Final Report Summary - MEGA-XUV (Efficient megahertz coherent XUV light source)
Coherent extreme ultraviolet (XUV) light sources open up numerous opportunities for science and technology. Femtosecond laser-driven high harmonic generation (HHG) in gases is the most successful method for coherent table-top XUV generation. Initial HHG systems were limited to low repetition rates in the kilohertz range, which is a strong limitation for many experiments. High repetition rates in the megahertz range can strongly reduce measurement time, improve signal-to-noise ratio, and enable XUV frequency comb metrology. Due to this large scientific potential, the last years have seen a tremendous increase in research efforts targeting HHG with high photon flux at MHz repetition rate.
In this ERC project, we developed a new approach for coherent XUV sources operating at megahertz repetition rates. We developed a compact XUV source that generates high harmonics directly inside the cavity of an ultrafast thin-disk laser oscillator, a laser concept that is power-scalable and that has been achieving the highest power levels from any femtosecond oscillator technology since its invention in the year 2000. Instead of amplifying a laser beam to high power levels and dumping it after the HHG interaction, the generation of high harmonics is placed directly inside the intra-cavity beam of the high power femtosecond laser. Thus, the unconverted light is “recycled”, and the laser medium only needs to compensate for the low losses of the resonator. In contrast to passive enhancement cavities, no complex input coupling of femtosecond pulses is required.
During the project, we strongly improved the ultrafast thin-disk laser technology, achieving new records in average output power, pulse energy, and pulse duration. Since their invention 17 years ago, ultrafast thin-disk lasers have been delivering longer pulses than lasers based on the same gain materials used in the bulk geometry. In our project, we demonstrated that the combination of broad-band Yb-gain materials with Kerr-lens modelocking enables 30-fs pulses, which is the shortest duration ever obtained from ultrafast thin disk lasers and equal to the shortest pulses obtained from Yb-bulk oscillators. Our results show that the ultrafast thin disk laser configuration is superior for the generation of ultrashort pulses compared to the standard bulk geometry thanks to its advantages with respect to thermal effects, nonlinearity, and pump geometry, and we expect that few-cycle pulses will be feasible by optimized dispersion and cavity engineering. We also proved that ultrafast thin disk lasers are well suited for ultrastable operation, demonstrating Carrier Envelope Offset (CEO) frequency measurement and stabilization. In addition, we invented a new method for CEO stabilization via opto-optical modulation (OOM) of a semiconductor saturable absorber, which overcomes limitations imposed by the gain lifetime.
In our final demonstration of intracavity HHG inside a thin disk laser, we drive HHG in a xenon gas jet at a repetition rate of 17.4 MHz. Despite the high pressure of the gas jet, the laser operates at high stability. We detect harmonics up to the 17th order (wavelength 60.8 nm, 20.4 eV photon energy). Due to the power-scalability of the thin disk concept, we expect that this new class of compact XUV sources will become a versatile tool for areas such as structural analysis of matter, attosecond science, XUV spectroscopy, and high resolution imaging.
In this ERC project, we developed a new approach for coherent XUV sources operating at megahertz repetition rates. We developed a compact XUV source that generates high harmonics directly inside the cavity of an ultrafast thin-disk laser oscillator, a laser concept that is power-scalable and that has been achieving the highest power levels from any femtosecond oscillator technology since its invention in the year 2000. Instead of amplifying a laser beam to high power levels and dumping it after the HHG interaction, the generation of high harmonics is placed directly inside the intra-cavity beam of the high power femtosecond laser. Thus, the unconverted light is “recycled”, and the laser medium only needs to compensate for the low losses of the resonator. In contrast to passive enhancement cavities, no complex input coupling of femtosecond pulses is required.
During the project, we strongly improved the ultrafast thin-disk laser technology, achieving new records in average output power, pulse energy, and pulse duration. Since their invention 17 years ago, ultrafast thin-disk lasers have been delivering longer pulses than lasers based on the same gain materials used in the bulk geometry. In our project, we demonstrated that the combination of broad-band Yb-gain materials with Kerr-lens modelocking enables 30-fs pulses, which is the shortest duration ever obtained from ultrafast thin disk lasers and equal to the shortest pulses obtained from Yb-bulk oscillators. Our results show that the ultrafast thin disk laser configuration is superior for the generation of ultrashort pulses compared to the standard bulk geometry thanks to its advantages with respect to thermal effects, nonlinearity, and pump geometry, and we expect that few-cycle pulses will be feasible by optimized dispersion and cavity engineering. We also proved that ultrafast thin disk lasers are well suited for ultrastable operation, demonstrating Carrier Envelope Offset (CEO) frequency measurement and stabilization. In addition, we invented a new method for CEO stabilization via opto-optical modulation (OOM) of a semiconductor saturable absorber, which overcomes limitations imposed by the gain lifetime.
In our final demonstration of intracavity HHG inside a thin disk laser, we drive HHG in a xenon gas jet at a repetition rate of 17.4 MHz. Despite the high pressure of the gas jet, the laser operates at high stability. We detect harmonics up to the 17th order (wavelength 60.8 nm, 20.4 eV photon energy). Due to the power-scalability of the thin disk concept, we expect that this new class of compact XUV sources will become a versatile tool for areas such as structural analysis of matter, attosecond science, XUV spectroscopy, and high resolution imaging.