Optically driving the radiative Auger transition

ARTICLE https://doi.org/10.1038/s41467-021-26875-8 OPEN Optically driving the radiative Auger transition 1234567890():,; Clemens Spinnler 1,5, Liang Zhai 1,5, Giang N. Nguyen 1, Julian Ritzmann2, Andreas D. Wieck Arne Ludwig 2, Alisa Javadi1, Doris E. Reiter 3, Paweł Machnikowski 4, Richard J. Warburton Matthias C. Löbl 1,5 ✉ 2, 1 & In a radiative Auger process, optical decay leaves other carriers in excited states, resulting in weak red-shifted satellite peaks in the emission spectrum. The appearance of radiative Auger in the emission directly leads to the question if the process can be inverted: simultaneous photon absorption and electronic demotion. However, excitation of the radiative Auger transition has not been shown, neither on atoms nor on solid-state quantum emitters. Here, we demonstrate the optical driving of the radiative Auger transition, linking few-body Coulomb interactions and quantum optics. We perform our experiments on a trion in a semiconductor quantum dot, where the radiative Auger and the fundamental transition form a Λ-system. On driving both transitions simultaneously, we observe a reduction of the fluorescence signal by up to 70%. Our results suggest the possibility of turning resonance fluorescence on and off using radiative Auger as well as THz spectroscopy with optics close to the visible regime. 1 Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland. 2 Lehrstuhl für Angewandte Festkörperphysik, Ruhr-Universität Bochum, 44780 Bochum, Germany. 3 Institut für Festkörpertheorie, Universität Münster, 48149 Münster, Germany. 4 Department of Theoretical Physics, Wrocław University of Science and Technology, 50-370 Wrocław, Poland. 5These authors contributed equally: Clemens Spinnler, Liang Zhai, Matthias C. Löbl. ✉email: NATURE COMMUNICATIONS | (2021)12:6575 | https://doi.org/10.1038/s41467-021-26875-8 | www.nature.com/naturecommunications 1 ARTICLE N NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-26875-8 on-radiative Auger processes have been observed in both atoms1 and solid-state quantum emitters2,3. They play an important role in determining the efficiency of semiconductor light-emitting diodes and lasers4. In the non-radiative Auger process, one electron reduces its energy by transferring it to a second electron that is promoted to a high-energy state. In the radiative Auger process (shake-up), in contrast, one electron makes an optical decay, creating a photon. Part of the photon energy is transferred to a second electron such that the radiative Auger emission is red-shifted with respect to the main emission line. Both radiative and non-radiative Auger processes arise as a consequence of the Coulomb interactions between electrons in close proximity5–7. Non-radiative Auger is a purely Coulomb scattering process. In contrast, radiative Auger involves both Coulomb scattering and electron-photon interactions. It can either be viewed as a higher-order scattering process or interpreted in terms of Coulomb-induced admixtures of higher singleparticle states to the multi-electron wave function7,8. What appears to be an optical relaxation of one electron in the singleparticle picture involves, in fact, a sudden change of the manyparticle configuration. Radiative Auger emission has been observed over a large spectral range: in the X-ray emission of atoms9; close to visible frequencies on donors in semiconductors10 and quantum emitters11,12; and at infrared frequencies as shake-up lines in twodimensional systems13–17. Furthermore, radiative Auger connects carrier dynamics to the quantum optical properties of the emitted photons11, making it a powerful probe of multi-particle systems. Driving the fundamental transition between the electron ground state and an optically excited state is an important technique in quantum optics18,19. In contrast, driving the radiative Auger transition has not been achieved, neither on atoms nor on solidstate systems. Success here would significantly increase the number of quantum optics techniques that can be employed. We demonstrate driving the radiative Auger transition on an epitaxial GaAs quantum dot embedded in AlGaAs20,21. The quantum dot forms a potential minimum and confines charge carriers, resulting in discrete energy levels like in an atom. Without optical illumination, a single electron is trapped in the conduction band of the quantum dot and occupies the lowest possible shell (the s-shell, jsi). Upon resonant excitation of the fundamental transition, a second electron is promoted from the filled valence band to the conduction band and a negative trion X1− (jt i) is formed. This trion consists of two electrons in the conduction band and one electron-vacancy (hole) in the valence band. Figure 1a shows the possible optical decay paths: in the fundamental transition, one electron decays, removing the valence band hole while the other electron remains in the conduction band ground state jsi; in the radiative Auger process, the remaining electron is left in an excited state jpi. The emitted photon is red-shifted by the energy separation between jpi and jsi5,11. Figure 1b shows a typical emission spectrum from the trion decay. This spectrum is measured on resonantly driving the fundamental transition jsi–jt i at 384.7 THz (1.591 eV) with a narrow-bandwidth laser11. Red-shifted by 3.2 THz (13.2 meV) from the fundamental transition, there is a weak satellite line that arises from the radiative Auger process. Photons at the radiative Auger frequency have insufficient energy to excite the fundamental transition jsi–jt i. Figure 1c shows how the trion state jt i still can be excited with a laser at the Auger transition. The missing energy is provided by the electron, which initially occupies the excited state jpi. However, driving the radiative Auger transition is experimentally challenging for two main reasons: first, there is a fast non-radiative relaxation from the excited single-electron state jpi back to jsi11,22, and the state jpi is not occupied at thermal equilibrium. Second, the dipole 2 moment of the radiative Auger transition is small. Therefore, it is difficult to achieve high Rabi frequencies on driving the transition, plus the radiative Auger emission is very weak and hard to distinguish from the back-reflected laser light. Results We perform a two-laser experiment revealing optical driving of the radiative Auger transition. The fundamental transition jsi–jt i (at ~ 1.591 eV) is driven with one laser (labelled by ω1) while the radiative Auger transition (at ~ 1.578 eV) is simultaneously driven with a second laser (labelled by ω2). This Λ-configuration has the following advantages: First, on driving jsi–jti with ω1, there is a small chance of initializing the system in state jpi via the radiative Auger emission. Additionally, driving the jpi–jt i-transition with ω2, while transferring population to jt i with ω1 also leads to a finite occupation of jpi. Se (...truncated)