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Machining performance of extruded Ti-6Al-4 V prosthetic alloy was investigated throughelectrical discharge machining (EDM) using copper as electrode material. Experimental sequencing followed the B...

This work reports a scalable method to synthesize hierarchically porous, hetero-atom doped activated carbon nanosheet from waste biomass−human hair, and demonstrates the use of this carbon as an ultra-high performance electrode material for supercapacitor applications. Microscopic analyses reveal that sheet size ranges from 50 to 200 nm having a thickness of 15–27 nm. As-synthesized, carbon nanosheets possess a hierarchical porous structure having a specific surface area of 1548 m2 g−1. Heteroatom concentration (nitrogen, oxygen, and sulfur) of around 25% is confirmed from XPS analysis. Therefore, the novel interconnected hierarchical porous nanosheets structure enables fast adsorption and transportation of ions during electrochemical processes. Also, the abundant chemically available electroactive heteroatom species in the material enhance the wettability of ions and contribute to pseudocapacitance. The electrochemical analyses through cyclic voltammetry and galvanostatic charge-discharge measurements in 6 M KOH reveal the quasi-EDLC behavior of the activated carbons due to the presence of heteroatoms. A reprensentative KOH activated carbon shows an excellent specific capacitance value of 999 F g−1 at a current density of 1 A g-1. The symmetrically assembled two-electrode device also delivers a maximum energy density of 32 W h Kg-1 at a power density of 325 W Kg-1. In addition, excellent cyclic stability of 98% capacitance retention is observed after 10,000 continuous GCD cycles even at a high current density of 5 A g−1. Symmetric flexible supercapacitor device using KOH-PVA-K3FeCN6 as redox gel polymer electrolyte shows a synergistic specific capacitance of 145 mF cm−1 at a current density of 0.8 mA cm−1 exhibiting very less deviation in bending mode. Therefore, this waste biomass derived heteroatom doped hierarchical porous carbon nanosheets can be a promising material for cost-effective high- performance supercapacitor.

Optics manufacturing technology is predicted to play a major role in the future production of integrated photonic circuits. One of the major drawbacks in the realization of photonic circuits is the damage of optical materials by intense laser pulses. Here, we report on the preparation of a series of organic–inorganic hybrid photoresists that exhibit enhanced laser-induced damage threshold. These photoresists showed to be candidates for the fabrication of micro-optical elements (MOEs) using three-dimensional multiphoton lithography. Moreover, they demonstrate pattern ability by nanoimprint lithography, making them suitable for future mass production of MOEs.

The integration of nanomaterials with the micro-wave technology is a significant step toward the development of low-cost and low-power RF gas sensors. But, the issues associated with materials like insignificant surface area and poor conductivity, are barriers for the rapid detection of wide range of gas concentrations. In this paper, we demonstrate the proof-of concept using an RF complementary split ring resonator (CSRR) integrated ZnO/N-doped graphene (NGN) nano-hybrid to study the gas sensing properties. This RF device senses the presence of ammonia based on the change in the electrical conductivity of the nanohybrid material upon gas exposure. For this, we have designed and fabricated various new CSRR structures, namely, the meander (M-CSRR) and the CSRR, with the concentric-etched square pattern (SP-CSRR) other than the conventional CSRR (C-CSRR). The fabricated CSRRs are coated with the solvothermal methodically prepared ZnO/NGN nanomaterials and a comparison among them is also made. The comparison revealed that the SP-CSRR coated ZnO/NGN sensor had the maximum shift in the S21 resonance frequency ( $\Delta \text {f}_{\mathbf {S21}} = 13$ MHz at 100 ppm) among all at room temperature. Furthermore, the SP-CSRR-ZnO/NGN sensor exhibited a large-S21 frequency shift of 133 MHz for 500 ppm of ammonia and has shown excellent reproducibility with 0.05% of variation. We have also projected the plausible sensing mechanism for this RF sensing device.

This work presents the generation of pulsed THz radiation from a quantum dot photoconductive antenna (PCA) pumped at 800nm. The work investigates the output and characteristics of the generated THz from the QD PCA alongside a comparison with a commercial antenna from Teravil. The QD PCA outputs significantly higher THz power at low pump powers than the commercial PCA and would therefore be suitable for any application that would require a low-pump power such as the use of semiconductor lasers as pump sources for THz generation.

We report on the characterization and analysis of a GaN-based distributed feedback laser diode (DFB-LD) with 3 rd -order laterally etched sidewall gratings centered at a wavelength of 420 nm. We also compare the device parameters with two commonly used Fabry-Perot (FP) devices operating at 450 nm and 520 nm. Intrinsic properties of the devices were extracted, including damping factor, carrier and photon lifetimes, modulation efficiency, differential gain, and parasitic capacitance. These parameters showed that the DFB exhibits a lower damping rate and parasitic capacitance while demonstrating a higher modulation efficiency, indicating that the DFB shows good potential for communications applications. Additionally, spectral linewidth of a GaN DFB is reported. To the authors’ knowledge, this is the first demonstration of parameter extraction and spectral linewidth measurement for GaN-based DFB-LDs.

There is an ever-growing need for compact sources which can be used for the cooling process in high accuracy atomic clocks. Current systems make use of large, expensive lasers which are power-hungry and often require frequency doubling in order to hit the required wavelengths. Distributed feedback (DFB) lasers have been fabricated at a number of key wavelengths which would allow chip scale atomic devices with very high accuracy to become a reality. Two key atomic transitions analysed here are 88 Sr + and 87 Rb which require cooling at 422 nm and 780.24 nm, respectively. The vital parameter of the DFB lasers for this application is the linewidth, as very narrow linewidths are required in order for the atomic cooling process to occur. The lasers realised here produce the required power levels, with high side-mode suppression ratios and show good single mode tuning which is important for hitting precise wavelengths. This work will present the latest techniques and results using the DFB lasers at both wavelengths.

We present the overview of the results on the development of compact THz setups based on the quantum dot photoconductive antennas obtained during the past five years. We demonstrate the potential of the InAs/GaAs Quantum-Dot based setups to become an efficient approach to compact, room-temperature operating CW and pulsed terahertz setups for spectroscopy and imaging. We describe the photoelectronic processes in quantum dot substrates and reveal the role of quantum dots in free carrier lifetimes and the formation of the ultrafast photocurrent. We demonstrate the operation mode of the proposed antennas in pulsed and CW regimes under resonant (carriers are excited only inside the quantum dots) and off-resonant (carriers are excited in the bulk volume of the substrate) pumps with compact quantum dot semiconductor lasers. The results allow suggesting the quantum dot based setups as a new approach to field condition compact THz sources for imaging and spectroscopy.

Nickel particles were deposited on carbon fiber (CF) via a facile electroless coating method at various coating times intended for advanced structural application. Scanning electron microscopy demo...

This paper presents the nonlinear static analysis of circular cylindrical sandwich shells using a higher-order thickness and shear deformation theory with nine kinematic parameters. The sandwich shell consists of a functionally graded porous core perfectly bonded with two carbon nanotubes (CNT) reinforced composite face sheets. The gradation is done in the thickness direction using three different types of distributions. An equivalent single layer theory is adopted to develop the model by assuming continuous displacements and strain functions across the thickness of the shell. The in-plane displacements and transverse deflections are expanded as third and fourth-order functions of thickness coordinate, respectively. The governing differential equations for the circular cylindrical shell are derived using the principle of minimum total potential energy and further discretized into algebraic equations by employing the Galerkin's method. These equations of motion are solved by adopting the arc-length continuation method, and the shell is analyzed for both the cases of displacement independent (radial distributed load) and displacement dependent (actual pressure) static pressurization . A parametric study is conducted to see the effect of different porosity coefficients, core-to-face sheet thickness ratios, porosity distributions, and boundary conditions.

A geometrically nonlinear forced vibration analysis of circular cylindrical sandwich shells with open/closed cellular core using higher-order thickness and shear deformation theory is presented. The proposed sandwich shell comprises two thin face-sheets perfectly bonded to a functionally graded porous core. The face-sheets are made of carbon nanotubes (CNTs) reinforced composites. Three different types of porosity distribution, including two non-uniform and a uniform variation through the thickness, are considered. The effective mechanical properties of the core material, having open cells or closed cells, are determined using the Gibson and Ashby model. The rule-of-mixture, which includes efficiency parameters to account for scale-dependent properties of nanocomposite media, is adopted to obtain the mechanical properties of the face-sheets. The in-plane and transverse displacements of a generic point are assumed as a third and fourth-order polynomials of the through-the-thickness coordinate. The model is derived within the framework of an equivalent single layer (ESL) theory. Hamilton's principle is employed to obtain the nonlinear governing differential equations and further discretised by adopting the Galerkin method. Finally, the incremental harmonic balance (IHB) method, in conjunction with the arc-length continuation method, is used to solve the nonlinear system of coupled ordinary differential equations and compute the frequency-amplitude response. An extensive numerical study is carried out to examine the effects of the porosity coefficient, porosity distribution, core-to-face ratio and the volume fraction of CNTs in the face-sheets on the frequency-amplitude response of circular cylindrical sandwich shells.

Present analysis deals with nonlinear flexural-torsional vibration and dynamic instability of thin-walled stiffener beam with open section subjected to harmonic in-plane loading. The static and dynamic components of the applied harmonic in-plane loading are assumed to vary uniformly. A set of nonlinear partial differential equations (PDEs) describing the vibration of system is derived. Using Galerkin's method, these partial differential equations are reduced into coupled Mathieu equations. The steady state response of the system is determined by solving the condition for a non-trivial solution. The principal regions of parametric resonance are determined using the method suggested by Bolotin. The numerical results are presented to investigate the effect of aspect ratios, boundary conditions and static load factor on the frequency-amplitude responses and instability regions.

Development of printable and flexible energy storage devices is one of the most promising technologies for wearable electronics in textile industry. The present work involves the design of a printable and flexible all-solid-state rechargeable battery for wearable electronics in textile applications. Copper-coated carbon fiber is used to make a poly(ethylene oxide) (PEO)-based polymer nanocomposite for a flexible and conductive current collector layer. Lithium iron phosphate (LiFePO4) and titanium dioxide (TiO2) are utilized to prepare the cathode and anode layers, respectively, with PEO and carbon black composites. The PEO- and Li salt-based solid composite separator layer is utilized for the solid-state and safe electrolyte. Fabrication of all these layers and assembly of them through coating on fabrics are performed in the open atmosphere without using any complex processing, as PEO prevents the degradation of the materials in the open atmosphere. The performance of the battery is evaluated through char...

Owing to increased performance capabilities over their LED counterparts, Gallium Nitride (GaN) based Laser Diodes (LDs) are becoming more prevalent in several applications, including lighting, visible light communications, medical spectroscopy, and atom cooling. For many of these applications, a tunable, precise wavelength is necessary such that chromatic aberrations are minimized and that specific wavelengths with narrow linewidth can be utilized for atomic transitions or filtered communications systems. This paper discusses one method of achieving single-mode lasing operation in GaN LDs, namely the deeply-etched sidewall grating Distributed Feedback (DFB) LD. Optical characteristics of such devices will be discussed, as well as their feasibility for optical communications.

Over the past 20 years, research into Gallium Nitride (GaN) has evolved from LED lighting to Laser Diodes (LDs), with applications ranging from quantum to medical and into communications. Previously, off-the-shelf GaN LDs have been reported with a view on free space and underwater communications. However, there are applications where the ability to select a single emitted wavelength is highly desirable, namely in atomic clocks or in filtered free-space communications systems. To accomplish this, Distributed Feedback (DFB) geometries are utilised. Due to the complexity of overgrowth steps for buried gratings in III-Nitride material systems, GaN DFBs have a grating etched into the sidewall to ensure single mode operation, with wavelengths ranging from 405nm to 435nm achieved. The main motivation in developing these devices is for the cooling of strontium ions (Sr+ ) in atomic clock applications, but their feasibility for optical communications have also been investigated. Data transmission rates exceeding 1 Gbit/s have been observed in unfiltered systems, and work is currently ongoing to examine their viability for filtered communications. Ultimately, transmission through Wavelength Division Multiplexing (WDM) or Orthogonal Frequency Division Multiplexing (OFDM) is desired, to ensure that data is communicated more coherently and efficiently. We present results on the characterisation of GaN DFBs, and demonstrate their capability for use in filtered optical communications systems.

Gallium nitride (GaN) sources are becoming a regular part of today's world and are now key devices for lighting infrastructures, communications systems and quantum applications, amongst others. In particular, many applications have seen the shift from LEDs to laser diodes to make use of higher powers, higher bandwidths and increased transmission distances. Laser communication systems are well established, however there are applications where the ability to select a single emitted wavelength is highly desirable, such as quantum atomic clocks or in filtered communication systems. Distributed feedback (DFB) lasers have been realised emitting at a single wavelength where the grating structure is etched into the sidewall of the ridge. The main motivation in developing these lasers is for the cooling of ions in atomic clocks; however their feasibility for optical communications is also explored. Narrow linewidth lasers are desirable and this paper will explore how this is achieved. Data rates in excess of 1 Gbit/s have also been achieved in a directly modulated, unfiltered system. These devices lend themselves towards wavelength division multiplexing and filtered optical communications systems and this will be analysed further in the work presented here.

In this work, we explore the nonlinear pulse dynamics in a SESAM-free, mode-locked electrically-pumped (EP) VECSEL. The laser setup is illustrated in Fig. 1 (left) and includes an EP-VECSEL chip, a curved output-coupler with a radius of curvature of 75 mm and 10 % out-coupling, and an adjustable intracavity slit directly in front of the output-coupler. The output-coupler is placed on a translation stage to allow for variation of the cavity length and a polarizing isolator is used for suppression of back-reflections.

Silica-based luminescent materials have gained significant attention over the last several years primarily to be used under integrated photonics. In this context Ce-doped silica are particularly interesting due to their potential to be used as Si-based light emitting sources [1]. In this work we have prepared a Ce-doped silica fiber that is drawn from a preform which is prepared using standard MCVD technology. In this work we study the spontaneous lifetime and fluorescence parameters of a Ce-doped silica fiber when pumped with 405 nm continuous wave laser.

We investigate the fluorescence characteristics of a purely Ce-doped silica fiber and demonstrate broad-bandwidth fluorescence across the visible and near-infrared. The Ce-doped fiber is fabricated using standard modified chemical vapor deposition technology. Trace metal analysis by inductively coupled plasma mass spectrometry confirmed the purity of Ce-doping. The Ce valence state of 3+ was revealed by X-ray photoelectron spectroscopy. The optimum pump wavelength for the broadest luminescence from a fiber is scanned between 405 nm to 440 nm wavelength of diode lasers operating under continuous-wave regime. The strongest pump absorption is observed at the wavelength of 405 nm. Variation of pump power and fiber length results in the demonstration of broad-bandwidth fluorescence with spectral widths up to 301 nm (at -10 dB). The measured fluorescence spectra cover the wavelength range from ∼458 nm to ∼819 nm with spectral power densities of up to 2.4 nW/nm. publishedVersion

The future generation of modern illumination should not only be cheap and highly efficient, but also demonstrate high quality of light, light which allows better color differentiation and fidelity. Here we are presenting a novel approach to create a white solid-state light source providing ultimate color rendition necessary for a number of applications. The proposed semi-hybrid device combines a monolithic blue-cyan light emitting diode (MBC LED) with a green-red phosphor mixture. It has shown a superior color rendering index (CRI), 98.6, at correlated color temperature of around 3400 K. The MBC LED epi-structure did not suffer from the efficiency reduction typical for monolithic multi-color emitters and was implemented in the two most popular chip designs: “epi-up” and “flip-chip”. Redistribution of the blue and cyan band amplitudes in the white-light emission spectrum, using the operating current, is found to be an effective tool for fine tuning the color characteristics. (Figure presented.).

This study presents a semi-analytical solution of nonlinear vibrations of circular cylindrical shells made of carbon nanotube (CNT) fiber-reinforced composite (CNT-FRC). Vibrations are produced by a radial harmonic force and viscous structural damping is considered. The effective properties of a lamina of the CNT-FRC shell are evaluated in two steps. The elastic properties of randomly distributed CNTs in a polymeric matrix (i.e., hybrid matrix) are computed by the Eshelby-Mori-Tanaka/Voigt scheme to consider the CNTs agglomeration effect in the hybrid matrix. Then, the resulting hybrid matrix is reinforced with aligned fibers in order to prepare the lamina of the CNT-FRC shell; its effective properties are estimated by the Halpin Tsai homogenization approach. The CNT-FRC shell is modelled incorporating the von Karman geometric nonlinearity and first-order shear deformation theory (FSDT). The nonlinear governing partial differential equations (PDEs) of the CNT-FRC shells are derived by the Hamilton’s principle. These PDEs are discretized into ordinary differential equations (ODEs) by using the Galerkin’s method. The ODEs are solved by incremental harmonic balance method (IHB) in conjunction with the arclength continuation method to obtain the frequency-amplitude response of the shell. The effect of different types of CNT agglomeration models, CNT mass fraction, agglomeration parameters and stacking sequence of laminates on the frequency-amplitude curves corresponding to forced and free nonlinear vibrations of the CNT-FRC shell are studied in detail.

Aluminum (Al3+) doped manganese (Mn)/copper (Cu) ferrite impregnated rGO hybrids are verified for photo-electrocatalytic water-splitting and super-capacitor electrode applications. Energy band gaps of 2.17 eV (Mn) and 2.25 eV (Cu) confirm photocatalytic activity in the visible-band. High anodic photo-currents around 18–40 and 2–9 mA/cm2 (1 V vs. Ag/AgCl) are observed for Mn and Cu systems. Formation of oxygen vacancies due to aluminum doping assisted in efficient charge transfer behavior both at surface and lattice orders due to the structural adjustments. 2-D rGO surface disorders (ID/I2D) emphasized the retardation of the charge recombination at the metal-oxide and rGO junction. During visible-light irradiation current densities are increased by 0.7–9.2 mA/cm2 with a better redox behavior. The difference in transient currents are observed around 0.01–0.02 mA/cm2 at 0 V vs. Ag/AgCl [0.7 mA/cm2 at 0.5 V]. Intrinsic photo-current at zero voltage infers built-in potentials at the hybrid and electrolyte interface (Schottky- junction). Impedance study implicates better electron-transfer/ion diffusion characteristics during photon irradiation with a semi-infinite diffusion rate-limiting step. Specific-capacities (Three-electrode) are observed around 216 F g−1 (Mn), 142 F g−1 (Cu) with combined pseudo-capacitance and EDLC characteristics. Li+-intercalation results in ultrahigh stability up to 100,000 cycles (t ~ 69.4 h) at 0.8 V s−1 with a capacity retention of 127%. Two-Electrode performance shows 99.1% capacity retention after 5000 cycles with decrease in the impedance (1.38 Ω to 1.06 Ω), and a maximum areal energy density of 0.54 μWh cm−2 at power density of 50 μW cm−2.

Flexible wire shaped energy storage devices grab tremendous attention due to the emergence of wearable and portable electronics markets. Herein, the design principles (fabrication and assembly process) and device performance (electrochemical performance in static and bending conditions) of the lithium-ion capacitor in core-shell wire topology are described. The wire shaped lithium-ion capacitor (WSLIC) assembly consists of multi-walled carbon nanotubes coated carbon fibers (CNT-CFs) core electrode, polymer electrolytes, lithium titanium oxide (LTO) composite outer electrode, helically wrapped current collectors and protective outer covering. This 2.7 V device exhibits a maximum energy density of 1.85 mWh/cm3, comparable to the energy densities of other reported wire-shaped batteries. WSLIC exhibits excellent safety at a high temperature of 60 °C due to plasticized polymer electrolytes of non-volatile composition, challenges may arise if flammable organic electrolytes were employed. The device also exhibits excellent electrochemical performance under different deforming and repeated bending conditions, attributed to unique properties of CNT-CF core electrode and helically wrapped current collectors. Therefore, WSLIC has excellent potential for implementation in wearable applications.

Laser diodes based on Gallium Nitride (GaN) are useful devices in a wide range of applications including atomic spectroscopy, data storage and optical communications. To fully exploit some of these application areas there is a need for a GaN laser diode with high spectral purity, e.g. in atomic clocks, where a narrow linewidth blue laser source can be used to target the atomic cooling transition. We report on the continuous wave, room temperature operation of a distributed feedback laser diode (DFB-LD) with high-order notched gratings. The design, fabrication and characterization of DFB devices based on the (Al,In) GaN material system is described. A single peak emission at 408.6 nm with an optical power of 20 mW at 225 mA and a side mode suppression ratio (SMSR) of 35 dB was achieved. Additionally, we demonstrate the use of a GaN DFB-LD as a transmitter in visible optical communications system. We also present results from a DFB-LD optimized for laser cooling of Sr+.

Synergistic Mn-Al-Fe impregnated rGO hybrid (MAF-rGO) is developed and verified for arsenic remediation. Preliminary adsorption studies are observed through vibrational spectroscopy and morphological tools. Adsorption isotherms are fitted by Freundlich model multilayer sorption with a loading of 402 mg g−1 [As(III)], and 339 mg g−1 [As(V)]. Adsorption kinetics study in competing ion environment [SO42−, PO43−, NO3−, CO32−] confirms the PSO model-controlled chemisorption as the rate-limiting step. The irradiation of white light (> 420 nm) in adsorption kinetics study shows a two-fold increase in the arsenic loading parameters (5–10 min). Multiplet peaks in As3d spectra (XPS) confirm the transformation of arsenic species in the near-surface region of hybrid [i.e., As(III), As(V)]. The occurrence of redox-active, ligand-exchange reactions are confirmed by individual Mn2p, Fe2p, Al2p, O1s, and C1s spectra. The dedicated electrochemical study confirms arsenic decontamination occurred through both reduction and sorption (i.e., electrosorption). An energy bandgap (Eg) of 2.17 eV resulted in the photocatalytic activity, which enhanced the arsenic remediation. Photo-electrocatalytic synergism resulted in high photocurrent densities (0.2–0.9 mA cm−2) with charge separation [e−–h+], and built-in potential abilities (Schottky-junction; Water-splitting). Arsenic-treated hybrid (i.e., sludge) is observed with an ultra-high stable charge–discharge cyclic performance for 100,000 cycles (~77 %) with improved specific-capacity of 125 F g−1 over pristine hybrid (92 F g−1). The better electrochemical parameters of sludge are due to the efficient lithiation/de-lithiation process supported by adsorbed arsenic. This work proposes sustainable approach towards environmental remediation and energy storage applications.

A pure Ce-doped silica fiber is fabricated using modified chemical vapor deposition (MCVD) technique. Fluorescence characteristics of a Ce-doped silica fiber are experimentally investigated with continuous wave pumping from 440 nm to 405 nm. Best pump absorption and broad fluorescence spectrum is observed for ~ 405 nm laser. Next, the detailed analysis of spectral response as a function of pump power and fiber length is performed. It is observed that a -10dB spectral width of ~ 280 mn can be easily achieved with different combinations of the fiber length and pump power. Lastly, we present, for the first time to the best of our knowledge, a broadband fluorescence spectrum with -10dB spectral width of 301 nm, spanning from ~ 517.36 nm to ~ 818 nm, from such fibers with non-UV pump lasers.

Microsized and shape-versatile flexible and wearable lithium-ion batteries (LIBs) are promising and smart energy storage devices for next-generation electronics. In the present work, we design and fabricate the first prototype of microsized fibrous LIBs (thickness ≈ 22 μm) based on multilayered coaxial structure of solid-state battery components over flexible and electrically conductive carbon fibers (CFs). The micro coaxial batteries over the CF surface were fabricated via electrophoretic deposition and dip-coating methods. The microfiber battery showed a stable potential window of 2.5 V with an areal discharge capacity of ∼4.2 μA h cm-2 at 13 μA cm-2 of the current density. The as-assembled battery fiber delivered a comparable energy density (∼0.006 W h cm-3) with solid-state lithium thin-film batteries at higher power densities (∼0.0312 W cm-3). The fibrous batteries were also connected in parallel and in series to deliver large current and high voltage, respectively. The fibrous battery also retains up to 85% discharge capacity even after 100 charge-discharge cycles. Furthermore, these battery fibers performed well under both static and bending conditions, which shows the robustness of the battery fiber. Therefore, this type of fibrous microbattery can be used in advanced flexible and wearable microelectronics, bioelectronics, robotics, and textile applications.

Self-starting pulsed operation in an electrically pumped (EP) vertical-external-cavity surface-emitting-laser (VECSEL) without intracavity saturable absorber is demonstrated. A linear hemispherical cavity design, consisting of the EP-VECSEL chip and a 10% output-coupler, is used to obtain picosecond output pulses with energies of 2.8 pJ and pulse widths of 130 ps at a repetition rate of 1.97 GHz. A complete experimental analysis of the generated output pulse train and of the transition from continuous-wave to pulsed operation is presented. Numerical simulations based on a delay-differential-equation (DDE) model of mode-locked semiconductor lasers are used to reproduce the pulse dynamics and identify different laser operation regimes. From this, the measured single pulse operation is attributed to FM-type mode-locking. The pulse formation is explained by strong amplitude-phase coupling and spectral filtering inside the EP-VECSEL.

We present a wavelength-tunable, external-cavity GaSb-based quantum-well laser operating near 3.2 $\mu \text{m}$ . The laser setup consists of an intra-cavity grating in Littman–Metcalf configuration and a cascade pumped GaSb-based gain chip with a narrow-ridge waveguide. The narrow-ridge waveguide has a length of 2 mm and width of 7.5 $\mu \text{m}$ . Cascade pumping is realized with three type-I quantum-wells, using one quantum-well per cascade stage. The laser provides continuous-wave output powers up to 8 mW and slope-efficiencies of 13% at room temperature. Laser operation is demonstrated over a wavelength range of more than 300 nm, using continuous-wave and pulsed operation regimes.

We demonstrate a phosphor free, dichromatic GaN-based monolithic white LED with vertically stacked green and blue emitting multiple quantum wells. The optimal thickness of GaN barrier layer between green and blue quantum wells used is 8 nm. This device can be tuned over a wide range of correlated color temperature (CCT) to achieve warm white (CCT = 3600 K) to cool white (CCT = 13,000 K) emission by current modulation from 2.3 A/cm2 to 12.9 A/cm2. It is also demonstrated for the first time that a color rendering index (CRI) as high as 67 can be achieved with such a dichromatic source. The observed CCT and CRI tunability is associated with the spectral power evolution due to the pumping-induced carrier redistribution.

We demonstrate a single wavelength operation from an InGaN/GaN distributed feedback (DFB) blue laser at 42X nm. The 39th order grating is etched in the sidewall to achieve single wavelength. The laser has a FWHM of ~ 25 pm at 500mA pulsed current with 15 mW output power.

We investigate the wavelength-tuning of cascade quantum-well GaSb-based diode lasers emitting at 3.2 µm. An external-cavity setup is used to demonstrate wavelength-tuning over a range of 250 nm with a maximum output power of 8.4 mW.

Strong interfacial bonding is necessary between carbon fiber and polymer matrix to take advantage of carbon fiber-reinforced polymer composites in structural applications. An attempt has been made to improve the interfacial bonding by coating coiled carbon nanotubes on carbon fiber surface through a single-step chemical vapor deposition process. Coiled structures were synthesized on nickel-coated carbon fiber by using thiophene as a sulphur impurity and acetylene as a carbon precursor. The coiled carbon nanotube-coated carbon fiber and epoxy were used, respectively, as the reinforcement and the matrix to form carbon fiber-reinforced polymer composites. The role of coiled carbon nanotubes on thermo-mechanical properties of carbon fiber-reinforced polymer composites was investigated using dynamic mechanical thermal analysis in three different modes of deformation. Unsized carbon fiber (carbon fiber heat-treated to remove sizing agents)/epoxy composites were used as the reference to evaluate the enhancement due to coiled carbon nanotubes. Substantial improvements in viscoelastic properties of coiled carbon nanotube/carbon fiber/polymer composites over unsized carbon fiber/polymer composites were observed in all of the deformation modes. Coiled Carbon nanotube composites in shear mode exhibit highest enhancements in both storage as well as loss moduli due to superior mechanical interlocking between coiled carbon nanotubes and polymer matrix.

We have realised InGaN/GaN distributed feedback laser diodes emitting at a single wavelength in the 42X nm wavelength range. Laser diodes based on Gallium Nitride (GaN) are useful devices in a wide range of applications including atomic spectroscopy, data storage and optical communications. To fully exploit some of these application areas there is a need for a GaN laser diode with high spectral purity, e.g. in atomic clocks, where a narrow line width blue laser source can be used to target the atomic cooling transition. Previously, GaN DFB lasers have been realised using buried or surface gratings. Buried gratings require complex overgrowth steps which can introduce epi-defects. Surface gratings designs, can compromise the quality of the p-type contact due to dry etch damage and are prone to increased optical losses in the grating regions. In our approach the grating is etched into the sidewall of the ridge. Advantages include a simpler fabrication route and design freedom over the grating coupling strength.Our intended application for these devices is cooling of the Sr+ ion and for this objective the laser characteristics of SMSR, linewidth, and power are critical. We investigate how these characteristics are affected by adjusting laser design parameters such as grating coupling coefficient and cavity length.

Gallium nitride (GaN) laser diodes are becoming popular sources not only for lighting but for applications ranging from communications to quantum. This paper presents the use of a commercial, off-the-shelf laser diode, with an emission wavelength of 450 nm, for visible light communication, both in free space and for underwater scenarios. Data rates up to 15 Gbit/s have been achieved by making use of orthogonal frequency division multiplexing (OFDM). In addition, distributed feedback (DFB) lasers have been realised emitting at a single wavelength which lend themselves towards applications where high spectral purity is crucial such as atomic clocks or filtered free space transmission systems. These devices have the grating structure etched into the sidewall of the ridge and work is ongoing to measure the linewidth of these lasers with the intended application of cooling Sr+ ions.

We report on InGaN/GaN distributed feedback laser diodes with high order gratings emitting at a single wavelength around 428 nm. The 39th order notched gratings have the advantage of a simplified fabrication route with no need for overgrowth. The laser ridge and grating were formed by electron beam lithography followed by ICP etching. The as-cleaved lasers emitted in the pulsed regime with a peak single-mode output power of 15 mW. Optimization of the grating design should lead to higher power single wavelength operation.

External quantum efficiency of industrial-grade green InGaN light-emitting diodes (LEDs) has been measured in a wide range of operating currents at various temperatures from 13 K to 300 K. Unlike blue LEDs, the efficiency as a function of current is found to have a multi-peak character, which could not be fitted by a simple ABC-model. This observation correlated with splitting of LED emission spectra into two peaks at certain currents. The characterization data are interpreted in terms of non-uniformity of the LED active region, which is tentatively attributed to extended defects like V-pits. We suggest a new approach to evaluation of temperature-dependent light extraction and internal quantum efficiencies taking into account the active region non-uniformity. As a result, the temperature dependence of light extraction and internal quantum efficiencies have been evaluated in the temperature range mentioned above and compared with those of blue LEDs.

In this paper, the study of the photoconductivity in self-assembled InAs/GaAs quantum dot photoconductive antenna in the wavelength region between 1140 nm and 1250 nm at temperatures ranging from 13 to 400 K is reported. These antennas are aimed to work in conjunction with quantum dot semiconductor lasers to effectively generate pulsed and continuous wave terahertz radiation. For the efficient operation, laser wavelengths providing the highest photocurrent should be determined. To study the interband photoconductivity of quantum dot photoconductive antennas, at room and cryogenic temperatures, we employed a broadly-tunable InAs/GaAs quantum dot based laser providing a coherent pump with power exceeding 20 mW over a 100 nm tunability range. The quantum dot antenna structure revealed sharp temperature-dependent photoconductivity peaks in the vicinity of wavelengths, corresponding to the ground and excited states of InAs/GaAs quantum dots. The ground state photoconductivity peak vanishes with a temperature drop, whereas the excited state peak persists. We associate this effect with different mechanisms of photoexcited carriers extraction from quantum dots.

Internal quantum efficiency (IQE) of a blue high-brightness InGaN/GaN light-emitting diode (LED) was evaluated from the external quantum efficiency measured as a function of current at various temperatures ranged between 13 and 440 K. Processing the data with a novel evaluation procedure based on the ABC-model, we have determined the temperature-dependent IQE of the LED structure and light extraction efficiency of the LED chip. Separate evaluation of these parameters is helpful for further optimization of the heterostructure and chip designs. The data obtained enable making a guess on the temperature dependence of the radiative and Auger recombination coefficients, which may be important for identification of dominant mechanisms responsible for the efficiency droop in III-nitride LEDs. Thermal degradation of the LED performance in terms of the emission efficiency is also considered.

The analysis of the non-linear vibration response is carried out for functionally graded (FG) circular cylindrical shells subjected to thermal environment along with mechanical in-plane non-uniformly distributed loading along the edges and harmonic radial force. The temperature dependent material properties of the simply supported shell are assumed to vary in the radial direction according to power-law distribution. Based on the first-order shear deformation theory and von-Karman type geometric nonlinearity, the strain-displacement relationships are established for circular cylindrical shells. The coupled governing equations of motion for functionally graded cylindrical shells are then derived using Hamilton’s principle. Employing Galerkin’s method, the coupled partial differential equations of motions are reduced to a set of non-linear ordinary differential equations. In order to obtain the free and forced vibration response of the FG shell, the incremental harmonic balance method, in conjunction with the arc-length method, is used. The non-uniform in-plane loading is converted to Fourier series and the pre-buckling analysis is performed to determine the stress distribution within the shell. The non-linear frequency-amplitude response is studied to examine the effects of volume fractions of the constituents, static partial edge loadings, thermal loads, and radial periodic loadings.

Separation of alcohols from dilute aqueous solutions is important to enable continuous microbial production of biofuels. Pervaporation is a process with great potential for continuous extraction of alcohols from biological fermentation broths. Here, we report on the synthesis and pervaporation performance of thin, supported pure polydimethylsiloxane (PDMS) and silicalite-1/PDMS nanocomposite membranes with nanoparticle contents of 10, 20, and 30 wt %. Using a batch pervaporation system, we measured flux and separation factors of the membranes for solutions of 4 wt % ethanol in water at 25 °C, 50 °C, and 65 °C. With increased nanosilicalite loadings in the nanocomposite membranes, we obtained both increased flux and increased alcohol separation factors.

We report on the continuous-wave, room-temperature operation of a distributed-feedback laser diode (DFB-LD) with high-order notched gratings. The design, fabrication, and characterization of DFB devices, which are based on the (Al,In)GaN material system, is described. The uncoated devices exhibited single-wavelength emission at 408.6 nm with an optical power of 20 mW at 225 mA. A side-mode suppression ratio (SMSR) of 35 dB was achieved, with a resolution-limited full-width at half maximum of 6.5 pm.

In this paper, current-dependent emission spectra and efficiency measured on the same AlGaInP red light-emitting diode (LED) pumped with the current pulses of very different durations are recorded. This enabled for the first time distinguishing between high-carrier concentration and self-heating effects on the efficiency decline at high current magnitudes. The electron leakage to the p-side of the LED structure, which is the major mechanism of the efficiency reduction, is found to rise substantially when the device self-heating starts to develop. As a result, in comparison to continuous-wave excitation, driving the LED with sub-microsecond current pulses allows suppressing the device self-heating and, eventually, increasing the operating current by an order of magnitude without noticeable efficiency losses. Based on the reduced ABC-model, neglecting Auger recombination, the light extraction efficiency, injection efficiency, and internal quantum efficiency of the LED are estimated, suggesting light extraction to be the most critical factor for the overall efficiency of the LED. The coupled spectral/power LED characterization using the variable-duration current pulse pumping is found to be an effective approach for analyzing mechanisms of the device operation.

This paper presents triply coupled vibrations and instability analysis of thin-walled columns having a non-symmetrical open cross-section. Vlasov’s theory is used to derive the governing differential equations for coupled flexural and torsional vibrations. A numerical method is presented to determine the exact natural frequencies and corresponding mode shapes in terms of real functions. Employing Galerkin’s method, the coupled partial differential equations are reduced to a set of coupled Mathieu type equations. Following Bolotin’s method, the principal instability regions of thin-walled column with a non-symmetrical cross-section having various boundary conditions are determined. Numerical examples are presented to examine the effect of different boundary conditions, aspect ratios, static and dynamic load factors on principal instability regions. The study of response and corresponding phase plot in stable and unstable regions are carried out to identify the dynamic instability behavior.

Efficiency of commercial 620 nm InAlGaP Golden Dragon-cased high-power LEDs has been studied under extremely high pump current density up to 4.5 kA/cm2 and pulse duration from microsecond down to sub-nanosecond range. No efficiency decrease and negligible red shift of the emission wavelength is observed in the whole range of drive currents at nanosecond-range pulses with duty cycles well below 1%. Analysis of the pulse-duration dependence of the LED efficiency and emission spectrum suggests the active region overheating to be the major mechanism of the LED efficiency reduction at higher pumping, dominating over the electron overflow and Auger recombination.

Two blue (450 nm) light–emitting diodes (LED), which only differ in top p-GaN layer growth conditions, were comparatively investigated. I-V, C-V, TLM, Electroluminescence (EL) and Photoluminescence (PL) techniques were applied to clarify a correlation between MOCVD carrier gas and internal properties. The A-structure grown in the pure N2 environment demonstrated better parameters than the B-structure grown in the N2/H2 (1:1) gas mixture. The mixed growth atmosphere leaded to an increase of sheet resistances of p-GaN layer. EL and PL measurements confirmed the advantage of the pure N2 utilization, and C(VR) measurement pointed the increase of static charge concentration near the p-GaN interface in the B structure.