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.

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.

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.

Switching of Photonic Crystal Lasers by Graphene.

Unique features of graphene have motivated the development of graphene-integrated photonic devices. In particular, the electrical tunability of graphene loss enables high-speed modulation of light and tuning of cavity resonances in graphene-integrated waveguides and cavities. However, efficient control of light emission such as lasing, using graphene, remains a challenge. In this work, we demonstrate on/off switching of single- and double-cavity photonic crystal lasers by electrical gating of a monolayer graphene sheet on top of photonic crystal cavities. The optical loss of graphene was controlled by varying the gate voltage Vg, with the ion gel atop the graphene sheet. First, the fundamental properties of graphene were investigated through the transmittance measurement and numerical simulations. Next, optically pumped lasing was demonstrated for a graphene-integrated single photonic crystal cavity at Vg below -0.6 V, exhibiting a low lasing threshold of ∼480 µW, whereas lasing was not observed at Vg above -0.6 V owing to the intrinsic optical loss of graphene. Changing quality factor of the graphene-integrated photonic crystal cavity enables or disables the lasing operation. Moreover, in the double-cavity photonic crystal lasers with graphene, switching of individual cavities with separate graphene sheets was achieved, and these two lasing actions were controlled independently despite the close distance of ∼2.2 µm between adjacent cavities. We believe that our simple and practical approach for switching in graphene-integrated active photonic devices will pave the way toward designing high-contrast and ultracompact photonic integrated circuits.

Stitching Flexible Electronics into the Brain.

Understanding complex neuronal networks requires monitoring long-term neuronal activity in various regions of the brain. Significant progress has been made in multisite implantations of well-designed probes, such as multisite implantation of Si-based and polymer-based probes. However, these multiprobe strategies are limited by the sizes and weights of interfaces to the multiple probes and the inability to track the activity of the same neurons and changes in neuronal activity over longer time periods. Here, a long single flexible probe that can be implanted by stitching into multiple regions of the mouse brain and subsequently transmit chronically stable neuronal signals from the multiple sites via a single low-mass interface is reported. The probe at four different sites is implemented using a glass capillary needle or two sites using an ultrathin metal needle. In vitro tests in brain-mimicking hydrogel show that multisite probe implantations achieve a high connection yield of >86%. In vivo histological images at each site of probes, implanted by stitching using either glass capillary or ultrathin metal insertion needles exhibit seamless tissue-probe interfaces with negligible chronic immune response. In addition, electrophysiology studies demonstrate the ability to track single neuron activities at every injection site with chronic stability over at least one month. Notably, the measured spike amplitudes and signal-to-noise ratios at different implantation sites show no statistically significant differences. Multisite stitching implantation of flexible electronics in the brain opens up new opportunities for both fundamental neuroscience research and electrotherapeutic applications.

Nanograin network memory with reconfigurable percolation paths for synaptic interactions.

The development of memory devices with functions that simultaneously process and store data is required for efficient computation. To achieve this, artificial synaptic devices have been proposed because they can construct hybrid networks with biological neurons and perform neuromorphic computation. However, irreversible aging of these electrical devices causes unavoidable performance degradation. Although several photonic approaches to controlling currents have been suggested, suppression of current levels and switching of analog conductance in a simple photonic manner remain challenging. Here, we demonstrated a nanograin network memory using reconfigurable percolation paths in a single Si nanowire with solid core/porous shell and pure solid core segments. The electrical and photonic control of current percolation paths enabled the analog and reversible adjustment of the persistent current level, exhibiting memory behavior and current suppression in this single nanowire device. In addition, the synaptic behaviors of memory and erasure were demonstrated through potentiation and habituation processes. Photonic habituation was achieved using laser illumination on the porous nanowire shell, with a linear decrease in the postsynaptic current. Furthermore, synaptic elimination was emulated using two adjacent devices interconnected on a single nanowire. Therefore, electrical and photonic reconfiguration of the conductive paths in Si nanograin networks will pave the way for next-generation nanodevice technologies.

Recent advances in nanocavities and their applications.

High quality factor and small mode volume in nanocavities enable the demonstration of efficient nanophotonic devices with low power consumption, strong nonlinearity, and high modulation speed, due to the strong light-matter interaction. In this review, we focus on recent state-of-the-art nanocavities and their applications. We introduce single nanocavities including semiconductor nanowires, plasmonic cavities, and nanostructures based on quasi-bound states in the continuum (quasi-BIC), for laser, photovoltaic, and nonlinear applications. In addition, nanocavity arrays with unique feedback mechanisms, including BIC cavities, parity-time symmetry coupled cavities, and photonic topological cavities, are introduced for laser applications. These various cavity designs and underlying physics in single and array nanocavities are useful for the practical implementation of promising nanophotonic devices.

Electrically driven strain-induced deterministic single-photon emitters in a van der Waals heterostructure.

Quantum confinement in transition metal dichalcogenides (TMDCs) enables the realization of deterministic single-photon emitters. The position and polarization control of single photons have been achieved via local strain engineering using nanostructures. However, most existing TMDC-based emitters are operated by optical pumping, while the emission sites in electrically pumped emitters are uncontrolled. Here, we demonstrate electrically driven single-photon emitters located at the positions where strains are induced by atomic force microscope indentation on a van der Waals heterostructure consisting of graphene, hexagonal boron nitride, and tungsten diselenide. The optical, electrical, and mechanical properties induced by the local strain gradient were systematically analyzed. The emission at the indentation sites exhibits photon antibunching behavior with a g(2)(0) value of ~0.3, intensity saturation, and a linearly cross-polarized doublet, at 4 kelvin. This robust spatial control of electrically driven single-photon emitters will pave the way for the practical implementation of integrated quantum light sources.

Room-temperature lasing from nanophotonic topological cavities.

The study of topological phases of light underpins a promising paradigm for engineering disorder-immune compact photonic devices with unusual properties. Combined with an optical gain, topological photonic structures provide a novel platform for micro- and nanoscale lasers, which could benefit from nontrivial band topology and spatially localized gap states. Here, we propose and demonstrate experimentally active nanophotonic topological cavities incorporating III-V semiconductor quantum wells as a gain medium in the structure. We observe room-temperature lasing with a narrow spectrum, high coherence, and threshold behaviour. The emitted beam hosts a singularity encoded by a triade cavity mode that resides in the bandgap of two interfaced valley-Hall periodic photonic lattices with opposite parity breaking. Our findings make a step towards topologically controlled ultrasmall light sources with nontrivial radiation characteristics.

Multipolar lasing modes from topological corner states.

Topological photonics provides a fundamental framework for robust manipulation of light, including directional transport and localization with built-in immunity to disorder. Combined with an optical gain, active topological cavities hold special promise for a design of light-emitting devices. Most studies to date have focused on lasing at topological edges of finite systems or domain walls. Recently discovered higher-order topological phases enable strong high-quality confinement of light at the corners. Here, we demonstrate lasing action of corner states in nanophotonic topological structures. We identify several multipole corner modes with distinct emission profiles via hyperspectral imaging and discern signatures of non-Hermitian radiative coupling of leaky topological states. In addition, depending on the pump position in a large-size cavity, we generate selectively lasing from either edge or corner states within the topological bandgap. Our studies provide the direct observation of multipolar lasing and engineered collective resonances in active topological nanostructures.

Electrical modulation of a photonic crystal band-edge laser with a graphene monolayer.

The electrical control of photonic crystal (PhC) lasers has been an attractive but challenging issue. Laser operation by electrical injection is of key importance for the viability and applicability of the PhC lasers. Another key factor is the electrical modulation of the laser output. The Fermi level of a graphene monolayer can be controlled by electrical gating, which adjusts its optical absorption. In this study, a graphene monolayer sheet is integrated on top of a two-dimensional PhC structure composed of InGaAsP multiple-quantum-wells (MQWs) in order to demonstrate the electrical modulation of a high-power (microwatt-scale) PhC band-edge laser. The introduced dielectric spacer layer presets the delicate balance between the optical gain from the MQWs and optical loss at the graphene monolayer. The proposed device is covered by an ion-gel film, which enables a low-voltage laser modulation at |Vg|≤1 V. The modulation is extensively investigated experimentally, and the obtained results are confirmed by performing numerical simulations.

Direct observation of exceptional points in coupled photonic-crystal lasers with asymmetric optical gains.

Although counter-intuitive features have been observed in non-Hermitian optical systems based on micrometre-sized cavities, the achievement of a simplified but unambiguous approach to enable the efficient access of exceptional points (EPs) and the phase transition to desired lasing modes remains a challenge, particularly in wavelength-scale coupled cavities. Here, we demonstrate coupled photonic-crystal (PhC) nanolasers with asymmetric optical gains, and observe the phase transition of lasing modes at EPs through tuning of the area of graphene cover on one PhC cavity and systematic scanning photoluminescence measurements. As the gain contrast between the two identical PhC cavities exceeds the intercavity coupling, the phase transition occurs from the bonding/anti-bonding lasing modes to the single-amplifying lasing mode, which is confirmed by the experimental measurement of the mode images and the theoretical modelling of coupled cavities with asymmetric gains. In addition, we demonstrate active tuning of EPs by controlling the optical loss of graphene through electrical gating.