Characterization and Tuning of Quantum Emitters in Hexagonal Boron Nitride

Citation

Akbari, Hamidreza (2024) Characterization and Tuning of Quantum Emitters in Hexagonal Boron Nitride. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/qz1v-3696. https://resolver.caltech.edu/CaltechTHESIS:02262024-184235077

Abstract

Hexagonal boron nitride (h-BN) is a two-dimensional material hosting atomic defects that serve as single-photon emitters, attributed to its large bandgap. Its high stability at room temperature, substantial Debye-Waller factor, and integrability into 2D devices make h-BN a compelling choice for quantum applications involving single-photon emitters.

Initially, we investigate the properties of emitters in h-BN to comprehend the limitations of their spectral linewidth. This study includes examining the effects of the host crystal's growth method, the emitter's environment (the substrate), and temperature. As a result, we identify two primary broadening regimes: thermal broadening and spectral diffusion. Secondly, we address spectral diffusion, the predominant broadening mechanism at cryogenic temperatures, which depends on local electrical charges near the emitter. We propose a device structure comprising graphene - emitter h-BN - buffer h-BN - graphene, designed to apply a DC electric field and suppress spectral diffusion. This approach leads to a dramatic two orders of magnitude reduction in linewidth, achieving Fourier transform-limited linewidth.

Moreover, we explored the 3D dipole orientation and axial location of emitters within an h-BN crystal slab by coupling them to a phase change material. We discovered that the dipole orientation of some emitters is predominantly out-of-plane, and these emitters tend to exist close to the crystal's surfaces. This insight aids in the quest to determine the atomic structure of the emitters.

Finally, we examine the photon statistics of single-photon beams generated by h-BN emitters. We demonstrate that these beams exhibit sub-Poissonian statistics with both pulsed and continuous-wave excitation. Our findings reveal that excitation power can serve as a control to alter photon statistics, and we utilize this dependency to illustrate how photon statistics influence the use of quantum emitters in quantum random number generation applications.

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