Two-dimensional metal-chalcogenide films in tunable optical microcavities

Monolayer-Based Single-Photon Source in a Liquid-Helium-Free Open Cavity Featuring 65% Brightness and Quantum Coherence

Solid-state single-photon sources are central building blocks in quantum information processing. Atomically thin crystals have emerged as sources of nonclassical light; however, they perform below the state-of-the-art devices based on volume crystals. Here, we implement a bright single-photon source based on an atomically thin sheet of WSe2 coupled to a tunable optical cavity in a liquid-helium-free cryostat without the further need for active stabilization. Its performance is characterized by high single-photon purity (g(2)(0) = 4.7 ± 0.7%) and record-high, first-lens brightness of linearly polarized photons of 65 ± 4%, representing a decisive step toward real-world quantum applications. The high performance of our devices allows us to observe two-photon interference in a Hong-Ou-Mandel experiment with 2% visibility limited by the emitter coherence time and setup resolution. Our results thus demonstrate that the combination of the unique properties of two-dimensional materials and versatile open cavities emerges as an inspiring avenue for novel quantum optoelectronic devices.

Solar cells based on 2D Janus group-III chalcogenide van der Waals heterostructures

Janus monolayers, realized by breaking the vertical structural symmetry of two-dimensional (2D) materials, pave the way for a new era of high-quality and high-performance atomically-thin vertical p-n heterojunction solar cells. Herein, employing first-principles computations, Janus group-III chalcogenide monolayers, MX, M₂XY, MM'X₂ and MM'XY (M, M' = Ga, In; X, Y = S, Se, Te), are deeply investigated in view of their implementation in 2D photovoltaic systems. Their stability analysis reveals that the 21 investigated monolayers are energetically, thermodynamically, mechanically, dynamically, and thermally stable, confirming their growth feasibility under ambient conditions. Furthermore, owing to their optimal band gap, high charge carrier mobilities, and strong light absorption, 2D Janus group-III monolayers are predicted as promising candidates for 2D excitonic solar cell applications. In fact, 46 type-II van der Waals (vdW) heterostructures with a lattice mismatch of less than 5% are identified by analyzing the band alignments of the investigated monolayers obtained through the HSE + SOC approach. In particular, 7 vertical vdW heterojunctions with a power conversion efficiency (PCE) higher than 20% are predicted and might be the focus of future experimental and theoretical studies. To further confirm the type II band alignment, the Ga₂STe-GaInS₂ vdW heterostructure, which reveals the highest PCE of 23.69%, is thoroughly investigated. Our results not only predict and evaluate stable 2D Janus group-III chalcogenide monolayers and vdW heterostructures, but also suggest that they could be used as materials for next-generation optoelectronic and photovoltaic devices.

2D-materials-integrated optoelectromechanics: recent progress and future perspectives

The discovery of two-dimensional (2D) materials has gained worldwide attention owing to their extraordinary optical, electrical, and mechanical properties. Due to their atomic layer thicknesses, the emerging 2D materials have great advantages of enhanced interaction strength, broad operating bandwidth, and ultralow power consumption for optoelectromechanical coupling. The van der Waals (vdW) epitaxy or multidimensional integration of 2D material family provides a promising platform for on-chip advanced nano-optoelectromechanical systems (NOEMS). Here, we provide a comprehensive review on the nanomechanical properties of 2D materials and the recent advances of 2D-materials-integrated nano-electromechanical systems and nano-optomechanical systems. By utilizing active nanophotonics and optoelectronics as the interface, 2D active NOEMS and their coupling effects are particularly highlighted at the 2D atomic scale. Finally, we share our viewpoints on the future perspectives and key challenges of scalable 2D-materials-integrated active NOEMS for on-chip miniaturized, lightweight, and multifunctional integration applications.

White-Light-Driven Resonant Emission from a Monolayer Semiconductor

Resonant emission in photonic structures is very useful to construct all-photonic circuits for optical interconnects and quantum computing. Optical generation of most resonant-emitting modes in photonic structures is obtained by coherent pumping rather than incoherent illumination. Particularly, the development of white-light- or even solar-powered on-chip light sources remains challenging but is very attractive in view of the much facile availability of these incoherent excitation sources. Here, net resonant emission from a monolayer semiconductor is demonstrated under simulated solar illumination by a white-light-emitting diode. The device is formed by embedding a 2D gain medium into a planar microcavity on a silicon wafer, which is compatible with the prevailing on-chip photonic technology. Coherent and white-light excitation sources are, respectively, selected for optical pumping, where the output light in two cases exhibits well-consistent resonant wavelength, linewidth, polarization, location, and Gaussian-beam profile. The fundamental TEM₀₀ mode behaves as a doublet emission, resulting from anisotropy-induced non-degenerate states with orthogonal polarizations. The extraordinary spectral flipping is attributed to the competitive interplay of resonant absorption and emission. This work paves a way toward white-light or solar-powered state-of-the-art photonic applications at the chip scale.

Engineering Plasmonic Environments for 2D Materials and 2D-Based Photodetectors

Two-dimensional layered materials are considered ideal platforms to study novel small-scale optoelectronic devices due to their unique electronic structures and fantastic physical properties. However, it is urgent to further improve the light–matter interaction in these materials because their light absorption efficiency is limited by the atomically thin thickness. One of the promising approaches is to engineer the plasmonic environment around 2D materials for modulating light–matter interaction in 2D materials. This method greatly benefits from the advances in the development of nanofabrication and out-plane van der Waals interaction of 2D materials. In this paper, we review a series of recent works on 2D materials integrated with plasmonic environments, including the plasmonic-enhanced photoluminescence quantum yield, strong coupling between plasmons and excitons, nonlinear optics in plasmonic nanocavities, manipulation of chiral optical signals in hybrid nanostructures, and the improvement of the performance of optoelectronic devices based on composite systems.

Localization of Laterally Confined Modes in a 2D Semiconductor Microcavity

Monolayer semiconductor embedded planar microcavities are becoming a promising light-matter interacting system to uncover a wealth of photonic, excitonic, and polaritonic physics at the two-dimensional (2D) limit. In these 2D semiconductor microcavities employing the longitudinal Fabry-Perot resonance, major attention has been paid to the coupling of excitons with vertically confined cavity photons; by contrast, the lateral confinement effect on exciton-photon interactions is still elusive. Here we observe the localized distribution of laterally confined modes with discrete energies in a 2D semiconductor embedded microcavity. Monolayer tungsten disulfides with equilateral triangular geometries but varied edge lengths are selected as the active media incorporated into a dielectric planar microcavity. With the shortening of the edge length, photoluminescence mappings of active regions present spatially localized emission patterns, which are attributed to the presence of in-plane triangular waveguiding resonance caused by total internal reflection at the one-dimensional closed boundary between the monolayer semiconductor and its surrounding cavity material. Unlike the conventional quantum confinement effect of native excitons appearing at the nanometer scale, the mode emission at the active-medium center exhibits apparent size-dependent features at the micrometer scale due to the optical confinement effect correlated with its photonic nature. By reducing the area of active media, single-mode dominant emission is achieved together with its nondispersive energy and improved directionality. Our work highlights the crucial role of lateral mode control in monolayer semiconductor embedded planar microcavities and encourages the investigation of the quantum billiard problem in 2D semiconductors.

Multiwavelength magnetic coding of helical luminescence in ferromagnetic 2D layered CrI3

Two-dimensional (2D) van der Waals (vdW) ferromagnets have opened new avenues for manipulating spin at the limits of single or few atomic layers, and for creating unique magneto-exciton devices through the coupling of ferromagnetic (FM) orders and excitons. However, 2D vdW ferromagnets explored so far have rarely possessed exciton behaviors; to date, FM CrI₃ have been revealed to show ligand-field photoluminescence correlated with FM ordering, but typically with a broad emission peak. Here, we report a straightforward approach to realize strong coupling of narrow helical emission and FM orders in CrI₃ through microsphere cavity. The resonant whispering-gallery modes (WGM) of SiO₂ microspheres cause strong oscillation helical emissions with a full width at half-maximum (FWHM) of ∼5 nm under continuous wave excitation. Reversible magnetic coding of helical luminescence is realized in the range of 950-1100 nm. This work enables numerous opportunities for creating magnetic encoding lasing for photonic integrated chips.

Hyperspectral study of the coupling between trions in WSe2 monolayers to a circular Bragg grating cavity

Circular Bragg gratings compose a very appealing photonic platform and nanophotonic interface for the controlled light-matter coupling of emitters in nanomaterials. Here, we discuss the integration of exfoliated monolayers of WSe2 with GaInP Bragg gratings. We apply hyperspectral imaging to our coupled system, and explore the spatio-spectral characteristics of our coupled monolayer-cavity system. Our work represents a valuable step towards the integration of atomically thin quantum emitters in semiconductor nanophotonic cavities.

Optically Pumped Monolayer MoSe2 Excitonic Lasers from Whispering Gallery Mode Microcavities

Developing integrable, nanoscale, and low-energy-consumption lasers is a crucial step toward on-chip optical communications and computing technologies. The strong exciton-photon interaction that emerged in monolayer transition metal dichalcogenides (TMDs) holds promise for engineering and integration. Herein, we prepare the MoSe₂/microsphere cavities excitonic lasers by placing SiO₂ microspheres on top of a monolayer MoSe₂ film. By virtue of continuous-wave exciting MoSe₂/microsphere whispering gallery mode (WGM) cavities, we realize multiple excitonic WGM lasing in the emission wavelength range of ∼750-875 nm at room temperature with tunable properties of free spectral range (FSR) and full width at half-maximum (fwhm) by varying the microsphere size. Theoretical calculations based on the finite element method (FEM) using COMSOL software were utilized to identify lasing modes and reveal the corresponding electric field distribution. These findings help to deepen fundamental understanding of excitonic WGM lasing and provide a promising research platform for integrable, scalable, and low-cost laser devices.

Strain Effect on Thermoelectric Performance of InSe Monolayer

Strain engineering is a practical method to tune and improve the physical characteristics and properties of two-dimensional materials, due to their large stretchability. Tensile strain dependence of electronic, phonon, and thermoelectric properties of InSe monolayer are systematically studied. We demonstrate that the lattice thermal conductivity can be effectively modulated by applying tensile strain. Tensile strain can enhance anharmonic phonon scattering, giving rise to the enhanced phonon scattering rate, reduced phonon group velocity and heat capacity, and therefore lattice thermal conductivity decreases from 25.9 to 13.1 W/mK when the strain of 6% is applied. The enhanced figure of merit indicates that tensile strain is an effective way to improve the thermoelectric performance of InSe monolayer.

Controlling the emission properties of solution-processed organic distributed feedback lasers through resonator design

Surface-emitting distributed feedback (DFB) lasers with both, resonator and active material based on solution-processable polymers, are attractive light sources for a variety of low-cost applications. Besides, the lasers should have competitive characteristics compared to devices based on high-quality inorganic resonators. Here, we report high performing all-solution-processed organic DFB lasers, consisting of water-processed photoresist layers with surface relief gratings located over the active films, whose emission properties can be finely tuned through resonator design. Their laser threshold and efficiency are simultaneously optimized by proper selection of residual resist thickness and grating depth, d. Lowest thresholds and largest efficiencies are obtained when there is no residual layer, while a trade-off between threshold and efficiency is found in relation to d, because both parameters decrease with decreasing d. This behaviour is successfully explained in terms of an overlap factor r, defined to quantify the interaction strength between the grating and the light emitted by the active film and traveling along it, via the evanescent field. It is found that optimal grating depths are in the range 100–130 nm (r ~ 0.5−0.4). Overall, this study provides comprehensive design rules towards an accurate control of the emission properties of the reported lasers.

Large area growth of few-layer In2Te3 films by chemical vapor deposition and its magnetoresistance properties

In this work we report a facile route to grow large area, uniform, continuous and few-layer α-In₂Te₃ film via chemical vapor deposition (CVD) methods. The characterizations show the large area of CVD-grown few-layer α-In₂Te₃. This method guarantees the precise control of thickness down to few layers and large area preparation. The magnetoresistance (MR) properties of few-layer In₂Te₃ was investigated from 2 to 300 K and its MR stability under long exposure to ambient air was studied for the first time. Few-layer of α-In₂Te₃ shows a positive MR and the largest transverse MR was observed to about 11% at 2 K and a high stability of MR to long time exposure in air up to 21 weeks.

Valley coherent exciton-polaritons in a monolayer semiconductor

Two-dimensional transition metal dichalcogenides (TMDs) provide a unique possibility to generate and read-out excitonic valley coherence using linearly polarized light, opening the way to valley information transfer between distant systems. However, these excitons have short lifetimes (ps) and efficiently lose their valley coherence via the electron-hole exchange interaction. Here, we show that control of these processes can be gained by embedding a monolayer of WSe₂ in an optical microcavity, forming part-light-part-matter exciton-polaritons. We demonstrate optical initialization of valley coherent polariton populations, exhibiting luminescence with a linear polarization degree up to 3 times higher than displayed by bare excitons. We utilize an external magnetic field alongside selective exciton-cavity-mode detuning to control the polariton valley pseudospin vector rotation, which reaches 45° at B = 8 T. This work provides unique insight into the decoherence mechanisms in TMDs and demonstrates the potential for engineering the valley pseudospin dynamics in monolayer semiconductors embedded in photonic structures.

The short exciton life time in atomically thin transition metal dichalcogenides poses limitations to efficient control of the valley pseudospin and coherence. Here, the authors manipulate the exciton coherence in a WSe₂ monolayer embedded in an optical microcavity in the strong light-matter coupling regime.

Lasing from lead halide perovskite semiconductor microcavity system

Organic-inorganic halide perovskite semiconductors are ideal gain media for fabricating laser and photonic devices due to high absorption, photoluminescence (PL) efficiency and low nonradiative recombination losses. Herein, organic-inorganic halide perovskite CH3NH3PbI3 is embedded in the Fabry-Perot (FP) microcavity, and a wavelength-tunable excitonic lasing with a threshold of 12.9 μJ cm-2 and the spectral coherence of 0.76 nm are realized. The lasing threshold decreases and the spectral coherence enhances as the temperature decreases; these results are ascribed to the suppression of exciton irradiative recombination caused by thermal fluctuation. Moreover, both lasing and light emission below threshold from the perovskite microcavity (PM) system demonstrate a redshift with the decreasing temperature. These results provide a feasible platform based on the PM system for the study of light-matter interaction for quantum optics and the development of optoelectronic devices such as polariton lasers.

Excitonic Emission of Monolayer Semiconductors Near-Field Coupled to High-Q Microresonators

We present quantum yield measurements of single layer WSe₂ (1L-WSe₂) integrated with high-Q ( Q > 10⁶) optical microdisk cavities, using an efficient (η > 90%) near-field coupling scheme based on a tapered optical fiber. Coupling of the excitonic emission is achieved by placing 1L-WSe₂ in the evanescent cavity field. This preserves the microresonator high intrinsic quality factor ( Q > 10⁶) below the bandgap of 1L-WSe₂. The cavity quantum yield is QY_c ≈ 10^-3, consistent with operation in the broad emitter regime (i.e., the emission lifetime of 1L-WSe₂ is significantly shorter than the bare cavity decay time). This scheme can serve as a precise measurement tool for the excitonic emission of layered materials into cavity modes, for both in plane and out of plane excitation.

Curvature-dependent flexible light emission from layered gallium selenide crystals

Flexible optoelectronics devices play an important role for technological applications of 2D materials because of their bendable, flexible and extended two-dimensional surfaces. In this work, light emission properties of layered gallium selenide (GaSe) crystals with different curvatures have been investigated using bending photoluminescence (BPL) experiments in the curvature range between R⁻¹ = 0.00 m⁻¹ (flat condition) and R⁻¹ = 30.28 m⁻¹. A bendable and rotated sample holder was designed to control the curvature (strain) of the layered sample under upward bending uniformly. The curvature-dependent BPL results clearly show that both bandgaps and BPL intensities of the GaSe are curvature dependent with respect to the bending-radius change. The main emission peak (bandgap) is 2.005 eV for flat GaSe, and is 1.986 eV for the bending GaSe with a curvature of 30.28 m⁻¹ (the maximum bending conditions in this experiment). An obvious redshift (i.e. energy reduction) for the GaSe BPL peak was detected owing to the c-plane lattice expansion by upward bending. The intensities of the corresponding BPL peaks also show an increase with increasing curvature. The correlations between BPL peak intensity, shiny area and bond-angle widening of the bent GaSe under laser excitation have been discussed. The lattice constant versus emission energies of the bending GaSe was also analyzed. An estimated lattice constant vs. bandgap relation was present for further application of the layered GaSe in bendable flexible light-emission devices.

Curvature-dependent luminescence enhancement and bandgap shift of 2D layered GaSe under upward bending have been clearly analyzed and demonstrated.

Unidirectional Doubly Enhanced MoS2 Emission via Photonic Fano Resonances

Atomically thin transition metal dichalcogenides like MoS₂ monolayers exhibit unique luminescent properties. However, weak quantum yield and low light absorption hinder their practical applications in two-dimensional light emitting devices. Here, we report 1300 times enhancement in photoluminescence emission from a MoS₂ monolayer via simultaneous Fano resonances in a dielectric photonic crystal. The spatially extended double Fano resonance scheme allows resonant enhancement of both the MoS₂ absorption and emission. We also achieve unidirectional emission within a narrow divergence angle of 5° by engineering the Fano resonance angular dispersion. Our approach provides a new platform for efficient light sources with high directionality based on emerging two-dimensional materials.

Strong-coupling of WSe2 in ultra-compact plasmonic nanocavities at room temperature

Strong coupling of monolayer metal dichalcogenide semiconductors with light offers encouraging prospects for realistic exciton devices at room temperature. However, the nature of this coupling depends extremely sensitively on the optical confinement and the orientation of electronic dipoles and fields. Here, we show how plasmon strong coupling can be achieved in compact, robust, and easily assembled gold nano-gap resonators at room temperature. We prove that strong-coupling is impossible with monolayers due to the large exciton coherence size, but resolve clear anti-crossings for greater than 7 layer devices with Rabi splittings exceeding 135 meV. We show that such structures improve on prospects for nonlinear exciton functionalities by at least 10⁴, while retaining quantum efficiencies above 50%, and demonstrate evidence for superlinear light emission.

Two-dimensional materials offer the prospect of excitonic devices operating at room-temperature. Here, Kleemann et al. demonstrate that by tuning the number of WSe₂ layers in a nanoparticle-on-mirror geometry, room-temperature plasmon strong-coupling can be achieved with large Rabi splittings.

Tuning Excitonic Properties of Monolayer MoS2 with Microsphere Cavity by High-Throughput Chemical Vapor Deposition Method

Tuning the optical properties of 2D direct bandgap semiconductors is crucial for applications in photonic light source, optical communication, and sensing. In this work, the excitonic properties of molybdenum disulphide (MoS₂ ) are successfully tuned by directly depositing it onto silica microsphere resonators using chemical vapor deposition method. Multiple whispering gallery mode (WGM) peaks in the emission wavelength range of ≈650-750 nm are observed under continuous wave excitation at room temperature. Time-resolved photoluminescence (TRPL) and femtosecond transient absorption (TA) spectroscopy are conducted to study light-matter interaction dynamics of the MoS₂ microcavities. TRPL study suggests radiative recombination rate of carrier-phonon scattering and interband transition processes in MoS₂ is enhanced by a factor of ≈1.65 due to Purcell effect in microcavities. TA spectroscopy study shows modulation of the interband transition process mainly occurs at PB-A band with an estimated F ≈ 1.60. Furthermore, refractive index sensing utilizing WGM peaks of MoS₂ is established with sensitivity up to ≈150 nm per refractive index unit. The present work provides a large-scale and straightforward method for coupling atomically thin 2D gain media with cavities for high-performance optoelectronic devices and sensors.

Room-temperature 2D semiconductor activated vertical-cavity surface-emitting lasers

Two-dimensional (2D) semiconductors are opening a new platform for revitalizing widely spread optoelectronic applications. The realisation of room-temperature vertical 2D lasing from monolayer semiconductors is fundamentally interesting and highly desired for appealing on-chip laser applications such as optical interconnects and supercomputing. Here, we present room-temperature low-threshold lasing from 2D semiconductor activated vertical-cavity surface-emitting lasers (VCSELs) under continuous-wave pumping. 2D lasing is achieved from a 2D semiconductor. Structurally, dielectric oxides were used to construct the half-wavelength-thick cavity and distributed Bragg reflectors, in favour of single-mode operation and ultralow optical loss; in the cavity centre, the direct-bandgap monolayer WS₂ was embedded as the gain medium, compatible with the planar VCSEL configuration and the monolithic integration technology. This work demonstrates 2D semiconductor activated VCSELs with desirable emission characteristics, which represents a major step towards practical optoelectronic applications of 2D semiconductor lasers.

Two-dimensional materials have recently emerged as interesting materials for optoelectronic applications. Here, Shang et al. demonstrate two-dimensional semiconductor activated vertical-cavity surface-emitting lasers where both the gain material and the lasing characteristics are two-dimensional.

Selective Chemical Response of Transition Metal Dichalcogenides and Metal Dichalcogenides in Ambient Conditions

To fabricate practical devices based on semiconducting two-dimensional (2D) materials, the source, channel, and drain materials are exposed to ambient air. However, the response of layered 2D materials to air has not been fully elucidated at the molecular level. In the present report, the effects of air exposure on transition metal dichalcogenides (TMD) and metal dichalcogenides (MD) are studied using ultrahigh-vacuum scanning tunneling microscopy (STM). The effects of a 1-day ambient air exposure on MBE-grown WSe₂, chemical vapor deposition (CVD)-grown MoS₂, and MBE SnSe₂ are compared. Both MBE-grown WSe₂ and CVD-grown MoS₂ display a selective air exposure response at the step edges, consistent with oxidation on WSe₂ and adsorption of hydrocarbon on MoS₂, while the terraces and domain/grain boundaries of both TMDs are nearly inert to ambient air. Conversely, MBE-grown SnSe₂, an MD, is not stable in ambient air. After exposure in ambient air for 1 day, the entire surface of SnSe₂ is decomposed to SnO_x and SeO_x, as seen with X-ray photoelectron spectroscopy. Since the oxidation enthalpy of all three materials is similar, the data is consistent with greater oxidation of SnSe₂ being driven by the weak bonding of SnSe₂.

Collective Optomechanical Effects in Cavity Quantum Electrodynamics

We investigate a cavity quantum electrodynamic effect, where the alignment of two-dimensional freely rotating optical dipoles is driven by their collective coupling to the cavity field. By exploiting the formal equivalence of a set of rotating dipoles with a polymer we calculate the partition function of the coupled light-matter system and demonstrate that it exhibits a second order phase transition between a bunched state of isotropic orientations and a stretched one with all the dipoles aligned. Such a transition manifests itself as an intensity-dependent shift of the polariton mode resonance. Our work, lying at the crossroad between cavity quantum electrodynamics and quantum optomechanics, is a step forward in the ongoing quest to understand how strong coupling can be exploited to influence matter internal degrees of freedom.

Recent Advances in Electronic and Optoelectronic Devices Based on Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional transition metal dichalcogenides (2D TMDCs) offer several attractive features for use in next-generation electronic and optoelectronic devices. Device applications of TMDCs have gained much research interest, and significant advancement has been recorded. In this review, the overall research advancement in electronic and optoelectronic devices based on TMDCs are summarized and discussed. In particular, we focus on evaluating field effect transistors (FETs), photovoltaic cells, light-emitting diodes (LEDs), photodetectors, lasers, and integrated circuits (ICs) using TMDCs.

Quantum size confinement in gallium selenide nanosheets: band gap tunability versus stability limitation

Gallium selenide is one of the most promising candidates to extend the window of band gap values provided by existing two-dimensional semiconductors deep into the visible potentially reaching the ultraviolet. However, the tunability of its band gap by means of quantum confinement effects is still unknown, probably due to poor nanosheet stability. Here, we demonstrate that the optical band gap band of GaSe nanosheets can be tuned by ∼120 meV from bulk to 8 nm thick. The luminescent response of very thin nanosheets (<8 nm) is strongly quenched due to early oxidation. Oxidation favors the emergence of sharp material nanospikes at the surface attributable to strain relaxation. Simultaneously, incorporated oxygen progressively replaces selenium giving rise to Ga₂O_3, with a residual presence of Ga₂Se₃ that tends to desorb. These results are relevant for the development and design of visible/ultraviolet electronics and optoelectronics with tunable functionalities based on atomically thin GaSe.

Bulk second-harmonic generation from thermally evaporated indium selenide thin films

We investigate bulk second-order nonlinear optical properties of amorphous indium selenide thin films fabricated by thermal evaporation. Such films are shown to exhibit strong and photostable second-harmonic generation (SHG). We report strong thickness dependence of the second-harmonic signals as characterized by the Maker-fringe method. The absolute value of the nonlinear susceptibility tensor of the film is addressed by analyzing the interference of SHG signals from the film and the glass substrate. The value of the joint non-diagonal component of the susceptibility is found to be 4 pm/V, which is comparable to that of widely used second-order nonlinear materials.

Room-temperature exciton-polaritons with two-dimensional WS2

Two-dimensional transition metal dichalcogenides exhibit strong optical transitions with significant potential for optoelectronic devices. In particular they are suited for cavity quantum electrodynamics in which strong coupling leads to polariton formation as a root to realisation of inversionless lasing, polariton condensation and superfluidity. Demonstrations of such strongly correlated phenomena to date have often relied on cryogenic temperatures, high excitation densities and were frequently impaired by strong material disorder. At room-temperature, experiments approaching the strong coupling regime with transition metal dichalcogenides have been reported, but well resolved exciton-polaritons have yet to be achieved. Here we report a study of monolayer WS2 coupled to an open Fabry-Perot cavity at room-temperature, in which polariton eigenstates are unambiguously displayed. In-situ tunability of the cavity length results in a maximal Rabi splitting of ħΩRabi = 70 meV, exceeding the exciton linewidth. Our data are well described by a transfer matrix model appropriate for the large linewidth regime. This work provides a platform towards observing strongly correlated polariton phenomena in compact photonic devices for ambient temperature applications.

Broadband Enhancement of Spontaneous Emission in Two-Dimensional Semiconductors Using Photonic Hypercrystals

The low quantum yield observed in two-dimensional semiconductors of transition metal dichalcogenides (TMDs) has motivated the quest for approaches that can enhance the light emission from these systems. Here, we demonstrate broadband enhancement of spontaneous emission and increase in Raman signature from archetype two-dimensional semiconductors: molybdenum disulfide (MoS2) and tungsten disulfide (WS2) by placing the monolayers in the near field of a photonic hypercrystal having hyperbolic dispersion. Hypercrystals are characterized by a large broadband photonic density of states due to hyperbolic dispersion while having enhanced light in/out coupling by a subwavelength photonic crystal lattice. This dual advantage is exploited here to enhance the light emission from the 2D TMDs and can be utilized for developing light emitters and solar cells using two-dimensional semiconductors.

Scanning Tunneling Microscopy and Spectroscopy of Air Exposure Effects on Molecular Beam Epitaxy Grown WSe2 Monolayers and Bilayers

The effect of air exposure on 2H-WSe2/HOPG is determined via scanning tunneling microscopy (STM). WSe2 was grown by molecular beam epitaxy on highly oriented pyrolytic graphite (HOPG), and afterward, a Se adlayer was deposited in situ on WSe2/HOPG to prevent unintentional oxidation during transferring from the growth chamber to the STM chamber. After annealing at 773 K to remove the Se adlayer, STM images show that WSe2 layers nucleate at both step edges and terraces of the HOPG. Exposure to air for 1 week and 9 weeks caused air-induced adsorbates to be deposited on the WSe2 surface; however, the band gap of the terraces remained unaffected and nearly identical to those on decapped WSe2. The air-induced adsorbates can be removed by annealing at 523 K. In contrast to WSe2 terraces, air exposure caused the edges of the WSe2 to oxidize and form protrusions, resulting in a larger band gap in the scanning tunneling spectra compared to the terraces of air-exposed WSe2 monolayers. The preferential oxidation at the WSe2 edges compared to the terraces is likely the result of dangling edge bonds. In the absence of air exposure, the dangling edge bonds had a smaller band gap compared to the terraces and a shift of about 0.73 eV in the Fermi level toward the valence band. However, after air exposure, the band gap of the oxidized WSe2 edges became about 1.08 eV larger than that of the WSe2 terraces, resulting in the electronic passivation of the WSe2.

Optomechanical Enhancement of Doubly Resonant 2D Optical Nonlinearity

Emerging two-dimensional semiconductor materials possess a giant second order nonlinear response due to excitonic effects while the monolayer thickness of such active materials limits their use in practical nonlinear devices. Here, we report 3300 times optomechanical enhancement of second harmonic generation from a MoS2 monolayer in a doubly resonant on-chip optical cavity. We achieve this by engineering the nonlinear light-matter interaction in a microelectro-mechanical system enabled optical frequency doubling device based on an electrostatically tunable Fabry-Perot microresonator. Our versatile optomechanical approach will pave the way for next generation efficient on-chip tunable light sources, sensors, and systems based on molecularly thin materials.

Broadband perfect light trapping in the thinnest monolayer graphene-MoS2 photovoltaic cell: the new application of spectrum-splitting structure

The light absorption of a monolayer graphene-molybdenum disulfide photovoltaic (GM-PV) cell in a wedge-shaped microcavity with a spectrum-splitting structure is investigated theoretically. The GM-PV cell, which is three times thinner than the traditional photovoltaic cell, exhibits up to 98% light absorptance in a wide wavelength range. This rate exceeds the fundamental limit of nanophotonic light trapping in solar cells. The effects of defect layer thickness, GM-PV cell position in the microcavity, incident angle, and lens aberration on the light absorptance of the GM-PV cell are explored. Despite these effects, the GM-PV cell can still achieve at least 90% light absorptance with the current technology. Our proposal provides different methods to design light-trapping structures and apply spectrum-splitting systems.

WSe₂ Light-Emitting Tunneling Transistors with Enhanced Brightness at Room Temperature

Monolayers of molybdenum and tungsten dichalcogenides are direct bandgap semiconductors, which makes them promising for optoelectronic applications. In particular, van der Waals heterostructures consisting of monolayers of MoS2 sandwiched between atomically thin hexagonal boron nitride (hBN) and graphene electrodes allows one to obtain light emitting quantum wells (LEQWs) with low-temperature external quantum efficiency (EQE) of 1%. However, the EQE of MoS2- and MoSe2-based LEQWs shows behavior common for many other materials: it decreases fast from cryogenic conditions to room temperature, undermining their practical applications. Here we compare MoSe2 and WSe2 LEQWs. We show that the EQE of WSe2 devices grows with temperature, with room temperature EQE reaching 5%, which is 250× more than the previous best performance of MoS2 and MoSe2 quantum wells in ambient conditions. We attribute such different temperature dependences to the inverted sign of spin-orbit splitting of conduction band states in tungsten and molybdenum dichalcogenides, which makes the lowest-energy exciton in WSe2 dark.

Fabrication and characterization of PbSe nanostructures on van der Waals surfaces of GaSe layered semiconductor crystals

The growth morphology, composition and structure of PbSe nanostructures grown on the atomically smooth, clean, nanoporous and oxidized van der Waals (0001) surfaces of GaSe layered crystals were studied by means of atomic force microscopy, x-ray diffractometry,photoelectron spectroscopy and Raman spectroscopy. Semiconductor heterostructures were grown by the hot-wall technique in vacuum. Nanoporous GaSe substrates were fabricated by the thermal annealing of layered crystals in a molecular hydrogen atmosphere. The irradiation of the GaSe(0001) surface by UV radiation was used to fabricate thin Ga(2)O(3) layers with thickness < 2 nm. It was found that the narrow gap semiconductor PbSe shows a tendency to form clusters with a square or rectangular symmetry on the cleanlow-energy (0001) GaSe surface, and (001)-oriented growth of PbSe thin films takes place on this surface. Using this growth technique it is possible to grow PbSe nanostructures with different morphologies:continuous epitaxial layers with thickness < 10 nm on the uncontaminated p-GaSe(0001)surfaces, homogeneous arrays of quantum dots with a high lateral density (more than 1011 cm(−2))on the oxidized van der Waals (0001) surfaces and faceted square pillar-like nanostructures with a low lateral density (∼10(8) cm(−2)) on the nanoporous GaSe substrates. We exploit the ‘vapor–liquid–solid’ growth with low-melting metal (Ga) catalyst of PbSe crystalline branched nanostructures via a surface-defect-assisted mechanism.

Exciton-polaritons in van der Waals heterostructures embedded in tunable microcavities

Layered materials can be assembled vertically to fabricate a new class of van der Waals heterostructures a few atomic layers thick, compatible with a wide range of substrates and optoelectronic device geometries, enabling new strategies for control of light-matter coupling. Here, we incorporate molybdenum diselenide/hexagonal boron nitride (MoSe2/hBN) quantum wells in a tunable optical microcavity. Part-light-part-matter polariton eigenstates are observed as a result of the strong coupling between MoSe2 excitons and cavity photons, evidenced from a clear anticrossing between the neutral exciton and the cavity modes with a splitting of 20 meV for a single MoSe2 monolayer, enhanced to 29 meV in MoSe2/hBN/MoSe2 double-quantum wells. The splitting at resonance provides an estimate of the exciton radiative lifetime of 0.4 ps. Our results pave the way for room-temperature polaritonic devices based on multiple-quantum-well van der Waals heterostructures, where polariton condensation and electrical polariton injection through the incorporation of graphene contacts may be realized.

Intrinsic homogeneous linewidth and broadening mechanisms of excitons in monolayer transition metal dichalcogenides

The band-edge optical response of transition metal dichalcogenides, an emerging class of atomically thin semiconductors, is dominated by tightly bound excitons localized at the corners of the Brillouin zone (valley excitons). A fundamental yet unknown property of valley excitons in these materials is the intrinsic homogeneous linewidth, which reflects irreversible quantum dissipation arising from system (exciton) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Here we use optical two-dimensional Fourier transform spectroscopy to measure the exciton homogeneous linewidth in monolayer tungsten diselenide (WSe₂). The homogeneous linewidth is found to be nearly two orders of magnitude narrower than the inhomogeneous width at low temperatures. We evaluate quantitatively the role of exciton–exciton and exciton–phonon interactions and population relaxation as linewidth broadening mechanisms. The key insights reported here—strong many-body effects and intrinsically rapid radiative recombination—are expected to be ubiquitous in atomically thin semiconductors.

The band-edge optical response of transition metal dichalcogenides is dominated by tightly bound valley excitons. Here, the authors use optical two-dimensional Fourier transform spectroscopy to determine the exciton homogeneous linewidth in monolayer tungsten diselenide.

Photocurrent generation with two-dimensional van der Waals semiconductors

We review photodetectors based on transition metal dichalcogenides, novel van der Waals materials, black phosphorus, and heterostructures.