Extremely short pulses of laser light with a peak power of 6 terawatts (6 trillion watts) (equivalent to the power produced by approximately 6,000 nuclear power plants) were performed by two RIKEN physicists. This achievement will help further develop attosecond lasers, for which three researchers won the Nobel Prize in Physics in 2023. The study was published in the journal Nature Photonics.

Just as a camera flash can “freeze” fast-moving objects, causing them to appear at rest in photographs, extremely short laser pulses can help illuminate ultrafast processes, giving scientists a powerful way to image and study them. .

For example, attosecond-order laser pulses (one attosecond = 10-18 seconds) are so short that they provide a new way to learn how chemical and biochemical reactions occur by detecting the movement of electrons in atoms and molecules. Even light appears to creep on very short time scales, taking about 3 attoseconds to travel a distance of one nanometer.

“Attosecond lasers that record the movement of electrons have made major contributions to basic science,” says Eiji Takahashi of the RIKEN Center for Advanced Photonics (RAP). “It is expected to be used in a wide variety of fields, from the observation of biological cells to the development of new materials and diagnosis of diseases.”

Power and influence

However, although it is possible to create ultrashort laser pulses, they do not have much power because their energy is low. The generation of ultrashort and high-energy laser pulses will significantly increase the possibilities of their use. “The current output energy of attosecond lasers is extremely low,” says Takahashi. “Therefore, it is vital to increase their output energy if they are to be used as light sources over a large area.”

Just as audio amplifiers are used to amplify audio signals, laser physicists use optical amplifiers to increase the energy of laser pulses. These amplifiers often use nonlinear crystals that exhibit a specific response to light. But these crystals can be irreparably damaged if they are used to amplify single-cycle laser pulses that are so short that the pulse ends before the light has a chance to oscillate through a full wavelength cycle.

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

A new record

Now, Takahashi and his RAP colleague Lu Xu have not only overcome this obstacle, they have surpassed it. They boosted single-cycle pulses to over 50 millijoules; this was more than 50 times the previous best effort. Because the resulting laser pulses are very short, this energy is converted into incredibly high power of several terawatts.

“We showed how to overcome the bottleneck by creating an effective method of amplifying the single-cycle laser pulse,” says Takahashi.

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

“Advanced DC-OPA for single-cycle laser pulse amplification is very simple, relying only on the combination of two types of nonlinear crystals; this seems like an idea anyone could think of,” says Takahashi. “I was surprised that such a simple concept enabled a new amplification technology and created a breakthrough in the development of high-energy ultrafast lasers.”

More importantly, the improved DC-OPA operates over a very wide wavelength range. Takahashi and Xu were able to amplify pulses whose wavelengths differed by more than twofold. “This new method has a revolutionary feature: the gain bandwidth can be made ultrawide without compromising the scaling characteristics of the output power,” says Takahashi.

amplification technique

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

Takahashi expects his techniques to lead to further development of attosecond lasers. “We were able to develop a new laser amplification method that can increase the intensity of single-cycle laser pulses to terawatt-class peak power,” he says. “This is clearly a major step forward in the development of powerful attosecond lasers.”

In the long term, it aims to go beyond attosecond lasers and create shorter pulses.

“By combining single-pulse lasers with high-order nonlinear optical effects, it is quite possible to produce pulses of light at zeptosecond time widths (one zeptosecond = 10-21 seconds),” he says. “My long-term goal is to leverage zeptosecond laser research and discover the next generation of ultrashort lasers beyond attosecond.”