Polarization-Dependent Memory and Erasure in Quantum Dots/Graphene Synaptic Devices.

We demonstrate excitatory and inhibitory properties in a single heterostructure consisting of two quantum dots/graphene synaptic elements using linearly polarized monochromatic light. Perovskite quantum dots and PbS quantum dots were used to increase and decrease photocurrent weights, respectively. The polarization-dependent photocurrent was realized by adding a polarizer in the middle of the PbS quantum dots/graphene and perovskite quantum dots/graphene elements. When linearly polarized light passed through the polarizer, both the lower excitatory and upper inhibitory devices were activated, with the lower device with the stronger response dominating to increase the current weight. In contrast, the polarized light was blocked by the polarizer, and the above device was only operated, reducing the current weight. Furthermore, two orthogonal polarizations of light were used to perform the sequential processes of potentiation and habituation. By adjustment of the polarization angle of light, not only the direction of the current weight but also its level was altered.

Enhancement of second-harmonic generation in a 64° stacked WSe2/WS2heterobilayer with local strain.

Transition metal dichalcogenides (TMDs) are actively studied in various fields of optics and optoelectronics, including nonlinear optics of second-harmonic generation (SHG). By stacking two different TMD materials to form a heterobilyaer, unique optical properties emerge, with stronger SHG at a twist angle of 0° between TMDs and weaker SHG at a twist angle of 60°. In this work, we demonstrate the enhancement of SHG in a heterobilayer consisting of WSe2and WS2monolayers stacked at a twist angle of 64.1°, using a nanoparticle to induce local strain. The interatomic spacing of the heterobilayer is deformed by the nanoparticle, breaking the inversion symmetry, resulting in a substantial increase in the SHG of the heterobilayer at room temperature. The SHG increases depending on the polarization of the pump laser: 15-fold for linear polarization, 9-fold for right-circular polarization, and up to 100-fold for left-circular polarization. In addition, the SHG enhanced in the heterobilayer with local strain satisfies the same chiral selection rule as in the unstrained TMD region, demonstrating that the chiral selection rule of SHG is insensitive to local strain. Our findings will increase the applicability of TMD heterobilayers in nonlinear optoelectronics and valleytronics.

Injectable Ventral Spinal Stimulator Evokes Programmable and Biomimetic Hindlimb Motion.

Spinal cord neuromodulation can restore partial to complete loss of motor functions associated with neuromotor disease and trauma. Current technologies have made substantial progress but have limitations as dorsal epidural or intraspinal devices that are either remote to ventral motor neurons or subject to surgical intervention in the spinal tissue. Here, we describe a flexible and stretchable spinal stimulator design with nanoscale thickness that can be implanted by minimally invasive injection through a polymeric catheter to target the ventral spinal space of mice. Ventrolaterally implanted devices exhibited substantially lower stimulation threshold currents and more precise recruitment of motor pools than did comparable dorsal epidural implants. Functionally relevant and novel hindlimb movements were achieved via specific stimulation patterns of the electrodes. This approach holds translational potential for improving controllable limb function following spinal cord injury or neuromotor disease.

Nontypical Wulff-Shape Silicon Nanosheets with High Catalytic Activity.

Nanostructured silicon with an equilibrium shape has exhibited hydrogen evolution reaction activity mainly owing to its high surface area, which is distinct from that of bulk silicon. Such a Wulff shape of silicon favors low-surface-energy planes, resulting in silicon being an anisotropic and predictably faceted solid in which certain planes are favored, but this limits further improvement of the catalytic activity. Here, we introduce nanoporous silicon nanosheets that possess high-surface-energy crystal planes, leading to an unconventional Wulff shape that bolsters the catalytic activity. The high-index plane, uncommonly seen in the Wulff shape of bulk Si, has a band structure optimally aligned with the redox potential necessary for hydrogen generation, resulting in an apparent quantum yield (AQY) of 12.1% at a 400 nm wavelength. The enhanced light absorption in nanoporous silicon nanosheets also contributes to the high photocatalytic activity. Collectively, the strategy of making crystals with nontypical Wulff shapes can provide a route toward various classes of photocatalysts for hydrogen production.

Advanced thin films of indium-tin oxide doping with photosensitive polymer via embossing process.

We propose a sol-gel thin film formation process involving nanoimprint lithography. First, indium tin oxide was dissolved in 2-methoxyethanol at a ratio of 5:5 and the mixture were mixed with 10 wt% of a UV-curable. Subsequently, a polydimethylsiloxane sheet prepared by covering a silicon wafer with a polydimethylsiloxane mold was attached to a InSnO thin film to duplicate the nanostructure through UV irradiation exposure. The replicated nanostructured thin films formed about morphological and chemical composition changes on the surface, we progressed to x-ray photoelectron spectroscopy and atomic force microscopy analysis. Furthermore, atomic force microscopy image analysis showed superior patterned grooves for a UV exposure time of 3 min. A suitability test involving the measurement of the transmittance was performed for examining the suitability of the thin film for use in display devices.

Scalable Three-Dimensional Recording Electrodes for Probing Biological Tissues.

Electrophysiological recording technologies can provide critical insight into the function of the nervous system and other biological tissues. Standard silicon-based probes have limitations, including single-sided recording sites and intrinsic instabilities due to the probe stiffness. Here, we demonstrate high-performance neural recording using double-sided three-dimensional (3D) electrodes integrated in an ultraflexible bioinspired open mesh structure, allowing electrodes to sample fully the 3D interconnected tissue of the brain. In vivo electrophysiological recording using 3D electrodes shows statistically significant increases in the number of neurons per electrode, average spike amplitudes, and signal to noise ratios in comparison to standard two-dimensional electrodes, while achieving stable detection of single-neuron activity over months. The capability of these 3D electrodes is further shown for chronic recording from retinal ganglion cells in mice. This approach opens new opportunities for a comprehensive 3D interrogation, stimulation, and understanding of the complex circuitry of the brain and other electrogenic tissues in live animals over extended time periods.

Enhanced Five-Photon Photoluminescence in Subwavelength AlGaAs Resonators.

Multiphoton processes of absorption photoluminescence have enabled a wide range of applications including three-dimensional microfabrication, data storage, and biological imaging. While the applications of two-photon and three-photon absorption and luminescence have matured considerably, higher-order photoluminescence processes remain more challenging to study due to their lower efficiency, particularly in subwavelength systems. Here, we report the observation of five-photon luminescence from a single subwavelength nanoantenna at room temperature enabled by the Mie resonances. We excite an AlGaAs resonator at around 3.6 µm and observe photoluminescence at around 740 nm. We show that the interplay of the Mie multipolar modes at the subwavelength scale can enhance the efficiency of the five-photon luminescence by at least 4 orders of magnitude, being limited only by sensitivity of our detector. Our work paves the way toward applications of higher-order multiphoton processes at the subwavelength scales enabled by the physics of Mie resonances.

Optical stimulation of cardiac cells with a polymer-supported silicon nanowire matrix.

Electronic pacemakers can treat electrical conduction disorders in hearts; however, they are invasive, bulky, and linked to increased incidence of infection at the tissue-device interface. Thus, researchers have looked to other more biocompatible methods for cardiac pacing or resynchronization, such as femtosecond infrared light pulsing, optogenetics, and polymer-based cardiac patches integrated with metal electrodes. Here we develop a biocompatible nongenetic approach for the optical modulation of cardiac cells and tissues. We demonstrate that a polymer-silicon nanowire composite mesh can be used to convert fast moving, low-radiance optical inputs into stimulatory signals in target cardiac cells. Our method allows for the stimulation of the cultured cardiomyocytes or ex vivo heart to beat at a higher target frequency.

Nonlinear Circular Dichroism in Mie-Resonant Nanoparticle Dimers.

We studied the nonlinear response of a dimer composed of two identical Mie-resonant dielectric nanoparticles illuminated normally by a circularly polarized light. We developed a general theory describing hybridization of multipolar modes of the coupled nanoparticles and reveal nonvanishing nonlinear circular dichroism (CD) in the second-harmonic generation (SHG) signal enhanced by the multipolar resonances in the dimer, provided its axis is oriented under an angle to the crystalline lattice of the dielectric material. We supported our multipolar hybridization theory by experimental results obtained for the AlGaAs dimers placed on an engineered substrate.

From Fano to Quasi-BIC Resonances in Individual Dielectric Nanoantennas.

Sharp optical resonances in high-index dielectric nanostructures have recently attracted significant attention for their promising applications in nanophotonics. Fano resonances, as well as resonances associated with bound states in the continuum (BIC), have independently shown a great potential for applications in nanoscale lasers, sensors, and nonlinear optical devices. Here, we demonstrate experimentally a close connection between Fano and quasi-BIC resonances excited in individual dielectric nanoantennas. We analyze systematically the resonant response of AlGaAs nanoantennas pumped with a structured light in the near-infrared range. We trace a variation of the scattering spectrum that fully agrees with an analytical expression governed by a Fano parameter and observe directly a transition to a quasi-BIC resonance. Our results suggest a unified approach toward the analysis of sharp resonances in subwavelength nanostructures originating from strong coupling of optical modes that can provide high energy localization for enhanced light-matter interactions.

All-Tissue-like Multifunctional Optoelectronic Mesh for Deep-Brain Modulation and Mapping.

The development of a multifunctional device that achieves optogenetic neuromodulation and extracellular neural mapping is crucial for understanding neural circuits and treating brain disorders. Although various devices have been explored for this purpose, it is challenging to develop biocompatible optogenetic devices that can seamlessly interface with the brain. Herein, we present a tissue-like optoelectronic mesh with a compact interface that enables not only high spatial and temporal resolutions of optical stimulation but also the sampling of optically evoked neural activities. An in vitro experiment in hydrogel showed efficient light propagation through a freestanding SU-8 waveguide that was integrated with flexible mesh electronics. Additionally, an in vivo implantation of the tissue-like optoelectronic mesh in the brain of a live transgenic mouse enabled the sampling of optically evoked neural signals. Therefore, this multifunctional device can aid the chronic modulation of neural circuits and behavior studies for developing biological and therapeutic applications.

Lasing Action from Anapole Metasurfaces.

We study active dielectric metasurfaces composed of two-dimensional arrays of split-nanodisk resonators fabricated in InGaAsP membranes with embedded quantum wells. Depending on the geometric parameters, such split-nanodisk resonators can operate in the optical anapole regime originating from an overlap of the electric dipole and toroidal dipole Mie-resonant optical modes, thus supporting strongly localized fields and high-Q resonances. We demonstrate room-temperature lasing from the anapole lattices of engineered active metasurfaces with low threshold and high coherence.

Polarization Control of Deterministic Single-Photon Emitters in Monolayer WSe2.

Single-photon emitters, the basic building blocks of quantum communication and information, have been developed using atomically thin transition metal dichalcogenides (TMDCs). Although the bandgap of TMDCs was spatially engineered in artificially created defects for single-photon emitters, it remains a challenge to precisely align the emitter's dipole moment to optical cavities for the Purcell enhancement. Here, we demonstrate position- and polarization-controlled single-photon emitters in monolayer WSe2. A tensile strain of ∼0.2% was applied to monolayer WSe2 by placing it onto a dielectric rod structure with a nanosized gap. Excitons were localized in the nanogap sites, resulting in the generation of linearly polarized single-photon emission with a g(2) of ∼0.1 at 4 K. Additionally, we measured the abrupt change in polarization of single photons with respect to the nanogap size. Our robust spatial and polarization control of emission provides an efficient way to demonstrate deterministic and scalable single-photon sources by integrating with nanocavities.

Monolithic Interface Contact Engineering to Boost Optoelectronic Performances of 2D Semiconductor Photovoltaic Heterojunctions.

In optoelectronic devices based on two-dimensional (2D) semiconductor heterojunctions, the efficient charge transport of photogenerated carriers across the interface is a critical factor to determine the device performances. Here, we report an unexplored approach to boost the optoelectronic device performances of the WSe2-MoS2 p-n heterojunctions via the monolithic-oxidation-induced doping and resultant modulation of the interface band alignment. In the proposed device, the atomically thin WOx layer, which is directly formed by layer-by-layer oxidation of WSe2, is used as a charge transport layer for promoting hole extraction. The use of the ultrathin oxide layer significantly enhanced the photoresponsivity of the WSe2-MoS2 p-n junction devices, and the power conversion efficiency increased from 0.7 to 5.0%, maintaining the response time. The enhanced characteristics can be understood by the formation of the low Schottky barrier and favorable interface band alignment, as confirmed by band alignment analyses and first-principle calculations. Our work suggests a new route to achieve interface contact engineering in the heterostructures toward realizing high-performance 2D optoelectronics.

Nanoenabled Direct Contact Interfacing of Syringe-Injectable Mesh Electronics.

Polymer-based electronics with low bending stiffnesses and high flexibility, including recently reported macroporous syringe-injectable mesh electronics, have shown substantial promise for chronic studies of neural circuitry in the brains of live animals. A central challenge for exploiting these highly flexible materials for in vivo studies has centered on the development of efficient input/output (I/O) connections to an external interface with high yield, low bonding resistance, and long-term stability. Here we report a new paradigm applied to the challenging case of injectable mesh electronics that exploits the high flexibility of nanoscale thickness two-sided metal I/O pads that can deform and contact standard interface cables in high yield with long-term electrical stability. First, we describe the design and facile fabrication of two-sided metal I/O pads that allow for contact without regard to probe orientation. Second, systematic studies of the contact resistance as a function of I/O pad design and mechanical properties demonstrate the key role of the I/O pad bending stiffness in achieving low-resistance stable contacts. Additionally, computational studies provide design rules for achieving high-yield multiplexed contact interfacing in the case of angular misalignment such that adjacent channels are not shorted. Third, the in vitro measurement of 32-channel mesh electronics probes bonded to interface cables using the direct contact method shows a reproducibly high yield of electrical connectivity. Finally, in vivo experiments with 32-channel mesh electronics probes implanted in live mice demonstrate the chronic stability of the direct contact interface, enabling consistent tracking of single-unit neural activity over at least 2 months without a loss of channel recording. The direct contact interfacing methodology paves the way for scalable long-term connections of multiplexed mesh electronics neural probes for neural recording and modulation and moreover could be used to facilitate a scalable interconnection of other flexible electronics in biological studies and therapeutic applications.

Photon-Triggered Current Generation in Chemically-Synthesized Silicon Nanowires.

A porous Si segment in a Si nanowire (NW), when exposed to light, generates a current with a high on/off ratio. This unique feature has been recently used to demonstrate photon-triggered NW devices including transistors, logic gates, and photodetection systems. Here, we develop a reliable and simple procedure to fabricate porous Si segments in chemically synthesized Si NWs for photon-triggered current generation. To achieve this, we employ 100 nm-diameter chemical-vapor-deposition grown Si NWs that possess an n-type high doping level and extremely smooth surface. The NW regions uncovered by electron-beam resist become selectively porous through metal-assisted chemical etching, using Ag nanoparticles as a catalyst. The contact electrodes are then fabricated on both ends of such NWs, and the generated current is measured when the laser is focused on the porous Si segment. The current level is changed by controlling the power of the incident laser and bias voltage. The on/off ratio is measured up to 1.5 × 104 at a forward bias of 5 V. In addition, we investigate the porous-length-dependent responsivity of the NW device with the porous Si segment. The responsivity is observed to decrease for porous segment lengths beyond 360 nm. Furthermore, we fabricate nine porous Si segments in a single Si NW and measure the identical photon-triggered current from each porous segment; this single NW device can function as a high-resolution photodetection system. Therefore, our fabrication method to precisely control the position and length of the porous Si segments opens up new possibilities for the practical implementation of programmable logic gates and ultrasensitive photodetectors.

Electro-Optical Behavior of Liquid Crystals Doped with Low Concentrations of Various Titanate Nanoparticles.

It is well known that the physical and electro-optical properties of liquid crystals can be controlled by doping with nanoparticles. In this study, we investigated the electro-optical properties of homogeneous aligned liquid crystal devices doped with various titanate nanoparticles, such as TiSiO4, BaTiO3, SrTiO3 and mixtures of BaTiO3 and SrTiO3. Doping liquid crystals with titanate nanoparticles decreased the threshold voltages of the liquid crystal mixture cells. In particular, the liquid crystal cells doped with SrTiO3 nanoparticles exhibited the lowest switching amplitude of 0.643 V. The high performance of the liquid crystal cells doped with various titanate nanoparticles indicates that the proposed liquid crystal devices have strong potential for use in materials for advanced liquid crystal displays.

Transparent, Flexible, Conformal Capacitive Pressure Sensors with Nanoparticles.

The fundamental challenge in designing transparent pressure sensors is the ideal combination of high optical transparency and high pressure sensitivity. Satisfying these competing demands is commonly achieved by a compromise between the transparency and usage of a patterned dielectric surface, which increases pressure sensitivity, but decreases transparency. Herein, a design strategy for fabricating high-transparency and high-sensitivity capacitive pressure sensors is proposed, which relies on the multiple states of nanoparticle dispersity resulting in enhanced surface roughness and light transmittance. We utilize two nanoparticle dispersion states on a surface: (i) homogeneous dispersion, where each nanoparticle (≈500 nm) with a size comparable to the visible light wavelength has low light scattering; and (ii) heterogeneous dispersion, where aggregated nanoparticles form a micrometer-sized feature, increasing pressure sensitivity. This approach is experimentally verified using a nanoparticle-dispersed polymer composite, which has high pressure sensitivity (1.0 kPa-1 ), and demonstrates excellent transparency (>95%). We demonstrate that the integration of nanoparticle-dispersed capacitor elements into an array readily yields a real-time pressure monitoring application and a fully functional touch device capable of acting as a pressure sensor-based input device, thereby opening up new avenues to establish processing techniques that are effective on the nanoscale yet applicable to macroscopic processing.

Long-range surface plasmon polariton detection with a graphene photodetector.

We present an integration of a single Ag nanowire (NW) with a graphene photodetector and demonstrate an efficient and compact detection of long-range surface plasmon polaritons (SPPs). Atomically thin graphene provides an ideal platform to detect the evanescent electric field of SPPs extremely bound at the interface of the Ag NW and glass substrate. Scanning photocurrent microscopy directly visualizes a polarization-dependent excitation and detects the SPPs. The SPP detection responsivity is readily controlled up to ∼17 mA/W by the drain-source voltage. We believe that the graphene SPP detector will be a promising building block for highly integrated photonic and optoelectronic circuits.

High-throughput patterning of photonic structures with tunable periodicity.

A patterning method termed "RIPPLE" (reactive interface patterning promoted by lithographic electrochemistry) is applied to the fabrication of arrays of dielectric and metallic optical elements. This method uses cyclic voltammetry to impart patterns onto the working electrode of a standard three-electrode electrochemical setup. Using this technique and a template stripping process, periodic arrays of Ag circular Bragg gratings are patterned in a high-throughput fashion over large substrate areas. By varying the scan rate of the cyclically applied voltage ramps, the periodicity of the gratings can be tuned in situ over micrometer and submicrometer length scales. Characterization of the periodic arrays of periodic gratings identified point-like and annular scattering modes at different planes above the structured surface. Facile, reliable, and rapid patterning techniques like RIPPLE may enable the high-throughput and low-cost fabrication of photonic elements and metasurfaces for energy conversion and sensing applications.