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...

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).