Van der Waals-Zeeman Colloquium

Tuesday, 5 October 2010, 16:00, room G2.10 (Science Park 904)

Extreme ultraviolet frequency comb metrology

Prof. Kjeld Eikema
Laser Centre, Vrije Universiteit, Amsterdam

Since the invention of frequency comb lasers, now 10 years ago, these devices have made a profound impact in many fields of physics. Comb lasers are based on mode-locked lasers, which produce a repetitive train of ultrafast pulses. With these devices it has become possible to control the electromagnetic waves of optical pulses, and to perform extremely precise frequency measurements over a wide range of the electromagnetic spectrum. This has e.g. resulted in the emergence of attosecond science, and made atomic clocks and tests of the basic laws of physics possible with unprecedented precision.

Up to now, application of frequency combs has been limited to wavelengths in the (far) infrared to ultraviolet range. In the lecture, after an introduction on comb lasers, it will be shown that the frequency comb principle can be extended to much shorter wavelengths in the extreme ultraviolet (XUV, wavelengths below 100 nm). Phase-coherent amplification and carefully controlled high-harmonic generation techniques enable comb generation over a wide range of wavelengths in the XUV. To illustrate the versatility of the method, XUV frequency comb excitation of helium at 51 nm will be discussed. With this experiment a nearly 10-fold improved ground state ionization energy was determined, challenging the accuracy of the most recent QED calculations in helium.