Beetle beauty captured in silicon

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Nov 20, 2010

Researchers in Canada have created a new material that mimics the brilliant iridescent colours seen in beetle shells. As the eye-catching effect can be switched off with the simple addition of water, the researchers believe their new material could lead to applications including "smart windows".

Structural colours, such as those on beetle shells and butterfly wings, differ from traditional pigments because the colour results from the interaction of light with periodic structures on the surface of the material. In certain biological materials, including the shells of scarab beetles, these exoskeletons take on a twisted or "chiral" structure, which causes reflected light to emerge circularly polarized.

Kevin Shopsowitz, working with colleagues at the University of British Columbia and FPInnovations, has now succeeded in mimicking this effect in a silica film. The breakthrough occurred with a certain degree of serendipity as the researchers were working with their industrial partner to develop forms of porous silica that could be used to store gases such as hydrogen. They were using nanocrystalline cellulose (NCC) as a template in silicon, which was then burned away to leave gaps within the silica.

But when Shopsowitz had forged the material, he discovered that is appeared to be iridescent. Analyzing the material with polarized optical microscopy (POM) revealed that the surface of the silica film had taken on a fingerprint-like texture during evaporation, with its associated spiralling pattern. Further analysis using transmission electron microscopy (TEM) confirmed that the individual nanocrystalline cellulose rods had organised into a "chiral nematic" structure.

"The eureka moment occurred when Kevin [Shopsowitz] discovered that the materials were iridescent," Mark MacLachlan, one of the researchers at the University of Columbia, told physicsworld.com. "Although NCC by itself forms iridescent films, we never thought it could be retained in the silica material."

Silica is usually a colourless material but modifying the surface in this way caused these films to reflect light at specific wavelengths. The researchers demonstrated that by changing the conditions of the synthesis, they could control how tightly wound the helix is (the pitch) and hence the wavelength of light that is reflected. In this way, they produced films that were a range of different colours.

What is more, Shopsowitz's team show that the iridescence can be turned off by the simple addition of water, before returning again when the material is dried out. They claim that this ability to switch between iridescent and colourless films, combined with the ability to control the pitch of the spirals, could be used to develop smart windows that respond to environmental conditions.

"It's fascinating research inspired by bio-mimetics," says Nicholas Roberts, a biologist at the University of Bristol, who specializes in neurobiology and sensory systems in nature. Roberts notes that liquid crystal chiral structures have been known for over 100 years and the similarities between cholesteric liquid crystals and beetle cuticles where noticed in the 1920s. "However, cholesteric liquid crystals are ordered fluids and the innovation here is to get the same self assembled structure be locked into something solid," he says.

The evolutionary significance of this ability of beetles is still not fully established. Writing in an article for the print edition of Physics World in August, zoologist David Pye of the University of London, UK, believes that – in the case of scarab beetles – it could be a tactic for improving communication within the species. It is widely accepted that these beetles take on bright colours to camouflage themselves within their forest environment: green for leaft backgrounds and metallic colours to imitate dappled sunlight. But if the eyes of these beetles have evolved to see polarized light, this would provide a system for these creatures to break the camouflage while remaining hidden.

This latest research is described in a paper in this week's Nature.

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Optical transistor in silicon is a first

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Nov 17, 2010

Researchers claim to have fabricated the first all-optical transistor on a silicon chip. This device allows the transmission of light emitted by one laser to be governed by the intensity of another.

This novel transistor was made by researchers at EPFL in Lausanne, Switzerland, and the Max Planck Institute for Quantum Optics in Garching, Germany. According to the team, the device promises to provide another building block for constructing all-optical integrated circuits. Such circuits could dramatically improve the efficiency of telecommunication networks because they would eliminate the need to convert optical information to electrical pulses – which can be processed easily – and then back to light.

The team employed standard nanofabrication methods to make the transistor, a taper consisting of a silicon dioxide disc with a rimmed edge sitting on a silicon pillar. The ability to make devices in silicon is important because the material is widely used in the electronics industry.

To operate the device, the frequency of one laser beam (the "probe") is tuned to an optical resonance of the silicon dioxide structure. The result is that the structure behaves like an optical cavity, with the incident light bouncing endlessly around its rim. "No light is transmitted through the taper since all the light is lost in the optical mode of the cavity," explains Tobias Kippenberg from EPFL.

A second "control" beam at a different frequency is then directed at the taper. Interaction between the two beams results in a beat frequency that also resonates with the disc and creates a mechanical oscillation. Interference between these three light fields results in the cancellation of the probe beam within the cavity.

"The presence of the control beam allows the probe beam to be transmitted through the taper as if it was not coupled anymore to an optical cavity. This is the optomechanically induced transparency effect," explains Kippenberg.

Cranking up the intensity of the control beam increases transmission of the light from the probe laser through the structure, but it is impossible to realize complete transmission – this would require an infinitely powerful control laser.

Kippenberg and his colleagues selected silicon dioxide for building their tapers, because this material combines very high transparency with very low optical losses.

"However, the optomechanically induced transparency effect can be realized in various optomechanical platforms that have been developed in recent years, based on many different materials such as silicon nitride and calcium fluoride."

Kippenberg believes that the optomechanically induced transparency effect might be able to control the quantum state of the transistor. "This would be a very important step towards the realization of quantum experiments on large-scale objects and tests of decoherence on unprecedentedly large systems."

The next goal for the team is to cool the mechanical oscillator into its quantum ground state using the optomechanical interaction. Kippenberg says that this will be a first step towards the preparation and control of a mesoscopic object in various quantum states.

The team's all-optical device joins a growing band of variants on the conventional electronic transistor, including one that converts an electrical input into an electrical and a laser output. Co-inventor of this "transistor laser", Milton Feng from the University of Illinois Urbana Champaign, is not particularly impressed by the all-optical variant built by the European team: "It is science, but it will never make it in the real world of integrated circuits like the semiconductor transistor did."

The research is reported at Science DOI: 10.1126/science.1195596.

Richard Stevenson is a freelance science and technology journalist based in Chepstow, Wales

View the original article here

Optical transistor in silicon is a first

To enjoy free access to all high-quality "In depth" content, including topical features, reviews and opinion sign up

Nov 17, 2010

Researchers claim to have fabricated the first all-optical transistor on a silicon chip. This device allows the transmission of light emitted by one laser to be governed by the intensity of another.

This novel transistor was made by researchers at EPFL in Lausanne, Switzerland, and the Max Planck Institute for Quantum Optics in Garching, Germany. According to the team, the device promises to provide another building block for constructing all-optical integrated circuits. Such circuits could dramatically improve the efficiency of telecommunication networks because they would eliminate the need to convert optical information to electrical pulses – which can be processed easily – and then back to light.

The team employed standard nanofabrication methods to make the transistor, a taper consisting of a silicon dioxide disc with a rimmed edge sitting on a silicon pillar. The ability to make devices in silicon is important because the material is widely used in the electronics industry.

To operate the device, the frequency of one laser beam (the "probe") is tuned to an optical resonance of the silicon dioxide structure. The result is that the structure behaves like an optical cavity, with the incident light bouncing endlessly around its rim. "No light is transmitted through the taper since all the light is lost in the optical mode of the cavity," explains Tobias Kippenberg from EPFL.

A second "control" beam at a different frequency is then directed at the taper. Interaction between the two beams results in a beat frequency that also resonates with the disc and creates a mechanical oscillation. Interference between these three light fields results in the cancellation of the probe beam within the cavity.

"The presence of the control beam allows the probe beam to be transmitted through the taper as if it was not coupled anymore to an optical cavity. This is the optomechanically induced transparency effect," explains Kippenberg.

Cranking up the intensity of the control beam increases transmission of the light from the probe laser through the structure, but it is impossible to realize complete transmission – this would require an infinitely powerful control laser.

Kippenberg and his colleagues selected silicon dioxide for building their tapers, because this material combines very high transparency with very low optical losses.

"However, the optomechanically induced transparency effect can be realized in various optomechanical platforms that have been developed in recent years, based on many different materials such as silicon nitride and calcium fluoride."

Kippenberg believes that the optomechanically induced transparency effect might be able to control the quantum state of the transistor. "This would be a very important step towards the realization of quantum experiments on large-scale objects and tests of decoherence on unprecedentedly large systems."

The next goal for the team is to cool the mechanical oscillator into its quantum ground state using the optomechanical interaction. Kippenberg says that this will be a first step towards the preparation and control of a mesoscopic object in various quantum states.

The team's all-optical device joins a growing band of variants on the conventional electronic transistor, including one that converts an electrical input into an electrical and a laser output. Co-inventor of this "transistor laser", Milton Feng from the University of Illinois Urbana Champaign, is not particularly impressed by the all-optical variant built by the European team: "It is science, but it will never make it in the real world of integrated circuits like the semiconductor transistor did."

The research is reported at Science DOI: 10.1126/science.1195596.

Richard Stevenson is a freelance science and technology journalist based in Chepstow, Wales

View the original article here