Keywords

1 Introduction

High Harmonic Generation (HHG), arising from the interaction of intense femtosecond laser pulses with noble gases, led to the realization of table-top sources of coherent Extreme UltraViolet (EUV) and Soft-X ray radiation. As a result, ultrafast spectroscopy can nowadays be performed with extreme temporal resolutions, down to the attosecond regime, and site and chemical selectivity. These peculiar features grant access to purely electronic dynamics initiated by ultrafast laser pulses in molecules and solids, and to fundamental processes of light-matter interaction.

One of the most promising all-optical techniques to perform these experiments is transient absorption in the EUV. However, the full exploitation of ultrafast spectroscopy in this spectral range is somehow hindered, even today, by the technological complexity of the required setups and the low generation efficiency of the HHG sources, particularly when moving towards higher photon energies.

Here, we report the efficient EUV generation inside a microfluidic device fabricated by femtosecond laser irradiation followed by chemical etching (FLICE) [1]. This microfluidic approach allows controlling and manipulating the harmonic generation conditions in gas on the micro-meter scale with unprecedented flexibility, enabling a high photon-flux and phase-matching on broadband harmonics above 100 eV.

We also report on the design and commissioning of a new beamline for transient absorption/reflectivity measurements in molecules and solids, equipped with a flexible EUV spectrometer for high-resolution and high dynamic range measurement, with a polarimeter for the characterization of the HHG polarization.

2 Efficient Generation of EUV in a Microfluidic Device

In our experimental setup, we excite the high-order harmonic generation by focusing ultrashort laser pulses in a gas-filled hollow waveguide realized inside a microfluidic glass device. The device is fabricated by using the advanced laser micromachining FLICE technique [2, 3]. FLICE allows building hollow three-dimensional structures embedded in the bulk of dielectric substrates. It provides a well-established platform to develop and prototype microfluidic systems for fine control of fluid dynamics on the micrometer scale. In our specific application, we realized a microfluidic system to accurately control and manipulate the gas density distribution inside the embedded capillary where the laser-gas interaction takes place. Specifically, we shaped the gas flow along the waveguide to achieve an almost uniform gas density along the laser propagation. The device is fabricated on a fused silica slab and is intended to work in a vacuum environment. Herein, we measured bright EUV generation on a spectral range between 40 and 160 eV.

2.1 Design of the Microfluidic Source

Figure 1a shows the structure of the device applied to HHG. It is realized on a 10 × 8 × 1 mm3 fused silica plate. It is composed of an embedded microchannel that acts as a hollow waveguide for ultrashort laser pulses. The microchannel is 8 mm long and has a diameter of 130 μm [1]. A 2 × 4.7 × 0.1 mm3 rectangular micro-chamber is dug on the upper surface working as a gas reservoir. The waveguide is connected to the reservoir through an array of four micro-channels, placed at a relative distance of 1.2 mm (see panel 1(b)). The gas, injected by these micro-channels, flows through the 8-mm-long hollow waveguide under the pressure gradient imposed by the high-vacuum environment (10−4 mbar) where the generation is performed. The device is operated in a continuous gas flow regime.

Fig. 1
A. An illustration of a chip with a length of 8 millimeters that has gas inlets, a gas reservoir, and a hollow waveguide. B. A schematic has 4 gas inlets with an I R laser and an I R laser + E U V beam on the left and right. C. A line graph of density versus capillary length.

(a) Structure of the chip used for EUV generation. (b) Scheme showing the working principle of the microfluidic system. Laser pulses are coupled inside the hollow waveguide that is filled with gas by small upper micro-channels. The interaction between the laser field and the gas lead to the generation of EUV radiation by HHG. (c) Numerical gas density along the central axis of the waveguide calculated at a backing pressure of 1 bar in the reservoir. The simulations are performed using COMSOL Multiphysics. (Figure adapted from [2])

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The steady-state gas dynamics inside the device were modeled by using the commercial Comsol Multiphysics™ CFD platform [4]. As predicted by numerical simulation, the flow is nearly isothermal, with the density profile smoothly varying from the center to the outputs of the waveguide, thus giving a central region with almost a constant gas density profile shown in Fig. 1c.

2.2 Characterization of the EUV Photon Flux

To generate the high-order harmonics, we used 800-nm pulses of amplified Ti:Sapphire laser source (Amplitude, Aurora laser system: 15 mJ, 25 fs, 1 kHz). The pulse duration for this experiment was 30 fs and the energy was 500 μJ. The beam is focused inside the devices by a lens with a 30-cm focal length and the measured full width at half maximum in the focus is 90 μm. The coupling efficiency is ~80%. The device is mounted in an aluminum holder and the gas is delivered to the hollow waveguide by a pipeline that is directly interfaced with the mechanical mounting. The gas backing pressure can be manually tuned by a needle valve and is monitored by a capacitive pressure gauge. The alignment of the device to the laser beam is performed with a high-precision five-axis motorized translational system. The EUV radiation is collected by a grazing incidence spectrometer (see Sect. 3).

Figure 2a, b show HHG spectra generated inside the microfluidic device filled with Argon and Helium, respectively. The spectral intensity reported in the graphs corresponds to the intensity per single pulse. By making a pressure scan up to 1 bar, we found that by using Argon as a target gas, the generation yield was maximized at a backing pressure of 100 mbar. In this condition, harmonics up to 60 eV were generated. The brightest harmonic peak is observed at 42 eV (H 27th), with an intensity of 1.6 × 109 photons/eV. The photon flux of H27 at a 1 kHz repetition rate approaches 1012 photons/s in 1% bandwidth, corresponding to a conversion efficiency of 8 × 10−6 in 1% bandwidth [5]. Compared to most advanced EUV sources [6,7,8,9,10], the conversion efficiency achieved inside the microfluidic source provides one of the highest reported in the literature, with the advantage of being obtained with low gas pressures (< 1 bar) and low pulse energies, on the sub-mJ scale.

Fig. 2
Two line graphs of H H G spectrum in argon and helium gas plot spectral intensity versus photon energy. Graph A plots a line that fluctuates with sharp peaks between 40 and 50 electron volts and then remains constant. Graph B plots a bell curve with fluctuations.

HHG spectra generated in the microfluidic cell (a) by filling the waveguide with Argon gas at a backing pressure of 100 mbar and (b) Helium gas at a backing pressure of 1 bar. The spectral intensity per single laser pulse is expressed in photons/eV. (Figure adapted from [5])

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HHG in Helium required a higher backing pressure (1 bar). The harmonics spectrum ranged from 80 eV up to 160 eV, with a photon flux peak-value of 8 × 108 photons/s in 1% bandwidth at 130 eV, corresponding to a conversion efficiency of 1.8 × 10−8 in 1% bandwidth. For He, we expected an improvement of the photon flux at backing pressures above 1 bar. For this purpose, a differential pumping system is currently under development, allowing continuous gas flow in the device at the multi-bar regime.

3 A Flexible EUV Spectrometer for Transient Absorption Spectroscopy

To detect EUV radiation we developed a flexible, versatile, and user-friendly spectrometer [11]. Our spectrometer is designed to work in the 1–100 nm spectral range in both stigmatic and astigmatic configurations, it is extremely sensitive to low-power signals and is equipped with a polarimeter to spectrally resolve the polarization state of the EUV radiation down to 12 nm. A schematic view of the spectrometer is reported in Fig. 3.

Fig. 3
A schematic view of the spectrometer. A laser and its light source pass through the L, i 1, chip, i 2 in an interaction chamber, then to T M, I P, and a X U V spectrometer with T M or S M, P S 1, P S 2, G R 1 or G R 2, M C P, and C C D.

Schematic view of the vacuum beamline and of the EUV spectrometer

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The EUV radiation after the interaction point (IP) undergoes focusing by a mirror. Here, a toroidal (TM) or a spherical mirror (SM) can be selected to switch from a stigmatic to astigmatic focusing onto the detector. The two mirrors are mounted on a precision motorized translation stage equipped with encoder for high reproducibility (Smaract SLLV42). After the focusing mirror, a polarimeter composed of two stages (PS1 and PS2) can be inserted into the beam path, allowing the full characterization of the polarization state of the radiation.

For a broadband detection, two Spherical Varied-Line-Space gratings can be selected (GR1 and GR2) depending on the spectral region of interest, the first one covering the range 120–5 nm and the second one dispersing the radiation up to 1 nm (1 keV). A high-precision remotely controlled translation stage allow accurate positioning of the two grating.

Finally, the EUV detection is performed with a double stack Micro-Channel Plate (MCP) in chevron configuration with independent voltages followed by a phosphor screen and a CCD, allowing a high dynamic range and sensitivity. Both the MCP and the visible CCD can be remotely moved to follow the whole dispersion curve of the two gratings.

4 Conclusion

We reported the progress on the development of an innovative HHG source based on a microfluidic device, realized on a glass substrate through the FLICE technique. The high-order harmonics generated inside this device exhibited a very high photon flux up to 160 eV, reaching the performances of the state-of-the-art EUV sources. Moreover, we showed the new design of a EUV spectrometer for full characterization of the spatial and polarization properties of the EUV radiation.

The high photon flux EUV source combined with the high flexibility and sensitivity of the detection system provides an ideal lab-scale beamline for HHG spectroscopy and EUV transient absorption experiments in molecules, solids and diluted samples.