Simulation of spatially resolved carrier dynamics in two-dimensional semiconductors induced by optical excitation
Final Report Abstract
In this project we have addressed the question how to use quantum kinetic approaches to describe the carrier dynamics in semiconductor nanostructures on the picosecond time and nanometer length scale. We have focussed mainly on atomically thin two-dimensional (2D) semiconductors as formed by monolayers of transition metal dichalcogenides (TMDCs). A main result of the work is the better understanding of carrier capture processes in TMDC monolayers with an embedded potential. The capture processes involved different dimensionalities. Due to the 2D natures of the electron dynamics in the TMDC, the geometry allows for more degrees of freedom due to the locality of the capture process. This opens the possibility for a spatial control of the carrier capture. In the project we showed that this can results in the formation of different dynamics of the electronic density, which can be interpreted as states on an electronic Poincare sphere. We modelled the dynamics using a realistic description of a potential formed by a nanobubble in the TMDC monolayer and demonstrated the initialization and control of the states of the electronic density. We have further analysed the effect of the Coulomb interaction on the carrier dynamics. Using a simpler geometry of a quantum wire-quantum dot system, we showed that depending on the carrier density the electrons and holes excited in the wire system travel almost independent of each other or form an excitonic wave packet. Accounting for the strong Coulomb interaction in TMDC monolayers, we have then studied the carrier capture of excitonic densities into a localized potential. For this, we have established a quantum kinetic description for the carrier dynamics and in particular for capture processes. We have demonstrated that the capture is mainly mediated by optical phonons. The exceptionally strong excitonphonon interaction resulted in a strong polaron binding energy, which strongly affects which carriers are captured. To enhance the light-matter interaction it is of high interest to embed a TMDC monolayer in a photonic structure. We have simulated the non-linear optical response of a TMDC monolayer in a nano-photonic cavity looking at the second harmonic generation. Combining finite-different time domain (FDTD) simulations with a microscopic description of the semiconductor material, we showed that by using a photonic structure the second harmonic generation can be strongly enhanced. We have further studied the interplay between the light field and the dynamics of an electronic wave packet in a simpler wirestructure showing that the FDTD simulation is capable of monitoring the dynamics. Another main aspect is the analysis of the phonon influence on optical absorption signals of TMDC monolayers. For this, we have developed a new approach using a time-convolutionless (TCL) approach based on a master equation. In comparison with a full quantum kinetic calculation using a Born approximation, the TCL is numerically much lighter. Though in the TCL approach several approximations are made, we showed that for the case of TMDCs it produces very reasonable results. This can be traced back to the fact, the in the TCL all phonon orders are accounted for approximately. Interestingly, if a Born approximation is performed with too low order, this may result in artifacts and unphysical approaches. We properly described the asymmetry in absorption line shapes and its temperature dependence. Using our TCL approach, we have analysed the optical signals of a TMDC monolayer in a high-Q cavity in the strong coupling regime. In such a system polaritons form. We have shown that the phonons also here lead to pronounced asymmetries. Interestingly, the two polariton branches are affected differently by the phonons. Altogether, we have pushed forwards the available methods for the theoretical description of TMDC monolayer, in particular with regards to the carrier-phonon interaction, the light-matter interaction and the spatio-temporal dynamics. We have gained several important insights in the phenomena found in TMDC monolayers due to the exceptionally strong interaction in these systems like strong polaron shifts or asymmetric line shapes. Our methods and results will allow future work to make better use of these unique 2D semiconductors.
Publications
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Dynamic theory of nanophotonic control of 2D semiconductor nonlinearities. Phys. Rev. B 98, 245307 (2018)
S. Guazzotti, A. Pusch, D. E. Reiter, O. Hess
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Spatial control of carrier capture in two-dimensional materials: Beyond energy selection rules. Phys. Rev. B 98, 195411 (2018)
R. Rosati, F. Lengers, D. E. Reiter, T. Kuhn
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Effective detection of spatio-temporal carrier dynamics by carrier capture. J. Phys.: Condens. Matter 31, 28LT01 (2019)
R. Rosati, F. Lengers, D. E. Reiter, T. Kuhn
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Spatiotemporal dynamics of Coulomb-correlated carriers in semiconductors. Phys. Rev. B 99, 155306 (2019)
F. Lengers, R. Rosati, T. Kuhn, D. E. Reiter
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Phonon-mediated exciton capture in Mo-based transition metal dichalcogenides. Phys. Rev. Research 2, 043160 (2020)
F. Lengers, T. Kuhn, D. E. Reiter
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Theory of absorption lineshape in monolayers of transition metal dichalcogenides. Phys. Rev. B, 101, 155304 (2020)
F. Lengers, R. Rosati, T. Kuhn, D. E. Reiter
