Excitation of optical whispering gallery modes in Yttrium Iron Garnet (YIG) microresonators

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master_Ngan_Pham_2025.pdf (30.52 MB)

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School of Science | Master's thesis
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Date

2025-09-22

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Materials Physics and Quantum Technology

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Master's Programme in Engineering Physics

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en

Pages

48

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Abstract

Optomagnonics studies the coherent interaction between optical photons and magnons. This is an important link in realizing quantum information storage and communication, where magnons play a central role of mediating the conversion from microwave to optical signal. To increase their coupling strength, one strategy is to confine both magnon and optical modes in a cavity, and at the same time, decrease each of the mode volumes and increase their mode overlap. This could be accomplished in yttrium iron garnet (YIG) disks, as YIG has the lowest Gilbert damping, which leads to a long-lived magnon mode, and simultaneously is transparent in the optical frequency range. In this thesis, we explore the method of a dimpled tapered fiber acting as a waveguide to excite whispering gallery modes (WGMs) in YIG microdisks via evanescent coupling. YIG disks of various diameters are grown using photolithography and Pulsed Laser Deposition methods. Their crystal structure and magnetic response are characterized by X-Ray Diffraction and Magneto-Optical Kerr Effect. In addition, the tapered fiber is fabricated by the heat-and-pull method utilizing both a CO2 laser and an alcohol burner as the heat source. Subsequently, the tapered fiber is dimpled by pressing down a polished pencil lead. The experiments are conducted in two phases: first, the coupling between tapered fiber and YIG sphere; and second, between dimpled fiber and YIG disks. In the first case, we observed periodic dips in the transmission signal for both TE and TM polarization. Furthermore, there is a shift in the resonance wavelengths between the two polarizations. This is consistent with theory and previous experiments with similar setups. Therefore, we concluded that we have successfully excited WGMs in the sphere. Moving on to the second phase, the dimpled fiber proves tricky to be positioned exactly near the edge of a YIG disk for evanescent coupling. Initial measurements have been carried out, but WGMs have not been observed in this geometry. For future research, we set out to tackle the problems that arise in the coupling of a dimpled fiber and YIG disk. Furthermore, we want to introduce the magnetic field and explore the coupling between magnons and photons in these structures.

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Supervisor

van Dijken, Sebastiaan

Thesis advisor

Peeters, Lars

Keywords

tapered dimpled fiber, whispering gallery modes, evanescence coupling, yttrium iron garnet disks, yttrium iron garnet sphere, optomagnonics

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https://urn.fi/URN:NBN:fi:aalto-202510228536

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[dipl] Perustieteiden korkeakoulu / SCI