Patrick Knüppel

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Knüppel

First Name

Patrick

ORCID

0000-0002-6193-3440

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Publications 1 - 8 of 8

Two-Dimensional Polaron-Polaritons: Interactions and Transport
Knüppel, Patrick (2021)
In this dissertation, we explore the interplay between strongly correlated electrons and optical polaritons confined to two dimensions. This is implemented experimentally with gallium arsenide quantum wells hosting a two-dimensional electron gas (2DEG). The structure is further embedded in an optical microcavity to reach the strong light-matter coupling regime. Optically created excitons in the quantum well act as impurities in the surrounding electron gas, forming collective excitations termed Fermi polarons. The resulting polaron-polariton modes are used to study the quantum Hall effect and allow tailoring photonic properties via control over the electrons. The electronic ground states in the integer and fractional quantum Hall regimes are investigated using polariton spectroscopy. This tool allows probing the spin polarization of the 2DEG by optical means. Many-body spin textures are studied around integer filling, in particular for the case of vanishing g-factor which is expected to favor large skyrmions. Using an optimized device structure, we advance polariton spectroscopy by drastically reducing unwanted modifications of the electron density upon optical illumination. We observe coupling of the polaron-polaritons to different fractional quantum Hall states as the filling factor is varied. In a second part, a polariton Hall bar device is fabricated to investigate the connection between electronic and polariton transport in two dimensions. We demonstrate acceleration of polaritons by shaping the electron density with external electric and magnetic fields. For a spin polarized electron gas, we demonstrate the creation of spin density gradients. They are used to route polaritons on the Hall bar device in a spin selective manner, reminiscent of an optical spin Hall effect. In the last part, we present four-wave mixing experiments performed with polaron-polaritons. The nonlinear optical response of polaritons is measured while they are coupled to different quantum Hall states. A surprising increase in nonlinear signal is found for the specific filling fractions 2/3 and 2/5. These results demonstrate enhanced polariton-polariton interactions both compared to other fillings and to the case of exciton-polaritons without 2DEG. This constitutes a step towards polariton blockade and suggests that nonlinear optics may allow us to extract properties of the correlated electronic states beyond linear spectroscopy.
Spin Reversal of a Quantum Hall Ferromagnet at a Landau Level Crossing
Lupatini, Mirko; Knüppel, Patrick; Faelt, Stefan; et al. (2020)
Physical Review Letters
When Landau levels (LLs) become degenerate near the Fermi energy in the quantum Hall regime, interaction effects can drastically modify the electronic ground state. We study the quantum Hall ferromagnet formed in a two-dimensional hole gas around the LL filling factor ν = 1 in the vicinity of a LL crossing in the heave-hole valence band. Cavity spectroscopy in the strong-coupling regime allows us to optically extract the spin polarization of the two-dimensional hole gas. By analyzing this polarization as a function of hole density and magnetic field, we observe a spin flip of the ferromagnet. Furthermore, the depolarization away from v=1 accelerates close to the LL crossing. This is indicative of an increase in the size of skyrmion excitations as the effective Zeeman energy vanishes at the LL crossing.
Interaction-Induced Shubnikov–de Haas Oscillations in Optical Conductivity of Monolayer MoSe2
Smolenski, Tomasz; Cotlet, Ovidiu; Popert, Alexander; et al. (2019)
Physical Review Letters
We report polarization-resolved resonant reflection spectroscopy of a charge-tunable atomically thin valley semiconductor hosting tightly bound excitons coupled to a dilute system of fully spin- and valley-polarized holes in the presence of a strong magnetic field. We find that exciton-hole interactions manifest themselves in hole-density dependent, Shubnikov–de Haas–like oscillations in the energy and line broadening of the excitonic resonances. These oscillations are evidenced to be precisely correlated with the occupation of Landau levels, thus demonstrating that strong interactions between the excitons and Landau-quantized itinerant carriers enable optical investigation of quantum-Hall physics in transition metal dichalcogenides.
Nonlinear optics in the fractional quantum Hall regime
Knüppel, Patrick; Ravets, Sylvain; Kroner, Martin; et al. (2019)
Nature
Engineering strong interactions between optical photons is a challenge for quantum science. Polaritonics, which is based on the strong coupling of photons to atomic or electronic excitations in an optical resonator, has emerged as a promising approach to address this challenge, paving the way for applications such as photonic gates for quantum information processing and photonic quantum materials for the investigation of strongly correlated driven–dissipative systems. Recent experiments have demonstrated the onset of quantum correlations in exciton-polariton systems, showing that strong polariton blockade—the prevention of resonant injection of additional polaritons in a well delimited region by the presence of a single polariton—could be achieved if interactions were an order of magnitude stronger. Here we report time-resolved four-wave-mixing experiments on a two-dimensional electron system embedded in an optical cavity, demonstrating that polariton–polariton interactions are strongly enhanced when the electrons are initially in the fractional quantum Hall regime. Our experiments indicate that, in addition to strong correlations in the electronic ground state, exciton–electron interactions leading to the formation of polaron-polaritons have a key role in enhancing the nonlinear optical response of the system. Our findings could facilitate the realization of strongly interacting photonic systems, and suggest that nonlinear optical measurements could provide information about fractional quantum Hall states that is not accessible through their linear optical response.
Proposal for a quantum interface between photonic and superconducting qubits
Tsuchimoto, Yuta; Knüppel, Patrick; Delteil, Aymeric; et al. (2017)
Physical Review B
Polaron Polaritons in the Integer and Fractional Quantum Hall Regimes
Ravets, Sylvain; Knüppel, Patrick; Fält, Stefan; et al. (2018)
Physical Review Letters
Spin-selective strong light-matter coupling in a 2D hole gas-microcavity system
Suarez-Forero, Daniel G.; Session, Deric W.; Jalali Mehrabad, Mahmoud; et al. (2023)
Nature Photonics
The interplay between time-reversal symmetry breaking and strong light-matter coupling in two-dimensional (2D) gases brings intriguing aspects to polariton physics. This combination can lead to a polarization/spin-selective light-matter interaction in the strong coupling regime. Here we report such a selective strong light-matter interaction by harnessing a 2D gas in the quantum Hall regime coupled to a microcavity. Specifically, we demonstrate circular-polarization dependence of the vacuum Rabi splitting, as a function of magnetic field and hole density. We provide a quantitative understanding of the phenomenon by modelling the coupling of optical transitions between Landau levels to the microcavity. This method introduces a control tool over the spin degree of freedom in polaritonic semiconductor systems, paving the way for new experimental possibilities in light-matter hybrids.
Accelerating Polaritons with External Electric and Magnetic Fields
Chervy, Thibault; Knüppel, Patrick; Abbaspour, Hadis; et al. (2020)
Physical Review X
It is widely assumed that photons cannot be manipulated using electric or magnetic fields. Even though hybridization of photons with electronic polarization to form exciton-polaritons has paved the way to a number of groundbreaking experiments in semiconductor microcavities, the neutral bosonic nature of these quasiparticles has severely limited their response to external gauge fields. Here, we demonstrate polariton acceleration by external electric and magnetic fields in the presence of nonperturbative coupling between polaritons and itinerant electrons, leading to formation of new quasiparticles termed polaron-polaritons. We identify the generation of electron density gradients by the applied fields to be primarily responsible for inducing a gradient in polariton energy, which in turn leads to acceleration along a direction determined by the applied fields. Remarkably, we also observe that different polarization components of the polaritons can be accelerated in opposite directions when the electrons are in ν=1 integer quantum Hall state.

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Patrick Knüppel

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