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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 µJ-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 µJ was obtained at 205 nm (average power 0.1 W), and at 1.92 MHz, a pulse energy of 0.54 µJ was obtained at 275 nm (average power 1.03 W).Ultraviolet (UV) laser pulses are in great demand for a wide range of applications, including photolithography [1], spectroscopy [2] and femtosecond pump-probe measurements [3]. Despite this, the range of available sources is limited. Apart from large-scale synchrotrons and free-electron lasers, very few lasers directly emit UV light, examples being excimer and some solid-state lasers (e.g., cerium-based [4]). An alternative approach involves upconversion of visible or near-infrared laser light via the generation of discrete harmonics in χ (2) and χ (3) media [5,6]. Using an optical parametric amplifier as pump laser provides wavelength-tunability in the UV, and with careful design very short pulse durations can be achieved [7]. Although such systems are flexible, they are also complex, and repetition rate scaling is challenging because of the high pump energies required. On the other hand, fiber and thin-disk technology allows repetition rate scaling in the near-infrared, pushing the frontiers of ultrafast lasers to unprecedented average power levels [8,9]. Much research is devoted to using these lasers for the generation of extreme UV light via high-harmonic generation [10,11]. To this end, pulse compression schemes are commonly employed, based for example on spectral broadening in bulk material [12] or gas-filled capillary and hollow-core photonic crystal fibers (HC-PCFs). This has allowed pulse compression from hundreds of fs to the few-cycle regime in set-ups involving two fiber stages [13,14]. In capillary fibers, spectral broadening normally occurs via self-phase modulation (SPM) in the normal dispersion regime, which requires the use of negatively chirped mirrors after the fiber and compresses the pulses in time by compensating for the induced chirp. If instead soliton-effect self-compression in the anomalous dispersion regime is used, there is no need for dispersion compensation. While this is difficult to achieve in large-bore capillary fibers (anomalous dispersion can only be achieved at very low gas pressures), it is straightforward in HC-PCFs, which deliver low loss even for small core di...

Gas-filled hollow-core photonic crystal fibre is being used to generate ever wider supercontinuum spectra, in particular via dispersive wave emission in the deep and vacuum ultraviolet, with a multitude of applications. Dispersive waves are the result of nonlinear transfer of energy from a self-compressed soliton, a process that relies crucially on phase-matching. It was recently predicted that, in the strong-field regime, the additional transient anomalous dispersion introduced by gas ionization would allow phase-matched dispersive wave generation in the mid-infrared—something that is forbidden in the absence of free electrons. Here we report the experimental observation of such mid-infrared dispersive waves, embedded in a 4.7-octave-wide supercontinuum that uniquely reaches simultaneously to the vacuum ultraviolet, with up to 1.7 W of total average power.

A double-nanospike As2S3-silica hybrid waveguide structure is reported. The structure comprises nanotapers at input and output ends of a step-index waveguide with a subwavelength core (1 μm in diameter), with the aim of increasing the in-coupling and out-coupling efficiency. The design of the input nanospike is numerically optimized to match both the diameter and divergence of the input beam, resulting in efficient excitation of the fundamental mode of the waveguide. The output nanospike is introduced to reduce the output beam divergence and the strong endface Fresnel reflection. The insertion loss of the waveguide is measured to be ∼2  dB at 1550 nm in the case of free-space in-coupling, which is ∼7  dB lower than the previously reported single-nanospike waveguide. By pumping a 3-mm-long waveguide at 1550 nm using a 60-fs fiber laser, an octave-spanning supercontinuum (from 0.8 to beyond 2.5 μm) is generated at 38 pJ input energy.

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 singlecycle 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 70%.

Dispersive wave emission (DWE) in gas-filled hollowcore 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 3 fs 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.Sources of very short laser pulses in the deep ultraviolet (DUV, 200 nm to 300 nm) are a key enabling technology for many areas of physics, such as ultrafast spectroscopy [1,2]. New approaches are made necessary by the limitations of sources based on harmonic generation, most importantly the lack of direct spectral tuneability, low conversion efficiency to the DUV, and the fact that only pulses of similar duration to the driving field can be generated [3]. One promising avenue is the use of dispersive wave emission (DWE), a phenomenon observable during optical soliton [4] and filamentation [5] dynamics among other effects. The extreme spectral broadening and nonlinear phase evolution experienced by a pulse undergoing soliton selfcompression allows for phase-matching and coherent energy transfer to a secondary pulse at a different frequency, known as a dispersive wave. Crucially, in a gas-filled waveguide the wavelength at which phase-matching occurs is continuously tuneable by way of the gas pressure, enabling the generation of broadband pulses from the visible to the vacuum ultraviolet spectral range [6,7].Numerical studies suggest that the dispersive wave is generated as a near transform-limited pulse with a duration of only a few femtoseconds, significantly shorter than the driving pulse [8]. Measurements of pulses which were strongly chirped by propagation through windows and ambient air before characterisation were consistent with few-fs pulse durations at generation, providing some support for this idea [9,10]. However, to date, no experiment has successfully measured a few-fs dispersive wave in the DUV or provided a route towards delivering such pulses to experiments without distortions.Here we present the full characterisation of 3 fs dispersivewave pulses with central wavelength tuneable from 225 nm to 300 nm using cross-correlation frequency-resolved optical gating (XFROG). Our measurements represent the first characterisation of a few-fs pulse generated via DWE and the shortest tuneable laser pulses in this spectral region to date, demonstrating the power of DWE-based sources for ultrafast science. The pulses are delivered to the characterisation set-up in vacuum; the measurements therefore faithfully reflect the pulses as generated in the waveguide. By numerically simulating the generation process, we further establish that the use of a pressure gradient along the waveguide is critical to the generation of few-fs pulses.The experimental layout is shown in Fig. 1(a)-(c) and consists ...