Ultrafast Thermodynamic Phenomena Involving Photons, Electrons and Phonons in (Twisted) Quantum (Meta)Materials

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, and the heat-induced decrease in terahertz (THz) absorption, to generate harmonics in the THz regime [1]. This THz harmonic generation is particularly efficient in quantum metamaterials that consist 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]. These results establish quantum (meta)materials as an excellent material platform for nonlinear terahertz photonics, with possible applications in next-generation wireless communication systems, among others.<br/><br/>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 ultrafast dynamics and found 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, where non-twisted bilayer graphene becomes increasingly slow for lower temperatures. We ascribe this ultrafast cooling in magic-angle twisted bilayer graphene to Umklapp-assisted electron-phonon cooling, facilitated by the moiré pattern in twisted bilayer graphene [5]. Whereas Umklapp scattering is a very common phenomenon for phonons, it is very rare to observe such scattering processes for electrons. 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.<br/><br/><b>References</b><br/>[1] H.A. Hafez et al, Nature <b>561</b>, 507 (2018).<br/>[2] J.C. Deinert et al, ACS Nano <b>15</b>, 1145 (2021).<br/>[3] A. Principi and K.J. Tielrooij, Phys. Rev. B. 106, 115422 (2022)<br/>[4] K.J. Tielrooij et al. Light Sci. Appl. <b>11</b>, 315 (2022)<br/>[5] J.D. Mehew et al. arXiv 2301.13742 (2023)