Color centers in hexagonal boron nitride (hBN) show stable single photon emission even at room temperature, making these systems a promising candidate for quantum information applications. Besides this remarkable property, also their interaction with longitudinal optical (LO) phonons is quite unique because they lead to dominant phonon sidebands (PSBs), well separated from the zero phonon line (ZPL). In this work we utilize this clear spectral separation to theoretically investigate the influence of phonon decay dynamics on time-dependent photoluminescence (PL) signals. Our simulations show, that by using tailored optical excitation schemes it is possible to create a superposition between the two LO modes, leading to a phonon quantum beat that manifests in the time-dependent PL signal.
Monolayer transition metal dichalcogenides(2D) and zero dimensional quantum dots(QD) are known to have unique optical properties in their individual limit such as high binding energy of excitons. The combination of these two systems is of particular interest in understanding various aspects of energy transfer, charge transfer, dynamics of excitons, etc. In this manuscript, we report the anomalous photoluminescence(PL) emission in one such heterostructure MoS2-CdSe QD. We observe multiple exciton emission peaks of the heterostructure on hBN substrate which are absent on SiO2. Our observation op…
Atomically thin materials, like semiconducting transition metal dichalcogenides, are highly sensitive to the environment. This opens up an opportunity to externally control their properties by changing their surroundings. In this work, high-quality van der Waals heterostructures assembled from hBN-encapsulated monolayer MoS$_2$ are studied with the aid of photoluminescence, photoluminescence excitation, and reflectance contrast experiments. We demonstrate that carrier concentration in MoS$_2$ monolayers, arising from charge transfer from impurities in the substrate, can be significantly tuned …
We present magneto-photoluminescence measurements in a hybrid 2D semiconductor/ferromagnetic structure consisting of MoSe2/hBN/Ni. When the Nickel layer is magnetized, we observe circularly polarized photoluminescence of the trion peak in MoSe2 monolayer under linearly polarized excitation. This build-up of circular polarization can reach a measured value of about 4% when the magnetization of Ni is saturated perpendicularly to the sample plane, and changes its sign when the magnetization is reversed. The circular polarization decreases when the hBN barrier thickness increases. These results ar…
The recently discovered spin defects in hexagonal boron nitride (hBN), a layered van der Waals material, have great potential in quantum sensing. However, the photoluminescence and the contrast of the optically detected magnetic resonance (ODMR) of hBN spin defects are relatively low so far, which limits their sensitivity. Here we report a record-high ODMR contrast of 46$\%$ at room temperature, and simultaneous enhancement of the photoluminescence of hBN spin defects by up to 17-fold by the surface plasmon of a gold-film microwave waveguide. Our results are obtained with shallow boron vacancy…
Heterostructures made from van der Waals materials provide a template to investigate proximity effects at atomically sharp heterointerfaces. In particular, near-field charge and energy transfer in heterostructures made from semiconducting transition metal dichalcogenides (TMD) have attracted interest to design model 2D "donor-acceptor" systems and new optoelectronic components. Here, using of Raman scattering and photoluminescence spectroscopies, we report a comprehensive characterization of a molybedenum diselenide (MoSe$_2$) monolayer deposited onto hexagonal boron nitride (hBN) and capped b…
Spin defects like the negatively charged boron vacancy color center ($V_B^-$) in hexagonal boron nitride (hBN) may enable new forms of quantum sensing with near-surface defects in layered van der Waals heterostructures. Here, we reveal the effect of strain associated with creases in hBN flakes on $V_B^-$ and $V_B$ color centers in hBN with correlative cathodoluminescence and photoluminescence microscopies. We observe strong localized enhancement and redshifting of the $V_B^-$ luminescence at creases, consistent with density functional theory calculations showing $V_B^-$ migration toward region…
Optical properties of atomically thin transition metal dichalcogenides are controlled by robust excitons characterized by a very large oscillator strength. Encapsulation of monolayers such as MoSe$_2$ in hexagonal boron nitride (hBN) yields narrow optical transitions approaching the homogenous exciton linewidth. We demonstrate that the exciton radiative rate in these van der Waals heterostructures can be tailored by a simple change of the hBN encapsulation layer thickness as a consequence of the Purcell effect. The time-resolved photoluminescence measurements together with cw reflectivity and …
By selective doping (Be) of the well and barrier regions of p-GaAs/AlGaAs structures we have realized the situation where the upper Hubbard band (A+ centers) has been occupied by holes in the equilibrium. We studied temperature behavior of the Hall effect, variable range hopping conductivity and the photoluminescence spectra of the corresponding structures. The experimental data demonstrated that the binding energy of the A+ states significantly increases with respect to 3D case and strongly depends on well width (9nm,15nm). The localization radii of the A+ states are of the order of well widt…
Quantum emitters in hexagonal boron nitride (hBN) have recently emerged as promising bright single photon sources. In this letter we investigate in details their optical properties at cryogenic temperatures. In particular, we perform temperature resolved photoluminescence studies and measure photon coherence times from the hBN emitters. The obtained value of 81(1) ps translates to a width of $\sim$12 GHz which is higher than the Fourier transform limited value of $\sim$32 MHz. To account for the photodynamics of the emitter, we perform ultrafast spectral diffusion measurements that partially a…
Ultralow-Tensile Strain Enables Exciton Funneling and Energy Transfer to Boost MoSe2 Photoluminescence Quantum Yield
Faculty of Physics, University of Warsaw（华沙大学物理学院）；SPEC, CEA, CNRS, Université Paris-Saclay（法国原子能和替代能源委员会、国家科学研究中心、巴黎萨克雷大学SPEC实验室）；Université Paris-Saclay, ONERA, CNRS, Laboratoire d’étude des microstructures (LEM)（巴黎萨克雷大学、ONERA、国家科学研究中心微结构研究实验室）；School of Basic Sciences, Indian Institute of Technology Bhubaneswar（布巴内斯瓦尔印度理工学院基础科学学院）；Research Center for Materials Nanoarchitectonics, National Institute for Materials Science（物质材料研究机构材料纳米架构研究中心）；Research Center for Electronic and Optical Materials, National Institute for Materials Science（物质材料研究机构电子与光学材料研究中心）
Quantum emitters based on atomic defects in layered hexagonal Boron Nitride (hBN) have emerged as promising solid state 'artificial atoms' with atom-like photophysical and quantum optoelectronic properties. Similar to other atom-like emitters, defect-phonon coupling in hBN governs the characteristic single-photon emission and provides an opportunity to investigate the atomic and electronic structure of emitters as well as the coupling of their spin- and charge-dependent electronic states to phonons. Here, we investigate these questions using photoluminescence excitation (PLE) experiments at T=…
Graphene/hBN heterostructure based Valley transistor: Dynamic Control of valley current in synchronized nonzero voltages, within the time-dependent regime
Competition between vacancy creation and filling in defect-engineering of hBN
University of Vienna（维也纳大学）；Univ. Paris-Saclay, CNRS, Laboratoire de Physique des Solides（巴黎萨克雷大学，法国国家科学研究中心，固体物理实验室）；Institute for Microelectronics, TU Wien（TU维也纳微电子研究所）
We investigated experimentally the valley Zeeman splitting of excitonic peaks in the photoluminescence (PL) spectra of high-quality hBN/WS2/MoSe2/hBN heterostructures at near-zero twist angles under perpendicular magnetic fields up to 20 T. We identify two neutral exciton peaks in the PL spectra: the lower energy one exhibits a reduced g-factor relative to that of the higher energy peak, and much lower than the recently reported values for interlayer excitons in other van der Waals (vdW) heterostructures. We provide evidence that such a discernible g-factor stems from the spatial confinement o…
The photoluminescence spectra of spherical CdTe nanocrystals with zincblende structure are studied by size-selective spectroscopic techniques. We observe a resonant Stokes shift of 15 meV when the excitation laser energy is tuned to the red side of the absorption band at 2.236 eV. The experimental data are analyzed within a symmetry-based tight-binding theory of the exciton spectrum, which is first shown to account for the size dependence of the fundamental gap reported previously in the literature. The theoretical Stokes shift presented as a function of the gap shows a good agreement with the…
Atomically thin semiconductors made from transition metal dichalcogenides (TMDs) are model systems for investigations of strong light-matter interactions and applications in nanophotonics, opto-electronics and valley-tronics. However, the photoluminescence spectra of TMD monolayers display a large number of features that are particularly challenging to decipher. On a practical level, monochromatic TMD-based emitters would be beneficial for low-dimensional devices but this challenge is yet to be resolved. Here, we show that graphene, directly stacked onto TMD monolayers enables single and narro…
Columbia University（哥伦比亚大学）；University of California San Diego（加州大学圣地亚哥分校）；Stony Brook University（石溪大学）；Brookhaven National Laboratory（布鲁克海文国家实验室）；Max Planck Institute for Structure and Dynamics of Matter（马克斯·普朗克物质结构动力学研究所）；The Flatiron Institute（平顿研究所）；Universidad del País Vasco UPV/EHU（巴斯克地区大学）
We report the investigation of a single quantum dot charge storage device. The device allows selective optical charging of a single dot with electrons, storage of these charges over timescales much longer than microseconds and reliable optical readout of the charge occupancy using a time gated photoluminescence technique. This device enables us to directly investigate the electric field dependent tunneling escape dynamics of electrons at high electric fields over timescales up to 4 us. The results demonstrate that such structures and measurement techniques can be used to investigate charge and…
Non-radiative energy transfer between boron vacancies in hexagonal boron nitride and other 2D materials
Université de Toulouse, INSA-CNRS-UPS（图卢兹大学，INSA-CNRS-UPS）；Ioffe Institute, Russian Academy of Sciences（约费研究所，俄罗斯科学院）；Laboratoire Charles Coulomb, Université de Montpellier and CNRS（夏尔·库隆实验室，蒙彼利埃大学和法国国家科学研究中心）；Institut Universitaire de France（法兰西学院）
Spectroscopic studies of semiconductor quantum dots (SQDs) addressing the problem of non-radiative carrier losses is vital for the improvement in the efficiency of various light-emitting devices. Various designs of SQDs emitter like doping, forming core-shell and alloying has been attempted to suppress non-radiative recombination. In this article, we show that forming a hybrid with metal nanoparticles (MNP) having localized surface plasmon resonance overlapped with the emission spectrum of SQD, the non-radiative carrier loss via Auger recombination can be mitigated. Using steady-state and time…
Tungsten-based monolayer transition metal dichalcogenides host a long-lived dark exciton, an electron-hole pair in a spin-triplet configuration. The long lifetime and unique spin properties of the dark exciton provide exciting opportunities to explore light-matter interactions beyond electric dipole transitions. Here we demonstrate that the coupling of the dark exciton and an optically silent chiral phonon enables the intrinsic photoluminescence of the dark-exciton replica in monolayer WSe2. Gate and magnetic-field dependent PL measurements unveil a circularly-polarized replica peak located be…
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…
Carrier dynamics of submonolayer (SML) InGaAs/GaAs quantum dots (QDs) were studied by micro-photoluminecence (MPL), selectively excited photoluminescence (SEPL), and time-resolved photoluminescence (TRPL). MPL and SEPL show the coexistence of localized and delocalized states, and different local phonon modes. TRPL reveal shorter recombination lifetimes and longer capture times for the QDs with higher emission energy. This suggests that the smallest SML QDs are formed by perfectly vertically correlated 2D InAs islands, having the highest In content and the lowest emission energy, while a slight…
We present an efficiently pumped single photon source based on single quantum dots (QD) embedded in photonic crystal nanocavities. Resonant excitation of a QD via a higher order cavity mode results in a 100$\times$ reduced optical power at the saturation onset of the photoluminescence, compared with excitation at the same frequency, after the cavity mode is detuned. Furthermore, we demonstrate that this excitation scheme leads to selective excitation of QDs coupled to the cavity by comparing photoluminescence and auto-correlation spectra for the same excitation wavelength with and without the …
We have fabricated and studied the photoluminescence from microdisks containing single, selected self-assembled quantum dots. Using two electron beam lithography exposures and a two-step selective wet etching process, the dots were positioned at the microdisks edges with sub-micron precision. The selection and positioning of quantum dots enables an optimum coupling of the dot emission to the microdisks whispering modes.
Monolayers of transition-metal dichalcogenides such as WSe2 have become increasingly attractive due to their potential in electrical and optical applications. Because the properties of these 2D systems are known to be affected by their surroundings, we report how the choice of the substrate material affects the optical properties of monolayer WSe2. To accomplish this study, pump-density-dependent micro-photoluminescence measurements are performed with time-integrating and time-resolving acquisition techniques. Spectral information and power-dependent mode intensities are compared at 290K and 1…
We investigate near-field energy transfer between chemically synthesized quantum dots (QDs) and two-dimensional semiconductors. We fabricate devices in which electrostatically gated semiconducting monolayer molybdenum disulfide (MoS2) is placed atop a homogenous self-assembled layer of core-shell CdSSe QDs. We demonstrate efficient non-radiative Förster resonant energy transfer (FRET) from QDs into MoS2 and prove that modest gate-induced variation in the excitonic absorption of MoS2 lead to large (~500%) changes in the FRET rate. This, in turn, allows for up to ~75% electrical modulation of QD…
We perform absorption and photoluminescence spectroscopy of trions in hBN-encapsulated WSe$_2$, WS$_2$, MoSe$_2$, and MoS$_2$ monolayers, depending on temperature. The different trends for W- and Mo-based materials are excellently reproduced considering a Fermi-Dirac distribution of bright and dark trions. We find a dark trion, $\rm{X_D^-}$ 19 meV $\textit{below}$ the lowest bright trion, $\rm{X}_1^-$ in WSe$_2$ and WS$_2$. In MoSe$_2$, $\rm{X_D^-}$ lies 6 meV $\textit{above}$ $\rm{X}_1^-$, while $\rm{X_D^-}$ and $\rm{X}_1^-$ almost coincide in MoS$_2$. Our results agree with GW-BSE $\textit{a…
We investigate the dynamics of chiral edge states in topological polariton systems under laser driving. Using a model system comprised of topolgically trivial excitons and photons with a chiral coupling proposed by Karzig et al. [Phys. Rev. X 5, 031001 (2015)], we investigate the real-time dynamics of a lattice version of this model driven by a laser pulse. By analyzing the time- and momentum-resolved spectral function, measured by time- and angle-resolved photoluminescence in analogy with time- and angle-resolved photoemission spectroscopy in electronic systems, we find that polaritonic state…
Single strain-induced quantum dots are isolated for optical experiments by selective removal of the inducing InP islands from the sample surface. Unpolarized emission of single, bi- and triexciton transitions are identified by power-dependent photoluminescence spectroscopy. Employing time-resolved experiments performed at different excitation powers we find a pronounced shift of the rise and decay times of these different transitions as expected from cascaded emission. Good agreement is found for a rate equation model for a three step cascade.
ZnO is a promising candidate as an environment friendly thermoelectric (TE) material. However, the poor TE figure of merit (zT) needs to be addressed to achieve significant TE efficiency for commercial applications. Here we demonstrate that selective enhancement in phonon scattering leads to increase in zT of RGO encapsulated Al-doped ZnO core shell nanohybrids, synthesized via a facile and scalable method. The incorporation of 1 at% Al with 1.5 wt% RGO into ZnO (AGZO) has been found to show significant enhancement in zT (=0.52 at 1100 K) which is an order of magnitude larger compared to that …
