Intense narrowband XUV pulses from a compact setup

We report on a compact and spectrally intense extreme-ultraviolet (XUV) source, which is based on high-harmonic generation (HHG) driven by 395 nm pulses. In order to minimize the XUV virtual source size and to maximize the XUV flux, HHG is performed several Rayleigh lengths away from the driving laser focal plane in a highdensity gas jet. As a result, a high focused XUV intensity of 5×10 W/cm is achieved, using a beamline with a length of only two meters and a modest driving laser pulse energy of 3 mJ. The high XUV intensity is demonstrated by performing a nonlinear ionization experiment in argon, using an XUV spectrum that is dominated by a single harmonic at 22 eV. Ion charge states up to Ar are observed, which requires the absorption of at least four XUV photons. The high XUV intensity and the narrow bandwidth are ideally suited for a variety of applications including photoelectron spectroscopy, the coherent control of resonant transitions and the imaging of nanoscale structures. ar X iv :2 10 6. 15 92 0v 1 [ ph ys ic s. op tic s] 3 0 Ju n 20 21 Intense narrowband XUV pulses from a compact setup 2 Ultrashort, intense extreme-ultraviolet (XUV) pulses are used in a growing number of applications, which include nonlinear multiphoton ionization of atoms [1– 6], molecules [7, 8] and clusters [9–11], second-harmonic generation in thin films [12] and the study of XUV strong-field physics [13, 14]. Intense XUV pulses are also a prerequisite for performing XUV-pump XUV-probe experiments where XUV pulse durations down to the attosecond regime have already been used [15–17]. While the study of electron dynamics on these extremely short timescales requires broadband XUV pulses, intense XUV pulses with a narrower bandwidth are advantageous for a range of applications including photoelectron spectroscopy [18, 19] and the study of resonant transitions, e.g. within four-wave mixing [20], superfluorescence [21] and the control of Rabi oscillations [22]. Furthermore, XUV pulses with a narrow bandwidth and a high spectral intensity are ideally suited for single-shot coherent diffractive imaging (CDI) of nanostructures and nanoscale targets [23–25]. Narrowband intense XUV pulses are available from free-electron laser (FEL) facilities [26–28], but the limited access and the large size of these facilities can make experiments very challenging. Alternatively, long high-harmonic generation (HHG) beamlines with lengths around 10 meters or even longer have been developed for the generation of intense XUV pulses [4–6, 11, 15, 16, 29, 30]. Intense XUV sources based on HHG are currently being developed, including the user facilities ELI beamlines in Prague [31] and ELI-ALPS in Szeged [32]. A disadvantage of these sources is that they require very powerful laser systems for driving the HHG process, often reaching the multi-terawatt range. At the same time, these large-scale setups lead to high demands regarding the laser stability. This can be challenging, especially when considering that the XUV pulses are typically focused to micrometer or even nanometer spot sizes [33, 34] and that some of these experiments require attosecond stability. A number of more compact XUV sources have been reported that were used to study two-photon ionization and absorption in He, resulting in the generation of singly-charged He ions [35–37]. In state-of-the-art setups devoted to the generation of intense XUV pulses based on HHG, the focus often lies on maximizing the XUV flux by using powerful driving lasers and by loosely focusing the driving laser pulses into the HHG medium [4, 5, 32, 38]. Recently, we have used a different approach, demonstrating that optimization of the XUV intensity on target requires a choice of parameters entirely different from the parameters needed to optimize the XUV pulse energy [6]. This approach was based on using a modest focal distance (≥ 5 m) for the near-infrared (NIR) driving laser, followed by a long propagation distance of the generated XUV beam. This enables large demagnification of the XUV source size and resulted in a high XUV intensity of 7×10 W/cm. Using these pulses for multiphoton ionization, charge states up to Ar were observed following the absorption of at least 10 XUV photons. At the same time, a moderate NIR pulse energy of 11 mJ was used [6]. However, this setup still required a lot of space, since overall an 18-m-long beamline was used. An important consideration for the generation of intense XUV pulses is that optical elements should only be used where absolutely necessary, because these typically suffer Intense narrowband XUV pulses from a compact setup 3 f = 1 m f = 75 mm gas jet