FLYER: ULTRAFAST ELECTRON-PHONON DYNAMICS IN SOLIDS FROM TRANSIENT PAULI BLOCKING TO THERMAL TRANSPORT MODULATION

Ultrafast optical excitation drives solids far from equilibrium, redistributing electronic occupation and initiating complex energy flow across electronic and lattice subsystems [1,2]. In this talk, I will discuss how transient electronic occupation can be engineered on femtosecond timescales and how this nonequilibrium state evolves into lattice heat transport across multiple timescales. Transient Pauli Blocking emerges under intense photoexcitation [2], where thermalized electron through electron-phonon scatterings rapidly accumulate in the conduction band, and suppress interband absorption through band filling effect. This nonequilibrium occupation enables broadband, sub-picosecond optical transparency modulation, highlighting transient Pauli blocking as a dynamic modulation mechanism in solids. Relaxation of this nonequilibrium electronic occupation is governed by electron–phonon coupling [1,2], which mediates energy transfer to the lattice. Using pump–probe thermoreflectance measurements [3], we quantify thermal transport and reveal how phonon-mediated thermal diffusion determines thermal redistribution on longer timescales [3,4]. Taken together, these findings provide a unified framework connecting ultrafast electronic occupation dynamics with lattice thermal transport, offering new perspectives for ultrafast optical modulation [1,2] and emerging energy-efficient thermal logic functionalities [5,6]. References: 1, J. Jia, et al., Multivalley optical switching in Germanium, Phys. Rev. Applied, 23, 024060 (2025). 2, J. Jia, et al., Transient Pauli blocking in an InN film as a mechanism for broadband ultrafast optical switching, Phys. Rev. B, 113, 045203 (2026).3, J. Jia, T. Yagi, Y. Shigesato, Thermal conduction in polycrystalline or amorphous transparent conductive oxide films, Solar Energy Materials and Solar Cells, 271, 112872 (2024). 4, H. Yagi, T. Yagi, Y. Yamashita, M. Kashiwagi, J. Jia, et al., Applied Physics Express, 16, 095503 (2023).5, J. Jia, et al., Tailoring thermal transport in (Sc,Yb)AlN thin films to the glassy limit, Acta Materialia, 304, 121767, (2026). 6, J. Jia, et al., Emerging solid-state thermal switching materials, Advanced Functional Materials, 34, 2406667, (2024).