Extreme-ultraviolet frequency combs have previously been used to realize spectroscopy with a megahertz level resolution, but higher resolutions are desired for precision-measurement applications. Now, a sub-hertz spectral resolution is demonstrated, which corresponds to coherence times of over 1 s at photon energies up to 20 eV; such coherence times are over six orders of magnitude longer than those previously reported.
- Craig Benko
- Thomas K. Allison
- Jun Ye