Periodic Reporting for period 1 - ILLUME (Infrared vaLLey control of moiré qUantum MatErials)
Reporting period: 2023-06-19 to 2025-06-18
The ILLUME project aimed at creating, manipulating and exploring novel phases with non-trivial topology and interplay between topology and electron-electron interactions in moiré materials. For this purpose, the project combined electronic transport techniques with polarisation-resolved mid-infrared (mid-IR) and terahertz (THz) excitations. The technique consists in irradiating a 2D material with mid-IR or THz continuous waves excitations under various conditions of power and polarisation and map the electronic response of moiré materials. The IR and THz wavelengths are particularly well suited for the study of moiré materials, as the photon energies are comparable to the inter- and intra-band transitions. The project was expected to significantly advance the fundamental understanding of strongly correlated electron systems, quantum geometry, and the critical role of lattice reconstruction in moiré materials. It was also expected to result in novel optoelectronic devices taking advantage of the unique absorptions of graphene-based heterostructures in these energy ranges.
The project used a combination of electronic transport, photocurrent and photoconductivity measurements to understand the interplay between topology and electron interactions in moiré quantum materials.
Infrared spectroscopy was employed to probe the band structure of twisted graphene bilayers (TGB) around the magic angle. Measurements revealed distinct spectral features originating from inter-band transitions. Building on this approach, polarisation-resolved IR and THz photocurrent measurements were utilised to probe magic-angle TGB. This technique leveraged the sensitivity of photocurrents to the Berry connection and used THz light resonant with the optical transition of magic-angle TGB’s flat bands. The research unveiled the interplay between quantum geometry and electron interactions in TGB. We detected inversion-breaking gapped states and observed sharp changes in photocurrent polarisation axes due to interaction-induced band renormalisation. Recurring photocurrent patterns allowed us to track the evolution of quantum geometry through a cascade of phase transitions. This work also laid the groundwork for innovative THz quantum technologies.
Following, we also took advantage of high-current features in bilayer graphene-hBN superlattices to engineer highly photosensitive non-equilibrium electron phases. This was based on the identification of a negative differential conductance region with a sensitive bistable state. A novel single-photon detection mechanism was demonstrated, capable of single-photon detection at mid-infrared and visible wavelengths. The detector operated at temperatures up to 30K, an order of magnitude above state-of-the-art single-photon detectors. The technology is compatible with standard CMOS fabrication processes and photonic integrated circuits.
References:
1. Li, G. et al. Infrared Spectroscopy for Diagnosing Superlattice Minibands in Twisted Bilayer Graphene near the Magic Angle. Nano Lett. 24, 15956–15963 (2024).
2. Krishna Kumar, R. et al. Terahertz photocurrent probe of quantum geometry and interactions in magic-angle twisted bilayer graphene. Nat. Mater. 24, 1034-1041 (2025).
3. Nowakowski, K. et al. Single-photon detection enabled by negative differential conductivity in moiré superlattices. Preprint at arXiv: 2505.13637 (2025).
Infrared spectroscopy was employed to probe the band structure of twisted graphene bilayers (TGB) around the magic angle. Measurements revealed distinct spectral features originating from inter-band transitions. Building on this approach, polarisation-resolved IR and THz photocurrent measurements were utilised to probe magic-angle TGB. This technique leveraged the sensitivity of photocurrents to the Berry connection and used THz light resonant with the optical transition of magic-angle TGB’s flat bands. The research unveiled the interplay between quantum geometry and electron interactions in TGB. We detected inversion-breaking gapped states and observed sharp changes in photocurrent polarisation axes due to interaction-induced band renormalisation. Recurring photocurrent patterns allowed us to track the evolution of quantum geometry through a cascade of phase transitions. This work also laid the groundwork for innovative THz quantum technologies.
Following, we also took advantage of high-current features in bilayer graphene-hBN superlattices to engineer highly photosensitive non-equilibrium electron phases. This was based on the identification of a negative differential conductance region with a sensitive bistable state. A novel single-photon detection mechanism was demonstrated, capable of single-photon detection at mid-infrared and visible wavelengths. The detector operated at temperatures up to 30K, an order of magnitude above state-of-the-art single-photon detectors. The technology is compatible with standard CMOS fabrication processes and photonic integrated circuits.
References:
1. Li, G. et al. Infrared Spectroscopy for Diagnosing Superlattice Minibands in Twisted Bilayer Graphene near the Magic Angle. Nano Lett. 24, 15956–15963 (2024).
2. Krishna Kumar, R. et al. Terahertz photocurrent probe of quantum geometry and interactions in magic-angle twisted bilayer graphene. Nat. Mater. 24, 1034-1041 (2025).
3. Nowakowski, K. et al. Single-photon detection enabled by negative differential conductivity in moiré superlattices. Preprint at arXiv: 2505.13637 (2025).
The research has yielded significant advancement in understanding and utilising moiré quantum materials, particularly twisted graphene bilayer (TGB) and related superlattices. The core results span three key areas:
1. Spectroscopic diagnosis of superlattice minibands. Infrared spectroscopy was used as a diagnostic tool for the understanding of single-particle band structure of TGB around the magic angle, with distinct spectral features originating from inter-band transition. We found that the energies are uniquely defined by the twist angle, and can be fitted with theoretical model.
2. Probing quantum geometry with photocurrent measurements. We used polarisation-resolved infrared and terahertz photocurrent measurements to probe the quantum geometry and electron interactions in magic-angle twisted graphene bilayers (TGBs). This technique is sensitive to the Berry connection, providing insights into the electron wavefunction texture. It detected inversion-breaking gapped states previously undetectable with conventional electron transport techniques. It also revealed interaction-induced band renormalisation and tracked the behaviour of quantum cascade of phase transitions.
3. Novel single-photon detection. We demonstrated a new mechanism for single-photodetection using highly photosensitive non-equilibrium electron phases in bilayer graphene-hexagonal boron nitride superlattices. This detector leverages negative differential conductance and a sensitive bistable state. It operates at temperatures up to 30K, surpassing current state-of-the art single-photon detectors by an order of magnitude, and could represent a significant improvement over many existing technologies.
1. Spectroscopic diagnosis of superlattice minibands. Infrared spectroscopy was used as a diagnostic tool for the understanding of single-particle band structure of TGB around the magic angle, with distinct spectral features originating from inter-band transition. We found that the energies are uniquely defined by the twist angle, and can be fitted with theoretical model.
2. Probing quantum geometry with photocurrent measurements. We used polarisation-resolved infrared and terahertz photocurrent measurements to probe the quantum geometry and electron interactions in magic-angle twisted graphene bilayers (TGBs). This technique is sensitive to the Berry connection, providing insights into the electron wavefunction texture. It detected inversion-breaking gapped states previously undetectable with conventional electron transport techniques. It also revealed interaction-induced band renormalisation and tracked the behaviour of quantum cascade of phase transitions.
3. Novel single-photon detection. We demonstrated a new mechanism for single-photodetection using highly photosensitive non-equilibrium electron phases in bilayer graphene-hexagonal boron nitride superlattices. This detector leverages negative differential conductance and a sensitive bistable state. It operates at temperatures up to 30K, surpassing current state-of-the art single-photon detectors by an order of magnitude, and could represent a significant improvement over many existing technologies.