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Abstract
Resonant enhancement inside an optical cavity has been a wide-spread approach to increase efficiency of nonlinear optical conversion processes while reducing the demands on the driving laser power. This concept has been particularly important for high harmonic generation XUV sources, where passive femtosecond enhancement cavities allowed significant increase in repetition rates required for applications in photoelectron spectroscopy, XUV frequency comb spectroscopy, including the recent endeavor of thorium nuclear clock development. In addition to passive cavities, it has been shown that comparable driving conditions can be achieved inside mode-locked thin-disk laser oscillators, offering a simplified single-stage alternative. This approach is less sensitive to losses thanks to the presence of gain inside the cavity and should thus allow higher conversion efficiencies through tolerating higher intensity in the gas target. Here, we show that the intra-oscillator approach can indeed surpass the much more mature technology of passive enhancement cavities in terms of XUV flux, even reaching comparable values to single-pass sources based on chirped-pulse fiber amplifier lasers. Our system operates at 17 MHz repetition rate generating photon energies between 60 eV and 100 eV. Importantly, this covers the highly attractive wavelength for the silicon industry of 13.5 nm at which our source delivers 60 nW of outcoupled average power per harmonic order.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ultrafast-laser-driven high harmonic generation (HHG) [1] has become an unusually polyvalent tool for modern physics. The compact laser setups have largely facilitated lab-scale experiments relying on coherent XUV and soft X-ray radiation [2–6] and provided access to the whole domain of attosecond physics [7–10]. This tremendous progress has been largely dependent on state-of-the-art lasers. The nonlinear conversion process of HHG in gas has represented one of the major challenges for the modern ultrafast laser technology in terms of wavelength, achievable pulse duration, pulse energy as well as average power and served as a catalyst for its development.
In between other directions, the laser driven HHG systems have always strived to increase the delivered photon flux. Initially, the technology was empowered by Ti:sapphire laser amplifier systems which have been capable of delivering very high pulse energies and peak power required for the highly nonlinear process of HHG. Although HHG strongly depends on these values, its efficiency saturates at a certain laser intensity and it became apparent that the technology would also benefit from an increase in average driving power [11]. This motivated the transition toward Yb-based lasers followed by a nonlinear pulse compression which nowadays provide orders of magnitude higher average power at a comparable pulse duration [12]. In terms of HHG yield, up to 13 mW in a single harmonic order at 26.5 eV have been obtained from a frequency-doubled nonlinearly compressed fiber laser amplifier system [13].
On top of the flux optimization, further requirement for higher driving power arises from applications benefiting from high repetition rates. Applications such as photoelectron spectroscopy and its time- and angle-resolved derivates are limited by space-charge effects due to the coulomb interaction between the emitted photoelectrons blurring the spectral resolution [14]. To mitigate this effect the pulse energy of the XUV light needs to be kept low to limit the number of emitted electrons. Similarly, experiments relying on coincidence detection of ions [15] require only a single ionization event per XUV pulse. In order to build sufficient statistics within a reasonable time, high repetition rate sources are favored in these applications. A further high repetition rate request comes from XUV frequency comb applications [2]. These typically require 100-MHz-scale repetition rates to provide sufficient line spacing and a high number of photons per comb line to allow reliable measurements. These requirements gave rise to the use of passive femtosecond enhancement cavities (fsEC) [16,17], which can maintain the necessary pulse energies for driving efficient HHG even at several hundred megahertz repetition rates [18]. Various instances of the aforementioned applications have benefited from this concept including photoelectron spectroscopy [19–21], molecular alignment experiments [22], XUV frequency comb spectroscopy [23,24] as well as the endeavor of thorium nuclear clock development [25].
Motivated by the same requirements our group has pioneered an alternative approach to fsECs based on driving HHG directly inside a cavity of a high-power thin-disk laser (TDL) oscillator, initially inspired by [26]. This conceptually simpler approach offers a single-stage system capable of driving HHG at a kilowatt-level average power, few tens of femtoseconds pulse duration and megahertz repetition rate without the necessity of coherent coupling into an external cavity or chirped pulse amplification and nonlinear pulse compression. The approach has held a promise that it can tolerate higher cavity losses thanks to the presence of gain inside the cavity and that the free-running soliton can better adapt to the nonlinearities induced by the plasma in the gas target. Thus, it should be capable of providing higher HHG efficiencies through tolerating higher intensities in the gas target as well as allowing higher XUV outcoupling efficiencies [27]. In this study, we show for the first time that this promise of the intra-oscillator HHG approach can be honored.
We focused on HHG in neon providing photon energies between 60 eV and 100 eV, covering the particularly interesting wavelength of 13.5 nm for industrial applications of actinic inspection of lithography masks and the produced semiconductors [5,28]. We show a comparison of our system to other high-repetition-rate sources in Fig. 1. The figure relates the outcoupled XUV flux of selected XUV sources to the repetition rate with respect to the photon energy and the type of the system. It also shows the historical progression of the intra-oscillator approach. The dashed connecting line follows the development of our system, starting from the first demonstration in 2017 [29]. A similar system has been demonstrated by the group of Midorikawa closely after in the same year [30,31].
Fig. 1. Overview of high-repetition-rate XUV sources with respect to outcoupled XUV power per harmonic and repetition rate. For enhancement cavities and intra-oscillator sources, the flux value corresponds to the position after the outcoupling mechanism (Brewster plate, grating, pierced mirror, etc.). For the single-pass systems, the generated flux is used in the graph. The dashed connecting line shows the progress of the system developed in our laboratory. The underlying data has been obtained from [2,5, 27,34,35].
Currently, our XUV source operates at 17 MHz and delivers ∼60 nW of average outcoupled power at 93 eV with a corresponding peak power spectral density of 3·109 photons/eV/s. Within the generated energy range, this by far exceeds the 1.3 nW at 94 eV obtained from an enhancement cavity [18] and is even comparable to 7·109 photons/eV/s at 92 eV obtained from a single-pass 50-kHz nonlinearly compressed fiber amplifier system [32]. Although still behind the 1.5 µW obtained from a frequency tripled Ti:sapphire laser amplifier system at 1 kHz repetition rate [33]. Our result, thus, manifests the potential of the intra-oscillator HHG approach to become a single-stage low-complexity alternative to HHG sources based on fsECs and high-power laser amplifiers.
2. Experimental setup
A schematic of the intra-oscillator HHG system is depicted in Fig. 2. The driving laser is a Kerr lens mode-locked (KLM) TDL oscillator built inside a vacuum chamber with a footprint of 0.8 × 1.6 m2. The heart of the laser system is a commercial thin-disk head with a standard Yb-doped disk from Trumpf. The disk is pumped at a 4-mm-diameter pump spot using a 969-nm volume-Bragg-grating-stabilized diode. The design of the linear laser oscillator is shown in Fig. 3. It implements two bounces over the disk and utilizes a 4-mm-thick sapphire Brewster plate and a 3.4-mm-diameter copper hard aperture for Kerr lens mode-locking. The laser operates in p-polarization determined by the Brewster plate orientation. Two dispersive mirrors of -500 fs2 introduce a roundtrip group delay dispersion (GDD) of −2000 fs2. The mode-locked operation is initiated using a mirror mounted on a piezo shaker with a 0.5-mm travel. A focusing section between a 150-mm radius of curvature (RoC) mirror CM3 and a 250-mm RoC pierced mirror PM creates a tight focus for the gas target of ∼40-µm diameter in the mode-locked operation. The hole in the mirror induces ∼0.2% IR losses per pass (0.4% per cavity roundtrip) for the laser in mode-locked regime, estimated from an overlap integral. The CM3 is mounted on a translation stage for compensating the lensing effect of the gas target and fine-tuning the cavity. The gas is injected into the focus using a 50-µm-diameter fused quartz nozzle at a backing pressure of up to 20 bar. The vacuum level in the laser chamber is kept below 10−2 mbar using two turbomolecular pumps with a total pumping power of 2900 l/s and a gas dump with a ∼1-mm opening placed directly below the nozzle directing most of the gas flow into the primary vacuum. The Brewster plate is purged by 15 l/h of oxygen to prevent surface contamination in vacuum. The generated XUV light is outcoupled from the cavity through the PM with a ∼120-µm-diameter on-axis hole. The XUV light further passes through a pair of grazing incidence plates and a 200-nm-thick zirconium filter to strip off the residual infrared light. The XUV spectrum is acquired using a home-developed spectrometer built around a Hitachi 001-0639 aberration-corrected concave grating and an Andor DO940P CCD.
Fig. 2. Experimental setup of the intra-oscillator HHG source based on a KLM TDL oscillator together with an XUV detection line. The XUV light is generated in a neon gas target and outcoupled from the cavity using a pierced mirror with a ∼120-µm opening diameter. Element abbreviations: DM dispersive mirror, CM curved mirror, HA hard aperture, BP Brewster plate, OC output coupler, PM pierced mirror, GIP grazing incidence plate.
Fig. 3. Design of the cavity in mode-locked and continuous-wave operation. The oscillator operates in a negative dispersion regime set by two -500 fs2 dispersive mirrors. Element abbreviations: HA 3.4-mm diameter hard aperture; BP 4-mm-thick sapphire Brewster plate; CM1, CM2 2-m-RoC mirrors; CM3 150-mm-RoC mirror; PM 250-mm-RoC pierced mirror with ∼120-µm diameter on-axis hole; OC 0.8% output coupler.
3. Results
The mode-locked laser oscillator operates at a repetition rate of 17.2 MHz and delivers 54-fs nearly transform-limited soliton pulses. This short pulse duration is achieved thanks to high amount of self-phase modulation inside the cavity as explained in [36]. The measured optical spectrum and the autocorrelation trace are shown in Fig. 4. The time-bandwidth product of the pulse amounts to 0.334 corresponding to 1.06 times the ideal soliton value. The laser reaches intracavity average and peak powers of 930 W and 880 MW, respectively. The corresponding intracavity pulse energy is 50 µJ. The laser pump power at this performance is 280 W.
Fig. 4. Laser characterization. a) Optical spectrum measured at 0.1-nm resolution bandwidth with a sech2 fit. b) Intensity autocorrelation trace with a corresponding soliton fit. The time-bandwidth product amounts to 0.334, 1.06× the ideal soliton value.
HHG is driven in a neon gas target formed by a 50-µm-diameter nozzle at 20 bar of backing pressure corresponding to the maximum of our gas installation. Up to this limit the obtained XUV flux was still increasing suggesting that we did not reach the optimal phase-matched pressure for the used target size. We did not try to increase the nozzle size in this experiment to prevent overloading of our turbomolecular pumps. The peak intensity inside the 40-µm-diameter HHG laser focus was estimated to 1.5 × 1014 W/cm2. Due to the use of a linear laser cavity, XUV light is generated both in the forward and backward direction. However, since the phase matching is experimentally optimized by adjusting the nozzle position along the focus through the Gouy phase shift, the backward direction reaching the CM3 is much weaker compared to the forward one. The obtained XUV spectrum is shown in Fig. 5(a). It was acquired using an Andor CCD detector cooled to −30°C at 100 ms acquisition time in a full vertical binning mode. The raw CCD counts were converted to photons / (s·eV) using the provided quantum efficiency of the detector and the spectrogram resolution. We further estimated the transmission of our XUV detection consisting of two grazing incidence plates with a Ta2O5 top layer, a 200-nm-thick zirconium filter, and an XUV grating as shown in Fig. 5(b). The total transmission in the figure is multiplied by a factor of 100 for better visibility. The estimated transmission allows recalculating the spectrum to the position after the pierced mirror as shown in Fig. 5(a) in green. To facilitate comparison to other systems in the literature we also provide a common metric of integrated flux per harmonic in several harmonic orders expressed in terms of XUV average power and photon flux, listed in Table 1.
Fig. 5. a) XUV spectra measured by the CCD detector (blue) and corrected to the position after the pierced mirror (green). b) XUV reflection/transmission efficiencies of the components in the detection line in p-polarization. The reflectivity of the Ta2O5 top layer of the GIPs and the transmission of the Zr filter were obtained from the Henke database [38]. The total transmission curve (red) was multiplied by a factor of 100 to fit the graph scale.
Table 1. Integrated values per harmonic order. The integration is performed over a 2.2-eV window centered around each harmonic. OC – outcoupled, CCD – on the CCD detector.
The influence of the neon gas target on the operation of the laser is small compared to our previous studies with heavier gases [34,35]. The SPM induced by the neon nonlinearity is negligible compared to the sapphire Brewster plate. A weak lensing effect is observable on the operation of the laser, which can be compensated by slight displacement of the CM3 mounted on a translation stage. The laser reaches a comparable intracavity peak power with and without the gas target. Also, the often-observed plasma-induced blue shift [37] is not apparent in the optical spectrum, which stay centered at 1030 nm as shown in Fig. 4. A significant thermal lens, however, arises from the presence of the pierced mirror in the cavity. The mode-locked operation of the laser is stable, nevertheless, the system requires a fine realignment on a 5-minute basis due to thermal drifts in order to remain centered on the pierced mirror for optimal XUV outcoupling. The system was typically operated in mode-locked regime at the highest ∼1-GW peak power for around 20 minutes.
4. Conclusion
We have demonstrated a single-stage 100-eV coherent XUV source based on intra-oscillator HHG. The system is particularly interesting for its conceptual simplicity. The KLM oscillator is based on a handful of dielectric mirrors, a sapphire Brewster plate and a hard aperture, built around a commercially available TDL head from Trumpf with their standard Yb-doped disk. It operates at ∼50-fs pulse duration and 17 MHz repetition rate, driving HHG inside its cavity at ∼1 GW of peak power and ∼1 kW of average power relying only on the passive KLM mechanism. It generates XUV light with photon energies between 60 eV and 100 eV covering the particularly interesting wavelength for the silicon industry of 13.5 nm. At this wavelength the outcoupled power per harmonic amounts to ∼60 nW which exceeds the flux of fsEC [18] and is even comparable to 100-kHz-class single-pass fiber amplifier systems [32,39]. This proves that the intra-oscillator HHG concept represents a simple and powerful megahertz-repetition-rate coherent XUV source, which might become a viable option for variety of photon-hungry experiments and applications, both in science and in industry. We expect that further power scaling is within reach since we have already operated our laser at 2 GW of intracavity peak power [40] which should bring the XUV flux beyond the microwatt level. With the development of 2-µm Holmium thin-disks [41–43], we might also expect higher photon energies in the soft-X-ray regime available from these sources. Furthermore, the approach is also applicable to nonlinear conversion toward many other frequency domains. The access to short pulse durations at kW-level average power inside a TDL oscillator together with the tolerance to cavity losses is highly attractive to drive nonlinear processes suffering from low conversion efficiencies. It also allows the use of thinner crystals offering broader phase matching. Such concept has been recently demonstrated in the THz domain through optical rectification in a 50-µm-thin lithium niobate plate inside a TDL oscillator [44] showing the progress in this direction.
Funding
H2020 European Research Council (279545); Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (200020_179146, 200020_200774, 206021_144970, 206021_170772).
Acknowledgment
The authors thank Tobias Ullsperger from the group of Prof. Dr. Stefan Nolte for drilling the pierced mirrors used in this work.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available in Dataset 1 (Ref. [45]).
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