Continuous ultra-wideband signal regeneration in random optoelectronic oscillators through injection locking.

By using an external injection locking method, for what we believe to be the first time, we experimentally demonstrate continuous ultra-wideband signal regeneration in random optoelectronic oscillators, achieving more adaptable signal processing capabilities than self-oscillation methods. Supported by theoretical analysis and experimental evidences, this system can regenerate any signal with sufficient gain in a random-feedback cavity, independent of cavity filters. Remarkably, enhanced phase noise performance with over 35.2 dB side mode suppression and a phase noise better than -86 dBc@1 kHz at higher injecting powers are demonstrated. Additionally, we successfully process complex multi-frequency communication signals, indicating potential applications in radar, remote sensing, and data communications.

Thermal-mechanical-photo-activation effect on silica micro/nanofiber surfaces: origination, reparation and utilization.

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.

Efficient higher-order nonlinear optical effects in CdSe nanowaveguides.

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.

Electrospun polymer bottle microresonators for stretchable single-mode lasing 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.

Mode tailoring in subwavelength-dimensional semiconductor micro/nanowaveguides by coupling optical microfibers.

Benefitted from large fraction of evanescent wave and high endface reflectivity, we have realized mode tailoring in subwavelength-dimensional semiconductor micro/nanowaveguides (MN-WGs) by coupling optical silica microfibers. By investigating the reflection spectra, it was found that the microfiber tips could offer effective reflection and can been used to continuously and reversibly tune the interference wavelengths by changing the contact points with the MN-WGs. The measured extinction ratio in the interference patterns was as high as ~10 dB. In addition, tunable free spectral range of photoluminescence emissions and humidity sensing were also demonstrated. Its advantages of non-destructively tuning, simple fabrication, easy interrogation, and remote monitoring, offer great possible prospects for developing miniature tunable lasers, sensors, and biological endoscopy.

Multicolour laser from a single bandgap-graded CdSSe alloy nanoribbon.

Multicolour lasing with wavelength varying from 578 nm to 640 nm is realized from a single bandgap-graded CdSSe alloy nanoribbon, by selecting the excited spot at room temperature. Though reabsorption is a serious problem to achieve lasing at short wavelength, multiple scatters on the nanoribbon form localized cavities, and thus lasing at different wavelengths is realized. By increasing the excitation area, multicolour lasing from the same nanoribbon is also observed simultaneously.

Metal single-nanowire plasmonic sensors.

We demonstrated a general approach for plasmonic sensing using metal single nanowires based on an evanescent coupling technique, where light was efficiently launched into and picked up from the metal nanowires using silica fiber tapers. A high amplitude sensitivity of ~13 dB to 1.2% hydrogen was obtained using palladium-coated Au nanowires for hydrogen sensing, and a fast response of ~5 ms was obtained using a polyacrylamide film-supported Ag nanowire for relative humidity sensing. Our results may help develop metal nanowire-based plasmonic sensors.

Autonomous nanorobots with powerful thrust under dry solid-contact conditions by photothermal shock.

Nanorobotic motion on solid substrates is greatly hindered by strong nanofriction, and powerful nanomotors‒the core components for nanorobotic motion‒are still lacking. Optical actuation addresses power and motion control issues simultaneously, while conventional technologies with small thrust usually apply to fluid environments. Here, we demonstrate micronewton-thrust nanomotors that enable the autonomous nanorobots working like conventional robots with precise motion control on dry surfaces by a photothermal-shock technique. We build a pulsed laser-based actuation and trapping platform, termed photothermal-shock tweezers, for general motion control of metallic nanomaterials and assembled nanorobots with nanoscale precision. The thrust-to-weight ratios up to 107 enable nanomotors output forces to interact with external micro/nano-objects. Leveraging machine vision and deep learning technologies, we assemble the nanomotors into autonomous nanorobots with complex structures, and demonstrate multi-degree-of-freedom motion and sophisticated functions. Our photothermal shock-actuation concept fundamentally addresses the nanotribology challenges and expands the nanorobotic horizon from fluids to dry solid surfaces.

Low-threshold supercontinuum generation in semiconductor nanoribbons by continuous-wave pumping.

We report the first observation of supercontinuum (SC) generation in single semiconductor nanoribbons (NRs). By launching a continuous wave (CW) 532-nm pump light along a 200-µm-length CdS NR for waveguiding excitation, SC generation is realized with a threshold down to sub-milliwatt level, which is ~3 orders lower compared with previous CW-pumped SC generated in glass fibers. The low threshold is benefitted from the favorable material properties and waveguide geometries including high Raman gains, strong light confinement, more optical guided modes and phonon modes. Our work paves the way to low-threshold nanoscale SC sources and may find widespread applications ranging from spectroscopic analysis and biological imaging to material research.

Single-nanowire single-mode laser.

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.

Electrostatic control of photoluminescence from A and B excitons in monolayer molybdenum disulfide.

Tailoring excitonic photoluminescence (PL) in molybdenum disulfide (MoS2) is critical for its various applications. Although significant efforts have been devoted to enhancing the PL intensity of monolayer MoS2, simultaneous tailoring of emission from both A excitons and B excitons remains largely unexplored. Here, we demonstrate that both A-excitonic and B-excitonic PL of chemical vapor deposition (CVD)-grown monolayer MoS2 can be tuned by electrostatic doping in air. Our results indicate that the B-excitonic PL changed in the opposite direction compared to A-excitonic PL when a gate voltage (V g) was applied, both in S-rich and Mo-rich monolayer MoS2. Through the combination of gas adsorption and electrostatic doping, a 12-fold enhancement of the PL intensity for A excitons in Mo-rich monolayer MoS2 was achieved at V g = -40 V, and a 26-fold enhancement for the ratio of B/A excitonic PL was observed at V g = +40 V. Our results demonstrate not only the control of the conversion between A0 and A-, but also the modulation of intravalley and intervalley conversion between A excitons and B excitons. With electrostatic electron doping, the population of B excitons can be promoted due to the enhanced intravalley and intervalley transition process through electron-phonon coupling. The electrostatic control of excitonic PL has potential applications in exciton physics and valleytronics involving the B excitons.

Direct-Bandgap Bilayer WSe2 /Microsphere Monolithic Cavity for Low-Threshold Lasing.

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.

Spatial bandgap engineering along single alloy nanowires.

Bandgap engineering of semiconductor nanowires is important in designing nanoscale multifunctional optoelectronic devices. Here, we report a facile thermal evaporation method, and realize the spatial bandgap engineering in single CdS(1-x)Se(x) alloy nanowires. Along the length of these achieved nanowires, the composition can be continuously tuned from x = 0 (CdS) at one end to x = 1 (CdSe) at the other end, resulting in the corresponding bandgap (light emission wavelength) being modulated gradually from 2.44 eV (507 nm, green light) to 1.74 eV (710 nm, red light). In spite of the existing composition (crystal lattice) transition along the length, these multicolor nanowires still possess high-quality crystallization. These bandgap engineered nanowires will have promising applications in such as multicolor display and lighting, high-efficiency solar cells, ultrabroadly spectral detectors, and biotechnology.

Optical quenching of photoconductivity in CdSe single nanowires via waveguiding excitation.

We demonstrate broadband optical quenching of photoconductivity in CdSe single nanowires with low excitation power. Using 1550-nm-wavelength light with 10-nW power for waveguiding excitation, we observe a typical responsivity of 0.5 A/W for quenching the photoconductivity established by 10-µW 660-nm-wavelength background light in a 403-nm-diameter CdSe nanowire, with detectable limit of the quenching power down to pW level at room temperature, which is several orders of magnitude lower than those reported previously. This large quenching effect originates from the enhanced light-defect interaction in the nanowires via waveguiding excitation. These results open new opportunities for noninvasive characterization of deep-level defect states in low-dimensional semiconductor nanomaterials, and novel optoelectronic applications of semiconductor nanowires such as high-sensitive broadband photodetection.

Large defect-induced sub-bandgap photoresponse in semiconductor nanowires via waveguiding excitation.

A large defect-induced sub-bandgap photoresponse over a broad spectral range is observed in semiconductor single nanowires via optical waveguiding excitation. Using an evanescent coupling technique, the excitation sub-bandgap light is efficiently transferred from a silica fiber taper into a CdS single nanowire (bandgap ∼ 2.46 eV), and is tightly confined and guided through the whole length of the nanowire, which significantly enhances the light-defect interaction compared with the conventional irradiation excitation scheme. Under 593 nm wavelength (∼2.09 eV) waveguiding excitation with an input power of 10 pW level at room temperature, a 350 nm diameter 150 µm-length CdS nanowire shows a responsivity of 250 A W( - 1), offering a sub-bandgap photosensitivity five orders of magnitude larger than by irradiation excitation. These results may open opportunities for noninvasive characterization of defect states in semiconductor nanowires, as well as for enabling novel sub-bandgap nanowire devices.

Polymer microfiber rings for high-sensitivity optical humidity sensing.

We demonstrate microrings assembled with polyacrylamide (PAM) microfibers for high-sensitivity relative humidity (RH) sensing. When exposed to moisture, the PAM microfiber absorbs water molecules and inflates monotonically with the increasing humidity, resulting in evident spectral shifts of the resonance peaks of the microring. By measuring the spectral shifts, the microring shows sensitivity as high as 490 pm/%RH and a response time of about 120 ms, within a dynamic range from 5% to 71% RH.

A wafer-scale synthesis of monolayer MoS2 and their field-effect transistors toward practical applications.

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.

Plasmon-driven nanowire actuators for on-chip manipulation.

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.

ZnO subwavelength wires for fast-response mid-infrared detection.

Room temperature operating thermal detection for mid-infrared light based on ZnO subwavelength wires has been demonstrated. Electric resistance in ZnO wires increases linearly with the intensity of incident light. Noise equivalent power (NEP) of 5.8 microW/Hz(1/2) (at 1 kHz) with typical response times as fast as 1.3 ms is obtained at 10.6-microm wavelength. The sensitivity and response time of the detector are also found to be insensitive to the ambient.

Polyaniline/polystyrene single-nanowire devices for highly selective optical detection of gas mixtures.

By using spectral analysis in the visible/near-infrared region, we demonstrate a single waveguiding polyaniline/polystyrene nanowire for highly selective detection of gas mixtures with a NH(3) detection limit on parts-per-million level and relative humidity sensing ranging from 37% to 84%. The compact and flexible sensing scheme shown here may be attractive for highly selective optical detection in complex chemical or biological environments with a single nanowire.