Lasers: Beyond Star Trek

Part of an ultrafast laser in the Kapteyn-Murnane laboratories at the University of Colorado-Boulder. (Photo/Dan Hickstein).*

by Paul Vallet

While big budget movies portray lasers only as futuristic weapons and plot devices, the reality is that lasers enable scientists to observe and control molecules on a scale never before imagined.

Typical Hollywood blockbusters may exaggerate some of the abilities of lasers to enhance a scene. While such exaggeration makes for good cinematography, it gives audiences the wrong impression of what lasers are and what they can do, and sweeps aside some of the incredible scientific advances lasers have made possible.

Commonly, movies portray lasers as loud (see Congo, Star Trek, anything involving laser guns, phasers, ray guns), visible beams of light (see Goldfinger, Ocean’s Twelve, every movie ever involving laser security systems), and able to cause explosions (see Star Wars – Destruction of Alderaan).

But this is not the case. Here’s the truth:

First of all, if you’ve played around with a laser pointer you know they don’t make any noise. “Pew pew” noises are fun to imagine with laser guns but unfortunately sound effects are just hype. For lasers used in typical research, or industrial lasers, the loudest sound will originate from the machinery in the cooling system.

Secondly, a laser beam is typically invisible unless it hits something or it is aimed directly into your eye, in which case you are probably looking at some eyesight damage so that is to be avoided. Normal light sources – such as incandescent bulbs or candles – cast off light in all directions. Laser light is special, however, because it is coherent, meaning all of the light is emitted in the same direction as a tight beam. Until the beam hits something it is completely unseen. In the vacuum of space this means that one would never be able to see the beam (a beam fired from the Enterprise, for example). On Earth, specks of dust, or air molecules if the intensity is high enough, can scatter light from the beam making it visible.

Finally, lasers don’t immediately cause explosions. While it’s fairly easy to burn and cut things with lasers, it’s quite difficult to get anything to explode with a laser. Light is energy, so anything absorbing the energy from a laser beam simply increases in temperature until it melts, vaporizes, or burns.

While one could cause some destruction with an extremely high-powered laser, it’s typically simpler and more effective to use conventional explosives. It’s true that the U.S. Navy is pursuing laser based weapons – their latest demonstration showed a 15-kilowatt laser setting fire to a small motorboat – but it’s expected that it will be more than a decade before more powerful lasers could melt through steel hulls and destroy incoming missiles. Even then, the desired outcome is cutting and disabling from a distance – not explosions.

A typical depiction of laser science in movies. (Illustration/Paul Vallett).

Movies use lasers to portray an environment of extreme technological control and power. While their depiction on the silver screen may be slightly exaggerated, it’s a shame that movies cannot capture the incredible versatility and usefulness of lasers.

“Lasers today produce much higher power densities than were previously possible, more precise measurements of distances, gentle ways of picking up and moving small objects such as individual microorganisms, the lowest temperatures ever achieved, new kinds of electronics and optics, and many billions of dollars worth of new industries,” notes Dr. Charles Townes, Nobel Prize winning physicist in 2003 publication “A Century of Nature: Twenty-One Discoveries that Changed Science and the World.”

While the breadth of technological and scientific advances made possible by lasers may not be flashy enough for Hollywood blockbusters, the unique properties of lasers allow scientists to gain a more complete understanding of the molecular world.

Here at CU-Boulder, in the JILA research laboratories on the University of Colorado’s campus, a group of students working under professors Henry Kapteyn and Magaret Murnane is performing some of the leading research into generation of new types of laser light sources.

One of these research areas is the generation of laser pulses in the ultrafast time scale. This refers to short laser pulses that last for only a few femtoseconds. A femtosecond is tiny – one quadrillionth of a second. As a comparison, there are as many femtoseconds in one second as there are seconds in 31 million years.

“There is nothing else humans can manipulate that is as short as an ultrafast laser pulse,” notes Daniel Hickstein, a PhD candidate working in the Kapteyn-Murnane labs. Hickstein says that molecular events such as the vibrations of atomic bonds or the movement of electrons within a molecule used to be “moving so fast that we could never see it – but now we can.”

Researchers are looking at making laser pulses even shorter, shrinking the duration of the pulse by one thousand times to reach the attosecond time scale. “I’m excited about this because we have absolutely no idea what goes on in the molecule at these time scales,” says Hickstein.

A second area of laser research in the Kapteyn-Murnane labs, known as strong-field laser physics, involves observing the effects of extremely high laser intensity on small gas molecules.

“In normal life, most molecules absorb one photon of light,” explains Hickstein. “Absorbing two photons is unheard of. Under intense illumination by a laser in the lab I can see signals corresponding to the absorption of two hundred photons. This high light intensity regime turns normal physics on its head.”

One of the unexpected results from such strong-field lasers is that visible laser light can be transformed into light at much shorter wavelengths in the extreme ultraviolet and x-ray region in a process known as high harmonic generation. During this process the extreme intensity of the laser blasts an electron off of a small gas molecule. The electron accelerates, turns around, and then crashes back into the molecule, releasing a huge amount of energy in the form of laser light in the extreme ultraviolet and X-ray wavelengths.

Lasers at these extremely short wavelengths can then be used for a variety of scientific applications that visible light lasers are unsuited for, from high resolution imaging of nanoscale objects to observing how energy flows through molecules.

And this makes researchers like Hickstein excited about how lasers will be used in future.

“New discoveries with lasers lead to new technologies,” he says.

*Correction: Updated March 12 at 12:55 p.m.

The original version of this article incorrectly attributed the picture of the ultrafast laser in the Kapteyn-Murnane lab to Paul Vallett. Dan Hickstein took the picture. Also, the original version spelled Kapteyn-Murnane as “Kapteyn-Murname.” Both errors have been rectified.