| 1. |
- 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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| 2. |
- 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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| 3. |
- 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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| 4. |
- Perea Causin, Raul, 1995, et al.
(författare)
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Exciton Propagation and Halo Formation in Two-Dimensional Materials
- 2019
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 19:10, s. 7317-7323
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Tidskriftsartikel (refereegranskat)abstract
- The interplay of optics, dynamics, and transport is crucial for the design of novel optoelectronic devices, such as photodetectors and solar cells. In this context, transition-metal dichalcogenides (TMDs) have received much attention. Here, strongly bound excitons dominate optical excitation, carrier dynamics, and diffusion processes. While the first two have been intensively studied, there is a lack of fundamental understanding of nonequilibrium phenomena associated with exciton transport that is of central importance (e.g., for high-efficiency light harvesting). In this work, we provide microscopic insights into the interplay of exciton propagation and many-particle interactions in TMDs. On the basis of a fully quantum mechanical approach and in excellent agreement with photoluminescence measurements, we show that Auger recombination and emission of hot phonons act as a heating mechanism giving rise to strong spatial gradients in excitonic temperature. The resulting thermal drift leads to an unconventional exciton diffusion characterized by spatial exciton halos.
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| 5. |
- Semnani, Behrooz, 1987, et al.
(författare)
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Anomalous optical saturation of low-energy Dirac states in graphene and its implication for nonlinear optics
- 2019
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Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 6:3
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Tidskriftsartikel (refereegranskat)abstract
- We reveal that optical saturation of the low-energy states takes place in graphene for arbitrarily weak electromagnetic fields. This effect originates from the diverging field-induced interband coupling at the Dirac point. Using semiconductor Bloch equations to model the electronic dynamics of graphene, we argue that the charge carriers undergo ultrafast Rabi oscillations leading to the anomalous saturation effect. The theory is complemented by a many-body study of the carrier relaxations dynamics in graphene. It will be demonstrated that the carrier relaxation dynamics is slow around the Dirac point, which in turn leads to a more pronounced saturation. The implications of this effect for the nonlinear optics of graphene are then discussed. Our analysis shows that the conventional perturbative treatment of the nonlinear optics, i.e. expanding the polarization field in a Taylor series of the electric field, is problematic for graphene, in particular at small Fermi levels and large field amplitudes.
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