Extremely short laser pulses, in the order of attoseconds, provide a powerful way to probe and image ultrashort processes like the motion of electrons in atoms and molecules. While it is possible to create ultrashort laser pulses, creating pulses that are both ultrashort and high-energy is a persistent challenge. To scale up the photon energy, photon flux, and continuum bandwidth for isolated attosecond pulses, it is necessary to develop a stable, high-energy, single-cycle laser source with a long wavelength. Researchers at the RIKEN Center for Advanced Photonics developed a way to generate high-energy, single-cycle, MIR pulses. The method, called advanced dual-chirped optical parametric amplification (advanced DC-OPA), increases the energy of single-cycle laser pulses by a factor of 50, and can be used to generate extremely short pulses with a peak power of 6 terawatts. “The current output energy of attosecond lasers is extremely low,” researcher Eiji Takahashi said. “It’s vital to increase their output energy if they are to be used as light sources in a wide range of fields.” The researchers used two types of nonlinear crystals to develop advanced DC-OPA — bismuth triborate oxide (BiB3O6) and lithium niobate doped with magnesium oxide (MgO:LiNbO3). The crystals amplify complementary regions of the spectrum. A new technique, called advanced dual-chirped optical parametric amplification (advanced DC-OPA), has increased the energy of single-cycle laser pulses by a factor of 50. The technique uses two crystals (shown as clear cubes), which amplify complementary regions of the spectrum. Courtesy of RIKEN. “Advanced DC-OPA for amplifying a single-cycle laser pulse is very simple, being based on just a combination of two kinds of nonlinear crystals,” Takahashi said. “I was surprised that such a simple concept provided a new amplification technology and caused a breakthrough in the development of high-energy, ultrafast lasers.” The damage threshold of nonlinear crystals has limited the energy scalability of OPA at larger pulse energies. “The biggest bottleneck in the development of energetic, ultrafast infrared laser sources has been the lack of an effective method to directly amplify single-cycle laser pulses,” Takahashi said. “This bottleneck has resulted in a one-millijoule barrier for the energy of single-cycle laser pulses.” The advanced DC-OPA method overcomes the bottleneck of pulse energy scalability using a single-cycle IR/MIR laser system. The researchers used advanced DC-OPA with a 10 Hz, joule-class Ti:sapphire pump laser. The nonlinear crystals were combined in each stage of the parametric amplifiers. With this setup, the researchers were able to amplify over-one-octave-bandwidth MIR pulses with a pulse energy of 53 millijoules centered at 2.44 µm. After enforcing pulse compression using a sapphire bulk, the temporal pulse duration went to 8.58 femtoseconds, which corresponds to 1.05 cycles at 2.44 µm. “We’ve demonstrated how to overcome the bottleneck by establishing an effective method for amplifying a single-cycle laser pulse,” Takahashi said. Advanced DC-OPA can work across a broad range of wavelengths. Takahashi and his colleague, researcher Lu Xu, demonstrated the method’s ability to amplify pulses whose wavelengths differed by more than a factor of two. “This new method has the revolutionary feature that the amplification bandwidth can be made ultrawide without compromising the output energy-scaling characteristics,” Takahashi said. The team expects that the advanced DC-OPA method will move attosecond laser technology forward. “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,” Takahashi said. “It’s undoubtedly a major leap forward in the development of high-power attosecond lasers.” Due to the excellent energy scalability of the advanced DC-OPA method, it is possible that laser pulses with higher pulse energy and fewer cycle numbers of pulse duration, based on different crystal combinations and a higher pump energy, could be achieved. The expansion of pulse energy could facilitate high-flux detection conditions for research in strong-field physics. Takahashi believes that, by making it possible to capture the motion of electrons, attosecond lasers have made a major contribution to basic science. “They’re expected to be used in a wide range of fields, including observing biological cells, developing new materials, and diagnosing medical conditions,” he said. Takahashi’s ultimate aim is to go beyond the speed of attosecond lasers and create even shorter pulses. “By combining single-cycle lasers with higher-order nonlinear optical effects, it could well be possible to generate pulses of light with a time width of zeptoseconds (one zeptosecond = 10-21 second),” he said. “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.” The research was published in Nature Photonics (www.doi.org/10.1038/s41566-023-01331-9).
