We present a Raman scattering study of optical phonons in hexagonal BN for temperatures ranging from80 to 600 K. The experiments were performed on high-quality, single-crystalline hexagonal BN platelets. Theobserved temperature dependence of the frequencies and linewidths of both Raman active E2g optical phonons isanalyzed in the framework of anharmonic decay theory, and possible decay channels are discussed in the light ofdensity-functional theory calculations. With increasing temperature, the Ehigh mode displays strong anharmonic 2ginteractions, with a linewidth increase that indicates an important contribution of four-phonon processes and amarked frequency downshift that can be attributed to a substantial effect of the four-phonon scattering processes(quartic anharmonicity). In contrast, the Elow mode displays a very narrow linewidth and weak anharmonic 2ginteractions, with a frequency downshift that is primarily accounted for by the thermal expansion of the interlayer spacing.

Among a variety of layered materials used as building blocks in van der Waals heterostructures, hexagonal boron nitride (hBN) appears as an ideal platform for hosting optically-active defects owing to its large bandgap ($\sim 6$~eV). Here we study the optical response of a high-purity hBN crystal under green laser illumination. By means of photon correlation measurements, we identify individual defects emitting a highly photostable fluorescence under ambient conditions. A detailed analysis of the photophysical properties reveals a high quantum efficiency of the radiative transition, leading to a single photon source with very high brightness ($\sim 4\times 10^6$ counts.s$^{-1}$). These results illustrate how the wide range of applications offered by hBN could be further extended to photonic-based quantum information science and metrology.

We report on the ultraviolet optical response of a color center in hexagonal boron nitride. We demonstrate a mapping between the vibronic spectrum of the color center and the phonon dispersion in hexagonal boron nitride, with a striking suppression of the phonon assisted emission signal at the energy of the phonon gap. By means of nonperturbative calculations of the electron-phonon interaction in a strongly anisotropic phonon dispersion, we reach a quantitative interpretation of the acoustic phonon sidebands from cryogenic temperatures up to room temperature. Our analysis provides an original method for estimating the spatial extension of the electronic wave function in a point defect.

I will discuss our recent experiments by two-photon spectroscopy demonstrating that hBN is an indirect bandgap material. I will show that the optical properties of hBN are profoundly determined by phonon-assisted transitions involving either virtual or real excitonic states. I will present our measurements of the exciton binding energy by two-photon excitation, leading to the estimation of 6.08 eV for the single-particle bandgap in hBN. I will also discuss our experimental evidence that transverse optical phonons at the K point of the Brillouin zone assist inter-K valley scattering in hexagonal boron nitride.

I will discuss our recent studies showing that the optical response in hBN displays prominent evidence for phonon-assisted optical transitions. First of all, by two-photon spectroscopy, we have demonstrated that the intrinsic optical properties at the band edge are characteristic of an indirect bandgap material. I will further discuss our experimental evidence that transverse optical phonons at the K point of the Brillouin zone assist inter-K valley scattering in hexagonal boron nitride, thanks to the presence of a density of final electronic states coming from extended stacking faults. Finally, I will present our measurements of the vibronic spectrum in a point defect in hBN, displaying a remarkable mapping with the phonon density of states, and in particular a suppression of the phonon-assisted recombination signal at the phonon gap energy.

I will review our recent experiments revisiting the optical properties of G centers in silicon.

I will discuss our recent experiments by two-photon spectroscopy demonstrating that hBN is an indirect bandgap material. I will show that the optical properties of hBN are profoundly determined by phonon-assisted transitions involving either virtual or real excitonic states. I will present our measurements of the exciton binding energy by two-photon excitation, leading to the estimation of 6.08 eV for the single-particle bandgap in hBN. I will also show that transverse optical phonons at the K point of the Brillouin zone assist intervalley scattering, which becomes observable because stacking faults in bulk hexagonal boron nitride provide a density of final electronic states.