Effects of two-photon absorption on all optical logic operation based on quantum-dot semiconductor optical amplifiers

We investigate all-optical logic operation in quantum-dot semiconductor optical amplifier (QD-SOA) based Mach–Zehnder interferometer considering the effects of two-photon absorption (TPA). TPA occurs during the propagation of sub-picosecond pulses in QD-SOA, which leads to a change in carrier recovery dynamics in quantum-dots. We utilize a rate equation model to take into account carrier refill through TPA and nonlinear dynamics including carrier heating and spectral hole burning in the QD-SOA. The simulation results show the TPA-induced pumping in the QD-SOA can reduce the pattern effect and increase the output quality of the all-optical logic operation. With TPA, this scheme is suitable for high-speed Boolean logic operation at 320 Gb/s.     

Spectral Properties of a Doped Antiferromagnet with Pairing Correlations

In this paper, we study the spectral properties of a phenomenological model for a weakly doped antiferromagnet. In this model, it is assumed that each carrier moves in one of the two sublattices where it was introduced. Such a situation corresponds to a case of underdoped high-temperature superconductors with the free carrier spectra maximum at k=(±π/2,±π/2) and with a four-pocket Fermi surface. We study the spectral properties of the model by taking into account both the fluctuations of the phases of the superconducting order parameter and spins of the antiferromagnetic background. It is shown that the hole spectral function and the density of states are strongly affected by these fluctuations. In particular, we argue that these fluctuations can be responsible for the temperature evolution of the Fermi pockets in cuprate superconductors.     

Sub-10-nm wide trench, line, and hole fabrication using pressed self-perfection

We report a new approach to adjust and improve nanostructures after their initial fabrication, which can reduce the trench width and hole diameter to sub-10 nm, while smoothing edge roughness and perfecting pattern shapes. In this method, termed pressed self-perfection by liquefaction (P-SPEL), a flat guiding plate is pressed on top of the structures (which are soften or molten transiently) on a substrate to reduce their height and guide the flow of the materials into the desired geometry before hardening. P-SPEL results in smaller spacing between two structures or smaller holes in a thin film.     

Highly stable persistent spectral hole burning in Eu ions doped oxy-fluoride glasses of 30CaF2–10Al2O3–60B2O3

High temperature persistent spectral hole burning up to room temperature has been observed in Eu³⁺ ions doped oxy-fluoride glasses with a composition of 30CaF₂–10Al₂O₃–60B₂O₃ (mol%) melted in a reducing atmosphere. The hole stability was studied through light-induced hole refilling and temperature cycling experiments. The burned holes survive thermal cycling to 300 K due to a high barrier height of 0.69 eV in the sample.     

Hole shape optimisation in circular discs with two neighbouring holes

Using a two-dimensional photoelastic technique, hole shapes have been optimised in diametrically loaded circular discs with two neighbouring holes leading to minimum s.c.f. Results are given for a range of disc diameter/hole diameter ratios (15.39 $ D/d $ 3.74). In comparison with circular holes, the s.c.f's have been reduced upto about 21 per cent at regions where peak stresses act. The results include the stress distribution around the holes, optimum geometries developed and the effect of tilting the load direction.     

Digitized measurements of the shape of corner cracks at fastener holes

This paper describes digitized profile measurements of fatigue cracks located at the corner of an open hole in a plate loaded in remote tension. The growth and transition of the initial corner crack into a through-the-thickness flaw is well documented. With this information, it is possible to compute crack growth rates along the flaw border, and to determine the shapes of “natural” fatigue cracks.     

On the effective spectral densities for a superconductor with paramagnons

Solutions of generalized Eliashberg equations containing an explicit paramagnon contribution are generated and used to obtain tunneling characteristics, which are then inverted in order to recover an effective spectral density. We consider several shapes for the paramagnon spectral density and vary the characteristic paramagnon energy. We invert the information on the gap using both the generalized and conventional Eliashberg equations and compare the effective spectral functions obtained by these methods with each other and with the actual kernels for the electron paramagnon and electron phonon spectral density. Scaling laws relating approximately the inverted and actual kernels are established and limitations on these laws described.     

Two neighbouring quasi-trapezoidal holes in discs with minimum s.c.f.

Using a two dimensional photoelastic technique, hole shapes have been optimised in diametrically loaded circular discs with two neighbouring holes leading to minimum stress concentration factor (s.c.f.). Two neighbouring holes symmetrically located side by side with the load axis perpendicular to the line joining the centres of the holes are considered. Results are given for a range of disc diameter/hole diameter ratios (11.44≥D/d≥4.16). Optimised quasi-trapezoidal hole geometries, stress distributions around these holes and the effect of tilting the load direction are presented. In comparison with circular holes, the s.c.fs. have been reduced up to about 14% with quasi-trapezoidal holes at regions of peak stresses and up to 23% at peak tensile stress regions.     

Highly Efficient and Bright Inverted Top‐Emitting InP Quantum Dot Light‐Emitting Diodes Introducing a Hole‐Suppressing Interlayer

InP quantum dots (QDs) based light‐emitting diodes (QLEDs) are considered as one of the most promising candidates as a substitute for the environmentally toxic Cd‐based QLEDs for future displays. However, the device architecture of InP QLEDs is almost the same as the Cd‐based QLEDs even though the properties of Cd‐based and InP‐based QDs are quite different in their energy levels and shapes. Thus, it is highly required to develop a proper device structure for InP‐based QLEDs to improve the efficiency and stability. In this work, efficient, bright, and stable InP/ZnSeS QLEDs based on an inverted top emission QLED (ITQLED) structure by newly introducing a “hole‐suppressing interlayer” are demonstrated. The green‐emitting ITQLEDs with the hole‐suppressing interlayer exhibit a maximum current efficiency of 15.1–21.6 cd A^?1 and the maximum luminance of 17 400–38 800 cd m^?2, which outperform the recently reported InP‐based QLEDs. The operational lifetime is also increased when the hole‐suppressing interlayer is adopted. These superb QLED performances originate not only from the enhanced light‐outcoupling by the top emission structure but also from the improved electron–hole balance by introducing a hole‐suppressing interlayer which can control the hole injection into QDs.     

Demonstration of a radio-frequency spectrum analyser based on spectral hole burning

We demonstrate application of spectral hole burning to the spectral analysis of broad-band rf signals. In quite the same way as an acousto-optic spectrometer, the device operates on an optically carried rf signal and achieves angular separation of the signal spectral components. An instantaneous bandwidth of 2.5 GHz has been achieved, with a power dynamic range of 35 dB, limited by the detector. Extension to greater than 10 GHz instantaneous bandwidth with greater than 1000 channels is consistent with the active material capabilities.     

Broadband radio-frequency spectrum analysis in spectral-hole-burning media

We demonstrate a 20 GHz spectrum analyzer with 1 MHz resolution and >40 dB dynamic range using spectral-hole-burning (SHB) crystals, which are cryogenically cooled crystal hosts lightly doped with rare-earth ions. We modulate a rf signal onto an optical carrier using an electro-optic intensity modulator to produce a signal beam modulated with upper and lower rf sidebands. Illuminating SHB crystals with modulated beams excites only those ions resonant with corresponding modulation frequencies, leaving holes in the crystal's absorption profile that mimic the modulation power spectrum and persist for up to 10 ms. We determine the spectral hole locations by probing the crystal with a chirped laser and detecting the transmitted intensity. The transmitted intensity is a blurred-out copy of the power spectrum of the original illumination as mapped into a time-varying signal. Scaling the time series associated with the transmitted intensity by the instantaneous chirp rate yields the modulated beam's rf power spectrum. The homogeneous linewidth of the rare-earth ions, which can be <100 kHz at cryogenic temperatures, limits the fundamental spectral resolution, while the medium's inhomogeneous linewidth, which can be >20 GHz, determines the spectral bandwidth.     

Improvement of transmittance by fabricating broadband subwavelength anti-reflection structures for polycarbonate

We report on how to increase transmittance of a 0.2 mm thick polycarbonate (PC) film by periodic subwavelength anti-reflection structures in the visible spectral range. Subwavelength anti-reflection structures like moth-eyes are fabricated into the polycarbonate substrate itself by thermal nano-imprinting lithography (TH-NIL), which uses silicon stamps that have periodic structures such as gratings (lines and spaces) and pillared dots, and are fabricated by laser interference lithography (LIL) and transformer coupled plasma etching. To increase transmittance of a polycarbonate film, we control the periods and shapes of patterns, the number of patterned surfaces, and the overlapping direction of patterns that are fabricated into its surfaces. As a result of this, we show that average transmittance improves as the pattern period gets shorter and as both surfaces of the film are patterned. We also show that the spectrum range gets larger as the pattern period gets shorter and is determined by the longer pattern period in the case of designing a film to have different pattern period on its surfaces. The maximum average transmittance of a polycarbonate film increases up to approximately 6% compared to a bare sample in the 470-800 nm spectral range.     

Experimental research on the influences of smoothing by spectral dispersion on the Technical Integration Line

We describe one-dimensional smoothing by spectral dispersion (SSD) in high fluence on the Technical Integration Line. The experimental results indicate that SSD did not influence the load capacity of the laser facility. The near- and far-field analysis prove that adopting SSD could smooth the high-frequency modulations in the near field and dramatically suppress 10 μm-100 μm spatial modulations in the far field. The focal spot contrast decreases from 2.58 to 0.80 after using SSD and adaptive optics. Adopting a 0.31 nm bandwidth frequency-modulated laser pulse and a 1200 l/mm dispersion grating, experimental results proved that 97% 3ω energy passed through the laser entrance hole using a 4 m focal length wedged lens and gold foil target with an 1100 μm hole.     

The structural intensities of composite plates with a hole

The structural intensities in isotropic and orthotropic laminae and composite laminated plates with and without a hole of different rim conditions are studied using the finite element method. Patterns of superposition among structural stress fields, intensity vectors, mode shapes and streamlines of energy flow are obtained for the various cases. It is found that the hole completely changes the distributions of the stress and energy flow path in plates. Despite its nature of mode dependent the pattern of energy flow may be different from each other despite having similar structural mode shapes, moreover the position of maximum stress is not corresponding to the position of maximum structural intensity.     

Interference phenomena in polymer light-emitting diodes: photoluminescence and modelling

We report an experimental and theoretical study of the effects of interference in polymeric light-emitting diodes (LEDs). These effects are due to the complex optical structures of the devices, which include many layers of materials with different refractive indices, and are of considerable importance since they affect spectral distribution and intensity of the absorption and emission in a significant way. By way of comparison, they can also provide a flexible, noninvasive optical probe of the electroluminescent processes, such as, for example, the spatial distribution of the recombination inside the LED. In this paper we analyse single-layer diodes with indium-tin oxide (ITO) and Al electrodes, where poly(p-phenylene vinylene) (PPV) is the luminescent polymer. We find that photo-induced excitation of the radiative species produce different spectral shapes depending on the excitation energy (and hence on the profile of excited chromophores) which we can describe in terms of interference phenomena. The theoretical analysis is conducted by means of multilayer stack theory and transfer matrix calculations, and takes into account additional quenching effects due to In contaminations from the ITO electrode. The theoretical results are in good agreement with the experiment.     

Droplet‐Based Microfluidic Synthesis of Anisotropic Metal Nanocrystals

A droplet‐based microfluidic method for the preparation of anisotropic gold nanocrystal dispersions is presented. Gold nanoparticle seeds and growth reagents are dispensed into monodisperse picoliter droplets within a microchannel. Confinement within small droplets prevents contact between the growing nanocrystals and the microchannel walls. The critical factors in translating macroscale flask‐based methods to a flow‐based microfluidic method are highlighted and approaches are demonstrated to flexibly fine tune nanoparticle shapes into three broad classes: spheres/spheroids, rods, and extended sharp‐edged structures, thus varying the optical resonances in the visible–near‐infrared (NIR) spectral range.     

Performance Simulation of a Quadrupole Mass Filter Operating in the First and Third Stability Zones

A method of computation that accurately simulates the performance of quadrupole mass filters (QMFs) is described. Behavior is described by determining the individual trajectories of a large number of ions (10⁸) as they are injected into the QMF. The effects of the ratio of circular electrode radius r to electric field radius r0 on the performance characteristics have been investigated for zone 1 (a ap 0.237 and q ap 0.706) and zone 3 (a ap 3.16 and q ap 3.23) operation. We demonstrate that performance sensitivity to the r/r0 ratio is different for zone 3 than those previously reported for zone 1. The magnitude and variation of the ldquotailrdquo in the mass spectral peak shapes that are apparent for zone 1 is much decreased for zone 3 and does not influence QMF resolution. Variation in ion trajectories and associated power-spatial frequency spectra when operated in zones 1 and 3 with varying r/r0 geometrical ratios are also presented. We demonstrate that these provide an alternative method in determining an ideal value for r/r0.     

Estimation of longitudinal resolution in optical coherence imaging

The spectral shape of a source is of prime importance in optical coherence imaging because it determines several aspects of image quality, especially longitudinal resolution. Wide spectral bandwidth, which provides short coherence length, is sought to obtain high-resolution imaging. To estimate longitudinal resolution, the spectral shape of a source is usually assumed to be Gaussian, although the spectra of real sources are typically non-Gaussian. We discuss the limit of this assumption regarding the estimation of longitudinal resolution. To this end, we also investigate how coherence length is related to longitudinal resolution through the evaluation of different definitions of the coherence length. To demonstrate our purpose, the coherence length for several theoretical and real spectral shapes of sources having the same spectral bandwidth and central wavelength is computed. The reliability of coherence length computations toward the estimation of longitudinal resolution is discussed.     

Numerical analysis of dynamic debonding under 2D in-plane and 3D loading

We present a numerical scheme specially developed for 2D and 3D dynamic debonding problems. The method, referred to as spectral scheme, allows for a precise modeling of stationary and/or spontaneously expanding interfacial cracks of arbitrary shapes and subjected to an arbitrary combination of time- and space-dependent loading conditions. It is based on a spectral representation of the elastodynamic relations existing between the displacement components along the interface plane and the corresponding dynamic stresses. A general stress-based cohesive failure model is introduced to model the spontaneous progressive failure of the interface. The numerical scheme also allows for the introduction of a wide range of contact relations to model the possible interactions between the fracture surfaces. Simple 2D problems are used to investigate the accuracy and stability of the proposed scheme. Then, the spectral method is used in various 2D and 3D interfacial fracture problems, with special emphasis on the issue of the limiting speed for a spontaneously propagating debonding crack in the presence of frictional contact. This revised version was published online in July 2006 with corrections to the Cover Date.