  We present comprehensive optical studies of recently discovered single photon
sources in boron nitride, which appear in form of narrow lines emitting
centres. Here, we aim to compactly characterise their basic optical properties,
including the demonstration of several novel findings, in order to inspire
discussion about their origin and utility. Initial inspection reveals the
presence of narrow emission lines in boron nitride powder and exfoliated flakes
of hexagonal boron nitride deposited on Si/SiO2 substrates. Generally rather
stable, the boron nitride emitters constitute a good quality visible light
source. However, as briefly discussed, certain specimens reveal a peculiar type
of blinking effects, which are likely related to existence of meta-stable
electronic states. More advanced characterisation of representative stable
emitting centres uncovers a strong dependence of the emission intensity on the
energy and polarisation of excitation. On this basis, we speculate that rather
strict excitation selectivity is an important factor determining the character
of the emission spectra, which allows the observation of single and
well-isolated emitters. Finally, we investigate the properties of the emitting
centres in varying external conditions. Quite surprisingly, it is found that
the application of a magnetic field introduces no change in the emission
spectra of boron nitride emitters. Further analysis of the impact of
temperature on the emission spectra and the features seen in second-order
correlation functions is used to provide an assessment of the potential
functionality of boron nitride emitters as single photon sources capable of
room temperature operation.
