Significance
Since its inception, attosecond science has entirely been driven by titanium:sapphire lasers; the initial enabling technology, combined with an active stabilization of the carrier-envelope phase. At the dawn of this decade, however, the development of high energy sources of longer-wavelength radiation launched a revolution in attosecond science. This revolution was necessary due to the fact that the previously named sources tend to limit the achievable photon energy to a mere ~100eV which is too low to access X-ray absorption edges of most second- and third-row elements which are central to chemistry, biology and material science. To this note, immense efforts have been put in place to extend the attosecond metrology to the Soft-X-ray domain using mid-infrared drivers.
A team of researchers led by professor Hans Jakob Wörner from the Laboratory of Physical Chemistry at ETH Zürich, in Switzerland has now established a new world record in the shortest laser pulse ever measured. They determined their soft-X-ray pulse to be a mere 43 attoseconds long (1 as = 10-18 s), improving the previous record by 10 as. To measure this pulse, they had to solve the long-standing problem of the complete temporal characterization of ultra-broadband (>10 eV) attosecond pulses. They accomplished this by generalizing the recently proposed Volkov-transform generalized projection algorithm to the case of multiple overlapping photoelectron spectra. Their work is currently published in the research journal Optics Express.
Their studies involved first, the generation of femtosecond laser pulses at 800 nm using a titanium:sapphire laser system. The researchers then proceeded to create isolated attosecond pulses through high-harmonic generation in a high-pressure gas cell filled with neon/argon. The residual mid-IR pulse was then separated from the attosecond Soft-X-ray pulse by means of a perforated mirror. Broadband reflection of the Soft-X-ray super continuum was achieved through grazing-incidence reflection on 3 flat mirrors coated with diamond-like carbon and one gold-coated toroidal mirror that focused the Soft-X-ray beam into the interaction region of a photoelectron time-of-flight spectrometer. A 100-nm Zirconium foil was utilized as a high-pass filter and simultaneously compensated the attochirp. Eventually, recombination of mid-IR and Soft-X-ray pulses was achieved by use of another perforated mirror that focused the mid-IR beam to the same spot.
The authors observed that their novel approach overcame all key limitations of previous attosecond-pulse reconstruction methods, in particular the central-momentum approximation. Furthermore, their new approach incorporates the physical, complex-valued and energy-dependent photoionization matrix elements. To be more specific, it was these properties that made their approach general and particularly suitable for attosecond supercontinua of arbitrary bandwidth.
The study by Hans J. Wörner and colleagues has successfully presented a prolific demonstration of the scalability of attosecond metrology to mid-IR drivers, including both amplitude gating and the attosecond streak camera. The results obtained have helped establish that the popular and broadly available scheme of post-compressing the output of white-light seeded optical parametric amplifiers is adequate to produce high-contrast isolated attosecond pulses covering the L-edges of silicon, phosphorous and sulfur. This new reconstruction technique and experimental results will open the path to the production and characterization of attosecond pulses lasting less than one atomic unit of time (24 as) and covering the X-ray absorption edges of lightest elements.
Reference
Thomas Gaumnitz, Arohi Jain, Yoann Pertot, Martin Huppert, Inga Jordan, Fernando Ardana-Lamas, Hans Jakob Wörner. Streaking of 43-attosecond soft-X-ray pulses generated by a passively CEP-stable mid-infrared driver. Volume 25, Number 22 |2017 | Optics Express
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