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Photonic crystal puts brakes on light

17 Jun 2002

NTT scientists in Japan have used a photonic crystal to slow 1550 nm light down to 1% of its normal speed.

Japanese researchers at the NTT Basic Research and Telecoms Energy Laboratories have created a photonic crystal slab that slows light down to one hundredth of its usual speed (Physical Review Letters 87 (253902)). By adding line-defects to the slab and creating a novel waveguide Masaya Notomi and colleagues are closer to realizing all-optical circuits.

The researchers first fabricated a two-dimensional silicon photonic crystal slab using electron-beam lithography and plasma etching. The slab contained an array of holes that were a fraction of a micron in size and created a wide photonic bandgap from 1.22 to 1.58 µm.

To manipulate the waveguiding properties of the slab, the researchers went on to create a range of slabs that contained a row of missing holes known as a line defect. By altering the width of the line defect, the researchers were able to change the 'wavelength window' within which light propagated through the line defect waveguides (LDWG). They also achieved single-mode waveguiding.

"We coupled 1200 to 1700 nm light [from a wide-band superluminescent diode] to a LDWG via a single-mode tapered fiber," explained Notomi. "We found that each LDWG had a clear waveguide mode with cut-offs, and the width and position of the waveguiding mode depends strongly on the line defect [width]."

Keen to investigate any light-decelerating effects, Notomi and his colleagues investigated the dispersion properties of the silicon LDWG. They found that the dispersion of light was very large compared to conventional waveguides and photonic crystal fibers. The researchers say that this indicates that the waveguide is slowing down the light.

"The speed of light propagation along a LDWG is up to 90 times slower than in air," said Notomi. "Another recent report shows that light velocity is greatly reduced with laser cooling. Although the basic physics is different, our results show another way to control braking light propagation."

The researchers believes that their waveguides enhance photon-matter interaction and will find use in device applications that need optical non-linearity and amplification. "We plan to make use of this unusual waveguide within ultra-small photonic-crystal-based integrated circuits in the future," added Notomi.

Rebecca Pool is news editor on Optics.org and Opto & Laser Europe magazine.

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