Papers › Phonon Mean Free Path Spectroscopy By Raman Thermometry
Phonon Mean Free Path Spectroscopy By Raman Thermometry
Katharina Dudde, Mahmoud Elhajhasan, Guillaume Würsch, Julian Themann, Jana Lierath, Dwaipayan Paul, Nakib H. Protik, Giuseppe Romano, Gordon Callsen
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In this work, we exemplify on a bulk silicon sample that Raman thermometry is capable of phonon mean free path (PMFP) spectroscopy. Our experimental approach is similar to the variation of different characteristic length scales l_c during thermal reflectance measurements in the time or frequency domain and transient thermal grating spectroscopy. In place of l_c, we vary the laser focus spot size (wₑ) and the light penetration depth (h_α) during one-laser Raman thermometry (1LRT) measurements. For our largest wₑ values, the derived effective thermal conductivities κ_(eff) converge towards the bulk thermal conductivity κ_(bulk) for silicon. However, towards smaller wₑ values, we observe a pronounced increase for the κ_(eff) values, which amounts up to a factor of 5.3 at 293K and even 8.3 at 200K. We mainly assign this phenomenon to quasi-ballistic phonon transport. As a result, we can compare our measured κ_(eff)(wₑ) trends with the thermal accumulation function κ_(cum) and its dependence on the phonon mean free path lₚₕ, which we derive from ab initio solutions of the linearized phonon Boltzmann transport equation (BTE). Since the variation of wₑ can be experimentally cumbersome, we also suggest varying h_α(λ) via the applied Raman laser wavelength λ during 1LRT. In this regard, we present proof-of-principle 1LRT measurements, yielding a step-like κ_(eff)(λ) trend for four different λ values, which we also interpret in terms of quasi-ballistic phonon transport. Our results shall seed future PMFP spectroscopy based on 1LRT, which can directly be benchmarked against state-of-art theory by comparison of κ_(cum) trends and not only κ values, aiming to test our understanding of the intricate phonon transport physics.
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