Emission Dynamics of Hexagonal-Boron Nitride (hBN) Single Photon Emitters (SPEs) Pumped by High Power Density Laser Excitation

Color centers in thin hBN crystals are promising candidate single photon emitters (SPEs), as they exhibit bright single photon emission at room temperature. We find that at high pump power densities (~70 kW/cm² at 532 nm wavelength), single photon emitting defects in hBN exhibit unstable emission around 600 nm wavelength under pulsed or continuous wave lasers. We observe both blinking (switching between a state of radiative emission and no radiative emission on long time scales), and long-term irreversible degradation of the SPE character of individual defects. To characterize the SPE at a given power, we scanned over the area containing the color center (~ 9 μm²) with a confocal luminescence microscope to measure the emission of the color center at each wavelength. This measurement produces both a visible emission spectrum for each scan point, as well as the emission integrated over wavelength for each point, which can be arranged in a photoluminescence (PL) map indicating how total emission changes spatially. We found, the full width half maximum (FWHM) at the zero-phonon line (ZPL) increases from 27 nm to over 88 nm when the power is increased from 25 kW/cm² to 165 kW/cm² at 532 nm. This broadening coincides with a transition from photon anti-bunching to bunching, quantified with second-order correlation (g²) measurements. The PL maps also show that under increased pump powers the radius of strong emission increases over 3-fold from 0.4 μm to 1.25 μm. This implies that at high pump powers, the color centers evolve from localized SPEs to classically emissive defects, capable of transporting the excited charge on the order of microns prior to radiative emission. Further, increasing the power causes a blue shift in the emission wavelength of the photons, indicating modification of the radiative emission pathway. We also observe a self-recovery of radiative emission over extended (336 hour) times. Fast, self-recovery of emitters, suggests there may be a relation between the blinking and apparent irreversible degradation, with blinking occurring over minutes or hours and degradation occurring over days or weeks. The observed self-recovery phenomenon points to the dynamics of photoexcited trapped charges in emission. This hypothesis will be further addressed by Kelvin probe force microscopy to reveal the spatial profile and role of trapped charges on hBN single photon emitter electrostatic charging and discharging.