Search

We combine soliton dynamics in gas-filled hollow-core photonic crystal fibers with a state-of-the-art fiber laser to realize a turnkey system producing few-femtosecond pulses at 8-MHz repetition rate at pump energies as low as 220 nJ. Furthermore, by exploiting the soliton self-frequency shift in a second hydrogen-filled hollow-core fiber, we efficiently generate pulses as short as 22 fs, continuously tunable from 1100 to 1474 nm.

High-brightness sources of coherent and few-cycle-duration light waveforms with spectral coverage from the ultraviolet to the terahertz would offer unprecedented versatility and opportunities for a wide range of applications from bio-chemical sensing to time-resolved and nonlinear spectroscopy, and to attosecond light-wave electronics. Combinations of various sources with frequency conversion and supercontinuum generation can provide relatively large spectral coverage, but many applications require a much broader spectral range and low-jitter synchronization for time-domain measurements. Here, we present a carrier-envelope-phase (CEP)-stable light source, seeded by a mid-infrared frequency comb with simultaneous spectral coverage across seven optical octaves, from the ultraviolet (340¿nm) into the terahertz (40,000¿nm). Combining soliton self-compression and dispersive wave generation in an anti-resonant-reflection photonic-crystal fibre with intra-pulse difference frequency generation in BaGa2GeSe6, the spectral brightness is two to five orders of magnitude above that of synchrotron sources. This will enable high-dynamic-range spectroscopies and provide numerous opportunities in attosecond physics and material sciences. J.B. and his group acknowledge financial support from the European Research Council via ERC Advanced Grant ‘TRANSFORMER’ (788218) and ERC Proof of Concept Grant ‘mini-X’ (840010), the European Union’s Horizon 2020 for FET-OPEN ‘PETACom’ (829153), FET-OPEN ‘OPTOlogic’ (899794), Laserlab-Europe (EU-H2020 654148), Marie Skłodowska-Curie grant no. 860553 (‘Smart-X’), MINECO for Plan Nacional FIS2017-89536-P, AGAUR for 2017 SGR 1639, MINECO for ‘Severo Ochoa’ (SEV- 2015-0522), Fundació Cellex Barcelona, CERCA Programme/Generalitat de Catalunya and the Alexander von Humboldt Foundation for the Friedrich Wilhelm Bessel Prize. We thank I. Tyulnev and M. Enders for their assistance.

We demonstrate spectral self-compression in a hollow-core gas-filled photonic crystal fiber for the generation of high energy ps pulses with limited bandwidths.

We report a turn-key system producing few-fs pulses at 8 MHz repetition rate for pump energies as low as 220 nJ, and shifting their central wavelength continuously between 1100 nm and 1400 nm.

We will discuss a carrier-to-envelope phase-controlled high brightness source of light covering 7 optical octaves. The spectral brightness up to 5 orders of magnitude higher than the brightest synchrotron and ranges from 340 nm to 40,000 nm.

We generate bandwidth limited 10 µJ pulses of 92 fs pulse width using an adaptive fiber Bragg grating stretcher (FBG) in conjunction with a Lyot filter. The temperature controlled FBG is used to optimize the group delay, whereas the Lyot filter counteracts gain narrowing in the amplifier chain. Soliton compression in a hollow core fiber (HCF) allows for access to the few-cycle pulse regime. Adaptive control further enables the generation of nontrivial pulse shapes.

We present an ultra-high brightness source of carrier-to-envelope phase-controlled light waveforms providing simultaneous spectral coverage across 7 optical octaves. The spectral brightness 2–5 orders of magnitude above synchrotron sources ranging from 340 nm to 40,000 nm

Soliton dynamics can be used to temporally compress laser pulses to few fs durations in many different spectral regions. Here we study analytically, numerically and experimentally the scaling of soliton dynamics in noble gas-filled hollow-core fibers. We identify an optimal parameter region, taking account of higher-order dispersion, photoionization, self-focusing, and modulational instability. Although for single-shots the effects of photoionization can be reduced by using lighter noble gases, they become increasingly important as the repetition rate rises. For the same optical nonlinearity, the higher pressure and longer diffusion times of the lighter gases can considerably enhance the long-term effects of ionization, as a result of pulse-by-pulse buildup of refractive index changes. To illustrate the counter-intuitive nature of these predictions, we compressed 250 fs pulses at 1030 nm in an 80-cm-long hollow-core photonic crystal fiber (core radius 15 µm) to ∼5 fs duration in argon and neon, and found that, although neon performed better at a repetition rate of 1 MHz, stable compression in argon was still possible up to 10 MHz.

Low-noise trains of few-fs pulses are widely used for studying and controlling ultrafast dynamics in matter. Microjoule-level pulse energies are often more than sufficient to trigger and observe these dynamics, and repetition rates in the few MHz range allow reduced acquisition times and improved signal-to-noise ratios—crucial for studies of processes with low-interaction cross-sections. The latest generation of fibre lasers can provide ultrashort pulses with stable carrier-envelope phase (CEP) at unprecedented energies and hundreds of kHz repetition rates [1] , [2] . However, because the generated pulses are hundreds of fs long, multiple compression stages are required to reach the few fs regime, thus increasing the system complexity and reducing the throughput efficiency.

Gas-based systems (e.g. based on hollow-core fibres) are extensively used over a wide spectral range to temporally compress ultrashort pulses. Recent progress in solid-state lasers and compression schemes has enabled the generation of sub-femtosecond pulses at unprecedented energies and average powers [1] – [3] . However, the latest generation of lasers deliver mJ pulses at hundreds-of-kHz repetition rates [4] , which raises new challenges for gas-based pulse compression. At high repetition rates, even weak single-shot energy deposition in the gas (e.g. by ionisation-driven heating) is sufficient to cause a transversely non-uniform gas density depression to build up pulse-by-pulse [5] , leading to thermal instabilities. Using lighter noble gases is often insufficient to mitigate this, despite the weaker nonlinear absorption. This is because sufficient nonlinearity for spectral broadening requires substantially higher pressures, resulting in much slower gas relaxation (the thermal diffusivity scales inversely with gas density).

Short wavelength light sources in the blue and ultraviolet (UV) are extremely useful for applications such as semiconductor metrology, photolithography, molecular spectroscopy and high performance liquid chromatography [1] . Gas-filled single-ring hollow-core photonic crystal fibre (SR-PCF), guiding by anti-resonant-reflection, is highly suitable for efficient generation of broadband supercontinua and tunable narrowband UV light because its nonlinearity and dispersion can be tuned by varying the species and pressure of gas [2] . We report the use of tapered SR-PCF filled with 15 bar krypton, pumped with ultrashort (220 fs) 1030 nm pulses carrying 7.8 μJ of energy from a compact laser source, to extend the UV bandwidth and achieve significant spectral power in the 200-350 nm region. The pulses are launched into the untapered end of a SR-PCF with core diameter of 36 μm and an average core-wall thickness of 386 nm (standard deviation 23 nm). Anti-crossings between the core modes and the resonances in the cladding walls result in loss peaks that interrupt the broad transmission window of the SRPCF. Upon propagation, the pulses break up due to modulational instability, resulting in a broad supercontinuum, the blue side of which forms by dispersive wave (DW) emission. By tapering the SR-PCF to half its original dimensions, the phase-matching wavelength for DW emission shifts to higher frequencies [3] , following the shift in zero dispersion wavelength indicated in Fig. 1(a) by dashed lines. As the thickness of the capillary walls falls, the loss peaks (marked by the shaded grey areas in Fig. 1(b) ) shift further into the blue, as previously reported [4] . As a result, the generated UV band extends down to 220 nm in the tapered fibre, a considerable improvement over untapered fibre, where the high frequency edge ends at 310 nm ( Fig. 1(a) ). The tapers were fabricated by brushing an oxy-butane flame across the fibre while pulling it apart axially, keeping the core at near vacuum pressure (60 mbar) and the cladding capillaries at atmospheric pressure [4] . The insets in Fig. 1(b) show optical micrographs of the untapered (above) and tapered (below) ends of the SR-PCF.

Femtosecond-laser-based micromachining is actively used in a broad range of applications, including photonic device fabrication and cell ablation [1] . Laser ablation of optically trapped particles has been explored as a means of creating particles with defined shapes [2] , [3] . During those experiments it was found that the particles were pushed away from the trapping point by ablation-induced momentum transfer. Dual-beam optical traps in hollow-core photonic crystal fibre (HC-PCF) permit particles to be stably trapped and moved over extended distances along the fibre axis [4] . Here we report fs laser ablation of particles levitated in HC-PCF, uniquely offering a means of coating the (normally inaccessible) core surfaces with thin films of different materials.

Soliton dynamics can be used for compressing optical pulses to few fs durations over a wide spectral range, and can be conveniently scaled in gas-filled hollow-core fibre by suitable design of the fibre structure and choice of gas [1] , [2] . In this fashion, similar patterns of pulse propagation can be obtained at pulse energies ranging from a few tens of nJ up to the mJ-level, and for pulse durations τ 0 up to hundreds of fs. The dynamics are controlled by the soliton order N and the spectral distance of the pulse frequency ν 0 from the zero dispersion frequency ν ZD . Here we report the existence of a region in ( N , ν ZD , τ 0 )-space where soliton-effect self-compression is optimal, provided the transmission loss of the fibre is negligible. We assess the limits set by modulational instability (MI) [3] , self-focusing (SF), and photoionisation (ION) [1] and derive a new limit set by third-order dispersion (TOD). By numerical simulation (not shown) we validate these limits, observing that if they are exceeded the compression quality degrades. Furthermore, we investigate, both theoretically and experimentally, scaling to MHz-level repetition rates, when inter-pulse effects cannot be neglected.

We present the use of linearly down-tapered gas-filled hollow-core photonic crystal fiber in a single-stage, pumped with pulses from a compact infrared laser source, to generate a supercontinuum carrying significant spectral power in the deep ultraviolet (200 - 300 nm). The generated supercontinuum extends from the near infrared down to around 213 nm with up to 0.83 mW/nm in the deep ultraviolet., Comment: 4 pages (+1 page for full reference list), 3 figures

Gas-filled hollow-core photonic crystal fiber (PCF) is used for efficient nonlinear temporal compression of femtosecond laser pulses, two main schemes being direct soliton-effect self-compression and spectral broadening followed by phase compensation. To obtain stable compressed pulses, it is crucial to avoid decoherence through modulational instability (MI) during spectral broadening. Here, we show that changes in dispersion due to spectral anti-crossings between the fundamental-core mode and core wall resonances in anti-resonant-guiding hollow-core PCF can strongly alter the MI gain spectrum, enabling MI-free pulse compression for optimized fiber designs. The results are important, since MI cannot always be suppressed by pumping in the normal dispersion regime.

The optical properties of hollow-core photonic crystal fibre (HC-PCF) can be conveniently tailored by modifying the transverse microstructure and selecting the species and pressure of the gas filling the hollow regions. Additionally, absorption of light by the gas can result in transient changes in refractive index, with characteristic timescales varying from a few hundred femtoseconds to hundreds of microseconds. Understanding the dynamics of these effects is essential for scaling the power and the repetition rate of laser pulse trains launched into HC-PCF, is relevant for pump-probe experiments, and can suggest new ways of controlling the guiding properties of these fibres. Here we review our recent studies, showing measurements of the temporal evolution of local changes in refractive index and describing the novel experimental techniques we developed to investigate the effects.

The evolution of a recombination-driven density depression in a krypton-gas-filled capillary, photoionized at MHz repetition rates, is interferometrically tracked. The msec-long buildup increases in amplitude with pulse energy and repetition rate.

Over the past years, ultrafast lasers with average powers in the 100 W range have become a mature technology, with a multitude of applications in science and technology. Nonlinear temporal compression of these lasers to few- or even single-cycle duration is often essential, yet still hard to achieve, in particular at high repetition rates. Here we report a two-stage system for compressing pulses from a 1030 nm ytterbium fiber laser to single-cycle durations with 5 µJ output pulse energy at 9.6 MHz repetition rate. In the first stage, the laser pulses are compressed from 340 to 25 fs by spectral broadening in a krypton-filled single-ring photonic crystal fiber (SR-PCF), subsequent phase compensation being achieved with chirped mirrors. In the second stage, the pulses are further compressed to single-cycle duration by soliton-effect self-compression in a neon-filled SR-PCF. We estimate a pulse duration of ∼3.4 fs at the fiber output by numerically back-propagating the measured pulses. Finally, we directly measured a pulse duration of 3.8 fs (1.25 optical cycles) after compensating (using chirped mirrors) the dispersion introduced by the optical elements after the fiber, more than 50% of the total pulse energy being in the main peak. The system can produce compressed pulses with peak powers >0.6 GW and a total transmission exceeding 66%.

We present a pump-probe technique for monitoring ultrafast polarizability changes. In particular, we use it to measure the plasma density created at the temporal focus of a self-compressing higher-order pump soliton in gas-filled hollow-core photonic crystal fiber. This is done by monitoring the wavelength of the dispersive wave emission from a counter-propagating probe soliton. By varying the relative delay between pump and probe, the plasma density distribution along the fiber can be mapped out. Compared to the recently introduced interferometric side-probing for monitoring the plasma density, our new technique is relatively immune to instabilities caused by air turbulence and mechanical vibration. The results of two experiments on argon- and krypton-filled fiber are presented, and compared to numerical simulations. The technique provides an important new tool for probing photoionization in many different gases and gas mixtures as well as ultrafast changes in dispersion in many other contexts., Comment: 7 pages, 3 figures, peer-reviewed and published

A dielectric microparticle, optically trapped within an air-filled hollow-core photonic crystal fiber (PCF), is accelerated backwards close to the speed of sound when a single guided femtosecond pulse is incident upon it. Acting as a spherical lens, the particle focuses a fraction of the pulse energy onto its inner rear surface, causing the material to ablate. The resulting plasma and vapor jet act like a rocket motor, driving the particle backward at peak accelerations conservatively estimated at more than a million times gravity. Using counter-propagating pulses to suppress particle motion, the effect may permit the inner core walls to be coated locally with different materials, allowing optical devices to be created at otherwise inaccessible points inside long lengths of hollow-core PCF.

Short laser pulses undergoing soliton self-compression in gas-filled hollow-core photonic crystal fibre (HC-PCF) can reach intensities sufficient to significantly ionise the gas under many experimental conditions. Due to the long-lived nature of the plasma, this may affect the propagation dynamics of subsequent pulses and thus limit scaling to higher repetition rates [1, 2]. Here we report experimental measurements and a numerical study of the free electron density distribution along the length of a gas-filled HC-PCF including its temporal evolution (with a few fs resolution) over 1 ns after the ionisation event. The plasma causes the effective refractive index of the modes to decrease which, in turns, alters the phase-matching conditions for dispersive wave (DW) generation from counter-propagating probe pulse, which shifts to higher frequencies in the ultraviolet. By measuring this shift we can detect free electron densities as low as ∼1016 cm−3, corresponding to a drop of refractive index of only ∼10−6.

Thermal tapering of step-index and photonic crystal fibres (PCF) has recently emerged as an effective tool for controlling nonlinear optical effects through offering axially varying dispersion and high effective nonlinearity [1,2]. At the same time, rapid developments in hollow-core PCF have opened up new possibilities for gas-based nonlinear optics. In particular, single-ring PCF (SR-PCF), consisting of a circular ring of thin-walled capillaries surrounding a central hollow core, can offer low-loss, easily predictable windows of transparency, effectively single-mode guidance and is relatively simple to fabricate [3]. Here we show, for the first time to our knowledge, the fabrication and non-invasive characterization of tapered SR-PCF.

We investigate refractive index changes caused by femtosecond photoionization in a gas-filled hollow-core photonic crystal fiber. Using spatially-resolved interferometric side-probing, we find that these changes live for tens of microseconds after the photoionization event - eight orders of magnitude longer than the pulse duration. Oscillations in the megahertz frequency range are simultaneously observed, caused by mechanical vibrations of the thin-walled capillaries surrounding the hollow core. These two non-local effects can affect the propagation of a second pulse that arrives within their lifetime, which works out to repetition rates of tens of kilohertz. Filling the fiber with an atomically lighter gas significantly reduces ionization, lessening the strength of the refractive index changes. The results will be important for understanding the dynamics of gas-based fiber systems operating at high intensities and high repetition rates, when temporally non-local interactions between successive laser pulses become relevant., Comment: 5 pages with four figures and one table

Recombination-driven acoustic pulses and heating in a photoionized gas transiently alter its refractive index. Slow thermal dissipation can cause substantial heat accumulation and impair the performance and stability of gas-based laser systems operating at strong-field intensities and megahertz repetition rates. Here we study this effect by probing the pulse-by-pulse buildup of refractive index changes in gases spatially confined inside a capillary. A high-power repetition-rate-tunable femtosecond laser photoionizes the gas at its free-space focus, while a transverse-propagating probe laser interferometrically monitors the resulting time-dependent changes in refractive index. The system allows convenient exploration of the nonlinear regimes used to temporally compress pulses with durations in the ∼30 to ∼300 fs range. We observe thermal gas-density depressions, milliseconds in duration, that saturate to a level that depends on the peak intensity and repetition rate of the pulses, in good agreement with numerical modelling. The dynamics are independently confirmed by measuring the mean speed-of-sound across the capillary core, allowing us to infer that the temperature in the gas can exceed 1000 K. Finally, we explore several strategies for mitigating these effects and improving the stability of gas-based high-power laser systems at high repetition rates.

Ultrafast broadband ultraviolet radiation is of importance in spectroscopy and photochemistry, since high photon energies enable single-photon excitations and ultrashort pulses allow time-resolved studies. Here we report the use of gas-filled hollow-core photonic crystal fibers (HC-PCFs) for efficient ultrafast nonlinear optics in the ultraviolet. Soliton self-compression of 400 nm pulses of (unprecedentedly low) ~500 nJ energies down to sub-6-fs durations is achieved, as well as resonant emission of tunable dispersive waves from these solitons. In addition, we discuss the generation of a flat supercontinuum extending from the deep ultraviolet to the visible in a hydrogen-filled HC-PCF. Comparisons with argon-filled fibers show that the enhanced Raman gain at high frequencies makes the hydrogen system more efficient. As HC-PCF technology develops, we expect these fiber-based ultraviolet sources to lead to new applications.

We demonstrate broadband, frequency-tunable, phase-locked terahertz (THz) generation and detection based on difference frequency mixing of temporally and spectrally structured near-infrared (NIR) pulses. The pulses are prepared in a gas-filled hollow-core photonic crystal fiber (HC-PCF), whose linear and nonlinear optical properties can be adjusted by tuning the gas pressure. This permits optimization of both the spectral broadening of the pulses due to self-phase modulation (SPM) and the generated THz spectrum. The properties of the prepared pulses, measured at several different argon gas pressures, agree well with the results of numerical modeling. Using these pulses, we perform difference frequency generation in a standard time-resolved THz scheme. As the argon pressure is gradually increased from 0 to 10 bar, the NIR pulses spectrally broaden from 3.5 to 8.7 THz, while the measured THz bandwidth increases correspondingly from 2.3 to 4.5 THz. At 10 bar, the THz spectrum extends to 6 THz, limited only by the spectral bandwidth of our time-resolved detection scheme. Interestingly, SPM in the HC-PCF produces asymmetric spectral broadening that may be used to enhance the generation of selected THz frequencies. This scheme, based on a HC-PCF pulse shaper, holds great promise for broadband time-domain spectroscopy in the THz, enabling the use of compact and stable ultrafast laser sources with relatively narrow linewidths (

We demonstrate a frequency-tunable phase-locked terahertz (THz) source relying on nonlinear optical propagation of a femtosecond near-infrared pulse inside a gas-filled hollow-core photonic-crystal fiber (HC-PCF).

We study the effect on nonlinear pulse propagation of anti-crossings between hollow core modes and cladding resonances in anti-resonant-reflecting photonic crystal fibers and report how their effect can be minimized by tuning the core-wall thickness.

We investigate long-lived photoionization-induced refractive index changes in gas-filled photonic crystal fiber by observing the frequency shift of a dispersive wave emitted by a soliton propagating against the ionizing pulse.

We observe long-lived refractive index changes in the hollow core of an argon-filled anti-resonant-guiding photonic crystal fiber, caused by plasma formation through femtosecond pulse compression and probed interferometrically through the fiber cladding.

Dispersive wave emission (DWE) in gas-filled hollow-core dielectric waveguides is a promising source of tuneable coherent and broadband radiation, but so far the generation of few-femtosecond pulses using this technique has not been demonstrated. Using in-vacuum frequency-resolved optical gating, we directly characterise tuneable 3fs pulses in the deep ultraviolet generated via DWE. Through numerical simulations, we identify that the use of a pressure gradient in the waveguide is critical for the generation of short pulses., Comment: 5 pages, 4 figures

We demonstrate efficient generation of 1.35-optical-cycle (14.5 fs) and 60 µJ mid-IR pulses at 160 kHz repetition rate. The CEP-stable, 21 W mid-IR waveforms are self-compressed inside a gas-filled antiresonant-reflection photonic crystal fiber (ARR-PCF).

We report the generation of high harmonics in a gas jet pumped by pulses self-compressed in a He-filled hollow-core photonic crystal fiber through the soliton effect. The gas jet is placed directly at the fiber output. As the energy increases, the ionization-induced soliton blueshift is transferred to the high harmonics, leading to emission bands that are continuously tunable from 17 to 45 eV.

Wavelength-tunable high-energy deep UV pulses are generated in gas-filled PCF pumped by a 20 µJ 1030 nm fiber laser: 1.05 µJ at 205 nm (100 kHz repetition rate) and 1.03 W at 275 nm (1.92 MHz).

We present 60-µJ, 1.35-optical-cycle pulse generation at 3.3 µm wavelength and 160 kHz repetition rate. The CEP-stable mid-IR waveforms are generated solely from self-compression inside a gas-filled ARR-PCF from a mid-IR, 131-µJ, sub-9-cycle OPCPA system.

We identify a novel regime of soliton-plasma interactions in which high-intensity ultrashort pulses of intermediate soliton order undergo coherent plasma-induced fission. Experimental results obtained in gas-filled hollow-core photonic crystal fiber are supported by rigorous numerical simulations. In the anomalous dispersion regime, the cumulative blueshift of higher-order input solitons with ionizing intensities results in pulse splitting before the ultimate self-compression point, leading to the generation of robust pulse pairs with PHz bandwidths. The novel dynamics closes the gap between plasma-induced adiabatic soliton compression and modulational instability.

In this Letter, we report the generation of a femtosecond supercontinuum extending from the ultraviolet to the near-infrared spectrum and detection of its carrier-envelope-phase (CEP) variation by f-to-2f interferometry. The spectrum is generated in a gas-filled hollow-core photonic crystal fiber, where soliton dynamics allows the CEP-stable self-compression of the optical parametric chirped-pulse amplifier pump pulses at 800 nm to a duration of 1.7 optical cycles, followed by dispersive wave emission. The source provides up to 1 μJ of pulse energy at the 800 kHz repetition rate, resulting in 0.8 W of average power, and it can be extremely useful, for example in strong-field physics, pump-probe measurements, and ultraviolet frequency comb metrology.

We demonstrate a spectral broadening and compression setup for carrier-envelope phase (CEP) stable sub-10-fs Ti:sapphire oscillator pulses resulting in 3.9 fs pulses spectrally centered at 780 nm. Pulses from the oscillator with 2 nJ energy are launched into a 1 mm long all-normal dispersive solid-core photonic crystal fiber and spectrally broadened to more than one octave. Subsequent pulse compression is achieved with a phase-only 4f pulse shaper. Second harmonic frequency resolved optical gating with a ptychographic reconstruction algorithm is used to obtain the spectral phase, which is fed back as a phase mask to the shaper display for pulse compression. The compressed pulses are CEP stable with a long term standard deviation of 0.23 rad for the CEP noise and 0.32 rad for the integrated rms phase jitter. The high total throughput of 15% results in a remaining pulse energy of about 300 pJ at 80 MHz repetition rate. With these parameters and the ability to tailor the spectral phase, the system is well suited for waveform sensitive photoemission experiments with needle tips or nanostructures and can be easily adapted to other sub-10 fs ultra-broadband Ti:sapphire oscillators.

We report on the properties of tapered single-ring hollow-core photonic-crystal fibers, with a particular emphasis on applications in nonlinear optics. The simplicity of these structures allows the use of non-invasive side-illumination to assess the quality of the tapering process, by observing the scattered far-field spectrum originating from excitation of whispering-gallery modes in the cladding capillaries. We investigate the conditions that ensure adiabatic propagation in the up- and down-tapers, and the scaling of loss-bands (created by anti-crossings between the core mode and modes in the capillary walls) with taper ratio. We also present an analytical model for the pressure profile along a tapered hollow fiber under differential pumping.

We experimentally demonstrate photoionization-induced mid-infrared dispersive wave emission in gas-filled hollow-core PCF. Launching few μJ, 30 fs pulses at 1030 nm, we generated a 4.2-octave-spanning spectrum, including plasma-induced resonant radiation between 3.2 and 3.8 μm.

We experimentally demonstrate plasma-induced soliton fission in a gas-filled photonic crystal fiber. Launching few μJ, 30 fs pulses at 1030 nm, PHz-wide spectral beating is observed as a result of pulse splitting and spectral interference.

Spectral anti-crossings between the fundamental guided mode and core-wall resonances alter the dispersion in hollow-core anti-resonant-reflection photonic crystal fibers. Here we study the effect of this dispersion change on the nonlinear propagation and dynamics of ultrashort pulses. We find that it causes emission of narrow spectral peaks through a combination of four-wave mixing and dispersive wave emission. We further investigate the influence of the anti-crossings on nonlinear pulse propagation and show that their impact can be minimized by adjusting the core-wall thickness in such a way that the anti-crossings lie spectrally distant from the pump wavelength.

Although ultraviolet (UV) light is important in many areas of science and technology, there are very few if any lasers capable of delivering wavelength-tunable ultrashort UV pulses at MHz repetition rates. Here we report the generation of deep-UV laser pulses at MHz repetition rates and \mu J-energies by means of dispersive wave (DW) emission from self-compressed solitons in gas-filled single-ring hollow-core photonic crystal fiber (SR-PCF). Pulses from an ytterbium fiber laser (~300 fs) are first compressed to ~25 fs in a SR-PCF-based nonlinear compression stage, and subsequently used to pump a second SR-PCF stage for broadband DW generation in the deep UV. The UV wavelength is tunable by selecting the gas species and the pressure. At 100 kHz repetition rate, a pulse energy of 1.05 \mu J was obtained at 205 nm (average power 0.1 W), and at 1.92 MHz, a pulse energy of 0.54 \mu J was obtained at 275 nm (average power 1.03 W).

In attosecond and strong-field physics, the acquisition of data in an acceptable time demands the combination of high peak power with high average power. We report a 21 W mid-IR optical parametric chirped pulse amplifier (OPCPA) that generates 131 μJ and 97 fs (sub-9-cycle) pulses at a 160 kHz repetition rate and at a center wavelength of 3.25 μm. Pulse-to-pulse stability of the carrier envelope phase (CEP)-stable output is excellent with a 0.33% rms over 288 million pulses (30 min) and compression close to a single optical cycle was achieved through soliton self-compression inside a gas-filled mid-IR antiresonant-guiding photonic crystal fiber. Without any additional compression device, stable generation of 14.5 fs (1.35-optical-cycle) pulses was achieved at an average power of 9.6 W. The resulting peak power of 3.9 GW in combination with the near-single-cycle duration and intrinsic CEP stability makes our OPCPA a key-enabling technology for the next generation of extreme photonics, strong-field attosecond research, and coherent x-ray science.