Focus: Pulsating Pulses
Keeping an intense pulse of light from diverging or focusing to a point as it travels through a material can be tricky. In the 21 July PRL, researchers demonstrate how a laser pulse passing through a series of flat, thin glass plates and air gaps alternately contracts and expands, maintaining roughly the same average width as it proceeds. The experiment is the first demonstration of an idea proposed a decade ago and might lead to techniques for optical signal transmission and switching. Similar principles might also be used to control Bose-Einstein condensates–ultracold clouds of atoms maintained in a unique quantum state.
Usually, the refractive index of a material is a fixed property that tells you how a light ray will bend when it enters or leaves the material. But extremely intense light can cause the index to change, so that rays bend and focus inside the material. For ordinary glass to show this so-called nonlinear effect, light intensities of around 1,000 terawatts per square centimeter are needed. A beam of that power in glass will focus to a point in a runaway fashion–the more the beam narrows, the greater its intensity and therefore the focusing induced by the nonlinearity. But in practice, explains Martin Centurion of the California Institute of Technology in Pasadena, the light becomes so intense that it creates a filament of hot plasma–detached atoms and electrons–putting a stop to further focusing.
Over the past decade, theorists have proposed ways of manipulating nonlinearity to control intense light pulses and other analogous systems, such as the ultracold gases called Bose-Einstein condensates or BECs. For light, “nonlinearity management” would rely on a series of alternating media with different properties, so that the traveling pulse would focus, then spread, then focus again, and never collapse into a plasma. Stationary BECs, by contrast, can collapse or explode because of nonlinear attractive or repulsive forces among the atoms. Nonlinearity management would mean applying an alternating magnetic field to make the atoms alternate between expansion and contraction, preserving the cloud indefinitely.
Although the theory has not been controversial, no one has demonstrated that nonlinearity management works. To show it in a simple experiment, Centurion and his colleagues set up a sequence of plain glass microscope slides, each 1 millimeter thick, separated by 1-millimeter air gaps. Through this arrangement the researchers blasted laser pulses delivering over 10 gigawatts within a diameter of less than 50 microns. Each pulse shrank significantly as it passed through the glass slides but then spread out in the air gaps, so its width “breathed” in and out as it moved through the layers.
The pulse width oscillations agreed reasonably well with the team’s theoretical analysis of this system, says Caltech team member Mason Porter. Because energy is lost at each glass-air boundary, the maximum traverse in these experiments included nine glass slides, with pulses losing about half their power along the way.
Although this demonstration of the technique is exciting, says Frank Wise of Cornell University in Ithaca, New York, “it only really gets interesting if the resulting propagation can be stable indefinitely,” with hardly any pulse energy lost at the air-glass interfaces. Centurion responds that their experiment was a “simple demonstration with readily available materials,” and says that they have already obtained better results using glass slides with a non-reflective coating that reduces energy loss. Custom-made materials could have lower losses still, he adds, and also could work at lower intensities. Such technology might one day allow the manipulation of “light bullets” that could be transmitted and switched faster than electronics, Centurion says.
David Lindley is a freelance writer in Alexandria, Virginia, and author of Uncertainty: Einstein, Heisenberg, Bohr, and the Struggle for the Soul of Science (Doubleday, 2007).