We investigate the ultrafast photoinjection process initiated by a few-femtosecond optical pulse in monocrystalline undoped germanium with attosecond transient reflectivity spectroscopy. By comparison with theoretical calculations, we decouple several distinct but concurring physical phenomena that are found to exhibit different timing within the pump envelope. As a result of their complex interplay, we found that intra-band motion hinders charge injection, in contrast with what has previously been observed in other semiconductors.
Excitons, quasi-particles generated by the Coulomb interaction between an excited electron and a hole, have been proposed as a possibility to overcome the limits of electronics. To fully exploit them, a deeper understanding of their interaction with light is required. In this study, the interaction of a core exciton with an intense, few-femtosecond infrared pulse is investigated with attosecond transient reflection spectroscopy in a bulk monocrystalline MgF2 sample. The distinct few- and sub-femtosecond optical responses attest the dual, atomic- and solid-like, nature of core excitons. Sub-femtosecond dynamics, in particular, are dictated by the interplay between the exciton and the conduction band of the crystal. Theoretical simulations allow to propose the exciton binding energy as a lever to control exciton dynamics on an ultrafast timescale.
Miniaturized optical diagnostics might be greatly favored by the availability of effective, conformable UV light sources combining reduced size with mechanical flexibility. Here we report on our recent results on ZnO-incorporated nanofibers, exhibiting optical gain and polarized emission, used to obtain flexible UV lasers operating at room-temperature. The research leading to these results has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 682157, “xPRINT”), from the Italian Minister of University and Research (PRIN 2017PHRM8X) and from the University of Pisa (PRA “ANISE”).
We demonstrate the utilization of Dammann lenses encoded onto a spatial light modulator (SLM) for triggering nonlinear effects. For continuous illumination Dammann lenses generate a multifocal pattern characterized by a set of N foci diffraction orders, all with the same intensity. We theoretically show that for pulses shorter than 100 femtosecond (fs) the effects of chromatic aberrations influence the uniformity of the generated pattern. Multifocal second harmonic generation (SHG) and on-axis multiple filamentation are produced and actively controlled in β-BaB2O4 (BBO) and fused silica samples, respectively, with an amplified Ti:Sapphire femtosecond laser (30 fs at FWHM). Our proposal allows us to dynamically control both the quantity of foci and the distance among them. The output diffraction pattern is in good agreement with theoretical calculations. The measured spectra at the rear face of the supercontinuum sample for different separation among foci are also provided. The potential of this technique is very promising in different fields of nonlinear optics or in applications of in-depth materials microprocessing.
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