Alternative test method for interlaminar fracture toughness of composites

Two methods of determining the mode I interlaminar fracture toughness for fiber-reinforced polymer matrix (FRPM) composites using a double cantilever beam (DCB) test are compared. The standard method of determining G _IC is based in linear-elastic fracture mechanics theory and requires a visual measurement of the crack length, presenting data acquisition and analysis difficulties. The proposed method makes use of elastic–plastic fracture mechanics theory and an analytical closed form solution to the J-integral to relate the fracture toughness J _IC , load, and angular displacement at the load application points. This method has the advantage of replacing visually acquired data with data easily obtained using inexpensive transducers as well as being applicable to a broader class of materials.     

Nanoscale spatial phase modulation of GaN on a V-grooved Si Substrate-cubic phase GaN on Si(001) for monolithic integration

Nanoscale spatial phase modulation of GaN grown on a 355-nm period array of V-grooves fabricated in a Si(001) substrate is reported. Orientation-dependent selective nucleation of GaN in metal-organic vapor phase epitaxy begins from the opposing Si{111} sidewalls and rapidly fills each V-groove. At the initial stages of growth, the GaN deposited on the sidewalls has hexagonal phase with the c-axis normal to the Si{111}. As the growth continues, the filling of the V-groove over these misaligned hexagonal phase regions results in a transition to a cubic phase with its principal crystal axes parallel to those of the Si substrate. In a cross-sectional view perpendicular to the grooves, the defected hexagonal phase region and the clean cubic phase region above it form a boundary at the inside of each V-groove which is parallel to the Si{111} sidewalls. The GaN surface is almost planarized for only 75-nm deposition and is parallel to the original [001] plane of the Si substrate. The GaN clearly exhibits nanoscale spatial phase modulation with a periodic separation of hexagonal and cubic crystal structures across the groove direction for 600-nm deposition, implying a possibility of cubic phase GaN on an isolated single V-groove fabricated in a Si(001) substrate for monolithic integration. The structural/optical properties and stress measurements of this phase-modulated GaN grown on a nanoscale faceted Si surface are presented.     

High-Power Continuous-Wave Single-Longitudinal-Mode Operation of an Optically Pumped DFB Laser at $lambda sim 3.64~ mu$m

High-power continuous-wave (CW) single-longitudinal-mode emission at 3.64 mum is obtained from an optically pumped distributed-feedback (DFB) laser. The Bragg stopband and two degenerate DFB modes are observed at certain pump powers. The laser incorporates 14 InAs-InGaSb-InAs type-II quantum wells imbedded in an InGaAsSb waveguide. The index-coupled 1-D grating is fabricated in the top clad using interference lithography and plasma etching. A 110-mum-wide stripe from a 1.9-mum CW laser provides both optical pumping and gain guiding. Record high output power of more than 560 mW per side is obtained at 80 K. The wavelength is tunable over a 6.8-nm range by varying the pump power from 1 to 8.1 W.     

External cavity CO2-pumped InSb spin-flip laser

A pulsed CO2-pumped external cavity spin-flip laser has been operated with the output frequency governed to a large extent by the Fabry-Perot (FP) modes of resonant output couplers. An output spectral width of 0.027 ± 0.003 cm^-1has been attained.     

Large area nanopatterning of alkylphosphonate self-assembled monolayers on titanium oxide surfaces by interferometric lithography

We demonstrate that interferometric lithography offers a fast, simple route to nanostructured self-assembled monolayers of alkylphosphonates on the native oxide of titanium. Exposure at 244 nm using a Lloyd's mirror interferometer caused the spatially periodic photocatalytic degradation of the adsorbates, yielding nanopatterns that extended over square centimetre areas. Exposed regions were re-functionalised by a second, contrasting alkylphosphonate, and the resulting patterns were used as templates for the assembly of molecular nanostructures; we demonstrate the fabrication of lines of polymer nanoparticles 46 nm wide. Nanopatterned monolayers were also employed as resists for etching of the metal film. Wires were formed with widths that could be varied between 46 and 126 nm simply by changing the exposure time. Square arrays of Ti dots as small as 35 nm (λ/7) were fabricated using two orthogonal exposures followed by wet etching.     

Radiation from a dipole embedded in a dielectric slab

A unified analytical treatment of the radiation from an electric dipole of arbitrary orientation embedded at an arbitrary location within a symmetrically clad dielectric slab is presented. Both the emission into three-dimensional (3-D) radiation modes, corresponding to emission within the critical angle escape cone within the dielectric slab, and into the two-dimensional (2-D) waveguide modes are evaluated from a single calculation. The model is valid for arbitrary dielectric contrast between the slab and the cladding. The mathematical approach uses well-known complex analysis techniques: the 3-D radiation is described by a steepest descents integration around branch cuts while the 2-D waveguide modes correspond to simple poles. The division of the radiated power between the 3-D and 2-D modes is evaluated across the entire range from small dielectric contrast appropriate to diode lasers (≲1,1) to the very large dielectric contrast of free-standing semiconductor slabs (~12-19). Both enhancement and suppression, depending on position, slab width, dielectric contrast, and wavelength, of the total radiated power in comparison with that in an unbounded dielectric-medium are found for slab widths on the order of a wavelength with a maximum enhancement of ~30% for these one-dimensional Fabry-Perot structures. For thicker slabs the total radiation is almost constant and equal to that in the unbounded medium for low dielectric contrast while still exhibiting some modulation as increasing thickness allows additional waveguide modes