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Abstract_Klaas-Jan TIELROOIJ
Quantum materials exhibit several exciting ultrafast physical phenomena that are moreover potentially technologically useful. This is particularly true for quantum materials with massless Dirac electrons, such as graphene and topological insulators. When light is absorbed in these materials, electron heating occurs through electron-electron interactions on a 10-100 fs timescale, followed by electron cooling, typically involving the emission of phonons on a picosecond timescale. We have exploited these ultrafast thermodynamics to generate harmonics in the terahertz (THz) regime [1], which is particularly efficient in “quantum metamaterials” consisting of a quantum material and a metallic grating [2]. Thanks to an efficient “Coulomb cooling” mechanism between surface and bulk electronic states in topological insulators [3], we have recently demonstrated that the ultrafast thermodynamics can give rise to third-order terahertz harmonic generation approaching the milliwatt regime [4]. Furthermore, quantum metamaterials enable fast and gate-tunable conversion from THz light to visible light [5]. These results establish quantum materials as an excellent material platform for nonlinear terahertz photonics, with possible applications in next-generation wireless communication systems, among others.
Whereas these ultrafast thermodynamics in graphene and topological insulators are relatively well understood, this is not the case for twisted bilayer graphene near the magic angle. Using time-resolved photocurrent measurements, we have studied these dynamics, finding that the electron cooling dynamics in twisted bilayer graphene near the magic angle is very distinct from the dynamics in monolayer or non-twisted bilayer graphene. Specifically, the cooling time in near-magic twisted bilayer graphene is a few picoseconds all the way from room temperature down to 10 K. We ascribe these observations to Umklapp-assisted electron-phonon cooling, facilitated by the moiré pattern in twisted bilayer graphene [6]. These results establish twist angle as control knob for steering the cooling dynamics and flow of electronic heat, and have possible implications for the development of ultrafast detectors operating at cryogenic temperatures, among others.
Besides interesting phenomena related to the thermodynamics of electrons in quantum materials, there are also important questions that require answers related to the transport and dynamics of phonons in quantum materials and 2D layered materials. This is particularly true for transition metal dichalcogenides, such as MoSe2, which are projected to be used in future transistor technologies. We studied how phonon heat transport changes with thickness [7,8], and have recently observed interesting novel heat transport phenomena in these ultrathin semiconductors, by exploiting our novel experimental technique to study heat transport directly in space and time [9, 10].
Reference
[1] H.A. Hafez et al, Nature 561, 507 (2018).
[2] J.C. Deinert et al, ACS Nano 15, 1145 (2021).
[3] A. Principi and K.J. Tielrooij, Phys. Rev. B. 106, 115422 (2022)
[4] K.J. Tielrooij et al. Light Sci. Appl. 11, 315 (2022)
[5] I. Ilyakov et al, Nano Lett. 23, 3872 (2023)
[6] J.D. Mehew et al. Sci. Adv. 10, eadj1361 (2024)
[7] D. Saleta Reig et al. Adv. Mater. 34, 2108352 (2022)
[8] R. Farris et al. Phys. Rev. B. 109, 125422 (2024)
[9] S. Varghese et al. Rev. Sci. Instr. 94, 034903 (2023)
[10] S. Varghese et al. to be submitted