Scientists make movies of slow light

15 Mar 2005

A European collaboration watches light pulses crawl through a photonic crystal waveguide.

Scientists have demonstrated a new way of putting the brakes on light and have used a high-resolution imaging technique to directly observe the phenomenon (Physical Review Letters 94 073903).

Their experiments with a photonic crystal waveguide and a phase-sensitive near-field scanning optical microscope (PS-NSOM) could accelerate the development of devices such as all-optical memory chips and sensitive biosensors.

PC waveguide

“The big thing here is that you can actually see what is happening,” explained Thomas Krauss from the University of St Andrews. “We can produce images and movies with nanoscale resolution that show the propagation of the slowed light pulses in the waveguide.”

The work was performed under a partnership called the Ultrafast Photonics Collaboration and involved scientists from the University of Twente and FOM Institute for Atomic and Molecular Physics in Holland, Ghent University in Belgium, and the University of St Andrews in Scotland.

Tim Karle and Thomas Krauss from St Andrews designed the photonics crystal waveguide while Henkjan Gersen and Kobus Kuipers from the University of Twente and FOM built the PS-NSOM.

slow light

Although light has already been slowed down in semiconductors and gases, the braking has previously relied on an effect called electromagnetically induced transparency (EIT) which is difficult to observe directly and has its limitations.

The advantage of using a photonic crystal waveguide instead is that it offers integration with fibre optics and can be designed to suit a particular wavelength of interest.

“Although you slow light down much more using EIT it has two catches -- there’s residual absorption and it only works in a very narrow bandwidth,” explained Krauss. “In our case we simply rely on the structure of the waveguide to slow down the light and we potentially have terahertz bandwidth. We are now thinking of adding optical gain [into the waveguide] so that the effect is lossless.”

To date, the team have performed their tests at a wavelength of 1.3 µm with a silicon-on-insulator (Si on SiO₂) waveguide which was riddled with tiny holes just 260 nm in diameter. When 120 fs duration pulses were launched into the waveguide, images captured by the PS-NSOM showed that they were travelling at about 1/1000th of the speed of light in vacuum.

Author
Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.