Colautti, Maja (2016) Optical Manipulation of Magnetization of a Ferrimagnet YIG Sphere. [Magistrali biennali]
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This thesis describes a novel method for the full-optical control of the magnetization precession in a ferrimagnet, based on the utilization of a mode-locked laser system with repetition rates in the ~GigaHertz range. This technique allows for sustaining the magnetization precession in the steady-state regime, condition that has not been realized in previous opto-magnetic experiments. The magnetic system responds to the optical laser excitation with a radiation field which is used to systematically investigate the phenomenon. The analysis is conducted in Yittrium-Iron garnet (YIG) samples by means of ferromagnetic resonance (FMR) techniques. Measurements are conducted in two different configurations, i.e. with the samples in free field couple to a single loop antenna (free field measurements) and in a cavity-QED framework, which is realized by enclosing the magnetic sample in a microwave resonator. In both cases the magnetization precession is optically driven by tuning the repetition rate of the picosecond laser pulses to the Larmor frequency of the magnetic sample. This condition is achieved through a previous characterization of the system in the frequency domain by means of microwave network analysis. As compared to the free field scheme, detection of the radiated field in the cavity during the laser action allows to estimate the radiated field amplitude by measuring the power absorbed by the sample.\par In the cavity we accomplish a strong coupling regime between the magnetostatic modes of the YIG sample and the cavity mode. This regime is the so-called hybridization that can be described with a simple classical model of a pair of coupled harmonic oscillators. \par In order to avoid thermal effects, investigation of the photoinduced magnetization precession is conducted using $1550\,$nm-wavelength laser pulses, in the transparency window of the YIG samples. The phenomenon is explained as originating from the non-linear inverse Faraday effect (IFE) where by laser light can modify the magnetization of the material. In the model that has been developed it is possible to simplify the theoretical description of the photoinduced magnetization vector, and the model is tested by investigating the intensity dependence of the emitted microwave field amplitude. Most importantly, the signal dependence from the incident light polarization gives relevant signatures of the IFE.
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