The Laser at 50
It's the golden anniversary of this fundamental technology
Photo: The Texas Center for High Intensity Laser Science
Petawatt Power: The world's highest-power laser delivers nearly 200 joules in less than 200 femtoseconds.
Fifty years after the first beam of coherent light shone out of a ruby crystal, lasers have expanded in every direction, ranging from as big as three football fields to as small as a few layers of atoms, producing wavelengths from deep in the X-ray regime to far into the infrared. Engineers are pushing the technology ever further, and here are some of the records they have set.
The laser with the highest peak power in the world—1.1 petawatts, about 2200 times the power output of the entire U.S. electrical grid—is run by the University of Texas at Austin. The laser starts with a short, low-energy laser pulse and stretches it to 10 000 times its length, amplifies it to 186 joules, then recompresses it to 167 femtoseconds. The laser provides scientists with enough power to study thermonuclear fusion as well as to examine the nature of plasmas and the properties of dense matter in brown dwarf stars.
An attosecond is to a second what a second is to the age of the universe. Pulses of X-ray laser light lasting 80 attoseconds—fast enough to take snapshots of quantum-level events, such as the motion of electrons in an atom—have been produced by scientists at the Max Planck Institute of Quantum Optics, in Munich. They fire a femtosecond laser into a neon gas, where extreme nonlinear effects upshift the photons from visible light to X-rays. The process of combining X-rays into a coherent beam shortens the pulse to tens of attoseconds.
One megajoule—enough energy to boil about three quarts of water—is the current record energy output of what is also the largest laser in the world: the 192-beam setup at the National Ignition Facility, in Livermore, Calif. In January, NIF produced that much ultraviolet light in just a few nanoseconds; ultimately it will provide 1.8 MJ. Later this year, scientists expect to shine the combined beams on test capsules containing deuterium and tritium, in the hope of igniting thermonuclear fusion. Project director Ed Moses expects to reach scientific breakeven—when more energy comes out of the system than what goes in—within a couple of years.
A particle of gold in a silica shell only 44 nm across is the smallest laser yet devised. Scientists from Cornell, Norfolk State, and Purdue universities designed the laser, which exploits surface plasmons—oscillations of electrons that occur where a metal touches an insulator. The laser emitted light in the green spectrum at a wavelength of 530 nm, far larger than the laser itself, which is tiny enough to place on a computer chip or attach to a cancer cell.
Longest wavelength in a diode laser
Terahertz radiation, in the infrared spectrum just short of microwaves, is the specialty of the quantum cascade laser—a semiconductor inscribed with a ladderlike series of quantum wells that confine electrons to desired energy states. An electron entering a well creates a photon and a lower-energy electron, and then the electron tunnels to another well and repeats the process. Qing Hu of MIT shaped the wells to achieve long wavelengths, then used a magnetic field to further control the electrons, getting an output of 0.68 terahertz, or 440 micrometers—long enough to serve as an investigative tool in biochemistry and explosives detection.
The shortest wavelength of any laser—0.15 nanometers, deep in the X-ray spectrum—comes from the Linac Coherent Light Source, at Stanford.
X-ray wavelengths of 0.15 nanometer, in coherent pulses lasting just 100 femtoseconds, come out of the Linac (for linear accelerator) Coherent Light Source, at Stanford University. The 3-kilometer-long Linac takes short pulses of electrons and accelerates them to nearly the speed of light, then runs them through alternating magnets that jerk the electrons back and forth, making them emit X-ray photons. Physicists use the device to investigate the structure of matter.
About the Author
Neil Savage writes about technology from Lowell, Mass. In March 2010, he reported on ways to reduce the radiation used in medical scanners.