Optoelectronics: Tapering off for efficiency

A compact ‘on-silicon-chip’ laser has been developed by A*STAR researchers that boasts both excellent confinement of light for lasing and the ability to efficiently share the laser light with nearby components. Compact lasers small enough to be integrated on chips are in great demand for a diverse range of applications, including data communication and storage. Lasers made from a combination of silicon and semiconductors containing elements from the third and fifth columns in the periodic table (dubbed III–V silicon lasers) are particularly attractive as on-chip light sources. To be used in applications, such lasers must tightly confine light to maximize the lasing efficiency and should effectively share, or ‘couple’, light with optical waveguides — the optical equivalent of electrical wiring — located under the laser. Jing Pu and co-workers at the A*STAR Data Storage Institute have demonstrated a III–V silicon laser that meets both criteria ("Heterogeneously integrated III-V laser on thin SOI with compact optical vertical interconnect access"). Their structure realizes efficient lasing through the smart control of light — light is tightly confined to the III–V semiconductor layer in which lasing occurs. Furthermore, both laser ends are tapered to facilitate the coupling of light with underlying silicon waveguides. A new design for a compact on-chip laser showing the two tapered ends that allow light to be efficiently coupled with structures on the chip. (Image: A*STAR Data Storage Institute) “Our laser exhibits a high efficiency as well as efficient light coupling between the III–V semiconductor and silicon layers, which is the thinnest reported to date,” says Pu. The new structure is promising as an on-chip light source for current silicon photonics technology but also as a potential new integration platform. It improves on conventional fabrication procedures, in which components are made separately and then combined, and enables fully integrated optoelectronic systems to be fabricated that take up less space on a chip. “This new technology could replace the current approach of integrating a laser diode to an optical system through assembling and then bonding of components,” explains Pu. “The laser diodes can be fabricated exactly where they are needed, which will cut the manufacturing cost and reduce the size and weight of light sources by a factor of hundreds.” These advantages are very attractive for many applications, including next-generation high-density magnetic data storage, where laser diodes need to be integrated on writing heads that are smaller than 0.1 square millimeters. The team plans to improve the manufacturing process and device performance so that the technology can advance from prototype to manufacture for industrial applications. “We also aim to reduce the laser size and power consumption for use as vital components for high-performance computing,” Pu adds.

read more "Optoelectronics: Tapering off for efficiency"

Batteries: Power of marine inspiration

A*STAR scientists have drawn on nature for a breakthrough that significantly enhances the electrochemical performance of lithium-ion batteries. The researchers have developed hierarchical porous carbon spheres to be used as anodes after being inspired by the templated formation of unicellular algae or ‘diatoms’ ("Bioinspired Synthesis of Hierarchical Porous Graphitic Carbon Spheres with Outstanding High-Rate Performance in Lithium-Ion Batteries"). A transmission electron microscopy image of graphitic carbon spheres with a hierarchical pore structure. Inset: A microscopy image of a marine diatom. (© American Chemical Society) “In nature, a great number of microorganisms, like diatoms, can assemble biominerals into intricate hierarchical three-dimensional architectures with great structural control over nano- to millimeter length scales,” explains Xu Li, who heads the research team at the A*STAR Institute of Materials Research and Engineering. “These organisms contain organic macromolecules, which can be used as templates to induce and direct the precise precipitation of silica building blocks to form the complex structures.” This natural phenomenon inspired Li and colleagues to develop biomimetic strategies based on self-assembled molecular templates to produce hierarchical carbon materials for use as anodic components of batteries. These materials contain mesopores, which form an interconnected network of channels within the carbon spheres, and have a microporous surface (see image). These three-dimensional features promote ion transport and high storage capacity within the carbon spheres. Li and the team used organic macromolecules, an aggregate of polymers and cobalt-containing molecules, as templates to make the interconnected mesopores — in a similar way that diatoms create their siliceous structure. The carbon scaffold of the spheres is derived from rings of sugar molecules, which thread on to the pendant polymer chains and form ‘soft’ carbon spheres after hydrothermal treatment. Pyrolysis causes a cobalt species to catalyze the graphitization process, creating the ‘hard’ carbon spheres. If urea is added before pyrolysis, nitrogen-doped graphitic carbon spheres are made. “The carbon spheres can only be prepared on a laboratory scale, however, we are optimizing the synthetic conditions to scale up fabrication,” says Li. Next, Li and co-workers tested the carbon spheres as anodes in lithium-ion batteries. The batteries showed high reversible capacity, good cycling stability and outstanding high-rate performance. Even when the current density is increased 600-fold, 57 per cent of the original capacity is retained. The nitrogen-doped carbon spheres have a higher reversible capacity because of more facile transport of ions and electrons within the doped carbon spheres. “These results are among the best output to date compared with pure carbon materials,” says Li. “We envisage that batteries composed of these anode materials could be charged faster than those fabricated using conventional carbon materials,” he adds. The next stage of the research is to extend the application of these materials to other energy storage or conversion systems, and other electrochemical applications, such as electrocatalysis.

read more "Batteries: Power of marine inspiration"

Building materials one atomic layer at a time aids the search for exotic phases of matter

A new technique for identifying exotic states of matter in crystalline materials has been demonstrated by RIKEN researchers. The electrons in some crystalline materials work together to create a host of unusual states, or phases. This collective behavior in these so-called correlated electron materials occurs because the electrons interact with each other via an intrinsic property known as spin, which is related to the rotation of the electrons about their axes. Although correlated electron materials have attracted much interest in the past decade, it has proved difficult to identify the exact mechanisms that give rise to specific phases and to determine what drives a material to switch from one phase to another. Now, Jobu Matsuno from the RIKEN Advanced Science Institute and his colleagues have investigated these mechanisms in a class of materials called iridates ("Engineering a spin-orbital magnetic insulator by tailoring superlattices"). These materials are interesting because their behavior is predominantly governed by two effects that are roughly equal in magnitude: the repulsive Coulomb force between electrons arising from their electric charge and the spin–orbit interaction, which arises because the spin of an electron interacts with its orbital motion. Theoretical analysis indicates that competition or cooperation between these two effects gives rise to a number of exotic phases in iridates. Changing the number of atomic layers of strontium iridate (red) sandwiched between single layers of strontium titanate (blue) in a superlattice offers a means of controlling the phase of the material. (© American Physical Society) To explore the influence of the spin–orbit interaction on the formation of these phases, Matsuno and his team created a series of iridate-based samples and measured their resistivity and magnetism. Each sample had a repeating structure consisting of one, two, three or four atomic layers of strontium iridate and a single layer of strontium titanate; the researchers also measured a sample without strontium titanate (Fig. 1). “This superlattice technique allows us to control these iridates,” says Matsuno, “and we can thus realize exotic phenomena.” The scientists found that the magnetic ordering temperature and the resistivity decreased with increasing number of strontium iridate layers. They also discovered that, in the sample containing three atomic layers of strontium iridate, a transition from a semimetal phase to an insulating state was closely linked to the appearance of magnetism. These results indicate the potential for finding unusual states of matter using the superlattice approach. The team hopes to use the method to identify a phase known as a topological insulator — a recently discovered class of materials that usually have a large spin–orbit interaction. “Theorists say that some iridates might host an even more exotic state called a topological Mott insulator,” Matsuno notes.

read more "Building materials one atomic layer at a time aids the search for exotic phases of matter"

Building materials one atomic layer at a time aids the search for exotic phases of matter

A new technique for identifying exotic states of matter in crystalline materials has been demonstrated by RIKEN researchers. The electrons in some crystalline materials work together to create a host of unusual states, or phases. This collective behavior in these so-called correlated electron materials occurs because the electrons interact with each other via an intrinsic property known as spin, which is related to the rotation of the electrons about their axes. Although correlated electron materials have attracted much interest in the past decade, it has proved difficult to identify the exact mechanisms that give rise to specific phases and to determine what drives a material to switch from one phase to another. Now, Jobu Matsuno from the RIKEN Advanced Science Institute and his colleagues have investigated these mechanisms in a class of materials called iridates ("Engineering a spin-orbital magnetic insulator by tailoring superlattices"). These materials are interesting because their behavior is predominantly governed by two effects that are roughly equal in magnitude: the repulsive Coulomb force between electrons arising from their electric charge and the spin–orbit interaction, which arises because the spin of an electron interacts with its orbital motion. Theoretical analysis indicates that competition or cooperation between these two effects gives rise to a number of exotic phases in iridates. Changing the number of atomic layers of strontium iridate (red) sandwiched between single layers of strontium titanate (blue) in a superlattice offers a means of controlling the phase of the material. (© American Physical Society) To explore the influence of the spin–orbit interaction on the formation of these phases, Matsuno and his team created a series of iridate-based samples and measured their resistivity and magnetism. Each sample had a repeating structure consisting of one, two, three or four atomic layers of strontium iridate and a single layer of strontium titanate; the researchers also measured a sample without strontium titanate (Fig. 1). “This superlattice technique allows us to control these iridates,” says Matsuno, “and we can thus realize exotic phenomena.” The scientists found that the magnetic ordering temperature and the resistivity decreased with increasing number of strontium iridate layers. They also discovered that, in the sample containing three atomic layers of strontium iridate, a transition from a semimetal phase to an insulating state was closely linked to the appearance of magnetism. These results indicate the potential for finding unusual states of matter using the superlattice approach. The team hopes to use the method to identify a phase known as a topological insulator — a recently discovered class of materials that usually have a large spin–orbit interaction. “Theorists say that some iridates might host an even more exotic state called a topological Mott insulator,” Matsuno notes.

read more "Building materials one atomic layer at a time aids the search for exotic phases of matter"

Batteries: Power of marine inspiration

A*STAR scientists have drawn on nature for a breakthrough that significantly enhances the electrochemical performance of lithium-ion batteries. The researchers have developed hierarchical porous carbon spheres to be used as anodes after being inspired by the templated formation of unicellular algae or ‘diatoms’ ("Bioinspired Synthesis of Hierarchical Porous Graphitic Carbon Spheres with Outstanding High-Rate Performance in Lithium-Ion Batteries"). A transmission electron microscopy image of graphitic carbon spheres with a hierarchical pore structure. Inset: A microscopy image of a marine diatom. (© American Chemical Society) “In nature, a great number of microorganisms, like diatoms, can assemble biominerals into intricate hierarchical three-dimensional architectures with great structural control over nano- to millimeter length scales,” explains Xu Li, who heads the research team at the A*STAR Institute of Materials Research and Engineering. “These organisms contain organic macromolecules, which can be used as templates to induce and direct the precise precipitation of silica building blocks to form the complex structures.” This natural phenomenon inspired Li and colleagues to develop biomimetic strategies based on self-assembled molecular templates to produce hierarchical carbon materials for use as anodic components of batteries. These materials contain mesopores, which form an interconnected network of channels within the carbon spheres, and have a microporous surface (see image). These three-dimensional features promote ion transport and high storage capacity within the carbon spheres. Li and the team used organic macromolecules, an aggregate of polymers and cobalt-containing molecules, as templates to make the interconnected mesopores — in a similar way that diatoms create their siliceous structure. The carbon scaffold of the spheres is derived from rings of sugar molecules, which thread on to the pendant polymer chains and form ‘soft’ carbon spheres after hydrothermal treatment. Pyrolysis causes a cobalt species to catalyze the graphitization process, creating the ‘hard’ carbon spheres. If urea is added before pyrolysis, nitrogen-doped graphitic carbon spheres are made. “The carbon spheres can only be prepared on a laboratory scale, however, we are optimizing the synthetic conditions to scale up fabrication,” says Li. Next, Li and co-workers tested the carbon spheres as anodes in lithium-ion batteries. The batteries showed high reversible capacity, good cycling stability and outstanding high-rate performance. Even when the current density is increased 600-fold, 57 per cent of the original capacity is retained. The nitrogen-doped carbon spheres have a higher reversible capacity because of more facile transport of ions and electrons within the doped carbon spheres. “These results are among the best output to date compared with pure carbon materials,” says Li. “We envisage that batteries composed of these anode materials could be charged faster than those fabricated using conventional carbon materials,” he adds. The next stage of the research is to extend the application of these materials to other energy storage or conversion systems, and other electrochemical applications, such as electrocatalysis.

read more "Batteries: Power of marine inspiration"

Optoelectronics: Tapering off for efficiency

A compact ‘on-silicon-chip’ laser has been developed by A*STAR researchers that boasts both excellent confinement of light for lasing and the ability to efficiently share the laser light with nearby components. Compact lasers small enough to be integrated on chips are in great demand for a diverse range of applications, including data communication and storage. Lasers made from a combination of silicon and semiconductors containing elements from the third and fifth columns in the periodic table (dubbed III–V silicon lasers) are particularly attractive as on-chip light sources. To be used in applications, such lasers must tightly confine light to maximize the lasing efficiency and should effectively share, or ‘couple’, light with optical waveguides — the optical equivalent of electrical wiring — located under the laser. Jing Pu and co-workers at the A*STAR Data Storage Institute have demonstrated a III–V silicon laser that meets both criteria ("Heterogeneously integrated III-V laser on thin SOI with compact optical vertical interconnect access"). Their structure realizes efficient lasing through the smart control of light — light is tightly confined to the III–V semiconductor layer in which lasing occurs. Furthermore, both laser ends are tapered to facilitate the coupling of light with underlying silicon waveguides. A new design for a compact on-chip laser showing the two tapered ends that allow light to be efficiently coupled with structures on the chip. (Image: A*STAR Data Storage Institute) “Our laser exhibits a high efficiency as well as efficient light coupling between the III–V semiconductor and silicon layers, which is the thinnest reported to date,” says Pu. The new structure is promising as an on-chip light source for current silicon photonics technology but also as a potential new integration platform. It improves on conventional fabrication procedures, in which components are made separately and then combined, and enables fully integrated optoelectronic systems to be fabricated that take up less space on a chip. “This new technology could replace the current approach of integrating a laser diode to an optical system through assembling and then bonding of components,” explains Pu. “The laser diodes can be fabricated exactly where they are needed, which will cut the manufacturing cost and reduce the size and weight of light sources by a factor of hundreds.” These advantages are very attractive for many applications, including next-generation high-density magnetic data storage, where laser diodes need to be integrated on writing heads that are smaller than 0.1 square millimeters. The team plans to improve the manufacturing process and device performance so that the technology can advance from prototype to manufacture for industrial applications. “We also aim to reduce the laser size and power consumption for use as vital components for high-performance computing,” Pu adds.

read more "Optoelectronics: Tapering off for efficiency"