Group sets record for optical delay with new nanocavity-array waveguide

Dec. 29, 2008
December 29, 2008--At NTT Basic Research Laboratories (Atsugi, Japan), scientists have developed a waveguide--an array of more than 100 photonic crystal cavities--capable of slowing light to one-hundredth its normal speed. The coupled optical microresonator shows promise for advanced signal processing and optical memory applications.

December 29, 2008--At NTT Basic Research Laboratories (Atsugi, Japan), scientists have developed an array of more than 100 photonic-crystal cavities capable of slowing light to one-hundredth its normal speed. The result, reported in Nature Photonics (2 741), is promising for applications in optical signal processing and optical memory / data storage.

The work of Masaya Notomi and his co-investigators represents "a noteworthy advance in photonic-crystal (PC) research," said Richard M. De La Rue, a member of the Optoelectronics Research Group, Department of Electronics and Electrical Engineering, University of Glasgow (Scotland, UK) in a commentary on the work. "The device is a specific example of a coupled-resonator optical waveguide (CROW), which has been investigated by various research groups. As measured by the large number of resonator elements used and the small size of the device footprint, the results of Notomi and co-workers are impressive."

"This is the first demonstration of large-scale coupled resonator arrays based on wavelength-sized cavities, in which tight-binding sinusoidal dispersion is seen," the researchers explain. "We confirm that an ultrahigh value of Q( 1 106) is maintained, even when N is large, and the resonators exhibit very low loss characteristics with regard to light propagation. The ultrahigh value of Q and small size has enabled us to achieve ultraslow light pulse propagation with a group velocity well below 0.01c and a long group delay."

The team has worked in the past with photonic crystals (periodically modulated nanofabricated dielectric structures) to produce high-Q coupled cavities, and has shown that single photonic crystal cavities can confine light for long periods. But the performance of real coupled cavities has been limited because it is difficult to assemble large arrays of very small, high-Q cavities. Light speed slows the more cavity size decreases and cavity Q increases.

The researchers' design is based on a uniform PC lattice with identical sections of uniform channel guide, which link identical resonator regions formed by small, but carefully designed and fabricated, regions of deformed PC lattice.

"We have now arrived at the start of the era of silicon photonics," says De La Rue, explaining that the technology borrows heavily from silicon electronics. Slow light propagation in silicon waveguide structures offers many advantages, among them compactness, acceptably small device insertion loss, and the possibility of compact gain structures integrated on-chip. But there are continuing challenges, he notes. The delay-line storage capacity achieved so far is modest when compared with the capabilities of optical fibre. Another issue is propagation loss.

De La Rue says that Notomi and co-workers have gone well below the benchmark group velocity of 0.01c (where c is the speed of light in a vacuum) while retaining good device performance. The structures have as many as 150 cavities coupled sequentially, and delays as large as 125 ps could be obtained.

The accessibility to different points on the device provides switchable and tunable delay, and this will probably be a key factor in the successful exploitation of CROW structures. "We should now expect the emergence of PC coupled-resonator delay lines with as many as 100 distinct resonators together with individual resonator control. If these structures can hold at least a byte of light pulses, such delay lines might provide an interesting direct manipulation capability for information streams that systems engineers will find useful. The technological challenges remain large, but the potential rewards are commensurate with these challenges," De La Rue explains.

Going forward, the team will work to optimize the coupled cavities and enlarge the bandwidth and delay by increasing the number of cavities. "At the same time, we will try to enhance light–matter interactions, theoretically expected in our slow light coupled cavities," said Notomi in an article by nanotechweb.org.

For more information, please see the paper, Large-scale arrays of ultrahigh-Q coupled nanocavities and De La Rue's commentary, Optical delays: Slower for longer, both in Nature Photonics; and the article at nanotechweb.org, Nanocavities put the brakes on light .

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