Extreme Ultraviolet Frequency Comb Metrology

Abstract

The remarkable precision of frequency-comb (FC) lasers is transferred to the extreme ultraviolet (XUV, wavelengths shorter than 100 nm), a frequency region previously not accessible to these devices. A frequency comb at XUV wavelengths near 51 nm is generated by amplification and coherent upconversion of a pair of pulses originating from a near-infrared femtosecond FC laser. The phase coherence of the source in the XUV is demonstrated using helium atoms as a ruler and phase detector. Signals in the form of stable Ramsey-like fringes with high contrast are observed when the FC laser is scanned over P states of helium, from which the absolute transition frequency in the XUV can be extracted. This procedure yields a 4 He ionization energy at h  5 945 204 212ð6Þ MHz, improved by nearly an order of magnitude in accuracy, thus challenging QED calculations of this two-electron system. Mode-locked frequency-comb (FC) lasers [1,2] have revolutionized the field of precision laser spectroscopy. Optical atomic clocks using frequency combs are about to redefine the fundamental standard of frequency and time [3]. FC lasers have also vastly contributed to attosecond science by providing a way to synthesize electric fields at optical frequencies [4], made long distance absolute length measurements possible [5], and have recently been employed to produce ultracold molecules [6]. FC based precision spectroscopy on simple atomic systems has provided one of the most stringent tests of bound state quantum electrodynamics (QED) as well as upper bounds on the drift of fundamental constants [7]. Extending these methods into the extreme ultraviolet (XUV, wavelengths below 100 nm) spectral region is highly desirable since this would, for example, allow novel precision QED tests [8].Currently the wavelength range below 120 nm is essentially inaccessible to precision frequency metrology applications due to a lack of power of single frequency lasers and media for frequency up-conversion. Spectroscopic studies on neutral helium using amplified nanosecond laser pulses [9,10] are notoriously plagued by frequency chirping during amplification and harmonic conversion which limits the accuracy. These kind of transient effects can be avoided if a continuous train of high power laser pulses (produced by a FC) can be coherently up-converted. This would transfer the FC modes, at frequencies f n ¼ f CEO þ nf rep , where f CEO is the carrier-envelope offset frequency, f rep is the repetition frequency of the pulses, and n an integer mode number, to the XUV. Similar to what was shown in the visible [11,12], the up-converted pulse train could be used to directly excite a transition, with each of the up-converted modes acting like a single frequency laser.By amplification of a few pulses from the train, and producing low harmonics in crystals and gasses, sufficient coherence has been demonstrated down to 125 nm to perform spectroscopic experiments [13,14]. To reach wavelengths below 120 nm in the extreme ultraviolet or even x rays, high harmonic generati...

Towards measuring the ionisation and dissociation energies of molecular hydrogen with sub-MHz accuracy

“…Single-photon XUV metrology at an absolute accuracy of better than 100 kHz would necessitate the use of XUV frequency combs. 29 However, the high density of Rydberg states at high n values precludes such measurements. Although the application of dual-frequency-comb spectroscopy, which has been shown to be able to resolve dense molecular spectra, 30 may enable one to overcome this problem, it appears beyond reach at present.…”

Revealing quantum path details in high-field physics

“…Also, the approach can shed light on several strong-field light-matter interaction processes resulting in coherent light emission, including harmonic generation from atoms and molecules, surface plasma, and bulk crystals [7][8][9][42][43][44][45][46]. Attosecond science [1][2][3][4][5][10][11][12], high-resolution spectroscopy studies in the EUV [13][14][15] spectral region, and molecular tomography methods [7][8][9] are among the research topics that can benefit from the in situ control of the emitted EUV phase distribution and/or the spatial selection of the EUV-radiation-atom interaction products.…”

“…New pulse characterization techniques have been developed for measuring the duration of these pulses [1,3], which have been successfully utilized in the observation of a number of new processes in all states of matter [3,4,[10][11][12]. At the same time, frequency combs have been recently developed in the EUV spectral region [13][14][15], pushing the frequency resolution in the MHz range, thus improving the precision of the measurements by an order of magnitude [14,15]. Thus, the detailed study of the strong-field light-matter interaction is essential for an in-depth understanding of the harmonic generation mechanism and the further development of the above-mentioned research directions.…”

A simple 2 W continuous-wave laser system for trapping ultracold metastable helium atoms at the 319.8 nm magic wavelength

“…Frequency metrology has been employed as a sensitive test of QED calculations, both from the ground state [1,2] and from the long-lived (lifetime ~8000 s) metastable 2 3 S state (He * ) [3][4][5]. Another interesting target for spectroscopy is to probe the influence on level energies of the finite size of the nucleus.…”