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Laser frequency combs and new frontiers of precision spectroscopy
- Femtosecond pulses from a laser are coupled into a bow-tie enhancement resonator in a vacuum vessel. A jet of xenon atoms is introduced that gives rise to a XUV pulse (blue) by the process of high harmonic generation.
Femtosecond laser frequency combs have revolutionised the art of measuring the frequency of light. They provide the long-missing clockwork for optical atomic clocks that will achieve unprecedented accuracies by slicing time into a hundred thousand times finer intervals than microwave caesium atomic clocks, the current standard of time and frequency1). Precision atomic spectroscopy with femtosecond laser combs has already established new limits for possible slow variations of fundamental constants2).
Within the MAP collaboration, new sources of high-harmonic radiation are being developed that can extend frequency comb techniques into the extreme ultraviolet and soft X-ray spectral regions. In a first proof-of-principle experiment we have already generated pulses of high-harmonic radiation down to wavelengths of 60 nm at a repetition frequency of 112 MHz by accumulating femtosecond laser pulses from a mode-locked titanium-sapphire laser in a passive build-up cavity and placing a xenon gas jet at an intracavity focus3) (see Fig. B.2.1).
Within this project we will characterise the spectral properties and limits of high-harmonic frequency combs. We will explore novel spectroscopic techniques that can extend the present ones into new spectral territory. In particular, we will adapt Fourier transform spectroscopy, spectral interferometry, or microscopic holography to the novel sources.
In a specific ambitious project, we plan to apply frequency combs in the XUV directly to precision spectroscopy of sharp resonances in laser-cooled trapped ions. The hydrogen-like helium ion with a 1S-2S two-photon transition near 60 nm is a particularly interesting candidate, since this simple atom permits unique confrontations between quantum electrodynamic theory and experiment. In one envisioned scenario, helium ions will be sympathetically cooled by laser-cooled magnesium ions in the same trap, and the signal may be detected via the production of doubly charged helium ions due to photoionisation.
References
1) Th. Udem et al., “Optical frequency metrology”, Nature 416, 233 (2002)
2) M. Fischer et al., “New limits on the drift of fundamental constants from laboratory measurements”, Phys. Rev. Lett. 92, 230802 (2004)
3) C. Gohle et al., “A frequency comb in the extreme ultraviolet”, Nature 436, 234 (2005)
Dr. habil. Thomas Udem