Theory of phonon-assisted luminescence in solids: Application to hexagonal boron nitride (original) (raw)
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Phonon-assisted emission and absorption of individual color centers in hexagonal boron nitride
2D Materials
Defect centers in hexagonal boron nitride represent room-temperature single-photon sources in a layered van der Waals material. These light emitters appear with a wide range of transition energies ranging over the entire visible spectrum, which renders the identification of the underlying atomic structure challenging. In addition to their eminent properties as quantum light emitters, the coupling to phonons is remarkable. Their photoluminescence exhibits significant side band emission well separated from the zero phonon line (ZPL) and an asymmetric broadening of the ZPL itself. In this combined theoretical and experimental study we show that the phonon side bands can be well described in terms of the coupling to bulk longitudinal optical (LO) phonons. To describe the ZPL asymmetry we show that in addition to the coupling to longitudinal acoustic (LA) phonons also the coupling to local mode oscillations of the defect center with respect to the entire host crystal has to be considered. By studying the influence of the emitter's wave function dimensions on the phonon side bands we find reasonable values for the size of the wave function and the deformation potentials. We perform photoluminescence excitation measurements to demonstrate that the excitation of the emitters is most efficient by LO-phonon assisted absorption. PAPER Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.
Physical Review B, 2007
Cathodoluminescence and photoluminescence spectroscopies have been performed on hexagonal boron nitride powders. The combination of these techniques allows us to analyze the two observed luminescence bands. A deep-level UV emission at about 4 eV is attributed to defects or impurities, and a near-band-gap UV emission is observed at about 5.5 eV. The deep-level band is composed of four peaks, which are attributed to phonon replica due to localized vibrations. In the near-band-gap region, six components are observed between 5.2 and 5.96 eV, in agreement with the recent experiments performed on h-BN single crystals by Watanabe et al. ͓Nat. Mater. 3, 404 ͑2004͔͒, but they are assigned here to Frenkel excitons.
Exciton and Phonon Radiative Linewidths in Monolayer Boron Nitride
Physical Review X, 2022
The light-matter interaction in bulk semiconductors is in the strong-coupling regime with hybrid eigenstates, the so-called exciton polaritons and phonon polaritons. In two-dimensional (2D) systems, the translational invariance is broken in the direction perpendicular to the plane of the 2D system. The lightmatter interaction switches to the weak-coupling regime with a finite radiative lifetime of the matter excitations in 2D. Radiative phenomena have been extensively studied for 2D excitons in quantum wells and 2D crystals, but their counterpart has never been addressed for optical phonons in 2D. Here we present a parallel study of the exciton and phonon radiative linewidths in atomically thin layers of hexagonal boron nitride (h-BN), epitaxially grown on graphite. Reflectivity experiments are performed either in the deep ultraviolet for the excitonic resonance or in the midinfrared for the phononic one. A quantitative interpretation is implemented in the framework of a transfer matrix approach generalized to the case of monolayers with the inclusion of Breit-Wigner resonances of either excitonic or phononic nature. For the exciton we find a giant radiative broadening in comparison to other 2D crystals, with a value of ∼25 meV related to the strong excitonic effects in h-BN. For the phonon we provide the first estimation of the radiative linewidth of a 2D phonon, with a value of ∼0.2 meV in monolayer h-BN. Our results are found to be in good agreement with first-principles calculations. Our study unravels the existence of radiative states for optical phonons in 2D, with numerous perspectives for fundamental physics, optoelectronic applications in the midinfrared spectral range, and advanced thermal management, and h-BN is emerging as a model system in this novel topic.
Nano Letters, 2016
We investigate the distribution and temperature-dependent optical properties of sharp, zero-phonon emission from defect-based single photon sources in multilayer hexagonal boron nitride (h-BN) flakes. We observe sharp emission lines from optically active defects distributed across an energy range that exceeds 500 meV. Spectrally-resolved photon-correlation measurements verify single photon emission, even when multiple emission lines are simultaneously excited within the same h-BN flake. We also present a detailed study of the temperature-dependent linewidth, spectral energy shift, and intensity for two different zero-phonon lines centered at 575 nm and 682 nm, which reveals a nearly identical temperature dependence despite a large difference in transition energy. Our temperature-dependent results are best described by a lattice vibration model that considers piezoelectric coupling to in-plane phonons. Finally, polarization spectroscopy measurements suggest that whereas the 575 !!! nm emission line is directly excited by 532 nm excitation, the 682 nm line is excited indirectly.
LOW-TEMPERATURE 450 nm LUMINESCENCE OF HEXAGONAL BORON NITRIDE
Latvian Journal of Physics and Technical Sciences, 2011
The low-temperature luminescence of bulk hexagonal boron nitride (hBN) powder and boron nitride multiwall nanotubes (BNNTs) was investigated. It was found that, apart from the well-known luminescence at 300 nm and 400 nm, a broad luminescence band at 450 nm appears under low temperatures (8 K), and that heating the sample results in its quenching-at 190 K the luminescence intensity becomes negligible. The 450 nm luminescence is characterized with its own excitation band at 240 nm and is excited also at 270 nm and 350 nm. Besides, this luminescence can be excited through excitonic processes with light around 205 nm. The same features of 450 nm luminescence are observed in both the hBN and BNNTs.
Huge Excitonic Effects in Layered Hexagonal Boron Nitride
Physical Review Letters, 2006
The calculated quasiparticle band structure of bulk hexagonal boron nitride using the all-electron GW approximation shows that this compound is an indirect-band-gap semiconductor. The solution of the Bethe-Salpeter equation for the electron-hole two-particle Green function has been used to compute its optical spectra and the results are found in excellent agreement with available experimental data. A detailed analysis is made for the excitonic structures within the band gap and found that the excitons belong to the Frenkel class and are tightly confined within the layers. The calculated exciton binding energy is much larger than that obtained by Watanabe et al[1] using a Wannier model to interpret their experimental results and assuming that h-BN is a direct-band-gap semiconductor.
Quantum Emission from Defects in Single-Crystalline Hexagonal Boron Nitride
Physical Review Applied, 2016
Bulk hexagonal boron nitride (hBN) is a highly nonlinear natural hyperbolic material that attracts major attention in modern nanophotonics applications. However, studies of its optical properties in the visible part of the spectrum and quantum emitters hosted by bulk hBN have not been reported to date. In this work we study the emission properties of hBN crystals in the red spectral range using sub-bandgap optical excitation. Quantum emission from defects is observed at room temperature and characterized in detail. Our results advance the use of hBN in quantum nanophotonics technologies and enhance our fundamental understanding of its optical properties.
Hexagonal boron nitride: optical properties in the deep ultraviolet
Comptes Rendus. Physique, 2021
We review the recent advance in the understanding of the optoelectronic properties of hexagonal boron nitride (hBN) in the deep ultraviolet. The comparison between bulk hBN and monolayer hBN highlights some of their major differences such the bandgap nature, the excitonic binding energy and the phononassisted broadening of the excitonic lines. Perspectives point out the relevance of addressing the regime of hBN samples made of a very few number of monolayers, including twisted hBN monolayers in the context of twistronics. Résumé. Cette revue est dédiée aux progrès récents dans la compréhension des propriétés optoélectroniques du nitrure de bore hexagonal (hBN) dans l'ultraviolet profond. La comparaison entre un cristal massif de hBN et une monocouche de hBN montre quelques unes de leurs différences majeures telles que la nature de la bande interdite, l'énergie de liaison excitonique ainsi que l'élargissement des raies excitoniques assisté par phonons. Les perspectives mettent en avant la nécessité d'étudier le régime d'échantillons composés de quelques monocouches de hBN, y compris des monocouches tournées dans le contexte actuel de la twistronique.