Near band gap photoluminescence (PL) of hBN single crystal has been studied at cryogenic temperatures with synchrotron radiation excitation. The PL signal is dominated by the D-series previously assigned to excitons trapped on structural defects. A much weaker S-series of self-trapped excitons at 5.778 eV and 5.804 eV has been observed using time-window PL technique. The S-series excitation spectrum shows a strong peak at 6.02 eV, assigned to free exciton absorption. Complementary photoconductivity and PL measurements set the band gap transition energy to 6.4 eV and the Frenkel exciton binding energy larger than 380 meV.
Excitons in van der Waals (vdW) stacking interfaces can be trapped in ordered moire potential arrays giving rise to attractive phenomenons of quantum optics and bosonic many-body effects. Compare to the prevalent transition metal dichalcogenides (TMDs) systems, due to the wide bandgap and low dielectric constant, excitons in twist-stacked hexagonal boron nitride (hBN) are anticipated trapped in deeper moire potential, which enhances the strength of interactions. However, constrained by the common low detectivity of weak light-emitting in the deep-ultraviolet (DUV) bands, the moire excitons in …
Using a new time-resolved cathodoluminescence system dedicated to the UV spectral range, we present a first estimate of the radiative lifetime of free excitons in hBN at room temperature. This is carried out from a single experiment giving both the absolute luminescence intensity under continuous excitation and the decay time of free excitons in the time domain. The radiative lifetime of indirect excitons in hBN is equal to 27 ns, which is much shorter than in other indirect bandgap semiconductors. This is explained by the close proximity of the electron and the hole in the exciton complex, an…
Microcavity exciton-polaritons are two-dimensional bosonic quasiparticles composed by excitons and photons. Using model Hamiltonian with parameters generated from ab initio density-functional theory and Bathe-Salpeter Equation calculations, we investigate the exciton and the exciton-polariton properties of hexagnonal boron nitride (hBN) based microcavity. We show that hBN based microcavities, including monolayer and all-dielectric ones, are promising in optoelectronic applications. Room temperature exciton-polariton Bose-Einstein Condensation can be achieved because of the large oscillating st…
Hexagonal boron nitride (hBN) is a wide band gap material with both strong excitonic light emission in the ultraviolet and strong exciton-phonon coupling. Luminescence experiments performed on the recently synthesized monolayer form (m-hBN) present emission spectra that differ from one another, with some suggesting a coexistence between phonon-assisted and direct emission channels. Motivated by these results, we investigated the optical response of (m-hBN) using a new \textit{ab initio} approach that takes into account the effects of atomic vibrations on the luminescence spectra. We construct …
Excitons in van der Waals materials: from monolayer to bulk hexagonal boron nitride
我们提出了一个一般的图片的激子性质的层状材料的单层积木的激发。为此，我们推导出一个模型激子哈密顿绘制一个类比与分子晶体，这是其他原型范德华材料。我们采用这个简化模型详细分析了六方氮化硼（hBN）的激发光谱，我们已经从多体Bethe-Salpeter方程的{ \it ab initio}解作为动量的函数得到。通过这种方法，我们确定了hBN中最低能量激子的性质，讨论了层间跳跃和电子-空穴交换相互作用对激子色散的影响，阐明了层状材料中激子与等离子体激元之间的关系。
Interlayer excitons (IXs) possess a much longer lifetime than intralayer excitons due to the spatial separation of the electrons and holes; hence, they have been pursued to create exciton condensates for decades. The recent emergence of two-dimensional (2D) materials, such as transition metal dichalcogenides (TMDs), and of their van der Waals heterostructures (HSs), in which two different 2D materials are layered together, has created new opportunities to study IXs. Here we present the observation of IX gases within two stacked structures consisting of hBN/WSe$_2$/hBN/p: WSe$_2$/hBN. The IX en…
In the wide world of 2D materials, hexagonal boron nitride (hBN) holds a special place due to its excellent characteristics. In addition to its thermal, chemical and mechanical stability, hBN demonstrates high thermal conductivity, low compressibility, and wide band gap around 6 eV, making it promising candidate for many groundbreaking applications and more specifically for optoelectronic devices. Millimeters scale hexagonal boron nitride crystals are obtained through a disruptive dual method (PDC/PCS) consisting in a complementary coupling of the Polymer Derived Ceramics route and a Pressure-…
The dielectric environment of atomically-thin monolayer (ML) of semiconducting transition metal dichalcogenides affects both the electronic band gap and the excitonic binding energy in the ML. We investigate the effect of the environment on the in-plane magnetic field brightening of neutral and charged dark exciton emissions in the WSe$_2$ ML. The monolayers placed in three dielectric environments are studied, in particular, the ML encapsulated in hexagonal BN (hBN) flakes, the ML deposited on a hBN layer, and the ML embedded between the hBN flake and SiO$_2$/Si substrate. We observe that the …
Transition metal dichalcogenides (TMDs) are materials that can exhibit intriguing optical properties like a change of the bandgap from indirect to direct when being thinned down to a monolayer. Well-resolved narrow excitonic resonances can be observed for such monolayers, however only for materials of sufficient crystalline quality, so far mostly available in the form of micrometer-sized flakes. A further significant improvement of optical and electrical properties can be achieved by transferring the TMD on hexagonal boron nitride (hBN). To exploit the full potential of TMDs in future applicat…
Monolayer and few-layer phosphorene are anisotropic quasi-two-dimensional (quasi-2D) van der Waals (vdW) semiconductors with a linear-dichroic light-matter interaction and a widely-tunable direct-band gap in the infrared frequency range. Despite recent theoretical predictions of strongly-bound excitons with unique properties, it remains experimentally challenging to probe the excitonic quasiparticles due to the severe oxidation during device fabrication. In this study, we report observation of strongly-bound excitons and trions with highly-anisotropic optical properties in intrinsic bilayer ph…
A phonoriton is an elementary excitation that is predicted to emerge from hybridization between exciton, phonon and photon. Besides the intriguing many-particle structure, phonoritons are of interest as they could serve as functional nodes in devices that utilize electronic, phononic and photonic elements for energy conversion and thermal transport applications. Although phonoritons are predicted to emerge in an excitonic medium under intense electromagnetic wave irradiation, the stringent condition for their existence has eluded direct observation in solids. In particular, on-resonance, inten…
$WSe_2$ Monolayers Grown by Molecular Beam Epitaxy on hBN
Institute of Experimental Physics, Faculty of Physics, University of Warsaw（华沙大学物理学院实验物理研究所）；Institute of Physics, Polish Academy of Sciences（波兰科学院物理研究所）；Research Center for Materials Nanoarchitectonics, National Institute for Materials Science（国家材料科学研究所材料纳米构造研究中心）；Research Center for Electronic and Optical Materials, National Institute for Materials Science（国家材料科学研究所电子与光学材料研究中心）
Optical spectroscopy of excited exciton states in MoS2 monolayers in van der Waals heterostructures
MoS 2单分子膜的光学性质主要由激子，但对于直接剥离到SiO2衬底上的单分子膜中的光谱宽的光学跃迁，激发激子状态的详细信息是不可访问的。六方氮化硼（hBN）的封装可以接近均匀的激子线宽，但范德华异质结构中的干扰使得直接比较具有不同振子强度的光谱中的跃迁更具挑战性。在这里，我们揭示了反射率和光致发光激发光谱中的A-激子的激发态在封装在hBN层的校准厚度的二硫化钼单层的存在下，允许外推激子结合能约220毫电子伏。我们从理论上再现的能量分离和振子强度测量的反射率相结合的激子共振计算的屏蔽二维库仑势与转移矩阵计算的范德华结构的反射率。我们的分析表明，与理想的二维氢模型相比，屏蔽库仑势的激子振子强度与主量子数的演变非常不同。
Many linked processes occur concurrently in strongly excited semiconductors, such as interband and intraband absorption, scattering of electrons and holes by the heated lattice, Pauli blocking, bandgap renormalization and the formation of Mahan excitons. In this work, we disentangle their dynamics and contributions to the optical response of a ZnO thin film. Using broadband pump-probe ellipsometry, we can directly and unambiguously obtain the real and imaginary part of the transient dielectric function which we compare with first-principles simulations. We find interband and excitonic absorpti…
Hyperspectral imaging at cryogenic temperatures is used to investigate exciton and trion propagation in MoSe$_2$ monolayers encapsulated with hexagonal boron nitride (hBN). Under a tightly focused, continuous-wave laser excitation, the spatial distribution of neutral excitons and charged trions strongly differ at high excitation densities. Remarkably, in this regime the trion distribution develops a halo shape, similar to that previously observed in WS2 monolayers at room temperature and under pulsed excitation. In contrast, the exciton distribution only presents a moderate broadening without …
It is shown that the three-fold rotational symmetry in transition metal dichalcogenides leads to a Coulomb induced renormalization of the effective electron and hole masses near the $K$-points of the Brillouin zone. The magnitude of the renormalization depends on the dielectric configuration. The effective exciton mass $m=0.4 m_0$ of a freely suspended MoS$_2$ monolayer changes to $m= 0.35 m_0$ with hBN encapsulation. The mass renormalization increases the excitonic binding energy and reduces the exciton diamagnetic shift and cyclotron frequency. Detailed comparisons with high field measuremen…
Integration of 2D Materials in Radial van der Waals Heterostructure Metasurfaces
Ludwig-Maximilians-Universität München（慕尼黑大学）；Politecnico di Milano（米兰理工大学）；Universidade Federal de Pernambuco（伯南布哥联邦大学）；IFN-CNR, Istituto di Fotonica e Nanotecnologie（意大利国家研究委员会光子学与纳米技术研究所）
Monolayer transition metal dichalcogenide (TMD) semiconductors such as MoSe$_2$ host strongly bound excitons which are known to exhibit a strong resonant third-order nonlinear response. Although there have been numerous studies of the ultrafast nonlinear response of monolayer TMDs, a study of the steady-state nonlinear response is lacking. We report a comprehensive study of the steady-state two-color nonlinear response of excitons in hBN-encapsulated monolayer MoSe$_2$ at 7 K. We observe differential transmission (DT) signals associated with the neutral and charged exciton species, which is st…
The propagation of Dirac fermions in graphene through a long-period periodic potential would result in a band folding together with the emergence of a series of cloned Dirac points (DPs). In highly aligned graphene/hexagonal boron nitride (G/hBN) heterostructures, the lattice mismatch between the two atomic crystals generates a unique kind of periodic structure known as a moiré superlattice. Of particular interests is the emergent phenomena related to the reconstructed band-structure of graphene, such as the Hofstadter butterfly, topological currents, gate dependent pseudospin mixing, and ball…
