Ph.D defense of Balint Eles"Energy relaxation mechanisms in plasmonic nanocomposite materials under femtosecond laser excitation"

"Energy relaxation mechanisms in plasmonic nanocomposite materials under femtosecond laser excitation"

Abstract

The present thesis is devoted to analyze the multitime scale response of plasmonic nanocomposite materials submitted to femtosecond laser irradiation. Understanding the energy relaxation pathways following the laser excitation in such materials paves the way for well-controlled laser processing that is particularly important for the laser-based micro- and nanostructuring of metasurfaces. The material studied throughout the work is a composite of silver nanoislands encapsulated between two titania layers, deposited on glass substrate. The system exhibits rich dynamics under reversible and irreversible relaxation conditions that serve as the main focus of the present work.

The thesis relies on three main pillars. The first part describes the complex interplay of the mechanisms leading to laser-induced anisotropic shape transformation and formation of self-organized nanopatterns. The irreversible material shape transformation is demonstrated to origin from the accumulative effect of tens of thousands of laser pulses with distinctive intermediate states of matter. These states are characterized via a broad range of experimental techniques revealing the structural transformations and corresponding optical response of the system.
The decisive role of laser-induced temperature rise is deeply analyzed in this multipulse phenomenon using numerical methods. Additionally, the quantitative investigation of characteristic self-organized nanogratings with drastically varying spatial profiles is discussed by means of combining the structural observations with rigorous electromagnetic calculations. The provided general description of the course of fundamental physico-chemical mechanisms contributes to the development of plasmonic metasurface applications in domains requiring anisotropic optical response and organized nanoparticle arrangement on the nanometer scale.

The subsequent experiments aim at characterizing the ultrafast dynamics of the shape transformations using pump-probe microscopy experiments. The ultrafast phenomena reflect the pulse-by-pulse transformation of the optical response in the accumulative regime, and the observed dynamics is interpreted based on the literature of photoexcited plasmonic systems, as well as ex situ structural characterizations. The latter clearly reveal traces of ultrafast phenomena driving the shape transformations. The results demonstrate the variation of the ultrafast electron dynamics due to pulse-by-pulse system transformations. Additionally, the ultrafast dynamics of ablation processes of plasmonic nanocomposites are discussed with combined quantitative phase-sensitive measurements.

The material response in the weak excitation regime below reshaping energy threshold is investigated using transient spectroscopic methods. The experiments reveal the ability of a single layer of silver nanoislands with broad shape dispersion and characteristic size variations to excite coherent acoustic pulses. Various samples with strongly different nanoisland morphologies are tested, and the propagation of acoustic pulses in glass substrate is monitored. Quantitative analysis of the acoustic wave features is conducted using a simplified theoretical model revealing the key parameters affecting the acoustic pulse emission properties of such inhomogeneous nanoparticle ensembles.

COMMITTEE

• Jörn Bonse, BAM,  Reviewer
• David Grojo, LP3 Laboratory,  Reviewer
• Jan Siegel, IO-CSIC,  Examiner
• Razvan Stoian, Hubert Curien Laboratory, Examiner
• Aurélien Crut, Institut Lumière Matière,  Examiner
• Nathalie Destouches, Hubert Curien Laboratory, Director of the thesis
• Christophe Hubert,  Hubert Curien Laboratory, Co-director of the thesis