Abstract
We have developed an extreme ultraviolet (XUV) frequency comb for performing ultra-high precision spectroscopy on the many XUV transitions found in highly charged ions (HCI). Femtosecond pulses from a 100 MHz phase-stabilized near-infrared frequency comb are amplified and then fed into a femtosecond enhancement cavity (fsEC) inside an ultra-high vacuum chamber. The low-dispersion fsEC coherently superposes several hundred incident pulses and, with a single cylindrical optical element, fully compensates astigmatism at the w0 = 15 µm waist cavity focus. With a gas jet installed there, intensities reaching ∼ 1014 W/cm2 generate coherent high harmonics with a comb spectrum at 100 MHz rate. We couple out of the fsEC harmonics from the 7th up to the 35th (42 eV; 30 nm) to be used in upcoming experiments on HCI frequency metrology.
Highlights
IntroductionThe frequency comb (FC) revolutionized the field of optical frequency metrology [1]
Soon after its invention, the frequency comb (FC) revolutionized the field of optical frequency metrology [1]
We have developed an extreme ultraviolet (XUV) frequency comb for performing ultra-high precision spectroscopy on the many XUV transitions found in highly charged ions (HCI)
Summary
The frequency comb (FC) revolutionized the field of optical frequency metrology [1]. In 2005, the FC spectral range was extended to the extreme ultraviolet (XUV) region below 100 nm using the process of high-harmonic generation (HHG) to produce odd harmonics of the central wavelength of the original comb [7, 8] displaying the same mode spacing. This technique has rapidly evolved since its introduction; the generated powers have increased by roughly six orders of magnitude and are currently on the order of mW per harmonic [9]; the highest photon energies have surpassed 100 eV [10]; and it was shown that such combs can generate radiation with coherence times exceeding 1 s [11]
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