Shedding light on ultrafast heat transport in graphene

August 28, 2025

Managing heat efficiently is one of the great challenges in the development of next-generation electronic and optoelectronic devices. At the nanoscale, energy moves in ways that defy classical descriptions of heat transfer. Houssem Rezgui, researcher from the Sotomayor research group at INL, has now provided new insights into how energy flows in graphene when excited with ultrafast laser pulses. Their work, published as a Letter in Journal of Physics D: Applied Physics, explores how electrons and vibrations in the lattice (known as phonons) interact in timeframes shorter than a trillionth of a second.

Graphene, with its remarkable electronic and thermal properties, is an ideal platform for probing these processes. When struck by a femtosecond laser pulse, graphene enters a highly non-equilibrium state: electrons absorb the incoming energy almost instantly, and only afterward begin to share it with the lattice. Traditionally, this sequence has been described by the two-temperature model, which assumes that electrons and phonons equilibrate in a straightforward way. However, the INL researcher shows that reality is more complex.

Using an extended temperature model, Rezgui revealed that different phonon branches respond at distinct timescales, with longitudinal acoustic phonons playing a key role in energy absorption and redistribution. Strikingly, their results show that under certain conditions the electron temperature can briefly dip below the lattice temperature – a behaviour classical models fail to capture. This wave-like energy transfer resembles hydrodynamic “second-sound” transport, a phenomenon of growing interest in nanoscale physics. It is somewhat like watching a ripple race across the surface of water instead of heat simply diffusing like steam in the air.

“Our study highlights the importance of resolving energy transfer processes at the level of individual phonon branches,” says Houssem Rezgui. “By understanding these ultrafast dynamics, we can better predict and control how materials manage heat, which is crucial for high-speed, high-performance devices.”

The findings deepen our understanding of fundamental heat transport mechanisms in low-dimensional materials. Beyond the physics, they carry practical implications: improved models of electron–phonon interactions could inform the design of faster and more reliable nanoelectronics, where overheating is often a limiting factor. In simple terms, this work shows how energy at the smallest scales doesn’t just flow like a steady current – it can bounce, ripple, and lag, reshaping the way we think about heat in the world of nanotechnology.