Phonon Mean Free Path Spectroscopy By Raman Thermometry
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| Main Authors: | , , , , , , , , |
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| Format: | Preprint |
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2025
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| _version_ | 1866918026884939776 |
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| author | Dudde, Katharina Elhajhasan, Mahmoud Würsch, Guillaume Themann, Julian Lierath, Jana Paul, Dwaipayan Protik, Nakib H. Romano, Giuseppe Callsen, Gordon |
| author_facet | Dudde, Katharina Elhajhasan, Mahmoud Würsch, Guillaume Themann, Julian Lierath, Jana Paul, Dwaipayan Protik, Nakib H. Romano, Giuseppe Callsen, Gordon |
| contents | 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 ($w_{e}$) and the light penetration depth ($h_α$) during one-laser Raman thermometry (1LRT) measurements. For our largest $w_{e}$ values, the derived effective thermal conductivities $κ_{eff}$ converge towards the bulk thermal conductivity $κ_{bulk}$ for silicon. However, towards smaller $w_{e}$ 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 $κ_{eff}(w_{e})$ trends with the thermal accumulation function $κ_{cum}$ and its dependence on the phonon mean free path $l_{ph}$, which we derive from ab initio solutions of the linearized phonon Boltzmann transport equation (BTE). Since the variation of $w_{e}$ 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 $κ_{\text{cum}}$ trends and not only $κ$ values, aiming to test our understanding of the intricate phonon transport physics. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2505_14506 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Phonon Mean Free Path Spectroscopy By Raman Thermometry Dudde, Katharina Elhajhasan, Mahmoud Würsch, Guillaume Themann, Julian Lierath, Jana Paul, Dwaipayan Protik, Nakib H. Romano, Giuseppe Callsen, Gordon Optics Materials Science 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 ($w_{e}$) and the light penetration depth ($h_α$) during one-laser Raman thermometry (1LRT) measurements. For our largest $w_{e}$ values, the derived effective thermal conductivities $κ_{eff}$ converge towards the bulk thermal conductivity $κ_{bulk}$ for silicon. However, towards smaller $w_{e}$ 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 $κ_{eff}(w_{e})$ trends with the thermal accumulation function $κ_{cum}$ and its dependence on the phonon mean free path $l_{ph}$, which we derive from ab initio solutions of the linearized phonon Boltzmann transport equation (BTE). Since the variation of $w_{e}$ 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 $κ_{\text{cum}}$ trends and not only $κ$ values, aiming to test our understanding of the intricate phonon transport physics. |
| title | Phonon Mean Free Path Spectroscopy By Raman Thermometry |
| topic | Optics Materials Science |
| url | https://arxiv.org/abs/2505.14506 |