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Metastructure-enhanced terahertz magnon-polaritons

  • Author(s): Wu, Yu
  • Advisor(s): Williams, Benjamin S
  • et al.

Magnons, i.e. the quanta of spin waves, are considered to be promising information carriers. Unlike electric currents, magnon-based spin currents could be used to transport information coherently over long distance without generating any Joule heat. Magnons in antiferromagnetic materials exhibit resonance frequencies extending up to the THz frequency range, which promises rapid response of magnon-based devices. However, it becomes difficult to control such rapid oscillations of magnetization using circuit-based electronic control. Instead, optical techniques have been investigated for the generation and control of AF magnons – however most techniques have been based upon ultrafast near-IR pump-probe techniques.

In this thesis, I investigate the feasibility of designing electromagnetic metastructures with subwavelength effective cavity volumes to realize strong light-matter coupling between suitable antiferromagnetic materials operating at over 1 THz. In these systems, light and material excitations are strongly coupled and mixed into superposition states with hybrid dispersion relation, which are termed as polaritons. Such magnon-polariton systems are of interest as they enable coherent information transferring between distinct physical platforms. While polaritons have been demonstrated between GHz-frequency photons and ferromagnetic magnons, only limited reports have been found on antiferromagnetic magnon-polaritons.

Moreover, as a potential application, the feasibility of THz magnon-polariton lasers is studied. In this thesis, I propose an idea which combines metal-metal waveguide with LC circuit- based microcavity, and introduce a design of metastructure with strong evanescent magnetic field, enabling the generation of magnon-polaritons even in a 200 nm thin antiferromagnetic film. Quantum cascade active region with intersubband transitions falling into THz frequency range can be applied into the metastructure, enabling direct amplification of magnon-polaritons. Magnon-polariton lasing becomes possible when this hybrid active metastructure/ antiferromganet is paired with an output coupler, building up a quantum cascaded vertical external cavity surface emitting laser (QC-VECSEL). The opportunity of magnon-polariton quantum cascade laser is also discussed when a thick antiferromagnetic slab is inserted in a standard VECSEL cavity.

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