Scalar gradient behaviour in MILD combustion

The results of three-dimensional Direct Numerical Simulation (DNS) of Moderate, Intense Low-oxygen Dilution (MILD) and conventional premixed turbulent combustion conducted using a skeletal mechanism including the effects of non-unity Lewis numbers and temperature dependent transport properties are analysed to investigate combustion characteristics using scalar gradient information. The DNS data is also used to synthesise laser induced fluorescence (LIF) signals of OH, CH2O, and CHO. These signals are analysed to verify if they can be used to study turbulent MILD combustion and it has been observed that at least two (OH and CH2O) LIF signals are required since the OH increase across the reaction zone is smaller in MILD combustion compared to premixed combustion. The scalar gradient PDFs conditioned on the reaction rate obtained from the DNS data and synthesised LIF signals suggests a strong gradient in the direction normal to the MILD reaction zone with moderate reaction rate implying flamelet combustion. However, the PDF of the normal gradient is as broad as for the tangential gradient when the reaction rate is high. This suggests a non-flamelet behaviour, which is due to interaction of reaction zones. The analysis of the conditional PDFs for the premixed case confirms the expected behaviour of scalar gradient in flamelet combustion. It has been shown that the LIF signals synthesised using 2D slices of DNS data also provide very similar insights. These results demonstrate that the so-called flameless combustion is not an idealised homogeneous reactive mixture but has common features of conventional combustion while containing distinctive characteristics. © 2013 The Combustion Institute.

Mode-locked and single-longitudinal-mode waveguide lasers fabricated by femtosecond laser pulses in Er: Yb-doped phosphate glass

Mode-locked and single-longitudinal-mode waveguide lasers, manufactured by femtosecond laser writing in Er-Yb-doped phosphate glasses, are presented. Transform-limited 1.6-ps pulses and a cw output power exceeding 50 mW have been obtained in the two regimes. © 2007 Optical Society of America.

Annealing study of carrier concentration in gradient-doped GaAs/GaAlAs epilayers grown by molecular beam epitaxy

We measured the carrier concentration distribution of gradient-doped GaAs/GqAlAs epilayers grown by molecular beam epitaxy before and after annealing at 600 degrees C, using electrochemical capacitance voltage profiling, to investigate the internal variation of transmission-mode GaAs photocathodes arising from the annealing process. The results show that the carrier concentration increased after annealing. As a result, the total band-bending energy in the gradient-doped GaAs emission layer increased by 25.24% after annealing, which improves the pbotoexcited electron movement toward the surface. On the other hand, the annealing process resulted in a worse carrier concentration discrepancy between the GaAs and the GaAlAs, which causes a lower back interface potential barrier, decreasing the amount of high-energy photoelectrons. (C) 2009 Optical Society of America

Comparison between gradient-doping GaAs photocathode and uniform-doping GaAs photocathode

We compared two reflection-mode negative electron affinity (NEA) GaAs photocathode samples that are grown by molecular beam epitaxy with p-type beryllium doping. One sample is uniform doping, and another is gradient doping. Experimental curves of spectral response sensitivity and quantum efficiency are obtained. The thicknesses of the two cathodes are both 2.6 mu m. The integrated sensitivity of the uniform doping one is 1966 mu A/lm, and that of the gradient-doping one is 2421 mu A/lm. The escape probability and diffusion length are fitted from the spectral response curves. For the uniform-doping sample, the escape probability is 0.45 and the diffusion length is 5 mu m. For the gradient-doping sample, the escape probability is 0.55 and the diffusion length is 5.5 mu m. (c) 2007 Optical Society of America.

Effect of the longitudinal and transverse stacking period of InAs/GaAs quantum dots on the distribution of strain field

A systematic investigation is made on the influence of the longitudinal and transverse period distributions of quantum dots on the elastic strain field. The results showed that the effects of the longitudinal period and transverse period on the strain field are just opposite along the direction of center-axis of the quantum dots, and under proper conditions, both effects can be eliminated. The results demonstrate that in calculating the effect of the strain field on the electronic structure, one must take into account the quantum dots period distribution, and it is inadequate to use the isolated quantum dot model in simulating the strain field.

Influence of longitudinal electric field on the hot-phonon effect in quantum wells

Using the Huang-Zhu model [K. Huang and B.-F. Zhu, Phys. Rev. B 38, 13377 (1988)] for the optical phonons and associated carrier-phonon interactions in semiconductor superlattices, the effects of longitudinal electric field on the energy-loss rates (ELRs) of hot carriers as well as on the hot-phonon effect (HPE) in GaAs/AlAs quantum wells (QWs) are studied systematically. Contributions of various bulklike and interface phonons to the hot-carrier relaxation are compared in detail, and comprehensively analyzed in relation to the intrasubband and intersubband scatterings for quantum cascade lasers. Due to the broken parity of the electron (hole) states in the electric field, the bulklike modes with antisymmetric potentials are allowed in the intrasubband relaxation processes, as well as the modes with symmetric potentials. As the interface phonon scattering is strong only in narrow wells, in which the electric field affects the electron (hole) states little, the ELRs of hot carriers through the interface phonon scattering are not sensitive to the electric field. The HPE on the hot-carrier relaxation process in the medium and wide wells is reduced by the electric field. The influence of the electric field on the hot-phonon effect in quantum cascade lasers is negligible. When the HPE is ignored, the ELRs of hot electrons in wide QWs are decreased noticeably by the electric field, but slightly increased by the field when considering the HPE. In contrast with the electrons, the ELRs of hot holes in wide wells are increased by the field, irrespective of the HPE. (c) 2006 American Institute of Physics.

Growth of low-density InAs/GaAs quantum dots on a substrate with an intentional temperature gradient by molecular beam epitaxy

InAs was deposited by molecular beam epitaxy (MBE) on a GaAs substrate with an intentional temperature gradient from centre to edge. Two-dimensional (2D) to three-dimensional (3D) morphology evolution was found along the direction in which the substrate temperature was decreasing. Quantum dots (QDs) with density as low as similar to 8 x 10(6) cm(-2) were formed in some regions. We attribute the morphological evolution to the temperature-dependent desorption of deposited indium and the intermixing between deposited indium and gallium from the buffer.

Longitudinal spin transport in diluted magnetic semiconductor superlattices: The effect of the giant Zeeman splitting

Longitudinal spin transport in diluted magnetic semiconductor superlattices is investigated theoretically. The longitudinal magnetoconductivity (MC) in such systems exhibits an oscillating behavior as function of an external magnetic field. In the weak magnetic-field region the giant Zeeman splitting plays a dominant role that leads to a large negative magnetoconductivity. In the strong magnetic-field region the MC exhibits deep dips with increasing magnetic field. The oscillating behavior is attributed to the interplay between the discrete Landau levels and the Fermi surface. The decrease of the MC at low magnetic field is caused by the s-d exchange interaction between the electron in the conduction band and the magnetic ions. The spin polarization increases rapidly with increasing magnetic field and the longitudinal current becomes spin polarized in strong magnetic field. The effect of spin-disorder scattering on MC is estimated numerically for low magnetic fields and found to be neglectible for our system.

Longitudinal optic phonon-plasmon coupling in delta-doped metamorphic InAlAs/InGaAs high-electron-mobility transistor structures on GaAs substrates

InAlAs/InGaAs metamorphic high-electron-mobility transistor structures with different spacer layers on GaAs substrates are characterized by Raman measurements. The influence of In0.52Al0.48As spacer thickness on longitudinal optic phonon-plasmon coupling is investigated. It is found that the intensity of GaAs-like longitudinal optic phonon, which couples with collective intersubband transitions of two-dimensional electron gas, is strongly affected by the different subband energy spacings, subband electron concentrations, and wave function distributions, which are determined by different spacer thicknesses. (C) 2001 American Institute of Physics.

Investigation on a pressure-gradient fiber laser hydrophone

In this paper, a pressure-gradient fiber laser hydrophone is demonstrated. Two brass diaphragms are installed at the end of a metal cylinder as sensing elements. A distributed feedback fiber laser, fixed at the center of the two diaphragms, is elongated or shortened due to the acoustic wave. There are two orifices at the middle of the cylinder. So this structure can work as a pressure-gradient microphone in the acoustic field. Furthermore, the hydrostatic pressure is self-compensated and an ultra-thin dimension is achieved. Theoretical analysis is given based on the electro-acoustic theory. Field trials are carried out to test the performance of the hydrophone. A sensitivity of 100 nm MPa-1 has been achieved. Due to the small dimensions, no directivity is found in the test.

Room temperature operation of photonic-crystal distributed-feedback quantum cascade lasers with single longitudinal and lateral mode performance

We demonstrate room temperature operation of photonic-crystal distributed-feedback quantum cascade lasers emitting at 4.7 mu m. A rectangular photonic crystal lattice perpendicular to the cleaved facet was defined using holographic lithography. The anticrossing of the index- and Bragg-guided dispersions of rectangular lattice forms the band-edge mode with extended mode volume and reduced group velocity. Utilizing this coupling mechanism, single mode operation with a near-diffractive-limited divergence angle of 12 degrees is obtained for 33 mu m wide devices in a temperature range of 85-300 K. The reduced threshold current densities and improved heat dissipation management contribute to the realization of devices' room temperature operation.

Investigation of Interfacial Tensions for Carbon Dioxide Aqueous Solutions by Perturbed-Chain Statistical Associating Fluid Theory Combined with Density-Gradient Theory

The perturbed-chain statistical associating fluid theory and density-gradient theory are used to construct an equation of state (EOS) applicable for the phase behaviors of carbon dioxide aqueous solutions. With the molecular parameters and influence parameters respectively regressed from bulk properties and surface tensions of pure fluids as input, both the bulk and interfacial properties of carbon dioxide aqueous solutions are satisfactorily correlated by adjusting the binary interaction parameter (k(ij)). Our results show that the constructed EOS is able to describe the interfacial properties of carbon dioxide aqueous solutions in a wide temperature range, and illustrate the influences of temperature, pressure, and densities in each phase on the interfacial properties.

A novel ultra-narrow transmission-band fiber Bragg grating and its application in a single-longitudinal-mode fiber laser with improved efficiency

A novel design approach to ultra-narrow transmission-band fiber Bragg grating (FBG) is proposed and demonstrated for the first time. The new grating consists of multiple identical distributed-Bragg reflector (DBR) cavities and a it-phase-shifted gap, and hence, the proposed laser is constructed by the cascade of these identical DBR fiber lasers. By manufacturing the proposed grating in a piece of Er-Yb codoped fiber, a single-wavelength single-longitudinal-mode (SLM) fiber laser with improved efficiency is demonstrated experimentally. The experimental results show that the pump-to-signal conversion efficiency of the proposed laser is improved by a factor of two in comparison with the optimized distributed-feedback (DFB) fiber lasers. (c) 2007 Elsevier B.V. All rights reserved.

Phase reconstruction from three interferograms based on integral of phase gradient

A novel algorithm of phase reconstruction based on the integral of phase gradient is presented. The algorithm directly derives two real-valued partial derivatives from three phase-shifted interferograms. Through integrating the phase derivatives, the desired phase is reconstructed. During the phase reconstruction process, there is no need for an extra rewrapping manipulation to ensure values of the phase derivatives lie in the interval [-pi, pi] as before, thus this algorithm can prevent error or distortion brought about by the phase unwrapping operation. Additionally, this algorithm is fast and easy to implement, and insensitive to the nonuniformity of the intensity distribution of the interferogram. The feasibility of the algorithm is demonstrated by both computer simulation and experiment.