Exciton and Phonon Radiative Linewidths in Monolayer Boron Nitride
Exciton and Phonon Radiative Linewidths in Monolayer Boron Nitride
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DOI：
10.1103/physrevx.12.011057
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发表时间：
2022-03-24
影响因子：
12.5
通讯作者：
Novikov, S., V
中科院分区：
文献类型：
--
作者：
Cassabois, G.;Fugallo, G.;Novikov, S., V
块体半导体中的光-物质相互作用处于具有混合本征态的强耦合区域，即所谓的激子极化子和声子极化子。在二维(2D)系统中，平移不变性在垂直于2D系统平面的方向上被破坏。在二维激发下，光-物质相互作用转变为辐射寿命有限的弱耦合区域。量子阱和二维晶体中的二维激子的辐射现象已被广泛研究，但其对应的二维光学声子的辐射现象却从未被研究过。在这里，我们提出了一个平行研究的激子和声子辐射线宽在原子薄层氮化硼(h-BN)，外延生长在石墨。激子共振的反射率实验是在深紫外区进行的，声子共振的反射率实验是在中红外下进行的。定量解释是在转移矩阵方法的框架下推广到单分子膜的情况，其中包括激子或声子性质的Breit-Wigner共振。对于激子，我们发现与其他2D晶体相比有一个巨大的辐射展宽，其值类似于25 meV，这与h-BN中的强激子效应有关。对于声子，我们首次估计了二维声子的辐射线宽，其值类似于单层h-BN中的0.2 meV。我们的结果与第一性原理的计算结果是一致的。我们的研究揭示了二维光学声子辐射态的存在，在基础物理、中红外光谱范围的光电子学应用和先进的热管理方面有着广泛的前景，h-BN正在成为这一新课题中的一个模型系统。
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 light-matter 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 similar to 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 similar to 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.