The attractive potential of hexagonal boron nitride (hBN) as wide-band gap materials was realized after obtaining high quality single crystals by using high pressure synthesis process. Taking advantage of the highly luminous properties of hBN, a stableoperation of far ultraviolet -plane-emission device was demonstrated. It is also emphasized that hBN exhibits superior properties as a substrate of graphene devices.
In order torealize these newly developed potential of hBN crystals, more preciseinsight for its quality control is important. Although the major impurities affects the optical properties of hBN are carbon and oxygen, the details of their contribution are still the subject of study. For the application of graphene’s substrates, some unknown point defects in hBN are considered to still affect its device qualities.In order to figure out this issue, this paper focus onaspatial distribution of residual carbon impurity within hBN single crystals and modification of their properties depend upon post heat treatments.
Another issue is to fabricate fine hBN crystals with high quality via conventional route. Although high pressure synthesis process has an advantage to use reactive alkali-base solvent such as Ba-BN, search for the alternative synthesis route without pressure is important for the practical application of hBN. Since hBN is thermodynamically stable at high temperatures and at atmospheric pressure, it should be possible to obtain high-quality hBN crystals at atmospheric pressure by using an appropriate solvent. Ni or Co-base metal base solvents seem useful to obtain high quality hBN crystals, though the yield of the crystals is less than those of high pressure process.
On the other hands, liquid phase crystal growth process is also applicable for other 2D materials such as graphite, black phosphor (BP) and TMDs.Also, controlling of boron and nitrogen isotope ratio (10B,11B and 15N) in hBN and cBN crystals can be now carried out by methatheisis reaction under HPHT.
In this paper, recent studies for synthesis of BN and other 2D single crystals underhigh pressure with respect to impurity control will be reported. Particularly the present issue to improve properties of BN further is to eliminate residual carbon and oxygen impurities. Furthermore, recent challenge for obtaining boron and nitrogen isotope control of BN crystals will also be introduced.

Hexagonal boron nitride (h-BN), which has a graphite-like crystal structure with stacked honeycomb sheets of boron and nitrogen, is expected to be one of the key materials in the future electronics devices. Its unique crystallographic property, excellent electric insulation and efficient luminescence at 215 nm have proven the potential as a substrate and an insulating layer for Van der Waals heterostructures, a light emitting material for the deep ultraviolet region and so on. Aiming at the development of such devices, we have paid attention to the growth of h-BN thin films on a c-plane sapphire substrate by chemical vapor deposition (CVD) with BCl3 and NH3 as boron and nitrogen sources, respectively. This source combination, which has been used to fabricate pyrolytic BN, is expected to be suitable for fast growth of h-BN on a large-size substrate. We were also motivated by the observation of intrinsic excitonic emission at room temperature from h-BN films grown on a nickel substrate by CVD with the same source combination.
The CVD apparatus used in this study consisted of a horizontal hot-wall reactor tube made of BN ceramics with an inner diameter of 40 mm and a high-temperature tubular furnace with graphite heaters. The typical growth temperature was 1200 C. We have found that the growth pressure (Pg) has a major impact on the sample properties. The morphology of the grown film was improved by reducing Pg from atmospheric pressure to 20 kPa. The samples grown at 20 kPa consisted of columnar grains with a flat top surface, some of which coalesced each other, whereas those with rough surfaces were only grown at atmospheric pressure. Low pressure growth also achieved the formation of single crystal h-BN films with uniform out-of- and in-plane orientations.
Furthermore, it was found that the growth at 5 kPa improves the cathodoluminescence (CL), resulting in the observation of intrinsic exciton emission at room temperature (RT). It should be noted that its spectral width, 5 nm in FWHM, is almost same as that observed for the high quality bulk crystal fabricated by a high-pressure and high-temperature solution technique [4]. The fine structures corresponding to the indirect excitonic line and its phonon replicas were clearly observed at 12 K. The CL images at RT indicated that the intrinsic exciton emission was observed only from the columnar crystal grains, revealing high crystalline quality of the columnar grains. On the other hand, the deep luminescence at 350 nm was observed from the entire surface with relatively high intensities from the valley region between the columnar grains, where the randomly oriented grains were formed. Attempts to further improve the film quality will be reported in the presentation.

Hexagonal boron nitride (h-BN), an isostructural material to graphene, is a wide bandgap semiconductor of interest for electronics, deep UV photonics and two-dimensional heterostructures. A scalable technique of metal-organic vapor phase epitaxy (MOVPE)has been utilized to epitaxial growth of III-V compounds including h-BN. However, sapphire and Si, the most common substrates for semiconductors, are notvery suitable for deposition of h-BN, which requires growth temperatures in the range of 1500°C. SiC would be one choice and indeed has been used for h-BN epitaxy. AlN could be an alternative option, as it is also thermally stable up to 1600°C even under ammonia, optical transparent until 200nm, in theory smaller lattice mismatch than sapphire to h-BN, and thus is perfectly compatible with BN epitaxy by MOVPE. The growth of h-BN on AlN provides important knowledgeto integrationof h-BN into AlN-or high Al-content AlGaN-based UV devices. H-BN as sacrificing layer and grown on AlN may further increase the versatility in the transfer technique since AlN can be grown on a wide variety of substrates. Due to high thermal stability and exfoliation property, it could also serve as protective coating material for ion-implantation and hightemperature post-healing process in AlN.
In this work, h-BN layers were directly grown on AlN/sapphire template by MOVPE. The layers were formed with different cycles and each alternatingly supplies NH₃ for 2 sec and triethylboron for 1 sec.The growth pressure and nominal V/III ratio were maintained at 3.85 kPa and 3000, respectively.The impact of flow rates, process temperatures, and supply time on h-BN growth were comprehensively investigated. At initial growth stage, the lateral 2D growth rate of nuclei is ~25 nm/min and the vertical growth rate (nuclei height) ~0.3 nm/min, indicating the process is not self-limiting. On those nuclei, highly ordered 2D h-BN layers were grown as confirmed by X-ray diffraction (XRD) and transmission electron microscopy. The nucleation and growth mechanism will be further discussed in the conference.

Graphene is known as a prototypical two-dimensional material with unique physical properties. However, the difficulty of creating an optical band gap stimulated the search for other monolayer materials.
Atomically thin transition metal dichalcogenides serve as a promising new material class for opto-electronics. In contrast to thicker crystals, monolayers of MoS₂, WS₂, MoSe₂, and WSe₂ exhibit prominent photoluminescence. Recently, we have discovered bright and stable single-photon emitters in single layers of WSe₂, which renders atomically thin semiconductors also interesting for quantum optics. In my talk, I will show that these quantum light sources are strain-induced and demonstrate deterministic positioning of the emitters on the nanoscale. Furthermore, I will present single-photon emission from the layered monochalcogenide semiconductor GaSe and provide evidence that the incorporated non-classical light sources are also strain-induced. Next, I will demonstrate that these single photons originating from GaSe emitters can be routed in dielectric waveguides on a photonic chip. Finally, I will discuss the nature and prospects of newly discovered single-photon emitters in hBN.

Van der Waals (vdW) heterostructures, in which a wide range of unique atomic layers can easily be combined, offer novel prospects to engineer and manipulate quantum confined states. I will present evidence for quantum confined states engineered by the periodic potential landscape of interlayer valley excitons in a vdW heterostructure. Here, the moiré potential of a twisted MoSe₂/WSe₂ heterostructure traps interlayer excitons, and we show that the trapped interlayer excitons inherit the magneto-optical properties of the type-II band-structure. Finally, we will discuss the ability to charge the trapped states with electrons or holes in tunable electronic devices. These results highlight new opportunities to engineer quantum confined spins in vdW heterostructures.

Monolayer transition metal dichalcogenides (TMDs), such as WSe2, are atomically thin semiconductors with a "valley" degree of freedom, which can be optically addressed, thus opening up exciting possibilities for "valleytronics". Recently, naturally occurring single quantum emitters, believed to be excitons trapped in shallow potentials, were reported in TMDs. They seem to inherit the valley degree of freedom from the host TMD and owing to their longer lifetimes, appear promising for quantum information processing applications. In this talk, I will begin by highlighting some unique properties of TMDs excitons which result from the off-Gamma-point origin of the constituent single particle electronic states. After describing the basic properties of quantum dots in TMDs, I will present evidence for quantum entanglement between chiral phonons of the 2D host and single photons emitted from the quantum dots. I will also present evidence for optical initialization of single spin-valley in trapped, positively charged WSe₂ excitons. Due to quenching of electron-hole exchange, which is the main mechanism of valley-mixing in neutral excitons, the valley lifetime of excess spin-valley is prolonged to at least nanoseconds timescale. Our work extends the field of two-dimensional valleytronics to the level of single spin- valleys, with implications for quantum information and sensing applications.

Solid-state, quantum emitters (QEs) are critical components for future quantum photonics with potential applications in quantum technologies. Hexagonal boron nitride (hBN) has recently found to host bright and optically stable QEs at room temperature. Despite all the efforts so far, reliable techniques to prepare highly pure QEs in large scale has yet shown, which hinder the development of applications in practical devices. In this work, we first demonstrate chemical vapor deposition growth of large-area, few layer hBN films that host large quantity of QEs, 85% of which have a zero-phonon line (ZPL) at 580±10nm. We then develop two methods to process hBN after transfer which significantly improve SPEs in hBN films. The emitters exhibit narrow linewidths of ~3nm and photon purity in excess of 90%. Finally, we demonstrate tuning of the ZPL wavelength using ionic liquid devices over a spectral range of up to 15 nm--the largest obtained to date from any solid-state QEs. Our work lays a foundation for producing high-quality emitters in an ultra-compact 2D material system, and paves the way for deployment of hBN SPEs in scalable on-chip photonic and quantum devices.

We demonstrate the deterministic generation of single defect emitters in a monolayer MoS2van der Waals heterostructure. We bombard monolayer MoS2 with helium ions to generate optically active defect luminescence and encapsulate the defective MoS2 within hBN to greatly enhance optical quality. The encapsulation of the defective MoS2 reveals narrow spatially localized spectral linesXL that exhibit emission that is redshifted by 90-220meV with respect tothe neutral 2D exciton. We spectroscopically investigate single emitters by performing photoluminescence excitation spectroscopy and temperature dependent measurements. The line shape reveals a strong asymmetry resembling the interaction with LA/TA phonons. Employing the independent Boson model to our emission lines, we find that the emitters are spatially localized to a length scale of 2nm. We attribute the emission to atomistic defects induced by the helium ion bombardment and discuss their origin in the light of scanning tunneling microscopy measurements. Our work paves the way towards a controlled and deterministic generation of single quantum emitters in monolayer TMDC van der Waals heterostructures.

Being monoatomic chains of carbon atoms, carbynes represent ultimate one-dimensional crystals. Unfortunately, free standing carbon chains are instable in vacuum, and fluctuations almost necessarily lead to their deformation and folding. The synthesis of free-standing carbon chains remains one of the Holy Graals of nano-physics and nano-chemistry. Here we report on the experimental realization of long straight free-standing carbon chains stabilized by metallic nanoparticles. We synthesize linear carbon chains (carbynes) in a colloidal solution, then deposit them on a surface and study free standing carbyne films with the transmission electron microscopy (TEM). TEM images of the carbon threads with golden nanoparticles attached to their ends demonstrate straight monoatomic carbon chains with full lengths of over 100 nm and linear parts of about 12 nm length (52 atoms), in average. Parallel linear chains of carbon atoms form a quasi one-dimensional crystal structure with a lattice constant of 0.256 nm as confirmed by the X-ray diffraction measurements. This method paves the way to fabrication of an ultimate one-dimensional crystal: a monoatomic carbon wire. Currently, we are performing the low-temperature photoluminescence measurements to reveal excitonic features in the optical spectra of carbynes. The spectra taken at the resonant excitation at 390 nm show the appearance of ultranarrow peaks forming regular triplet groups in the range of 430-490 nm. We cautiously attribute these spectral features to the excitonic transitions and phonon replica in carbon chains of different lengths.

Band insulators with time reversal symmetry can be classified into normal insulators (NI), weak topological insulators (WTI) and strong topological insulators (STI) based on their Z₂ topological indices. Changing Z₂ indices requires closing and reopening the bandgap, and topologically distinct insulating phases are separated by a gapless Dirac or Weyl semimetal phase.The van der Waal layered material ZrTe₅ is a prototypical example that the emergence of massive 3D Dirac fermions is due to its proximity to the STI-WTI phase boundary. In this talk, I will demonstrate that a STI-WTI topological phase transition can be induced by applying uniaxial stress to ZrTe₅, and make the case that the in-situ tunable strain is a powerful to study and control topological materials. In addition, due to the low carrier density in this material, the lowest Landau level can be reached within a few tesla of magnetic field. I will describe the unusual magnetotransport behavior in the quantum limit magnetic fields.

Nanotubes (NTs) of transition metal dichalcogenides (TMD) attract an increasing interest due to possible applications in nanoelectronics and nanophotonics. The radiation ability of NTs has been discovered recently, in spite of the fact that they were synthesized long enough ago. Their planar allotrope of a monolayer thickness demonstrates bright photoluminescence (PL) related to direct bandgap excitons, while multilayered structures exhibit the rather weak emission of indirect exciton states. In common opinion, the multiwalled NTs should exhibit similar behavior. Contrary to that, we observe the bright PL from the single TMD NTs (MoS2 and WS2) with diameters of 0.2 - 2 microns, whose walls contain several dozens of monolayers. In contrast to the flakes, synthesized during the same process, the NTs display predominantly the emission of direct excitons. PL temperature dependencies in the NTs and flakes differ by the decrease or rise, respectively, in PL intensity with increasing temperature. However, they have a common feature that is the suppression of the indirect exciton transitions at low temperatures. This has been observed in the flakes that were exfoliated from a high-quality bulk crystal as well. The threshold appearance of the indirect-exciton emission is suggestive of a spin-forbidden lowest state in indirect transitions. Transmission electron microscopy studies have revealed that the NT walls are not homogeneous but consist of domains comprising 4-7 monolayers, the thickness of whose and mutual shift (chirality) are controlled by the internal strain. Thus the optical properties can be discussed in the context of the formalism of excitons in the chiral stacks. Such architecture promotes the localization of excitons within isolated spatial regions. In other words, the condition for exciton confinement is rather 2D than 3D. A micron-sized tube can act as an optical resonator supporting whispering gallery modes (WGMs) with the quality factor of several hundreds. The observed sharp peaks of WGMs in the emission spectra are strongly polarized along NT axis (TM polarization). The reason of that is the TE modes possess low quality factor in the spectral range of interest. Theoretical consideration shows that the polarization of excitation affects the initial distribution of excitons in the NT walls, which can be probed using different detection regimes. At oblique light incidence on the NT, the coupling between optical modes and direct exciton is possible with the formation of exciton-polariton, propagating within the NT walls. However, the observation of such polariton needs the tubular structure of higher quality, synthesized by either chemical reaction or grown by molecular beam epitaxy. The first attempts towards exploiting of the latter method are done. In particular, it is demonstrated that such method can create nanostructures of different shapes varied from nanoplatelets to nanotubes, and that some of them can radiate strongly polarized narrow PL lines. In general, our studies open a way towards combination of an effective emitter and a resonator in single tubular structure.

Graphene is a promising candidate for future nano-electronic devices including building blocks for quantum information processing. Reasons are the expected long spin lifetimes and high carrier mobility. Recent improvements in fabrication technologies for graphene nanostructures, namely, the encapsulation between boron nitride, edge-contacting, graphite back-gates and the use of electrostatic gating of bilayer graphene, have leveraged the quality of quantum dots to such an extent, that few-electron or -hole quantum dots have been realized that are comparable to the best devices in gallium arsenide.
Here we confine charge carriers laterally by applying strong displacement fields forcing charge carriers to flow through a narrow channel. In transport direction, charge carriers are confined by pn-junctions forming natural tunnel barriers, thus creating a p-type quantum dot coupled to n-type leads, or vice versa. In this ambipolar system, we can realize both single electron and single hole occupation of the respective quantum dots showing charging energies on the order of 5 meV. In addition, we can use our design to form multi-dots.
We use finite bias spectroscopy to study and identify the single-particle and many-body ground-and excited states of electrostatically-defined quantum dotsin bilayer graphene trapping only one or two charge carriers. While the properties of the material bear similarities to carbon nanotubes silicon because of the two-fold valley and spin-degeneracies, the results of our experiments allow us to propose a remarkably clear level scheme for two-particle spectra, in which the spin-and valley-entanglement, as well as exchange interactions play a crucial role. With this level scheme at hand, future experiments can investigate spin-and valley-coherence and relaxation times, which are key parameters to be compared to other material systems.

In integer quantum Hall systems, optical excitation can be used to investigate a variety of physics; for example, the Landau-level density of states through absorption, and non-equilibrium physics through the generation of excited electrons and holes. Optical techniques are attractive because the length scale associated with photo-excited charge carries lies between that of local probes and global transport measurements, perhaps revealing phenomena in a meso-scale environment. In addition, optical approaches are particularly attractive in Dirac materials where the Landau-level spacing is nonlinear, allowing for more restrictive selection rules for optical transitions.

Semiconducting two-dimensional materials are found to exhibit a number of highly intriguing phenomena, including spin-valley locking, efficient light-matter coupling, and strong Coulomb interaction. Spin-valley physics in particular remain in the focus of the ongoing research, further motivated by a variety of recently available heterostructures to tune and harness the associated phenomena. This strongly motivates increased attention to the consequences of environmental influences and the role of proximity effects for the key properties of local electronic structure.
Here, I will present an alternative, fundamental source of disorder in 2D materials based entirely on the local changes of the Coulomb interaction due to fluctuations of the external dielectric environment. The consequences for optical and transport properties of excitons and free charge carriers will be discussed. The importance of dielectric homogeneity in heterostructures will be further highlighted, demonstrated by spatial mapping of the fluctuations and by identifying the elimination of the dielectric disorder as a main consequence of material's encapsulation. Finally, I will present direct, time-resolved monitoring of exciton transport in 2D semiconductors, discuss the influence of disorder, and address strong non-linearities beyond linear regime.

Electromagnetically induced transparency (EIT)occurs in atomic systems and shows versatile applications in slow-light generation, gain without inversion and optical quantum-information processing. We demonstrate a cavity-free, atomic-like EIT effect in single-layer crystals of WSe2, probed by exploiting the intrinsic second-harmonic generation (SHG) arising from the breaking of inversion symmetry. Under conditions of double resonance of the driving and radiated field with the fundamental transitions, the SHG spectrum bifurcates. The feature follows a pump-wavelength-dependent spectral anticrossing, accurately described by aladder-type three-level model. Crucially, the SHG power-law exponentdiverges from the canonical value of 2 to follow a Fano-like dispersion with wavelength. This signature of quantum interference is retained at room temperature, opening up opportunities in solid-state quantum nonlinear optics for optical mixing and gain without inversion.

We discuss the electronic and optical properties of monolayer 2D hexagonal crystals, transition metal dichalcogenites (TMDCs). The ab-initio calculations establish monolayer TMDCs as direct gap semiconductors. In order to develop a better understanding of the electronic properties and their response to external magnetic field a tight binding model involving Mo and W metal d-orbitals and sulfur dimer S2 p orbitals is developed based on input from ab-initio calculations. The role of d- and p-orbitals, nearest and next nearest neighbor hopping and the origin of Q-points and their role in band-nesting is clarified. The effective tight binding model is further reduced to the massive Dirac Fermion model at K-points which allows introduction of the magnetic field and explicit derivation of valley-selective light-matter interaction. Using the microscopic wavefunctions the direct and exchange Coulomb matrix elements are computed. The exciton spectrum is obtained from the solution of Bethe-Salpeter equation. The role of screening, valleys and band-nesting on the exciton spectrum is described. We next discuss robust trions and the role of electron-phonon interaction, the possibility a Valley Polarized Electron Gas (VPEG) and potential for laser cooling of 2D crystals.

The present talk will review our recent work advances on optical properties calculations of transition metal dichalcogenide based structures using beyond standard DFT method. Indeed the use of the so-called GW+BSE approachis mandatory, since the optical properties of two-dimensional TMD monolayers such as MoS₂ or WSe₂ are dominated by excitons, Coulomb bound electron-hole pairs. As a first example, we will show that the sign and the amplitude of the splitting between bright and dark exciton states can be determined, showing the influence of the spin-orbit coupling on the optical spectra and demonstrating the strong impact of the combined intra-valley Coulomb exchange term and effective mass changes on the dark-bright exciton fine structure splitting. Using the same methodology we have recently reported the existence of interlayer excitons in MoS₂ bilayer structures also observed experimentally. Finally, screening effects due the presence of hexagonal-BN surrounding layers will be presented, since we have calculated the dependence of both the quasi-particle gap and the binding energy of the neutral exciton ground state E_b as a function of the hBN layer thickness. This study demonstrates that the effects of screening at this level of theory are more short-ranged that it is widely believed. The encapsulation of a WSe₂ monolayer by three sheets of hBN (∼1 nm) already yields a 20 % decrease of E_b whereas the maximal reduction is 27% for thick hBN. Similar calculations have been performed in the case of a WSe₂ monolayer deposited on stacked hBN layers. These results will be compared to the recently proposed Quantum Electrostatic Heterostructure approach.

The energy structure of direct bandgap excitons which appear at K-points of the Brillouin zone in mono- and few-layers of semiconducting transition metal dichalcogenides (S-TMD) will be reviewed.
The s-type Rydberg series of bright excitonic states in S-TMD monolayers will be shown to follow a simple energy ladder εn= − Ry*/(n+ δ)2, n=1, 2 ,..., which is well accounted by using the modified Kratzer potential to describe the Coulomb interaction in a non-uniform dielectric medium.
Next, the optical activity, the magnetic brightening and polarization properties of the so called dark excitons (pairs of electrons and holes with opposite spin) in tungsten based S-TMD monolayers will be presented. The scheme of probing and manipulating the valley degree of freedom of dark excitons will be discussed.
Finally, the fine structure of K-excitons in S-TMD multilayers will be examined with the results of experimental and theoretical studies of bi- and tri-layer MoS₂. Particular attention will be focused on properties of inter-layer excitons which appear in S-TMD multilayers, in addition to more conventional intra-layer excitons.

We present experimental evidence for a spin-polarized electronic ground-state in a two-dimensional semiconductor, monolayer MoS₂>. In a two-dimensional electron gas (2DEG), Coulomb effects dominate over single-particle effects in the limit of low electron-density. In this regime, the average inter-electron distance is larger than the Bohr radius in the host material. In gallium arsenide and silicon 2DEGs, electrons are localized at these low electron densitiessuch that Coulomb effectst end to be obscured. New opportunities arise in transition-metal dichalcogenides (TMDs). A TMD represents a natural realizationof a 2DEG. Significantly, the extremely small TMD Bohr-radius suggests that Coulomb effects play an important role at experimentally relevant electron-densities. We investigate the optical absorption of a monolayer MoS₂, a member of the optically active TMD-family.

Understanding and controlling defects in crystalline materials is an everlasting theme in material science and engineering. For two dimensional materials at atomic limit, defects are more influential and have come to the forefront in the pursuit of electronic and optoelectronic applications. In particular, intrinsic defects in semiconducting transition metal dichalcogenide (TMD) monolayers lower the carrier mobility and photoluminescence quantum yield despite of their direct semiconducting bandgap. Furthermore, defects in WSe₂ monolayer host localized excitons that could behave as single quantum emitters (SQEs) at low temperature. However, the exact nature of these defects remains elusive. In this talk, I will introduce our recent progress in study of the point defect in TMDs. By using low temperature scanning tunneling microscopy and spectroscopy, in corroboration with density functional theory calculations, we unveiled the atomic structure of the most abundant defects in WSe₂ monolayer grown by chemical vapor deposition. Instead of chalcogen vacancies that prevail in other TMD materials, such defects in WSe2arise from single tungsten vacancies, leading to the hole (p-type) doping and modifying electronic structure locally (~1.5 nm). Moreover, we found these defects to dominate the optical emission of WSe2monolayer at low temperature, providing the first real-space peek into bound excitons. In the end of the talk, I will also discuss the relationship between the bound exciton trapped by the tungsten vacancy and the SQEs. We conjecture that the tungsten vacancy defect is a likely candidate, at least a precursor, for SQEs in the WSe₂ monolayer.

The quantum spin Hall (QSH) phase is a quantum state of matter, proposed to exist in two-dimensional (2D) semiconductors with inverted band structure, i.e., topological insulators (TIs). Such a quantum phase should exhibit a spin Hall (SH) conductance while the charge Hall conductance vanishes. Since no external magnetic field is present, it is due to the (strong) spin-orbit interaction (SOI). As central questions we investigate if such a QSH phase is indeed related to a quantized static SH conductivity, as indicated by the characterization as quantum phase, and how the answer depends on translational and point-group symmetry.

The optical properties of atomically thin transition metal dichalcogenide (TMDC) semiconductors are shaped by the emergence of correlated many-body complexes due to strong Coulomb interaction. Exceptional electron-hole exchange predestines TMDCs to study fundamental and applied properties of Coulomb complexes such as valley depolarization of excitons and fine-structure splitting of trions.
Biexcitons in these materials are less understood and it has been established only recently that they are spectrally located between exciton and trion. We show that biexcitons in monolayer TMDCs exhibit a distinct and rich fine structure on the order of meV due to electron-hole exchange. Ultrafast pump-probe experiments on monolayer WSe₂ reveal decisive biexciton signatures and a fine structure in excellent agreement with a microscopic theory. We provide a pathway to understand the complex spectral structure of higher-order Coulomb complexes in TMDCs going beyond the usual classification scheme in terms of four-particle configurations.

In monolayer transition metal dichalcogenides (TMDs), the interplay of spin-orbit interactionand circular dichroism enables valley and spin selective optical excitation of excitons. In this talk, we develop, based on a fully excitonic picture, a theoretical description of the excitation, coherent dynamics and relaxation pathways of TMD excitons on ultrashort time scales. The analysis includes the optical preparation of excitons, the subsequent formation of polarons and momentum dark excitons due to exciton-phonon coupling as well as excitonic coupling due to Coulomb interaction (exchange interaction, mean-field and biexcitonic states). As experimental observables we address phonon sidebands in absorption, pump-probe signals and photoelectron emission.

In this paper, we discuss theoretically the exciton fine-structure in transition-metal dichalcogenides (TMD) mono-layers such as MoSe₂. While bulk MoSe₂ is an indirect-gap material, MoSe₂ mono-layers (which have D_3h point-group symmetry) possess a direct energy gap at the K±-points of the two-dimensional Brillouin zone. Depending on their coupling to the electromagnetic radiation field optical active (bright) and inactive (dark) exciton states can be formed. Thus, two exciton series (called "A" and "B" series) are observed in optical measurements.
In order to discuss their fine structure, we define excitons in the product space of electron- and hole states, including the lowest conduction band (LCB) and the uppermost valence band (UVB) where, at the Γ-point, they are both spin degenerate. All other states are neglected. On this basis exciton states are constructed and analyzed in the framework of an invariant expansion of a model Hamiltonian: The spin-orbit coupling in the conduction- and valence band is first simulated by introducing an effective magnetic field, giving rise to a splitting of the electron- and hole states outside the Γ-point. Then the Coulomb electron-hole exchange-interaction is introduced into the exciton Hamiltonian. It is due to the fact that electron and hole are indistinguishable particles in the exciton problem. In $D_{3h}$ crystal symmetry this electron-hole exchange-interaction (named "intra-valley exchange") has two different contributions: A first term accounts for an energy re-normalization of the different dark- and bright-exciton states in both series. A second term does not influence the bright states but affects only the dark states of both series, which become mixed. The importance of this mixing depends also on the spin-orbit coupling within the bands. As a result, exchange interaction and spin-orbit coupling of the conduction-band electrons lift together by different amounts the degeneracy of the bright- and dark states.
These calculations (starting from the Γ-point) can be extended up to the K_±- points at the edges of the Brillouin zone. In transition-metal dichalcogenides mono-layers the wave-vector group the K_± points is C_3h. K₊ and K_- points being separated by a reciprocal lattice vector, they are equivalent points of the Brillouin zone. Their corresponding wave-vectors are connected to each other by time-reversal symmetry. In this situation, the exchange interaction introduced above leads also to exchange interaction between electrons and holes from different K± valleys. The latter is called "inter-valley exchange interaction". Outside the K± points this results into a mixing of exciton states belonging to different valleys and leads to an inter-valley transfer of electron-hole excitation that has been studied e. g. in the framework of k.p perturbation theory.

Excitons in thin layers of semiconducting transition metal dichalcogenides such as MoS₂, WS₂, MoSe₂ and WSe₂ due to their high binding energy and oscillator strength are perfect systems for investigation of light–matter interactions.
In this work we demonstrate a tunable optical cavity incorporating tungsten diselenide (WSe₂) monolayer. Our open cavity consists of two dielectric TiO₂ / SiO₂ distributed Bragg reflectors (DBRs). One of them is mounted directly to the piezoelectric chip, which allows for precise control over the intermirror distance, thus energy of the cavity mode. Exfoliated WSe₂ single-layers are placed directly on the surface of the DBR. For optimal coupling the DBRs are specially designed to ensure the antinode of the electric field at the position of the flake. Our setup allows for angle resolved experiments with a control over the cavity mode energy over 100 meV.
We report observation of the strong coupling regime between excitons in WSe₂ and the cavity photons. Both spectra show two anticrossing upper and lower polariton branches with the Rabi splitting of 26 meV and up to 5 splitting-to-linewidth ratio.
In this system we investigated polariton valley polarization with circularly-polarized excitation laser beam resonant with the WSe₂ exciton. Tunability of the photonic mode allowed us to probe a wide spectral range and intersect polariton modes with other excitonic transitions such as dark and charged excitons. We observed that the degree of circular polarization of the emitted light strongly depends on polaritons energy, but is almost constant for different wavevectors. We demonstrate that depending on the intersecting resonance we can retain up to 40% of initial laser polarization or completely lose it.

Research on lead halide semiconductors with perovskite lattices is a rapidly growing field semiconductor physics and optics. They have great potential for low-cost yet efficient solar cells and light-emitting devices. Their optical properties can be tuned by tailoring the chemical composition and/or the nanostructure spatial dimension with very high precision and high quality. Moreover, the hybrid nature and soft lattice of organic-inorganic lead halide perovskites render their structural changes and optical properties susceptible to external driving forces such as temperature and pressure, remarkably different from conventional semiconductors. Herein, we study the optical properties and strong light-matter coupling in high-quality two-dimensional perovskite crystals. We report multiple intrinsic optical transitions originating from the radiative recombination of coupled electron-hole pairs (excitons). An extraordinary fine-structure splitting has been experimentally resolved on these atomically thin semiconductors thanks to the unusual Coulomb interaction. This fine-structure splitting and anisotropic polarized emission are supported by first-principle calculations. The optical spectra exhibit a large tunability of up to 320 meV within a moderate pressure range of below 3.5 GPa, while the quantum-yield remains constant. Such a large tunable range originates from the anisotropic structural deformation that effectively modulates the quantum confinement effect by 250 meV via barrier height lowering. Moreover, due to the large binding energy and oscillator strengths, two-dimensional perovskites are ideal candidates for studying strong light-matter interaction. We further successfully demonstrate room-temperature strong coupling in exfoliated 2D perovskite semiconductors embedded into a planar microcavity, exhibiting a large energetic splitting-to-linewidth ratio of 34.2. Our findings advocate a considerable promise of 2D perovskite to explore not only fundamental optical properties but also quantum phenomena such as Bose-Einstein condensation, superfluidity and exciton-polariton networks.

I will report on the optical properties of metal halide perovskite nano-platelets with controllable thickness down to one monolayer. Pronounced quantum confinement effects, large excitonic binding energies and comparably high radiative recombination rates have been found, all depending on the number of monolayers present in the respective nano-platelets. Ultrafast optical experiments provide further insight into characteristic charge carrier and spin relaxation scenarios in 2D perovskites. Finally, the assembly of halide perovskite nanocrystals into ordered supercrystals leads to remarkable changes of their linear and nonlinear optical properties.

Organic–inorganic halide perovskites have become the "next big thing" in emerging semiconductor materials, with their unprecedented rapid development and successful application in high-performance photovoltaics. Yet, their inherent instabilities over moisture, remain a crucial challenge for these materials. This directed the interest of the scientific community to perovskites derivatives such as 2D perovskites. These materials are significantly more stable and possesses higher tunability of their properties which expand the field of their application from energy harvesting through LED to single materials white light emitters.

Two-dimensional (2D) Van der Waals materials have emerged as a very attractive class of optoelectronic material due to the unprecedented strength in its interaction with light. In this talk I will discuss approaches to enhance and control this interaction by integrating these 2D materials with microcavities. I will first discuss the formation of strongly coupled half-light half-matter quasiparticles (microcavity exciton-polaritons) and their spin-optic and electrical excitation and control in the 2D transition metal dichalcogenide (TMD) systems. Prospects of realizing condensation and few photon nonlinear switches using Rydberg states in TMDs will also be discussed. Finally, I will talk about room temperature single photon emission from hexagonal boron nitride and the prospects of developing deterministic quantum emitters using them.

Many classes of two dimensional (2D) quantum systems, e.g., transition metal dichalcogenide (TMD) monolayers,have recently emerged as potential candidates for novel optoelectronic and photonicdevices. Hence, understanding the physical properties and light-matter interactions of 2D quantum systems are of fundamental significance. In this talk, I will introduce a hybrid nano-opto-mechanical tip-enhanced spectroscopy and imaging approach combining tip-enhanced Raman scattering (TERS) and tip-enhanced photoluminescence (TEPL) to probe the heterogeneous optical responses of TMD monolayer at nanoscale defects and control the local bandgap of crystal through atomic force local strain manipulation.This approach is further extended to probe and control the radiative emission of dark excitons and localized excitons. Based on nano-tip enhanced spectroscopy with ~600,000-fold PL enhancement induced by the plasmonic Purcell effect and few-fs radiative dynamics of the optical antenna tip, we can directly probe and actively modulate the dark exciton and localized exciton emissions in time (~ms) and space (< 15 nm) at room temperature.

Transition metal dichalcogenides (TMDs) have emerged as novel two-dimensional semiconductors with a direct band gap in their monolayer limit and remarkable opto-valleytronic properties. While extensive research has been dedicated to TMD monolayers in the past decade, more recent work has focused on novel aspects provided by combinations of dissimilar monolayers in stacks of van der Waals heterobilayers. Vertical MoSe2-WSe2 heterobilayers, for example, host layer separated excitons with finite dipole moments and long radiative lifetimes 3 that can be subjected to moire effects in twisted heterobilayer systems.
In our experiments, we studied the cryogenic photophysics of excitons in as-grown moire-free MoSe2-WSe2 heterobilayers in cavity-modified photonic environments and in response to external magnetic field. We demonstrate cavity-control of the radiative decay channels associated with different types of interlayer excitons, and quantify the respective light-matter coupling strengths. Moreover, we discuss the distinct roles of bright and dark interlayer excitons for the dynamics of photoexcited spin orientation in the presence of external magnetic fields. The findings will be compared with observations made on exfoliation-stacked MoSe2-WSe2 heterostructures with different orientation angles.

Van der Waals heterostructures based on transition metal dichalcogenide monolayers are new promising objects of research in semiconductor physics. By choosing the material of semiconductor layers, the mutual orientation of the crystallographic axes of different layers and the distances between them, the optical transition energy and the lifetime of photoexcited states can be varied over a wide range, which allows engineering of various photoelectric devices based on them. The complex structure of spin sublevels associated with the presence of several degenerate valleys and strong spin-valley interaction requires separate study of the dynamics of the spin degrees of freedom of charge carriers in such heterostructures.
In this work, the spin relaxation of excitons in a heterostructure based on WSe​₂​ / Mo_0.5​​W​_0.5​Se​₂ monolayers was studied using the method of micro-Kerr rotation. A strong difference in the spin relaxation time of photogendered excitons between the WSe2 monolayer and the heterostructure was found. We conclude that such an increase in the spin relaxation time in the heterostructure is associated with the excitation of an indirect exciton, when the electron and hole are localized in different layers of the heterostructure. Since the decay rate of the signal of Kerr rotation is determined by the sum of the recombination and spin relaxation rates, this means that the one and the other decreases tenfold. The change in the recombination rate is due to the spatial separation of electrons and holes, and the decrease in the spin relaxation rate is due to the stabilization of the hole spin by a strong spin-orbit interaction. The rate of depolarization of the electron spins does not change in this case, which manifests itself as a rapid decrease in the signal on the scale of the first picoseconds.

In heterostructures of transition metal dichalcogenides, electrons and holes residing in adjacent monolayers can bind into spatially indirect excitons. These interlayer bound pairs have attracted tremendous interest due to their strong promise for novel optoelectronics, valleytronics and moiréinduced nanodot lattices. Since these quasiparticles couple only weakly to light, their binding energies have not been directly measured yet. Here we introduce a direct ultrafast access to Coulomb correlations acting between monolayers. For the prototypical case of WSe₂/WS₂ hetero-bilayers, we optically excite 1s A excitons in the WSe₂ monolayer. Phase-locked mid-infrared (MIR) pulses subsequently interrogate characteristic intra-excitonic transitions independently of the center-of-mass momentum of the excitons or interband optical selection rules. Our measurements reveal a novel transition between 1s and 2p orbitals of interlayer excitons at an energy of 67 ± 6 meV. This value coincides with our numerical solution of the Wannier equation and implies a binding energy of itinerant interlayer excitons as 126 ± 7 meV. Since the complex-valued spectral response functions of inter- and intralayer excitons differ strongly, we can sensitively track the ultrafast evolution of the respective exciton densities by monitoring the MIR response as a function of the pump-probe delay time t_pp. Interestingly, intralayer excitons photoinjected into the WSe₂ monolayer transform into interlayer species by direct electron tunnelling without a strong intermediate phase of unbound electron-hole pairs. Depending on the stacking angle of the individual monolayers, intra- and interlayer species coexist on picosecond scales and relax into quantum confined states in moiré-induced nanodots.

Exciton polaritons in semiconductor microcavity constitute the archetypal system to investigate condensate in solid state materials with room temperature. Together with the consideration of microscale size, polariton condensates appears promising for polaritonic devices for all-optical information and communication processing. To implement these quantum technologies, quantum memory to store and recall the quantum state on demand is one of the most important and challenging issues. In polariton condensates, however, due to the inherently dissipative property and non-equilibrium character, only several researches around the manipulation of solitons and vortices are explored. With the investigation of temporal coherence in polariton condensate, the study of polariton memory appears possible but remains still unaddressed.
Here we address the quantum memory based on temporal pulse train of phase locking in microcavity polariton condensate. Two significant outcomes are: (a) by firstly exploiting the quantum interference in time-frequency domain, which has been reported only in spatial domain in prior works, we generate the temporal pulse train with tunable periodicity around several picoseconds in polariton condensate. The results are helpful not only for the signal storage mentioned following but also for the study of polariton laser. (b) Based on the temporal pulse train of π jump phase, we implement the memory with high efficiency and fidelity in polariton condensate, which is checked to be independent of the material parameters and experimentally accessible within a quasi-1D microcavity. It allows the very first observation of quantum memory in a polariton condensate.

Optical bound states in the continuum (BICs) are peculiar resonances that can exist in photonic crystals. Characterized by diverging giant Q-factors and field enhancement due to their non-radiative character, as well as formation of momentum-space vortices, BICs are promising candidates for applications in sensing, lasing, and nonlinear optical devices. As planar photonic crystals can be interfaced with 2D transition metal dichalcogenides (TMDs), BICs can be efficiently coupled to TMD excitons creating polaritons.
We study experimentally and theoretically exciton-polaritons formed in the strong coupling regime between TMD excitons and BICs. Our hybrid structures consist of Ta₂O₅/SiO₂/Si photonic crystal slabs supporting BIC resonances at Г point in the momentum space, hBN spacer layer, and monolayer MoSe₂.

We present the design and realization of a hybrid MoSe₂-GaAs Tamm structure operating in the strong coupling regime. This device combines the unique physics inherent to transition metal dichalcogenide (TMDC) monolayers (e.g. spin-valley locking) with the mature III-V device platform in optoelectronics and polaritonics. We observe collective strong coupling of the different kinds of excitons (Wannier type excitons in GaAs and strongly bound valley excitons in the MoSe₂ monolayer) to the same cavity mode and the formation of hybrid polaritons with their three characteristic dispersion branches via angle resolved PL and reflection measurements. That manifests the first successful observation of this new kind of quasiparticle.
Furthermore, we observe bosonic condensation in this device driven by the excitons hosted in the atomically thin layer of MoSe₂, visualized via power dependent PL measurements at T = 4.2 K. Additionally we demonstrate that the effects of spin-valley locking are conserved in the condensate. Our work paves the way towards highly efficient, ultra-compact polariton-based light sources and valleytronic devices hosting bosonic quantum fluids in atomically thin materials which we hope can be ultimately operated at room temperature.

The absence of strict lattice matching requirement in van der Waals heterostructures allows for coupling between monolayer crystals with incommensurate lattices and arbitrary mutual orientation. The introduction of rotational misalignment has been shown to modify mechanical and electronic coupling between the layers, as well as give rise o long-range periodic variation of the local atomic registry known as moiré superlattice. The latter has attracted considerable research interest in recent years due to its ability to significantly alter the energy spectrum of heterostructures, leading to the formation of topological, superconducting, and quantum-confinedstates.
We demonstrate that rotational alignment can be used to efficiently and systematically control the optical properties of van der Waals heterostructures built of incommensurate transition metal dichalcogenide monolayers. We investigate the effects of interlayer rotation by studying a large number of MoSe₂/WS₂ and MoSe₂/MoS₂ heterobilayers with various interlayer orientations, fabricated from either CVD-grown or mechanically exfoliated monolayers. We show that in MoSe₂/WS₂ heterostructures, the near-degeneracy of the conduction band edges leads to twist-angle-dependent hybridization between intra-and interlayer excitons, which manifests as a gradual redshift of the photoluminescence peaks of the hybridized exciton states of up to 60 meV. For heterostructures in which the monolayer pairs are nearly aligned, resonant mixing of the electron states leads to pronounced effects of the geometrical moiré pattern of the heterostructure on the dispersion and optical spectra of the hybridized excitons. While in MoSe₂/MoS₂ heterobilayers the hybridization between the intra-and interlayer exciton statesis suppressed by the large band offset, the mutual layer orientation strongly affects the properties of interlayer exciton, leading to 100 meV variation of its emission energy. Our findings underpin new strategies for band structure engineering in semiconductor devices based on van der Waals heterostructures.

At the few-atom-thick limit, transition-metal dichalcogenides (TMDs) exhibit strongly interconnected structural and optoelectronic properties. The possibility to tailor the latter by controlling the former is guaranteed to have a great impact. Here, we present a route toward the patterning of TMDs based on the effects ofproton irradiation on the structural and electronic properties of bulk WS₂, WSe₂, WTe₂, MoS₂, MoSe₂ and MoTe₂.Suitable irradiation conditions let protons penetrate the top layerand trigger the hydrogen evolution reaction just beneath the first chalcogen-metal-chalcogenbasal plane. This results in the blistering of one-monolayer thick domes, which stud the crystal surfaceand locally turn the dark bulk material into an efficient light emitter. These stable (>2-year lifetime), robust domes host strong, non-trivial strain fields thatmodify profoundly the electronic band structure of the curved monolayerplanes. The domes can be produced with well-ordered positions and sizes tunable from the nanometer to the micrometer scale, with important prospects in nanomechanics, pressure sensing and nanophotonics, as well as for the strain engineering of two-dimensional materials.

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 MoSe₂ in hexagonal boron nitride (hBN) yields narrow optical transitions approaching the homogeneous 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 aconsequence of the Purcell effect. The time-resolved photoluminescence measurements together with cw reflectivity and photoluminescence experiments show that the neutral exciton spontaneous emission time can be tuned by one order of magnitude depending on the thickness of the surrounding hBN layers. The inhibition of the radiative recombination can yield spontaneous emission time up to 10 ps. These results are in very good agreement with the calculated recombination rate in the weak exciton-photon coupling regime. The analysis shows that we are also able to observe a sizeable enhancement of the exciton radiative decay rate.Understanding the role of these electrodynamical effects allow us to elucidate the complex dynamics of relaxation and recombination for both neutral and charged excitons.

Ensembles of indirect or interlayer excitons (IXs) are intriguing systems to explore classical and quantum phases of interacting bosonic ensembles. IXs feature enlarged lifetimes due to the reduced overlap of the electron-hole wave functions. A field effect structure with few layer hexagonal boron nitrite (hBN) as insulator and few-layer graphene as gate-electrodes facilitates an electric field control of the IXs in a MoS2/WS2 heterobilayer. A multiplet structure in the IX emission band can be observed even at room temperature. Stark shift measurements reveal the presence of a finite out-of plane dipole of the IXs. Due to a different strength of the dipole and a distinct temperature dependence, we identify the IXs to stem from optical interband transitions with electrons and holes in different valleys of the heterostructures. We observe a field dependent level anti-crossing for the energetically lowest emission line, forming hybridized indirect excitons at low temperatures. We discuss this behavior in terms of a finite coupling of the electronic states of the two TMDC monolayers. Our results demonstrate the design of novel nano-quantum materials prepared from artificial van der Waals solids with the possibility to in-situ control their physical properties via external stimuli such as electric fields.