2025/05/20 by Katharina Dudde, Mahmoud Elhajhasan, Dudde, Katharina +15
Physics and Astronomy · Biochemistry, Genetics and Molecular Biology · #Spectroscopy and Quantum Chemical Studies #Spectroscopy Techniques in Biomedical and Chemical Research #Laser-Matter Interactions and Applications
paper · pdf · doi:10.48550/arxiv.2505.14506
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 lc during thermal reflectance measurements in the time or frequency domain and transient thermal grating spectroscopy. In place of lc, we vary the laser focus spot size (we) and the light penetration depth (hα) during one-laser Raman thermometry (1LRT) measurements. For our largest we values, the derived effective thermal conductivities κeff converge towards the bulk thermal conductivity κbulk for silicon. However, towards smaller we values, we observe a pronounced increase for the κeff 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 κeff(we) trends with the thermal accumulation function κcum 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 hα(λ) via the applied Raman laser wavelength λ during 1LRT. In this regard, we present proof-of-principle 1LRT measurements, yielding a step-like κeff(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 κcum trends and not only κ values, aiming to test our understanding of the intricate phonon transport physics.