Light's hidden holdup: reflected laser beams loiter a little.
"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.
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|Title Annotation:||This Week|
|Date:||Mar 19, 2005|
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