Focus: Small Laser Beam Blazes Long Path
A French team of physicists has, for the first time, made a hairline laser beam travel for long distances inside a solid. The researchers took advantage of an effect called self-guiding, in which a high-power laser beam travels hundreds of times farther than a ordinary one would. Their results, appearing in the 19 November print issue of PRL, show that self-guided pulses obey the same rules in solids as they do in gases, and may improve prospects for computers that use light instead of electric signals.
Under normal circumstances, the distance laser light travels without spreading depends on the beam’s diameter. This intrinsic property prevents most micrometer-sized beams from traveling even a few centimeters, but in the mid-1990s, physicists discovered that high-intensity laser pulses a few micrometers across could travel through air over surprisingly long distances. This effect, called “self-guiding,” is the result of two competing phenomena within the pulse. First, the pulse alters the atmosphere’s index of refraction–causing the air to act as a lens and focus the pulse’s light into a small area. Second, the focused light’s high intensity ionizes some of the air molecules, which lowers the index of refraction and causes the pulse to spread out. The balance between focusing and spreading allows a beam that would normally travel a few millimeters to propagate for hundreds of meters.
“The question was: Is there a similar phenomenon in solids?” says Stelios Tzortzakis of the National School of Advanced Techniques in Palaiseau, France. Because solids have an optical density thousands of times higher than air, it’s more difficult to induce self-guiding, he explains. The team had to carefully adjust the intensity and diameter of the 800 nm light in their beam to create self-guided, ultrashort (160 fs) pulses that would travel about a centimeter through a block of glass. They used a CCD camera and other tools to record the shape and duration of the pulses. They fitted these shapes with computer models and found that self-guiding in solids is almost identical to the process in air.
Learning how to control tiny pulses of laser light is a first step towards all-optical computers that would use light to perform computations, explains Alexander Gaeta of Cornell University. But, he adds, the real value of the work is in a more fundamental understanding of self-guided pulses.