Light-matter coupling in low-dimensional semiconductors and fiber-based microcavities

Husel, Lukas (2025): Light-matter coupling in low-dimensional semiconductors and fiber-based microcavities. Dissertation, LMU München: Fakultät für Physik

Abstract

The coupling between photons and excitons, bound electron-hole pairs in semiconductors, is of central interest in solid-state based quantum science and technology. Many applications and experiments require enhanced light-matter interactions, frequently achieved in optical resonators with microscopic mode volumes. A promising platform to emerge from recent miniaturization efforts are open fiber-based Fabry-Perot cavities, in which one resonator mirror is machined on the tip of an optical fiber. This dissertation reports on the coupling of such cavities to two different low dimensional semiconductor platforms, where the confinement of charge carriers gives rise to strongly-bound excitons. Monolayer transition metal dichalcogenides are two-dimensional semiconductors, whose large exciton oscillator strength renders them prime candidates for studies of coherent light-matter coupling in optical resonators. In this regime, exciton-polaritons are formed, hybrid light-matter quantum states with promise for experiments and devices in solid state nonlinear optics and quantum simulation. In many applications, local control of polariton energies is required, forming the basis for engineered potential landscapes. In this dissertation, we locally control polaritons formed by transition metal dichalcogenide excitons coupled to a cryogenic fiber cavity via their excitonic fraction. By nanostructuring the dielectric semiconductor environment, we define disk-shaped areas of excitons with modified resonance energies. The local modulation in exciton energy translates to shifts in the polariton energy, tunable in magnitude via the cavity-exciton detuning. We also demonstrate the dispersive regime of cavity-coupling, where an effective hopping is mediated between highly exciton-like polaritons localized to different disk-shaped areas in the device. Fluorescent defects in functionalized carbon nanotubes constitute zero-dimensional quantum systems capable of emitting room-temperature single photons at technologically relevant telecom wavelengths. In these emitters, large room-temperature dephasing limits the photon indistinguishability, an important resource in the design of single photon sources. In this work, we implement a recently proposed strategy to overcome this limitation by coupling individual nanotube defects to a room-temperature fiber cavity operated in the incoherent good cavity regime. Here, the spectral linewidth of the emitter greatly exceeds that of the cavity, resulting in a drastic spectral purification of single photons emitted from the cavity. Our experimental results indicate a corresponding increase of the photon indistinguishability by two orders of magnitude compared to free-space emission, accompanied by an enhancement of the emission spectral density by at least a factor of four. The results of this dissertation provide perspectives for the design of polaritonic devices based on dielectrically tailored transition metal dichalcogenides, as well as improved sources of quantum light built on functionalized carbon nanotubes. The corresponding experiments are performed in different regimes of cavity-coupling, thus highlighting the versatility of fiber-based cavities for solid-state based studies of light-matter coupling.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: nanophotonics, low-dimensional semiconductors, carbon nanotubes, transition metal dichalcogenides, fiber cavities
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 14. März 2025
• 1. Bericht­erstatter:in: Högele, Alexander