Effect of annealing temperature on the optoelectrical synapse behaviors of A-ZnO microtube.

Optoelectronic synapses with fast response, low power consumption, and memory function hold great potential in the future of artificial intelligence technologies. Herein, a strategy of annealing in oxygen ambient at different temperatures is presented to improve the optoelectronic synaptic behaviors of acceptor-rich ZnO (A-ZnO) microtubes. The basic synaptic functions of as-grown and annealed A-ZnO microtubes including excitatory postsynaptic current (EPSC), short-term memory (STM) to long-term memory (LTM) conversion, and paired-pulse facilitation (PPF), were successfully emulated. The results show that the annealing temperature of 600 °C yields high figures of merit compared to other annealed A-ZnO microtubes. The 4-fold and 20-fold enhancement dependent on the light pulse duration time and energy density have been achieved in the 600 °C annealed A-ZnO microtube, respectively. Furthermore, the device exhibited a PPF index of up to 238% and achieved four cycles of "learning-forgetting" process, proving its capability for optical information storage. The free exciton (FX) and donor-acceptor pair (DAP) concentrations significantly influenced the persistent photoconductivity (PPC) behavior of A-ZnO microtubes. Therefore, the LTM response can be controlled by the adjustment of numbers, powers, and interval time of the optical stimulation. This work outlines a strategy to improve the EPSC response through defect control, representing a step towards applications in the field of optoelectronic synaptic device.

Diagnosis of neuropsychiatric systemic lupus erythematosus by label-free serum microsphere-coupled SERS fingerprints with machine learning.

Surface-enhanced Raman spectroscopy (SERS) is a powerful optical technique for non-invasive and label-free bioanalysis of liquid biopsy, facilitating to diagnosis of potential diseases. Neuropsychiatric systemic lupus erythematosus (NPSLE) is one of the subgroups of systemic lupus erythematosus (SLE) with serious manifestations for a high mortality rate. Unfortunately, lack of well-established gold standards results in the clinical diagnosis of NPSLE being a challenge so far. Here we develop a novel Raman fingerprinting machine learning (ML-) assisted diagnostic method. The microsphere-coupled SERS (McSERS) substrates are employed to acquire Raman spectra for analysis via convolutional neural network (CNN). The McSERS substrates demonstrate better performance to distinguish the Raman spectra from serums between SLE and NPSLE, attributed to the boosted signal-to-noise ratio of Raman intensities due to the multiple optical regulation in microspheres and AuNPs. Eight statistically-significant (p-value <0.05) Raman shifts are identified, for the first time, as the characteristic spectral markers. The classification model established by CNN algorithm demonstrates 95.0% in accuracy, 95.9% in sensitivity, and 93.5% in specificity for NPSLE diagnosis. The present work paves a new way achieving clinical label-free serum diagnosis of rheumatic diseases by enhanced Raman fingerprints with machine learning.

Anisotropic carrier dynamics and laser-fabricated luminescent patterns on oriented single-crystal perovskite wafers.

Perovskite materials and their applications in optoelectronics have attracted intensive attentions in recent years. However, in-depth understanding about their anisotropic behavior in ultrafast carrier dynamics is still lacking. Here we explore the ultrafast dynamical evolution of photo-excited carriers and photoluminescence based on differently-oriented MAPbBr3 wafers. The distinct in-plane polarization of carrier relaxation dynamics of the (100), (110) and (111) wafers and their out-of-plane anisotropy in a picosecond time scale were found by femtosecond time- and polarization-resolved transient transmission measurements, indicating the relaxation process dominated by optical/acoustic phonon interaction is related to photoinduced transient structure rearrangements. Femtosecond laser two-photon fabricated patterns exhibit three orders of magnitude enhancement of emission due to the formation of tentacle-like microstructures. Such a ultrafast dynamic study carried on differently-oriented crystal wafers is believed to provide a deep insight about the photophysical process of perovskites and to be helpful for developing polarization-sensitive and ultrafast-response optoelectronic devices.

Determination of Enantiomeric Excess by Optofluidic Microlaser near Exceptional Point.

Enantiomeric excess (ee) is an essential indicator of chiral drug purification in the pharmaceutical industry. However, to date the ee determination of unknown concentration enantiomers generally involves two separate techniques for chirality and concentration measurement. Here, a whispering-gallery mode (WGM) based optofluidic microlaser near exceptional point to achieve the ee determination under unknown concentration with a single technique is proposed. Exceptional point induces the unidirectional WGM lasing, providing the optofluidic microlaser with the novel capability to measure chirality by polarization, in addition to wavelength-based concentration detection. The dual-parameters detection of optofluidic microlaser empowers it to achieve ee determination of various unknown enantiomers without additional concentration measurements, a feat that is challenging to accomplish with other methods. Featuring the sensitivity enhancement and miniature structure of the WGM sensors, the obtained chiroptical response of the present approach is ≈30-fold higher than that of the conventional optical rotation-based polarimeter, and the reagent consumption is reduced by three orders of magnitude.

Microsphere Photonic Superlens for a Highly Emissive Flexible Upconversion-Nanoparticle-Embedded Film.

Increasing upconversion luminescence (UCL) to overcome the intrinsically low conversion efficiency of upconversion nanoparticles (UCNPs) poses a fundamental challenge. Photonic nanostructures are the efficient approaches for UCL enhancement by tailoring the local electromagnetic fields. Unfortunately, such nanostructures are sensitive to environmental conditions, and the regulation strength is varied in flexible applications. Here, we report giant UCL enhancement from a flexible UCNP-embedded film coupled with a microsphere photonic superlens (MPS), by which the enhancement ratio of UCL is over 104-fold under 808 nm excitation down to 0.72 mW. The enhancement pathways of MPS-enhanced UCL are attributed to Mie-resonant nanofocusing for high excitation-photon density, optical whispering-gallery modes (WGMs) for fast radiative decay, and the directional antenna effect for far-field emission confinement. The contribution of optical resonance in the MPS to suppressing the phonon-induced nonradiative transition and thermal quenching is experimentally validated. The UCL quantum yield is therefore improved by 3-fold to 4.20% under 120 mW/cm2 near-infrared excitation, consistent with the enhancement ratio via the Purcell effect of WGMs. Furthermore, the MPS demonstrates the robust optical regulation capability toward flexible applications, opening up new opportunities for facilitating multiphoton upconversion in wearable optoelectrical devices for nanoimaging, biosensing, and energy conversion in the future.

In situ SERS monitoring of plasmon-driven catalytic reaction on gap-controlled Ag nanoparticle arrays under 785 nm irradiation.

Plasmon-enhanced photocatalysis has attracted considerable attention due to its low energy consumption and high energy throughput. Surface-enhanced Raman scattering (SERS) is a highly sensitive and label-free nondestructive tool to investigate plasmon-driven photocatalytic reactions. Herein, we present a facile method to fabricate gap-controlled Ag nanoparticle (NP) arrays with uniform and high-density distribution of hot spots, which can be employed as both efficient plasmonic photocatalysts and stable SERS platforms. The plasmon-driven catalytic reaction of 4-nitrobenzenethiol (4NBT), which transforms it into p, p'-dimercaptoazobenzene (DMAB), is detected by using an in situ SERS technique at the excited wavelength of 785 nm. According to the temperature and laser power density dependent photocatalytic reaction rates observed on the Ag NP arrays, we quantitatively determined that the reductive coupling of 4NBT is more likely to occur as the gap decreases. The finite-difference time-domain (FDTD) simulation results demonstrate that the plasmonic hot spots are significantly enhanced with a decrease in gap, which in turn reduces activation energy. The gap-controlled Ag NP arrays are efficient for both promotion and detection of plasmon-driven catalytic reactions, and may pave a pathway for implementing efficient plasmonic photocatalytic platforms.

Current-Induced Thermal Tunneling Electroluminescence in a Single Highly Compensated Semiconductor Microrod.

Here we demonstrate a novel and robust mechanism, termed as "current-induced Joule heating activated thermal tunneling excitation," to achieve electroluminescence (EL) by the hot electron-hole-pair recombination in a single highly compensated semiconductor microrod. The radiative luminescence is electrically excited under ambient conditions. The current-induced Joule heating reduces the thermal tunneling excitation threshold of voltage down to 8 V and increases the EL efficiency ~4.4-fold at 723 K. We interpret this novel phenomenon by a thermal tunneling excitation model corrected by electric-induced Joule heating effect. The mechanism is confirmed via theoretical calculation and experimental demonstration, for the first time. The color-tunable EL emission is also achieved by regulation of donor concentration. This work opens up new opportunities for design of novel multi-color light-emitting devices by homogeneous defect-engineered semiconductors in future.

Facile and efficient preparation of high-quality black phosphorus quantum dot films for sensing applications.

In this study, black phosphorus quantum dots (BPQDs) are synthesized by improved ultrasonic-assisted liquid exfoliation. The as-prepared BPQDs are deposited on large-area conductive substrates by electrophoretic deposition, and exhibit high-sensitivity humidity sensing and excellent electrical properties. The results offer not only a facile and efficient method to synthesise stable BPQDs films, but also a new opportunity to develop humidity sensors and nano-electronic devices.

Ultraviolet luminescence enhancement of planar wide bandgap semiconductor film by a hybrid microsphere cavity/dual metallic nanoparticles sandwich structure.

Here we report a novel hybrid structure composing of microsphere array (MA), Al nanoparticles (Al-NPs), ZnO thin film (luminescence layer), Au nanoparticles (Au-NPs), and substrate (sapphire) for ultraviolet (UV) luminescence enhancement of planar wide bandgap semiconductor film. The plasmonic sandwich structure of Al-NPs/ZnO/Au-NPs boosts the hot electron state density in the conduction band by electron trapping from deep-defect level of ZnO and localized surface plasmon resonances (LSPRs) coupling around dual metallic NPs, enhancing UV emission and suppressing visible emission efficiently. The dielectric microsphere array capping on the plasmonic sandwich structure further increases UV emission intensity via photonic nanojets, optical whispering-gallery modes (WGMs), and directional antenna effect, by which the interaction between photon and exciton is strengthened. The contribution of microsphere cavity coupling with LSPRs to UV luminescence enhancement is therefore revealed. The maximum enhancement ratio of up to two orders of magnitude (~250-fold) is achieved by the optimized 5.06-µm-diameter-MA/Al-NPs/ZnO/Au-NPs/sapphire structure and the UV emission is highly directional with a divergent angle of ~5°. The present work provides a simple and easily-prepared structure incorporating optical WGMs and LSPRs to manipulate UV luminescence of planar wide-bandgap semiconductors for potential optoelectronic applications.

Controllable plasmon-induced catalytic reaction by surface-enhanced and tip-enhanced Raman spectroscopy.

The controllable catalytic reaction plays a pivotal role in heterogeneous catalysis. Surface-enhanced Raman scattering (SERS) and tip enhanced Raman spectroscopy (TERS) are considered promising techniques for the study of catalytic reactions due to the highly localized sensitivity of SERS and the nanoscale spatial resolution of TERS. Herein, Ag/Au composite films were employed as catalyst for in situ monitoring of the catalytic reaction of 4­nitrobenzenethiol (4NBT) to p, p'­dimercaptoazobenzene (DMAB). The catalytic reaction of 4NBT adsorbed on Au film can be manipulated at the nanoscale using TERS by controlling the height between the tip-apex and the sample surface in Ag tip-Au substrate geometry. According to finite difference time domain (FDTD) simulations, the 'hot electron' induced by the localized surface plasmon is sufficient for promoting the catalytic reaction. These findings provide a novel way for controllable graph drawing of molecules at the nanoscale level.

Parametric study on photoluminescence enhancement of high-quality zinc oxide single-crystal capping with dielectric microsphere array.

A dielectric microsphere is a multifunctional platform to manipulate light in microscale by nanofocusing, optical whispering gallery resonance, and unidirectional antenna. Dielectric microsphere arrays (MSAs) have demonstrated the capability for photoluminescence (PL) and Raman enhancement without plasmonics. In this work, we investigate the effects of excitation power, tilting angle, and temperature on PL enhancement of high-quality zinc oxide (ZnO) single-crystal capping with fused silica MSAs. The microsphere diameter is optimized to 3.5-5.5 µm, achieving the maximum UV-PL enhancement ratio of intensity (ERI) up to tenfold by strong focusing and unidirectional antenna effects. Under the excitation power <0.2 mW, the incident light focused by the MSA increases the localized exciton state density for a higher ERI of ∼15-fold. The angle-sensitive PL intensity from the MSA enhancer provides a simple approach achieving unidirectional UV emission from planar ZnO. The 16-fold enhancement for UV-PL near 130°C is also demonstrated, for the first time, owing to thermal ionization of hydrogen-related donor that increases free-exciton concentration. The high temperature stability and reproducibility of PL enhancement up to 400°C promote the nonplasmonic MSAs superior to surface plasmon-related metal nanostructures for ZnO-based highly efficient luminescence and highly sensitive photon detection above room temperature.

Flexible Microsphere-Embedded Film for Microsphere-Enhanced Raman Spectroscopy.

Dielectric microspheres with extraordinary microscale optical properties, such as photonic nanojets, optical whispering-gallery modes (WGMs), and directional antennas, have drawn interest in many research fields. Microsphere-enhanced Raman spectroscopy (MERS) is an alternative approach for enhanced Raman detection by dielectric microstructures. Unfortunately, fabrication of microsphere monolayer arrays is the major challenge of MERS for practical applications on various specimen surfaces. Here we report a microsphere-embedded film (MF) by immersing a highly refractive microsphere monolayer array in the poly(dimethylsiloxane) (PDMS) film as a flexible MERS sensing platform for one- to three-dimensional (1D to 3D) specimen surfaces. The directional antennas and wave-guided whispering-gallery modes (WG-WGMs) contribute to the majority of Raman enhancement by the MFs. Moreover, the MF can be coupled with surface-enhanced Raman spectroscopy (SERS) to provide an extra >10-fold enhancement. The limit of detection is therefore improved for sensing of crystal violet (CV) and Sudan I molecules in aqueous solutions at concentrations down to 10-7 M. A hybrid dual-layer microsphere enhancer, constructed by depositing a MF onto a microsphere monolayer array, is also demonstrated, wherein the WG-WGMs become dominant and boost the enhancement ratio >50-fold. The present work opens up new opportunities for design of cost-effective and flexible MERS sensing platforms as individual or associated techniques toward practical applications in ultrasensitive Raman detection.

Sandwich-structure-modulated photoluminescence enhancement of wide bandgap semiconductors capping with dielectric microsphere arrays.

Here we investigated the effect of substrate and film thickness on photoluminescence (PL) enhancement of wide bandgap semiconductor (i.e. ZnO) by dielectric microsphere array/luminescence film/substrate (MLS) sandwich structures. The PL enhancement channels in the sandwich structure were revealed, for the first time, including the focusing property of microsphere array (MSA) distinctly enhancing free-exciton recombination, anti-reflection effect of MSA increasing excitation cross-section area, MLS-supported TW-/SW-WGMs inducing ASE and Purcell's effect, and optical directional antenna effect for high equivalent NA of objective lens as well as out-coupling efficiency. The enhancement ratio of intensity (ERI) for ZnO UV-PL from free-exciton recombination in the sandwich structure was found to be strongly dependent upon the refractive index of substrate and luminescence film thickness. In order to achieve high ERI for PL emission, the refractive index of substrate should differ from luminescence film and the film thickness needs to be chosen to support WGMs in the sandwich structure. The maximum ERI beyond one order of magnitude for ZnO UV-PL was therefore predicted theoretically and validated experimentally, where 11.25-fold UV PL enhancement ratio was achieved in ~650-nm-thick ZnO film grown on SiC substrate and capped with 5.06-µm-diameter MSA. The ERI could further be increased by improving above-mentioned enhancement channels. The present work provides a novel platform to manipulate light by low-loss dielectric microstructures for enhancing photon-matter interaction, which would be employed for other semiconductors achieving energy-saving luminescence and high-sensitivity photoelectric detection in future.

Free-Standing Undoped ZnO Microtubes with Rich and Stable Shallow Acceptors.

Fabrication of reliable large-sized p-ZnO is a major challenge to realise ZnO-based electronic device applications. Here we report a novel technique to grow high-quality free-standing undoped acceptor-rich ZnO (A-ZnO) microtubes with dimensions of ~100 µm (in diameter) × 5 mm (in length) by optical vapour supersaturated precipitation. The A-ZnO exhibits long lifetimes (>1 year) against compensation/lattice-relaxation and the stable shallow acceptors with binding energy of ~127 meV are confirmed from Zn vacancies. The A-ZnO provides a possibility for a mimetic p-n homojunction diode with n(+)-ZnO:Sn. The high concentrations of holes in A-ZnO and electrons in n(+)-ZnO make the dual diffusion possible to form a depletion layer. The diode threshold voltage, turn-on voltage, reverse saturated current and reverse breakdown voltage are 0.72 V, 1.90 V, <10 µA and >15 V, respectively. The A-ZnO also demonstrates quenching-free donor-acceptor-pairs (DAP) emission located in 390-414 nm with temperature of 270-470 K. Combining the temperature-dependent DAP violet emission with native green emission, the visible luminescence of A-ZnO microtube can be modulated in a wide region of colour space across white light. The present work opens up new opportunities to achieve ZnO with rich and stable acceptors instead of p-ZnO for a variety of potential applications.

Self-assembled dielectric microsphere array enhanced Raman scattering for large-area and ultra-long working distance confocal detection.

Here we report enhanced confocal Raman detection with large-area and ultra-long working distance by capping dielectric microsphere array. Microspheres have been found to provide three channels for Raman scattering enhancement, including localized photonic nanojets, directional antenna effects, and whispering-gallery modes. The maximum enhancement ratio of Raman intensity is up to 14.6 using 4.94-µm-diameter polystyrene (PS) microspheres. Investigation on the directional antenna effect of microsphere reveals that the microsphere array confines electromagnetic (EM) waves to a narrow distribution with small divergent angles, by which the signal-to-noise ratio is retained and the offset of focal plane position from sample surface can be up to ± 7.5 mm. The present work reduces the requirement of focusing in confocal Raman detection and hence makes the large-area detection possible via rapid mapping. It opens up a simple approach for high-sensitivity Raman detection of 3D-structured surface.

Ten-fold enhancement of ZnO thin film ultraviolet-luminescence by dielectric microsphere arrays.

Here we report strong enhancement in ultraviolet-photoluminescence (UV-PL) of ZnO thin films (grown on a SiC substrate) covered by monolayer dielectric fused silica or polystyrene microspheres with diameters ranging from 0.5 to 7.5 µm. The excited light scatted in the film is collected by the microspheres to stimulate whispering gallery modes, by which the internal quantum efficiency of spontaneous emission is enhanced. Meanwhile, the microsphere monolayer efficiently couples emitted light energy from the luminescent film to the far-field for PL detection. A UV-PL enhancement up to 10-fold via a 5-µm-diameter microsphere monolayer is experimentally demonstrated in this work. The unique optical property of microsphere in photoluminescence (PL) enhancement makes them promising for high-sensitivity PL measurements as well as design of photoelectric devices with low loss and high efficiency.

Microsphere-coupled scanning laser confocal nanoscope for sub-diffraction-limited imaging at 25 nm lateral resolution in the visible spectrum.

We report a direct optical super-resolution imaging approach with 25 nm (∼ λ/17) lateral resolution under 408 nm wavelength illumination by combining fused silica and polystyrene microspheres with a conventional scanning laser confocal microscope (SLCM). The microsphere deposited on the target surface generates a nanoscale central lobe illuminating a sub-diffraction-limited cross-section located on the target surface. The SLCM confocal pinhole isolates the reflected light from the near-field subdiffractive cross-section and suppresses the noises from the side lobe and the far-field paraxial focal point. The structural detail of the subdiffractive cross-section is therefore captured, and the 2D target surface near the bottom of microspheres can be imaged by intensity-based point scanning.

Immersed transparent microsphere magnifying sub-diffraction-limited objects.

The resolution of an optical microscope is restricted by the diffraction limit, which is approximately 200 nm for a white light source. We report that sub-diffraction-limited objects can be resolved in immersion liquids using a microsphere optical nanoscopy (MONS) technique. Image magnifications and resolutions were obtained experimentally and compared in different immersion liquids. We show that a 100 µm diameter barium titanate (BaTiO(3)) glass microsphere combined with a standard optical microscope can image sub-diffraction-limited objects with halogen light in three different media: water, 40% sugar solution, and microscope immersion oil. In this paper, the super-resolution imaging performance has been described with the three immersion liquid types and the mechanisms are discussed with Mie theory calculation in the field of a Poynting vector.