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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., Though laser action has been reported for optical bound states in the continuum (BIC) cavities with high quality factors, these BIC lasers lacked practical applicability. Here, the authors report an ultralow-threshold super-BIC laser featuring merged symmetry-protected and accidental BICs.

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.

Photolithography is the prevalent microfabrication technology. It needs to meet resolution and yield demands at a cost that makes it economically viable. However, conventional far-field photolithography has reached the diffraction limit, which imposes complex optics and short-wavelength beam source to achieve high resolution at the expense of cost efficiency. Here, we present a cost-effective near-field optical printing approach that uses metal patterns embedded in a flexible elastomer photomask with mechanical robustness. This technique generates sub-diffraction patterns that are smaller than 1/10th of the wavelength of the incoming light. It can be integrated into existing hardware and standard mercury lamp, and used for a variety of surfaces, such as curved, rough and defect surfaces. This method offers a higher resolution than common light-based printing systems, while enabling parallel-writing. We anticipate that it will be widely used in academic and industrial productions., Photolithography is an established microfabrication technique but commonly uses costly shortwavelength light sources to achieve high resolution. Here the authors use metal patterns embedded in a flexible elastomer photomask with mechanical robustness for generation of subdiffraction patterns as a cost effective near-field optical printing approach.

Two-dimensional transition-metal dichalcogenide (TMD) crystals are a versatile platform for optoelectronic, catalytic and quantum device studies. However, the ability to tailor their physical properties through explicit synthetic control of their morphology and dimensionality is a major challenge. Here we demonstrate a gas-phase synthesis method that substantially transforms the structure and dimensionality of TMD crystals without lithography. Synthesis of MoS2 on Si(001) surfaces pre-treated with phosphine yields high-aspect-ratio nanoribbons of uniform width. We systematically control the width of these nanoribbons between 50 and 430 nm by varying the total phosphine dosage during the surface treatment step. Aberration-corrected electron microscopy reveals that the nanoribbons are predominantly 2H phase with zig-zag edges and an edge quality that is comparable to, or better than, that of graphene and TMD nanoribbons prepared through conventional top-down processing. Owing to their restricted dimensionality, the nominally one-dimensional MoS2 nanocrystals exhibit photoluminescence 50 meV higher in energy than that from two-dimensional MoS2 crystals. Moreover, this emission is precisely tunable through synthetic control of crystal width. Directed crystal growth on designer substrates has the potential to enable the preparation of low-dimensional materials with prescribed morphologies and tunable or emergent optoelectronic properties. Synthesis of MoS2 on a silicon surface pre-treated with phosphine enables the growth of one-dimensional MoS2 nanocrystals with tunable dimensions and optical properties.

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.

We investigated a photon-triggered nanowire transistor with near-infrared spectral photoresponse. To understand the mechanism of operation of the nanowire transistor consisting of crystalline Si and porous Si segments, we performed a qualitative analysis of trapped carriers in the porous Si and modeling of a transistor operated by photon gating. We then fabricated a photon-triggered transistor device with a nanowire diameter of 200 nm and a porous Si length of 400 nm. Systematic measurements and analyses demonstrate that the wavelength dependence of the photon-triggered current generation is caused by a broad range of energy levels for the localized trap states of the porous Si segment. We believe that these results pave the way for simplification of the fabrication and operation of ultracompact nanoprocessors and development of efficient nanoscale photodetectors for high-resolution imaging.

We demonstrate lasing with ultralow threshold from small-scale dielectric metasurfaces with only ~10×10 periods operating at a super-BIC regime. Engineered high-Q resonance originates from topological merging of accidental and symmetry-protected bound states in the continuum.

Efficient integration of a single-photon emitter with an optical waveguide is essential for quantum integrated circuits. In this study, we integrated a single-photon emitter in a hexagonal boron nitride (h-BN) flake with a Ag plasmonic waveguide and measured its optical properties at room temperature. First, we performed numerical simulations to calculate the efficiency of light coupling from the emitter to the Ag plasmonic waveguide, depending on the position and polarization of the emitter. In the experiment, we placed a Ag nanowire, which acted as the plasmonic waveguide, near the defect of the h-BN, which acted as the single-photon emitter. The position and direction of the nanowire were precisely controlled using a stamping method. Our time-resolved photoluminescence measurement showed that the single-photon emission from the h-BN flake was enhanced to almost twice the intensity as a result of the coupling with the Ag nanowire. We expect these results to pave the way for the practical implementation of on-chip nanoscale quantum plasmonic integrated circuits.

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., Topological photonics: integrated nanolasers Active topological cavities that can lase at room temperature could bring new opportunities for controlling light in integrated nanophotonic circuits. Smirnova et al. etched a special pattern of nanoscale holes into a 250 nm thick slab of the compound semiconductor InGaAsP to create a triangle-shaped cavity with optical behaviour governed by the band topology. The presence of quantum wells in the slab provides the cavity with optical gain allowing it to lase in the near-infrared when excited with nanosecond pump pulses at 980 nm. The laser emission is observed to be spectrally narrow with high coherence and hosts a donut-shaped singularity in Fourier space. The achievement opens the way to a new type of nanophotonic light source which exhibits unusual radiation characteristics and suits integration with metasurfaces.

The high stability and strong coupling nature of gold nanoparticles (Au-NPs) than other metal counter parts have attracted the solar cell industry to pursue enhanced performances. Herein, we report on the improved performance of polymer bulk hetero-junction (BHJ) solar cells by the incorporation of large-size Au-NPs in the hole transport layer of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). To examine the reproducibility of the enhancement parameters, two different donor photoactive materials have been adapted and the role of larger-size (>70 nm, i.e. 71, 80, 87, 103 nm) Au-NPs in BHJ solar cells have been studied extensively. Significantly, when employing Au-NPs smaller than 80 nm, near-field coupling (localized surface plasmon resonance; LSPR) was prevalent, while the infusion of Au-NPs with sizes greater than 87 nm resulted in far-field scattering enhancement as the dominant effect, which was clearly determined using time resolved photo luminescence studies. The superior power conversion efficiency of 5.35% and 8.58% was achieved with PBDTTT-C: PC61BM and PTB7: PC71BM BHJs respectively, by employing 87 nm Au-NPs due to the balanced contribution of near- and far-field plasmonic effects, improved vertical coverage and better interfacial properties. This study illustrates that 87 nm Au-NPs is the maximum size to attain the improved efficiency, above which the rate of enhancement reduces dramatically.

We demonstrate selective pump focusing for highly isolated single-mode lasers in microdisk and microring cavities, and achieve lasing action from a microdisk cavity underneath a scattering medium. The spatial profile of the pumping light evolves by an iterative feedback process and is optimized to maximize the field overlap with a selected cavity mode. The high order of mode selectivity and high resolving power are obtained in a multimode cavity in the presence of significant modal overlaps. As a result of the adaptive optical pumping, we successfully achieve the efficient energy transfer to a microdisk underneath a random scattering medium and observe lasing action through the scattering medium. We believe that our selective pumping procedure will pave the way for the development of low-threshold, single-mode nanolasers embedded in various materials.

Liquid crystal (LC) alignment on inorganic films has been found to be affected by surface modification via ion-beam irradiation. In this study, ZnO films treated by rubbing with a velvet cloth were shown to be capable of aligning LC molecules in the direction of the rubbing. Uniform and homogeneous LC alignment was achieved on the rubbed ZnO films. By analysing the optical axes before and after the rubbing treatment, we confirmed an increase in the anisotropy of the ZnO films; this optical anisotropy contributed to the uniform orientation of LC molecules. Further, the electro-optical characteristics of the twisted nematic cells based on ZnO films were superior to those based on conventional polyimide layers. Our results indicate that the rubbing approach could be applied in the fabrication of high-performance LC displays.

We investigated homogeneous liquid crystal alignment on hafnium strontium oxide films deposited by a solution process, where the liquid crystal alignment was induced by ion-beam irradiation. Uniform liquid crystal alignment was achieved on the HfSrO films grown at 300 °C. We confirmed the effect of surface morphology and chemical composition of HfSrO films using field-emission scanning electron microscopy and X-ray photoelectron spectroscopy. In addition, we observed the electro-optical characteristics of the twisted-nematic liquid crystal cells based on hafnium strontium oxide films to verify the possibility of their application in liquid crystal displays.

Gate-tunable Schottky junction solar cells (SJSCs) based on graphene and graphene mesh electrodes on n-type Si are fabricated and the effect of the external gate voltage (Vg) on the photovoltaic characteristics is investigated. The power conversion efficiencies (PCEs) of both devices continuously increase with increasing absolute values of Vg. Importantly, despite the slightly lower PCE values at Vg = 0 V, the graphene mesh on Si SJSC shows more rapid enhancement of PCE values, from 5.7% to 8.1%, with Vg varied from 0 V to −1 V. The finite element simulation highlights the benefits of the graphene mesh electrodes from the non-uniform and dynamic modulation of potential distributions driven correlatively by a work function change in the graphene regions and electric-field penetration through the hole regions.

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., Non-Hermitian optical systems have been shown to sustain lasing when they go from a PT-symmetric to a PT-symmetry-broken phase. Here, Kim et al. study this phase transition of lasing modes in partially graphene-covered coupled microcavities and show tuning of an exceptional point.

We fabricated a single U-shaped nanowire (NW) and tangent-coupled straight and U-shaped NWs using a novel technique of shape-controlled deterministic assembly of NWs with nanoscale controllability. To examine the optical properties of these structures, we calculated the transmitted and crossover optical powers of the fundamental guided mode in both the single and coupled CdS NWs by varying the key structural parameters such as the radius of curvature of the U-shaped head and the gap between the NWs. In addition, we experimentally studied active waveguiding in the assembled CdS NWs, which exhibited light coupling to the waveguide mode and direct transmission in the single U-shaped NW. Furthermore, the crossover coupling was measured for the tangent-coupled straight and U-shaped CdS NWs with a gap width of 70 nm. The shape-controlled deterministic assembly technique can open up a new opportunity to demonstrate subwavelength-scale photonic waveguides and couplers in compact nanophotonic integrated circuits.

Doping with cerium (IV) oxide (CeO2) nanoparticles can significantly enhance the thermal stability and electro-optical (EO) properties of nematic liquid crystal (NLC) systems. Thermal stability was improved without aggregation and reduction of transmittance by adding CeO2 nanoparticles in a liquid crystal medium. In particular, the EO properties greatly improved in CeO2-dispersed NLC cells. The threshold voltage reduced from 3.027 to 2.279 V and the response time decreased from 13.097 to 9.970 ms with increased CeO2 nanoparticles in the NLC cells. The improved properties of liquid crystals doped with CeO2 nanoparticles depend on the anchoring energy and the electric field of the CeO2-dispersed liquid crystal displays.

We present the characteristics of annealing temperature-dependent, zinc-tin-oxide (ZTO) films deposited by a solution process for application in liquid crystal displays (LCDs). ZTO surfaces supported homogeneously-aligned liquid crystal (LC) molecules based on an ion beam irradiation system. Uniform LC alignment and a precise pretilt angle were obtained at an annealing temperature greater than 300 °C. The oxidation of ZTO films was confirmed using field-emission-scanning electron microscopy and X-ray photoelectron spectroscopy. The electro-optical characteristics of electrically controlled birefringence (ECB) cells based on the ZTO films were superior to those based on polyimide. Especially, IB-irradiated ZTO films exhibited superior performance with respect to response time. This result indicates that this approach will allow for the fabrication of advanced LCDs with high performance.

We demonstrate two-dimensional mapping of current flow in graphene devices by using a single-spin scanning magnetometer based on a nitrogen-vacancy defect center in diamond. We first image the stray magnetic field generated by the current and then reconstruct the current density map from the field data. We focus on the visualization of current flow around a small sized current source of ∼500 nm diameter, which works as an effective point contact. In this paper, we study two types of point-contacted graphene devices and find that the overall current profiles agree with the expected behavior of electron flow in the diffusive transport regime. This work could offer a route to explore interesting carrier dynamics of graphene including ballistic and hydrodynamic transport regimes.

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.

Atomically-thin two-dimensional (2D) materials including graphene and transition metal dichalcogenide (TMD) atomic layers (e.g. Molybdenum disulfide, MoS2) are attractive materials for optoelectronic and plasmonic applications and devices due to their exceptional flexural strength led by atomic thickness, broadband optical absorption, and high carrier mobility. Here, we show that crumple nanostructuring of 2D materials allows the enhancement of the outstanding material properties and furthermore enables new, multi-functionalities in mechanical, optoelectronic and plasmonic properties of atomically-thin 2D materials. Crumple nanostructuring of atomically thin materials, graphene and MoS2 atomic layers are used to achieve flexible/stretchable, strain-tunable photodetector devices and plasmonic metamaterials with mechanical reconfigurability. Crumpling of graphene enhances optical absorption by more than an order of magnitude (~12.5 times), enabling enhancement of photoresponsivity by 370% to flat graphene photodetectors and ultrahigh stretchability up to 200%. Furthermore, we present a novel approach to achieve mechanically reconfigurable, strong plasmonic resonances based on crumple-nanostructured graphene. Mechanical reconfiguration of crumple nanostructured graphene allows wide-range tunability of plasmonic resonances from mid- to near-infrared wavelengths. The mechanical reconfigurability can be combined with conventional electrostatic tuning. Our approach of crumple nanostructuring has potential to be applicable for other various 2D materials to achieve strain engineering and mechanical tunability of materials properties. The new functionalities in mechanical, optoelectronic, plasmonic properties created by crumple nanostructuring have potential for emerging flexible electronics and optoelectronics as well as for biosensing technologies and applications.

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.

Graphene nanostructures with complex geometries have been widely explored for plasmonic applications, as their plasmonic resonances exhibit high spatial confinement and gate tunability. However, edge effects in graphene and the narrow range over which plasmonic resonances can be tuned have limited the use of graphene in optical and optoelectronic applications. Here we present a novel approach to achieve mechanically reconfigurable and strongly resonant plasmonic structures based on crumpled graphene. Our calculations show that mechanical reconfiguration of crumpled graphene structures enables broad spectral tunability for plasmonic resonances from mid- to near-infrared, acting as a new tuning knob combined with conventional electrostatic gating. Furthermore, a continuous sheet of crumpled graphene shows strong confinement of plasmons, with a high near-field intensity enhancement of ~1 × 104. Finally, decay rates for a dipole emitter are significantly enhanced in the proximity of finite-area biaxially crumpled graphene flakes. Our findings indicate that crumpled graphene provides a platform to engineer graphene-based plasmonics through broadband manipulation of strong plasmonic resonances., Plasmonics: Exploiting the properties of crumpled graphene An innovative approach for making tunable plasmonic nanoparticles using crumpled graphene opens the door to new optical and optoelectronic devices. Graphene is a promising plasmonic material due to its low optical loss, high spatial confinement, and tunable plasmonic resonances. However, its structural patterns fix the resonance wavelengths, making broadband tunability across near- to mid-infrared wavelengths very challenging, limiting its range of applications. This led Pilgyu Kang from the University of Illinois at Urbana-Champaign in America, in collaboration with fellow American and South Korean colleagues, to turn to crumpled graphene, a form of graphene with unique optical and physical properties. By modelling the plasmonic resonances of crumpled graphene, the researchers demonstrated mechanically reconfigurable structures that enable a much broader range of spectral tunability, leading to new applications from optical sensors to photovoltaic devices.

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.

Strontium oxide (SrO) films were fabricated via solution processing and used as a liquid crystal (LC) alignment layer. The effect of the molar concentration of Sr on the properties of these films was analyzed. Some treatment on the alignment surface is generally required to align LC molecules. However, LC molecules aligned spontaneously on SrO films that were fabricated using our solution process; this alignment occurred along the direction of LC injection. Uniform LC alignment was achieved on SrO films with Sr molar concentrations between 0.1 and 0.3 mol; however, randomly aligned LCs were observed at 0.4 mol Sr. In addition, LC cells with excellent electro-optical characteristics were obtained. The threshold voltages and response times of LC cells fabricated by SrO films decreased as the molar concentration of the Sr component increased. We were able to obtain values that are competitive with those demonstrated by conventional rubbed LC cells. It is important to investigate the alignment property and electro-optical property for realization of a suitable LC application. Graphical abstract exhibits the alignment property (upper POM image) and electro-optical property (applied voltage-transmittance graph) of LC cell fabricated by SrO films. LC cell with 0.1, 0.2 and 0.3 M shows uniform alignment state. Moreover, voltage-transmittance of LC cells based on SrO film was measured and compared to that of rubbed PI. The results show the possibility of advanced LC applications.

Liquid crystal (LC) alignment layers were prepared by fabricating solution-processed HfZnO films, annealing them, and treating them with ion beam (IB) irradiation, and the effect of annealing temperature upon the resulting film properties was studied. Homogeneous LC alignment was achieved on IB-irradiated HfZnO films. Topographical changes were observed from field-emission scanning electron microscopy as annealing temperature increased. X-ray photoelectron spectroscopy analysis showed that IB irradiation resulted in oxidation of HfZnO surfaces, which caused the LCs to be oriented more uniformly. The best electro-optical characteristics observed corresponded to the annealing temperature of 200°C. The low optimal annealing temperature for fabricating the HfZnO films suggested that this material has remarkable potential for LCD applications.

Graphene has been widely used for optical and optoelectronic applications due to the capability of achieving electrostatically tunable plasmonic resonances. However, patterning of graphene in complex geometries and a narrow tunable range of plasmonic resonances have been a challenge for practical use of graphene in the applications. Here, we present a novel approach to achieve mechanically reconfigurable, strong plasmonic resonances based on crumple-nanostructured graphene. We show that the mechanical reconfiguration of crumple nanostructured graphene enables broad spectral tunability of plasmonic resonances from mid- to near-infrared, introducing a new tuning method combined with conventional electrostatic tuning. Our results demonstrate that crumple nanostructured graphene provides an innovative platform to graphene-based plasmonics for strong, broadband tunable plasmonic resonances.

This letter demonstrates liquid crystal (LC) alignment on solution-derived Zn-doped GaO (ZnGaO) films using an ion beam (IB) method. Homogeneous and uniform LC alignment was achieved on IB-irradiated ZnGaO films. X-ray photoelectron spectroscopy was used to analyze the chemical bonding states of the IB-irradiated ZnGaO surfaces to verify the compositional behavior for LC alignment. In addition, ZnGaO films with twisted nematic cells exhibited electro-optical characteristics comparable with those of a conventional polyimide layer for potential LC display applications.

Y2Sn2O7 nanoparticle layer fabricated by ion beam (IB) irradiation after annealing is reported here. Spherical particles were induced after IB irradiation as demonstrated by field emission scanning electron microscopy. When this layer was used as an alignment layer, homogeneous alignment of nematic liquid crystals (LCs) was observed with a small pretilt angle due to van der Waals interactions between LCs and the Y2Sn2O7 layer and interactions between benzene rings of the LCs and broken chemical bonds. The resulting electric-controlled twist nematic cell showed excellent electrical-optical properties, indicating that our approach holds great potential for creating commercial LC displays (LCDs).

We report the doping effect of graphene quantum dots (QDs) in a nematic liquid crystal (NLC) device. The use of graphene QDs leads to improved electro-optical (EO) properties, such as faster response times and a decreased threshold voltage. A 14.8% reduction in the threshold voltage (3.165 V), a 30% decrease in the falling time (4.9 ms) and a 63.6% decrease in the rising time (2.4 ms) were observed for twisted nematic (TN) LC devices (LCDs) that were doped with QDs. We confirm that the doping of the TN-LCD with graphene QDs reduces the field-screening effect, which is caused by impurity ions, whereas the other properties of conventional TN-LCDs are maintained.

We studied external and internal patterns for boosting light extraction from GaN light-emitting diodes (LEDs). With a transparent conductive layer (TCL) inserted between an external pattern and an upper GaN plane, light extraction increased in a thin, high-refractive-index TCL. A periodic array of air voids embedded in a GaN medium yielded an extraction efficiency similar to the external pattern’s one. For double-layer patterns, as the internal pattern was close to multiple quantum wells within a wavelength, the extraction efficiency was enhanced ∼30% over that of a single-layer pattern. This work will be useful for designing blue-to-ultraviolet-emitting GaN-based LEDs with multi-layer patterns.

We propose a multi-walled carbon nanotube (MWCNT)-doped liquid crystal (LC) device with enhanced electro-optical properties. MWCNT-doped LC cells were successfully operated as a twisted nematic LC device with high performance. The LC cells fabricated with MWCNT-doped LCs and a polyimide layer exhibited the best electro-optical properties, such as a low threshold voltage (1.60 V) and fast response time (13.95 ms). MWCNT doping decreased the pre-tilt angles of LC mixtures. Additionally, UV-visible transmittances and photomicrograph images of LC cells indicated LCD application potential with clear and uniform LC alignment.

We report a theoretical investigation of light coupling between a semiconductor nanowire (NW) and a plasmonic waveguide. The light emitted from the NW is coupled to the waveguide and excites a symmetric plasmonic waveguide mode propagating along the top surface of the waveguide. Using numerical simulations, coupling efficiencies were calculated for various coupling structures consisting of tapered/non-tapered NWs and single-strip/double-strip gold waveguides. The highest coupling efficiency of >2.8% was obtained between a tapered NW and a double-strip plasmonic waveguide with a height of 200 nm. This study offers a new opportunity for the efficient integration of nanoscale light sources with plasmonic waveguides and enables further miniaturization of ultracompact photonic integrated circuits.

Porous silicon nanowires enable optical switching and electrical current amplification in a photon-triggered transistor. Photon-triggered electronic circuits have been a long-standing goal of photonics. Recent demonstrations include either all-optical transistors in which photons control other photons1,2 or phototransistors with the gate response tuned or enhanced by photons3,4,5. 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.

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 abo...

We demonstrate the efficient integration of an electrically driven nanowire (NW) light source with a double-strip plasmonic waveguide. A top-down-fabricated GaAs NW light-emitting diode (LED) is placed between two straight gold strip waveguides with the gap distance decreasing to 30 nm at the end of the waveguide and operated by current injection through the p-contact electrode acting as a plasmonic waveguide. Measurements of polarization-resolved images and spectra show that the light emission from the NW LED was coupled to a plasmonic waveguide mode, propagated through the waveguide, and was focused onto a subwavelength-sized spot of surface plasmon polaritons at the tapered end of the waveguide. Numerical simulation agreed well with these experimental results, confirming that a symmetric plasmonic waveguide mode was excited on the top surface of the waveguide. Our demonstration of a plasmonic waveguide coupled to an electrically driven NW LED represents important progress toward further miniaturization and practical implementation of ultracompact photonic integrated circuits.

We propose rationally designed 3D grating nanowires for boosting light-matter interactions. Full-vectorial simulations show that grating nanowires sustain high-amplitude waveguide modes and induce a strong optical antenna effect, which leads to an enhancement in nanowire absorption at specific or broadband wavelengths. Analyses of mode profiles and scattering spectra verify that periodic shells convert a normal plane wave into trapped waveguide modes, thus giving rise to scattering dips. A 200 nm diameter crystalline Si nanowire with designed periodic shells yields an enormously large current density of ∼28 mA/cmsup2/suptogether with an absorption efficiency exceeding unity at infrared wavelengths. The grating nanowires studied herein will provide an extremely efficient absorption platform for photovoltaic devices and color-sensitive photodetectors.

This report discusses homogeneously aligned liquid crystals (LCs) subjected to ion-beam (IB) irradiation of the amorphous(a)-Al2O3 alignment layer. We studied the relationship between LC alignment and IB alignment conditions acting on homogeneous surfaces. We attained homogeneous LC orientations on a- Al2O3 alignment layer by optimization between the van der Waals interactions caused by Al-O bonding breaking with x-ray photoelectron spectroscopy. The voltage-transmittance characteristics of the twisted nematic – liquid crystal displays (LCDs) on the a-Al2O3 film indicate potential for use as the LC alignment layer in various LCD display applications.

Electrical performance of thin-film transistors (TFTs) is important for their applications. Solution-processed TFTs have low mobility and high sub-threshold swings (S.Ss) because there are many pores and pin-holes in the films. These characteristics are attributed to electron trapping in the YInZnO (YIZO) channel, the SiO2 gate insulator, or their interface. We fabricated hybrid YIZO TFTs with and without UV radiation, and observed that UV-curing of the film affected TFT performance through promoting a response to reactive mesogen (RM). The UV irradiated TFT showed better performance because of the alignment of the channel materials in the source-drain direction. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0031503ssl] All rights reserved.

The electrooptical properties of single-walled carbon-nanotube (SWCNT) mixed liquid-crystal (LC) cells fabricated by rubbing and ion-beam (IB) process are investigated. The increase of SWCNT concentration in the LC mixture led to a decrease in the threshold voltage of twisted nematic cells by reducing the screening effect. Threshold voltages of 1.3 and 1.7 V are lowered to 0.8 and 1.2 V, respectively, with SWCNT mixtures up to 10×10-3 wt%. LC alignment characteristics including LC orientation state, transmittance spectra, pretilt angle, and thermal stability of LC cells are also observed.

The authors introduce variable liquid crystal (LC) properties via ion beam (IB) irradiation on the ZrO2 layers. A uniform LC alignment characteristic was achieved on the ZrO2 thin film, and the pretilt angle was about 1.2°. We have shown that the pretilt angle can be controlled by changing ion beam parameters, such as the ion beam energy, the angle of incidence, and the irradiation time. The low pretilt angle for the nematic liquid crystal (NLC) on the ZrO2 thin film treated by ion beam irradiation could be adopted in planer alignment LCD liquid crystal display (LCD) applications as in-plane switching and fringe-field switching modes. The thermal stability of the ZrO2 thin film was sustained until 210°C. We also have shown that a liquid crystal cell aligned homogeneously by the ion beam irradiation exhibits the voltage-transmittance curve similar to that of a rubbed polyimide cell.

We present the characteristics of titanium dioxide (TiO2) inorganic film deposited by RF magnetron sputtering for liquid crystal display applications in this paper. TiO2 films supported vertically aligned liquid crystal (LC) molecules based on ion-beam (IB) irradiation time dependence. Uniform alignment was obtained only when the incident time was two minutes with 1.8 keV of incident energy and an incident angle of 45°. Energy from uniform high-density plasma by IB irradiation affected the interaction between the TiO2 layer and LC molecules to produce vertical alignment of the LC molecules.

This paper introduces a novel alignment method for control of pretilt angles in liquid crystals (LCs) using an etchant dipping effect on a mixed polyimide layer as an alternative alignment layer for liquid crystal display applications. A rubbing process was used to induce LC alignment and induce the measured LC pretilt angle on the etchant dipped mixture layer. The resulting thermal stabilities were good, however thermal stresses decreased as the etchant dipping time increased, indicating that the LC anchoring energy became weaker as a function of etchant dipping time after treatment. Excellent voltage-transmittance characteristics for vertically aligned cells were observed for all conditions.

We report the demonstration of subwavelength plasmonic lasers from a semiconductor nanodisk with a silver nanopan cavity. Full 3-D surface-plasmon-polariton (SPP) lasing was achieved because the nanodisk/nanopan structure enables excitation of high-quality SPP modes with subwavelength mode volumes. The optical properties of all possible resonant modes including SPP and optical modes excited in the nanodisk/nanopan were calculated and analyzed systematically using the finite-difference time-domain method. To fabricate the nanodisk/nanopan structure with an ultra-smooth silver surface, conformal deposition of silver was performed on the nanodisk. Rich SPP lasing actions were demonstrated through optical pumping of the fabricated structures. The observed SPP lasing modes were indentified unambiguously from measurements of the spectrum, mode image, and polarization state. These measurements compared well with the simulation results. In particular, the significant temperature-dependent threshold of the SPP lasers, which distinguishes SPP modes from conventional optical modes, was measured. This subwavelength SPP laser is a significant step toward the further miniaturization of a coherent light source in ultra-compact photonic integrated circuits.

The realization of nonlinear photonic circuits to achieve the control of light-by-light is contingent upon a strong nonlinear response that can be captured in a guided-wave geometry. There remains a need to further scale down waveguides while maintaining a strong nonlinear response. In this study, we report second-harmonic generation and optical parametric generation using the second-order nonlinear response in an 80 nm thick CdS nanowire subwavelength waveguide. Moreover, our three-dimensional finite-difference time-domain (FDTD) simulations demonstrate that it is possible to enhance the coherence length due to the very nature of the subwavelength geometry. Nonlinear mixing in a nanowire subwavelength waveguide represents an advance toward all-optical processing and all-optical switching in integrated photonic circuits.

Vertical alignment (VA) of a liquid crystal display (LCD) device with a stable pretilt angle has been achieved on ion-bombarded silicon nitride (SiN x) surfaces. The surface energy variation of the SiN x surface with the bonding conversion achieved via ion beam bombardment makes it possible to control the liquid crystal (LC) pretilt angle. X-ray photoelectron spectroscopy revealed that the vertical orientation of the LC molecules occurs on the SiO 2 surface and that this orientation is directionally transformed upon ion bombardment of SiN x thin films. Moreover, the upgraded electro-optical characteristics of VA-LCDs showed potential for high-performance LCD applications on modified SiN x surfaces.