Frequency comb metrology at PHz frequencies: Precision in the extreme ultraviolet

The capability of frequency-comb (FC) lasers to precisely measure optical frequencies is extended to the multiple-PHz domain. This frequency region, which covers the extreme ultraviolet (XUV, wavelengths shorter than 100 nm), was previously not accessible to these devices. Frequency comb generation is shown for 51-85 nm by amplification and coherent up-conversion of a pair of pulses originating from a near-infrared femtosecond FC laser. Moreover, Ramsey-like signals with up to 61% contrast are observed when the XUV comb is scanned over transitions in argon, neon and helium, resulting in an 8-fold improved determination of the ground state ionization energy of helium

Frequency comb metrology in the extreme ultraviolet

Frequency comb generation and excitation of argon, neon, and helium is shown from 51 to 85 nm with amplified and harmonically upconverted comb laser pulses, resulting in an 8fold improved helium ground state ionization energy

Widely tunable extreme ultraviolet frequency comb generation

Frequency comb lasers [1] have led to great advances in fields such as precision spectroscopy, optical atomic clocks, and attosecond science. We transfer the remarkable precision of frequency combs to extreme ultraviolet (XUV) wavelengths by parametric amplification and high-harmonic generation (HHG) of two subsequent Ti:Sapphire comb laser pulses (see Fig. 1b). As a result a pair of phase-locked extreme ultraviolet pulses is generated, which can be used directly for precision spectroscopy without the need for an additional spectroscopy laser. Viewed in the frequency domain, the spectrum of the upconverted pulse sequence in the XUV still resembles a frequency comb, but now in the form of a cosine-modulated spectrum (see Fig. 1a). From a timedomain perspective, excitation with phase-locked pulses is a form of Ramsey excitation (see e.g. [2,3])

Cavity-enhanced dual-comb spectroscopy

The sensitivity of molecular fingerprinting is dramatically improved when placing the absorbing sample in a high-finesse optical cavity, thanks to the large increase of the effective path-length. As demonstrated recently, when the equidistant lines from a laser frequency comb are simultaneously injected into the cavity over a large spectral range, multiple trace-gases may be identified within a few milliseconds. Analyzing efficiently the light transmitted through the cavity however still remains challenging. Here, a novel approach, cavity-enhanced frequency comb Fourier transform spectroscopy, fully overcomes this difficulty and measures ultrasensitive, broad-bandwidth, high-resolution spectra within a few tens of $\mu$s. It could be implemented from the Terahertz to the ultraviolet regions without any need for detector arrays. We recorded, within 18 $\mu$s, spectra of the 1.0 $\mu$m overtone bands of ammonia spanning 20 nm with 4.5 GHz resolution and a noise-equivalent-absorption at one-second-averaging per spectral element of 3 10^-12 cm^-1Hz^-1/2, thus opening a route to time-resolved spectroscopy of rapidly-evolving single-events