Photonic crystal waveguide stops light in its tracks

Light, which normally travels at an enormous speed, has been slowed down by a factor of 300 by a scientific collaboration involving a St Andrews physics research group. The light travels so slowly, that it seems almost frozen into position.

 

This feat was achieved with a "photonic crystal waveguide". This is a microscopic slice of semiconductor material that has an array of tiny holes etched into it. There is a special track through these holes where light is free to travel. We would normally expect the light just to travel from the open end of this track on one side and to leave the structure at the other end of the track. However, the structure traps light through extreme interference effects.

Observing these effects requires the use of a new type of microscope, termed a "Phase Sensitive Near field Scanning Optical Microscope" (PS-NSOM) . This microscope can produce images and movies with a resolution on the nanoscale and allows us to directly observe the propagation of ultrashort light pulses in these exotic microstructures. It yields an unprecedented method of viewing the way lightwaves are slowed down and captured.

This work has recently been published in the prestigous journal "Physical Review Letters". The image above was the featured image in the 25 February issue. It shows images of a light pulse travelling through an 84 micrometre long photonic-crystal waveguide, using "time-resolved near-field microscopy". Each image is taken a (very) short time after the one above it in this diagram. The first four images show the lgiht pulse propagating unhindered through the waveguide. Behind this pulse is its wake; a long-lived optical field is observed captured in the first 25 µm of the waveguide, slowed down to the extent that its movement is not discernable.

Prof Thomas Krauss, who leads the St Andrews team working on this said "Reducing the speed of light is not only of interest as an ultimate challenge in fundamental science, it also opens exciting prospects for applications. If it is travelling more slowly, light can intreract longer with its environment, yielding increased light-matter interactions that are of benefit in a range of applications such as light emitters, optical switches and even  the ability to sense low concentrations of molecules. Also, the control of the timing of optical signals is crucial for managing the ever-increasing data flow in  the telecommunications network. We can even envisage using these structures as optical buffer and memory elements. Our slow-light photonic crystal structures are therefore expected to become crucial elements in the next generation of optical microchips."

The work has also been reported as a news item at Optics.org and PhysicsWeb. This work later featured in the "Newsline" Magazine of the Engineering and Physical Sciences Research Council.

Prof Krauss is pictured with research fellow Tim Karle looking at images of the structure obtained with their scanning electron microscope.

First posted BDS 15.3.05

update with Optics.org 23.3.05

and Physics Web 11.4.05

and Newsline 11.11.05