Excitons bound by photon exchange

Abstract

In contrast to interband excitons in undoped quantum wells, doped quantum wells do not display sharp resonances due to excitonic bound states. The effective Coulomb interaction between electrons and holes in these systems typically leads to only a depolarization shift of the single-electron intersubband transitions1. Non-perturbative light–matter interaction in solid-state devices has been investigated as a pathway to tuning optoelectronic properties of materials2,3. A recent theoretical work4 predicted that when the doped quantum wells are embedded in a photonic cavity, emission–reabsorption processes of cavity photons can generate an effective attractive interaction that binds electrons and holes together, leading to the creation of an intraband bound exciton. Here, we spectroscopically observe such a bound state as a discrete resonance that appears below the ionization threshold only when the coupling between light and matter is increased above a critical value. Our result demonstrates that two charged particles can be bound by the exchange of transverse photons. Light–matter coupling can thus be used as a tool in quantum material engineering, tuning electronic properties of semiconductor heterostructures beyond those permitted by mere crystal structures, with direct applications to mid-infrared optoelectronics.

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Fig. 1: Coulomb effect in doped and undoped quantum wells.
Fig. 2: Schematics of the experimental set-up.
Fig. 3: Bound-to-continuum nature of the optical transition in bare QWs with no surrounding photonic resonator.
Fig. 4: Experimental reflectivity data.
Fig. 5: Calculation of P.

Data availability

The data that support the findings of this study are available from the corresponding authors on reasonable request. Source data are provided with this paper.

Code availability

The codes that support the findings of this study are available from the corresponding author (S.D.L.) on reasonable request.

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Acknowledgements

S.D.L. is a Royal Society Research Fellow and was partly funded by the Philip Leverhulme Prize of the Leverhulme Trust. R.C., J.M.-M., G.B. and I.C. were partly funded by the European Union FET-Open Grant Number MIR-BOSE 737017. R.C. and A.B. were partly funded by the French National Research Agency (project IRENA). This work was partly supported by the French RENATECH network.

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S.D.L. supervised the project and led the theoretical work. I.C., R.C. and S.D.L. designed the experiment. R.C. led the experimental work. G.B. grew the sample and N.-L.T fabricated the devices. N.-L.T., J.-M.M. and A.B. carried out the optical characterization. E.C. performed the data analysis. All authors discussed the data and contributed to the manuscript.

Corresponding authors

Correspondence to Raffaele Colombelli or Simone De Liberato.

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The authors declare no competing interests.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–3 and text.

Source data

Source Data Fig. 3

Source Data for Figure 3.

Source Data Fig. 4

Source Data for Figure 4.

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Cortese, E., Tran, NL., Manceau, JM. et al. Excitons bound by photon exchange. Nat. Phys. (2020). https://doi.org/10.1038/s41567-020-0994-6

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