Magnons and phonons in nanostructures
Publication Date
December 11, 2026
Creator
Carr, Alex
University of Nottingham
Abstract
Coupling between quantum systems allows for the discovery of interference-based effects in materials or between different systems, including Fano interferences and amplification due to constructive interference. Strong coupling is a regime in which interacting quasiparticles can exchange energy coherently, with potential applications in quantum computing for manipulation and for extending the decoherence time. To be useful for application, these devices need to be fabricated on-chip. In this thesis, the coupling between magnons and phonons and between phonons and excitons is investigated in nanoscale systems fabricated on-chip.
For the magnon-phonon coupling, the material of interest is Galfenol, which is chosen due to its high magnetostriction coefficient of 395 ppm and ease of growth and device fabrication. A nanograting with a trough depth of 7 nm, a width of 48 nm, and a period of 120 nm is fabricated in a 20 nm thick Galfenol layer such that the excited magnon and phonon modes are spatially confined and overlap, and have similar frequencies. The device can then be excited optically via a pump laser with a wavelength of 1050 nm and probed by a probe laser with a wavelength of 780nm, enabling the use of the polar magneto-optical Kerr effect and the photoelastic effect to observe magnons and phonons, respectively. Using the polar magneto-optical Kerr effect and increasing the external magnetic field from 200 mT t0 400 mT in 10 mT steps, the dynamics of magnons are investigated, including the magnon-phonon interaction. It is observed that at high pump laser fluence (10 mJ/cm2) the magnon-phonon interaction exhibits a Fano-like interference effect which is controllable by adjusting the pump laser fluence.
Alternatively, the device can be electrically excited via a microwave antenna which operates at frequencies between 10 to 20 GHz, and the microwaves generated have a power of 18 dBm. The microwave-generated magnons are then detected via the inverse spin Hall effect. In this work, the magnon spectra change with the orientation of the external field relative to the Galfenol bar, with measured orientations of 0◦, 45◦ and 90◦. The origins of this spectral change are explored, including the localisation of different magnon modes within the bar; this was achieved using Ubermag to simulate the magnons.
This thesis also explores the coupling of phonons and excitons, where the material of choice is WSe2, which is a transition metal dichalcogenide and a van der Waals material. This material is chosen because the direct exciton resonance (1.62 eV) is close in energy to the probe laser photon energy (1.60 eV). This results in amplification of the phonon resonance via the photoelastic effect, which depends on the exciton resonance and the Bragg condition.
Item Type
ethesis
Thesis Type
PhD
Supervisors
Subjects (LC)
Associated Schools / Departments
UoN Repository URI
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