Cavity Magnon-Polariton Spectroscopy
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Abstract
The work which has been conducted in the course of this doctoral thesis with the title
“Cavity Magnon Polariton Spectroscopy” can be classified as part of the recent and active
research area of cavity spintronics. Among others, cavity spintronics aims to study
light-matter interaction by the examination of the properties of the cavity-magnon polariton
(CMP) and harness the CMP for applications bridging concepts of cavity quantum
electrodynamics (CQED) with spintronics. The CMP is the associated quasiparticle resulting
from the hybridisation of strongly coupled cavity photon-magnon (quasiparticles
from a collective spin excitation in a magnetic material) states. In close collaboration
with co-workers from the Karlsruhe Institute for Technology (KIT) who provided for the
millikelvin data at 30 mK, the work on the CMP started with conducting a study on the
temperature dependence of key properties of the CMP such as the cooperativity C, that
is, the coupling strength and the magnon linewidth from 30mK to 290 K. This work connects
the quantum regime at millikelvin temperatures with the classical regime at room
temperature and shows the persistence of the strong coupling regime, that is the existence
of a coherent exchange of information, as C > 1 for the entire temperature range.
However, beyond the realisation of strong coupling, real applications using CMPs require
the ability to tune and control of the key property, i.e. the coupling strength reliably and
reproducibly. For instance, it is necessary to switch the coherent exchange of information
on and off deliberately. Therefore, as the next step, a new experimental scheme allowing
such control over the coupling strength has been developed in the course of this thesis.
Instead of the generally used approach of a single-tone driven CMP in the field of cavity
spintronics, an approach driving the CMP with two tones is realised. The coupling
strength is controlled by the relative phase and amplitude between the intracavity fields
from the two inputs. It is shown that the coupling strength can be increased, decreased
to zero accompanied by a strong amplitude enhancement and the decrease of the signal’s
linewidth, or transferred to a regime of level merging where the coupling strength is a
complex quantity. Also, that method does not require any intrusion into the experimental
apparatus during measurements. Thus, by the combination of realising strong coupling
and the control of the coupling of the CMP, this work contributes to cavity spintronics.
Specifically, the control over the coupling strength opens an avenue for generalized control
of the coupling of other solid-state polaritons towards the development of applications for
data storage and information processing technologies.