Optical Stimuli Research Articles

A Flexible Organic Electrochemical Synaptic Transistor With Dopamine-Mediated Plasticity

Artificial synapses relying on electrical and optical stimuli to implement neuromorphic tasks have been intensively developed, but very few of them can directly respond to real biological neurotransmitter stimulus, thus limiting the seamless integration of bio-electronic interfaces. In this letter, we report a flexible organic electrochemical transistor-based synaptic device, which demonstrates not only short-term synaptic behaviors dominated by the kinetics of ions but also long-term synaptic behaviors derived from the redox reaction of dopamine neurotransmitters. More importantly, we systematically studied the dopamine-mediated synaptic plasticity under both flat and bending states, which shows reliable synaptic response and excellent mechanical flexibility. These results reveal that our device providing a promising way for direct biohybrid integration with soft biological tissues.

Self-powered optoelectronic artificial synapses based on a lead-free perovskite film for artificial visual perception systems

We demonstrate a two-terminal and self-powered optoelectronic synaptic device based on a 2D lead-free perovskite, Cs3Bi2Br9. The fabricated device successfully mimics typical biological synaptic functions under programmed optical stimuli.

Flexible multiterminal photoelectronic neurotransistors based on self‐assembled rubber semiconductors for spatiotemporal information processing

AbstractA significant step toward constructing high‐efficiency neuromorphic systems is the electronic emulation of advanced synaptic functions of the human brain. While previous studies have focused on mimicking the basic functions of synapses using single‐gate transistors, multigate transistors offer an opportunity to simulate more complex and advanced memory‐forming behaviors in biological synapses. In this study, a simple and general method is used to assemble rubber semiconductors into suspended two‐phase composite films that are transferred to the surface of the ion‐conducting membrane to fabricate flexible multiterminal photoelectronic neurotransistors. The suspended ion conductive film is used as the gate dielectrics and supporting substrate. The prepared devices exhibit excellent electrical stability and mechanical flexibility after being bent. Basic photoelectronic synaptic behavior and pulse‐dependent plasticity are emulated. Furthermore, the device realizes the spatiotemporally integrated electrical and optical stimuli to mimic spatiotemporal information processing. This study provides a promising direction for constructing more complex spiking neural networks and more powerful neuromorphic systems with brain‐like dynamic spatiotemporal processing functions.

Multifunctional 2D MoS2 Optoelectronic Artificial Synapse with Integrated Arithmetic and Reconfigurable Logic Operations for In‐Memory Neuromorphic Computing Applications

AbstractThe hardware implementation of advanced artificial intelligence (AI) technology based on complex deep learning and machine learning algorithms is constricted by the limitation of conventional Von–Neuman architecture. Emerging neuromorphic computing architecture based on the human brain with in‐memory computing capability could instigate unprecedented breakthroughs in AI technology. In this pursuit, 2D MoS2 optoelectronic artificial synapse imitating complex biological neuromorphic behavior such as short/long‐term memory, paired‐pulse facilitation, and long‐term depression‐potentiation is proposed and demonstrated. Furthermore, the broadband sensitivity of the device can be utilized to emulate Pavlov's classical conditioning for associative learning of the biological brain. More importantly, reconfigurable Boolean AND and OR logic gate operation is demonstrated within the same device by synergistically modulating the device conductance via the persistent photoconductivity and electrical gate stress. The linear response of the photocurrent to the optical stimulus can perform arithmetic operations such as counting, addition, and subtraction within a single device. This novel integration of memory, synaptic behavior, and processing within a single monolayer MoS2 device is believed to put forth a new horizon for the Non‐Von–Neuman type in‐memory computing architecture for highly advanced AI applications based on 2D materials.

Synaptic plasticity in self-powered artificial striate cortex for binocular orientation selectivity

Get in-depth understanding of each part of visual pathway yields insights to conquer the challenges that classic computer vision is facing. Here, we first report the bioinspired striate cortex with binocular and orientation selective receptive field based on the crossbar array of self-powered memristors which is solution-processed monolithic all-perovskite system with each cross-point containing one CsFAPbI3 solar cell directly stacking on the CsPbBr2I memristor. The plasticity of self-powered memristor can be modulated by optical stimuli following triplet-STDP rules. Furthermore, plasticity of 3 × 3 flexible crossbar array of self-powered memristors has been successfully modulated based on generalized BCM learning rule for optical-encoded pattern recognition. Finally, we implemented artificial striate cortex with binocularity and orientation selectivity based on two simulated 9 × 9 self-powered memristors networks. The emulation of striate cortex with binocular and orientation selectivity will facilitate the brisk edge and corner detection for machine vision in the future applications.

Photoconductor-metadevices: an enabling platform for dual-optical/electrical modulation of terahertz waves

We report a photoconductor-based terahertz metadevice whose transmission characteristics can be comprehensively controlled by a combination of an optical excitation and electrical bias. A metasurface with interdigitated electrodes is proposed to simultaneously support a terahertz resonance and the photoconductive effect, allowing efficient local and global dual-tuning of the carriers in the semiconductor and thus the terahertz wave, based on the simultaneous external optical and electrical stimuli. Experimental results reveal the opposite tuning trend of optical and electrical excitations, leading to the comprehensive transmission modulation of terahertz wave. Our study demonstrates a proof-of-concept device for sophisticated manipulation of terahertz radiation.

Nanoscale metal-organic frameworks as photosensitizers and nanocarriers in photodynamic therapy.

Photodynamic therapy (PDT) is a new therapeutic system for cancer treatment that is less invasive and offers greater selectivity than chemotherapy, surgery, and radiation therapy. PDT employs irradiation light of known wavelength to excite a photosensitizer (PS) agent that undergoes photochemical reactions to release cytotoxic reactive oxygen species (ROS) that could trigger apoptosis or necrosis-induced cell death in tumor tissue. Nanoscale metal–organic frameworks (NMOFs) have unique structural advantages such as high porosity, large surface area, and tunable compositions that have attracted attention toward their use as photosensitizers or nanocarriers in PDT. They can be tailored for specific drug loading, targeting and release, hypoxia resistance, and with photoactive properties for efficient response to optical stimuli that enhance the efficacy of PDT. In this review, an overview of the basic chemistry of NMOFs, their design and use as photosensitizers in PDT, and as nanocarriers in synergistic therapies is presented. The review also discusses the morphology and size of NMOFs and their ability to improve photosensitizing properties and localize within a targeted tissue for effective and selective cancer cell death over healthy cells. Furthermore, targeting strategies that improve the overall PDT efficacy through stimulus-activated release and sub-cellular internalization are outlined with relevance to in vitro and in vivo studies from recent years.

Light-Triggered Manipulations of Droplets All in One: Reversible Wetting, Transport, Splitting, and Merging.

Here, we establish different ways of light-triggered droplet manipulation such as reversible wetting, splitting, merging, and transport. The unique features of our approach are that the changes in the wetting properties of microscopic droplets of isotropic (oil) or anisotropic (liquid crystalline) liquids adsorbed on photoswitchable films can be triggered just by application of soft optical stimuli, which lead to dynamical, reversible changes in the local morphology of the structured surfaces. The adaptive films consist of an azobenzene-containing surfactant ionically attached to oppositely charged polymer chains. Under exposure to irradiation with light, the azobenzene photoisomerizes between two states, nonpolar trans-isomer and polar cis-isomer, resulting in the corresponding changes in the surface energy and orientation of the surfactant tails at the interface. Additionally, the local increase in the surface temperature due to absorption of light by the azobenzene groups enables diverse processes of manipulation of the adsorbed small droplets, such as the reversible increase of the droplet basal area up to 5 times, anisotropic wetting during irradiation with modulated light, and precise partition of the droplet into many small pieces, which can then be merged on demand to the desired number of larger droplets. Moreover, using a moving focused light spot, we experimentally demonstrate and theoretically explain the locomotion of the droplet over macroscopic distances with a velocity of up to 150 μm·s-1. Our findings could lead to the ultimate application of a programmable workbench for manipulating and operating an ensemble of droplets, just using simple and gentle optical stimuli.

Controllable and Gradient Wettability of Bilayer Two-Dimensional Materials Regulated by Interlayer Distance.

Surfaces with controllable and gradient wettability often require an elaborate design of the microstructure or its response under electrical, thermal, optical, pH, and other stimuli. Generally, the wettability change under these physical or chemical effects relies on a complex mechanism that is difficult to be quantitatively described. In this study, an online controlling strategy for surface wettability and the corresponding theoretical model are put forward based on a bilayer graphene-like atomic structure. Molecular dynamics results indicate that the surface wettability varies toward hydrophilicity after sticking a bottom material regardless of its wettability. But such an influence becomes weak with increasing interlayer distance, and the overall wettability approaches that of the upper layer material gradually. This variation is elucidated by the increase of the work of adhesion, providing new insight into the wetting transparency of graphene. A theoretical model of the governing relationship is established based on the work of adhesion, which correlates the overall surface wettability with the interlayer distance and the wettabilities of individual materials. Moreover, a surface with a uniform wettability gradient is achieved by inclining the bottom material. The spontaneous and steady motion of droplets can be induced by this gradient wettability. The relevant speedup behavior is evaluated through a theoretical model considering the varying interlayer distance, which reveals the critical role of the lower layer. This study proposes a novel strategy for controllable wetting and relevant gradient surfaces using prevailing two-dimensional materials, paving new routes to many applications such as microfluidic chips, virus diagnosis, and intelligent sensors.

Optically Responsive Protein Coating of DNA Origami for Triggered Antigen Targeting.

DNA nanostructures have emerged as modular building blocks in several research fields including biomedicine and nanofabrication. Their proneness to degradation in various environments has led to the development of a variety of nature-inspired protection strategies. Coating of DNA origami nanostructures with proteins can circumvent degradation and alter their properties. Here, we have used a single-chain variable antibody fragment and serum albumin to construct positively charged and stimuli-responsive protein-dendron conjugates, which were complexed with DNA origami through electrostatic interactions. Using a stepwise assembly approach, the coated nanostructures were studied for their interaction with the corresponding antigen in fluorescence-based immunoassays. The results suggest that the antibody–antigen interaction can be disturbed by the addition of the bulky serum albumin. However, this effect is fully reversible upon irradiation of the structures with an optical stimulus. This leads to a selective dissociation of the serum albumin from the nanostructure due to cleavage of a photolabile group integrated in the dendron structure, exposing the antibody fragment and enabling triggered binding to the antigen, demonstrating that serum albumin can be considered as an externally controlled “camouflaging” agent. The presented stimuli-responsive complexation approach is highly versatile regarding the choice of protein components and could, therefore, find use in DNA origami protection, targeting, and delivery as well as their spatiotemporal control.

Solid‐State Optoelectronic Synapse Transistor Using a LaF3 Gate Dielectric

As a new generation of core components for neuromorphic computing, optoelectronic synaptic devices have the advantages of high bandwidth and less energy consumption. They can combine the functions of visual sensing, signal processing, and memory in one device. Herein, an all‐solid‐state optoelectronic synaptic transistor using indium gallium zinc oxide (IGZO) as the channel and lanthanum fluoride (LaF3) as the gate dielectric is designed and fabricated. Its synaptic plasticity induced by electric and optical stimuli is investigated. LaF3 is a solid superionic conductor with plenty of mobile fluoride ions, which can be used to achieve a high on/off ratio of more than 105 and a low subthreshold swing less than 150 mV dec−1 in this novel synaptic transistor. Due to the ion‐trapping effect at the interface, synaptic plasticity can be achieved upon electrical stimulation. Synaptic plasticity by light pulse stimuli can also be accomplished with the assistance of the persistent photoconductivity effect in IGZO channel. Furthermore, it is found that the synaptic plasticity induced by light stimuli can be modulated by gate voltage. These results provide a new way to control synaptic function and extend the function of optoelectronic synaptic devices.

Active and tunable nanophotonic metamaterials

AbstractMetamaterials enable subwavelength tailoring of light–matter interactions, driving fundamental discoveries which fuel novel applications in areas ranging from compressed sensing to quantum engineering. Importantly, the metallic and dielectric resonators from which static metamaterials are comprised present an open architecture amenable to materials integration. Thus, incorporating responsive materials such as semiconductors, liquid crystals, phase-change materials, or quantum materials (e.g., superconductors, 2D materials, etc.) imbue metamaterials with dynamic properties, facilitating the development of active and tunable devices harboring enhanced or even entirely novel electromagnetic functionality. Ultimately, active control derives from the ability to craft the local electromagnetic fields; accomplished using a host of external stimuli to modify the electronic or optical properties of the responsive materials embedded into the active regions of the subwavelength resonators. We provide a broad overview of this frontier area of metamaterials research, introducing fundamental concepts and presenting control strategies that include electronic, optical, mechanical, thermal, and magnetic stimuli. The examples presented range from microwave to visible wavelengths, utilizing a wide range of materials to realize spatial light modulators, effective nonlinear media, on-demand optics, and polarimetric imaging as but a few examples. Often, active and tunable nanophotonic metamaterials yield an emergent electromagnetic response that is more than the sum of the parts, providing reconfigurable or real-time control of the amplitude, phase, wavevector, polarization, and frequency of light. The examples to date are impressive, setting the stage for future advances that are likely to impact holography, beyond 5G communications, imaging, and quantum sensing and transduction.

Hybrid mixed-dimensional perovskite/metal-oxide heterojunction for all-in-one opto-electric artificial synapse and retinal-neuromorphic system

Human central nervous system and the peripheral nervous system have played significant roles in mediating the interactions with the outside world. Inspired by the human nervous systems, artificial sensory and neuromorphic innovations have been developed to mimic nervous functions. A hybrid mixed-dimensional perovskite/metal-oxide heterojunction has been demonstrated in this work for three-terminal all-in-one opto-electric artificial synapse to integrate opto-electric synaptic emulations and optical perception functions. Based on the well-designed layer configuration, an all-in-one device, consisting of the ion-electrolyte layer, ion-permeable metal-oxide semiconductor channel layer, the mixed-dimensional perovskite optical perception layer, and the amorphous ZnO passivation layer, has been demonstrated with superior electrical performance. Utilizing an ion-electrolyte and ion-permeable metal-oxide semiconductor structure, the synaptic emulation modulated by electrical gate-stimulus could be effectively achieved. The optical perception and synaptic plasticity modulated by the optical stimulus have been integrated into the all-in-one device. Furthermore, the electrolyte gated device enables artificial visual adaptation with adaptive behavior of environmental lightness under dim and bright conditions. Addition, artificial visual persistence has been emulated by the device utilizing the optical synaptic behavior. Based on those properties, a cascaded near-sensor face recognition access control retinal-neuromorphic computing system has been developed based on the all-in-one device. The retinal-neuromorphic system based on all-in-one devices could recognize the face of a requester with an accuracy rate over 90 %, and ignore the passers with the lingering phenomenal trace as the result of the artificial visible persistence.

Optoelectronic synaptic plasticity mimicked in ZnO-based artificial synapse for neuromorphic image sensing application

Memristor-based optoelectronic artificial synapse enables future neuromorphic computing to be more efficient. Hence the development of better optoelectronic artificial synapses becomes considerably important for the era of next-generation neuromorphic computing and neuromorphic visual systems. Optoelectronic artificial synapses can empower neuromorphic visual systems even beyond the visible light region, which has substantial potential to mimic the essential functions of the human visual system. This paper demonstrates a better optoelectronic artificial synapse that shows many fundamental bio-synaptic characteristics. Fabricated devices exhibit excellent paired-pulse facilitation with both electrical and optical input stimuli. The transition from short-term memory to long-term memory was observed for the frequent and high number of input stimuli similar to bio-synapse. Essential Hebbian learning protocols, such as spike-timing-dependent plasticity and spike-rate-dependent plasticity, were successfully emulated in this artificial synapse. It also possesses light-tunable synaptic plasticity, which enables real-time neuromorphic visual pre-processing for realizing neuromorphic visual systems.

Noise-induced collective dynamics in the small-world network of photosensitive neurons.

Photosensitive neurons can capture and convert external optical signals, and then realize the encoding signal.It is confirmed thata variety of firing modes could be induced under optical stimuli. As a result, it is interesting to explore the mode transitions of collective dynamics in the photosensitive neuron network under external stimuli. In this work, the collective dynamics of photosensitive neurons in a small-world network with non-synaptic coupling will be discussed with spatial diversity of noise and uniform noise applied on, respectively. The results prove that a variety of different collective electrical activities could be induced under different conditions. Under spatial diversity of noise applied on, a chimera state could be observed in the evolution, and steady cluster synchronization could be detected in the end; even the nodes in each cluster depend on the degree of each node. Under uniform noise applied on, the complete synchronization window could be observed alternately in the transient process, and steady complete synchronization could be detected finally. The potential mechanism is that continuous energy is pumped in the phototubes, and energy exchange and balance between neurons to form the resonance synchronization in the network with different noise applied on. Furthermore, it is confirmed that the evolution of collective dynamical behaviors in the network depends on the external stimuli on each node. Moreover, the bifurcation analysis for the single neuron model is calculated, and the results confirm that the electrical activities of single neuron are sensitive to different kinds of noise.

The Integration of Optical Stimulation in a Mechanically Dynamic Cell Culture Substrate.

A cell culture well with integrated mechanical and optical stimulation is presented. This is achieved by combining dielectric elastomer soft actuators, also known as artificial muscles, and a varifocal micro-electromechanical mirror that couples light from an optical fiber and focuses it onto the transparent cell substrate. The device enables unprecedented control of in vitro cell cultures by allowing the experimenter to tune and synchronize mechanical and optical stimuli, thereby enabling new experimental assays in optogenetics, fluorescent microscopy, or laser stimulation that include dynamic mechanical strain as a controlled input parameter.

Self-organized lasers from reconfigurable colloidal assemblies

Non-equilibrium assemblies, where units are able to harness available energy to perform tasks, can often self-organize into dynamic materials that uniquely blend structure with functionality and responsiveness to their environment. The integration of similar features in photonic materials remains challenging but is desirable to manufacture active, adaptive and autonomous photonic devices. Here we show the self-organization of programmable random lasers from the reversible out-of-equilibrium self-assembly of colloids. The random lasing originates from the optical amplification of light undergoing multiple scattering within the dissipative colloidal assemblies and therefore depends crucially on their self-organization behaviour. Under external light stimuli, these dynamic random lasers are responsive and present a continuously tuneable laser threshold. They can therefore reconfigure and cooperate by emulating the ever-evolving spatiotemporal relationship between structure and functionality that is typical of many non-equilibrium assemblies. Experiments inspired by the behaviour of active matter show that an external optical stimulus can spatially reconfigure colloidal random lasers and continuously tune their lasing threshold.

Photo‐Synaptic Oxide Transistors with Al2O3/SiOx Stacked Gate Dielectric Exhibiting 1024 Conduction States with Good Linearity

AbstractTiO2‐based photo‐synaptic transistors employing a stacked SiOX/Al2O3 gate dielectric are fabricated. The trapped charges at the interface between the gate dielectric and the channel induce a channel current threshold shift that displays synaptic behavior. When a positive gate voltage pulse is applied, the transfer curve shifts positively. In contrast, the curve shifts negatively under 365 nm wavelength UV pulses. The magnitude of the threshold shift is dependent on the optical pulse time. A 5 ms optical pulse shifts the transfer curve to the negative direction but the semiconducting properties of the channel are maintained. Increasing the optical pulse time to 0.5 s causes the channel to exhibit metallic behavior. When optical and electrical pulses are applied alternately, reversible shifts between the metallic and semiconducting states can be induced. The pulse‐paired facilitation time constants of the device for an optical pulse stimuli of 2 ms are 0.9 s and 26.6 s, verifying excellent photo‐synaptic properties. Short‐term plasticity measurements revealed 1024 distinctly separate conduction states with good linearity. Synaptic potentiation and depression are simulated by applying a continuous supply of optical pulses followed by an idle dark condition and good symmetric behavior is observed.

Electrically tunable metasurfaces: from direct to indirect mechanisms

Intensive development of nanofabrication processes has opened a new window to control electromagnetic waves using subwavelength nanostructures array, named metasurfaces. Although the metasurfaces have succeeded in achieving unprecedented functionality by arranging various shapes of nanostructures to modulate the properties of the incident light, inherent passive characteristics make it impossible to alter the engraved functions after it is fabricated. To give tunability to metasurfaces, various methods have been proposed by using a thermal, chemical, optical and physical stimulus. In particular, electrically tunable metasurfaces are attractive in that they are easy to control precisely and could be integrated into electronic devices. In this review, we categorize the representative electrical tuning mechanisms and research into three; voltage-operated modulation, electrochemical-driven modulation, and externally mediated modulation. Voltage-operated modulation uses materials that could be directly reorganized by an electric field, including liquid crystals and Drude materials. Electrochemical-driven modulation adjusts the optical properties of metasurfaces through electrochemical responses such as electrochromism and electrodeposition. Lastly, externally mediated modulation causes a change in the geometric parameters of metasurfaces or in the phase of the constituent materials by converting electrical energy into thermal or mechanical stimulation. This paper concludes after explaining the pros and cons of each mechanism and the new possibilities which electrically-responsive metasurfaces could bring about.

Nonvolatile reconfigurable dynamic Janus metasurfaces in the terahertz regime

Metasurfaces, especially tunable ones, have played a major role in controlling the amplitude, phase, and polarization of electromagnetic waves and attracted growing interest, with a view toward a new generation of miniaturized devices. However, to date, most existing reconfigurable devices are bounded in volatile nature with sustained external energy to maintain and single functionality, which restrict their further applications. Here, we demonstrate for the first time, to our knowledge, nonvolatile, reconfigurable, and dynamic Janus metasurfaces by incorporating phase-change material Ge 2 Se 2 Te 5 (GST) in the terahertz (THz) regime. First, we experimentally show the reversible switching characteristic of GST on large areas by applying a single nanosecond laser pulse, which exhibits excellent contrast of THz properties in both states. Then, we present a multiplex metasurface scheme. In each metasurface, three sets of structures are adopted, in which two sets integrate GST. The effective structures can be reversely modulated by the amorphization and crystallization of GST. As a proof of concept, the dynamic beam splitter, bifocal metalens, dual-mode focusing optical vortex generators, and switchable metalens/focusing optical vortex generators are designed, fabricated, and experimentally characterized, and can be switched reversibly and repeatedly with the help of optical and thermal stimuli. Our scheme will pave the way toward the development of multifunctional and compact THz devices and may find use for applications in THz imaging, sensing, and communications.