Focus: Bullets of Light
A new technique allows a “bullet” of light to maintain its shape in all three dimensions as it travels through a material. In the 29 August PRL, a team describes a light configuration that forms spontaneously from a laser pulse moving through a crystal. The technique could lead to improvements in microscopes and systems for carving microscale devices.
Compact pulses of light that are brief in time and small in size tend to spread out as they move through a medium like glass or biological tissue. A pulse that doesn’t spread–a light bullet–could be used to precisely focus a microscope on a small region in the center of a thick sample or to give lasers greater control in micromachining.
A light pulse spreads out along the direction it’s moving because it’s composed of many different frequencies (colors), and each one moves at a slightly different speed. As the components get out of sync, the pulse becomes more diffuse–an effect known as dispersion. Pulses also exhibit diffraction–spreading perpendicular to their propagation direction–just as a laser pointer’s beam gets wider on the way to a distant screen.
Researchers have attempted to create light bullets with two main approaches. The so-called non-linear techniques use materials where the speed of a light wave depends on its intensity: bright light travels more slowly than dim light. The speed difference causes the dimmer edges of a pulse to remain focused toward the brighter central portion, countering diffraction. And because the speed also depends on the color of each component, researchers can arrange for the slower colors to focus toward the front of the pulse, and the faster ones toward the rear, canceling dispersion. The result is a so-called soliton that keeps its shape over time. But solitons are difficult to create in three dimensions and often quickly collapse in on themselves.
One example of the other approach–linear techniques–is the linear X-shaped wave. This configuration is made of two cones of light pointing in opposite directions, making an X from a side view. The light is concentrated on the conical surface and at the point where the cones meet, but there’s no light inside the cones. This bizarre pulse shape can travel without spreading because different frequency components move at different angles in a way that cancels the diffraction. But canceling dispersion at the same time takes a complicated set-up and might be impossible in many cases.
Paolo Di Trapani of the University of Insubria in Como, Italy, and his colleagues, came up with a pulse shape that combines attributes of both solitons and linear X waves. They found that when a conventional laser pulse is launched into a lithium triborate crystal, the pulse spontaneously “reshapes” into what the team calls a nonlinear X wave. This X wave is similar to the linear variety, but it maintains its shape using the non-linear principles of a soliton. The arms of the X wave act as an energy reservoir that continually “refuels” the intense central spot, preventing collapse, says Di Trapani. He believes that these spontaneous X waves may appear often in other labs, but most are not equipped to observe them.
The result is “a significant step forward in the field,” says Lluis Torner of the Photonic Sciences Institute in Barcelona. Frank Wise, of Cornell University in Ithaca, New York, says that solitons could be used to send information in future ultrafast optical computers, so a soliton-like wave that works in three dimensions may be very useful.
Kim Krieger is a freelance science writer in Norwalk, Connecticut.