Advanced Gallium Nitride based Quantum Devices
Final Report Abstract
The project focused on realization and in-depth understanding of nitride-based nanophotonics structures and devices. The topics include the theoretical description of two-photon processes in GaN quantum dots, the development and fabrication of GaN quantum well and quantum dots and their integration into high-quality nanobeam cavities, and the optical and quantum optical study of the realized devices. The theoretical work considered an inherent feature of wurtzite III-nitride heterostructures, namely a pronounced quantum confined Stark effect due to strong pyro- and piezoelectric fields along the [0001] crystallographic axis, and its effect on the probability of two-photon absorption and two-photon emission. The studies were performed using advanced 8-band 𝑘 ∙ 𝑝-modelling of the involved electronic states. It turned out that the resulting separation of electron and hole ground state wavefunction weakens parity selection rules for optical transitions which fosters two-photon transitions in GaN based quantum dots. Detailed investigations revealed for instance that the associated transition rate increases exponentially with the ground state transition energy. Moreover, when enhancing two-photon transition by light-matter interaction in the regime of cavity quantum electrodynamics singly-induced two-photon lifetimes below 1 µs are predicted for technologically and experimentally accessible parameters. The technological work was mainly performed by the Swiss project partner at EPFL Lausanne. The group of Professor Nicolas Grandjean realized for instance GaN nanobeam cavities with InGaN quantum well acting has high-β nanolasers. The main subject of their activities was the growth of high-quality GaN quantum dots for optical studies on two-photon excitation and emission. This included also the development of site-controlled GaN quantum dots on etched nanohole surfaces. Due to unforeseen delays in the sample growth only part of the samples was available for spectroscopic studies at TU Berlin. In addition to the sample preparation at EPFL the Berlin team developed a piezo-tuning technology and successfully applied it to strain-tuning of InGaAs quantum dots deterministically integrated into monolithic microlenses. It will be interesting to transfer this technology to GaN quantum dots in the future. First tuning tests on bulk GaN revealed the potential of strain tuning also for nitride-based semiconductors. The spectroscopic work at TUB focused on optical and quantum-optical studies of GaN-based nanocavities and single-photon emitters. In the first part of the project nanobeam lasers based on single InGaN quantum wells were studied. These ultra-high β nanolasers are of great interest from a fundamental physics point of view because they promise access to the ultimate limit of thresholdless lasing. Moreover, they operate at room temperature which makes them highly interesting also for applications. In the present project a comprehensive power and temperature dependent study of these lasers revealed interesting physics such as an excitation power dependent transition from 0D gain to 2D gain which makes it virtually impossible to identify the laser transition by excitation dependent studies of the emission intensity and linewidth alone. Instead, only quantum-optical studies on the photon statistics of emission can unambiguously reveal thresholdless lasing in the nanobeam cavities. Further spectroscopic work focused on the experimental study of single-QD two-photon processes in GaN. These studies showed that the experimental observation of QD ensemble emission under degenerate two-photon excitation is possible. However, two photon transitions on the single quantum dot level could not be observed with the available samples in the timeframe of the project. Beyond that, the optical properties of positioned InGaN quantum discs were evaluated. These interesting nanostructures were realized by an external partner (Professor J. Massies at Université Côte d’Azur) using selective area sublimation and show promising emission properties in terms of non-classical light emission, as revealed by second-order autocorrelation measurements.
Publications
- A deterministically fabricated spectrally-tunable quantum dot based single-photon source
Fischbach, S., von Helversen, M., Schmidt, M., Kaganskiy, A., Schmidt, R., Schliwa, A., Heindel, T., Rodt, S., Reitzenstein, S.
- A quantum optical study of thresholdless lasing features in high-β nitride nanobeam cavities. Nat. Commun. 9. 564 (2018)
Jagsch, S. T., Vico Triviño, N., Lohof, F., Callsen, G., Kalinowski, S., Rousseau, I. M., Barzel, R., Carlin, J.-F., Jahnke, F., Butté, R., Gies, C., Hoffmann, A., Grandjean, N., Reitzenstein, S.
(See online at https://doi.org/10.1038/s41467-018-02999-2) - Ground State Resonant Two-Photon Transitions in Wurtzite GaN/AlN Quantum Dots
Jagsch, S. T., Greif, L. A. T., Reitzenstein, S., Schliwa, A.
(See online at https://doi.org/10.1103/PhysRevB.99.245303)
