The LEON project has developed and demonstrated a new class of optoelectronic devices based on graphene quantum dots (QDs) operating at THz frequencies. By combining advanced theoretical modelling, nanofabrication, and state-of-the-art THz spectroscopy, the project establishes large graphene QDs as a versatile platform for compact and integrable THz devices.
On the theoretical side, LEON provided a detailed understanding of the electronic and optical properties of large graphene QDs (50–100 nm). Using tight-binding modelling, the project revealed strong and tunable THz absorption resonances arising from intraband electronic transitions in doped graphene QDs. An interesting finding is that electronic transitions involving edge states do not couple to THz photons, eliminating a major source of loss and confirming the suitability of graphene QDs for THz optoelectronic and laser applications.
Experimentally, LEON demonstrated ultrasensitive THz detection using a single graphene QD integrated into a single-electron transistor. At cryogenic temperatures (170 mK) and in the Coulomb blockade regime, these devices show strong photoresponses to incoherent THz radiation over a broad frequency range (<0.1–10 THz). Furthermore, their interaction with coherent THz field revealed a remarkably large electric dipole moment (d≈230 nm). This unique property highlights the strong potential of graphene QDs for THz devices and for quantum optics, as they involve only single-electron transition that can be ultra-strongly coupled to THz cavities. The project also realized graphene double QDs, and by combining quantum transport spectroscopy with modeling based on Green's functions formalism, demonstrated a two-level system with transition frequencies of up to 0.14 THz. This constitutes the first on-chip compatible two-level system operating in the THz range and represents an important milestone toward future solid-state THz quantum technologies.
Beyond, LEON extended its approach to other quantum materials, including crenellated hBN-graphene heterostructures and mercury telluride (HgTe) nanocrystals. These studies provided new insights into ballistic transport, THz intraband absorption, carrier lifetimes, and coherent THz emission, thereby broadening the range of material for THz quantum devices.
With a view toward future THz lasers based on graphene QDs, LEON developed innovative fabrication processes for arrays of large graphene QDs and demonstrated their broad THz intraband absorption.
To probe these quantum materials using THz spectroscopy system, the project also built powerful table-top THz sources delivering ultra-broadband, high-field THz pulses (100 kV/cm) at intermediate repetition rates (200 kHz), creating a unique experimental capability for nonlinear and time-resolved THz spectroscopy with high signal-to-noise ratios.
A major technological outcome of LEON is also the design and realization of original THz waveguides and microcavities. In particular, novel THz Tamm resonators with high quality factors were demonstrated, together with hybrid cavity architectures enabling fine frequency tuning, polarization control, and enhanced light–matter interaction. Zero-dimensional (0D) THz Tamm cavities were developed to confine THz modes in all three spatial dimensions, maximizing coupling to a single or a double QD. Devices based on double QDs coupled to 0D Tamm cavity are under investigation.
Finally, LEON investigated how optical pumping conditions control non-equilibrium carrier dynamics in graphene/hBN heterostructures, identifying regimes with long carrier lifetimes (~30 ps ) and switchable relaxation pathways upon ignition of HPhP relaxation. These results open promising perspectives toward graphene-based THz amplifiers and lasers.
The results of the project were disseminated through peer-reviewed scientific publications, invited presentations at international conferences, and close collaborations with national and international research groups. In addition, the technologies and methodologies developed within the LEON project have contributed significantly to the training and development of young researchers.
