Thèses

Vendredi 29 Novembre 2024 à 14h00.

Ultrafast energy transfers at the nanoscale : cooling and vibrational dynamics of single supported metal nano-objects

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Directeur de thèse / thesis director :
Aurélien Crut et Natalia Del Fatti

Membres du jury / jury members :
Stefan Dilhaire, Université de Bordeaux (Rapporteur)
Pascal Ruello, Le Mans Université (Rapporteur)
Nathalie Destouches, Université Jean Monnet Saint-Etienne (Examinatrice)
Gianpietro Cagnoli, iLM (examinateur)
Aurélien Crut, iLM (directeur de thèse)
Natalia Del Fatti, iLM (co-Directrice de thèse)
Noëlle Lascoux, iLM (invitée)

Résumé / Abstract :
This thesis work focuses on the optical study of the thermal and vibrational properties of metallic nano-objects. Two optical techniques operating at the scale of a single nano-object (spatial modulation spectroscopy and time-resolved pump-probe spectroscopy) were used to localize single gold nanodisks randomly deposited on a solid substrate, characterize their optical properties, and study their ultrafast dynamics.
The first part of this work describes the optical study of the cooling of colloidal gold nanodisks deposited on substrates of different compositions and thicknesses, complemented by numerical simulations. Experiments and simulations demonstrated that the thermal dynamics, governed by thermal transfer at the nanodisk-substrate interface and heat diffusion in the substrate, strongly depend on the characteristics of the substrate, with a significant slowdown in the case of nanometric membranes.
The second part of this work allowed to characterize the transient optical response of a single gold nanodisk induced by its thermal dynamics. A strong dependence of the sensitivity of the optical measurements to nanodisk heating on the wavelength of the light beam used to probe this heating was demonstrated and quantified. A dependence of the temporal dynamics of the measured optical signals on the probe wavelength was also observed in the spectral range where the contribution of the heating of the membrane is not negligible compared to that associated with nanodisk heating, and was quantitatively reproduced by a thermo-optical model.
The final section of this thesis focuses on the study of the vibrational properties of single gold nanodisks. The experiments carried out allowed to confirm the strong dependence of the vibrational quality factors of deposited nano-objects on their morphology, demonstrated in previous work. In particular, improved quality factors were observed for nanodisks with a diameter/thickness ratios between 2 and 5. The sensitivity of measured optical signals to the vibrations of a nanodisk was also measured. It strongly depends on the probe wavelength, while presenting a very different spectral shape from that of optical sensitivity to nanodisk heating.
This research work has thus led to a better understanding of the cooling and vibrational dynamics of single gold nanodisks deposited on nanometer-thick membranes, and has allowed to precisely quantify the sensitivity of signals measured by optical pump-probe spectroscopy to these dynamics. Future work could focus on studying nanoparticles deposited on membranes of different compositions that are more favorable for demonstrating non-diffusive modes of heat transfer, or investigate the nature of internal vibrational damping phenomena, which determine the vibrational quality factors of nano-objects in the absence of coupling with the environment.

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