Collective Coupling of 3D Confined Optical Modes in Monolithic Twin Microtube Cavities Formed by Nanomembrane Origami.

We report the monolithic fabrication of twin microtube cavities by a nanomembrane origami method for achieving collective coupling of 3D confined optical modes. Owing to the well-aligned twin geometries, two sets of 3D confined optical modes in twin microtubes are spectrally and spatially matched, by which both the fundamental and higher-order axial modes are respectively coupled with each other. Multiple groups of the coupling modes provide multiple effective channels for energy exchange between coupled microcavities illustrated by the measured spatial optical field distributions. The spectral anticrossing and changing-over features of each group of coupled modes are revealed in experiments and calculations, indicating the occurrence of strong coupling. In addition, the simulated 3D mode profiles of twin microcavities confirm the collective strong coupling behavior, which shows good agreement with experiments. The collective coupling of 3D confined resonant modes promises broad applications in multichannel optical signal processing, nanophotonics, and 3D non-Hermitian systems.

Verification and Analysis of Single-Molecule SERS Events via Polarization-Selective Raman Measurement.

We propose polarization-selective Raman measurement as a decent method for single-molecule surface-enhanced Raman scattering (SMSERS) verification. This approach features rapid acquisition of SMSERS events and appeals liberal requirements for analyte concentration. It is demonstrated as an efficient tool in sorting out dozens of SMSERS events from a large-scale plasmonic dimer array. In addition, it allows identification of a mixed SMSERS event containing two different individual molecules. In this article, the RPM method is employed to explore the underlying mechanisms of signal blinking, spectral wandering, and other unique characteristics in SMSERS. We observed synchronized blinking of different modes from one Rhodamine 6G (R6G) molecule, but a disagreement is found in a mixed SMSERS event containing one R6G molecule and one crystal violet molecule. Our approach offers a reliable means to interpret SMSERS events in statistical terms and facilitate the fundamental understanding of SMSERS.

Interfacial Chemistry Triggers Ultrafast Radiative Recombination in Metal Halide Perovskites.

Efficient radiative recombination is essential for perovskite luminescence, but the intrinsic radiative recombination rate as a basic material property is challenging to tailor. Here we report an interfacial chemistry strategy to dramatically increase the radiative recombination rate of perovskites. By coating aluminum oxide on the lead halide perovskite, lead-oxygen bonds are formed at the perovskite-oxide interface, producing the perovskite surface states with a large exciton binding energy and a high localized density of electronic state. The oxide-bonded perovskite exhibits a ≈500 fold enhanced photoluminescence with a ≈10 fold reduced lifetime, indicating an unprecedented ≈5000 fold increase in the radiative recombination rate. The enormously enhanced radiative recombination promises to significantly promote the perovskite optoelectronic performance.

Imperceptible Supercapacitors with High Area-Specific Capacitance.

Imperceptible electronics will make next-generation healthcare and biomedical systems thinner, lighter, and more flexible. While other components are thoroughly investigated, imperceptible energy storage devices lag behind because the decrease of thickness impairs the area-specific energy density. Imperceptible supercapacitors with high area-specific capacitance based on reduced graphene oxide/polyaniline (RGO/PANI) composite electrodes and polyvinyl alcohol (PVA)/H2 SO4 gel electrolyte are reported. Two strategies to realize a supercapacitor with a total device thickness of 5 µm-including substrate, electrode, and electrolyte-and an area-specific capacitance of 36 mF cm-2 simultaneously are implemented. First, the void volume of the RGO/PANI electrodes through mechanical compression is reduced, which decreases the thickness by 83% while retaining 89% of the capacitance. Second, the PVA-to-H2 SO4 mass ratio is decreased to 1:4.5, which improves the ion conductivity by 5000% compared to the commonly used PVA/H2 SO4 gel. Both advantages enable a 2 µm-thick gel electrolyte for planar interdigital supercapacitors. The impressive electromechanical stability of the imperceptible supercapacitors by wrinkling the substrate to produce folds with radii of 6 µm or less is demonstrated. The supercapacitors will be meaningful energy storage modules for future self-powered imperceptible electronics.

Topological Defect Engineering and PT Symmetry in Non-Hermitian Electrical Circuits.

We employ electric circuit networks to study topological states of matter in non-Hermitian systems enriched by parity-time symmetry PT and chiral symmetry anti-PT (APT). The topological structure manifests itself in the complex admittance bands which yields excellent measurability and signal to noise ratio. We analyze the impact of PT-symmetric gain and loss on localized edge and defect states in a non-Hermitian Su-Schrieffer-Heeger (SSH) circuit. We realize all three symmetry phases of the system, including the APT-symmetric regime that occurs at large gain and loss. We measure the admittance spectrum and eigenstates for arbitrary boundary conditions, which allows us to resolve not only topological edge states, but also a novel PT-symmetric Z_{2} invariant of the bulk. We discover the distinct properties of topological edge states and defect states in the phase diagram. In the regime that is not PT symmetric, the topological defect state disappears and only reemerges when APT symmetry is reached, while the topological edge states always prevail and only experience a shift in eigenvalue. Our findings unveil a future route for topological defect engineering and tuning in non-Hermitian systems of arbitrary dimension.

Dynamic tuning of photon-plasmon interaction based on three-dimensionally confined microtube cavities.

We present tunable coupling between surface plasmon resonances supported by a metal-nanoparticle-coated tip and three-dimensionally (3D) confined optical modes supported by a microtube cavity. The competition and transition between two types of coupling mechanisms, i.e., dielectric-dielectric and plasmon-dielectric coupling, are observed in the tunable system. Owing to the competition between the two coupling mechanisms, the resonant modes can be dynamically tuned to first shift from higher to lower energies and then revert to higher energy. Moreover, the unique spatial field distribution of 3D confined modes allows selective coupling of odd and even order axial modes with surface plasmon resonances.

Curved Nanomembrane-Based Concentric Ring Cavities for Supermode Hybridization.

We report the mode interactions and resonant hybridization in nanomembrane-formed concentric dual ring cavities supporting whispering gallery mode resonances. Utilizing a rolled-up nanomembrane with subwavelength thickness as an interlayer, dual concentric microring cavities are formed by coating high-index nanomembranes on the inner and outer surfaces of the rolled-up dielectric nanomembrane. In such a hybrid cavity system, the conventional fundamental mode resonating along a single ring orbit splits into symmetric and antisymmetric modes confined by concentric dual ring orbits. Detuning of the coupled supermodes is realized by spatially resolved measurements along the cavity axial direction. A spectral anticrossing feature is observed as a clear evidence of strong coupling. Upon strong coupling, the resonant orbits of symmetric and antisymmetric modes cross over each other in the form of superwaves oscillating between the concentric rings with opposite phase. Notably, the present system provides high flexibilities in controlling the coupling strength by varying the thickness of the spacer layer and thus enables switching between strong and weak coupling regimes. Our work offers a compact and robust scheme using curved nanomembranes to realize novel cavity mode interactions for both fundamental and applied studies.

Multiplexing and tuning of a double set of resonant modes in optical microtube cavities monolithically integrated on a photonic chip.

In this Letter, we experimentally demonstrate a monolithic integration of two vertically rolled-up microtube resonators (VRUMs) on polymer-based 1×5 multimode interference waveguides to achieve 3D multi-channel coupling. In this configuration, different sets of resonant modes are simultaneously excited at S-, C-, and L- telecom bands, demonstrating an on-chip multiplexing, based on a vertical-coupling configuration. Moreover, the resonant wavelength tuning and consequently the overlapping of resonant modes are accomplished via covering the integrated VRUMs by liquid. A maximum sensitivity of 330 nm/refractive index unit is achieved. The present work would be a critical step for the realization of massively parallel optofluidic sensors with higher sensitivity and flexibility for signal processing, particularly in a 3D-integrated photonic chip.

Optical microtube cavities monolithically integrated on photonic chips for optofluidic sensing.

Microtubular optical resonators are monolithically integrated on photonic chips to demonstrate optofluidic functionality. Due to the compact subwavelength-thin tube wall and a well-defined nanogap between polymer photonic waveguides and the microtube, excellent optical coupling with extinction ratios up to 32 dB are observed in the telecommunication relevant wavelength range. For the first time, optofluidic applications of fully on-chip integrated microtubular systems are investigated both by filling the core of the microtube and by the microtube being covered by a liquid droplet. Total shifts over the full free spectral range are observed in response to the presence of the liquid medium in the vicinity of the microtube resonators. This work provides a vertical coupling scheme for optofluidic applications in monolithically integrated so-called "lab-in-a-tube" systems.

Localized Surface Plasmons Selectively Coupled to Resonant Light in Tubular Microcavities.

Vertical gold nanogaps are created on microtubular cavities to explore the coupling between resonant light supported by the microcavities and surface plasmons localized at the nanogaps. Selective coupling of optical axial modes and localized surface plasmons critically depends on the exact location of the gold nanogap on the microcavities, which is conveniently achieved by rolling up specially designed thin dielectric films into three-dimensional microtube cavities. The coupling phenomenon is explained by a modified quasipotential model based on perturbation theory. Our work reveals the coupling of surface plasmon resonances localized at the nanoscale to optical resonances confined in microtubular cavities at the microscale, implying a promising strategy for the investigation of light-matter interactions.

Vertical optical ring resonators fully integrated with nanophotonic waveguides on silicon-on-insulator substrates.

We demonstrate full integration of vertical optical ring resonators with silicon nanophotonic waveguides on silicon-on-insulator substrates to accomplish a significant step toward 3D photonic integration. The on-chip integration is realized by rolling up 2D differentially strained TiO(2) nanomembranes into 3D microtube cavities on a nanophotonic microchip. The integration configuration allows for out-of-plane optical coupling between the in-plane nanowaveguides and the vertical microtube cavities as a compact and mechanically stable optical unit, which could enable refined vertical light transfer in 3D stacks of multiple photonic layers. In this vertical transmission scheme, resonant filtering of optical signals at telecommunication wavelengths is demonstrated based on subwavelength thick-walled microcavities. Moreover, an array of microtube cavities is prepared, and each microtube cavity is integrated with multiple waveguides, which opens up interesting perspectives toward parallel and multi-routing through a single-cavity device as well as high-throughput optofluidic sensing schemes.

Ultra-dense plasmonic nanogap arrays for reorientable molecular fluorescence enhancement and spectrum reshaping.

Understanding interactions between molecular transition and intense electromagnetic fields confined by plasmon nanostructures is of great significance due to their huge potential in fundamental cavity quantum electrodynamics and practical applications. Here, we report reorientable plasmon-enhanced fluorescence leveraging the flexibilities in densely-packed gold nanogap arrays by template-assisted depositions. By finely adjusting the symmetry of the unit structure, arrays of nanogaps along two nearly-orthogonal axes can be tailored collectively with spacing down to sub-10 nm on a single chip, facilitating distinct "inter-cell" and "intra-cell" plasmon couplings. Through engineering two sets of nanogaps, the varying hybridization-induced plasmonic bonding modes lead to adjustable splitting of the fluorescence emission peak with a width up to 81 nm and narrowing of linewidths up to a factor of 3. Besides, polarization anisotropy with a ratio up to 63% is obtained on the basis of spectrally separated local hotspots with discrepant oscillation directions. The developed plasmonic nanogap array is envisaged to provide a promising chip-scale, cost-effective platform for advancing fluorescence-based detection and emission technologies in both classical and quantum regimes.

Fast-Response Micro-Phototransistor Based on MoS2/Organic Molecule Heterojunction.

Over the past years, molybdenum disulfide (MoS2) has been the most extensively studied two-dimensional (2D) semiconductormaterial. With unique electrical and optical properties, 2DMoS2 is considered to be a promising candidate for future nanoscale electronic and optoelectronic devices. However, charge trapping leads to a persistent photoconductance (PPC), hindering its use for optoelectronic applications. To overcome these drawbacks and improve the optoelectronic performance, organic semiconductors (OSCs) are selected to passivate surface defects, tune the optical characteristics, and modify the doping polarity of 2D MoS2. Here, we demonstrate a fast photoresponse in multilayer (ML) MoS2 by addressing a heterojunction interface with vanadylphthalocyanine (VOPc) molecules. The MoS2/VOPc van der Waals interaction that has been established encourages the PPC effect in MoS2 by rapidly segregating photo-generated holes, which move away from the traps of MoS2 toward the VOPc molecules. The MoS2/VOPc phototransistor exhibits a fast photo response of less than 15 ms for decay and rise, which is enhanced by 3ordersof magnitude in comparison to that of a pristine MoS2-based phototransistor (seconds to tens of seconds). This work offers a means to realize high-performance transition metal dichalcogenide (TMD)-based photodetection with a fast response speed.

Nanomaterial-decorated micromotors for enhanced photoacoustic imaging.

Micro-and nanorobots have the potential to perform non-invasive drug delivery, sensing, and surgery in living organisms, with the aid of diverse medical imaging techniques. To perform such actions, microrobots require high spatiotemporal resolution tracking with real-time closed-loop feedback. To that end,  photoacoustic imaging has appeared as a promising technique for imaging microrobots in deep tissue with higher molecular specificity and contrast. Here, we present different strategies to track magnetically-driven micromotors with improved contrast and specificity using dedicated contrast agents (Au nanorods and nanostars). Furthermore, we discuss the possibility of improving the light absorption properties of the employed nanomaterials considering possible light scattering and coupling to the underlying metal-oxide layers on the micromotor's surface. For that, 2D COMSOL simulation and experimental results were correlated, confirming that an increased spacing between the Au-nanostructures and the increase of thickness of the underlying oxide layer lead to enhanced light absorption and preservation of the characteristic absorption peak. These characteristics are important when visualizing the micromotors in a complex in vivo environment, to distinguish them from the light absorption properties of the surrounding natural chromophores. Supplementary Information: The online version contains supplementary material available at 10.1007/s12213-023-00156-7.

Enhanced optical axial confinement in asymmetric microtube cavities rolled up from circular-shaped nanomembranes.

Asymmetric cone-like microtube cavities have been fabricated by unevenly rolling-up prestrained SiO/SiO(2) circular-shaped nanomembranes. Spatially localized axial resonant modes are obtained due to an axial confinement mechanism that is defined by the variation of the tube radius and windings along the tube axis. A theoretical model is applied to quantitatively explain and confirm our experimental results.

Direct Thermal Enhancement of Hydrogen Evolution Reaction of On-Chip Monolayer MoS2.

MoS2 has drawn great attention as a promising alternative to Pt-based catalysts for the hydrogen evolution reaction (HER). However, it suffers from sluggish kinetics to drive the HER process because of inert basal planes. Here, an on-chip MoS2 monolayer (MoS2 ML) HER reactor was designed and fabricated to reveal direct thermal enhancement of MoS2 ML for the HER. The thermal effects generated efficient electron transfer in the atomic MoS2 ML and at the interface between the electrolyte and the catalyst, leading to enhanced HER activity. The MoS2 ML measured at a higher temperature (60 °C) possesses a significantly enhanced HER activity with a lower overpotential (90 mV at current densities of 10 mA cm-2), lower Tafel slope (94 mV dec-1), and higher turnover frequency (73 s-1 at an overpotential of 125 mV) compared to the results obtained at room temperature. More importantly, the findings are attractive toward understanding the thermal effect on 2D monolayers as well as the development of next-generation electrocatalysts.

Lasing in Two-Dimensional Tin Perovskites.

Two-dimensional (2D) perovskites have been proposed as materials capable of improving the stability and surpassing the radiative recombination efficiency of three-dimensional perovskites. However, their luminescent properties have often fallen short of what has been expected. In fact, despite attracting considerable attention for photonic applications during the last two decades, lasing in 2D perovskites remains unclear and under debate. Here, we were able to improve the optical gain properties of 2D perovskite and achieve optically pumped lasing. We show that the choice of the spacer cation affects the defectivity and photostability of the perovskite, which in turn influences its optical gain. Based on our synthetic strategy, we obtain PEA2SnI4 films with high crystallinity and favorable optical properties, resulting in amplified spontaneous emission (ASE) with a low threshold (30 µJ/cm2), a high optical gain above 4000 cm-1 at 77 K, and ASE operation up to room temperature.

Single "Swiss-roll" microelectrode elucidates the critical role of iron substitution in conversion-type oxides.

Advancing the lithium-ion battery technology requires the understanding of electrochemical processes in electrode materials with high resolution, accuracy, and sensitivity. However, most techniques today are limited by their inability to separate the complex signals from slurry-coated composite electrodes. Here, we use a three-dimensional "Swiss-roll" microtubular electrode that is incorporated into a micrometer-sized lithium battery. This on-chip platform combines various in situ characterization techniques and precisely probes the intrinsic electrochemical properties of each active material due to the removal of unnecessary binders and additives. As an example, it helps elucidate the critical role of Fe substitution in a conversion-type NiO electrode by monitoring the evolution of Fe2O3 and solid electrolyte interphase layer. The markedly enhanced electrode performances are therefore explained. Our approach exposes a hitherto unexplored route to tracking the phase, morphology, and electrochemical evolution of electrodes in real time, allowing us to reveal information that is not accessible with bulk-level characterization techniques.

Tuning of optical resonances in asymmetric microtube cavities.

We tune optical resonances in rolled-up SiO/SiO(2) microtube cavities by gradually modifying the tube structure through asymmetrical postdeposition of SiO(2). Spectral blueshifts followed by redshifts of the resonant modes are observed in a thin-walled microtube (tube-I), which is attributed to a competition between shape deformation and effective increase of tube wall thickness. In contrast, only a monotonic redshift is detected when asymmetrical deposition is performed on a thick-walled microtube (tube-II). Distinct wavelength-dependent tuning was revealed in both kinds of tubes. Numerical calculations based on perturbation theory are carried out to explain and confirm the experimental results.

Efficient coding of natural images.

Efficient coding theory proposes that sensory systems in the brain have been adapted to process natural scenes efficiently over the long history of evolution. Computational modeling the statistical regularities of natural images is therefore beneficial to our understanding of the mechanisms of visual information processing. In this paper, we briefly review the recent progress in using efficient coding approaches to study the encoding of natural images in the visual system.