We develop an analytical model based on the WKB approach to evaluate the experimental results of the femtosecond pump-probe measurements of the transmittance and reflectance obtained on thin membranes of porous silicon. The model allows us to retrieve a pump-induced nonuniform complex dielectric function change along the membrane depth. We show that the model fitting to the experimental data requires a minimal number of fitting parameters while still complying with the restriction imposed by the Kramers-Kronig relation. The developed model has a broad range of applications for experimental data analysis and practical implementation in the design of devices involving a spatially nonuniform dielectric function, such as in biosensing, wave-guiding, solar energy harvesting, photonics and electro-optical devices.
The authors report single-shot femtosecond laser ablation experiments on the highly oriented pyrolitic graphite surface. The yield of positively charged carbon clusters has been measured by time-of-flight mass spectrometry. The reconstructed velocity distribution of atomic carbon ions indicates that the observed Coulomb explosion is most pronounced for fluences slightly above the damage threshold. Atomic force microscopy studies of the damage spots indicate nanoscopic removal of intact monolayers. The results imply a significant degree of charge localization on the surface graphite layers.
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