Browsing by Author "Tay, Fu Yang"

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    Quantum Semiconductor Structures Dressed with Terahertz Cavity Photons
    (2024-04-17) Tay, Fu Yang; Kono, Junichiro
    Cavity quantum electrodynamics (cQED) deals with an ensemble of two-level atoms coupled with a single mode of electromagnetic fields in a cavity. The theory of atomic cQED is well-developed, allowing one to make precise predictions for the quantum dynamics of atom–photon hybrids, indispensable for realizing quantum transduction, entanglement generation, and single-photon emission. Recent years have witnessed significant advancements in condensed matter cQED, i.e., studies of coherent coupling of solids with cavity photons. Because of the giant dipole moments associated with resonances in solids, compared to atomic dipoles, uncharted regimes of strong and ultrastrong coupling are currently explored in various systems, which in turn offer unconventional ways of modifying and controlling material properties with light. Semiconductors – especially quantum semiconductor structures – offer a variety of engineerable platforms for cQED studies due to their highly tunable properties and well-established fabrication techniques. Both high-quality-factor cavities and resonant transitions can be designed and achieved using artificial semiconductor structures. In this dissertation work, we have investigated several cQED phenomena in semiconductor-based cavities containing low-dimensional semiconductor materials. First, we demonstrated the simultaneous ultrastrong coupling of two photonic modes with the cyclotron resonance of a two-dimensional electron gas in GaAs in a three-dimensional photonic-crystal cavity. Second, we designed and simulated a chiral one-dimensional photonic-crystal cavity with broken time-reversal symmetry using magnetoplasmons in lightly doped semiconductors. Finally, we investigated heavy-mass Landau polaritons in a wide-gap semiconductor GaN in the ultrastrong coupling regime. These results highlight the advantages of semiconductor platforms in uncovering novel phenomena and phases in condensed matter systems dressed with cavity photons and developing cavity-based devices for quantum technology.
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    Terahertz Magnetospectroscopy of Lanthanum Strontium Manganite
    (2022-07-25) Tay, Fu Yang; Kono, Junichiro
    La0.875Sr0.125MnO3 is a mixed-valence manganite that exhibits a variety of nonintuitive many-body phenomena, including anomalous metal-insulator transitions and colossal magnetoresistance. To date, the underlying interactions behind the intriguing behaviors of this material have not been well understood. In this thesis work, we have mapped out the detailed frequency, temperature, and magnetic field dependence of the terahertz (THz) complex optical constants of a single crystal of La0.875Sr0.125MnO3. We revealed a series of novel effects, including simultaneous THz colossal magnetoresistance and magnetocapacitance for the first time. Our results suggest that the percolation theory plays an important role in La0.875Sr0.125MnO3 at temperatures above the charge-ordering temperature. Furthermore, our study provides supporting evidence for a new phase transition at low temperatures whose existence has recently been suggested. Finally, we observed an anisotropic collective excitation, possibly due to the charge density wave in the charge-ordered phase, along the b-axis of the crystal. These results provide considerable new insights into the complex interactions among different degrees of freedom – charge, orbital, spin, and lattice – in La0.875Sr0.125MnO3.