Manipulating the nonlinearity of transition-metal dichalcogenide polaritons.

The lithographically designed potential wells in monolayer WS2 microcavities are utilized to manipulate nonlinear transition-metal dichalcogenide polaritons and enhance the polariton-reservoir interaction strength.

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.

Ultralow-threshold laser using super-bound states in the continuum.

Wavelength-scale lasers provide promising applications through low power consumption requiring for optical cavities with increased quality factors. Cavity radiative losses can be suppressed strongly in the regime of optical bound states in the continuum; however, a finite size of the resonator limits the performance of bound states in the continuum as cavity modes for active nanophotonic devices. Here, we employ the concept of a supercavity mode created by merging symmetry-protected and accidental bound states in the continuum in the momentum space, and realize an efficient laser based on a finite-size cavity with a small footprint. We trace the evolution of lasing properties before and after the merging point by varying the lattice spacing, and we reveal this laser demonstrates the significantly reduced threshold, substantially increased quality factor, and shrunken far-field images. Our results provide a route for nanolasers with reduced out-of-plane losses in finite-size active nanodevices and improved lasing characteristics.

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.

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.

Coherent Interactions in One-Dimensional Topological Photonic Systems and Their Applications in All-Optical Logic Operation.

Topological photonics has become an active subfield of photonics analogous to the electronic counterpart, and the bulk-edge correspondence leads to robust topologically protected interfacial states. However, a single-topological interface mode with fixed energy cannot be easily manipulated, hindering its applications in optical devices. Here, we study coupled-waveguide arrays mapped to a one-dimensional Su-Schrieffer-Heeger system with two coupled topological interfaces. This configuration greatly increases device versatility and tunability while keeping the confinement of coupled-interface modes inherited from the topological properties nearly intact. Theoretically predicted oscillations between coupled interfaces is experimentally observed. The spatial and energetic isolation of the coupled interface states from the bulk modes is experimentally observed and theoretically confirmed by calculating the degree of localization of the eigenstates, which is found to be comparable to a single-interface state. Finally, a proof-of-principle, all-optical logic circuit is fabricated based on coupled interfaces, demonstrating its potential in assembling on-chip topological optical devices.

Generation of helical topological exciton-polaritons.

Topological photonics in strongly coupled light-matter systems offer the possibility for fabricating tunable optical devices that are robust against disorder and defects. Topological polaritons, i.e., hybrid exciton-photon quasiparticles, have been proposed to demonstrate scatter-free chiral propagation, but their experimental realization to date has been at deep cryogenic temperatures and under strong magnetic fields. We demonstrate helical topological polaritons up to 200 kelvin without external magnetic field in monolayer WS2 excitons coupled to a nontrivial photonic crystal protected by pseudo time-reversal symmetry. The helical nature of the topological polaritons, where polaritons with opposite helicities are transported to opposite directions, is verified. Topological helical polaritons provide a platform for developing robust and tunable polaritonic spintronic devices for classical and quantum information-processing applications.

Recent Progress in Nanolaser Technology.

Nanolasers are key elements in the implementation of optical integrated circuits owing to their low lasing thresholds, high energy efficiencies, and high modulation speeds. With the development of semiconductor wafer growth and nanofabrication techniques, various types of wavelength-scale and subwavelength-scale nanolasers have been proposed. For example, photonic crystal lasers and plasmonic lasers based on the feedback mechanisms of the photonic bandgap and surface plasmon polaritons, respectively, have been successfully demonstrated. More recently, nanolasers employing new mechanisms of light confinement, including parity-time symmetry lasers, photonic topological insulator lasers, and bound states in the continuum lasers, have been developed. Here, the operational mechanisms, optical characterizations, and practical applications of these nanolasers based on recent research results are outlined. Their scientific and engineering challenges are also discussed.

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.

Z2 Photonic Topological Insulators in the Visible Wavelength Range for Robust Nanoscale Photonics.

Topological photonics provides an ideal platform for demonstrating novel band topology concepts, which are also promising for robust waveguiding, communication, and computation applications. However, many challenges such as extremely large device footprint and functionality at short wavelengths remain to be solved which are required to make practical and useful devices that can also couple to electronic excitations in many important organic and inorganic semiconductors. In this letter, we report an experimental realization of Z2 photonic topological insulators with their topological edge state energies spanning across the visible wavelength range including in the sub-500 nm regime, which requires highly optimized nanofabrication. The photonic structures are based on deformed hexagonal lattices with preserved 6-fold rotational symmetry patterned on suspended SiNx membranes. The experimentally measured energy-momentum dispersion of the topological lattices directly shows topological band inversion by the swapping of the brightness of the bulk energy bands, and also the helical edge states when the measurement is taken near the topological interface. The robust topological transport of the helical edge modes in real space is demonstrated by successfully guiding circularly polarized light beams unidirectionally through sharp kinks without significant signal loss. This work paves the way for small footprint photonic topological devices working in the short wavelength range that can also be utilized to couple to excitons for unconventional light-matter interactions at the nanoscale.

Observation and Active Control of a Collective Polariton Mode and Polaritonic Band Gap in Few-Layer WS2 Strongly Coupled with Plasmonic Lattices.

Two-dimensional semiconductors host excitons with very large oscillator strengths and binding energies due to significantly reduced carrier screening. Two-dimensional semiconductors integrated with optical cavities are emerging as a promising platform for studying strong light-matter interactions as a route to explore a variety of exotic many-body effects. Here, in few-layered WS2 coupled with plasmonic nanoparticle lattices, we observe the formation of a collective polaritonic mode near the exciton energy and the formation of a complete polariton band gap with energy scale comparable to the exciton-plasmon coupling strength. A coupled oscillator model reveals that the collective mode arises from the cooperative coupling of the excitons to the plasmonic lattice diffraction orders via exciton-exciton interactions, leading to ultrastrong coupling. The emergence of the collective mode is accompanied by a superlinear increase of the polariton mode splitting as a function of the square root of the exciton oscillator strength. The presence of these many body effects, which are enhanced in systems which lack bulk polarization, not only allows the formation of a collective mode with periodically varying field profiles, but also further enhances the exciton-plasmon coupling. By integrating the hybrid WS2-plasmonic lattice device with a field-effect transistor, we demonstrate active tuning of the collective mode and the polariton band gap. We also report electrically tunable waveguiding in the polariton band gap region through a line defect, which can be turned off with gate bias that can extinguish the collective mode and the polariton band gap. These systems provide new opportunities for obtaining a deeper and systematic understanding of many body cooperative phenomena in two-dimensional materials coupled with periodic photonic systems and for designing more complex and actively controllable polaritonic devices including switchable polariton lasers, waveguides, and optical logical elements.

Unique Scattering Properties of Silicon Nanowires Embedded with Porous Segments.

The development of advanced imaging tools is important for the investigation of the fundamental properties of nanostructures composed of single or multiple nanomaterials. However, complicated preparation processes and irreversible alterations of the samples to be examined are inevitable in most current imaging techniques. In this work, we developed a simple method based on polarization-resolved light scattering measurements to characterize the structural and optical properties of complex nanomaterials. In particular, we examined a single Si nanowire embedded with porous Si segments, in which the porous Si could not be easily distinguished from solid Si by scanning electron microscopy. The dark-field optical images and polarization-resolved scattering spectra showed unique optical features of porous and solid Si. In particular, the porosity, diameter, and number of porous Si segments in the single Si nanowire were identified from the scattering measurements. In addition, we performed systematic optical simulations based on the effective medium model in individual porous and solid Si nanowires. A good agreement between the simulation and measurement results enabled the estimation of the structural parameters of the nanowires, such as diameter and porosity. We believe that our method will be useful for analyzing the structural and optical properties of nanomaterials prior to using complicated and uneconomical imaging tools.

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.

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.

Responsive Block Copolymer Photonic Microspheres.

Responsive photonic crystals (PCs) have attracted much attention due to their broad applications in the field of chemical and physical sensing through varying optical properties when exposed to external stimuli. In particular, assembly of block copolymers (BCPs) has proven to be a robust platform for constructing PCs in the form of films or bulk. Here, the generation of BCPs photonic microspheres is presented with 3D periodical concentric lamellar structures through confined self-assembly. The structural color of the spherical PCs can be tuned by selective swelling of one block, yielding large change of optical property through varying both layer thickness and refraction index of the domains. The as-formed spherical PCs demonstrate large reflection wavelength shift (≈400-700 nm) under organic solvent permeation and pH adjustment. Spherical shape and structural symmetry endow the formed spherical PCs with rotation independence and monochrome, which is potentially useful in the fields of displays, sensing, and diagnostics.

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.

Enhancement of Light Absorption in Silicon Nanowire Photovoltaic Devices with Dielectric and Metallic Grating Structures.

We report the enhancement of light absorption in Si nanowire photovoltaic devices with one-dimensional dielectric or metallic gratings that are fabricated by a damage-free, precisely aligning, polymer-assisted transfer method. Incorporation of a Si3N4 grating with a Si nanowire effectively enhances the photocurrents for transverse-electric polarized light. The wavelength at which a maximum photocurrent is generated is readily tuned by adjusting the grating pitch. Moreover, the electrical properties of the nanowire devices are preserved before and after transferring the Si3N4 gratings onto Si nanowires, ensuring that the quality of pristine nanowires is not degraded during the transfer. Furthermore, we demonstrate Si nanowire photovoltaic devices with Ag gratings using the same transfer method. Measurements on the fabricated devices reveal approximately 27.1% enhancement in light absorption compared to that of the same devices without the Ag gratings without any degradation of electrical properties. We believe that our polymer-assisted transfer method is not limited to the fabrication of grating-incorporated nanowire photovoltaic devices but can also be generically applied for the implementation of complex nanoscale structures toward the development of multifunctional optoelectronic devices.

Photon-triggered nanowire transistors.

Photon-triggered electronic circuits have been a long-standing goal of photonics. Recent demonstrations include either all-optical transistors in which photons control other photons or phototransistors with the gate response tuned or enhanced by photons. However, only a few studies report on devices in which electronic currents are optically switched and amplified without an electrical gate. Here we show photon-triggered nanowire (NW) transistors, photon-triggered NW logic gates and a single NW photodetection system. NWs are synthesized with long crystalline silicon (CSi) segments connected by short porous silicon (PSi) segments. In a fabricated device, the electrical contacts on both ends of the NW are connected to a single PSi segment in the middle. Exposing the PSi segment to light triggers a current in the NW with a high on/off ratio of >8 × 106. A device that contains two PSi segments along the NW can be triggered using two independent optical input signals. Using localized pump lasers, we demonstrate photon-triggered logic gates including AND, OR and NAND gates. A photon-triggered NW transistor of diameter 25 nm with a single 100 nm PSi segment requires less than 300 pW of power. Furthermore, we take advantage of the high photosensitivity and fabricate a submicrometre-resolution photodetection system. Photon-triggered transistors offer a new venue towards multifunctional device applications such as programmable logic elements 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.

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.