Thesis defense of Michal Kobecki

The goal of this work is to obtain better control over all-optical excitation and detection of high-frequency collective excitations, in particular - phonons and magnons. Phonons- a collective movement of atoms in the lattice, and magnons- excitations of a spin system in magnetically ordered materials, recently became the prospective alternative to electrons in quantum computing or in general, information technology. In the scope of this thesis, we present a novel approach to the resonant excitation of fundamental magnon mode in a thin ferromagnetic film of Iron-Gallium alloy, as well as enhanced detection sensitivity of propagating coherent phonon wave packet exploiting giant photo-elasticity of exciton-polaritons in GaAs/AlAs superlattice. Additionally, a possible way to miniaturize the all-optical set-ups for manipulation of collective excitations is proposed by implementing a passively mode-locked semiconductor laser diode. The measurements focused on increasing the detection sensitivity of coherent phonons were done in the time domain with a standard pump-probe set-up with a mechanical delay line. The coherent phonon wave packet was generated in the Al film deposited at the back of the sample and then it propagated towards the superlattice at the front. The probe beam energy was tailored to match the exciton resonance of the superlattice and the Time Domain Brillouin Signal was obtained revealing giant photo-elasticity of exciton-polaritons. The signals show three orders of magnitude increase in detection sensitivity compared to standard probing away from the resonance. Obtained results pave the way for the quantum sensitivity of coherent phonon detection. Resonant excitation and detection of the fundamental magnon mode in the ferromagnetic film were performed by the Asynchronous Optical Sampling method. The sample was placed in the external magnetic field at room temperature. It is shown that by tuning the magnetic field and matching the magnon frequency with the 10GHz excitation repetition rate of the laser we can obtain a 12-fold enhancement of magnetization precession amplitude detected by the magneto-optical Kerr effect. The demonstrated principle can be exploited in various nano-devices operating at GHz frequency ranges. Finally, as proof of the principle, a passively mode-locked semiconductor laser diode is used to conduct pump-probe experiments on exciting and detecting phonon echo in the thin aluminum film at room temperature. We hope it is the first step toward out-of-lab applications of all-optical pump-probe set-ups.

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