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Abstract:
An extreme ultraviolet frequency comb for highly charged ion metrology
Highly charged ions (HCI) have been proposed as extremely sensitive probes for physics
beyond the Standard Model, such as a possible α-variation, and as novel frequency standards,
due to their insensitivity to external fields. We aim at performing ultra-high precision
spectroscopy of HCI in the extreme ultraviolet (XUV) region, where many transitions
are located. Therefore, we have developed an XUV frequency comb. Femtosecond pulses
from a 100MHz phase-stabilized near-infrared comb are amplified and fed into an enhancement
cavity inside an ultra-high vacuum chamber. In the tight focus (w0 = 15 μm)
of the astigmatism-compensated cavity, intensities ~1014 W/cm2 are reached. As a first
application, we perform multi-photon ionization of xenon using the velocity-map imaging
technique. The high repetition rate facilitates fast data acquisition even at low intensities,
enabling future precision tests in nonlinear physics. Finally, we have observed outcoupled
XUV radiation, produced in the cavity focus, up to the 35th harmonic order (42 eV;
30 nm). No signs of mirror degradation were observed during five hours of continuous
operation. Using He:Xe gas mixtures, improved phase-matching conditions led to 49 μW
output power at 16 eV. This is sufficient to drive HCI transitions with kHz excitation rates
and is an important step towards XUV frequency metrology with HCI.
