Staff directory Alexander Block



  • Hot-Carrier Cooling in High-Quality Graphene Is Intrinsically Limited by Optical Phonons

    Pogna E.A.A., Jia X., Principi A., Block A., Banszerus L., Zhang J., Liu X., Sohier T., Forti S., Soundarapandian K., Terrés B., Mehew J.D., Trovatello C., Coletti C., Koppens F.H.L., Bonn M., Wang H.I., Van Hulst N., Verstraete M.J., Peng H., Liu Z., Stampfer C., Cerullo G., Tielrooij K.-J. ACS Nano; 15 (7): 11285 - 11295. 2021. 10.1021/acsnano.0c10864. IF: 15.881

    Many promising optoelectronic devices, such as broadband photodetectors, nonlinear frequency converters, and building blocks for data communication systems, exploit photoexcited charge carriers in graphene. For these systems, it is essential to understand the relaxation dynamics after photoexcitation. These dynamics contain a sub-100 fs thermalization phase, which occurs through carrier-carrier scattering and leads to a carrier distribution with an elevated temperature. This is followed by a picosecond cooling phase, where different phonon systems play a role: graphene acoustic and optical phonons, and substrate phonons. Here, we address the cooling pathway of two technologically relevant systems, both consisting of high-quality graphene with a mobility >10000 cm2 V-1 s-1 and environments that do not efficiently take up electronic heat from graphene: WSe2-encapsulated graphene and suspended graphene. We study the cooling dynamics using ultrafast pump-probe spectroscopy at room temperature. Cooling via disorder-assisted acoustic phonon scattering and out-of-plane heat transfer to substrate phonons is relatively inefficient in these systems, suggesting a cooling time of tens of picoseconds. However, we observe much faster cooling, on a time scale of a few picoseconds. We attribute this to an intrinsic cooling mechanism, where carriers in the high-energy tail of the hot-carrier distribution emit optical phonons. This creates a permanent heat sink, as carriers efficiently rethermalize. We develop a macroscopic model that explains the observed dynamics, where cooling is eventually limited by optical-to-acoustic phonon coupling. These fundamental insights will guide the development of graphene-based optoelectronic devices. ©

  • Observation of giant and tunable thermal diffusivity of a Dirac fluid at room temperature

    Block A., Principi A., Hesp N.C.H., Cummings A.W., Liebel M., Watanabe K., Taniguchi T., Roche S., Koppens F.H.L., van Hulst N.F., Tielrooij K.-J. Nature Nanotechnology; 2021. 10.1038/s41565-021-00957-6. IF: 39.213

    Conducting materials typically exhibit either diffusive or ballistic charge transport. When electron–electron interactions dominate, a hydrodynamic regime with viscous charge flow emerges1–13. More stringent conditions eventually yield a quantum-critical Dirac-fluid regime, where electronic heat can flow more efficiently than charge14–22. However, observing and controlling the flow of electronic heat in the hydrodynamic regime at room temperature has so far remained elusive. Here we observe heat transport in graphene in the diffusive and hydrodynamic regimes, and report a controllable transition to the Dirac-fluid regime at room temperature, using carrier temperature and carrier density as control knobs. We introduce the technique of spatiotemporal thermoelectric microscopy with femtosecond temporal and nanometre spatial resolution, which allows for tracking electronic heat spreading. In the diffusive regime, we find a thermal diffusivity of roughly 2,000 cm2 s−1, consistent with charge transport. Moreover, within the hydrodynamic time window before momentum relaxation, we observe heat spreading corresponding to a giant diffusivity up to 70,000 cm2 s−1, indicative of a Dirac fluid. Our results offer the possibility of further exploration of these interesting physical phenomena and their potential applications in nanoscale thermal management. © 2021, The Author(s).