Doctoral student John Beetar aligns a hollow-core capillary fiber on a laser in Michael Chini’s lab at the University of Central Florida, USA. [Image: Joshua Sweet, UCF]
Science at the timescale of attoseconds—a billionth of a billionth of a second—allows interrogation of electron dynamics, a study with huge promise for many disciplines. But accessing those ultrashort timescales today relies on custom-built lasers or access to world-class laser science facilities, which is often limited.
Now, a team led by Michael Chini, an associate professor at University of Central Florida (UCF), USA, has shown a path toward making science at such extreme timescales possible using workaday industrial lasers. The team reports that, by passing the pulse from such a laser through molecular gas-filled capillaries, it was able to compress the pulse below two optical cycles. (Sci. Adv., doi: 10.1126/sciadv.abb5375).
Being a leader in the attosecond field “has always required access to the best lasers,” says Chini. By using commercial lasers, “we’re hoping to get this technology into the hands of more people—chemists or materials scientists, for example—whose research problems would benefit from studies at these extreme time scales.”
Ironically, perhaps, Chini himself was driven by a similar quest for a powerful yet more readily available laser-research tool. As a new faculty member at UCF in 2015, he says, “I did not want to spend several years building my dream laser. I wanted to hit the ground running. I also had a limited budget, and a long list of things that I wanted to do with my startup funds.”
Chini says other groups had demonstrated light compression of a few-cycles duration with Yb-doped fiber lasers, and so he bought a commercial, Yb: KGW solid-state femtosecond laser. How to get the very short pulses needed for attosecond science wasn’t completely clear, he says, “but it seemed like it should only be a technical challenge.”
For light compression, Chini says, researchers commonly use self-phase modulation in noble-gas-filled hollow-core capillary fibers to generate a broad spectrum, and then compress the pulses using chirped mirrors. “Unfortunately, when we tried to push the techniques—using the typical knobs of noble gas species, pressure, capillary inner radius and capillary length—we would always get stuck at around 15 fs, which is too long for generating attosecond pulses,” Chini says.
And that’s where Chini’s doctoral student, John E. Beetar, and postdoctoral fellow, N. Nrisimhamurty (Murty) came in. Chini asked them to first try making changes to the technical setup. As xenon gas, the noble gas targeted for the experiment, is very expensive, Chini suggested that they do the test with nitrogen, and switch to xenon when the basic setup was working. “I still remember saying ‘You won’t find anything interesting with the nitrogen,’” he says.
An unexpected result
The initial result, Chini says, showed N2 generating three times broader spectra than Ar and two times broader than Xe—strange, he notes, because Ar and N2 have roughly the same nonlinear refractive index and ionization potential, while that of Xe is much higher than either. The spectra were also asymmetrical, being much more red-shifted in the molecular gas, Chini says. “I remembered some interesting work from Howard Milchberg’s group at the University of Maryland on the time dependence of the nonlinear refractive index of molecules, and this led us to consider N2O.”
The basic idea, he says, is that spectral broadening is most enhanced when the molecular axis becomes aligned to the laser polarization direction close to the peak of the pulse. “Since enhancement to the spectral broadening is caused by the molecular alignment, the duration of the laser pulses and the time taken to align with the laser field dictates how the pulses interact with the rotating molecules,” Beetar says. “This in turn determines the shape and energy distribution of the spectrum we obtain.”
Finding the right molecule
“For longer pulses, you want a molecule with a larger moment of inertia, since it takes a bit longer to align,” according to Chini. “We find that N2 works well for a pulse duration of about 150–200 fs, whereas N2O is perfect for the 280-fs pulses from commercial Yb:KGW lasers.”
“By taking advantage of the much larger rotational contribution to the nonlinear index of refraction of molecules and using relatively long laser pulses for which the rotational nonlinearity can be nearly instantaneous,” the authors write in their paper, “we achieve a record >45× pulse compression in a single stage.” The team adds that it was also able to demonstrate “the utility of these pulses for applications in attosecond science” by using high-harmonic generation (HHG) to generate a coherent extreme-ultraviolet (XUV) supercontinuum spectrum.
By working with a turnkey industrial laser system, Beetar says, he was able to spend all of his time working on developing the pulse-compression schemes necessary to get few-cycle—and, ultimately, near single-cycle—pulse durations.
Building an attosecond future
Chini says he can envision a new laboratory instrument based on his group’s technique. “Yb-doped laser-amplifier technology is progressing very quickly right now,” Chini says. “The high average power and repetition rate of Yb lasers are especially attractive for attosecond science for detailed studies of excited states of molecules.”
As another example, for solar energy and other optoelectronic devices, Chini says, “there are still major gaps in our understanding of charge transfer at metal-oxide semiconductor junctions. Attosecond spectroscopy, especially with high-repetition-rate sources, has an important advantage over other techniques for studying these dynamics due to the element specificity in the XUV absorption.”