Abstract
We realized a solid-state-based vacuum ultraviolet frequency comb by harmonics generation in an external enhancement cavity. Optical conversions were so far reported by only using gaseous media. We present a theory that allows the most suited solid generation medium to be selected for specific target harmonics by adapting the material's bandgap. We experimentally use a thin AlN film grown on a sapphire substrate to realize a compact frequency comb high-harmonic source in the Deep Ultraviolet (DUV) / Vacuum Ultraviolet (VUV) spectral range. By extending our earlier VUV source [Opt. Express26, 21900 (2018)] with the enhancement cavity, a sub-Watt level Ti:sapphire femtosecond frequency comb is enhanced to 24 W stored average power, its 3rd, 5th, and 7th harmonics are generated, and the targeted 5th harmonic's power at 160 nm increased by two orders of magnitude. The emerging nonlinear effects in the solid medium, together with suitable intra-cavity dispersion management, support optimal enhancement and stable locking. To demonstrate the realized frequency comb's spectroscopic ability, we report on the beat measurement between the 3rd harmonic beam and a 266 nm CW laser reaching about 1 MHz accuracy.
Highlights
The extension of frequency comb metrology beyond the UV spectral range into the DUV [1,2,3,4] and even into the VUV and XUV [5,6,7,8,9] gives an opportunity to measure new important atomic and molecular transitions for testing quantum electrodynamics or to look for new atomic clock transitions
We report the first realization of a VUV frequency comb using solid material as the medium for non-perturbative multi-harmonic generation within an enhancement cavity. (We refer to our observations as "multi-harmonic generation", as only 3rd, 5th and 7th orders are observed with the available detectors.) very early realizations [21,22] were reported, solids as generation media have recently attracted new attention because they require lower laser peak intensities (> 1011 W/cm2) than gases for reaching the VUV or XUV spectral ranges and they can operate even at MHz's repetition rates [23,24,25]
We found that AlN with its 6.3 eV band gap is an almost optimal material for the efficient generation of our target wavelength of 160 nm (5th harmonic)
Summary
The extension of frequency comb metrology beyond the UV spectral range into the DUV [1,2,3,4] and even into the VUV and XUV [5,6,7,8,9] gives an opportunity to measure new important atomic and molecular transitions for testing quantum electrodynamics or to look for new atomic clock transitions. To reach short wavelengths at high repetition rates, non-perturbative highharmonic generation is currently the only candidate Such high-harmonic sources were successfully realized by adding an enhancement cavity to fiber laser systems or Ti:sapphire oscillators [13,14,15,16,17,18]. Different noble gases were used for non-perturbative frequency conversion, which relies on the ionization of the gases and requires suitably high (> 1013 W/cm2) laser peak intensity Such intensity cannot be maintained at the very high repetition rate (typically > 100 MHz) of the frequency combs, especially those based on Ti:sapphire oscillators. Intense 5th harmonic is generated from a thin AlN crystalline film grown on a sapphire substrate within a compact setup and the appearance of the 7th harmonic is demonstrated (see Fig. 2)
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