Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Ju Mei,†,‡,∥ Nelson L. C. Leung,†,‡,∥ Ryan T. K. Kwok,†,‡ Jacky W. Y. Lam,†,‡ and Ben Zhong Tang*,†,‡,§ †HKUST-Shenzhen Research Institute, Hi-Tech Park, Nanshan, Shenzhen 518057, China ‡Department of Chemistry, HKUST Jockey Club Institute for Advanced Study, Institute of Molecular Functional Materials, Division of Biomedical Engineering, State Key Laboratory of Molecular Neuroscience, Division of Life Science, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China

Aggregation-Induced Emission: Together We Shine, United We Soar!

Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

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Light-emitting diodes

One-dimensional (1D) ZnO nanostructures have been studied intensively and extensively over the last decade not only for their remarkable chemical and physical properties, but also for their current and future diverse technological applications. This article gives a comprehensive overview of the progress that has been made within the context of 1D ZnO nanostructures synthesized via wet chemical methods. We will cover the synthetic methodologies and corresponding growth mechanisms, different structures, doping and alloying, position-controlled growth on substrates, and finally, their functional properties as catalysts, hydrophobic surfaces, sensors, and in nanoelectronic, optical, optoelectronic, and energy harvesting devices.

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One-dimensional ZnO nanostructures: Solution growth and functional properties

The trend towards miniaturization in optical imaging, diagnostics and lab-on-a-chip technology is creating a demand for sophisticated microlenses. A new type of smart liquid microlens has been developed that differs from most current devices in that it is self-focusing. The key component is a stimuli-responsive hydrogel integrated into a microfluidic system and acting as a container for a liquid droplet. The hydrogel simultaneously senses stimuli and actuates a change in droplet shape — and hence focal length. Stimuli can include biological and chemical agents and physical parameters. At this micrometre scale, pinned liquid-liquid interfaces are used to attain stable devices, and response times of ten to a few tens of seconds. Lenses can have virtually any focal length and are readily integrated into arrays. Despite its compactness, the human eye can easily focus on different distances by adjusting the shape of its lens with the help of ciliary muscles1. In contrast, traditional man-made optical systems achieve focusing by physical displacement of the lenses used. But in recent years, advances in miniaturization technology have led to optical systems that no longer require complicated mechanical systems to tune and adjust optical performance. These systems have found wide use in photonics, displays and biomedical systems. They are either based on arrays of microlenses with fixed focal lengths2,3,4,5, or use external control to adjust the microlens focal length6,7,8,9,10,11,12. An intriguing example is the tunable liquid lens, where electrowetting or external pressure manipulates the shape of a liquid droplet and thereby adjusts its optical properties. Here we demonstrate a liquid lens system that allows for autonomous focusing. The central component is a stimuli-responsive hydrogel13 integrated into a microfluidic system and serving as the container for a liquid droplet, with the hydrogel simultaneously sensing the presence of stimuli and actuating adjustments to the shape—and hence focal length—of the droplet. By working at the micrometre scale where ionic diffusion and surface tension scale favourably14, we can use pinned liquid–liquid interfaces to obtain stable devices and realize response times of ten to a few tens of seconds. The microlenses, which can have a focal length ranging from -∞ to +∞ (divergent and convergent), are also readily integrated into arrays that may find use in applications such as sensing, medical diagnostics and lab-on-a-chip technologies15,16,17,18,19.

Adaptive liquid microlenses activated by stimuli-responsive hydrogels

Micro-light emitting diode (LED) arrays with diameters of 4 to 20 mum have been fabricated and were found to be much more efficient light emitters compared to their broad-area counterparts, with up to five times enhancement in optical power densities. The possible mechanisms responsible for the improvement in performance were investigated. Strain relaxation in the microstructures as measured by Raman spectroscopy was not observed, arguing against theories of an increase in internal quantum efficiency due to a reduction of the piezoelectric field put forward by other groups. Optical microscope images show intense light emission at the periphery of the devices, as a result of light scattering off the etched sidewalls. This increases the extraction efficiency relative to broad area devices and boosts the forward optical output. In addition, spectra of the forward emitted light reveal the presence of resonant cavity modes [whispering gallery (WG) modes in particular] which appear to play a role in enhancing the optical output.

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Mechanism of enhanced light output efficiency in InGaN-based microlight emitting diodes

GaN micro-light-emitting diodes (micro-LEDs) with monolithically integrated microlenses have been demonstrated. Microlenses, with a focal length of 44 μm and a root mean square roughness of ∼1 nm, have been fabricated on the polished back surface of a sapphire substrate of an array of micro-LEDs by resist thermal reflow and plasma etching. The optical properties of the microlenses have been demonstrated to alter the emission pattern of the LED emitters. The cone of light emitted from this hybrid device is significantly less divergent than a conventional broad-area device. This combination of micro-LED and microlens technologies offers the potential for further improvement in the overall efficiency of GaN-based light emitters.

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GaN micro-light-emitting diode arrays with monolithically integrated sapphire microlenses

Arrays of pivoted GaN microdisks have been fabricated on a GaN∕Si material by a combination of dry and wet etching. The Si material beneath the GaN microdisks is removed by wet etching, leaving behind a fine pillar to support the disks. Raman spectroscopy reveals substantial strain relaxation in these structures. Resonant modes, corresponding to whispering gallery modes, are observed in the photoluminescence spectra. Stimulated emission is achieved at higher optical pumping intensities.

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Lasing in GaN microdisks pivoted on Si

The characteristics of monolithically integrated light-emitting diodes (LEDs), photodetectors (PDs), and waveguides on a GaN-on-Si wafer are investigated. The InGaN/GaN multi-quantum wells, which are responsible for blue light emission in LEDs, are also used for photodetection in PDs. Despite the Stokes shift, a spectral overlap of ∼25  nm between the emission and absorption spectra provides the PDs with sufficient photosensitivity to signals from the emitter while remaining insensitive to ambient lighting. Optical interconnects in the form of linear or bent suspended waveguides bridging the LEDs and PDs are formed by selective detachment of etched GaN mesas from the Si substrate. Additionally, the PDs can be detached from the substrate and remounted on an elevated platform, owing to the flexibility of the thin-film waveguide. The 150  μm×150  μm LEDs and PDs exhibit rapid response on nanosecond time scales, which is attributed to fast radiative recombinations as well as minimized resistive-capacitive (RC) delays, enabling transmission of pseudorandom binary sequence (PRBS) data signals at rates of 250 Mb/s with an opening in the eye diagram. Together with multichannel transmission free of crosstalk, the ability of the planar and three-dimensional monolithic photonic systems to handle visible-light communication (VLC) applications is demonstrated.

Monolithically Integrated InGaN/GaN Light-emitting Diodes, Photodetectors and Waveguides on Si Substrate

Light-emitting diodes (LEDs) based on an interconnected array of GaN/InGaN micro-ring elements have been demonstrated. The devices have electrical characteristics similar to those of conventional broad-area devices. However, due to the large surface areas provided by the sidewalls, the extraction efficiency is greatly enhanced. Intense light emission at the periphery of the micro-rings is observed upon excitation by an electron beam, suggesting scattering of the photons which are extracted through the sidewalls. The devices provide a doubling in total light output compared to a broad-area reference LED of equal light-generation area.

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Hoi Wai Choi

Papers

Mechanism of enhanced light output efficiency in InGaN-based microlight emitting diodes

GaN micro-light-emitting diode arrays with monolithically integrated sapphire microlenses

Lasing in GaN microdisks pivoted on Si

Monolithically Integrated InGaN/GaN Light-emitting Diodes, Photodetectors and Waveguides on Si Substrate

High extraction efficiency InGaN micro-ring light-emitting diodes