Seminar "Coherence-Correlations-Complexity", Dept. of Theoretical Physics, Wrocław University of Technology
Sala 320a bud. A-1, Politechnika Wrocławska
Picosecond dynamics of quantum dot excitons: Optical control of single dots and phonon control of lasers
dr Daniel Wigger
Institut für Festkörpertheorie, WWU Münster
Rapid technological progress leads to an increasing number of control schemes for nanosystems on and below the picosecond time scale. A mature nanosystem family are self-assembled semiconductor quantum dots (QDs). On the one hand QDs have already evolved into real applications. On the other hand they can still be seen as prototypical few-level systems functioning as test bed for up-and-coming systems, e.g., in the field of localized emitters in van der Waals materials. In my talk I will highlight two control schemes of QD excitons based on optical pulses and coherent phonons wave packets.
In the fi rst part I will focus on the powerful tool of heterodyne four-wave mixing (FWM) micro-spectroscopy to study single QDs. Here, the laser pulses commonly used to excite the QD exciton have durations in the sub-picosecond range. By scanning the delay between these pulses the resulting FWM signal reflects the dynamics of the exciton properties. By studying these signal dynamics it is possible to identify couplings between different states, which leads to characteristic beats [1]. In a recent collaboration with Jacek Kasprzak, who carried out the experiments at Institut Neel in Grenoble, we were able to resolve the optically driven dynamics of the exciton state in the form of Rabi oscillations [2]. This was realized by stretching the laser pulses into the ten picosecond range and investigating the FWM signal during the overlap of the pulses.
In the second part I will consider a QD ensemble that is used as active medium in a laser. In the group of Manfred Bayer at the TU Dortmund it was shown that coherent phonon wave packets can be used to dynamically change the detuning between the QD exciton energies and the cavity mode in a QD laser. This leads to enhancement and to quenching of the output intensity on the picosecond time scale [3]. We used a semiclassical laser theory to model the coupled phonon-exciton-light system and identi ed two basic mechanisms that change the output of the QD laser: (I) A strain induced rapid change ofthe excitons' energies brings highly occupied QDs into resonance with the cavity, which can dynamically contribute to the laser process. We call this shaking of the ensemble.(II) QD ensembles have usually a Gaussian energy distribution, which implies that the laser works most efficiently when the cavity is in resonance with the ensemble maximum.The so called adiabatic shift of the ensemble changes the detuning between cavity mode and ensemble maximum, which results in different output intensities [4]. I will discuss the characteristic features of the two regimes. In particular by comparing different initial detunings between cavity and ensemble we find that the relative contributions of the two mechanisms can be controlled. Finally I will directly compare the simulated laser output to the measurement and show that we achieved a very good agreement [5].
[1] Q. Mermillod et al., Optica 3(4), 377 (2016)
[2] D. Wigger et al., Optica 5(11), 1442 (2018)
[3] C. Bruggemann et al., Nat. Photon. 6, 30 (2012)
[4] D. Wigger et al., New J. Phys. 19, 073001 (2017)
[5] T. Czerniuk et al., Phys. Rev. Lett. 118, 133901 (2017)
