InGaN light-emitting diodes with indium-tin-oxide sub-micron lenses patterned by nanosphere lithography

Close-packed micro-lenses with dimensions of the order of wavelength have been integrated onto the indium-tin-oxide (ITO) layer of GaN light-emitting diodes employing nanosphere lithography. The ITO lens arrays are transferred from a self-assembled silica nanosphere array by dry etching, leaving the semiconductor layer damage-free. An enhancement of up to 63.5% on optical output power from the lensed light-emitting diode (LED) has been observed. Lens-patterned LEDs are also found to exhibit reduced emission divergence. Three-dimensional finite-difference time-domain simulations performed for light extraction and emission characteristics are found to be consistent with the observed results. © 2012 American Institute of Physics.published_or_final_versio

Spherical interface dynamos: Mathematical theory, finite element approximation, and application

Stellar magnetic activities such as the 11-year sunspot cycle are the manifestation of magnetohydrodynamic dynamo processes taking place in the deep interiors of stars. This paper is concerned with the mathematical theory and finite element approximation of mean-field spherical dynamos and their astrophysical application. We first investigate the existence, uniqueness, and stability of the dynamo system governed by a set of nonlinear PDEs with discontinuous physical coefficients in spherical geometry, and characterize the system by a saddle-point type variational form. Then we propose a fully discrete finite element approximation to the dynamo system and study its convergence and stability. For the astrophysical application, we perform some fully threedimensional numerical simulations of a solar interface dynamo using the proposed algorithm, which successfully generates the equatorially propagating dynamo wave with a period of about 11 years similar to that of the Sun. © 2006 Society for Industrial and Applied Mathematics.published_or_final_versio

Passive phloem loading and long-distance transport in a synthetic tree-on-a-chip

Vascular plants rely on differences of osmotic pressure to export sugars from regions of synthesis (mature leaves) to sugar sinks (roots, fruits). In this process, known as M\"unch pressure flow, the loading of sugars from photosynthetic cells to the export conduit (the phloem) is crucial, as it sets the pressure head necessary to power long-distance transport. Whereas most herbaceous plants use active mechanisms to increase phloem concentration above that of the photosynthetic cells, in most tree species, for which transport distances are largest, loading seems to occur via passive symplastic diffusion from the mesophyll to the phloem. Here, we use a synthetic microfluidic model of a passive loader to explore the nonlinear dynamics that arise during export and determine the ability of passive loading to drive long-distance transport. We first demonstrate that in our device, phloem concentration is set by the balance between the resistances to diffusive loading from the source and convective export through the phloem. Convection-limited export corresponds to classical models of M\"unch transport, where phloem concentration is close to that of the source; in contrast, diffusion-limited export leads to small phloem concentrations and weak scaling of flow rates with the hydraulic resistance. We then show that the effective regime of convection-limited export is predominant in plants with large transport resistances and low xylem pressures. Moreover, hydrostatic pressures developed in our synthetic passive loader can reach botanically relevant values as high as 10 bars. We conclude that passive loading is sufficient to drive long-distance transport in large plants, and that trees are well suited to take full advantage of passive phloem loading strategies

A study of aggregated 2D Gabor features on appearance-based face recognition

Author name used in this publication: Wai-Kin KongAuthor name used in this publication: David ZhangRefereed conference paper2004-2005 > Academic research: refereed > Refereed conference paperVersion of RecordPublishe

On nonlinear multiarmed spiral waves in slowly rotating systems

Stable nonlinear equilibria of convection in the form of quasistationary, multiarmed spiral waves, up to a maximum of six spiral arms, are found in a slowly rotating fluid confined within a thin spherical shell governed by the three-dimensional Navier–Stokes equation, driven by a radial unstable temperature gradient and affected by a weak Coriolis force. It is shown that three essential ingredients are generally required for the formation of the multiarmed spirals: the influence of slow rotation, large-aspect-ratio geometry and the effect of weak nonlinearity.published_or_final_versio

On hierarchical palmprint coding with multiple features for personal identification in large databases

Author name used in this publication: Wai-Kin KongAuthor name used in this publication: King Hong Cheung2003-2004 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

The non-resonant response of fluid in a rapidly rotating sphere undergoing longitudinal libration

published_or_final_versio

DNS of a diffusional jet flame in turbulent cross-flow using a low Mach number solver

Understanding of flame anchoring in a jet in crossflow (JICF) configuration is vital to the design of fuel injectors in combustion devices. The present study numerically investigates a hydrogen rich jet injecting perpendicularly into hot vitiated crossflow using direct numerical simulation (DNS). Development of the reacting flow field and flame shape along the jet trajectory is scrutinised. The flame is found to be anchored around the jet exit, and downstream only on the windward side. Heat release rate and Chemical Explosive Mode Analysis (CEMA) are used to identify combustion modes. Distinct from flames stabilizing in non-vitiated crossflow where combustion is mainly partially premixed, diffusion flame is significant under the current condition, though some premixed or partially premixed regions are found on the leeward side of the jet due to large scale turbulent mixing

Size effects on dynamics of nanodroplets in binary head-on collisions

Head-on collision dynamics of 10, 50 and 100 nm droplets are investigated in vacuum by molecular dynamics, involving 35,858, 4,506,410 and 36,051,466 molecules, respectively. A variety of droplet collision dynamics are observed, such as coalescence, hole formation and shattering, as a function of the Weber number. It is found for the first time that the collision and reflexive separation can occur in the nanodroplet regime when the droplet diameter reaches 100 nm but not for 10 or 50 nm droplets. The size effect in droplet collisions is studied based on the analysis of stretching factors, energy dissipation and collision outcomes for droplets of different diameters. The kinetic energy dissipation due to the atomic interactions at nanoscales is identified to significantly influence the occurrence or otherwise of reflexive separation. Through quantitative analysis of the evolution of the internal structure of the 100 nm nanodroplets collision at the Weber number of 277, it is revealed for the first time that molecules from both parent nanodroplets have penetrated the full length of the merged nanodroplet in the direction of collision, due to a combination of molecular mixing and internal currents. Consequently, all three child nanodroplets have molecules from both parent nanodroplets, contrary to the perception gained from common imaging techniques. The results show that the dynamics, outcomes and mechanisms of nanodroplet collisions have both similarities and differences compared with their micro- and macro-counterparts