Direct Measurement of Room-Temperature Nondiffusive Thermal Transport Over Micron Distances in a Silicon Membrane

TLDR

Silicon’s room‑temperature phonon mean free path is about 40 nm, yet low‑frequency phonons with much longer paths dominate thermal conductivity. The study demonstrates that silicon’s room‑temperature thermal transport deviates from diffusion at micron scales. The experiment uses crossed laser pulses to create a sinusoidal temperature profile in a freestanding silicon membrane, monitoring it by probe‑laser diffraction while varying the grating period to adjust heat transport distances from ~1–10 µm. At micron‑scale distances, the effective thermal conductivity drops, revealing a transition from diffusive to ballistic transport for low‑frequency phonons.

Abstract

The "textbook" phonon mean free path of heat carrying phonons in silicon at room temperature is ∼40 nm. However, a large contribution to the thermal conductivity comes from low-frequency phonons with much longer mean free paths. We present a simple experiment demonstrating that room-temperature thermal transport in Si significantly deviates from the diffusion model already at micron distances. Absorption of crossed laser pulses in a freestanding silicon membrane sets up a sinusoidal temperature profile that is monitored via diffraction of a probe laser beam. By changing the period of the thermal grating we vary the heat transport distance within the range ∼1-10 μm. At small distances, we observe a reduction in the effective thermal conductivity indicating a transition from the diffusive to the ballistic transport regime for the low-frequency part of the phonon spectrum.