| 10. |
- Wendler, Florian, 1982, et al.
(författare)
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Doping-dependent intraband carrier dynamics in Landau-quantized graphene
- 2016
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 93:3
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the intraband carrier dynamics in Landau-quantized graphene after an optical excitation with low-energetic terahertz pulses. Based on a microscopic theory, we calculate time-dependent differential transmission spectra reflecting the Landau-level dynamics. Our calculations reveal a strong dependence on the Fermi energy E-F of the graphene sample as well as on the applied magnetic field B. We find that the pump pulse can lead to both absorption bleaching and absorption enhancement depending on B and the position of E-F with respect to the resonant Landau-level transition. As a result, positive and negative contributions in differential transmission spectra appear, in good agreement with recent pump-probe measurements.
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