Journal Article

Direct frequency comb spectroscopy in the extreme ultraviolet

Cingöz, Arman, Hartl, Ingmar, Fermann, Martin E., Allison, Thomas K., Ruehl, Axel, Ye, Jun, Yost, Dylan C.

2012

Overview

By coupling a high-power, high-repetition-rate near-infrared frequency comb to a femtosecond optical cavity, a frequency comb operating in the extreme-ultraviolet spectral range has been produced, by high harmonic generation, and provides high-resolution spectroscopy in this spectral region. Precision spectroscopy in the extreme UV Laser-based optical frequency combs, so called because they emit evenly spaced spectral lines, are used in precision spectroscopy and other applications requiring accurate measurements, such as atomic clocks. Efforts to extend this capability to shorter wavelengths in the extreme ultraviolet — which would open up exciting new applications, including searches for variation in fundamental constants — have lacked sufficient power for the purpose until now. Jun Ye and co-workers demonstrate a new approach, using a high-power, high-repetition pulsed infrared laser coupled into an optical cavity, to produce an improved extreme UV comb. In a first precision spectroscopy demonstration, they use direct frequency comb spectroscopy to determine argon and neon atomic transitions with ultra-high precision. The development of the optical frequency comb (a spectrum consisting of a series of evenly spaced lines) has revolutionized metrology and precision spectroscopy owing to its ability to provide a precise and direct link between microwave and optical frequencies 1 , 2 . A further advance in frequency comb technology is the generation of frequency combs in the extreme-ultraviolet spectral range by means of high-harmonic generation in a femtosecond enhancement cavity 3 , 4 . Until now, combs produced by this method have lacked sufficient power for applications, a drawback that has also hampered efforts to observe phase coherence of the high-repetition-rate pulse train produced by high-harmonic generation, which is an extremely nonlinear process. Here we report the generation of extreme-ultraviolet frequency combs, reaching wavelengths of 40 nanometres, by coupling a high-power near-infrared frequency comb 5 to a robust femtosecond enhancement cavity. These combs are powerful enough for us to observe single-photon spectroscopy signals for both an argon transition at 82 nanometres and a neon transition at 63 nanometres, thus confirming the combs’ coherence in the extreme ultraviolet. The absolute frequency of the argon transition has been determined by direct frequency comb spectroscopy. The resolved ten-megahertz linewidth of the transition, which is limited by the temperature of the argon atoms, is unprecedented in this spectral region and places a stringent upper limit on the linewidth of individual comb teeth. Owing to the lack of continuous-wave lasers, extreme-ultraviolet frequency combs are at present the only promising route to extending ultrahigh-precision spectroscopy to the spectral region below 100 nanometres. At such wavelengths there is a wide range of applications, including the spectroscopy of electronic transitions in molecules 6 , experimental tests of bound-state and many-body quantum electrodynamics in singly ionized helium and neutral helium 7 , 8 , 9 , the development of next-generation ‘nuclear’ clocks 10 , 11 , 12 and searches for variation of fundamental constants 13 using the enhanced sensitivity of highly charged ions 14 .

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/766/25 / 639/766/930/527 / Argon / Atomic properties / Biological and medical applications / Exact sciences and technology / Femtosecond / Frequency conversion ; harmonic generation, including high-order harmonic generation / Frequency synthesizers / Fundamental areas of phenomenology (including applications) / Helium / Humanities and Social Sciences / Laser spectroscopy / Lasers / letter / Magnetic properties / multidisciplinary / Neon / Nonlinear optics / Optical frequency / Optics / Photons / Physics / Science / Science (multidisciplinary) / Spectra / Spectroscopy / Spectrum analysis / Ultraviolet / Ultraviolet radiation / Wavelengths