Interferometric Method Measures Interactions at Zeptosecond Speed

According to researchers at the Australian Attosecond Science Facility and the Centre for Quantum Dynamics of Griffith University, building an interferometer in the extreme ultraviolet (EUV) region is challenging for two reasons. First, it is itself a challenge to control the delay of the EUV pulses precisely between the two arms with subcycle precision. Second, the researchers said, necessary highly reflective EUV optics are not yet developed.

Overcoming these barriers, the Griffith team has now developed a novel interferometric technique that simplifies measurement of the ultrafast dynamics of light-induced processes. The researchers used the interferometric technique to measure time delays with zeptosecond — one trillionth of a billionth of a second — resolution.

The researchers’ technique generates two coherent high-harmonic pulses without splitting the driving laser and EUV beams. The two mutually coherent EUV pulses are generated by exploiting the inherent properties (Gouy phase) of a single Gaussian focused laser beam. Further, the all-optical, Gouy phase, direct EUV technique does not require EUV optics, and gas pressures do not need to be calibrated to ensure the same number densities. The technique has a resolution of approximately 300 microradians (μrad) — about 100 zeptoseconds.

“Such unprecedented time resolution is achieved via an interferometric measurement — overlapping the delayed lightwaves and measuring their combined brightness,” researcher Mumta Hena Mustary said.

To investigate the effect of nuclear dynamics on the electron motion in molecular hydrogen, the researchers used the EUV Gouy phase interferometry technique in experiment that aimed to measure a small phase difference of high harmonic generation (HHG) signals and gain insight into the correlated electron-nuclear dynamics for two different isotopes of hydrogen molecules.

The researchers made precise measurements of the high-harmonic phase difference and corresponding HHG phase delays produced in light (²H) and heavy (²D) hydrogen isotopes. They measured the emission time delay for all the harmonics observed in the HHG spectrum. The measured phase difference was nearly constant and was about 200 milliradians (mrad), or approximately 3 attoseconds (i.e., one quintillionth of a second long) phase delay for all the harmonics.

An interferometer and corresponding technique developed by researchers at Griffith University enables the measure of ultrafast dynamics of various light-induced processes in atoms and molecules at unprecedented time resolution. Courtesy of Griffith University.

Researchers at Shanghai Jiao Tong University simulated the experimental work of the Griffith team. The SJTU researchers used advanced theoretical methods to comprehensively model the HHG process taking place in the hydrogen isotopes, including all degrees of freedom for nuclear and electronic motion at various levels of approximation.

A strong correlation between experimental and theoretical results gave the Griffith team confidence in the ability of its interferometric technique to capture the most essential features of the underlying physical processes.

The Gouy phase EUV interferometric technique could open an avenue for measuring the phase of harmonic emission for different atoms and molecules. Together with isomeric or isotopic comparisons, it could also allow scientists to observe the subtle effects of molecular structures and nuclear motion on electron dynamics in strong laser fields.

The research was published in Ultrafast Science (www.doi.org/10.34133/2022/9834102).