Riken ultrafast pulses set to reveal how chemical reactions evolve

17 Apr 2024

Technique using two kinds of nonlinear crystal amplifies single-cycle pulses to new energy levels.

Researchers at Japan's Riken research center have created ultrashort mid-infrared laser pulses using a novel amplification architecture.

The work is a further step towards the development and applied use of attosecond lasers, a topic for which three of the pioneering researchers were awarded the Nobel Prize in Physics in 2023.

Attosecond laser pulses should be short enough to reveal new details about the motion of electrons in atoms and molecules, and offer a better understanding of how chemical and biochemical reactions proceed.

"By making it possible to capture the motion of electrons, attosecond lasers have made a major contribution to basic science," said Eiji Takahashi of the Riken Center for Advanced Photonics (RAP). "They’re expected to be used in a wide range of fields, including observing biological cells, developing new materials and diagnosing medical conditions."

One challenge to date has been creating pulses of suitably short duration that also have high enough pulse energies to be of practical use in the demanding experiments researchers wish to carry out.

Several approaches to this hurdle have been developed, including optical parametric amplification (OPA) in which energy from a separate secondary laser is used to amplify the energy of the main laser pulses.

OPA involves the use of nonlinear crystals, and the Riken project's breakthrough employed two different kinds of crystal in its architecture: MgO-doped lithium niobate, and bismuth triborate or BiBO, each amplifying a different part of the pulse's spectrum. The approach, described in Nature Photonics, has been termed advanced dual-chirped optical parametric amplification (DC-OPA).

The road to zeptosecond research

In trials the project was able to generate stable mid-infrared pulses with an output pulse energy of 53 mJ. The pulse duration was compressed to 8.58 femtoseconds and the peak power equated to 6 terawatts, results thought by the team to represent the highest pulse energy and peak power values for optical parametric amplification of single-cycle mid-infrared laser pulses.

Riken also found that its advanced DC-OPA approach worked over a broad range of wavelengths, and the project successfully amplified pulses whose wavelengths differed by more than a factor of two. This means that the amplification bandwidth can be made ultrawide without compromising the scaling of output energy, described as a "revolutionary feature" by Riken.

"We have succeeded in developing a new laser amplification method that can increase the intensity of single-cycle laser pulses to terawatt-class peak power," Eiji Takahashi commented. "It’s undoubtedly a major leap forward in the development of high-power attosecond lasers."

The same principle should also be applicable to even shorter pulse lengths, with Riken now assessing whether energetic zeptosecond pulses of 10^-21 seconds can be achieved in the same way.

"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." said Takahashi.