University of Minnesota
School of Physics & Astronomy

Academic Calendar

Friday, May 18th 2018
1:00 pm:
Thesis Defense in 110 PAN
Speaker: Li He, University of Minnesota
Subject: Exploiting the spin of photons and electrons in light-matter interaction
This is the public portion of Mr. He's Thesis Defense. His advisor is Mo Li.

In light-matter interaction, the conservation of angular momentum requires the angular momentum transfer either between light and matter or within various thermodynamic reservoirs of materials, which consequently gives rise to a plethora of intriguing phenomena such as mechanical motion, charge current and magnetization. This dissertation focuses on the roles of photon spin and electron spin in light-matter interaction. The angular momentum transfer in three different scenarios are studied.First, we present the measurement of spin angular momentum of light propagating in a silicon waveguide. The continuous evolution of light polarization along propagation results in the exchange of angular momentum between light and medium and thus an optical torque to twist the waveguide. We demonstrate the use of optical torque to excite the torsional motion of an on-chip optomechanical device, which enables the coupling between optical and mechanical degrees of freedom. Second, we show the optical manipulation of surface electrons in 3D topological insulator Bi2Se3 using circularly polarized light. The transfer of angular momentum manifests itself in the spin-dependent optical selection rules and the generation of helicity-dependent photocurrent at zero bias voltage. Finally, we present the all-optical manipulation of magnetic order in ferrimagnetic alloy GdFeCo using sub-picosecond laser pulses as the ultrafast stimuli. The instantaneous heating of the electron temperature due to light absorption triggers the energy and angular momentum exchange among electron, spin and lattice reservoirs and leads to the switching of magnetization. As a step towards device application, we demonstrate an magnetic tunnel junction that can be switched all-optically without any external magnetic fields.

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