A Visit To One Of The Most Powerful Lasers On Earth

A Visit To One Of The Most Powerful Lasers On Earth

BERKELEY, CALIFORNIA—The winners of this year’s Nobel Prize in Physics didn’t just make discoveries. Their revolutionary work turned powerful lasers into ubiquitous lab tools. The day of the announcement earlier this month, I’d already planned to visit the tennis-court sized Berkeley Lab Laser Accelerator, or BELLA, which uses one of the Nobel-winning methods to create one of the most powerful laser pulses on Earth.

Donna Strickland, Gérard Mourou, and Arthur Ashkin shared this year’s Nobel Prize in Physics for advances in laser technology. It was the first physics prize to feature a female laureate in over 50 years—Strickland is only the third female physics laureate in history. BELLA and other high-power lasers employ the technique developed by Strickland and Mourou, called chirped pulse amplification, to create their incredible laser pulses. These devices could one day power tabletop particle accelerators for medical use, act as microscopes to image atoms, and push the frontiers of physics even further.

Donna Strickland (Photo: University of Waterloo)

“When I saw talks by people building these lasers and the science done by these lasers, I thought, wow, this is mind-boggling that I could have done something that changed a field of science,” Strickland, a professor at the University of Waterloo, told Gizmodo in an interview.

Light bulbs send light in all directions with different wavelengths, but lasers create intense beams of light, with photons—the smallest units of light—whose electromagnetic fields are all synchronised. Lasers function using a process called stimulated emission.

Normally, atoms absorb radiation in the form of photons, which makes their electrons jump to higher energy states, then spontaneously drop to lower states and emit photons—that’s “spontaneous emission.” But if you put enough of the electrons in a medium into excited states, then new photons will cause the electrons to drop to a lower energy state and emit photons without absorbing them. In this case, a properly tuned photon with a given wavelength, phase, and direction induces the excited electrons to emit photons with the same properties. Modern lasers consist of an input energy source to excite the electrons, which are held in a medium like a crystal. The crystal sits between two mirrors, one of which only partially reflects light. The light bouncing between the mirrors continues the stimulated emission, resulting in a single-colour beam of light exiting the device through the partial mirror—a laser.

Prior to the invention of chirped pulse amplification, there seemed to be a limit to a laser pulse’s intensity. Increased power could change the laser medium’s optical properties, which could distort the beam or even damage the medium. This became a major problem that slowed the development of laser science and required building bulky lasers—until Strickland and Mourou came along.

Strickland and Mourou solved the problem with chirped pulse amplification for lasers in the 1980s. The process begins with a short laser pulse. The pulse bounces through a pair of gratings, which make it longer. The gratings act like a prism, causing different colours to take paths of different lengths. Since power is just energy delivered over time, stretching the light decreases its power, and allows it to be amplified without harming the laser medium. Finally, the amplified pulse passes through a compressor, which squeezes it into a shorter blip—a more powerful pulse. The method gave researchers access to powerful laser pulses that could sit on a tabletop, and made high-power laser-pulse tools like BELLA more feasible.

BELLA’s sapphire lasing crystal (Photo: Ryan F. Mandelbaum)

How does one of these laser pulses differ from, say, a common laser pointer? If you opened the shutter of a store-bought laser for one second, the pulse, if uninterrupted, would span three-quarters of the length from here to the Moon before you shut it off, Strickland explained. Lasers amplified through chirp pulse amplification could pack the same number of photons into a pulse the thickness of a piece of paper. “When you squeeze it all together, you get a tremendous number of photons,” she said. Yes, if your hand were in the way of such a beam, it could get a nasty burn. An especially powerful focused laser pulse could even shatter sapphire.

BELLA is chirped pulse amplification on steroids. It begins with a lasing medium—a synthetic sapphire crystal with added titanium atoms. The beam passes through the next portion, the stretcher, which spreads the pulse out in time. A series of smaller lasers activate more titanium-doped sapphire crystals, adding energy to the stretched pulse as it travels through six amplifiers. At the other end is the compressor, and finally a mirror to focus the beam before passing it into the experiment.

BELLA packs 40 joules of energy, a few times the energy in a camera flash, into an infrared pulse lasting just 40 femtoseconds, which is something like a trillion times faster than a single flap of a bee’s wing. BELLA director Wim Leemans was not willing to speculate as to what would happen if this laser pulse hit you, but it’s safe to say you would be seriously injured or even killed.

Lasers like these have plenty of applications, from industry to medical research. But Mourou and others have realised that these high-power lasers can serve as a whole new way to accelerate particles to high energies—and that’s what BELLA is used to study. Scientists dream that one day, these laser-plasma accelerators could shrink particle physics experiments to the point that they no longer require enormous infrastructure projects like the Large Hadron Collider in Switzerland. Laser-plasma accelerators may one day appear in everyday contexts, where the particles are used to target and break down tumours as a cancer treatment. The Nobel committee mentioned both laser-plasma acceleration and BELLA in their scientific background of the 2018 prize. 

As he walked me through a cavernous hallway documenting the history of particle accelerators at Berkeley National Lab, Leemans revealed a small block, about the size of half a stick of butter. BELLA’s laser pulse passes through a plasma stored in a tube etched into that tiny device, accelerating electrons in the plasma to nearly the energies of some of the world’s highest-energy electron accelerators.

BELLA accelerates electrons in a plasma held in this block (Photo: Ryan F. Mandelbaum)

There are plenty of kinks to work out before a tabletop, laser-driven particle accelerator could do something like treat cancer, including increasing how frequently the device can deliver pulses. Yet a particle accelerator for cancer treatment wouldn’t need a laser as large as BELLA. After all, chirped pulse amplification has allowed for tabletop-sized terawatt laser pulses. Theoretically, this could bring such a cancer treatment to anyone, without requiring them to visit a hospital. “You could put the whole system on a truck,” Leemans said.

When we say that Strickland and Mourou’s work transformed the field—we mean it. Though BELLA once held the record for the world’s most powerful laser pulse, other labs have since taken its place, with ambitious goals like “breaking the vacuum,” or squeezing enough energy into a region to generate particles out of empty space, something that’s never been done before. Entirely new fields of research have begun thanks to this laser breakthrough. “It’s remarkable,” Strickland said. “The pulses are shorter, the energy is higher. It’s changed everything.”