Inevitably, terms like “laser” and “light pulse” will trigger images from popular culture: movie scenes from Star Wars, Star Trek or The War of the Worlds, alien powers firing laser beams through the air, fighting with energy weapons and colored light sabers. But the so-called “ultrashort laser pulses” discussed at the University of Bern on Monday night are by no means science fiction. They are fired every day – not by Stormtroopers equipped with blaster rifles, but by researchers all over the world.

Science was handed this “weapon” by physicist Donna Strickland, among others. The Canadian was only the third woman to receive the Nobel Prize in Physics, in 2018, for groundbreaking inventions in the field of laser physics. In this year’s Einstein Lectures, Strickland talks about how, using new techniques, she continues to try and produce ever shorter and more intense laser pulses, what surprising applications are enabled by them, and why it is not all that wrong to think of science fiction when considering her work.

Einstein’s Optical Legacy

“I was asked to mention Albert Einstein at least once,” Strickland jokes at the beginning of her talk, prompting laughter from the audience. Indeed, there is no avoiding the namesake of the Einstein Lectures: like to many things in physics, the laser goes back to Einstein’s discoveries. Specifically, it was his work in the field of optics, and in particular on the photoelectric effect, explaining which earned him the 1921 Nobel Prize, that laid the foundation for this technology.

The clue is in the word: “laser” is an acronym for “light amplification by stimulated emission of radiation.” The “stimulated emission” of radiation may be regarded as a counterpart to Einstein’s photoelectric effect: By means of a short light pulse, electrons in the shells of particular atoms are made to transition from a higher to a lower energy level. The energy thus released is emitted in the form of light particles, so-called photons. By bundling many photons created in this way, it is possible to obtain light of a uniform wavelength and a specific direction: a laser beam.

Shorter, Stronger, Better

Donna Strickland’s great achievement in the history of laser research is the development of “chirped pulse amplification” (CPA), a technique for producing an ultrashort but high-powered light pulse. She accomplished this in the mid-1980s as a doctoral student in the laboratory of her fellow Nobel laureate, Gérard Mourou.

Unlike everyday applications like the laser pointer fittingly used by Strickland during her talk, CPA does not produce continuous beams but periodic flashes of light. These flashes have intriguing properties. For one thing, they can be ultrashort: a single laser pulse may last for as little as a few millionths of a billionth of a second. For another, they are extremely high-powered: unlike with a continuous laser, where the power is distributed along the beam, with a pulsating laser it is concentrated in the pulse peaks. At the same average power, single light pulses can thus reach several terawatts or even petawatts.

Before Strickland and Mourou, however, such immensely powerful pulses – a petawatt is 0.5% of the total solar energy reaching the earth at any given moment – posed a problem: the high-powered photons produced non-linear effects threatening to destroy the laser apparatus from the inside. It was only using CPA, with the laser pulse being stretched, then amplified, and then compressed again, that it became possible to produce ultrashort light pulses simply and safely.

Cameras for “Molecular Movies”

Nowadays, Strickland’s laser pulses have a variety of applications. They are used in laser eye surgery, for high-precision cuts; in materials processing, for manufacturing the smallest components; and they may be part of the next generation of particle accelerators.

Strickland herself has a completely different project: she wants to make “molecular movies”. These are about depicting the structure of complex molecules and their movements in the highest possible resolution. This will require even shorter and even more powerful laser pulses, as Strickland explains. She thus joins the pioneers of photographic technology, Eadweard Muybridge and Harold Edgerton. And she tries her hand at making movies: With modest pride, Strickland shows her own PowerPoint animations of her “movie stars”: vibrating atoms and circling molecules.

While it is already possible to produce pulses short enough for “molecular movies”, Strickland explains, the required intensity is not available yet. For the objective is to produce a so-called “Coulombic explosion”: this involves the high-intensity laser pulses ionizing the atoms in the targeted molecules, making them burst apart. Inferences about the structure of the molecule concerned can then be drawn from the speed of the escaping atoms.

The Hunt for Pulses Continues

What Strickland then says about her experiments for producing new laser pulses and the techniques used – such as “mode coupling,” “harmonic generation” and “Raman scattering” – may be a little too fast for the uninitiated. But one thing is clear: forty years on from her groundbreaking invention, Donna Strickland’s enthusiasm for the world of lasers remains undimmed, and for all the bustle around winning the Nobel Prize she continues to hunt for new pulses and applications.

In her upcoming talks, Strickland will be looking more closely at two such applications: Today, Tuesday, she will talk about fighting tumors, and tomorrow, Wednesday, about monitoring the environment and the climate using laser pulses. That, again, sounds a little like science fiction – but if Strickland and her colleagues are successful, it will soon be science fact.