Advanced control of ultrashort and high-power pulses in enhancement cavities

In the decade preceding this thesis, femtosecond enhancement cavities had emerged as a highly promising technology in the context of extreme ultraviolet light (XUV) sources for frequency comb metrology and attosecond physics. These applications require light of laser-like coherence, which can be provided by high-order harmonic generation (HHG), a highly nonlinear frequency conversion process driven by intense ultrashort laser pulses. The laser systems commonly used to drive HHG are limited to pulse repetition rates in the kilohertz range. In contrast, the enhancement of femtosecond pulses in passive optical cavities to average powers of many kilowatts delivers the necessary intensities even at repetition rates of tens to hundreds of megahertz. Achieving sufficient XUV flux with megahertz repetition rates would enable the extension of frequency comb metrology to the XUV, and dramatically reduce data acquisition times for experiments in attosecond physics. However, cavity-enhanced HHG comes with unique challenges, imposing cavity-related limitations to the power, peak intensity, and minimum duration of the driving pulses. In this thesis, several novel approaches to extending the capabilities of femtosecond enhancement cavities are presented. In a first experiment, we demonstrated the compensation of thermal lensing effects in enhancement cavities. Using intracavity Brewster plates, which also offer a robust solution for XUV output coupling in cavity-enhanced HHG setups, we gained control over the thermally-induced mode change at average powers of up to 160 kW. Subsequently, we investigated the effects of nonlinear phase modulations caused by ionization in an intracavity gas target, which is a prerequisite for HHG. We experimentally validated a numerical model of the plasma-cavity interaction, leading to a scaling law allowing for the layout of optimized cavity HHG systems, and a proposal for tailoring the spectral finesse of cavities to exploit the nonlinear phase modulation for intracavity pulse compression. In parallel, we worked on the design and characterization of highly reflective multilayer mirrors to optimize the cavity dispersion. Combining different mirrors with compatible spectral phase characteristics, we demonstrated enhancement cavities supporting waveform-stable pulses, and cavities supporting pulse durations approaching the few-cycle regime. These results represent vital technological developments towards the goal of isolated attosecond pulse generation with enhancement cavities. Finally, we applied the developed methods of dispersion control to design an enhancement cavity for intracavity pulse compression using self-phase modulation in a Brewster plate. Implementing a flexible locking scheme, we demonstrated for the first time the generation of temporal cavity solitons in free-space enhancement cavities. The temporal compression from 350 fs to 37 fs together with the spectrally tailored finesse resulted in a peak power enhancement factor of over 3000, significantly surpassing the enhancement in linear cavities supporting similar pulse durations. This intriguing result opens the door to a novel regime of nonlinear cavity operation, with potentially significant benefits to cavity-enhanced HHG. In addition, we proposed a concept for optomechanical cavity dumping, with the potential to aid efforts employing enhancement cavities for a new generation of high-pulse-energy lasers

Advanced control of ultrashort and high-power pulses in enhancement cavities

In the decade preceding this thesis, femtosecond enhancement cavities had emerged as a highly promising technology in the context of extreme ultraviolet light (XUV) sources for frequency comb metrology and attosecond physics. These applications require light of laser-like coherence, which can be provided by high-order harmonic generation (HHG), a highly nonlinear frequency conversion process driven by intense ultrashort laser pulses. The laser systems commonly used to drive HHG are limited to pulse repetition rates in the kilohertz range. In contrast, the enhancement of femtosecond pulses in passive optical cavities to average powers of many kilowatts delivers the necessary intensities even at repetition rates of tens to hundreds of megahertz. Achieving sufficient XUV flux with megahertz repetition rates would enable the extension of frequency comb metrology to the XUV, and dramatically reduce data acquisition times for experiments in attosecond physics. However, cavity-enhanced HHG comes with unique challenges, imposing cavity-related limitations to the power, peak intensity, and minimum duration of the driving pulses. In this thesis, several novel approaches to extending the capabilities of femtosecond enhancement cavities are presented. In a first experiment, we demonstrated the compensation of thermal lensing effects in enhancement cavities. Using intracavity Brewster plates, which also offer a robust solution for XUV output coupling in cavity-enhanced HHG setups, we gained control over the thermally-induced mode change at average powers of up to 160 kW. Subsequently, we investigated the effects of nonlinear phase modulations caused by ionization in an intracavity gas target, which is a prerequisite for HHG. We experimentally validated a numerical model of the plasma-cavity interaction, leading to a scaling law allowing for the layout of optimized cavity HHG systems, and a proposal for tailoring the spectral finesse of cavities to exploit the nonlinear phase modulation for intracavity pulse compression. In parallel, we worked on the design and characterization of highly reflective multilayer mirrors to optimize the cavity dispersion. Combining different mirrors with compatible spectral phase characteristics, we demonstrated enhancement cavities supporting waveform-stable pulses, and cavities supporting pulse durations approaching the few-cycle regime. These results represent vital technological developments towards the goal of isolated attosecond pulse generation with enhancement cavities. Finally, we applied the developed methods of dispersion control to design an enhancement cavity for intracavity pulse compression using self-phase modulation in a Brewster plate. Implementing a flexible locking scheme, we demonstrated for the first time the generation of temporal cavity solitons in free-space enhancement cavities. The temporal compression from 350 fs to 37 fs together with the spectrally tailored finesse resulted in a peak power enhancement factor of over 3000, significantly surpassing the enhancement in linear cavities supporting similar pulse durations. This intriguing result opens the door to a novel regime of nonlinear cavity operation, with potentially significant benefits to cavity-enhanced HHG. In addition, we proposed a concept for optomechanical cavity dumping, with the potential to aid efforts employing enhancement cavities for a new generation of high-pulse-energy lasers

Tailoring the transverse mode of a high-finesse optical resonator with stepped mirrors

Enhancement cavities (ECs) seeded with femtosecond pulses have developed into the most powerful technique for high-order harmonic generation (HHG) at repetition rates in the tens of MHz. Here, we demonstrate the feasibility of controlling the phase front of the excited transverse eigenmode of a ring EC by using mirrors with stepped surface profiles, while maintaining the high finesse required to reach the peak intensities necessary for HHG. The two lobes of a TEM01 mode of a 3.93m long EC, seeded with a single-frequency laser, are delayed by 15.6 fs with respect to each other before a tight focus, and the delay is reversed after the focus. The tailored transverse mode exhibits an on-axis intensity maximum in the focus. Furthermore, the geometry is designed to generate a rotating wavefront in the focus when few-cycle pulses circulate in the EC. This paves the way to gating isolated attosecond pulses (IAPs) in a transverse manner (similarly to the attosecond lighthouse), heralding IAPs at repetition rates well into the multi-10MHz range. In addition, these results promise high-efficiency harmonic output coupling from ECs in general, with an unparalleled power scalability. These prospects are expected to tremendously benefit photoelectron spectroscopy and extreme-ultraviolet frequency comb spectroscopy

Cumulative plasma effects in cavity-enhanced high-order harmonic generation in gases

Modern ultrafast laser architectures enable high-order harmonic generation (HHG) in gases at (multi-) MHz repetition rates, where each atom interacts with multiple pulses before leaving the HHG volume. This raises the question of cumulative plasma effects on the nonlinear conversion. Utilizing a femtosecond enhancement cavity with HHG in argon and on-axis geometric extreme-ultraviolet (XUV) output coupling, we experimentally compare the single-pulse case with a double-pulse HHG regime in which each gas atom is hit by two pulses while traversing the interaction volume. By varying the pulse repetition rate (18.4 and 36.8 MHz) in an 18.4-MHz roundtrip-frequency cavity with a finesse of 187, and leaving all other pulse parameters identical (35-fs, 0.6-mu J input pulses), we observe a dramatic decrease in the overall conversion efficiency (output-coupled power divided by the input power) in the double-pulse regime. The plateau harmonics (25-50 eV) exhibit very similar flux despite the twofold difference in repetition rate and average power. We attribute this to a spatially inhomogeneous plasma distribution that reduces the HHG volume, decreasing the generated XUV flux and/or affecting the spatial XUV beam profile, which reduces the efficiency of output coupling through the pierced mirror. These findings demonstrate the importance of cumulative plasma effects for power scaling of high-repetition-rate HHG in general and for applications in XUV frequency comb spectroscopy and in attosecond metrology in particular

Velocity- and pointing-error measurements of a 300 000-r/min self-bearing permanent-magnet motor for optical applications

Compact, ultra-high-speed self-bearing permanent-magnet motors enable a wide scope of applications including an increasing number of optical ones. For implementation in an optical setup the rotors have to satisfy high demands regarding their velocity and pointing errors. Only a restricted number of measurements of these parameters exist and only at relatively low velocities. This manuscript presents the measurement of the velocity and pointing errors at rotation frequencies up to 5 kHz. The acquired data allows to identify the rotor drive as the main source of velocity variations with fast fluctuations of up to 3.4 ns (RMS) and slow drifts of 23 ns (RMS) over ~120 revolutions at 5 kHz in vacuum. At the same rotation frequency the pointing fluctuated by 12 $\mu$rad (RMS) and 33 $\mu$rad (peak-to-peak) over ~10000 roundtrips. To our best knowledge this states the first measurement of velocity and pointing errors at multi-kHz rotation frequencies and will allow potential adopters to evaluate the feasibility of such rotor drives for their application

What do consumers want to know about drugs (medication and lifestyle) in their pregnancy journey?

We demonstrate the enhancement of 250-fs pulses in a passive resonator to 400 kW of average power. Thermal effects are investigated and mitigated by custom mirrors and optimized cavity designs