Exciton-phonon interaction in the strong-coupling regime in hexagonal boron nitride

Abstract

The temperature-dependent optical response of excitons in semiconductors is controlled by the exciton-phonon interaction. When the exciton-lattice coupling is weak, the excitonic line has a Lorentzian profile resulting from motional narrowing, with a width increasing linearly with the lattice temperature T . In contrast, when the exciton-lattice coupling is strong, the lineshape is Gaussian with a width increasing sublinearly with the lattice temperature, proportional to √ T . While the former case is commonly reported in the literature, here the latter is reported for the first time, for hexagonal boron nitride. Thus the theoretical predictions of Toyozawa [Progr. Theor. Phys. 20, 53 (1958)] are supported by demonstrating that the exciton-phonon interaction is in the strong coupling regime in this Van der Waals crystal.

Isotope engineering of van der Waals interactions in hexagonal boron nitride

“…However, the B 1g interlayer breathing mode contributes to the thermally-assisted broadening of the emission spectrum in hBN, as recently demonstrated in [40]. Above 70K, absorption of such phonons is in fact the dominant broadening process so that the study of the temperature-dependent linewidth in PL spectroscopy provides an accurate measure of the exciton-phonon coupling for this specific type of lattice vibrations.…”

Phonon wind and drag of excitons in monolayer semiconductors

“…(35) and the effective force due to the Seebeck effect caused by the exciton temperature gradient, second term in Eq. (27), see also second term in the brackets in Eq. (31).…”

“…It results in the enhancement of the phonon density of states. This together with the fact that the exciton radii in these monolayer materials are quite small results in rather strong the exciton-phonon interaction in two-dimensional materials [26][27][28][29][30][31][32][33].…”

Rotationally aligned hexagonal boron nitride on sapphire by high-temperature molecular beam epitaxy

“…Additionally, hBN has been demonstrated as a promising material for ultraviolet light emission [8,9]. Despite the indirect character of its bandgap, hBN exhibits highly efficient radiative recombination through phononmediated transitions due to strong electron-phonon interactions [6,7,[10][11][12][13]. It has also recently been shown to host point defects that function as bright, room temperature single photon sources, which can enable technologies such as quantum cryptography and precision sensing [14,15].…”

“…It has also recently been shown to host point defects that function as bright, room temperature single photon sources, which can enable technologies such as quantum cryptography and precision sensing [14,15]. On top of these potential applications, the strong electron-phonon coupling and hyperbolic dispersion of hBN make it an exciting platform to study rich new physical phenomena such as phonon-polaritons and the effects of isotopes [12,[16][17][18][19][20].…”