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Fast and furious : ultrasnabb elektrondynamik i oregelbundna nanostrukturer

Year: 2026
Type: dissertation
Abstract: This thesis investigates ultrafast charge carrier dynamics in disordered nanostructures using femtosecond optical pump probe spectroscopy. The main aim is to understand how photoexcited electronic distributions evolve on femtosecond to picosecond timescales and how nanoscale morphology reshapes the transient optical response and the associated relaxation pathways. Broadband pump probe measurements are combined with optical modelling to relate time dependent changes in transmission to transient modifications of the complex permittivity, electronic damping, and energy transfer to the lattice. Ultrafast dynamics are studied in two plasmonic metal systems. For nanoporous gold, the transient transmission response is strongly enhanced and broadened compared with a continuous film. The broadband negative signal extends below the equilibrium interband onset, consistent with higher transient electron temperatures in the porous network that increase Fermi smearing and enable additional 5d to 6sp excitation pathways at lower photon energies. The relaxation is slower than in bulk gold, and modelling with an extended two temperature description combined with an effective medium optical treatment captures both the broadened spectra and the modified recovery, linking the response to morphology-controlled energy deposition and electron to lattice energy flow. For disordered copper nano island films, the transient transmission is strongly dispersive in the visible range, with negative and positive contributions that evolve in time due to the interplay of pump induced absorption and bleaching. In both Au and Cu, the measurements show that disorder and nanoscale connectivity reshape the spectral line shape and modify the apparent relaxation dynamics by changing the effective optical response and the effective metal volume involved in energy deposition. A complementary case study on polycrystalline NiO thin films extends the investigation to a transition metal oxide under sub band gap excitation, The transient reflectivity shows a prompt negative response followed by recovery that is well described by a biexponential model with a fast component on the order of a few tens of femtoseconds and a slower sub picosecond component. In addition, the thesis documents the generation and characterization of few-cycle structured light pulses carrying orbital angular momentum with controlled polarization states, providing an experimental platform for future ultrafast studies with tailored excitation fields
Author: Tapani, Tlek
Language: English
Open Access status: green
OpenAlex ID: W7133591313
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