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We experimentally demonstrate an efficient and broadband extreme-ultraviolet light (XUV) out-coupling mechanism of intra-cavity generated high harmonics. The mechanism is based on a coated grazing-incidence plate (GIP), which utilizes the enhanced reflectivity of s-polarized light in comparison to p-polarized light for large angles of incidence (AoI). We design and produce a 60°-AoI coated GIP, tailored specifically for the high demands inside a sub-50-fs Kerr-lens mode-locked Yb:YAG thin-disk laser oscillator in which high harmonic generation (HHG) is driven at ∼450 MW peak power and 17 MHz repetition rate. The coated GIP features an XUV out-coupling efficiency of25% for photon energies ranging from 10 eV to 60 eV while being anti-reflective for the driving laser field. The XUV spectra reach up to 52 eV in argon and 30 eV in xenon. In a single harmonic, we out-couple 1.3 µW of XUV average power at 37 eV in argon and 5.4 µW at 25 eV in xenon. The combination of an improved HHG driving laser performance and the out-coupling via the coated GIP enabled us to increase the out-coupled XUV average power in a single harmonic by a factor of 20 compared to previous HHG inside ultrafast laser oscillators. Our source approaches the state-of-the-art out-coupled XUV power levels per harmonic of femtosecond enhancement cavities operating at comparable photon energies.

This thesis is devoted to the development of ultrafast thin-disk lasers (TDL) oscillators and their use for intra-oscillator high-harmonic generation (HHG). The main motivation was to develop a simple and compact source of coherent extreme ultraviolet (XUV) light. Conventionally, coherent XUV radiation can be found in large scale synchrotron or free-electron laser facilities. Laser driven HHG systems have long strived to become a lab-scale alternative to these sources. The major challenge has been the low conversion efficiency of the HHG process and the high demands on the driving laser. With the advancing laser technology, this situation is rapidly improving and several state-of-the-art HHG systems can nowadays reach an XUV flux which is in certain aspects comparable to synchrotrons. The intra-laser-oscillator HHG approach is inspired by femtosecond enhancement cavities. Driving HHG inside an enhancement cavity allows for recycling the unconverted pulse energy, while increasing the available power by the cavity enhancement factor. An enhancement cavity, however, requires coherent coupling of the femtosecond pulses into the cavity which is a demanding task. The intra-oscillator HHG approach offers a simplified concept using directly the cavity of the laser instead of an external enhancement cavity. To achieve sufficiently high driving power for efficient intra-oscillator HHG, the existing TDL technology had to be significantly improved and optimized for intracavity performance. Within the scope of this thesis, a strong focus was placed on the development of the driving laser and significant progress has been achieved. To this purpose, we investigated and demonstrated Kerr lens modelocked TDL oscillators based on the gain material Yb:YAG, which achieved several records in terms of their output performance. Using our system we have demonstrated the shortest pulse duration of any TDL oscillator of 27 fs, as well as the highest average power in the sub-100-fs regime of 100 W and the highest peak power of 100 MW. The corresponding increase of intracavity performance of our laser allowed us to significantly improve the HHG operation in terms of the generated XUV flux and photon energies. Starting from 0.5 nW generated at 13 eV demonstrated five years ago, we have improved the XUV flux to 10 μW at 30 eV at the current state. Although this is not yet the highest performance among other HHG systems, the flux value is becoming competitive to the results of enhancement cavities operating at similar photon energy. The improved output performance of the demonstrated TDLs is also highly attractive for nonlinear conversion toward longer wavelengths of the electromagnetic spectrum. A part of this thesis is, thus, dedicated to terahertz generation. We have demonstrated the first TDL driven terahertz generation in 2018, through optical rectification in GaP. Later, we used the tunability and the high average power of our self-built TDL oscillator to experimentally investigate in detail the properties of optical rectification in GaP driven by 1- μm high-power Yb-based lasers. Finally, having the experience from intra-oscillator HHG, we have shown that the high-power driving laser can be replaced by driving the THz generation directly inside the cavity of a small KLM bulk laser oscillator.

Ultrafast thin-disk lasers continue achieving higher average powers at shorter pulse durations, reaching 100-MW peak power directly from the laser oscillator output. We discuss recent progress and its application to intra-oscillator high harmonic generation.

We demonstrate efficient short-pulse generation from Yb:YAG in the regime of strong intracavity self-phase-modulation. We reach 102-MW, 52-fs pulses at 103-W average-power with 26% optical-to-optical efficiency, presenting the highest peak-power of any ultrafast laser oscillator.

We develop and implement a coated grazing-incidence-plate for broadband and efficient XUV-out-coupling of intra-cavity generated high-harmonics. Moreover, we power-scale our thin-disk laser and reach with 2-GW the highest intra-cavity peak-power of any ultrafast laser oscillator.

We develop and implement a coated grazing-incidence-plate for broadband out-coupling of intracavity generated high-harmonics. We reach >25% XUV out-coupling efficiency resulting in 1.2-µW out-coupled average XUV power in a single harmonic at 37-eV and 17-MHz.

High harmonic generation (HHG) in a gas target is the most common approach for table-top coherent extreme-ultraviolet (XUV) light generation. Progressing from kilohertz to megahertz repetition rate XUV sources is of high interest for reducing acquisition times and enabling XUV frequency comb applications [1] .

High-power laser oscillators in the sub-100-fs pulse duration regime at megahertz repetition rates are a compact and reliable alternative to multi-stage amplifier systems. Sub-100-fs Ti:sapphire bulk oscillators have been the working horse in ultrafast science, but up to nowadays they have been limited in average power to 3.5 W [ Fig. 1a) ]. Yb-doped gain materials are better suited for high-power operation thanks to their lower quantum defect. Sub-100-fs Yb-based bulk oscillators have already reached 12.5 W. However, further power-scaling in the bulk geometry is strongly limited by thermal effects. The thin-disk geometry is advantageous for higher average powers as it reduces thermal effects in the gain material. Ultrafast thin-disk laser (TDL) oscillators operated already with up to 350 W average power at 940 fs pulse duration [1] and 150 W at 140 fs [2] . However, operation in the sub-100-fs regime still remains challenging. In 2019, we presented an Yb:Lu 2 O 3 TDL oscillator generating 21 W in 95-fs pulses [3] .

Dual-comb systems are attractive sources for various applications like fast high-resolution spectroscopy [1] . Typically, such systems rely on the complex active stabilization of two independently generated asynchronous pulse trains. A simpler and promising alternative is the generation of the two pulse trains directly from a single laser cavity. As cavity components are shared, most of the noise in both pulse trains is correlated and cancels out in the beating process, allowing for dual-comb applications in free-running laser operation. Among various single-cavity dual-comb laser architectures, the dual-comb thin-disk laser (TDL) stands out by providing more than an order of magnitude higher average power, being highly attractive for nonlinear frequency conversion into the desired spectral range [2] . However, the increased noise level mainly induced by the turbulent water-cooling of the disk, severely challenges the suitability of dual-comb TDLs for high-resolution spectroscopic applications.

Powerful broadband THz sources have proven to be a promising tool for time-domain spectroscopy and imaging since a decade. Many efforts have been undertaken to push the limit of this technology towards higher THz average powers and broader spectral bandwidths. A popular way for powerful broadband THz generation is optical rectification (OR) of ultrafast near-infrared pulses in nonlinear crystals. However, as OR is a relatively low efficiency process, powerful and complex driving laser systems are typically required. For example, Meyer et al . demonstrated a milliwatt-class broadband THz source using a powerful 112-W sub-100-fs nonlinearly-compressed thin-disk laser oscillator [1] . Although the THz power level of this source is very promising, the complexity and the size of the system might be a limiting factor for many applications. A simpler and more compact alternative is driving OR directly inside the cavity of a standard ultrafast bulk laser oscillator. This way, the THz generation benefits from the high-power levels directly available inside the laser oscillator cavity. While the first intra-oscillator THz generation via OR has already been demonstrated a decade ago [2] , the potential of this technique for high average THz power with a broad spectrum has not yet been exploited.

We demonstrate that Kerr lens modelocking is well-suited for operating an ultrafast thin-disk laser with intra-oscillator high harmonic generation (HHG) in the 100-fs pulse duration regime. Exploiting nearly the full emission bandwidth of the gain material Yb:YAG, we generate 105-fs pulses with an intracavity peak power of 365 MW and an intracavity average power of 470 W. We drive HHG in argon with a peak intensity of ∼7⋅10

We demonstrate the suitability of dual-comb thin-disk lasers for comb-line-resolved spectroscopy by measuring the absorption spectrum of acetylene. Operating with 240-fs, 6-8 W and 97-MHz, such sources are highly attractive for frequency-conversion into the mid-infrared.

We discuss recent developments and the state-of-the-art of high-harmonic generation inside thin-disk laser oscillators and their potential for further scaling of the XUV performance.

We demonstrate intra-laser-oscillator HHG reaching 10 µW at 30 eV generated in argon. The laser operated at 550 MW of intracavity peak power with 105 fs pulses at 17 MHz repetition rate.

Operating in a regime of strong self-phase-modulation, 84-fs pulses are generated with 42-MW peak power and 12% efficiency from Yb:YAG. We demonstrate a 3x higher average power than any previous sub-100-fs laser oscillator.

We drive optical rectification in GaP directly inside a sub-100-fs Yb-oscillator at 22-W intracavity power and 7-W of pump power, leading to >10-fold increase of efficiency with respect to pump power compared to similar technologies.

We demonstrate the generation of 29-fs pulses at 2.5-W and 37-fs pulses at 17.1-W average power. This is the highest average power of any sub-50-fs laser oscillator and the shortest pulse duration of any TDL oscillator.

We present intra-oscillator HHG in argon with photon energies up to 37 eV and ~2 µW of total generated XUV flux. The 11-MHz laser operates at 400-MW intracavity peak power and 500-W intracavity average power.

We present a novel scheme for dual-pulse-train operation in a single thin-disk laser cavity. We report on the first fCEO measurement of such systems, investigate the noise-contributions, and expect sub-3-kHz optical linewidths within 1-millisecond measurement-times.

We demonstrate HHG with ~1014 W/cm2 peak intensity in argon and '5x103؛ W/cm2 in krypton inside a mode-locked laser oscillator. The system operates with up to 180-W intiacavity average power with 89-fs pulses at 11-2020

We present the first dual-comb thin-disk laser based on polarization splitting. It generates 6-W and 8-W outputs of 240-fs pulses with adjustable repetition rate. We investigate noise properties and demonstrate a proof-of-principle spectroscopy experiment.

We demonstrate HHG inside a thin-disk laser cavity at significantly decreased pulse duration, compared to the previous results. The system operates at around 100 fs with ~1014 W/cm2 peak intensity and 11 MHz repetition rate.

We experimentally investigate the limits of pulse duration in a Kerr-lens mode-locked Yb:YAG thin-disk laser (TDL) oscillator. Thanks to its excellent mechanical and optical properties, Yb:YAG is one of the most used gain materials for continuous-wave and pulsed TDLs. In mode-locked operation, its 8-nm wide gain bandwidth only directly supports pulses with a minimum duration of approximately 140 fs. For achieving shorter pulses, a Kerr-lens mode-locked TDL oscillator can be operated in the strongly self-phase modulation (SPM) broadened regime. Here, the spectral bandwidth of the oscillating pulse exceeds the available gain bandwidth by generating additional frequencies via SPM inside the Kerr medium. In this work, we study and compare different laser configurations in the strongly SPM-broadened regime. Starting with a configuration providing 84-fs pulses at 69 W average power at 17 MHz repetition rate, we reduce the pulse duration by optimizing various mode-locking parameters. One crucial parameter is the dispersion control which was provided by in-house-developed dispersive mirrors produced by ion-beam sputtering (IBS). We discuss trade-offs in average power, pulse duration, efficiency, and intra-cavity peak power. For the configuration operating at the highest SPM-broadening, we achieve a minimum pulse duration of 27 fs, which represents the shortest pulse duration directly generated by any ultrafast TDL oscillator. The corresponding full width at half maximum (FWHM) spectral bandwidth exceeds more than five times the FWHM gain bandwidth. The average output power of 3.3 W is moderate for ultrafast TDL oscillators, but higher than other Yb-based laser oscillators operating at this pulse duration. Additionally, the corresponding intra-cavity peak power of 0.8 GW is highly attractive for implementing intra-cavity extreme nonlinear optical interactions such as high harmonic generation.

We demonstrate broadband and powerful terahertz (THz) generation at megahertz repetition rate based on intra-oscillator optical rectification (OR) in gallium phosphide (GaP). By placing the nonlinear crystal directly inside the cavity of a Kerr-lens mode-locked ultrafast diode-pumped solid-state laser (DPSSL) oscillator, we demonstrate a compact and single-stage THz source. Using only 7 W of diode-pump power, we drive OR in a GaP crystal with 22 W of average power at ∼80 MHz repetition rate. In a first configuration, using a 0.3-mm-thick GaP and 105 fs driving pulses, we generate up to 150 µW of THz radiation with a spectrum extending to 5.5 THz. In a second configuration allowing for sub-50-fs pulse duration, we generate up to 7 THz inside a 0.1-mm-thick GaP crystal. This performance is well suited for THz time-domain spectroscopy and THz imaging. Intra-oscillator THz generation in sub-100-fs DPSSLs is a promising way to scale down footprint, complexity and cost of powerful broadband THz sources.

We demonstrate broadband high-power terahertz (THz) generation at megahertz repetition rates by optical rectification in GaP driven by an ultrafast Yb-based thin-disk laser (TDL) oscillator. We investigate the influence of pulse duration in the range of 50–220 fs and thickness of the GaP crystal on the THz generation. Optimization of these parameters with respect to the broadest spectral bandwidth yields a gap-less THz spectrum extending to nearly 7 THz. We further tailor the driving laser and the THz generation parameters for the highest average power, demonstrating 0.3 mW THz radiation with a spectrum extending to 5 THz. This was achieved using a 0.5 mm thick GaP crystal pumped with a 95 fs, 20 W TDL, operating at 48 MHz repetition rate. We also provide a simple method to estimate the THz spectrum, which can be used for design and optimization of similar THz systems.

We investigate power-scaling of a Kerr lens mode-locked (KLM) Yb:Lu

High-power laser oscillators in the sub-100-fs pulse duration regime at megahertz repetition rates are compact and reliable alternatives to multi-stage amplifier systems. Sub-100-fs bulk oscillators based on Ti:sapphire — the working horses in ultrafast science — are currently limited to 3.5 W of average power [1]. In comparison, sub-100-fs bulk oscillators based on other gain materials have already reached 12.5 W [2]. However, thermal effects in the gain material are a severe challenge for further power-scaling [Fig. 1(a)].

We demonstrate a Kerr lens mode-locked thin-disk laser oscillator operating with 95-fs pulses at 21.1 W of average power. This is the highest average power achieved by any oscillator in the sub-100-fs regime.

We demonstrate broadband THz generation driven by an ultrafast thin-disk laser (TDL) oscillator. By optical rectification of 50-fs pulses at 61 MHz repetition rate in a collinear geometry in crystalline GaP, THz radiation with a central frequency at around 3.4 THz and a spectrum extending from below 1 THz to nearly 7 THz are generated. We realized a spectroscopic characterization of a GaP crystal and a benchmark measurement of the water-vapor absorption spectrum in the THz range. Sub-50-GHz resolution is achieved within a 5 THz bandwidth. Our experiments show the potential of ultrafast TDL oscillators for driving MHz-repetition-rate broadband THz systems.

We present a proof-of-principle demonstration of THz generation driven by a short-pulse thin-disk laser (TDL) oscillator. The laser source is based on a directly diode-pumped Yb:Lu 2 O 3 gain crystal delivering 4 W average power in 50-fs pulses at 61 MHz repetition rate. Broadband THz radiation up to 6 THz is generated by optical rectification and detected via electrooptic sampling in GaP crystals. Our experiment shows the high potential of this power-scalable technology for driving MHz-repetition-rate broadband THz systems.

Morphing commonly refers to the smooth transition from a specific shape into another one, in which the initial and final shapes can be significantly different. In this study, we show that the concept of morphing applied to laser micro-manufacturing offers an opportunity to change the topology of an initial shape, and to turn it into something more complex, like for instance for creating self-sealed cavities. Such cavities could be filled with various gases, while also achieving an optical surface quality since being shaped by surface tension. Furthermore, we demonstrate that laser morphing can be accurately modelled and predicted. Finally, we illustrate the possible use of 'laser-morphed' shape to achieve high-quality resonators that can find applications, for instance, in ultra-small quantities molecules label-free detection through whispering gallery mode resonances.

The paper presents application of dielectrophoresis for manipulation of a microparticle in a liquid medium above a planar surface. The force on the microparticle is exerted by a non-uniform AC electric field. The field is generated by a set of micro-electrodes patterned on the planar surface and connected to a multi-channel voltage waveform generator. An novel yet simple control approach based on changing the phase delays among voltages on the electrodes is introduced together with a description of an efficient hardware implementation. Laboratory experiments with a 250-micron polystyrene bead are documented. This work was partly motivated by the series of international competitions called Mobile Microrobotics Challenge and organized by NIST and taken over by IEEE RAS recently.

THz-time-domain spectroscopy (THz-TDS) is a well-established tool for characterizing material properties and investigating dynamics of complex molecular systems. As the broadband THz generation typically relies on sub-100-fs laser pulses, most systems are currently driven by Ti:sapphire lasers. In the last years there has been a strong effort to replace Ti:sapphire systems with power-scalable Yb-based directly diode-pumped solid-state lasers (DPSSLs) [1]. Ultrafast DPSSLs generate higher average power with lower complexity and better efficiency. However, their potential for high-power broadband THz operation has not yet been fully exploited [2].

Emergence of products that feature functional surfaces with complex geometries, such as freeform optics in consumer electronics and augmented reality and virtual reality, requires high-accuracy non-contact surface measurement. However, large discrepancies are often observed between the measurement results of optical methods and contact stylus methods, especially for complex surfaces. For interference microscopy, such as coherence scanning interferometry, the three-dimensional surface transfer function provides information about the instrument spatial frequency passband and about lens aberrations that can result in measurement errors. Characterisation and phase inversion of the instrument's three-dimensional surface transfer function yields an inverse filter that can be applied directly to the three-dimensional fringe data. The inverse filtering is shown to reduce measurement errors without using any data processing or requiring any a priori knowledge of the surface. We present an experimental verification of the characterisation and correction process for measurements of several freeform surfaces and an additive manufactured surface. Corrected coherence scanning interferometry measurements agree with traceable contact stylus measurements to the order of 10 nm.