Light's hidden holdup: reflected laser beams loiter a little.

Physicists in France have timed the tiny pause between the arrival of light at a reflective surface and its departure from that surface. Ever since Newton made the suggestion, theorists have been aware that incoming light slightly overshoots some surfaces before bouncing away, but no one could time that excursion, until now.

"We all think of [reflection] as instantaneous, but it takes a little time," says Albert Le Floch of the University of Rennes 1, leader of the team that made the measurement.

For light ricocheting inside a glass prism, the team found the delay to be either 28 millionths of a billionth of a second (femtosecond, fs) or 57 fs, depending on the pulse's polarization.

Not all reflective surfaces cause delays--only those that light can penetrate a bit before reflecting, Le Floch says. Such an excursion--for instance, at an optical fiber's glass-air boundary--costs the pulse the time required to cross and recross that border. The emerging light hugs the surface in this out-in process. Beams of particles, such as neutrons and electrons, probably undergo similar delays, the researchers say.

To measure how long light pulses loiter, the team split a 150-fs infrared laser pulse into two pulses and sent each along a different path to the same detector. In the path of one pulse was a glass prism in which the light reflected internally off one of the prism's faces before heading back out. The other pulse passed through a delay line that enabled the researchers to control when the pulse would reach the detector.

Initially, the experimenters coated the prism's face with mercury to create a mirrored, no-delay, reflective surface. Then, the team tuned the delay line so that light traveling its course would arrive at the detector at the same time as the mercury-deflected light pulse did. The detector would trip only when both pulses arrived simultaneously.

Next, the scientists cleared the mercury from the prism, thereby replacing a metallic reflecting surface with a glass-air interface. To reestablish the pulses' synchronous arrival at the detector, the scientists had to dial into the prismfree path a delay presumably equal to that introduced by the glass-air boundary.

"Elegant," says John B. Pendry of the Imperial College London of the new measurement. The technique could prove useful as a way to probe properties of certain exotic materials, the Rennes group proposes in the March 14 Physics Letters A. Those materials include photonic crystals (SN: 8/21/04, p. 125), which block and manipulate specific wavelengths of light.