Search

Ag/NiO/Pt structures did (did not) exhibit reproducible resistive switching when a positive bias was applied to the Pt (Ag) electrode. X-ray photoemission spectra revealed that ultrathin NiO films on Pt (Ag) layers did (did not) undergo reversible chemical state change during heat treatment in a vacuum and oxygen ambient. Such differences in interfacial chemical interaction may affect filament formation and rupture processes near the electrode and hence alter the resistive switching behaviors. [ABSTRACT FROM AUTHOR]

Epitaxial NiO films have been fabricated on SrRuO3 films prepared on SrTiO3 single-crystal substrates. The x-ray diffraction spectra and transmission electron microscopy confirm the epitaxial growth of NiO with atomically flat surfaces on the SRO electrode. The I-V measurements of epitaxial NiO show the resistive memory switching behavior with a change in the polarity of the voltage bias, in contrast with the switching behavior of polycrystalline NiO by a single polarity. The I-V characteristics of epitaxial NiO prepared under various synthesis conditions and electrodes are presented, which suggests an important role of interfaces between NiO and electrodes on the resistive switching behavior. [ABSTRACT FROM AUTHOR]

Electronic dynamics in the excited state of transition-metal dichalcogenides (TMDs) has attracted great interest. To understand the ultrafast intraband scattering process of excited electrons in the conduction band, we demonstrated ultrafast time-resolved electron diffraction measurements with double-optical-pulse excitation and ultrafast transient reflectivity measurements of a TMD material, 2H-MoTe2. Due to the saturable absorption (or Pauli blocking) effect present in 2H-MoTe2, the system does not absorb the second excitation pulse until the excited electrons generated by the first excitation pulse with a specific fluence are scattered in the conduction band. By exploiting the Pauli blocking effect in ultrafast time-resolved electron diffraction measurements with double-optical-pulse excitation, we found that the excited electrons were scattered within 100 fs comparable to the excitation optical pulse duration. Furthermore, the excited electrons were relaxed to the lowest energy level of the conduction band (K- or Σ-valley) within 1–2 ps. [ABSTRACT FROM AUTHOR]

Semiconductor lasers are the heart for the development of technologies in many fields. In recent decades, a new type of large-area surface-emitting lasers based on the in-plane photonic crystal modulation and feedback have emerged to show superior advantages of high output power, low-divergence, high beam quality and brightness, and compact and monolithic structure. In this paper, we review the design and development of photonic crystal surface-emitting laser and discuss the future investigation and improvements. [ABSTRACT FROM AUTHOR]

Ultrahigh frequency ultrasound (>60 MHz) has emerged as a crucial actuating mechanism for non-contact manipulation, owing to its unique acoustic field properties. The acoustic field traps microparticles mainly via the gradient force of acoustic radiation force (ARF) on the transverse plane, which is up to hundreds of nanonewtons. In theory, directly using ARF can strengthen the manipulation force. Currently, the direct implementation of ARF is restricted to a specific frequency and magnitude range, which lacks a comprehensive and versatile control method. Additionally, it lacks an ARF calibration method that accommodates a wide magnitude and frequency range. We propose a variable frequency pulse width modulation-based control method for ARF, which uses an atomic force microscope microcantilever as a mechanical sensor to calibrate the magnitude and frequency. The precise control of ARF emitted by an 85 MHz transducer was achieved with a resolution of hundreds of piconewtons, magnitude range spanning from nanonewtons to micronewtons, and frequency range of 1 kHz to hundreds of kilohertz. This control method breakthrough holds great promise for expanding the application of ultrahigh frequency ultrasound in noncontact manipulation. [ABSTRACT FROM AUTHOR]

We demonstrate that absorption saturation of a mid-infrared intersubband transition can be engineered to occur at moderate light intensities of the order of 10–20 kW cm−2 and at room temperature. The structure consists of an array of metal–semiconductor–metal patches hosting a judiciously designed 253 nm thick GaAs/AlGaAs semiconductor heterostructure. At low incident intensity, the structure operates in the strong light–matter coupling regime and exhibits two absorption peaks at wavelengths close to 8.9 μm. Saturation appears as a transition to the weak coupling regime—and therefore, to a single-peaked absorption—when increasing the incident intensity. Comparison with a coupled mode theory model explains the data and permits to infer the relevant system parameters. When the pump laser is tuned at the cavity frequency, the reflectivity decreases with increasing incident intensity. When instead the laser is tuned at the polariton frequencies, the reflectivity non-linearly increases with increasing incident intensity. At those wavelengths, the system, therefore, mimics the behavior of a saturable absorption mirror in the mid-IR range, a technology that is currently missing. [ABSTRACT FROM AUTHOR]

The knowledge of the band alignment in semiconductor heterostructures is crucial, as it governs carrier confinement with many impacts on the performances of devices. By controlling the direction of the current flow in in-plane In0.53Ga0.47As/InP heterostructure nanowires, either horizontally along the nanowires or vertically into the InP substrate with low temperature multiple-probe tunneling spectroscopy, a direct measurement of the band offsets at the buried In0.53Ga0.47As/InP heterointerface is performed. Despite the unavoidable processing steps involved in selective area epitaxy, conduction and valence band offsets of 0.21 ± 0.01 and 0.40 ± 0.01 eV are, respectively, found, indicating the formation of an interface with a quality comparable to two-dimensional In0.53Ga0.47As/InP heterostructures. [ABSTRACT FROM AUTHOR]

Photonic topological insulators have recently received widespread attention mainly due to their ability to provide directions in the development of photonic integration platforms. The proposal for a topological bulk cavity with a single-mode expands upon previous research works on topological cavities; thus, interest in topological edge states and corner states is beginning to shift into analysis on bulk properties and their applications. However, there remains a gap in research on a multi-mode cavity of the topological photonic crystals (PCs). In this Letter, a cavity of the topological PCs is proposed involving pair-partitioned bulk localized states (BLSs) from a two-dimensional inner and outer nested square lattice (2D IONSL), which can enable a multi-mode cavity for the topological PCs. First, the topological characteristics are described in terms of a Zak phase, and band inversions are achieved by changing the size of scatterers in the inner and outer circles that reside within the unit cell. Afterwards, analogous to the tight-binding model for electronic systems, the Hamiltonian and topological phase transition conditions of 2D IONSL PCs are derived. Furthermore, it is proposed that the demonstrated optical field reflection and confinement mechanism induced by topological band inversions due to the opposite parities of wavefunctions may lead to the phenomenon of pair-partitioned BLSs. This research increases the research works of bulk topological effects, creating a route for photonic integration platforms for near-infrared. [ABSTRACT FROM AUTHOR]

The second-order χ (2) process underpins many important nonlinear optical applications in the field of classical and quantum optics. Generally, the χ (2) process manifests itself only in a non-centrosymmetric dielectric medium via an anharmonic electron oscillation when driven by an intense optical field. Due to inversion symmetry, group-IV semiconductors, such as silicon (Si) and germanium (Ge), are traditionally not considered as ideal candidates for second-order nonlinear optics applications. Here, we report the experimental observation of the second-harmonic generation (SHG) in a Ge-on-insulator (GOI) sample under femtosecond optical pumping. We especially, report the measurement of the SHG signal from a GOI sample in the telecom S-band by pumping at ∼ 3000 nm. [ABSTRACT FROM AUTHOR]

We consider the dynamics of micro-sized, asymmetrically coated thermoresponsive hydrogel ribbons (microgels) under periodic heating and cooling in the confined space between two planar surfaces. As the result of the temperature changes, the volume and, thus, the shape of the slender microgel change, which leads to repeated cycles of bending and elastic relaxation, and to net locomotion. Small devices designed for biomimetic locomotion need to exploit flows that are not symmetric in time (non-reciprocal) to escape the constraints of the scallop theorem and undergo net motion. Unlike other biological slender swimmers, the non-reciprocal bending of the gel centerline is not sufficient here to explain for the overall swimming motion. We show instead that the swimming of the gel results from the flux of water periodically emanating from (or entering) the gel itself due to its shrinking (or swelling). The associated flows induce viscous stresses that lead to a net propulsive force on the gel. We derive a theoretical model for this hypothesis of jet-driven propulsion, which leads to excellent agreement with our experiments. [ABSTRACT FROM AUTHOR]

The spectral dependence of the linewidth enhancement factor (αH-factor) of a multimode InAs/GaAs quantum dot laser is analyzed. Amplified spontaneous and high-frequency modulation methods are used to experimentally retrieve the αH-factor of each longitudinal mode below and above the threshold. A dispersion of the αH-factor is unlocked across the entire optical spectrum, which is further illustrated in the context of four wave mixing experiments. The results show that the induced conversion efficiency is increased at lasing wavelengths where the linewidth enhancement is lower. These results highlight the importance of carefully monitoring the linewidth enhancement factor in quantum dot lasers especially for frequency combs and mode-locking applications in future optical communication systems. [ABSTRACT FROM AUTHOR]

Strain-engineered diffusion masks deposited via plasma-enhanced chemical vapor deposition are demonstrated to control the curvature of the zinc diffusion front and, hence, disordering front, in disorder-defined vertical-cavity surface-emitting lasers (VCSELs) for enhanced high-power single-mode operation. Tensilely strained silicon nitride diffusion masks are applied to limit the lateral undercut of the disordering front, thereby minimizing the interaction between the disordered region of the distributed Bragg reflector and the fundamental mode. This results in higher threshold modal gain and absorption losses from the disordered region for higher-order modes while enabling greater output powers for fundamental-mode operation in single-mode impurity-induced disordered VCSEL designs. Using this technique, 850 nm AlGaAs VCSELs are shown to operate in a single fundamental mode with record optical output powers in excess of 10 mW and side-mode suppression ratios greater than 35 dB. Electrical and optical performances of these devices are presented in addition to near-field images confirming single-fundamental-mode lasing. [ABSTRACT FROM AUTHOR]

The rapid development of flexible electronics, human–computer interaction, wireless technology, the Internet of Things, and internet health is promoting fast-past innovation in the field of wearable medical devices. Wearable devices are a category of personalized devices that include specialized sensors, which can make conformal contact with the human body or tissue to collect biochemical or electrophysiological signals. Hence, the development of high-precision flexible devices is attracting increasing interest as they can provide real-time medical data for monitoring the physiological state of patients and their diagnosis and treatment, as well as help individuals to pursue a healthier lifestyle. This Perspective reviews the developments and requirements of wearable flexible electronic devices in medical monitoring and then discusses the possible applications and challenges of using flexible sensor technology for point-of-care devices. Finally, an up-to-date discussion of the flexible sensor, its future prospects, and solutions it could provide in medical and diagnostic equipment are summarized. [ABSTRACT FROM AUTHOR]

Surface plasmon polaritons (SPPs), elementary excitation of the hybrid states between collective motion of electrons and photons, are associated with strong local field enhancement and deep subwavelength mode confinement. The use of SPPs in nonlinear optics can amplify intrinsically weak nonlinear processes and shrink down the size of nonlinear optic devices to a nanometer scale. In this Perspective, we review the nonlinear optic processes using SPPs in the plasmonic waveguides and foresee their potential in developing compact nonlinear integrated circuits. We discuss the key factors to enhance the conversion efficiency from the plasmonic waveguide, including the spatial overlap between the interacting modes and the nonlinear materials, and the momentum conservation that allows the coherent constructive superposition. Strategies toward this goal include shrinking the effective mode area through adjusting the geometry of the plasmonic waveguide, proper incorporation of the nonlinear susceptibilities to the plasmonic near field, and the use of counter-propagating configurations or phase compensation techniques. We also forecast the future developments of nonlinear plasmonics based on propagating SPPs in active nonlinear devices. [ABSTRACT FROM AUTHOR]

We present an experimental realization and characterization of artificial spiking neuron based on an optoelectronic pair "microcavity laser-single photon avalanche diode" operating in few photon regime. We show that basic properties of biological neurons, such as an existence of the threshold and the refractory period, the insensitivity to the effect of the stimuli strength above the threshold, and the dependence of the neuron fire rate of the stimuli strength, can be realized with such a type of artificial neuron. To compare, we present corresponding results of the numerical simulation in the framework of the FitzHugh–Nagumo neuron model. [ABSTRACT FROM AUTHOR]

Formation of wrinkles is a common phenomenon in the large area growth of two-dimensional (2D) layered materials on metallic substrates. Wrinkles can significantly affect the working of 2D materials based large scale electronic devices, and therefore, it is of utmost importance to investigate local electrical properties of such wrinkled/folded structures on 2D materials. Here, we report local conductivity measurements by conducting atomic force microscopy and surface potential mapping by Kelvin probe force microscopy on large area wrinkled turbostratic graphene films grown on nickel foils. We show that the electrical transport current is several orders of magnitude higher on the wrinkles than that on the flat regions of the graphene films. Therefore, our results suggest that controlled engineering of such wrinkles on graphene may facilitate development of superior graphene-based nano-electronic devices, where transport of high current through narrow channels is desired. [ABSTRACT FROM AUTHOR]

We have demonstrated a vertical UV-C LED composed of the Ga-face n-contact electrodes using the circular shape holes on mesa. In order to understand the dependence of optical performance on the mesa hole density, we varied the number of holes. As the number of holes on mesa increased, both the light output power and the external quantum efficiency of vertical chips have enhanced while preserving the operation voltage. In spite of reduced active area, the efficiency of current injection could be improved by reducing the current density per hole. The increased mesa hole density of vertical chips induces the enhanced light extraction through the sidewall of holes. [ABSTRACT FROM AUTHOR]

We describe the effectiveness of filter layers, which displace misfit dislocation (MD) formation away from the active region, in improving high temperature reliability of epitaxially integrated InAs quantum dot lasers on on-axis silicon substrates. We find that inserting these "trapping layer (TL)" filters at either 80 nm or 180 nm from the active region substantially reduces device degradation at 60 °C. After 3000 h of continuous operation, the best trapping-layer-free device shows a 55% increase in threshold current while the best trapping layer (TL) devices each show less than a 9% increase. We explain these findings by correlating changes in individual device performance to changes in misfit dislocation (MD) structure. All MDs in devices without TLs show evidence of recombination enhanced dislocation climb (REDC); in contrast, adding trapping layers at 180 nm or 80 nm reduces the fraction of electrically active MDs to 9% and 1%, respectively. Reliability data after 3000 hours suggest that incorporating trapping layers a shorter distance from the active region (80 nm) is more effective than incorporating these layers further away. We conclude by identifying the mutually and self-reinforcing failure processes associated with REDC that TLs significantly remediate: increasing dislocation line length, increasing point defect densities, and increasing junction temperature. Overall, understanding and controlling crystal defects continues to be the most impactful avenue toward integrating light sources on photonic integrated circuits and closing the gap with native-substrate lasers. [ABSTRACT FROM AUTHOR]

We demonstrate a low threading dislocation density (TDD) and smooth surface InAs layer epitaxially grown on Si by suppressing phase separation of InxAl1−xAs (x = 0 to 1) graded buffer and by inserting a tensile-strained In0.95Al0.05As dislocation filter layer. While keeping the total III–V layer below 2.7 μm to avoid thermal cracks, we have achieved a sixfold reduction of TDD in InAs on Si compared to the unoptimized structure. We found a strong correlation between the metamorphic InAs surface roughness and TDD as a function of InxAl1−xAs buffer thickness. An optimal thickness of 175 nm was obtained where both phase separation and 3D islanding growth were suppressed. Moreover, a tensile-strained In0.95Al0.05As dislocation filter layer and high growth temperature of the InAs cap layer further assisted the dislocation reduction process, which led to a TDD to 1.37 × 108 cm−2. Finally, an InAs p-i-n photodetector grown on the optimized InAs/Si template confirmed its high quality by showing an improved responsivity from 0.16 to 0.32 A/W at a 2 μm wavelength. [ABSTRACT FROM AUTHOR]

Superconducting nanowires used in single-photon detectors have been realized on amorphous or poly-crystalline films. Here, we report the first use of single-crystalline NbN thin films for superconducting nanowire single-photon detectors (SNSPDs). Grown by molecular beam epitaxy (MBE) at high temperature on nearly lattice-matched AlN-on-sapphire substrates, the NbN films exhibit a high degree of uniformity and homogeneity. Even with relatively thick films, the fabricated nanowire detectors show saturated internal efficiency at near-IR wavelengths, demonstrating the potential of MBE-grown NbN for realizing large arrays of on-chip SNSPDs and their integration with AlN-based χ (2) quantum photonic circuits. [ABSTRACT FROM AUTHOR]

Epitaxially integrated III–V semiconductor lasers for silicon photonics have the potential to dramatically transform information networks, but currently, dislocations limit performance and reliability even in defect-tolerant InAs quantum dot (QD)-based lasers. Despite being below the critical thickness, QD layers in these devices contain previously unexplained misfit dislocations, which facilitate non-radiative recombination. We demonstrate here that these misfit dislocations form during post-growth cooldown due to the combined effects of (1) thermal-expansion mismatch between the III–V layers and silicon and (2) mechanical hardening in the active region. By incorporating an additional sub-critical thickness, indium-alloyed "misfit dislocation trapping layer," we leverage these mechanical hardening effects to our advantage, displacing 95% of misfit dislocations from the QD layer in model structures. Unlike conventional dislocation mitigation strategies, the trapping layer reduces neither the number of threading dislocations nor the number of misfit dislocations. It simply shifts the position of misfit dislocations away from the QD layer, reducing the defects' impact on luminescence. In full lasers, adding a misfit dislocation trapping layer both above and below the QD active region displaces misfit dislocations and substantially improves performance: we measure a twofold reduction in lasing threshold currents and a greater than threefold increase in output power. Our results suggest that devices employing both traditional threading dislocation reduction techniques and optimized misfit dislocation trapping layers may finally lead to fully integrated, commercially viable silicon-based photonic integrated circuits. [ABSTRACT FROM AUTHOR]

Practical applications of low-defect III–V materials grown on Si require large areas for patterning metal contacts and enhancing design flexibility. Here, we report selective area growth of bufferless and micrometer-scale InP on commercial (001)-oriented silicon-on-insulators. We obtained in-plane, centimeter-long and micrometer-wide InP single crystal stripes right atop the buried oxide layer through leveraging the lateral aspect ratio trapping (lateral ART) growth method. Using the extended InP grown by "lateral ART," we inserted InGaAs quantum wells emitting at the telecom bands. Numerical simulation suggests that the micrometer-scale InP can support the fundamental TE mode with an ultra-low metal-induced propagation loss of 3.2 dB/cm when patterned into ridge waveguides and introducing metal contacts at both ends. Our results here represent a leap toward electrically driven III–V lasers seamlessly interfaced with Si-photonics. [ABSTRACT FROM AUTHOR]

The direct growth of III–V lasers on the industry-standard Si-platforms is the key for fully integrated Si-photonics. Conventional III–V hetero-epitaxy on Si substrates is plagued by crystalline defects generated at the mismatched interface, and thick buffers are often used to mitigate the issues. Here, we report the direct deposition of room temperature III–V micro- and nano-lasers on amorphous SiO2 layers. Without the requirement of crystalline substrates nor having to cope with the long-lasting lattice-mismatch problem, we demonstrate the direct hetero-epitaxy of dislocation-free III–V nanopillars and micro-islands on amorphous SiO2 layers. As the epitaxial InP crystals are embedded inside a low-index environment, we observed strong room temperature lasing from the as-grown InP crystallites under optical excitation. [ABSTRACT FROM AUTHOR]

We demonstrate waveguide-integrated superconducting nanowire single-photon detectors on thin-film lithium niobate (LiNbO3, LN). Using a 250 lm-long NbN superconducting nanowire lithographically defined on top of a 125 lm-long LN nanowaveguide, an on-chip detection efficiency of 46% is realized with simultaneous high performance in dark count rates and timing jitter. As LN possesses high vð2Þ secondorder nonlinear and electro-optic properties, an efficient single-photon detector on thin-film LN opens up the possibility to construct a small-scale fully integrated quantum photonic chip, which includes single-photon sources, filters, tunable quantum gates, and detectors. [ABSTRACT FROM AUTHOR]

Quantum dot and quantum dash (QDash) lasers exhibit lower threshold, less temperature sensitivity, and larger modulation bandwidths than the conventional quantum well lasers. For III–V lasers monolithically grown on Si, the stronger carrier confinement and the discrete distribution of these three-dimensional (3D) quantum structures add to their immunity to material defects resulted from hetero-epitaxy. In this study, we report InAs/InAlGaAs/InP QDash lasers emitting at 1.3 μm directly grown on compliant InP/Si substrates by metalorganic chemical vapor deposition. Room-temperature lasing has been demonstrated on both nano-V-groove patterned and unpatterned planar (001) Si under pulsed electrical pumping, with a low threshold current density of 1.05 kA/cm2. A comparison of lasers grown on these two categories of InP/Si templates in terms of material quality and device performance is presented. Results presented in this work demonstrate the possibility of integrating both datacom and telecom lasers on Si, using the same InAs/InP quantum dash material system on a developed InP-on-Si virtual substrate. [ABSTRACT FROM AUTHOR]

We demonstrate a thin-film flip-chip (TFFC) light-emitting diode (LED) emitting in the ultraviolet B (UVB) at 311 nm, where substrate removal has been achieved by electrochemical etching of a sacrificial Al 0.37 Ga 0.63 N layer. The electroluminescence spectrum of the TFFC LED corresponds well to the as-grown LED structure, showing no sign of degradation of structural and optical properties by electrochemical etching. This is achieved by a proper epitaxial design of the sacrificial layer and the etch stop layers in relation to the LED structure and the electrochemical etch conditions. Enabling a TFFC UV LED is an important step toward improving the light extraction efficiency that limits the power conversion efficiency in AlGaN-based LEDs. [ABSTRACT FROM AUTHOR]

We study the impact of misfit dislocations on the luminescence from InAs quantum dots (QDs) grown on Si substrates. Electron channeling contrast imaging is used together with cathodoluminescence mapping to locate misfit dislocations and characterize the resulting nonradiative recombination of carriers via near-infrared light emission profiles. With a 5 kV electron beam probe, the dark line defect width due to a typical misfit dislocation in a shallow QD active layer is found to be approximately 1 μm, with a 40%–50% peak emission intensity loss at room temperature. Importantly, we find that at cryogenic temperatures, the dislocations affect the QD ground state and the first excited state emission significantly less than the second excited state emission. At the same time, the dark line defect width, which partially relates to carrier diffusion in the system, is relatively constant across the temperature range of 10 K–300 K. Our results suggest that carrier dynamics in the QD wetting layer control emission intensity loss at dislocations, and that these defects reduce luminescence only at those temperatures where the probability of carriers thermalizing from the dots into the wetting layer becomes significant. We discuss the implications of these findings toward growing dislocation-tolerant, reliable quantum dot lasers on silicon. [ABSTRACT FROM AUTHOR]

Efficient on-chip laser sources of Si photonics can be built from direct epitaxy of dislocation-free III–V alloys on industrial-standard (001) Si wafers. Here, we report on selective lateral epitaxy of InP on patterned (001) silicon-on-insulators (SOIs) by metal organic chemical vapor deposition. Based on the conventional "aspect ratio trapping" approach, we created undercut patterns to alter the growth front to the lateral direction. Growth of InP inside the nano-scale SOI trenches results in dislocation-free InP crystals right atop the buried oxide layer. The intimate placement of the InP crystals with the Si device layer points to the development of dislocation-free nano-ridges for integration of efficient III–V light emitters with Si-based photonic components on SOI. [ABSTRACT FROM AUTHOR]

We report on the response of graphene to high intensity mid-IR radiation and show that graphene exhibits saturable absorption and significant two-photon absorption in the spectral region of 1.55 μm to 3.50 μm (0.35 eV to 0.80 eV). We find that the effective modulation depth of multilayer graphene is limited by two-photon absorption, which will affect its performance as a laser mode-locking element. The measured saturation intensities of femtosecond pulses were found to depend on the third power of photon energy when we combined our results with others reported in the literature, while those of longer pulses were found to have a square root dependence. [ABSTRACT FROM AUTHOR]

Large normal material dispersion acts as an obstacle in the generation of visible optical frequency combs (OFCs) in microcavities. The trajectory separation of light propagation aggravates the dispersion unbalance in the visible region. Dispersion engineering devoted to compressing the separation reaches a bottleneck due to manufacturing difficulties. In this article, we demonstrated that the separation of the light trajectory can be used to manipulate the cavity dispersion. In a bilayer concentric spherical microcavity (CSM), the visible anomalous dispersion was achieved numerically by precisely matching the separated trajectory with the appropriate materials. Furthermore, we simulated the generation of an optical soliton in the visible region based on the proposed microcavity. Our work provides a clue for the visible dispersion control, and the proposed BK7/silica CSM will be an interesting platform for the visible OFC generation. [ABSTRACT FROM AUTHOR]

The realization of efficient mixing of samples inside a microfluidic channel is essential for performing numerous biological assays in miniaturized total analysis systems. The low Reynolds number flows at the microscale create laminar streams inside the microchannel, limiting flow mixing to a molecular diffusion level. In this paper, we propose a simple and efficient acoustofluidic mixing technique inside a single-layered polydimethylsiloxane (PDMS) microfluidic channel. The proposed surface acoustic wave (SAW)-based system composed of a straight interdigitated transducer (IDT) is positioned beneath the PDMS microchannel. Fluorescein dye dissolved in deionized water (sample fluid) and deionized water (sheath fluid) was introduced through the first and second inlets of the PDMS microchannel, respectively. Their flow rates were controlled such that the sample fluid with fluorescein dye was hydrodynamically focused close to the bottom of the microchannel by the sheath fluid. High-frequency (140 MHz) SAWs, generated from the IDT placed right beneath the first outlet, mixed the two fluids under the influence of strong acoustic streaming flows. The mixed samples were then collected at the two outlet ports for further analysis of the mixing efficiency. The developed acoustofluidic mixing device required an input voltage of 12 Vpp at a total flow rate of 50 μl/min to realize complete mixing. At a similar applied voltage, the throughput of the proposed device could be further increased to 200 μl/min with a mixing efficiency of >90%. [ABSTRACT FROM AUTHOR]

On-chip frequency upconversion of a near-infrared (NIR) Kerr comb in a χ(2) and χ(3) system provides a convenient route to extending the comb spectra into the visible band. Yet to date, only limited visible or near-visible comb lines have been obtained using this scheme. In this work, we demonstrate the generation of multiple near-visible comb lines based on spectral translation from a broadband NIR Kerr comb. This physical process is implemented in an aluminum nitride (AlN)-on-sapphire microring, where we achieve a wideband frequency upconversion by incorporating the phase-mismatched fundamental and first-order near-visible modes. Upon tuning the pump into the resonance with sufficient power, we attain a broadband NIR Kerr comb and 153 corresponding near-visible comb lines in 720–840 nm with a reasonable efficiency over 4.1 × 10−5%. The wideband frequency upconversion can be adapted to on-chip frequency stabilization of self-referenced microcombs, as required for precision optical clocks and frequency metrology. [ABSTRACT FROM AUTHOR]

High quality factor optical microcavities have been employed in a variety of material systems to enhance nonlinear optical interactions. While single-crystalline aluminum nitride microresonators have recently emerged as a low loss platform for integrated nonlinear optics such as four wave mixing and Raman lasing, few studies have investigated this material for second-harmonic generation. In this letter, we demonstrate an optimized fabrication of dually resonant phase-matched ring resonators from epitaxial aluminum nitride thin films. An unprecendented second-harmonic generation efficiency of 17 000%/W is obtained in the low power regime, and pump depletion is observed at a relatively low input power of 3.5 mW. This poses epitaxial aluminum nitride as the highest efficiency second-harmonic generator among current integrated platforms. [ABSTRACT FROM AUTHOR]

We investigate, both experimentally and theoretically, the gain characteristics of modulation p-doped 1.3 μm quantum dot lasers epitaxially grown on silicon. Gain spectra and transparency points are measured for structurally identical lasers with varying levels of p doping in the active region. A many-body model is employed to facilitate understanding of the material gain characteristics. It has been found that appropriate p doping greatly reduces transparency and improves differential gain. It is also found that the improvements saturate with excessive doping because of the increase in nonradiative carrier recombination. [ABSTRACT FROM AUTHOR]

Optical frequency comb direct generation on silicon by mode locked lasers (MLLs) is promising as it offers high wavelength channel counts and ultrashort pulses that will benefit future large-scale high capacity silicon photonic integrated circuits. Here, we demonstrate two-section quantum dot (QD) MLLs that are directly grown on a complementary metal–oxide–semiconductor compatible on-axis (001) silicon substrate by employing molecular beam epitaxy. The lasers, incorporating five layers of InAs QDs, operate in the O-band wavelength range with a pulse repetition rate around 9 GHz. A pulsewidth reduction of 48% of the narrowest achievable pulse from each QD MLL is obtained when the saturable absorber (SA) section length ratio is increased from 8% to 23%. The device with the longest SA section exhibits a more than 50 dB fundamental RF peak signal to noise floor ratio with 1.3 ps pulses. [ABSTRACT FROM AUTHOR]

Due to their strong light-matter interaction, monolayer transition metal dichalcogenides (TMDs) have proven to be promising candidates for nonlinear optics and optoelectronics. Here, we characterize the nonlinear absorption of chemical vapour deposition (CVD)-grown monolayer MoSe2 in the 720–810 nm wavelength range. Surprisingly, despite the presence of strong exciton resonances, monolayer MoSe2 exhibits a uniform modulation depth of ~80 ± 3% and a saturation intensity of ~2.5 ± 0.4 MW/cm2. In addition, pump-probe spectroscopy is performed to confirm the saturable absorption and reveal the photocarrier relaxation dynamics over hundreds of picoseconds. Our results unravel the unique broadband nonlinear absorptive behavior of monolayer MoSe2 under ultrafast excitation and highlight the potential of using monolayer TMDs as broadband ultrafast optical switches with customizable saturable absorption characteristics [ABSTRACT FROM AUTHOR]

We report the basic nonlinear optical properties, namely, two-photon absorption coefficient (β), three-photon absorption coefficient (γ), and Kerr nonlinear refractive index (nkerr), of GaN crystals in polar c-plane, nonpolar m-plane, and semipolar (2021) plane orientations. A typical Z-scan technique was used for the measurement with a femtosecond Ti:S laser from wavelengths of 724 nm to 840 nm. For the two-photon absorption coefficient (β), similar values were obtained for polar, nonpolar, and semipolar samples, which are characterized to be ~0.90 cm/GW at 724nm and ~0.65 cm/GW at 730 nm for all the three samples. For the Kerr nonlinear refractive index (nkerr), self-focusing features were observed in this work, which is different from previous reports where self-defocusing features were observed on GaN in the visible and near-UV spectral regions. At 724 nm, nkerr was measured to be ~2.5 0 x 10-14 cm²/W for all three samples. Three-photon absorption coefficients (γ) were also determined, which were found to be consistent with previous reports. This study provides valuable information on the basic nonlinear optical properties of III-nitride semiconductors, which are vital for a wide range of applications such as integrated photonics and quantum photonics. [ABSTRACT FROM AUTHOR]

The development of long-range order in disordered Co-Pt alloy nanoparticles has been atomically resolved in situ with an ultra-high voltage electron microscope equipped with a direct electron detection camera. Electron-irradiation-enhanced ordering occurred at 573K with 1MeV electrons at a dose rate of 8.9X1024 e/m2s. High-speed (400 frames/s) imaging revealed fluctuations of the c-axis orientation of the L10-type ordered structure. Specifically, the c-axis orientation changes occurred at 2.5-ms intervals. Thus, the atomic ordering rate at 573K is deduced to be 3×10-17 m2/s, which is 1013 times higher than that estimated for interdiffusion in a bulk Co-Pt alloy. The observed kinetic ordering temperature of 573K is significantly lower than that reported previously (>800 K). The low-temperature ordering may be the result of enhanced atom migration via excess vacancies, 106 times higher than that at thermal equilibrium, introduced by the high-energy electron irradiation. [ABSTRACT FROM AUTHOR]

Direct growth of graphene nanostructures by using concepts of self-assembly and intercalation without further lithography and transfer processes is beneficial for their integration into device applications. In this letter, we report on bilayer graphene nanoribbons, typically 100 nm in width, grown along step edges of SiC(0001) substrates. The ribbons are electrically decoupled from the substrate by an oxygen treatment. By means of a 4-tip STM system, the microscopic structure and transport properties were comprehensively studied. The ribbons reveal a robust hole concentration of around 1 x 1013 cm-2 and mobilities up to 700 cm2/Vs at room temperature. The comparably high mobilities are a consequence of interlayer hopping of the charge carriers. The transport is not limited by the step roughness; thus, this scalable process can be easily extended to arbitrarily shaped structures. [ABSTRACT FROM AUTHOR]

We examine the influence of variable oxygen concentration in TaOx active layers on the forming process and bipolar resistive switching (BRS) features of TaOx-based resistive switching cells. TaOx active layers prepared using various rf sputtering powers were systematically analyzed to identify the relation between initial compositions and BRS behavior. Proper control of oxygen vacancy concentration was clearly identified as a basic factor in ensuring typical BRS features without affecting the structural properties. We describe the possible origins of both conduction and switching based on the variation of oxygen concentrations initially provided by the growth conditions. [ABSTRACT FROM AUTHOR]

In this paper, we present the controlled modulation of the intracell diffusion energy barrier of silver (Ag) monomers on the Si(111)-(7×7) substrate through voltage pulses. After the voltage pulse is performed, the diffusion energy barrier of Ag monomers on Si(111)-(7×7) could be locally increased. Such changes consequently enable us to gradually restrict the diffusion area of Ag monomers. It provides a way to control the diffusion behavior of single atoms or molecules by modulating the diffusion energy barrier through indirectly performing voltage pulse on the substrate. [ABSTRACT FROM AUTHOR]

The electrical manipulation of graphene oxide (GO) alignment in aqueous dispersions is a useful technique with various applications. In particular, the electrical switching of GO particles can be used to devise optical birefringent liquid crystal displays. However, the electric switching of aqueous GO dispersions with a high ionic concentration requires driving voltages with high frequencies (~10 kHz), which is a challenging limitation. We demonstrate that stable electrooptical switching can be achieved at low frequencies (100 Hz) using GO dispersions in organic solvents instead of water. The hydrodynamic flow of the solvent and the electrophoretic drift of the GO particles are hindered in the GO dispersions in organic solvents with lower dielectric constants. Moreover, the electro-optical performance of these GO dispersions is similar to the aqueous GO dispersions, despite the lower magnitude of the ionization ratio for the GO particle functional groups. These results are crucial for developing a liquid crystal display device using GO dispersions. [ABSTRACT FROM AUTHOR]

Semiconductor surfaces facilitate the injection of highly mobile point defects into the underlying bulk, thereby offering a special means to manipulate bulk defect concentrations. The present work combines diffusion experiments and first-principles calculations for polar ZnO (0001) surface to demonstrate such manipulation. The rate behavior of oxygen interstitial injection varies dramatically between the Zn- and O-terminated ZnO surfaces. A specific injection pathway for the Zn-terminated surface is identified, and activation barrier determined from the first-principles calculations agrees closely with the experimental activation energy of 1.7 eV. [ABSTRACT FROM AUTHOR]

Here, we report a printing mechanism that utilizes mechanical stress for dry transfer of chemically reduced graphene oxide (r-GO) thin film. We discover that shear stress induced on the elastomeric stamp surface facilitates delamination of the deposited r-GO thin film from the stamp. Shear stress is introduced in a roll-to-roll printing system by rotating the stamp roller faster than the substrate roller. Energy-balance theory for thin film transfer is suggested to analyze the film delamination behavior with induced shear stress. Facile dry transfer of r-GO thin film onto flexible substrates is demonstrated in a roll-to-roll printing system at a printing rate of 5mm/min with calculated shear stress of 325.43 kPa. [ABSTRACT FROM AUTHOR]

Due to the lack of suitable optical modulators, directly generated Pr3+- and Dy3+-doped bulk visible lasers are limited in the continuous-wave operation; yet, pulsed visible lasers are only sparingly reported recently. It has been theoretically predicated that Au nanorods could modulate the visible light operation, based on the nonlinear optical response of surface plasmon resonance. Here, we demonstrate the saturable absorption properties of Au nanorods in the visible region and experimentally realized the pulsed visible lasers over the spectral range of orange (605 nm), red (639 nm), and deep red (721 nm) with Au nanorods as the optical modulator. We show that Au nanorods have a broad nonlinear optical response and can serve as a type of broadband, low-cost, and eco-friendly candidate for optical switchers in the visible region. Our work also advocates the promise of plasmonic nanostructures for the development of pulsed lasers and other plasmonic devices. [ABSTRACT FROM AUTHOR]

We grow carbon nanotube forests on piezoelectric AlN films and fabricate and characterize nanotube-based solidly mounted bulk acoustic wave resonators employing the forests as the top electrode material. The devices show values for quality factor at anti-resonance of ~430, and at resonance of ~100. The effective coupling coefficient is of ~6%, and the resonant frequencies are up to ~800 MHz above those observed with metallic top electrodes. AlN promotes a strong catalyst-support interaction, which reduces Fe catalyst mobility, and thus enforces the growth of forests by the base growth mechanism. [ABSTRACT FROM AUTHOR]

he electrical properties of organic field-effect transistors are governed by the quality of the constituting layers, and the resulting interfaces. We compare the properties of the same organic semiconductor film, 2,8-difluoro- 5,11-bis (triethylsilylethynyl) anthradithiophene, with bottom SiO2 dielectric and top Cytop dielectric and find a 10× increase in charge carrier mobility, from 0.17 ± 0.19 cm² V-1 s-1 to 1.5 ± 0.70 cm² V-1 s-1, when the polymer dielectric is used. This results from a significant reduction of the trap density of states in the semiconductor band-gap, and a decrease in the contact resistance. [ABSTRACT FROM AUTHOR]

Unipolar resistive switching (URS) as well as bipolar resistive switching (BRS) behaviors in a Cu/TaOx/Pt structure were investigated. Upon increasing the compliance current (Ic), the currentvoltage characteristics of the Cu/TaOx/Pt structure showed a URS behavior at Ic=0.1mA then experienced a non-reversible transition from the URS to a BRS mode at Ic=10 mA. Through a detailed analysis of the electrical properties in each resistance state of URS and BRS, we revealed that the permanent transition from the URS to the BRS mode was induced by the formation of stronger Cu metal conductive filaments within the TaOx thin film. More interestingly, both URS and BRS modes were governed by the formation and rupture of conductive filaments, whereas the rupture of these filamentary paths in BRS was proposed due to both Joule heating and electric field effects. [ABSTRACT FROM AUTHOR]

Low-dimensional materials, due to their unique and versatile properties, are very interesting for numerous applications in electronics and optoelectronics. Recently rediscovered black phosphorus, with a graphite-like layered structure, can be effectively exfoliated up to the single atomic layer called phosphorene. Contrary to graphene, it possesses a direct band gap controllable by the number of stacked atomic layers. For those reasons, black phosphorus is now intensively investigated and can complement or replace graphene in various photonics and electronics applications. Here, we demonstrate that black phosphorus can serve as a broadband saturable absorber and can be used for ultrashort optical pulse generation. The mechanically exfoliated 300 nm thick layers of black phosphorus were transferred onto the fiber core, and under pulsed excitation at 1560 nm wavelength, its transmission increases by 4.6%. We have demonstrated that the saturable absorption of black phosphorus is polarization sensitive. The fabricated device was used to mode-lock an Erdoped fiber laser. The generated optical solitons with the 10.2 nm bandwidth and 272 fs duration were centered at 1550 nm. The obtained results unambiguously show that black phosphorus can be effectively used for ultrashort pulse generation with performances similar or even better than currently used graphene or carbon nanotubes. This application of black phosphorus proves its great potential to future practical use in photonics. [ABSTRACT FROM AUTHOR]