Lasers: the still very promising prospects of CPA

The chirped-pulse amplification (CPA) technique allows lasers to become ever more powerful and to take up unprecedented challenges. As soon as the laser was invented in 1958, Gordon Gould described in his patent many applications that have emerged today, such as welding, drilling or telecommunications. But the invention of the CPA, whose inventors Donna Strickland and Gérard Mourou won a Nobel Prize in 2018, paved the way for the new era of high-power lasers, the full potential of which is still far from being explored.

Ophthalmology: a proven application

Serendipity - this lucky coincidence that makes science progress – instigated the use of lasers for eye surgery. Indeed, a student of Gérard Mourou unfortunately received a laser shot in the eye, first compromising the laboratory’s activities. However, this injury turned out to be benign and even stunned the surgeon who examines it: “how did you achieve such a perfect ablation”? It didn't take more to initiate a collaboration between surgeons and physicists to make the laser an essential instrument for eye surgery. A pioneer in this field, the Hungarian ophthalmologist Imola Ratkay -Traub, experienced the appearance of lasers in operating rooms. "Flap" which consists in creating a flap to operate the upper part of the cornea, ablation, correction of aberrations: the laser has revolutionized eye surgery thanks to its precision and the "cleanliness" of this light tool which does not carry any bacteria.

Particule acceleration

First theorized by Toshiki Tajima and John Dawson in 1979, plasma wake-field acceleration required a laser powerful enough to be demonstrated: the principle of this technique is to use a laser to create a wake - like a boat on a lake - in a plasma (ionized gas). Charged particles, such as plasma electrons, then "surf" on the plasma waves and are thus accelerated over very short distances to very high energies. The invention of CPA and the meeting between Toshiki Tajima and Gérard Mourou made the experiment possible and validated this concept in 1994. Their collaboration, still fruitful today, allows to design the next generation of particle accelerators: much more compact, they could replace installations like CERN by accelerating particles over a hundred meters (compared to 27 km currently).

The study of vacuum

The vacuum is actually not empty. It is filled by a magnetic field which interacts with the laser beams when their power is sufficient. While light seems to propagate without any disturbance in the low energy regime, the vacuum should reveal new properties when experiencing high photon radiation. Sergei Bulanov, one of the research program leaders of the ELI-beamlines* facility in the Czech Republic, worked on the study of vacuum properties in collaboration with Toshiki Tajima and Gérard Mourou. Experiments performed in this high light intensity regime have revealed that the vacuum then behaves like a plasma of virtual pairs of electrons and positrons which influence the propagation of light by dispersing or dissipating it. Beyond a certain power that lasers should reach very soon thanks to CPA, the electric field could then cross the Schwinger limit, thus making the vacuum "boil" and materialize these pairs of electrons and positrons from void.

The whole universe in the lab

The power of lasers enabled by the CPA now allows researchers to study the universe inside their laboratory. Jena Meineke, a young astrophysicist in the physics department of the University of Oxford, makes miniature supernovas in the laboratory* and tries to reconstruct the primordial conditions of the universe to better understand the magnetic field and its origin. Pisin Chen, professor of cosmology at the National University of Taiwan and winner of the Blaise Pascal International Chair, which allows him to currently conduct research at Ecole Polytechnique, studies black holes. He uses the wake-field acceleration mentioned above to reproduce the gravitational phenomena near the horizon of these astrophysical giants. Indeed, Einstein's general relativity demonstrates that by changing the reference frame, gravitation is equivalent to an acceleration. Both are bending time and space and are expected to produce Hawking radiation called Unruh radiation for the accelerating field. By combining this property of the universe with particle acceleration in a laser wake-field, Pisin Chen intends to unveil the mysteries surrounding black holes, in particular the Hawking information loss paradox.

Toward a clean energy

Besides fundamental research enabled by CPA and its subsequent medical applications, Gérard Mourou is also focusing on clean energy. Two axes are considered on nuclear energy, which produces no CO2 but whose waste unfortunately presents other risks for mankind and the planet.

One of the applications, presented as soon as he received his Nobel Prize, is the transmutation of nuclear waste. Gérard Mourou's idea is to use lasers to create a beam of neutrons in order to bombard radioactive waste. They "transmute", becoming new elements whose radioactivity decays much more quickly. From waste having a lifetime of several thousand years, the Nobel Prize hopes to produce elements with a lifetime of several tens of years.

The second application he is considering for nuclear power is the change of raw material from Uranium to Thorium. Gérard Mourou explains that to produce a power of 1GW for a year, 300 megatons of coal are needed, which corresponds to 300 tons of Uranium, or equivalently only 1 ton of Thorium. The laser would allow to exploit this material which he also sells the benefits for world peace, as it cannot be used to produce plutonium, basic element for atomic bombs.

CPA still has many other applications that could not be mentioned during the celebration of its anniversary, such as micro-machining, cancer treatment or dynamic measurements in chemistry. These applications and their perspectives are made possible by exchanges within the high-power lasers community and by the imagination of passionate researchers like Gérard Mourou who always push science to the limit for the knowledge and the good of mankind.

* ELI-beamlines is one of the three laser facilities making up the ELI network.

* Also discover the work carried out at Ecole Polytechnique in the field of astrophysics in the laboratory.

Learn more about the CPA technique:

Instead of directly amplifying a laser pulse whose intensity would damage the optical components, the pulse is spread over time before the amplification stage. Like a prism that separates the colors of a rainbow, the different colors - frequencies - that make up the laser pulse are delayed relative to each other and pass in turn through the amplifying medium. Once all the frequencies have been amplified, the pulse is recombined, thus restoring the total power of each of the frequencies. This technique allows to reach unprecedented peak powers compared to convention laser pulse amplification.