Exciton dynamics in transition metal dichalcogenides excited by cylindrically polarised light beams
Nieminen, Arttu (2022)
2022
Teknis-luonnontieteellinen DI-ohjelma - Master's Programme in Science and Engineering
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
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Hyväksymispäivämäärä
2022-05-05
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202204193293
https://urn.fi/URN:NBN:fi:tuni-202204193293
Tiivistelmä
In this thesis, we investigate the exciton dynamics in monolayer transition metal dichalcogenides (TMDs) when excited by cylindrically polarised light. We observe how the complex polarisation structure will excite transitions in the fine structure of exciton states in TMDs, also when an external magnetic field is present. Finally, we discuss how the tightly cylindrically polarised modes themselves could be used to create strong enough magnetic field needed to couple these dark excitons with light.
Our results indicate, that the co-rotating cylindrically polarised modes selectively excite the bright exciton bands, namely radially polarised beam only excites transitions rates into the linear light-like band and azimuthally polarised beam into the parabolic particle-like band, giving the possibility to control the exciton states with the choice of a polarisation mode. In the case when out-of-plane magnetic field is present, the spectrum of the parabolic band is broadened significantly, and, moreover, both co-rotating modes can excite both bands, contrary to the case when there is not an out-of-plane magnetic field.
When the in-plane magnetic field is present, we can excite transitions into the dark exciton states. The results lead to small amount of excitation of dark excitons, about 0.2\% of the bright exciton amount, a similar result to other works done on brightening of dark excitons. The counter-rotating azimuthally polarised field has smaller total transition rate of the dark excitons compared to the co-rotating ones, whereas counter-rotating radially polarised field has a larger transition rate, but these differences are small. When the out-of-plane magnetic field is also taken into account, we disregard, for simplicity, the valley exchange interactions, and only look at the single valley optical response, which leads to all cylindrical polarisations having the same optical transitions, whose magnitude is about the same as when the out-of-plane magnetic field is not present.
Last, we discuss how the tightly focused cylindrically polarised modes can be used in creating the strong magnetic fields, which then could be used to brighten the dark exciton states and cause change in the optical transitions by the out-of-plane field. This, however, might be a challenging task, as the peak power needed for the optical pulse to create the strong enough magnetic fields could damage the TMD sample upon impact. The magnetic fields for these modes are larger than for uniformly polarised beams, though, and using them to brighten the dark excitons would be desirable.
This analysis is purely done in the paraxial approximation of optical fields. Future work would include doing the same analysis for nonparaxial fields, where we would need to redefine the vector potential, to account for the nonparaxiality of the field, and redo the calculations with that potential, which could lead to different results compared to the paraxial case.
Our results indicate, that the co-rotating cylindrically polarised modes selectively excite the bright exciton bands, namely radially polarised beam only excites transitions rates into the linear light-like band and azimuthally polarised beam into the parabolic particle-like band, giving the possibility to control the exciton states with the choice of a polarisation mode. In the case when out-of-plane magnetic field is present, the spectrum of the parabolic band is broadened significantly, and, moreover, both co-rotating modes can excite both bands, contrary to the case when there is not an out-of-plane magnetic field.
When the in-plane magnetic field is present, we can excite transitions into the dark exciton states. The results lead to small amount of excitation of dark excitons, about 0.2\% of the bright exciton amount, a similar result to other works done on brightening of dark excitons. The counter-rotating azimuthally polarised field has smaller total transition rate of the dark excitons compared to the co-rotating ones, whereas counter-rotating radially polarised field has a larger transition rate, but these differences are small. When the out-of-plane magnetic field is also taken into account, we disregard, for simplicity, the valley exchange interactions, and only look at the single valley optical response, which leads to all cylindrical polarisations having the same optical transitions, whose magnitude is about the same as when the out-of-plane magnetic field is not present.
Last, we discuss how the tightly focused cylindrically polarised modes can be used in creating the strong magnetic fields, which then could be used to brighten the dark exciton states and cause change in the optical transitions by the out-of-plane field. This, however, might be a challenging task, as the peak power needed for the optical pulse to create the strong enough magnetic fields could damage the TMD sample upon impact. The magnetic fields for these modes are larger than for uniformly polarised beams, though, and using them to brighten the dark excitons would be desirable.
This analysis is purely done in the paraxial approximation of optical fields. Future work would include doing the same analysis for nonparaxial fields, where we would need to redefine the vector potential, to account for the nonparaxiality of the field, and redo the calculations with that potential, which could lead to different results compared to the paraxial case.