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Photonic crystal yields ultrafast all-optical switch

08 Dec 2006

Scientists at Singapore's Nanyang Technological University (NTU) talk to optics.org about the commercial opportunities for their prototype all-optical switch.

A team of researchers at NTU has used nanoscale photonic crystals to make an all-optical switch for the first time. Operating at the near-infrared region of the spectrum, the switch could have myriad applications in communication and consumer electronic devices.

In the past, MEMS mirrors have been used to realize optical switches, but such technology has speed limitations as it relies on moving mechanical components. A design based entirely on optics has the promise of achieving much higher switching rates - on the order of picoseconds.

Speaking to optics.org about the achievement, Dr Ai-Qun Liu of NTU said: "We have succeeded in developing the first ever all-optical switch based on the unique light absorption effects of photonic crystals, and are excited about the possibilities that this could have in the global ICT industry."

Photonic crystals are dielectrics that offer novel ways to control the flow of light. For this experiment, photonic-crystal rods were etched onto a layer of silicon -- which has a refractive index of 3.4 -- and packed in a dense, square lattice structure with a period of 100--200 nm. Each rod was 13 µm tall and had a diameter of between 100 and 230 nm.

The researchers then created a cross-shaped waveguide at the centre of the lattice by removing two perpendicular lines of photonic-crystal rods. The large difference in refractive index between the silicon rod and the surrounding air reflects the light along the rod's surface, causing it to propagate along the waveguide.

With this arrangement, a beam of light entering the photonic lattice along one of the waveguides can in theory be directed to flow out through any of the other three paths. This switching is achieved in practice by directing a pump laser beam over the cavity rod located at the centre of the lattice (see figure).

When high-energy photons from the pump laser are absorbed by the cavity rod, free electrons and holes are generated at the centre of lattice, which in turn alter the local refractive index and optical transmission properties. By precisely managing the intensity of the beam, the Singapore team could control the direction in which light exits from the cavity. Studies conducted by Canadian researchers have already demonstrated that free-charge carriers can be used to control the properties of photonic crystals (Phys. Rev. B 66 161102).

"Our main challenge was in the fabrication of high-aspect-ratio photonic-crystal structures with different radii," said Dr Liu. Structures with such high aspect ratios require specialized fabrication techniques, which has so far presented a major barrier to the exploitation of photonic-crystal devices. Dr Liu's team exploited deep-ultraviolet optical lithography and deep-reactive ion etching processes to fabricate their device.

An obvious application for the all-optical device lies in communications, as a modulator or high-speed switch, since its picosecond switching rate could push transmission speeds way beyond the limits of current-day microelectronics. "The working prototype has been demonstrated, and we are eager to collaborate with the industry to develop commercial applications for this technology," added Dr Liu. The team is now working on designs for new applications that use this nano-photonic bandgap circuit, which could even include an optical transistor.

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