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Engel, Lena

Lena Engel

MSc.

PhD-Student
Institut für Halbleiteroptik und Funktionelle Grenzflächen
Semiconductor Epitaxy Group

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+49 711 685 65189
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Allmandring 3
70569 Stuttgart
Deutschland
Raum: 0.074

Publikationen

2023
1. Engel, L., Kolatschek, S., Herzog, T., Vollmer, S., Jetter, M., Portalupi, S. L., & Michler, P. (2023). Purcell enhanced single-photon emission from a quantum dot coupled to a truncated Gaussian microcavity. Applied Physics Letters, 122(4), Article 4. https://doi.org/10.1063/5.0128631
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  @article{doi:10.1063/5.0128631, abstract = { Purcell enhancement of quantum dot (QD) single-photon emission and increased device brightness have been demonstrated with various types of microcavities. Here, we present the first realization of a truncated Gaussian-shaped microcavity coupled to a QD. The implementation is based on wet-chemical etching and epitaxial semiconductor overgrowth. The cavity modes and their spatial profiles are experimentally studied and agree well with simulations. The fundamental mode wavelength with Q-factors around 6000 and a small polarization splitting of 29 μeV can be reproducibly controlled via fabrication design, enabling the adaption of the cavity to a specific QD. Finally, transitions of a QD inside a cavity are tuned on and off resonance via temperature tuning. A reduced decay time by a factor above 3 on resonance clearly indicates Purcell enhancement while second-order correlation measurements of g(2)(0) = 0.057 prove that the QDs single-photon characteristic is preserved. }, author = {Engel, Lena and Kolatschek, Sascha and Herzog, Thomas and Vollmer, Sergej and Jetter, Michael and Portalupi, Simone L. and Michler, Peter}, doi = {10.1063/5.0128631}, eprint = {https://doi.org/10.1063/5.0128631}, journal = {Applied Physics Letters}, number = 4, pages = 043503, title = {Purcell enhanced single-photon emission from a quantum dot coupled to a truncated Gaussian microcavity}, url = {/brokenurl# https://doi.org/10.1063/5.0128631 }, volume = 122, year = 2023 }
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  /brokenurl# https://doi.org/10.1063/5.0128631
2. Strobel, T., Weber, J. H., Schmidt, M., Wagner, L., Engel, L., Jetter, M., Wieck, A. D., Portalupi, S. L., Ludwig, A., & Michler, P. (2023). A Unipolar Quantum Dot Diode Structure for Advanced Quantum Light Sources. Nano Letters, 23(14), Article 14. https://doi.org/10.1021/acs.nanolett.3c01658
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  @article{Strobel_2023, author = {Strobel, Tim and Weber, Jonas H. and Schmidt, Marcel and Wagner, Lukas and Engel, Lena and Jetter, Michael and Wieck, Andreas D. and Portalupi, Simone L. and Ludwig, Arne and Michler, Peter}, doi = {10.1021/acs.nanolett.3c01658}, journal = {Nano Letters}, month = {07}, number = 14, pages = {6574--6580}, publisher = {American Chemical Society ({ACS})}, title = {A Unipolar Quantum Dot Diode Structure for Advanced Quantum Light Sources}, url = {https://doi.org/10.1021%2Facs.nanolett.3c01658}, volume = 23, year = 2023 }
  Link
  https://doi.org/10.1021%2Facs.nanolett.3c01658
2020
1. Vural, H., Portalupi, S., & Michler, P. (2020). Perspective of self-assembled InGaAs quantum-dots for multi-source quantum implementations. Applied Physics Letters, 117(3), Article 3. https://doi.org/10.1063/5.0010782
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  @article{noauthororeditor, abstract = {In recent years, semiconductor quantum dots have demonstrated their potential to reach the goal of being an ideal source of single and entangled photon pairs. Exciting reports of near unity entanglement fidelity, close to unity photon indistinguishability, and high collection efficiency in nanophotonic structures have been demonstrated by several distinct groups, showing unequivocally the maturity of this technology. To achieve the required complexity and scalability in realistic quantum photonic implementations, two-photon interference of photons from multi-sources must be reached. While high indistinguishability values have been observed for photons generated from the same source within a relatively short time separation, achieving similar visibility for larger time separation or in multi-source experiments still requires intensive efforts. In fact, the coupling to the particular mesoscopic environment of charge carriers confined in the quantum dot leads to decoherence processes, which limit the quantum interference effects to a short time window. Here, we discuss the progress in studying the dynamics of this decoherence, which crucially depends on the evolution of line broadening in high-quality self-assembled InGaAs quantum dots. Characterization of line broadening mechanisms is the first fundamental step to be able to counteract them. Optimization of the growth and active and passive control of the radiative transitions are crucial for the technological readiness of non-classical light sources based on semiconductor platforms. The non-classical emission properties of self-assembled semiconductor quantum dots (QDs), which render them one of the most appealing platforms for quantum photonics, were already demonstrated at the beginning of the millennium: the emission of triggered single photons1 and the indistinguishability of successively emitted photons.2 These two properties provide the basis for many quantum optical technologies,3–9 while utilizing resonant π‐ pulse excitation is a promising scheme to push QD emission to its best performance. High-performance deterministic sources10–12 can bring significant advantages to the realization of quantum information tasks in comparison to the utilization of probabilistic emitters.13 To shed light on the limitations these emitters still face, it is instructive to have a look at the milestone works on the path to ever better non-classical interference visibilities of QD photons. The first observation of Hong–Ou–Mandel (HOM) two-photon interference14 of successively emitted QD photons was reached under the resonant excitation of higher QD states2 and demonstrated a visibility of 0.8. However, this high degree of indistinguishability could not be justified by the measured coherence time of the photons by means of Fourier spectroscopy (FS). This suggested that emission line broadening mainly occurs at a much slower rate than the radiative decay. While the pure dephasing on the photons' wavepacket15 tends to vanish, a diffusion of emission frequency is present, which is below the spectral resolution of standard spectrometers, and its temporal dynamics cannot be resolved by standard FS.16,17 A continuous improvement in the non-classical properties of semiconductor quantum dots and their efficient integration in nanophotonic devices were achieved over the years.10,11,18 Following the first demonstration of non-classical interference for successively emitted QD photons, it took a decade to observe close to unity HOM interference visibility. This was reached by the combination of a high-quality sample and a proper pumping scheme: resonant excitation into the s-shell under the π‐ pulse.19 However, the emission spectrum still showed line broadening with a strong dependence on the excitation power in the investigated range of up to the π‐ pulse. In contrast, negligible reduction of HOM visibility with the pulse power implied that the fluctuation amplitude of the slower line broadening mechanism is indeed independent of the intrinsic photonic coherence. The next significant step was the demonstration of indistinguishable photons from a resonantly pumped QD embedded in a micropillar structure.20,21 In these devices, near unity visibility was shown not only for a time separation between successively emitted photons of a few nanoseconds but also for photons hundreds of nanoseconds22 or up to 15 μs (Ref. 23) apart. The additional high brightness of such a source allowed very recently demultiplexed multi-photon boson sampling, approaching the performance of classical computers.9 Further improvements in quantum optical applications and their scalability crucially depend on the understanding and control of the broadening processes, which limit HOM visibility and thus the quantum interference and fidelity of many applications. For a single emitter, the dynamics of these decoherence processes translates in the time-wise reduction of HOM visibility, which limits the achievable photon number in multi-photon applications. For independent multi-source applications, the full integrated spectra due to the total extent of broadening processes directly determine their quantum interference. Despite its determining character and crucial importance for the future technological readiness, the decoherence dynamics of single-photon emitters translating into the emission spectrum are rarely studied. Further below, after a short overview on the noise sources, we discuss the approaches to study the broadening dynamics and attempts dedicated to their stabilization (Fig. 1). figure FIG. 1. (a)–(c) Illustration of an electron-hole pair confined in a self-assembled quantum dot (QD) under environmental coupling of (a) lattice vibrations, (b) nuclear spins constituting an effective magnetic field Beff referred to as the Overhauser-field, and (c) local charges. Their fluctuations induce dephasing and emission line broadening over the indicated timescales. Δνi/ΔνFL is the ratio between common environment-induced broadening and the Fourier-limited homogeneous linewidth. (d)–(f) Illustration of active and passive counteracting approaches including (d) a quantum dot in a microcavity at cryogenic temperature, (e) an external magnetic field Bext and self-locking effects through dynamic nuclear spin polarization (DNSP), and (f) an external electric field Eext and active feedback control.}, author = {Vural, Hüseyin and Portalupi, Simone and Michler, Peter}, doi = {https://doi.org/10.1063/5.0010782}, journal = {Applied Physics Letters}, language = {engl}, number = 3, title = {Perspective of self-assembled InGaAs quantum-dots for multi-source quantum implementations}, url = {https://aip.scitation.org/doi/full/10.1063/5.0010782?journalCode=apl}, volume = 117, year = 2020 }
  Link
  https://aip.scitation.org/doi/full/10.1063/5.0010782?journalCode=apl
2018
1. Sartison, M., Engel, L., Kolatschek, S., Olbrich, F., Nawrath, C., Hepp, S., Jetter, M., Michler, P., & Portalupi, S. L. (2018). Deterministic integration and optical characterization of telecom O-band quantum dots embedded into wet-chemically etched Gaussian-shaped microlenses. Appl. Phys. Lett., 113(3), Article 3. https://doi.org/10.1063/1.5038271
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  @article{sartison2018deterministic, abstract = {In the present study, we report on the deterministic integration of quantum dots, emitting in the telecom O-band, into wet-chemically fabricated Gaussian-shaped microlenses which exhibit a surface quality comparable to epi-ready wafers. The slow wet-chemical etching rate enables us to gain control of the lens aspect-ratio and the vertical position with respect to the quantum dot, allowing us to engineer the far field shape to better match the acceptance profile of single-mode fibers. Maximum light enhancement values of around 10 to 16 could be achieved for collection numerical apertures from 0.6 to 0.2, respectively. The current results constitute an important step forward in transferring state-of-the-art performances achieved in the near-infrared regime to the key wavelengths for long distance fiber communication.}, author = {Sartison, Marc and Engel, Lena and Kolatschek, Sascha and Olbrich, Fabian and Nawrath, Cornelius and Hepp, Stefan and Jetter, Michael and Michler, Peter and Portalupi, Simone Luca}, booktitle = {Applied Physics Letters}, comment = {doi: 10.1063/1.5038271}, doi = {10.1063/1.5038271}, issn = {00036951}, journal = {Appl. Phys. Lett.}, month = {07}, number = 3, pages = {032103--}, privnote = {doi: 10.1063/1.5038271}, publisher = {American Institute of Physics}, title = {Deterministic integration and optical characterization of telecom O-band quantum dots embedded into wet-chemically etched Gaussian-shaped microlenses}, url = {https://doi.org/10.1063/1.5038271}, volume = 113, year = 2018 }
  Link
  https://doi.org/10.1063/1.5038271

Lena Engel

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Lena Engel

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2023

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