Periodic Reporting for period 4 - LEON (Compact THz lasers based on graphene quantum dots)
Berichtszeitraum: 2024-03-01 bis 2025-08-31
THz radiation is extremely attractive for fundamental investigations of matter and emerging applications including, for example, security screening, medical imaging and spectroscopy. However, the THz spectral range remains one of the least exploited spectral regions, mainly due to the lack of compact powerful sources. The development of the typical semiconductor-laser scheme emitting at THz frequencies has been seriously hampered by the absence of an appropriate material with a sufficiently small bandgap.
The LEON project addresses this technological and scientific blocking point with new semiconductor-laser schemes for THz emission centered on the integration of graphene-based materials.
Indeed, graphene is potentially an excellent candidate for a THz semiconductor-laser model owing to its ‘zero’ bandgap. However, non-radiative recombination mechanisms, especially Auger recombination, reduce the lifetime of the optical gain to few hundreds of femtoseconds. This phenomenon drastically limits the feasibility of a THz laser. In order to suppress these detrimental non-radiative processes, a new concept is needed. The project proposes to exploit the full discretization of electronic states in graphene quantum dots.
The outcome of this project will overcome this major lack of modern THz technology by developing compact THz amplifiers and powerful lasers operating at room temperature. Furthermore, this project will have an industrial impact by improving the portability and mobility of THz systems and reducing cost. It will strongly contribute to elevate THz technology towards the levels that exist in the electronic and infrared regions. It will also lay the basis through the establishment of advanced know-how of a new physical system for future successive activities and opening new horizons in THz optoelectronics.
This project has three major cornerstones: i) the demonstration of THz amplifiers based on graphene quantum dots, ii) the demonstration of THz lasers based on graphene quantum dots in a microcavity, iii) the exploitation of these THz amplifiers/lasers for security and communication applications.
The LEON project addresses this technological and scientific blocking point with new semiconductor-laser schemes for THz emission centered on the integration of graphene-based materials.
Indeed, graphene is potentially an excellent candidate for a THz semiconductor-laser model owing to its ‘zero’ bandgap. However, non-radiative recombination mechanisms, especially Auger recombination, reduce the lifetime of the optical gain to few hundreds of femtoseconds. This phenomenon drastically limits the feasibility of a THz laser. In order to suppress these detrimental non-radiative processes, a new concept is needed. The project proposes to exploit the full discretization of electronic states in graphene quantum dots.
The outcome of this project will overcome this major lack of modern THz technology by developing compact THz amplifiers and powerful lasers operating at room temperature. Furthermore, this project will have an industrial impact by improving the portability and mobility of THz systems and reducing cost. It will strongly contribute to elevate THz technology towards the levels that exist in the electronic and infrared regions. It will also lay the basis through the establishment of advanced know-how of a new physical system for future successive activities and opening new horizons in THz optoelectronics.
This project has three major cornerstones: i) the demonstration of THz amplifiers based on graphene quantum dots, ii) the demonstration of THz lasers based on graphene quantum dots in a microcavity, iii) the exploitation of these THz amplifiers/lasers for security and communication applications.
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.
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.
Within this project, we have achieved major results on THz quantum dots, THz cavities, and instrumentation in the THz spectral range that go well beyond the state of the art, establishing the key foundations for future THz amplifiers and lasers based on graphene quantum dots. In parallel, we have paved the way toward quantum technologies operating at THz frequencies, a spectral domain that remains in its infancy despite the rapid advances achieved at microwave and optical frequencies.