Quantum emission from hexagonal boron nitride
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Realization of quantum technologies demands successful assembly of crucial building blocks. Quantum light sources, lying at the heart of this architecture, have attracted a great deal of research focus during the last several decades. Optically active defect-based centers in wide bandgap materials such as diamond and silicon carbide have been proven to be excellent candidates due to their high brightness and photostability. Integration of quantum emitters on an on-chip integrated circuit, however, favors low dimensionality of the host materials. Single photon sources embedded in two-dimensional lattices are, therefore, highly desired. In this thesis, we introduce a class of novel quantum systems hosted in hexagonal boron nitride (hBN) – a wide bandgap semiconductor in the two-dimensional (2D) limit. First, we demonstrate experimentally that the quantum systems possess a record high single photon count rate, exceeding 4 MHz at room temperature. Polarization and time-resolved spectroscopy reveal their full emission polarization and short excited state lifetime (~3 ns). Besides, the emitters from this class of quantum system also show extremely high stability under high excitation at ambient conditions. By employing spin-resolved density functional theory (DFT) calculation, we suggest that the defect center is an antisite nitrogen vacancy (NʙVɴ). A multicolor phenomenon where there is a wide distribution of zero-phonon lines (ZPL) from different emitters is also observed and can be attributed to strain field in the hBN lattice thanks to DFT calculation. Additionally, we demonstrate the ability to create the emitters by means of thermal treatment or electron beam induced etching. Under harsh environments, strikingly, most of the emitters survive and preserve their quantum properties. Resonant excitation spectroscopy reveals a linewidth of ~6 GHz, and a high single photon purity confirmed from an emitter by on-resonance antibunching measurements. Studies on bulk hBN crystals reveal that the emitters tend to locate at dislocations or stacking faults in the crystals. We also demonstrate ion implantation and laser ablation as means of increasing formation yield of the emitters in mechanically exfoliated hBN flakes. Next, the coupling of quantum emitters in hBN to plasmonic particles arrays is demonstrated, showing several times Purcell enhancement factor. Lastly, we show that another 2D material - tungsten disulfide (WS₂) – when being oxidized also hosts quantum emitters at room temperature. This observation, therefore, opens a new avenue for studying quantum emitters embedded in other 2D materials.
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