The speed of light under pressure

Measuring how fast light travels is not so easy, but inside opaque materials, like clouds, bone, skin, or paint the problem is particularly daunting. AMOLF researcher Faez et. al in a recent issue of Physical Review Letters have teased out this rate of transport using a simple, yet novel, trick: changing the ambient pressure.

As the pressure is slowly tuned so is the so-called effective refractive index, which determines the speed of light. Even though the light is following an extremely complex path as it bounces through the material, its speed can be characterized by tracking the influence of pressure on the outgoing light intensity pattern. So simple and direct is the technique that it offers an entirely new way for probing inside important biological materials such as bone or wood as well as complex photonic materials such as photonic crystals or metamaterials.

Ref: Faez S., Johnson P. M., Lagendijk A. 2009. Varying the effective refractive index to measure optical transport in random media. Phys. Rev. Lett. 103, 053903,

For more information, please contact Sanli Faez. Tel. no. (020-6081234.

Shining laser through a composite material, in this case a plastic air filter, produces a speckle pattern with areas of high (red) and low (blue) brightness. AMOLF scientists have worked out how fast light travels through such a material by varying pressure on it and measuring changes in the speckles.