Chuangtang Wang’s PhD Proposal Review

“All-optical Control of Magnetization in Nanostructures”

Committee:

Prof. Yongmin Liu (Advisor)

Prof. Don Heiman

Prof. Nian X. Sun

Abstract:
The switching of magnetization by a femtosecond laser within several picoseconds has recently gained substantial attention, because it promises next-generation, energy-efficient, and high-rate data storage technology. One of the most intriguing demonstrations is the helicity-dependent switching (HD-AOS) of a ferromagnet, in which the magnetization states can be deterministically written and erased using left- and right-circularly polarized light. However, the challenge is to realize a single-pulse HD-AOS. Controlling the spin angular momentum transfer from light to magnetic materials in nanostructures is the key to advance this field.
In my thesis research work, I will study the all-optical control of magnetization in different nanostructures, aiming to better understand the underlying mechanisms of HD-AOD and accelerate the technology development. Firstly, helicity-driven magnetization dynamics in heavy metal/ferromagnet Au(Pt)/Co bilayer by the optical spin transfer torque (OSTT) is experimentally explored. The wavelength-dependent measurement of OSTT reveals that the quantum efficiency of OSTT strongly depends on the interface electronic structure and pump energy. The Inverse Faraday effect (IFE), which is believed to be the driving mechanism of HD-AOS, is subsequently investigated in an Au thin film. The dependence of IFE on photon energy implies that the orbital angular momentum contribution to IFE is dominated by the excitation of laser pulses. To the best of our knowledge, it is the first demonstration of this phenomenon. Lastly, I will discuss our recent results on plasmonics-enhanced all-optical control of magnetization. Light can be tightly confined in plasmonic structures, which can potentially enable low-energy and high-density magnetic data storage.