Unraveling the strain tuning mechanism of interlayer excitons in WSe2/MoSe2heterostructure.

Atomically thin transition metal dichalcogenides (TMDs) exhibit rich excitonic physics, due to reduced dielectric screening and strong Coulomb interactions. Especially, some attractive topics in modern condensed matter physics, such as correlated insulator, superconductivity, topological excitons bands, are recently reported in stacking two monolayer (ML) TMDs. Here, we clearly reveal the tuning mechanism of tensile strain on interlayer excitons (IEXs) and intralayer excitons (IAXs) in WSe2/MoSe2heterostructure (HS) at low temperature. We utilize the cryogenic tensile strain platform to stretch the HS, and measure by micro-photoluminescence (µ-PL). The PL peaks redshifts of IEXs and IAXs in WSe2/MoSe2HS under tensile strain are well observed. The first-principles calculations by using density functional theory reveals the PL peaks redshifts of IEXs and IAXs origin from bandgap shrinkage. The calculation results also show the Mo-4d states dominating conduction band minimum shifts of the ML MoSe2plays a dominant role in the redshifts of IEXs. This work provides new insights into understanding the tuning mechanism of tensile strain on IEXs and IAXs in two-dimensional (2D) HS, and paves a way to the development of flexible optoelectronic devices based on 2D materials.

Dynamically Reconfigurable on-Chip Polarimeters Based on Nanoantenna Enabled Polarization Dependent Optoelectronic Computing.

On-chip polarization detectors have attracted extensive research interest due to their filterless and ultracompact architecture. However, their polarization-dependent photoresponses cannot be dynamically adjusted, hindering the development toward intelligence. Here, we propose dynamically reconfigurable polarimetry based on in-sensor differentiation of two self-powered photoresponses with orthogonal polarization dependences and tunable responsivities. Such a device can be electrostatically configured in an ultrahigh polarization extinction ratio (PER) mode, where the PER tends to infinity, a Stokes parameter direct sensing mode, where the photoresponse is proportional to S1 or S2 with high accuracy (RMSES1 = 1.5%, RMSES2 = 2.0%), or a background suppressing mode, where the target-background polarization contrast is singularly enhanced. Moreover, the device achieves a polarization angle sensitivity of 0.51 mA·W-1·degree-1 and a specific polarization angle detectivity of 2.8 × 105 cm·Hz1/2·W·degree-1. This scheme is demonstrated throughout the near-to-long-wavelength infrared range, and it will bring a leap for next-generation on-chip polarimeters.

Exploring the emerging of electronic and magnetic properties with adatom adsorption on a novel semiconductor monolayer: N2P6.

The effect of adsorbed adatoms on the structural stability and electronic properties of monolayer N2P6 have been systematically studied via first-principles simulation methods. It is found that pristine N2P6 is an indirect 0.21 eV band gap semiconductor, with a pleated honeycomb-like structure similar to phosphorene. The calculation results show that adsorbed adatoms can modify the properties of monolayer N2P6 effectively. The degree of local distortion strongly depends on the electronegativity and size of adatoms, also the adsorption energy ranges from 0.3 to 5.8 eV depending on the species of adatoms. The electronic properties show metallic behavior with several adsorbed metal atoms (Li, Na, Al, K, Cu, Ni, and Zn) and some non-metal atoms (H, F, P, and Cl), while adsorbed O, S, Ca, and Si atoms still remain semiconductors. The systems of Ni and Zn adatoms show ferromagnetic behavior, and adsorbed Mg exhibits a half-metallic character. Our theoretical studies indicate that N2P6 possesses potential application in the field of gas sensors.

Slowing Hot-Electron Relaxation in Mix-Phase Nanowires for Hot-Carrier Photovoltaics.

Hot carrier harvest could save 30% energy loss in solar cells. So far, however, it is still unreachable as the photoexcited hot carriers are short-lived, ∼1 ps, determined by a rapid relaxation process, thus invalidating any reprocessing efforts. Here, we propose and demonstrate a feasible route to reserve hot electrons for efficient collection. It is accomplished by an intentional mix of cubic zinc-blend and hexagonal wurtzite phases in III-V semiconductor nanowires. Additional energy levels are then generated above the conduction band minimum, capturing and storing hot electrons before they cool down to the band edges. We also show the superiority of core/shell nanowire (radial heterostructure) in extracting hot electrons. The strategy disclosed here may offer a unique opportunity to modulate hot carriers for efficient solar energy harvest.

Quantum Dot Self-Assembly Enables Low-Threshold Lasing.

Perovskite quantum dots (QDs) are of interest for solution-processed lasers; however, their short Auger lifetime has limited lasing operation principally to the femtosecond temporal regime the photoexcitation levels to achieve optical gain threshold are up to two orders of magnitude higher in the nanosecond regime than in the femtosecond. Here the authors report QD superlattices in which the gain medium facilitates excitonic delocalization to decrease Auger recombination and in which the macroscopic dimensions of the structures provide the optical feedback required for lasing. The authors develope a self-assembly strategy that relies on sodiumd-an assembly director that passivates the surface of the QDs and induces self-assembly to form ordered three-dimensional cubic structures. A density functional theory model that accounts for the attraction forces between QDs allows to explain self-assembly and superlattice formation. Compared to conventional organic-ligand-passivated QDs, sodium enables higher attractive forces, ultimately leading to the formation of micron-length scale structures and the optical faceting required for feedback. Simultaneously, the decreased inter-dot distance enabled by the new ligand enhances exciton delocalization among QDs, as demonstrated by the dynamically red-shifted photoluminescence. These structures function as the lasing cavity and the gain medium, enabling nanosecond-sustained lasing with a threshold of 25 µJ cm-2 .

Room temperature exciton-polariton Bose-Einstein condensation in organic single-crystal microribbon cavities.

Exciton-polariton Bose-Einstein condensation (EP BEC) is of crucial importance for the development of coherent light sources and optical logic elements, as it creates a new state of matter with coherent nature and nonlinear behaviors. The demand for room temperature EP BEC has driven the development of organic polaritons because of the large binding energies of Frenkel excitons in organic materials. However, the reliance on external high-finesse microcavities for organic EP BEC results in poor compactness and integrability of devices, which restricts their practical applications in on-chip integration. Here, we demonstrate room temperature EP BEC in organic single-crystal microribbon natural cavities. The regularly shaped microribbons serve as waveguide Fabry-Pérot microcavities, in which efficient strong coupling between Frenkel excitons and photons leads to the generation of EPs at room temperature. The large exciton-photon coupling strength due to high exciton densities facilitates the achievement of EP BEC. Taking advantages of interactions in EP condensates and dimension confinement effects, we demonstrate the realization of controllable output of coherent light from the microribbons. We hope that the results will provide a useful enlightenment for using organic single crystals to construct miniaturized polaritonic devices.

Realization of strong coupling between 2D excitons and cavity photons at room temperature.

Two-dimensional (2D) semiconductors of graphene, as well as transition-metal dichalcogenides, have performed strong interaction with light. Here the strong light-matter interaction between monolayer tungsten disulphide (WS2) excitons and microcavity photons at room temperature is well studied by the introduction of a gain material embedded dielectric optical microcavity structure. A Rabi splitting of about 36 meV is observed in angle-resolved reflectance spectra at room temperature, which agrees well with the theoretical results simulated by using the transfer matrix method. Since the cavity structures and 2D semiconductors can be prepared, the cavity and the gain materials, respectively, can be optimized separately in this platform. An all-dielectric Fabry-Pérot microcavity provides a simple but effective way to study the room temperature strong coupling between cavity photons and 2D excitons.

Strong fluorescence blinking of large-size all-inorganic perovskite nano-spheres.

We demonstrated strong fluorescence blinking on large all-inorganic perovskite (CsPbBr3) nano-spheres. By performing (time-resolved) micro-photoluminescence (µ-PL) measurements, the unique blinking characteristics of the as-grown nano-spheres with diameters of hundred nanometers, are clearly observed. Blinking has no obvious on/off states, which is different from the blinking characteristics of quantum dots. It is believed that the blinking of fluorescence is caused by metastable defect-induced trapping of carriers on the surface of the nano-spheres, because dramatically suppressed fluorescence blinking and the decay rates of ultrafast carriers are realized by surface passivation of the nano-spheres. Surface defects are closely related to the ambient atmosphere, which has been further confirmed by PL measurements of the as-grown nano-spheres in vacuum. Additionally, we also found that the fluorescence blinking was significantly suppressed as the sample size increased, which can be attributed to the large-size induced average effect on fluorescence blinking. These results may be important for understanding the mechanism of the fluorescence blinking of perovskite materials and for developing optical devices with good fluorescence stability.

Near-field imaging of the multi-resonant mode induced broadband tunable metamaterial absorber.

Metamaterial absorbers with tunability have broad prospects for mid-infrared absorption applications. While various methods have been proposed to control absorption, how to analyse and present the physical image of absorption mechanism in depth is still expected and meaningful. Here, we present experimental spatial near-field distributions of a multi-resonant mode induced broadband tunable metamaterial absorber by using near-field optical microscopy. The absorber is constructed by a metal double-sized unit cell and a metallic mirror separated by a thin Ge2Sb2Te5 (GST) spacer. To clearly obtain the physical images, we used a hybrid unit cell consisting of four square resonators to produce two absorption peaks at 7.8 µm and 8.3 µm. The resonance central-wavelength exhibits a redshift while switching the GST thin film from amorphous to crystalline phase. The near-field amplitude and phase optical responses of the absorber are directly observed at absorption frequencies when GST is in both phases, respectively. This work will pave the way for the fundamental science field and inspire potential applications in optical tunable absorption control.

Large-area, lithography-free, narrow-band and highly directional thermal emitter.

Thermal radiation with narrow bandwidth and well-defined emission directions is highly sought after for a variety of applications, ranging from infrared sensing and thermal imaging to thermophotovoltaics. Here, a large-area (4-inch-diameter) long-wavelength infrared thermal emitter is presented, which is spectrally selective, highly directional, and easily fabricated. The basic structure of the proposed thermal emitter is composed of a truncated one-dimensional photonic crystal and a continuous metallic film separated by a dielectric spacer. Experimental results show that the emitter exhibits a narrowband thermal emittance peak of 92% in the normal direction at the wavenumber of 943.4 cm-1 with a bandwidth of 12.5 cm-1 and a narrow angular emission lobe with a limited solid angle of 0.325 sr (0.115 sr) for s (p) polarization. Numerical simulation analyses are performed to corroborate the experimental observations. Temporal coupled-mode theory combined with transfer matrix method is employed to analytically investigate the emission properties of the structure, which not only can be used to understand the experimental results, but also plays a certain guidance role in designing a thermal emitter with the desired properties. The present thermal emitter can be implemented for thermal photonics management, allowing applications in thermal imaging and medical systems, etc.

An All-Inorganic Perovskite-Phase Rubidium Lead Bromide Nanolaser.

Rubidium lead halides (RbPbX3 ), an important class of all-inorganic metal halide perovskites, are attracting increasing attention for photovoltaic applications. However, limited by its lower Goldschmidt tolerance factor t≈0.78, all-inorganic RbPbBr3 has not been reported. Now, the crystal structure, X-ray diffraction (XRD) pattern, and band structure of perovskite-phase RbPbBr3 has now been investigated. Perovskite-phase RbPbBr3 is unstable at room temperature and transforms to photoluminescence (PL)-inactive non-perovskite. The structural evolution and mechanism of the perovskite-non-perovskite phase transition were clarified in RbPbBr3 . Experimentally, perovskite-phase RbPbBr3 was realized through a dual-source chemical vapor deposition and annealing process. These perovskite-phase microspheres showed strong PL emission at about 464 nm. This new perovskite can serve as a gain medium and microcavity to achieve broadband (475-540 nm) single-mode lasing with a high Q of about 2100.

Ultrasensitive Hybrid MoS2-ZnCdSe Quantum Dot Photodetectors with High Gain.

Recently, two-dimensional (2D) materials, especially transition-metal dichalcogenides (TMDCs), have attracted extensive interest owing to their potential applications in optoelectronics. Here, we demonstrate a hybrid 2D-zero-dimensional (0D) photodetector, which consists of a single-layer or few-layer molybdenum disulfide (MoS2) thin film and a thin layer of core/shell zinc cadmium selenide/zinc sulfide (ZnCdSe/ZnS) colloidal quantum dots (QDs). It is worth mentioning that the photoresponsivity of the hybrid 2D-0D photodetector is 3 orders of magnitude larger than the TMDC photodetector (from 10 to 104 A W-1). The detectivity of the hybrid structure detector is up to 1012 Jones, and the gain is up to 105. Due to an effective energy transfer from the photoexcited QD sensitizing layer to MoS2 films, light absorption is enhanced and more excitons are generated. Thus, this hybrid 2D-0D photodetector takes advantage of high charge mobility in the MoS2 layer and efficient photon absorption/exciton generation in the QDs, which suggests their promising applications in the development of TMDC-based optoelectronic devices.

Thickness-controlled direct growth of nanographene and nanographite film on non-catalytic substrates.

Metal-catalyzed chemical vapor deposition (CVD) has been broadly employed for large-scale production of high-quality graphene. However, a following transfer process to targeted substrates is needed, which is incompatible with current silicon technology. We here report a new CVD approach to form nanographene and nanographite films with accurate thickness control directly on non-catalytic substrates such as silicon dioxide and quartz at 800 °C. The growth time is as short as a few seconds. The approach includes using 9-bis(diethylamino)silylanthracene as the carbon source and an atomic layer deposition (ALD) controlling system. The structure of the formed nanographene and nanographite films were characterized using atomic force microscopy, high resolution transmission electron microscopy, Raman scattering, and x-ray photoemission spectroscopy. The nanographite film exhibits a transmittance higher than 80% at 550 nm and a sheet electrical resistance of 2000 ohms per square at room temperature. A negative temperature-dependence of the resistance of the nanographite film is also observed. Moreover, the thickness of the films can be precisely controlled via the deposition cycles using an ALD system, which promotes great application potential for optoelectronic and thermoelectronic-devices.

Single-Mode Lasers Based on Cesium Lead Halide Perovskite Submicron Spheres.

Single-mode laser is realized in a cesium lead halide perovskite submicron sphere at room temperature. All-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) microspheres with tunable sizes (0.2-10 µm) are first fabricated by a dual-source chemical vapor deposition method. Due to smooth surface and regular geometry structure of microspheres, whispering gallery resonant modes make a single-mode laser realized in a submicron sphere. Surprisingly, a single-mode laser with a very narrow line width (∼0.09 nm) was achieved successfully in the CsPbX3 spherical cavity at low threshold (∼0.42 µJ cm-2) with a high cavity quality factor (∼6100), which are the best specifications of lasing modes in all natural nano/microcavities ever reported. By modulating the halide composition and sizes of the microspheres, the wavelength of a single-mode laser can be continuously tuned from red to violet (425-715 nm). This work illustrates that the well-controlled synthesis of metal cesium lead halide perovskite nano/microspheres may offer an alternative route to produce a widely tunable and greatly miniaturized single-mode laser.

Exciton-Polariton Fano Resonance Driven by Second Harmonic Generation.

Angle-resolved second harmonic generation (SHG) spectra of ZnO microwires show characteristic Fano resonances in the spectral vicinity of exciton-polariton modes. We observe a resonant peak followed by a strong dip in SHG originating from the constructive and destructive interference of the nonresonant SHG and the resonant contribution of the polariton mode. It is demonstrated that the Fano line shape, and thus the Fano asymmetry parameter q, can be tuned by the phase shift of the two channels. We develop a model to calculate the phase-dependent q as a function of the radial angle in the microwire and achieve a good agreement with the experimental results. The deduced phase-to-q relation unveils the crucial information about the dynamics of the system and offers a tool for control on the line shape of the SHG spectra in the vicinity of exciton-polariton modes.

Direct observation of strong light-exciton coupling in thin WS2 flakes.

The strong coupling between excitons and Fabry-Pérot (F-P) cavity modes in tungsten sulfide (WS2) thin layers was studied. By using home-made micro-reflectance spectroscopic technique, we observed the anti-crossing behavior between cavity mode and excitons with flake's thickness dependence. Giant Rabi splitting of ~270 meV for A exciton and ~780 meV for B exciton were obtained from the fitting of the cavity polariton dispersions by use of the coupled oscillators model.

Emission energy, exciton dynamics and lasing properties of buckled CdS nanoribbons.

We report the modulation of emission energy, exciton dynamics and lasing properties in a single buckled CdS nanoribbon (NR) by strain-engineering. Inspired by ordered structure fabrication on elastomeric polymer, we develop a new method to fabricate uniform buckled NRs supported on polydimethylsiloxane (PDMS). Wavy structure, of which compressive and tensile strain periodically varied along the CdS NR, leads to a position-dependent emission energy shift as large as 14 nm in photoluminescence (PL) mapping. Both micro-PL and micro-reflectance reveal the spectral characteristics of broad emission of buckled NR, which can be understood by the discrepancy of strain-induced energy shift of A- and B-exciton of CdS. Furthermore, the dynamics of excitons under tensile strain are also investigated; we find that the B-exciton have much shorter lifetime than that of redshifted A-exciton. In addition, we also present the lasing of buckled CdS NRs, in which the strain-dominated mode selection in multi-mode laser and negligible mode shifts in single-mode laser are clearly observed. Our results show that the strained NRs may serve as new functional optical elements for flexible light emitter or on-chip all-optical devices.

Enhanced second-harmonic generation from metal-integrated semiconductor nanowires via highly confined whispering gallery modes.

Coherent and tunable nanoscale light sources utilizing optical nonlinearities are required for applications ranging from imaging and bio-sensing to on-chip all-optical signal processing. However, owing to their small sizes, the efficiency of nanostructures even with high nonlinear coefficients is poor, therefore requiring very high excitation energies. Although surface-plasmon resonances of metal nanostructures can enhance surface nonlinear processes such as second-harmonic generation, they still suffer from low conversion efficiencies owing to their intrinsically low nonlinear coefficients. Here we show highly enhanced and directional second-harmonic generation from individual CdS nanowires integrated with silver nanocavities (>1,000 times higher external efficiency compared with bare CdS), in which the lowest-order whispering gallery mode is engineered to concentrate light in the nonlinear material while minimizing Ohmic losses. The directional nonlinear signal is redirected into another waveguide, which is then utilized to configure an optical router that can potentially serve as a tunable coherent light source to enable on-chip signal processing for integrated nanophotonic systems.

Resolving parity and order of Fabry-Pérot modes in semiconductor nanostructure waveguides and lasers: Young's interference experiment revisited.

Semiconductor nanostructures such as nanowires and nanoribbons functioning as Fabry-Pérot (F-P)-type optical cavities and nanolasers have attracted great interest not only for their potential use in nanophotonic systems but also to understand the physics of light-matter interactions at the nanoscale. Due to their nanoscale dimensions, new techniques need to be continuously developed to characterize the nature of highly confined optical modes. Furthermore, the inadequacy of typical far-field photoluminescence experiments for characterizing the nanoscale cavity modes such as parity and order has precluded efforts to obtain precise information that is required to fully understand these cavities. Here, we utilize a modified Young's interference method based on angle-resolved microphotoluminescence spectral technique to directly reveal the parity of F-P cavity modes in CdS nanostructures functioning as waveguides and nanolasers. From these analyses, the mode order can be straightforwardly obtained with the help of numerical simulations. Moreover, we show that the Young's technique is a general technique applicable to any F-P type cavities in nanoribbons, nanowires, or other photonic and plasmonic nanostructures.

Strain-induced large exciton energy shifts in buckled CdS nanowires.

Strain engineering can be utilized to tune the fundamental properties of semiconductor materials for applications in advanced electronic and photonic devices. Recently, the effects of large strain on the properties of nanostructures are being intensely investigated to further expand our insights into the physics and applications of such materials. In this Letter, we present results on controllable buckled cadmium-sulfide (CdS) optical nanowires (NWs), which show extremely large energy bandgap tuning by >250 meV with applied strains within the elastic deformation limit. Polarization and spatially resolved optical measurements reveal characteristics related to both compressive and tensile regimes, while microreflectance spectroscopy clearly demonstrates the effect of strain on the different types of excitons in CdS. Our results may enable strained NWs-based optoelectronic devices with tunable optical responses.