Attosecond imaging now possible with ultrashort light pulses

April 16 () –

Two RIKEN physicists have achieved extremely short pulses of laser light with a maximum power of 6 terawatts, equivalent to the energy of 6,000 nuclear power plants.

This achievement will help to continue developing attosecond lasers, for which three researchers – Pierre Agostini, Ferenc Krausz and Anne L'Huillier – received the Nobel Prize in Physics in 2023. He work is published in the journal Nature Photonics.

In the same way that a camera flash can “freeze” fast-moving objects, making them appear as if they are still in photographs, extremely short laser pulses can help illuminate ultrafast processes, providing scientists with a powerful way to obtain images and probe them.

For example, laser pulses on the order of attoseconds (one attosecond = 10 to 18 seconds) are so short that they can reveal the movement of electrons in atoms and molecules, providing a new way to discover how chemical and biochemical reactions evolve. . Even light seems to crawl on such short time scales, taking about 3 attoseconds to traverse a single nanometer.

“By allowing the motion of electrons to be captured, attosecond lasers have made an important contribution to basic science,” he says. it's a statement Eiji Takahashi of the RIKEN Center for Advanced Photonics (RAP). “They are expected to be used in a wide range of fields, including the observation of biological cells, the development of new materials, and the diagnosis of medical conditions.”

But while it is possible to create ultrashort laser pulses, they lack much power and have low energies. Creating laser pulses that are ultrashort and high energy would greatly expand their potential uses. “The current output energy of attosecond lasers is extremely low”says Takahashi. “That is why it is vital to increase their energy production if they are to be used as light sources in a wide range of fields.”

Just as audio amplifiers are used to boost sound signals, laser physicists use optical amplifiers to boost the energy of laser pulses. These amplifiers usually use non-linear crystals that have special responses to light. But these crystals can be irreparably damaged if they are used to amplify single-cycle laser pulses, which They are so short that the pulse ends before the light can oscillate through a full wavelength cycle.

“The biggest obstacle in the development of ultrafast and energetic infrared laser sources has been the lack of an effective method to directly amplify single-cycle laser pulses,” explains Takahashi. “This bottleneck has resulted in a one-millijoule barrier for the energy of single-cycle laser pulses.”

Now, Takahashi and his RAP colleague Lu Xu have not only overcome this barrier, they have broken through it. They have amplified single-cycle pulses to more than 50 millijoules, more than 50 times the previous best effort. Because the resulting laser pulses are so short, this energy translates into incredibly high powers of several terawatts.

“We have shown how to overcome the bottleneck by establishing an effective method to amplify a single-cycle laser pulse,” says Takahashi.

Their method, called dual-chip advanced optical parametric amplification (DC-OPA), is surprisingly simple and involves just two crystals, which amplify complementary regions of the spectrum.

“The advanced DC-OPA for amplifying a single-cycle laser pulse is very simple, as it is simply based on a combination of two types of nonlinear crystals; it seems like an idea that anyone could have thought of,” says Takahashi . “I was surprised that such a simple concept provided a new amplification technology and “would lead to a breakthrough in the development of ultrafast, high-energy lasers.”

Importantly, advanced DC-OPA operates over a very wide range of wavelengths. Takahashi and Xu were able to amplify pulses whose wavelengths differed by more than a factor of two. “This new method has the revolutionary characteristic that the amplification bandwidth can be made ultra-wide without compromising output power scaling characteristics“says Takahashi.

Their technique is a variation of another optical pulse amplification technique, called “chirp pulse amplification”, for which three researchers from the United States, France and Canada received the Nobel Prize in Physics in 2018. There is an interesting connection between the and 2023 awards because chirped pulse amplification was one of the techniques that enabled the development of attosecond lasers.

Takahashi anticipates that his technique will further advance the development of attosecond lasers. “We have managed to develop a new laser amplification method that can increase the intensity of single-cycle laser pulses to a maximum terawatt-class power,” he says. “It is certainly a breakthrough in the development of high-power attosecond lasers.”

In the long term, their goal is to go beyond attosecond lasers and create even shorter pulses. “By combining single-cycle lasers with higher-order nonlinear optical effects, it may well be possible to generate light pulses with a duration of zeptoseconds (a zeptosecond is one trillionth of a second),” he says. “My long-term goal is to knock on the door of zeptosecond laser research and open up the next generation of ultrashort lasers after attosecond lasers.”