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In this work, we show that both A-excitonic and B-excitonic photoluminescence of the CVD-grown monolayer MoS2 can be tuned by electrostatic doping in air by applying a gate voltage.

Broadband and tunable microwave signals are widely used in modern communication systems and sensing systems. In this work, we demonstrate a stable and tunable optoelectronic oscillator (OEO) scheme based on external low-power beat note injection of the stimulated Brillouin signal and a phase modulation sideband of the original pump signal. The injection-locked system reduces the high-frequency disturbance induced by the beat note signal, and thus the power required only meets the OEO locking threshold requirement. In addition, the external stimulated Brillouin signal generator avoids the excessive length of the resonant cavity and guarantees the frequency stability, especially the mode stability in the OEO with a total path length of ~12 m. By varying the stimulated Brillouin signal frequency and selecting different pump modulation sidebands, a continuously tuned beat note is generated, and a beat note injected OEO with a maximum frequency up to 11.5 GHz is achieved, which is the intrinsic upper frequency limit of the devices used, even for an injection beat frequency power as low as −14 dBm ( $\sim 40~ {\mu } \text{W}$ ). Our work provides an important solution for the realization of widely tunable microwave systems.

The exploration relevant to the surface changes on optical micro- and nanofibers (MNFs) is still in infancy, and the reported original mechanisms remain long-standing puzzles. Here, by recognizing the combined interactions between fiber heating, mechanically tapering, and high-power pulsed laser guiding processes in MNFs, we establish a general thermal-mechanical-photo-activation mechanism that can explain the surface changes on MNFs. Our proposed activation mechanism can be well supported by the systematical experimental results using high-intensity nanosecond/femtosecond pulsed lasers. Especially we find large bump-like nanoscale cavities on the fracture ends of thin MNFs. Theoretically, on the basis of greatly increased bond energy activated by the fiber heating and mechanically tapering processes, the energy needed to break the silicon-oxygen bond into dangling bonds is significantly reduced from its intrinsic bandgap of ∼9 eV to as low as ∼4.0 eV, thus high-power pulsed lasers with much smaller photon energy can induce obvious surface changes on MNFs via multi-photon absorption. Finally, we demonstrate that using surfactants can repair the MNF surfaces and exploit them in promising applications ranging from sensing and optoelectronics to nonlinear optics. Our results pave the way for future preventing the performances from degradation and enabling the practical MNF-based device applications.

Chemically synthesized metal nanowires are promising building blocks for next-generation photonic integrated circuits, but technological implementation in monolithic integration will be severely hampered by the lack of controllable and precise manipulation approaches, due to the strong adhesion of nanowires to substrates in non-liquid environments. Here, we demonstrate this obstacle can be removed by our proposed earthworm-like peristaltic crawling motion mechanism, based on the synergistic expansion, friction, and contraction in plasmon-driven metal nanowires in non-liquid environments. The evanescently excited surface plasmon greatly enhances the heating effect in metal nanowires, thereby generating surface acoustic waves to drive the nanowires crawling along silica microfibres. Advantages include sub-nanometer positioning accuracy, low actuation power, and self-parallel parking. We further demonstrate on-chip manipulations including transporting, positioning, orientation, and sorting, with on-situ operation, high selectivity, and great versatility. Our work paves the way to realize full co-integration of various functionalized photonic components on single chips., Implementing metal nanowires in photonic circuits is challenging due to lack of suitable manipulation techniques. Here, the authors present an earthworm-like peristaltic crawling motion mechanism, based on surface plasmons and surface acoustic waves, and show on-chip manipulations of single nanowires.

Molybdenum disulfide (MoS2) has attracted considerable research interest as a promising candidate for downscaling integrated electronics due to the special two-dimensional structure and unique physicochemical properties. However, it is still challenging to achieve large-area MoS2 monolayers with desired material quality and electrical properties to fulfill the requirement for practical applications. Recently, a variety of investigations have focused on wafer-scale monolayer MoS2 synthesis with high-quality. The 2D MoS2 field-effect transistor (MoS2-FET) array with different configurations utilizes the high-quality MoS2 film as channels and exhibits favorable performance. In this review, we illustrated the latest research advances in wafer-scale monolayer MoS2 synthesis by different methods, including Au-assisted exfoliation, CVD, thin film sulfurization, MOCVD, ALD, VLS method, and the thermolysis of thiosalts. Then, an overview of MoS2-FET developments was provided based on large-area MoS2 film with different device configurations and performances. The different applications of MoS2-FET in logic circuits, basic memory devices, and integrated photodetectors were also summarized. Lastly, we considered the perspective and challenges based on wafer-scale monolayer MoS2 synthesis and MoS2-FET for developing practical applications in next-generation integrated electronics and flexible optoelectronics.

Subwavelength diameter micro/nanofiber (MNF) provides excellent platform for many applications. In this work, we fabricate subwavelength diameter MNFs with ultra-long waist and analyze the propagation loss origin for such fibers. Transmittance as high as 99.4% is measured for MNF with a waist length of 10 cm and diameter of $1.2\mu \text{m}$ , and the loss is mainly resulted from the modal leakage of the taper. However, when the MNF is getting longer and thinner, the intrinsic surface roughness induced during the fabrication process overrides the modal leakage and plays a major role in the propagation loss. Our study reveals existence of an ultimate transmission limitation for ultra-long MNFs.

The assessment of eco-environmental carrying capacity is the focus of planning environmental impact assessment and an important basis for determining whether the planning can be implemented. Based on the theory and method of ECO-environmental impact assessment, this paper applies the analytic hierarchy process (AHP) to analyze the ecological resilience, resource-environment carrying capacity and mining pressure of mineral resources. A comprehensive evaluation model of environmental impact of mineral resources exploitation is established. Taking Shenzhen City, Guangdong Province as an example, the difference of eco-environmental carrying capacity of the main planned mining areas is evaluated. The results show that the pressure bearing degree of most of the assessed blocks is less than 1, indicating that the pressure on the ecosystem is less than the carrying capacity. The ecosystem is not easy to change and can be restored to the original state in a short time after change, and the ecosystem can maintain normal development. This is of great guiding significance to the scientific formulation of mineral resources exploitation strategy.

We proposed a hybrid transfer method for continuous and triangular monolayer MoS2 and experimentally analyzed their excitonic photoluminescence and optoelectrical properties on Au interdigital electrodes. The photoluminescence intensities of both MoS2 on interdigital Au were lower than those of the MoS2 on SiO2/Si, and continuous MoS2 on interdigital Au showed the highest A–/A0 exciton ratio. Furthermore, the formed Au/MoS2/Au devices exhibited light-dependent Schottky-contact characteristics and the I D–V D hysteresis. The results provide a basis for understanding the photoluminescence and optoelectrical properties of different MoS2-Au contacts for improving the performance of MoS2-based optoelectrical devices.

Nonlinear optical effects, including second- and third-harmonic generation, and multi-photon luminescence, are found to be excited much more efficiently via evanescent wave coupling. Taking this advantage, we establish a microfiber-based compact device to extract the magnitude and phase information of ultrafast pulse laser. The sensitivity of this configuration can be down to femto-joule level.

Molybdenum disulfide (MoS

Single-crystal Pd nanowires with typically 100 nm diameters and lengths from several micrometers to tens of micrometers were grown on Al 2 O 3 substrates in a furnace with Ar atmosphere. Integral field emission measurements of such samples after Pd coating (~10 nm) were performed in a diode configuration with a 50 $\mu\mathrm{m}$ mica spacer in a vacuum chamber at pressures of about 10–9 mbar. The IV measurement of the sample shows an integral emission current up to 2 $\mu \mathrm{A}$ at a voltage of 500 V (10 MV/m) and an onset voltage for a current of 1 nA of about 275 V. The corresponding Fowler-Nordheim plot show a linear behavior, like expected for a metal needle.

We use a photoactivation method to enhance the room-temperature photoluminescence quantum yields in monolayer MoS2 grown on silica micro/nanofibres by more than two orders of magnitude, and realize continuous-wave lasing with greatly reduced thresholds.

Monolayer transition metal dichalcogenides (TMDs) have emerged as widely accepted 2D gain materials in the field of light sources owing to their direct bandgap and high photoluminescence quantum yield. However, the monolayer medium suffers from weak emission because only a single layer of molecules can absorb the pump energy. Moreover, the material degradation when transferring these fragile materials hinders their cooperation with the optical cavity further. In this study, for the first time, a high-quality monolithic structure is developed by directly growing single-domain tungsten diselenide (WSe2 ) bilayers on single silica microsphere (MS) cavities. Such a completely wrapped structure guides the indirect-to-direct bandgap transition of WSe2 bilayers, leading to a significantly improved photoluminescence intensity by about 60-fold. Moreover, the high-quality monolithic structure enhances the confinement factor of the cavity by more than 20-fold. Based on the above advantages, a bilayer WSe2 /MS microlaser is realized with an ultralow threshold of 0.72 W cm-2 , nearly an order of magnitude lower than the existing records. The results demonstrate the possibility of using multilayer TMD materials as 2D gain media and provide insights into a new ultracompact monolithic platform of TMD material/cavity for lasing devices.

Photoactivating oxygen dangling bonds on tapered micro/nanofibers enhances monolayer photoluminescence quantum yields., Although monolayer transition metal dichalcogenides (TMDs) have direct bandgaps, the low room-temperature photoluminescence quantum yields (QYs), especially under high pump intensity, limit their practical applications. Here, we use a simple photoactivation method to enhance the room-temperature QYs of monolayer MoS2 grown on to silica micro/nanofibers by more than two orders of magnitude in a wide pump dynamic range. The high-density oxygen dangling bonds released from the tapered micro/nanofiber surface are the key to this strong enhancement of QYs. As the pump intensity increases from 10−1 to 104 W cm−2, our photoactivated monolayer MoS2 exhibits QYs from ~30 to 1% while maintaining high environmental stability, allowing direct lasing with greatly reduced thresholds down to 5 W cm−2. Our strategy can be extended to other TMDs and offers a solution to the most challenging problem toward the realization of efficient and stable light emitters at room temperature based on these atomically thin materials.

Miniaturizing spectrometers Spectroscopy is a ubiquitous characterization tool spanning most scientific and many industrial disciplines. Most handheld spectrometers are based on tabletop optical components, which limits the scale to which these spectrometers can be shrunk. To address the desire for miniaturized spectrometers with a micrometer-scale (and smaller) footprint, Yang et al. developed such a microspectrometer based on single, compositionally engineered nanowire. This result is a practical step forward for the use of other light-sensitive nanomaterials for such ultra-miniaturized spectroscopy platforms. Science , this issue p. 1017

Based on gain and loss engineering approach, we realize single-whispering-gallery-mode lasing in polymer bottle microresonators and then demonstrate stretchable strain sensing and temperature sensing devices.

Figure S1. DAST NWs preparation. (DOC 2375 kb)

The compact cavity is always conducive to the integration and application of the devices based on whispering gallery modes. However, it is a challenge to strike a balance between the pursuit of a smaller cavity and higher quality. Here, we propose a low-loss cavity by combining a microsphere with a metal substrate, thereby compressing the cavity size by 24%. In addition, the substrate-modulated cavity is found to have a higher confinement factor and Purcell factor, showing the good prospect for realizing high-performance compact lasers.

The ability to manipulate microlaser performance is highly desirable so as to promote on-chip classical and quantum information-processing technology. Here, we demonstrate that mode manipulation of bottle microresonators is enabled by precise deposition of single gold nanoparticles in a reconfigurable and selective manner. Numerical investigation reveals the mechanism of introducing optical loss via single Au NP scattering. Experimental results show that the lasing action of cavity modes could be efficiently suppressed, and single mode lasing is successfully achieved with a high side mode suppression factor ∼13 dB.

High quality single-crystal GaSe nanoribbons (NRs) are fabricated and demonstrated as waveguides with good performance. By taking advantage of the strong confinement and long interaction length of the waveguiding approach, significantly enhanced nonlinear light–matter interaction is observed in GaSe NRs. Based on transverse SH generation, a single GaSe NR-configured optical auto-correlator is constructed for ultrashort pulse characterization. Because of the large second-order nonlinear susceptibility, high sensitivity with femto-joule-level pulse energy that has higher sensitivity than those reported before is demonstrated.

Stimulating higher-order nonlinear optical (HO-NLO) response from individual semiconductor nanostructures is challenging due to the low nonlinear coefficients and the small number of molecules within the nanostructures. In this work, we demonstrate efficient third harmonic generation and multi-photon luminescence in CdSe nanowaveguides by means of evanescent wave coupling technique. Under appropriate conditions, a coupling efficiency of 70% can be achieved from an optical microfiber to a single CdSe nanowaveguide, leading to the enhanced HO-NLO effects. Provided a high signal-to-noise ratio, we thus observe a fourth order excitation power dependence of 3-photon luminescence, and we attribute it to surface defect mechanism based on the recombination of free carriers. This work provides an alternative for efficient excitation for HO-NLO, which also makes these hard-to-produce signals more feasible in the applications of nonlinear optical devices.

Due to the broad scattering spectral profiles, localized surface plasmon resonances (LSPRs) of Pd nanoparticles have low resolution and limited sensitivity for hydrogen detection. In this work, we use a simple light-irradiation method to demonstrate that free-space light can be efficiently coupled into and from the microfiber whispering-gallery modes (WGMs) by the Pd nanoantennas. The nanoantenna–microfiber cavity system provides strong intermodal coupling between LSPRs and WGMs, and induces significant modulation of the scattering spectra. A measured full width at half-maximum of 3.2 nm at 622.7 nm is obtained, which is the narrowest in Pd nanoparticle-based LSPR structures reported up to now. The ultranarrow resonances offer enhanced sensitivity to hydrogen gas detection with a figure of merit reaching ∼2.22. Other advantages of the Pd nanoantenna–microfiber cavity system including independence of precise alignment of excitation light, large tunability of the resonant wavelengths, easy and low-cost fabrication of the system, have also been demonstrated.

Semiconductors have large optical nonlinear susceptibilities especially in the spectral range above material bandgaps. However, optical frequency conversion encounters large absorption above bandgaps and difficulty using common phase-matching techniques. The frequency conversion bandwidths are thus limited. Here, frequency up-conversion far above the bandgaps using surface emissions from semiconductor nanoribbons is demonstrated, wherein nanoscale waveguiding tightly confines fundamental waves for decreasing pump powers, and above-bandgap absorption is greatly decreased in nanoscale waveguide thickness. By using CdSe nanoribbons, efficient 532- and 404-nm second-harmonic and 459-nm sum-frequency generations are obtained with the continuous-wave pump power less than 100 μW. The normalized efficiency of 532-nm second-harmonic generation is about 2 × 10 -5 mm -1 at pump power of 300 μW. Attractive features such as tunable spatial distribution and highly polarization are observed. A broadband emission with a full width half maximum of ~10 nm is attained by frequency summing a continuous-wave laser and an amplified spontaneous emission source.

By using PdAu nanowires as plasmonic waveguides, hybrid photon-plasmon Mach-Zehnder interferometers by integrating single-crystal PdAu alloy nanowires with silica optical microfibers are demonstrated. Based on an evanescent wave coupling technique using optical fiber tapers, surface plasmon polaritons are efficiently excited and propagated in suspended PdAu nanowires. The interference spectra show attractive properties such as broad and flexible in situ tunability with wavelength spacings ranging from ∼1 to tens of nanometers, and high extinction ratios of over 20 dB. The hybrid Mach-Zehnder interferometers show a higher sensitivity to hydrogen gas than a single-nanowire sensing approach, and the lengths of PdAu nanowires used are less than 20 μm, which are 2 or 3 orders of magnitude shorter than the lengths of Pd coatings in existing fiber-optic hydrogen sensors. Other advantages including good reversibility and low-power operation are also obtained.

Based on engineering the pump intensity to modify the spatial gain profiles of bottle microresonators, we demonstrate a simple and general approach to realize single-whispering- gallery-mode (WGM) lasing in polymer bottle microresonators.

Transverse frequency conversion in the sub-bandgap spectral region is investigated in semiconductor nanowires and nanoribbons using CW lasers with a pump power less than 1 mW. It is found that the properties of the emissions are strongly dependent on the cross-sectional geometries and the surrounding media of nano-waveguides. The polarization is higher in nano-waveguides under single-mode conditions, and the spatial distribution is more tunable in nano-waveguides with higher-order modes involved. Compared with the birefringent approach, transverse frequency conversion shows lower divergence angles, higher polarization, and more tunable spatial distribution.

In this study, we propose a one-drop self-assembly method, which proved capable of successfully preparing 4-N, N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST) single-crystalline nanowires (NWs). The apparent roughness of the DAST NWs was determined to be less than 100 pm by using a high-resolution atomic force microscope, indicating their ultrafine quality. The DAST NWs also exhibited excellent nonlinear optical properties, including two-photon excited fluorescence and second harmonic generation, which could enable the production of low-cost, low-power-consumption wideband wavelength conversion devices. Thus, the described method may provide a new avenue for organic NW fabrication. Electronic supplementary material The online version of this article (10.1186/s11671-019-3103-y) contains supplementary material, which is available to authorized users.

We demonstrate an effective approach of mode suppression by simply using a tungsten probe to destroy the external neck surface of polymer microbottle resonators. The higher-order bottle modes with large axial orders, spatially located around the neck surface of the microresonator, will suffer large optical losses. Thus, excitation just with an ordinary free-space light beam will ensure direct generation of single fundamental bottle mode lasers. This method is with very low cost and convenient and can obtain high side-mode suppression factors. Our work demonstrated here may have promising applications such as in lasing and sensing devices.

In this work plasmon propagation and highly sensitive hydrogen sensing are demonstrated in single-crystal Pd and its alloy nanowires, based on an evanescent wave coupling technique using optical fi ber tapers. The plasmon propagation losses of Pd nanowires on an MgF 2 substrate are measured to be ∼ 2 dB/ μ m (at 635 nm). Using a suspension scheme of the nanowire, the propagation losses decrease to ∼ 0.6 dB/ μ m in air. Utilizing an alloying technique, single-crystal PdAg nanowires are fabricated and exhibit decreasing and tunable propagation losses. Furthermore, Pd nanowires are used for plasmonic gas sensing with amplitude sensitivity of ∼ 11 dB to 3.6% hydrogen gas, which is much higher than those in photonic nanowire sensors. The concentration detection limit is about 0.2%, which is lower than those in photonic nanowire sensors and most surface plasmon resonance sensors. The hysteresis effect of the Pd nanowire in hydrogen sensing is suppressed by using single-crystal PdAu alloy nanowires. This work may fi nd widespread application, ranging from sensing and integrated circuits to materials research.

The use of ultrashort pulses for fundamental studies and applications has been increasing rapidly in the past decades. Along with the development of ultrashort lasers, exploring new pulse diagnositic approaches with higher signal-to-noise ratio have attracted great scientific and technological interests. In this work, we demonstrate a simple technique of ultrashort pulses characterization with a single semiconductor nanowire. By performing a frequency-resolved optical gating method with a ZnO nanowire coupled to tapered optical microfibers, the phase and amplitude of a pulse series are extracted. The generated signals from the transverse frequency conversion process can be spatially distinguished from the input, so the signal-to-noise ratio is improved and permits lower energy pulses to be identified. Besides, since the nanometer scale of the nonlinear medium provides relaxed phase-matching constraints, a measurement of 300-nm-wide supercontinuum pulses is achieved. This system is highly compatible with standard optical fiber systems, and shows a great potential for applications such as on-chip optical communication.

Recently, handprint-based recognition system has been widely applied for security and surveillance purposes. The success of this technology has also demonstrated that handprint is a good approach to perform forensic identification. However, existing identification systems are nearly based on the handprints that could be easily prevented. In contrast to earlier works, we exploit the thermal handprint and introduce a novel distance metric for thermal handprint dissimilarity measure, called Heat-Earth Mover's Distance (HEMD). The HEMD is designed to classify heat-based handprints that can be obtained even when the subject wears a glove. HEMD can effectively recognize the subjects by computing the distance between point distributions of target and training handprints. Through a comprehensive study, our identification system demonstrates the performance even with the handprints obtained by the subject wearing a glove. With 20 subjects, our proposed system achieves an accuracy of 94.13%for regular handprints and 92.00% for handprints produced with latex gloves.

Free-space light is efficiently coupled into and from the palladium nanoantenna-microfiber whispering-gallery cavity systems. A measured full width at half-maximum of 3.2 nm at 622.7 nm and enhanced sensitivity to hydrogen detection are obtained.

We demonstrate single-mode laser emission in single nanowires. By folding a 200 nm diameter CdSe nanowire to form loop mirrors, single-mode laser emission around 738 nm wavelength is obtained with line width of 0.12 nm and low threshold. The mode selection is realized by the vernier effect of coupled cavities in the folded nanowire. In addition, the loop structure makes it possible to tune the nanowire cavity, opening an opportunity to realize a tunable single-mode nanowire laser.

We investigate the microscopic optical force density distributions respectively inside a subwavelength-diameter (SD) fiber with flat endface and inside one with oblique endface by using a finite-difference time-domain (FDTD) method. Optical force density distributions at the fiber endfaces can now be readily available. The complete knowledge of optical force density distributions not only reveal features regarding the microscopic near-field optomechanical interaction, but also provide straightforward explanations for the sideway deflections and other mechanical motions. Our results can provide a useful reference for better understanding the mechanical influence when light transports in a microscale or nanoscale structure and for developing future highly-sensitive optomechanical devices.

We report a simple electrospinning method to fabricate polymer bottle microresonators, which are doped with a lasing gain material and supported by electrospun polymer micro/nanofibers on a flexible grooved polymer substrate. The fabricated bottle microresonators have smooth outer surfaces and high quality. By using an interference light pump approach, single whispering gallery mode lasing is obtained, with a side-mode suppression factor over 20 dB. By mechanically stretching the grooved substrate, tunability of the lasing peaks is demonstrated. Our method has the advantages of saving time and being low in cost and may have promising applications in stretchable lasing and sensing devices.

We report highly versatile nanosensors using polymer single nanowires. On the basis of the optical response of waveguiding polymer single nanowires when exposed to specimens, functionalized polymer nanowires are used for humidity sensing with a response time of 30 ms and for NO 2 and NH 3 detection down to subparts-per-million level. The compact and flexible sensing scheme shown here may be attractive for very fast detection in physical, chemical, and biological applications with high sensitivity and small footprint.

Nanofluidic systems are attracting a great deal of interest due to their fundamental significance and potential applications in chemistry, biology and physics. However, high fabrication cost, expensive equipments and complicated fabrication process of most current fabrication techniques prevent lots of researchers from entering the nanofluidic field. Here we present a quick, simple and cost-effective method for fabricating two-dimensional (2D) nanochannel in polycarbonate (PC) substrates. Silica nanowires, taper-drawn from commercially available single-mode fiber were used as templates and embedded in the PC substrate by hot embossing. The nanochannels were created after removing the nanowires by hydrofluoric acid (HF) etching. Poly(dimethylsiloxane) (PDMS) was used to seal the nanochannel reversibly. Nanochannels with widths range from 100 to 900 nm and lengths up to several millimeters were obtained. Various nanostructures including integrated micro and nanochannels, nanochannel array, bent nanochannel and cross-shaped nanochannel were fabricated and characterized by fluorescent microscope, scanning electron microscope (SEM) and atomic force microscope (AFM).

Exploring new nanowaveguide materials and structures is of great scientific interest and technological significance for optical and photonic applications. In this work, high-quality single-crystal MoO3 nanoribbons (NRs) are synthesized and used for optical guiding. External light sources are efficiently launched into the single MoO3 NRs using silica fiber tapers. It is found that single MoO3 NRs are as good nanowaveguides with loss optical losses (typically less than 0.1 dB/μm) and broadband optical guiding in the visible/near-infrared region. Single MoO3 NRs have good Raman gains that are comparable to those of semiconductor nanowaveguides, but the second harmonic generation efficiencies are about 4 orders less than those of semiconductor nanowaveguides. And also no any third-order nonlinear optical effects are observed at high pump power. A hybrid Fabry-Pérot cavity containing an active CdSe nanowire and a passive MoO3 NR is also demonstrated and the ability of coupling light from other active nanostructures and fluorescent liquid solutions has been further demonstrated. These optical properties make single MoO3 NRs attractive building blocks as elements and interconnects in miniaturized photonic circuitries and devices.