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We report recent advances in pulse compression, frequency upconversion to the ultraviolet, polarisation control, and flexible pulse delivery experiments in the ultrafast regime, using different types of hollow-core fibres.

We report the on-target delivery of 40 fs, 200 µJ laser pulses at 800 nm through a 10 m long hollow-core nested anti-resonant fibre, with a guided peak intensity of 3×1015 W/cm2.

We report the generation and temporal characterization of wavelength-tunable sub-15 fs pulses from a commercial 220 fs Yb laser, in a single stage, through soliton-plasma interactions in an argon-filled hollow-core anti-resonant fiber.

We investigate soliton self-compression and photoionization effects in an argon-filled antiresonant hollow-core photonic crystal fiber pumped with a commercial Yb:KGW laser. Before the onset of photoionization, we demonstrate self-compression of our 220 fs pump laser to 13 fs in a single and compact stage. By using the plasma driven soliton self-frequency blueshift, we also demonstrate a tunable source from 1030 to ∼700 nm. We fully characterize the compressed pulses using sum-frequency generation time-domain ptychography, experimentally revealing the full time-frequency plasma–soliton dynamics in hollow-core fiber for the first time.

We experimentally demonstrate the efficient generation of circularly polarized pulses tunable from the vacuum to deep ultraviolet (160–380 nm) through resonant dispersive wave emission from optical solitons in a gas-filled hollow capillary fiber. In the deep ultraviolet, we measure up to 13 µJ of pulse energy, and from numerical simulations, we estimate the shortest output pulse duration to be 8.5 fs. We also experimentally verify that simply scaling the pulse energy by 3/2 between linearly and circularly polarized pumping closely reproduces the soliton and dispersive wave dynamics. Based on previous results with linearly polarized self-compression and resonant dispersive wave emission, we expect our technique to be extended to produce circularly polarized few-fs pulses further into the vacuum ultraviolet, and few to sub-fs circularly polarized pulses in the near infrared.

Transparent conducting oxides exhibit giant optical nonlinearities in the near-infrared window where their linear index approaches zero. Despite the magnitude and speed of these nonlinearities, a “killer” optical application for these compounds has yet to be found. Because of the absorptive nature of the typically used intraband transitions, out-of-plane configurations with short optical paths should be considered. In this direction, we propose an alternative frequency-resolved optical gating scheme for the characterization of ultra-fast optical pulses that exploits near-zero-index aluminium zinc oxide thin films. Besides the technological advantages in terms of manufacturability and cost, our system outperforms commercial modules in key metrics, such as operational bandwidth, sensitivity, and robustness. The performance enhancement comes with the additional benefit of simultaneous self-phase-matched second and third harmonic generation. Because of the fundamental importance of novel methodologies to characterise ultra-fast events, our solution could be of fundamental use for numerous research labs and industries.

Circularly polarized ultra-short laser pulses in the deep ultraviolet region (DUV, 200-400 nm) are commonly obtained via a two-step process requiring frequency up-conversion in nonlinear crystals followed by polarization conversion using quarter-wave plates. Due to strong dispersion in bulk media, this method suffers from limitations in phase-matching bandwidth, phase compensation and achromatic birefringence. Here, we demonstrate a direct process for the generation of ultra-short, circularly polarized DUV pulses via soliton dynamics in gasfilled stretched hollow capillary fibers [1] , driven by circularly polarized pulses centered at 800 nm. Frequency up-conversion occurs via resonant dispersive wave (RDW) emission with inherent spectral tunability (here we demonstrate 223-377 nm) through control of the gas (Ar) pressure. Our technique overcomes the limitations inherent to crystal-based approaches and allows energy up-scaling and extension to the vacuum UV (100-200 nm) and other spectral regions while permitting ultra-short duration, because the polarization conversion is performed at 800 nm, where material dispersion is low and the quality of commercial phase retarders is high.

Hardware-based machine learning for photoinjector manipulation is a promising solution for real-time adaptive electron-beam manipulation. We present preliminary studies towards this goal including simulations of the optical system and early machine learning results.

We report on recent advances in versatile and efficient light sources based on gas-filled hollow-core fibers.

Background Elevated plantar pressures represent a significant risk factor for neuropathic diabetic foot (NDF) ulceration. Foot offloading, through custom-made insoles, is essential for prevention and healing of NDF ulcerations. Objective quantitative evaluation to design custom-made insoles is not a standard method. Aims: 1) to develop a novel quantitative-statistical framework (QSF) for the evaluation and design of the insoles’ offloading performance through in-shoe pressure measurement; 2) to compare the pressure-relieving efficiency of traditional shape-based total contact customised insoles (TCCI) with a novel CAD-CAM approach by the QSF. Methods We recruited 30 neuropathic diabetic patients in cross-sectional study design. The risk-regions of interest (R-ROIs) and their areas with in-shoe peak pressure statistically ≥200kPa were identified for each patients’ foot as determined on the average of peak pressure maps ascertained per each stance phase. Repeated measures Friedman test compared R-ROIs’ areas in three different walking condition: flat insole (FI); TCCI and CAD-CAM insoles. Results As compared with FI (20.6±12.9 cm2), both the TCCI (7±8.7 cm2) and the CAD-CAM (5.5±7.3 cm2) approaches provided a reduction of R-ROIs mean areas (p Conclusions The CAD-CAM strategy achieves better offloading performance than the traditional shape-only based approach. The introduced QSF provides a more rigorous method to the direct 200kPa cut-off approach outlined in the literature. It provides a statistically sound methodology to evaluate the offloading insoles design and subsequent monitoring steps. QSF allows the analysis of the whole foot’s plantar surface, independently from a predetermined anatomical identification/masking. QSF can provide a detailed description about how and where custom-made insole redistributes the underfoot pressure respect to the FI. Thus, its usefulness extends to the design step, helping to guide the modifications necessary to achieve optimal offloading insole performances.

We demonstrate broadband wavelength up-conversion (240-320 nm) based on a seeded four-wave mixing scheme in gas-filled stretched hollow-capillary fibers with 50% conversion efficiency. Our technique is scalable in energy from the nJ to mJ level.

By exploiting soliton dynamics in gas-filled hollow capillary fibres, we generate tuneable few-femtosecond pulses from the vacuum ultraviolet to the infrared as well as sub-cycle field transients in the visible and near infrared.

Resonant dispersive wave (RDW) emission in gas-filled hollow waveguides is a powerful technique for the generation of bright few-femtosecond laser pulses from the vacuum ultraviolet to the near infrared. Here we investigate deep-ultraviolet RDW emission in a hollow capillary fibre filled with a longitudinal gas pressure gradient. We obtain broadly similar emission to the constant-pressure case by applying a surprisingly simple scaling rule for the gas pressure and study the energy-dependent dispersive-wave spectrum in detail using simulations. We further find that in addition to enabling dispersion-free delivery to experimental targets, a decreasing gradient also reduces the pulse stretching within the waveguide itself, and that transform-limited pulses with 3 fs duration can be generated by using short waveguides. Our results illuminate the fundamental dynamics underlying this frequency conversion technique and will aid in fully exploiting it for applications in ultrafast science and beyond., Comment: 4 pages, 5 figures

We demonstrate an efficient scheme for the generation of broadband, high-energy, circularly polarized femtosecond laser pulses in the deep ultraviolet through seeded degenerate four-wave mixing in stretched gas-filled hollow capillary fibers. Pumping and seeding with circularly polarized 35 fs pulses centered at 400 nm and 800 nm, respectively, we generate idler pulses centered at 266 nm with more than 25 microjoule of energy and over 95% spectrally averaged ellipticity. Even higher idler energies and broad spectra (27 nm bandwidth) can be obtained at the cost of reduced ellipticity. Our system can be scaled in average power and used in different spectral regions, including the vacuum ultraviolet.

We demonstrate the generation of sub-cycle pulses in the infrared (1340 nm) with a peak power of 27 GW using soliton self-compression. The envelope duration of 2 fs corresponds to an 840 as field transient.

Soliton dynamics in large-core gas-filled hollow capillary fibres can create high-energy sub-femtosecond and few-femtosecond pulses tuneable across the vacuum ultraviolet to infrared. Our work provides a new platform technology for ultrafast science.

Soliton-driven resonant dispersive-wave (RDW) emission is an established route to the generation of frequency tunable ultrafast pulses. In gas-filled anti-resonant guiding hollow-core photonic-crystal fibre (HC-PCF), continuous tuning of RDW generation in both the vacuum (VUV) and deep ultraviolet (DUV) to visible (110 nm to 550 nm) has been achieved [1] and fully characterised to have a temporal duration of a few fs in DUV [2]. Due to the high conversion efficiency (up to 10%), relatively low pump energies are required (few μJ) to get useful UV energy, and so high repetition-rate pump sources can be used [3]. Furthermore, the generated UV emission reproduces the polarization state of the pump pulse. Combined, all of these features make RDW-emission in gasfilled HC-PCF an ideal, and rather unique, source for ultrafast pump-probe spectroscopy in the ultraviolet region.

Recently we have demonstrated soliton effects at high energy (0.3 mJ) in helium- and neon-filled hollow capillary fibres (HCF) [1]. We observed pulse self-compression to single-cycle durations and the generation of deep (DUV) and vacuum ultraviolet (VUV) radiation at record energy levels for a tunable source. Here, we demonstrate that soliton dynamics in capillary fibres can also be accessed with argon- and krypton-filled HCF, although ionisation effects then start to play a larger role. We also find that the DUV and VUV generation through resonant dispersive-wave (RDW) emission can be achieved at wavelengths considerably shorter than the two-photon resonances in each gas—something surprising, given previous work in gas-filled photonic crystal fibres HC-PCF [2], where soliton-related effects have been previously extensively explored at pJ pulse energy levels [3].

The creation of shorter and shorter optical pulses at higher and higher energies has been one of the primary aims of laser science and nonlinear optics. At high energies (few mJ), the dominant technique is currently the use of self-phase modulation (SPM) and self-steepening in a gas-filled hollow capillary fibre (HCF) to spectrally broaden pump pulses (typically ∼30 fs), followed by chirped mirrors to compensate the phase variation across the spectrum and hence achieve pulse compression [1]. In this way high-energy single-cycle pulses are routinely generated. An alternative route to shorter pulses is soliton self-compression, where the dispersion of the fibre is tuned to be anomalous such that it continuously compensates the phase of the pulse as its spectrum expands. This technique is widely established at low power in solid-core optical fibres and at few energies in hollow-core photonic-crystal fibres (HC-PCF) [2]. Here we experimentally demonstrate soliton self-compression in conventional gas-filled HCF at much higher energy [3]. We will describe how our technique opens the door to generating 1 fs, 1 TW optical pulses.

Soliton dynamics underlie a wide range of phenomena in nonlinear fibre optics. In particular, higher-order solitons in gas-filled hollow-core photonic crystal fibre (HC-PCF) have been applied to self-compression of ultrafast laser pulses [1] and the generation of widely tuneable resonant dispersive waves (RDW) from the vacuum ultraviolet (VUV) to the visible spectral range [2]. We recently demonstrated that by moving to long, large-core hollow capillary fibres (HCF) and shorter driving pulses, these effects can be scaled by up to three orders of magnitude in pulse energy, providing unprecedented peak power in ultrafast VUV pulses as well as a route towards terawatt-scale optical attosecond pulses [3]. Here we show that by further decreasing the initial pulse duration, high-energy soliton dynamics can be obtained in HCF as short as 35 cm.

We demonstrate, for the first time, the application of rare-gas-filled hollow-core photonic crystal fibers (HC-PCFs) as tunable ultraviolet light sources in femtosecond pump-probe spectroscopy. A critical requirement here is excellent output stability over extended periods of data acquisition, and we show this can be readily achieved. The time-resolved photoelectron imaging technique reveals nonadiabatic dynamical processes operating on three distinct time scales in the styrene molecule following excitation over the 242-258 nm region. These include ultrafast (100 fs) internal conversion between the S

We report on a highly-efficient experimental scheme for the generation of deep-ultraviolet ultrashort light pulses using four-wave mixing in gas-filled kagom\'e-style photonic crystal fiber. By pumping with ultrashort, few $\mu$J, pulses centered at 400 nm, we generate an idler pulse at 266 nm, and amplify a seeded signal at 800 nm. We achieve remarkably high pump-to-idler energy conversion efficiencies of up to 38%. Although the pump and seed pulse durations are ~100 fs, the generated ultraviolet spectral bandwidths support sub-15 fs pulses. These can be further extended to support few-cycle pulses. Four-wave mixing in gas-filled hollow-core fibres can be scaled to high average powers and different spectral regions such as the vacuum ultraviolet (100-200 nm)., Comment: 4 pages, 5 figures

We show that soliton dynamics scale to millijoule energies and terrawatt peak powers in simple hollow capillary fibers. We numerically model sub-femtosecond pulse self-compression, and experimentally demonstrate high-brightness μJ-scale ultraviolet (125-330 nm) pulse generation.

Using soliton dynamics in 250 µm diameter Ne-filled hollow capillaries, we generate tunable, > 5 µJ, ultrafast pulses in the deep ultraviolet (200-330 nm). Further energy scaling and extension to the vacuum ultraviolet is predicted.

Soliton dynamics in the visible and near-infrared can be scaled to millijoule energy levels and terrawatt peak powers in simple hollow capillary fibers. We describe sub-femtosecond pulse self-compression and very high-brightness vacuum ultraviolet generation.

We show that soliton dynamics scale to millijoule energies in simple hollow capillary fibers. We numerically model sub-femtosecond pulse self-compression, and experimentally demonstrate high-brightness multiple-μJ-scale ultraviolet (115–330 nm)pulse generation.

We review the physics and techniques for the extension of multi-octave near-infrared pumped supercontinuum generation to the deep ultraviolet in solid-core photonic-crystal fibers (200 nm) and the vacuum-ultraviolet in gas-filled hollow-core fibers (113 nm).

We present the results of an experimental and numerical investigation into temporally nonlocal coherent interactions between ultrashort pulses, mediated by Raman coherence, in a gas-filled kagome-style hollow-core photonic-crystal fiber. A pump pulse first sets up the Raman coherence, creating a refractive index spatiotemporal grating in the gas that travels at the group velocity of the pump pulse. Varying the arrival time of a second, probe, pulse allows a high degree of control over its evolution as it propagates along the fiber through the grating. Of particular interest are soliton-driven effects such as self-compression and dispersive wave (DW) emission. In the experiments reported, a DW is emitted at $\ensuremath{\sim}300\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ and exhibits a wiggling effect, with its central frequency oscillating periodically with pump-probe delay. The results demonstrate that a strong Raman coherence, created in a broadband guiding gas-filled kagome photonic-crystal fiber, can be used to control the nonlinear dynamics of ultrashort probe pulses, even in difficult-to-access spectral regions such as the deep and vacuum ultraviolet.

We demonstrate high-energy resonant dispersive-wave emission in the deep ultraviolet (218 to 375 nm) from optical solitons in short (15 to 34cm) hollow capillary fibres. This down-scaling in length compared to previous results in capillaries is achieved by using small core diameters (100 and 150 $\mu$m) and pumping with 6.3 fs pulses at 800 nm. We generate pulses with energies of 4 to 6 $\mu$J across the deep ultraviolet in a 100 $\mu$m capillary and up to 11 $\mu$J in a 150 $\mu$m capillary. From comparisons to simulations we estimate the ultraviolet pulse to be 2 to 2.5 fs in duration. We also numerically study the influence of pump duration on the bandwidth of the dispersive wave., Comment: 5 pages, 3 figures

Soliton dynamics, coupled with either plasma formation or molecular modulation, enable the efficient generation of ultrafast vacuum-ultraviolet light pulses in gas-filled hollow-core photonic-crystal fibres. We review the theory, experiment and future prospects of these sources.

Raman effect in gases can generate an extremely long-living wave of coherence that can lead to the establishment of an almost perfect temporal periodic variation of the medium refractive index. We show theoretically and numerically that the equations, regulate the pulse propagation in hollow-core photonic crystal fibers filled by Raman-active gas, are exactly identical to a classical problem in quantum condensed matter physics - but with the role of space and time reversed - namely an electron in a periodic potential subject to a constant electric field. We are therefore able to infer the existence of Wannier-Stark ladders, Bloch oscillations, and Zener tunneling, phenomena that are normally associated with condensed matter physics, using purely optical means.

A recently developed source of ultraviolet radiation, based on optical soliton propagation in a gas-filled hollow-core photonic crystal fiber, is applied here to angle-resolved photoemission spectroscopy (ARPES). Near-infrared femtosecond pulses of only few {\mu}J energy generate vacuum ultraviolet (VUV) radiation between 5.5 and 9 eV inside the gas-filled fiber. These pulses are used to measure the band structure of the topological insulator Bi2Se3 with a signal to noise ratio comparable to that obtained with high order harmonics from a gas jet. The two-order-of-magnitude gain in efficiency promises time-resolved ARPES measurements at repetition rates of hundreds of kHz or even MHz, with photon energies that cover the first Brillouin zone of most materials., Comment: 8 pages, 3 figures

A cross-correlation frequency-resolved optical gating (FROG) concept, potentially suitable for characterizing few or sub-cycle pulses in a single shot, is described in which a counter-propagating transient grating is used as both the gate and the dispersive element in a FROG spectrometer. An all-reflective setup, which can operate over the whole transmission range of the nonlinear medium, within the sensitivity range of the matrix sensor, is also proposed, and proof-of-principle experiments for the ultraviolet and visible-to-near-infrared spectral ranges are reported.

We report two techniques for the efficient generation of tunable ultrafast pulses in the vacuum-ultraviolet, covering at least 117-200 nm, by pumping gas-filled kagome-style photonic crystal fibers with few-µJ, 35 fs, 800 nm laser pulses.

We numerically and experimentally explore spatiotemporal effects during ultrashort pulse propagation in gas-filled kagome-PCF. These include self-focusing, intermodal dispersive-wave emission, and multi-mode transient Raman frequency-comb generation.

Hollow-core photonic crystal fibre (HC-PCF) is unique host for gas-based nonlinear optical experiments. This is because it offers low-loss single-mode guidance in a micron-sized hollow core along with pressure-tunable dispersion and nonlinearity. In previous work, noble gases have been used as Raman-free nonlinear media, permitting efficient soliton-based pulse compression where the interplay between Kerr nonlinearity and anomalous dispersion results in dramatic self-compression of an ultrashort pulse. Novel phenomena such as UV wavelength conversion and even plasma generation from ~50 fs laser pulses of ~1 μJ energy have been reported [1]. In a different context, HC-PCF filled with molecular gases offers excellent performance as an ultra-low threshold modulator and frequency shifter for nano- and picosecond laser pulses [2]. Motivated by this, here we study experimentally and numerically the propagation of a 40 fs laser pulse in a hydrogen-filled HC-PCF [3]. Since the pump pulse duration is well below the period of one rotational cycle of ortho-hydrogen (57 fs), so that the bandwidth of the pulse is broader than the corresponding Raman frequency shift (18 THz), the interaction takes place in the impulsive regime. In other words, the pump pulse already contains Stokes shifted photons and the Raman process is self-seeded. Moreover, since the pulse duration is much shorter than the phase relaxation time T2 of the molecular coherence, the (Raman) response of the medium is highly non-instantaneous (i.e., nonlocal in time) and affected by the whole pre-history of the interaction [4].

Although supercontinuum sources are readily available for the visible and near infrared (IR), and recently also for the mid-IR, many areas of biology, chemistry, and physics would benefit greatly from the availability of compact, stable, and spectrally bright deep-ultraviolet and vacuum-ultraviolet (VUV) supercontinuum sources. Such sources have, however, not yet been developed. Here we report the generation of a bright supercontinuum, spanning more than three octaves from 124 nm to beyond 1200 nm, in hydrogen-filled kagome-style hollow-core photonic crystal fiber (kagome-PCF). Few-microjoule, 30 fs pump pulses at wavelength of 805 nm are launched into the fiber, where they undergo self-compression via the Raman-enhanced Kerr effect. Modeling indicates that before reaching a minimum subcycle pulse duration of ∼1 fs, much less than one period of molecular vibration (8 fs), nonlinear reshaping of the pulse envelope, accentuated by self-steepening and shock formation, creates an ultrashort feature that causes impulsive excitation of long-lived coherent molecular vibrations. These phase modulate a strong VUV dispersive wave (at 182 nm or 6.8 eV) on the trailing edge of the pulse, further broadening the spectrum into the VUV. The results also show for the first time that kagome-PCF guides well in the VUV.

A noise-seeded transient comb of Raman sidebands spanning three octaves from 180 to 2400 nm, is generated by pumping a hydrogen-filled hollow-core photonic crystal fiber with 26-μJ, 300-fs pulses at 800 nm. The pump pulses are spectrally broadened by both Kerr and Raman-related self-phase modulation (SPM), and the broadening is then transferred to the Raman lines. In spite of the high intensity, and in contrast to bulk gas-cell based experiments, neither SPM broadening nor ionization are detrimental to comb formation.