  We report a comprehensive study on the optical properties of cubic boron
nitride (c-BN) and its optically active defects. Using electron energy-loss
spectroscopy (EELS) within a monochromated scanning transmission electron
microscope (STEM) on the highest-quality crystals available, we demonstrate
unequivocally that the optical-gap energy of c-BN slightly exceeds 10 eV.
Further theoretical analysis in the framework of the Bethe-Salpeter equation of
many-body perturbation theory supports this result. The spatial localization of
defect-related emissions has been investigated using nanometric resolved
cathodoluminescence (nano-CL) in a STEM. By high-temperature annealing a c-BN
powder, we have promoted phase transitions in nanometric domains which have
been detected by the appearance of specific hexagonal-phase signatures in both
EELS and CL spectra. A high number of intragap optically active centers are
known in c-BN, but the literature is rather scattered and hence has been
summarized here. For several emission lines we have obtained nano-CL maps which
show emission spot sizes as small as few tens of nanometers. Finally, by
coupling nano-CL to a Hanbury-Brown-Twiss intensity interferometer, we have
addressed individual spots in order to identify the possible presence of
single-photon sources. The observed CL bunching effect is compatible with a
limited set of single-photon emitters and it permits obtaining emission
lifetimes of the order of the nanosecond.
