Down-conversion and frequency spectrum convolution of the broadband signal based on active mode-locking technology.

A multifunction processor for a broadband signal based on the active mode-locking optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The central frequency down-conversion and frequency spectrum convolution of the target broadband signal (TBS) are realized by just tuning the wavelength of the optical carrier or by the time domain product, respectively. To achieve the central frequency down-conversion of the TBS, an optical tunable delay line (OTDL) is adopted to match the delay time of the OEO loop with the repetition period of the TBS. Then the spectrum convolution of the TBS is produced by just injecting a lower frequency signal consistent with the free spectral range (FSR) of the OEO loop. Moreover, the frequency convolution repetition is also greatly increased by harmonic mode-locking injection. The equivalent bandwidth of the TBS is enlarged by ∼50 times, benefiting from the frequency convolution. The central frequency conversion flexibility and the bandwidth compatibility are also discussed in detail. This work provides a multifunction processor system and may have potential usage in multifunctional integrated radar systems.

Robust Two-Dimensional Ferromagnetism in Cr5Te8/CrTe2 Heterostructure with Curie Temperature above 400 K.

The discovery of ferromagnetism in two-dimensional (2D) van der Waals crystals has generated widespread interest. The seeking of robust 2D ferromagnets with high Curie temperature (Tc) is vitally important for next-generation spintronic devices. However, owing to the enhanced spin fluctuation and weak exchange interaction upon the reduced dimensionalities, the exploring of robust 2D ferromagnets with Tc > 300 K is highly demanded but remains challenging. In this work, we fabricated air-stable 2D Cr5Te8/CrTe2 vertical heterojunctions with Tc above 400 K by the chemical vapor deposition method. Transmission electron microscopy demonstrates a high-quality-crystalline epitaxial structure between tri-Cr5Te8 and 1T-CrTe2 with striped moiré patterns and a superior ambient stability over six months. A built-in dual-axis strain together with strong interfacial coupling cooperatively leads to a record-high Tc for the CrxTey family. A temperature-dependent spin-flip process induces the easy axis of magnetization to rotate from the out-of-plane to the in-plane direction, indicating a phase-dependent proximity coupling effect, rationally interpreted by first-principles calculations of the magnetic anisotropy of a tri-Cr5Te8 and 1T-CrTe2 monolayer. Our results provide a material realization of effectively enhancing the transition temperature of 2D ferromagnetism and manipulating the spin-flip of the easy axis, which will facilitate future spintronic applications.

Anisotropic charge transfer and gate tuning for p-SnS/n-MoS2 vertical van der Waals diodes.

2D-material-based van der Waals heterostructures (vdWhs) have shown great potential in next-generation multi-functional microelectronic devices. Thanks to their sharp interface and ultrathin thickness, 2D p-n junctions with high rectification properties have been established by combining p-type monochalcogenides with n-type transition metal dichalcogenides. However, the anisotropic rectification together with the charge transfer and gate effect has not been clarified. Herein, the electrical anisotropy of p-SnS/n-MoS2 diodes was studied. Optimum ideality factors within 1.08-1.18 have been achieved for the diode with 6.6 nm thick SnS on monolayer MoS2, and a high rectification ratio of 3.1 × 104 with strong in-plane anisotropy is observed along the zigzag direction of SnS. A strong gate effect on the anisotropic series resistance has been verified and an effective tuning over the transport length of the SnS channel can be established through adjustment of the current orientation and gate voltage. A thickness-dependent minority carrier transport mechanism has also been demonstrated for the reverse drain current, and Fowler-Nordheim tunneling and direct tunneling are proposed for the increase of the reverse current of the thicker and thinner diodes, respectively. This work will provide another strategy for high-performance diodes based on vdWhs via the control of the current orientation and the gate effect.

Highly improved side mode suppression ratio and a low phase noise optoelectronic oscillator.

By adopting self-injection locking (SIL) technology in an external injection locking (EIL) optoelectronic oscillator (OEO), a highly improved side mode suppression ratio (SMSR) and low phase noise microwave signal generator is designed. The EIL ranging is closely related to the frequency spacing ranging of the free-running OEO, which is the reverse of the oscillation loop length, and limits the phase noise performance. Here SIL technology is introduced to significantly increase the Q-factor of the OEO without degrading the SMSR by setting the longer loop without oscillation. Both the simulation and experimental results are carried out to confirm the conclusion. Additionally, an SMSR up to 86 dB and phase noises as low as -88.80dBc/Hz@100Hz and -122.83dBc/Hz@10kHz, respectively, are demonstrated. Furthermore, the frequency overlapping Allan deviation of the proposed OEO scheme is also enhanced by 103 times, which benefits from the external injection technology compared with the free-running OEO. In addition, the SMSR and phase noise modification dependence on the fiber length, the RF source quality and external injection power, as well as the frequency tunability, are detailed and discussed to reveal the compatibility combination mechanism of the EIL and the SIL.

Active mode lock optoelectronic oscillator based on the simulated Brillouin scattering effect.

An active mode-locked optoelectronic oscillator (AML-OEO) based on the simulated Brillouin scattering (SBS) effect without an electrical filter is demonstrated here. By using phase modulation and SBS-based selective sideband amplification, the central frequency of the proposed SBS-AML-OEO is easily adjusted by simply changing the pump laser frequency instead of the filters. A microwave frequency comb signal with an adjustable central frequency and fixed bandwidth are generated by injecting a mode-locking external RF synchronizing with the free spectral ranging FSR. In addition, the harmonic SBS-AML-OEO is also achieved by harmonic signal injection. The proposed method reveals a simple solution to tune the central frequency and has the potential to be integrated on a chip since there is no structure changing in the scheme.

Self-Powered CsPbBr3 Perovskite Nanonet Photodetector with a Hollow Vertical Structure.

For most commercial photodetectors (PDs), incident light is illuminated from the top or side of the device, but the opaque electrode (gold, copper, or aluminum, etc.) on the top will block part of the light from entering, wasting the efficiency of light utilization. Herein, to solve this issue, we introduced perovskite nanonet PDs with a hollow vertical structure by using a polystyrene microsphere template. Compared with ordinary thin film devices, our reticulated hollow vertical structure devices not only can enable easy entrance of the light from the reticulated hollow surface of the devices but also can reduce the reflection of light, resulting in better device performance. For our optimal CsPbBr3 perovskite PDs, high photoelectric performances were achieved with the switching ratio up to 4.17 × 104, a detectivity of 7.44 × 1011 Jones, a linear dynamic range of 108 dB, and the rise/fall time of 0.1/0.16 ms. More importantly, because of the reticulated hollow structure, our device performance showed less reduction when the incident light was illuminated from the top than from the bottom. These results may be of great reference value for improving the photoelectric performance of silicon-based devices or deep ultraviolet PDs.

Contribution of Cation Addition to MnO 2 Nanosheets on Stable Co 3 O 4 Nanowires for Aqueous Zinc-Ion Battery.

Zinc-based electrochemistry attracts significant attention for practical energy storage owing to its uniqueness in terms of low cost and high safety. In this work, we propose a 2.0-V high-voltage Zn-MnO2 battery with core@shell Co3O4@MnO2 on carbon cloth as a cathode, an optimized aqueous ZnSO4 electrolyte with Mn2+ additive, and a Zn metal anode. Benefitting from the architecture engineering of growing Co3O4 nanorods on carbon cloth and subsequently deposited MnO2 on Co3O4 with a two-step hydrothermal method, the binder-free zinc-ion battery delivers a high power of 2384.7 W kg-1, a high capacity of 245.6 mAh g-1 at 0.5 A g-1, and a high energy density of 212.8 Wh kg-1. It is found that the Mn2+ cations are in situ converted to Mn3O4 during electrochemical operations followed by a phase transition into electroactive MnO2 in our battery system. The charge-storage mechanism of the MnO2-based cathode is Zn2+/Zn and H+ insertion/extraction. This work shines light on designing multivalent cation-based battery devices with high output voltage, safety, and remarkable electrochemical performances.

Strategic modulation of energy transfer in Au-TiO2-Pt nanodumbbells: plasmon-enhanced hydrogen evolution reaction.

Owing to the capacity of efficiently harvesting and converting incident energy, localized surface-plasmon resonance of noble metals was introduced into a metal-semiconductor design for promoting hydrogen evolution. In this study, a plasmonic nanodumbbell structure was employed to strategically modulate the energy transfer in the water reduction reaction. A maximum H2 evolution rate of 80 µmol g-1 h-1 was obtained in the Au-TiO2 nanodumbbells, and further improvement was achieved through surface modification with Pt nanoparticles functioning as active sites, leading to ∼4.3 times enhanced photocatalytic activity. Compared with similar nanostructures reported previously, the present superior photoactivity response is ascribed to the injection process of the energetic hot electrons generated from the excitation and decay of the longitudinal surface-plasmon resonance (LSPR) and transverse surface-plasmon resonance (TSPR) in the Au nanorods, which corresponds to the electric field distribution of the finite-difference-time-domain simulation. These intriguing results, originating from the positive synergistic effect of the plasmon and co-catalyst, demonstrated the mechanism of the plasmon-assisted photochemistry and provided a promising strategy for the rational design of novel plasmonic photocatalysts.

Enhanced Photocatalytic Hydrogen Evolution by Loading Cd0.5Zn0.5S QDs onto Ni2P Porous Nanosheets.

Ni2P has been decorated on CdS nanowires or nanorods for efficient photocatalytic H2 production, whereas the specific surface area remains limited because of the large size. Here, the composites of Cd0.5Zn0.5S quantum dots (QDs) on thin Ni2P porous nanosheets with high specific surface area were constructed for noble metal-free photocatalytic H2 generation. The porous Ni2P nanosheets, which were formed by the interconnection of 15-30 nm-sized Ni2P nanoparticles, allowed the uniform loading of 7 nm-sized Cd0.5Zn0.5S QDs and the loading density being controllable. By tuning the content of Ni2P, H2 generation rates of 43.3 µM h- 1 (1 mg photocatalyst) and 700 µM h- 1 (100 mg photocatalyst) and a solar to hydrogen efficiency of 1.5% were achieved for the Ni2P-Cd0.5Zn0.5S composites. The effect of Ni2P content on the light absorption, photoluminescence, and electrochemical property of the composite was systematically studied. Together with the band structure calculation based on density functional theory, the promotion of Ni2P in charge transfer and HER activity together with the shading effect on light absorption were revealed. Such a strategy can be applied to other photocatalysts toward efficient solar hydrogen generation.

Loading Cd0.5Zn0.5S Quantum Dots onto Onion-Like Carbon Nanoparticles to Boost Photocatalytic Hydrogen Generation.

Carbon dots (C dots, size < 10 nm) have been conventionally decorated onto semiconductor matrixes for photocatalytic H2 evolution, but the efficiency is largely limited by the low loading ratio of the C dots on the photocatalyst. Here, we propose an inverse structure of Cd0.5Zn0.5S quantum dots (QDs) loaded onto the onionlike carbon (OLC) matrix for noble metal-free photocatalytic H2 evolution. Cd0.5Zn0.5S QDs (6.9 nm) were uniformly distributed on an OLC (30 nm) matrix with both upconverted and downconverted photoluminescence property. Such an inverse structure allows the full optimization of the QD/OLC interfaces for effective energy transfer and charge separation, both of which contribute to efficient H2 generation. An optimized H2 generation rate of 2018 µmol/h/g (under the irradiation of visible light) and 58.6 µmol/h/g (under the irradiation of 550-900 nm light) was achieved in the Cd0.5Zn0.5S/OLC composite samples. The present work shows that using the OLC matrix in such a reverse construction is a promising strategy for noble metal-free solar hydrogen production.

Plasmon-enhanced light harvesting of chlorophylls on near-percolating silver films via one-photon anti-Stokes upconversion.

There exists a wealth of means of efficient utilization of solar energy in nature, with photosynthesis of chlorophylls as a prime example. Separately, artificially structured plasmonic materials are versatile in light harvesting and energy conversion. Using a simple and scalable design of near-percolating silver nanostructures, we demonstrate that the light-harvesting efficiency of chlorophylls can be drastically enhanced by tuning the plasmon frequency of the constituent silver nanoparticles to coincide with the maximal photon flux of sunlight. In particular, we show that the photon upconversion efficiency can be readily enhanced by over 20 folds, with the room-temperature fluorescence quantum yield increased by a factor of 2.63. The underlying mechanism for the upconversion enhancement is attributed to a one-electron-per-photon anti-Stokes process, involving absorption of a characteristic phonon mode of the chlorophylls. These findings suggest that chlorophylls can serve as molecular building blocks for high-efficiency light harvesting and solar energy conversion.

A new "turn-on" naphthalenedimide-based chemosensor for mercury ions with high selectivity: successful utilization of the mechanism of twisted intramolecular charge transfer, near-IR fluorescence, and cell images.

For the first time, a new near-IR "turn-on" fluorescent chemosensor with high selectivity for Hg(2+) ions was designed according to the twisted intramolecular charge transfer (TICT) mechanism. The selective fluorescence enhancement effect can be optimized by modulating the solvent systems. And this naphthalenedimide-based sensor with long wavelength absorption and emission can be used to image intracellular Hg(2+) ions in living Hela cells.

Dynamically tuning emission band of CdSe/ZnS quantum dots assembled on Ag nanorod array: plasmon-enhanced Stark shift.

We demonstrate tuning emission band of CdSe/ZnS semiconductor quantum dots (SQDs) closely-packed in the proximity of Ag nanorod array by dynamically adjusting exciton-plasmon interaction. Large red-shift is observed in two-photon luminescence (TPL) spectra of the SQDs when the longitudinal surface plasmon resonance (LSPR) of Ag nanorod array is adjusted to close to excitation laser wavelength, and the spectral red-shift of TPL reaches as large as 101 meV by increasing excitation power, which is slightly larger than full width at half-maximum of emission spectrum of the SQDs. The observed LSPR-dependent spectral shifting behaviors are explained by a theoretical model of plasmon-enhanced quantum-confined Stark effect. These observations could find the applications in dynamical information processing in active plasmonic and photonic nanodevices.

Tuning gold nanorod-nanoparticle hybrids into plasmonic Fano resonance for dramatically enhanced light emission and transmission.

We investigate the optical response of a gold nanorod array coupled with a semicontinuous nanoparticle film. We find that, as the gold nanoparticle film is adjusted to the percolating regime, the nanorod-film hybrids are tuned into plasmonic Fano resonance, characterized by the coherent coupling of discrete plasmonic modes of the nanorod array with the continuum band of the percolating film. Consequently, optical transmission of the percolating film is substantially enhanced. Even more strikingly, electromagnetic fields around the nanorod array become much stronger, as reflected by 2 orders of magnitude enhancement in the avalanche multiphoton luminescence. These findings may prove instrumental in the design of various plasmonic nanodevices.

Plasmon-mediated radiative energy transfer across a silver nanowire array via resonant transmission and subwavelength imaging.

Efficient plasmon-mediated excitation energy transfer between the CdSe/ZnS semiconductor quantum dots (QDs) across the silver nanowire array up to 560 nm in length is observed. The subwavelength imaging and spectral response of the silver nanowire arrays with near-field point-source excitations are revealed by theoretical simulations. Our studies demonstrate three advantages of the nanosystem: efficient exciton-plasmon conversion at the input side of the array through near-field strong coupling, directional waveguidance and resonant transmission via half-wave plasmon modes of the nanowire array, and subwavelength imaging at the output side of the array. These advantages allow a long-range radiative excitation energy transfer with a high efficiency and a good directionality.

A positively charged QDs-based FRET probe for micrococcal nuclease detection.

A new strategy for quantitatively detecting micrococcal nuclease (MNase) is proposed using electrostatic interaction-based fluorescence resonance energy transfer (FRET) between positively charged QDs and negatively charged dye-labeled single-stranded DNA (dye-ssDNA). Herein, we have made our attempt to develop a strategy where the variation of FRET efficiency is due to the change of the electrostatic interaction between QDs and the ssDNA that result from the cleavage of dye-ssDNA by a single-strand-specific nuclease. To demonstrate the feasibility of this design, positively charged QDs (lysozyme modified QDs, Lyz-QDs) are prepared as the energy donor, with the fluorescent dye 6-carboxy-X-rhodamine (ROX) that is labeled to ssDNA serving as the energy acceptor. The ROX-labeled probe ssDNA (ROX-ssDNA) is absorbed to the surface of the QDs through electrostatic interaction, which results in resonance energy transfer between the QDs and the dye. In the presence of MNase which cleaves the ROX-ssDNA into small fragments, the weakened interaction between QDs and the shortened ssDNA causes the decrease of FRET efficiency. At given amounts of donor and acceptor, the ratio of fluorescence intensity of QDs to ROX changes in a MNase concentration-dependent manner. Under optimized conditions, the ratio is linear with MNase concentration over the range of 8 x 10(-3) to 9.0 x 10(-2) unit mL(-1), with a limit of detection of 1.6 x 10(-3) unit mL(-1). This new detection strategy features straightforward design and easy operation, which is capable of expanding the application of the positively charged QDs-based FRET in DNA-related bioassays.

Plasmon-enhanced Förster energy transfer between semiconductor quantum dots: multipole effects.

We experimentally demonstrated plasmon-assisted energy transfer (ET) between CdSe semiconductor quantum dots (QDs) self-assembled in a monolayer by using time-resolved micro-photoluminescence (PL) technique. The enhancements of PL intensity and ET efficiency were manipulated by adjusting thickness (Delta) of SiO(2) coating on large Ag nanoparticles. The PL enhancement factor of the acceptor QDs and the PL intensity ratio of acceptor-to-donor reached their maxima approximately 47 and approximately 14 when Delta = 7 nm, the corresponding ET efficiency reached 86%. We also presented theoretical analysis based on the rate equation. The theoretical calculations agreed with experimental data and revealed interesting physics of multipole effect, and metal nanoparticle induced quench effect and plasmon-enhanced Förster ET.

Homogeneous competitive hybridization assay based on two-photon excitation fluorescence resonance energy transfer.

Recently, we have successfully developed a two-photon excitation fluorescence resonance energy transfer (TPE-FRET)-based homogeneous immunoassay using two-photon excitable small organic molecule as the energy donor. In the present work, the newly emerging TPE-FRET technique was extended to the determination of oligonucleotide. A new TPE molecule with favorable two-photon action cross section was synthesized [2-(2,5-bis(4-(dimethylamino)styryl)-1H-pyrrol-1-yl)acetic acid, abbreviated as TP-COOH], with the tagged reactive carboxyl group allowing facile conjugation with streptavidin (SA). Employing the TP-COOH molecule as energy donor and black hole quencher 1 (BHQ-1) as acceptor, a TPE-FRET-based homogeneous competitive hybridization model was constructed via a biotin-streptavidin bridge. Through the hybridization between a biotinylated single-stranded DNA (ssDNA) and a BHQ-1-linked ssDNA, and the subsequent capture of the as-formed hybrid by TP-COOH labeled SA, the donor fluorescence was quenched due to the FRET between TP-COOH and BHQ-1. Upon the competition between a target ssDNA and the quencher-linked ssDNA toward the biotinylated oligonucleotide, the donor fluorescence was recovered in a target-dependent manner. Good linearity was obtained with the target oligonucleotide ranging from 0.08 to 1.52 microM. The method was applied to spiked serum and urine samples with satisfying recoveries obtained. The results of this work verified the applicability of TPE-FRET technique in hybridization assay and confirmed the advantages of TPE-FRET in complicated matrix.

Sublinear and superlinear photoluminescence from Nd doped anodic aluminum oxide templates loaded with Ag nanowires.

We first time prepared Nd(3+) ions doped anodic aluminum oxide (Nd:AAO) templates, reported linear, sublinear and superlinear photoluminescence (PL) from Nd:AAO templates loaded with Ag nanowires in different excitation power regions, in which, the excitation laser with wavelength 805 nm resonantly pumped the population to (4)F5/2 states of Nd(3+), and the radiative transitions (4)F(3/2) -->(4)I(9/2) of Nd(3+) centered at 880 nm. The excitation power dependences of emission polarization ratio and the spectral width were also investigated. The observed nonlinear amplifications of the PL intensity implied strong interaction between randomly-dispersed Nd(3+) ions and ordered-arrayed Ag nanowires in AAO templates.