Tunable narrow-linewidth lasers, as required for precision spectroscopy, are available only in the visible and infrared wavelength ranges, but not in the VUV range (below 200 nm). Unfortunately, many of the most important transitions lie in this specific wavelength range: building a laser for the VUV range would allow one to perform spectroscopy on the Lyman-alpha transition in hydrogen (121 nm), on He+ ions (60 nm), on a variety of highly charged ions which are relevant for cosmology, and (of course), the Th-229 nuclear transition.
Such lasers build on high-harmonic generation in a gas jet, which is quite an inefficient nonlinear process. As a consequence, lasers with both high average power and high peak power (short pulses) are required. The combination of short pulselength, high repetition rate, and high average power is hard to fulfill. Researchers at MPQ in Garching now made an important step forward: Instead of using Ti:Sa lasers (which are common in the field), they used a pulsed Yb-doped laser at 370 W average power, however with a comparably long pulse length of 860 fs. Using a scheme called multi-pass cell spectral broadening (MPCSB), they were able to shorten the pulse length to 115 fs, which is an increase in peak power by a factor of about 7. The specific laser developed here will be used for spectroscopy of He+ ions, but the technology could also be transferred to a laser system dedicated to Th-229 research.
The work has recently been published with Optics Express and can be found here.