Point Defects in h-BN as efficient UV quantum emitters
Single photons sources (SPS) play a central role in the experimental foundation of quantum computing. Currently, there is a large scientific effort in identifying new bright and stable single-photon emission sources, with the aim of extending the spectral range achievable by quantum emitters. In the last years low dimensional layered semiconductors appeared as new promising optical materials and low energy SPS have been identified in transition metal dichalcogenides. In this work, thanks to a newly developed optical set-up integrated in a scanning transmission electron microscope, we have identified a new very bright UV single photon emitter in hexagonal boron nitride.
The characteristic size of SPS is in the nanometer range for quantum dots, and even smaller for point defects in semiconductors. Therefore, the quest for new quantum emitters faces the difficulty of combining an efficient excitation/detection optical setup with the capability of addressing individual color centers in potentially highly defective materials. This problem can in principle be overcome by illuminating the system with a fast electron beam, which can be focused in a subwavelength spot size. The LPS electron microscopy group has developed an original cathodoluminescence system integrated into a scanning transmission electron microscope (STEM) which allows a spatial selectivity as high as a few nanometers while providing a relatively easy access to a wide spectral range from the infrared to the ultraviolet. The CL signal can then be guided into a light intensity Hanbury Brown and Twiss (HBT) interferometer. The potential nonclassical nature of highly localized emissions can be easily demonstrated by the appearance of an antibunching signature in the second-order correlation function.
Using this experimental set up we have obtained nanometric resolved hyperspectral cathodoluminescence maps of few layers h-BN flakes. We observed very high spatially localized spots (~80 nm) in correspondence to a bright 4.1 eV emission presenting a typical zero-phonon line plus phonon replica spectroscopic signature. Thanks to the high spatial resolution of the STEM microscope, we were able to illuminate an individual spot while acquiring the CL second order correlation function. The appearance of an antibunching dip indicates clearly the presence of a single-photon source associated with point defects in the flakes. An additional non-single-photon broad band, spatially localized but not correlated with the SPE emission, can also be observed in the same energy range. The intensity of this broad band increases upon electron irradiation and therefore vacancies and interstitial defects most probably play a role in it.
These results shown how an innovative approach combining STEM cathodoluminescence to HBT interferometry can be extremely effective in the quest for new SPEs in emerging optical materials.