Optoelectronic control of light-matter interactions in TMDCs using plasmonic waveguides

To circumvent the heat management issues currently inhibiting further downscaling of information processing technologies, optical interconnects have attracted huge scientific attention. Whereas conventional photonic devices have a size ultimately limited by diffraction, plasmonics promises to open the way towards deep-subwavelength optical confinement. For the building blocks of optical interconnects, such as integrated light sources, routing elements and optical detectors, passive plasmonic structures have to be interfaced with optically active materials. In this respect, transition metal dichalcogenides (TMDCs) have caught the attention of many research groups worldwide due to their extraordinary optical, electrical and mechanical properties. The observation of single-photon emitters in these materials demonstrates the possibility of reaching the quantum limit in terms of energy-per-bit in optical interconnects. In this thesis, we demonstrate the feasibility of achieving the fundamental building blocks for on-chip optical interconnects by combining deterministically fabricated plasmonic slot waveguides with TMDC monolayers. In a series of proof of concept experiments, we report on increased light-matter interaction facilitating a two-fold increase in luminescence intensity of molybdenum diselenide (MoSe2) coupled to a plasmonic waveguide. The coupling of TMDC photoluminescence to propagating plasmonic modes with propagation lengths of (380 ± 60) nm is demonstrated in deep-subwavelength confined systems with a transverse mode volume of 0.02 λ². Polarization resolved measurements and complimentary numerical calculations are in good agreement supporting our conclusion that we observe the coupling between the free excitons and bound plasmonic modes. In a next step, we use the topography of plasmonic waveguides to enhance the formation probability of emitters in tungsten diselenide (WSe2) by a factor of (3.3 ± 0.7) in the immediate vicinity ≤ 0.5µm of the waveguides. We characterize the formed emitters and demonstrate sub-Poissonian photon statistics yielding a second-order autocorrelation value of g(2)(0) = 0.27. By performing spatially and spectrally resolved experiments, we prove the coupling of these emitters to single propagating surface plasmon polaritons. Time-resolved studies reveal increased light-matter interaction with Purcell factors of up to (15 ± 3) times. Furthermore, we employ focused ion-beam fabrication methods to obtain more control over emitter formation yielding enhancements by a factor of (10 ± 1) close to the waveguides and we reach enhancements of up to ~ 30 times when applying spectral filtering. To address the photodetection component, we electrically contact individual waveguides and extract photocurrents from MoSe2 monolayers with responsivity up to 18 A/W. From spectrally resolved measurements, we can identify the photocurrent generation with the excitation of neutral excitons in the monolayer. We demonstrate the electrical detection of propagating surface-plasmon polaritons yielding a power linearity of 86% and a device footprint of 0.5µm². Additional numerical simulations promise detection areas of < 0.1µm² for detectors based on TMDC multilayers. Our findings demonstrate proof-of-concept components based on plasmonic waveguides and TMDC materials suitable for optical interconnects with nanoscale device footprint.