When an electron-hole pair is optically excited in a semiconductor quantum dot the host crystal lattice needs to adapt to the presence of the generated charge distribution. Therefore the coupled exciton-phonon system has to establish a new equilibrium, which is reached in the form of a quasiparticle called polaron. Especially, when the exciton is abruptly generated on a timescale faster than the typical lattice dynamics, the lattice displacement cannot follow adiabatically. Consequently, a rich dynamics on the picosecond timescale of the coupled system is expected. In this study we combine simulations and measurements of the ultrafast, coherent, nonlinear optical response, obtained by four-wave mixing spectroscopy, to resolve the formation of this polaron. By detecting and investigating the phonon sidebands in the four-wave mixing spectra for varying pulse delays and different temperatures we have access to the influence of phonon emission and absorption processes which finally result in the emission of an acoustic wave packet out from the quantum dot.
Using a simplified hydrodynamic model of the free electron gas of a metal, we theoretically investigate optically induced DC current loops in a plasmonic nanostructure. Such current loops originate from an optical rectification process relying on three electromotive forces, one of which arises from an optical spin–orbit interaction. The resulting static magnetic field is found to be maximum and dramatically confined at the corners of the plasmonic nanostructure, which reveals the ability of metallic discontinuities to concentrate and tailor static magnetic fields on the nanoscale. Plasmonics can thus generate and tune static magnetic fields and strong magnetic forces on the nanoscale, potentially impacting small scale magnetic tweezing and sensing as well as the generation of magneto-optical effects and spin waves.
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