Abstract
Many-body phenomena in quantum materials emerge from the interplay among a broad continuum of electronic states and controlling these interactions is critical for engineering new phases. One promising approach exploits light confined within optical cavities to tailor electronic properties1. Here we demonstrate that terahertz cavity photons can mediate attractive interactions in a tunable van der Waals (vdW) material and reorganize a continuum of electron–hole transitions into an exciton-like state. We introduce a broadband, sub-wavelength time-domain microscope that integrates exfoliated, dual-gated 2D quantum materials into a terahertz cavity. This approach enables the spectroscopic measurement of the field-tunable bandgap of bilayer graphene2 (BLG) in the terahertz range and, at resonance, reveals ultrastrong coupling3 (USC) with an effective interaction strength exceeding g/ωc ≈ 40% of the bare photon energy. Crucially, we identify a cavity-induced resonance emerging from the interband continuum that resembles Coulomb-bound excitons and remains stable across a broad temperature range. Our findings propose an experimental platform for designing and investigating hybrid light–matter phases in 2D quantum matter.
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Data availability
The numerical simulations and measurement datasets underlying the figures in this paper are available from the corresponding author on request. The raw data that support the findings of this article are also available in the ETH Research Collection (https://doi.org/10.3929/ethz-c-000797558).
Code availability
The code used in the theoretical part of this study is available from the corresponding author on request and in the ETH Research Collection (http://doi.org/10.5905/ethz-1007-932).
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Acknowledgements
We thank E. Jöchl for discussions on lens assembly and sample processing. F.H. acknowledges discussions on sample fabrication with the Ensslin group (J. Gerber, M. Niese and M. Ruckriegel) and Z. Hao, support from the FIRST cleanroom staff, as well as M. Baer for the design of the sample holder. T.F.N. thanks M. Buzzi and M. Schiro for fruitful discussions. The BNA terahertz generation crystals were provided by Swiss Terahertz LLC. T.F.N. discloses support for the research of this work from the ETH Postdoctoral Fellowship programme, which also supported approximately 30% of the experimental set-up. F.H. and A.İ. disclose support from the Swiss National Science Foundation (SNSF; grant nos. 200020 and 207520). J.F. and G.S. disclose support from the SNSF (grant no. 10000397). H.S.A. discloses support from the Swiss Government Excellence Scholarship. I.K. acknowledges the financial assistance of the Rothschild Post-Doctoral Fellowship from Yad HaNadiv, the Helen Diller Quantum Center and the Viterbi Post-Doctoral Fellowships from Technion.
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T.F.N. and F.H. conceived the project, with input from J.F., G.S. and A.İ. All five interpreted the data. F.H. and T.F.N. designed and built the terahertz set-up, fabricated the devices, planned and performed the transport and terahertz measurements, analysed the experimental data, performed finite-element method and transfer matrix simulations and developed the quantum mechanical model. H.S.A. contributed to the development of the theoretical model. M.K. assisted during the experimental campaign. I.K. contributed to the initial development of the terahertz set-up. G.S. and J.F. contributed to the development of the lens system. T.F.N. and F.H. wrote the manuscript, with input from all authors. T.F.N. supervised the project.
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Extended data figures and tables
Extended Data Fig. 1 Band-edge absorption.
a, Schematic of THz absorption by a band edge (top) and the corresponding transmission spectrum (bottom), with an arrow marking the location of the band edge. b, Experimental transmission spectra at various displacement fields |D| > 0 and n = 0 referenced to the insulating state D = −0.36. The arrows mark the onset of absorption, similarly to panel a. Data obtained from device D1 at 5 K.
Extended Data Fig. 2 UP peak as a function of displacement field.
The data are adapted from Fig. 5a. The frequency window is restricted to 2.3–5.4 THz (that is, from just above the cavity frequency up to near the upper end of our spectral window), thereby removing the LP contribution and excluding the noisy high-frequency tails. The vertical axis is rescaled independently for each D (min-to-max within the shown frequency window) to highlight the UP peak. The dashed lines highlight the peak position. a, D = 0 V nm−1. b, D = −0.037 V nm−1. c, D = −0.073 V nm−1. d, D = −0.11 V nm−1.
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Supplementary Sections 1–11 including Supplementary Figs. 1–26 and Supplementary references.
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Helmrich, F., Adlong, H.S., Kroner, M. et al. Cavity-driven attractive interactions in quantum materials. Nature (2026). https://doi.org/10.1038/s41586-026-10609-1
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DOI: https://doi.org/10.1038/s41586-026-10609-1
