Hercules' Power Increased

Thanks to an additional amplifier, a laser at the University of Michigan can now produce a concentrated beam of light so intense it is like holding a giant magnifying glass in space and focusing all the sunlight shining toward Earth onto one grain of sand.

“That’s the instantaneous intensity we can produce,” with the Hercules (high-energy repetitive CUOS laser system) titanium:sapphire laser, said Karl Krushelnick, a physics and engineering professor at the University of Michigan (U-M) and associate director of its Center for Ultrafast Optical Science (CUOS). “I don’t know of another place in the universe that would have this intensity of light. We believe this is a record.”

The pulsed laser beam cannot be seen by the naked eye, as it lasts just 30 femtoseconds. It takes the human eye one-thirtieth of a second to react to light; a femtosecond is a millionth of a billionth of a second.

The new amplifier of the Hercules laser fires. The laser is now capable of producing a beam so intense University of Michigan scientists believe it sets a record. (Photo courtesy Anatoly Maksimchuk, associate research scientist, U-M Department of Electrical Engineering and Computer Science)

Such intense beams could help scientists develop better proton and electron beams for radiation treatment of cancer, among other biomedical applications.

The record-setting beam measures 20 billion trillion watts per square centimeter. It contains 300 terawatts of power, about 300 times the capacity of the entire US electricity grid. The laser beam’s power is concentrated to a 1.3-µm speck about 100th the diameter of a human hair.

This intensity is about two orders of magnitude higher than any other laser in the world can produce, said Victor Yanovsky, a research scientist in the U-M Department of Electrical Engineering and Computer Science who built the ultrahigh-power system over the past six years.

The laser can produce this intense beam once every 10 seconds, whereas other powerful lasers can take an hour to recharge.

“We can get such high power by putting a moderate amount of energy into a very, very short time period,” Yanovsky said. “We’re storing energy and releasing it in a microscopic fraction of a second.”

To achieve this beam, the research team added another amplifier to Hercules laser system, which previously operated at 50 terawatts.

The intensity of the beam is now "twice as high as those previous measurements, but still 100 times higher than that produced by any other laser system," Krushelnick said. "Also, by the addition of the new amplifier, the power of the Hercules laser has been increased by a factor of six or seven. Because of the increased power it is, in principle, also possible to increase the intensity in our lab by another factor of three or so merely by using different focusing optics, although this hasn't been done yet."

Hercules takes up several rooms at CUOS. Light fed into it bounces like a pinball off a series of mirrors and other optical elements. It gets stretched, energized, squeezed and focused along the way.

Hercules uses the technique of chirped pulse amplification developed by U-M engineering professor emeritus Gerard Mourou in the 1980s. Chirped pulse amplification relies on grooved surfaces called diffraction gratings to stretch a very short duration laser pulse so that it lasts 50,000 times longer. This stretched pulse can then be amplified to much higher energy without damaging the optics in its path. After the beam is amplified to a higher energy by passing through titanium:sapphire crystals, an optical compressor reverses the stretching, squeezing the laser pulse until it’s close to its original duration. The beam is then focused to ultrahigh intensity.

In addition to medical applications, intense laser beams like these could help researchers explore new frontiers in science. At even more extreme intensities, laser beams could potentially “boil the vacuum,” which scientists theorize would generate matter by merely focusing light into empty space. Some scientists also see applications in inertial confinement fusion research, coaxing low-mass atoms to join together into heavier ones and release energy in the process.

A paper on the research is published online in the journal Optics Express; Yanovsky and Krushelnick are authors. Their team includes associate research scientist Vladimir Chvykov and assistant research scientist Galina Kalinchenko of the U-M Department of Electrical Engineering and Computer Science.

For more information, visit: www.umich.edu