Apparatus and method for generating X-rays by laser irradiation of superfluid helium droplets.

Abstract

An X-ray laser apparatus 100 for generating X-rays 1 comprises an excitation laser apparatus 10 configured to generate drive laser pulses 2, and a conversion material source apparatus 20 configured to provide a conversion material in droplet form that is capable of generating X-rays 1 by nonlinear frequency conversion in response to irradiation with the drive laser pulses 2, the excitation laser apparatus 10 being configured for focused irradiation of the conversion material in droplet form, and the conversion material source apparatus 20 being configured to provide superfluid helium droplets 3 that provide the conversion material. Further, a method of generating X-rays 1 utilizing the superfluid helium droplets 3 as the conversion material is described.

Description

[Technical Field]

[0001] The present invention relates to an X-ray laser device for generating X-rays by laser irradiation of a droplet-shaped conversion material. Furthermore, the present invention relates to a method for generating X-rays by nonlinear frequency conversion, comprising laser irradiation of a droplet-shaped conversion material. Applications of the present invention can be found, for example, in the fields of X-ray lithography (e.g., structuring processes in semiconductor and microsystem technology), laser processing of materials, materials research and X-ray imaging. [Background technology]

[0002] Reference is made herein to the following prior art which provides background to the present invention, particularly with respect to X-ray generation by nonlinear frequency conversion. [1]PBCorkum “Plasma perspective on strong field multiphoton ionization” in “Phys.Rev.Lett.”71,1994(1993) [2] U.S. Patent No. 7,729,403 [3]TTLuu et al. “Extreme-ultraviolet high-harmonic generation in liquids” in “Nat.Commun.”9,3723(2018) [4]J. Seres et al. “Source of coherent kiloelectronvolt X-rays” in “Nature” 433,596(2005) [5]C. Wagner et al. “Lithography gets extreme” in “Nature Photonics” 4,24(2010) [6] U.S. Patent No. 7,897,947 [7] U.S. Patent No. 7,372,056 [8] U.S. Patent No. 6,304,630 [9]S.Uetake et al.“Nonlinear optics with liquid hydrogen droplet” in “Proc.SPIE 4270,Laser Resonators IV”(24 April 2001),p.19;doi:10.1117 / 12.424665

[10] K.von Haeften et al.“Size and Isotope Effects of Helium Clusters and Droplets:Identification of Surface and Bulk-Volume Excitations” in “J.Phys.Chem.A” 115,7316(2011)

[11] D.Pentlehner et al.“Rapidly pulsed helium droplet source” in “Rev.Sci.Instrum.”80,043302(2009)

[12] M.Joppien et al.“Electronic Excitations in Liquid Helium:The Evolution from Small Clusters to Large Droplets” in “Phys.Rev.Lett.”71,2654-2657(1993)

[0003] To realize high-power X-ray laser sources on a laboratory scale, intense laser pulses, typically in the visible or near-infrared spectral range, interact with a conversion material. During this interaction, coherent short-wavelength X-ray laser light can be generated, and electrons are emitted from the conversion material, gaining significant energy in the optical field of the driving laser. If the emitted high-energy electrons recombine with their nuclei in the material in a phase-matched manner, the high accumulated kinetic energy of the electrons in the laser field can be emitted as X-ray pulses [1]. This mechanism is known as the "high-harmonic generation" (HHG) process. Typically, rare gas atoms (He, Ne, Ar, Kr, Xe) are used as the conversion material, prepared in gas cells, gas capillaries, or effusion particle beams (e.g., [2]).

[0004] The following parameters substantially affect the X-ray generation: the particle density ρ in the conversion section, the conversion efficiency of particles (atoms) A, and the signal strength S of the X-ray source based on high-order harmonic generation (HHG) on the gas scale, which depends on the effective length L of the conversion section of the conversion medium. HHG teeth,

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[0005] Furthermore, the conversion efficiency strongly depends on the available intensity I of the driving laser, which is very unfavorably determined at long wavelengths in the mid-infrared. In particular, at the shortest X-ray wavelengths (maximum photon energy E max ), the so-called "blocking"

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[0006] This means that, on average, trillions of photons must be injected to produce a single X-ray photon using conventional processes.

[0007] Therefore, the HHG process is usually determined by the driving laser parameters, the conversion material [3], and the wavelength of the X-ray laser to be achieved (λ -13 @1000eV)[4], 10 -5 It is not very effective, with efficiency significantly lower than that.

[0008] Similar considerations play an essential role in current CO2 laser-pumped extreme ultraviolet (EUV) sources with a wavelength of 13.5 nm for lithography applications, where highly charged ions (Sn q+ Microplasmas of liquid tin (Sn) are generated, which exhibit characteristic emission lines of . Alternatively, laser pulses can be irradiated onto water droplets to generate EUV radiation [8]. However, the EUV pulses from these plasma sources have low temporal coherence and are therefore of limited applicability. Another nonlinear frequency conversion by laser irradiation is described in [9], where hydrogen droplets are used as the conversion material. However, the shortest wavelengths produced by hydrogen droplets, based on the underlying Raman scattering process, are limited to UV radiation. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Patent No. 7,729,403 [Patent Document 2] U.S. Patent No. 7,897,947 [Patent Document 3] U.S. Patent No. 7,372,056 [Patent Document 4] U.S. Patent No. 6,304,630 [Non-patent literature]

[0010] [Non-licensed document 1] PBCorkum “Plasma perspective on strong field multiphoton ionization” in “Phys.Rev.Lett.” 71,1994(1993) [Non-licensed document 2] TTLuu et al. “Extreme-ultraviolet high-harmonic generation in liquids” in “Nat.Commun.” 9, 3723 (2018) [Non-licensed document 3] J.Seres et al. “Source of coherent kiloelectronvolt X-rays” in “Nature” 433,596(2005)

Non-licensed Document 4

Non-licensed Document 5

Non-licensed Document 6

Non-licensed Document 7

[0011] It is an object of the present invention to provide an improved X-ray laser apparatus and method for generating X-rays, which apparatus and method avoid the drawbacks of the prior art and / or provide new or expanded uses for X-ray sources. In particular, X-rays are generated with increased power, particularly an increased product of X-ray pulse power and repetition rate, increased efficiency, increased photon energy, and / or improved coherence. [Means for solving the problem]

[0012] The above object is solved by an X-ray laser device and a method for generating X-rays, each comprising the features of the independent claims. Preferred embodiments and applications of the invention are defined in the dependent claims.

[0013] According to a first general aspect of the present invention, the above object is solved by an X-ray laser apparatus configured to generate X-rays, the X-ray laser apparatus comprising: an excitation laser apparatus configured to generate drive laser pulses; and a conversion material source apparatus configured to provide a droplet-shaped conversion material capable of generating X-rays by nonlinear frequency conversion in response to irradiation with the drive laser pulses, the excitation laser apparatus being arranged for focused irradiation of the droplet-shaped conversion material.

[0014] In accordance with the X-ray laser device of the present invention, the conversion material source device is configured to provide superfluid helium droplets that provide the conversion material.

[0015] According to a second general aspect of the present invention, the above object is solved by a method for generating x-rays, comprising the steps of generating drive laser pulses using a pump laser device, providing conversion material in the form of droplets using a conversion material source device, and focused irradiation of the conversion material in the form of droplets with the drive laser pulses, wherein x-rays are generated by nonlinear frequency conversion.

[0016] According to the method of the present invention, the conversion material comprises superfluid helium droplets. Preferably, the method is carried out using an X-ray laser device according to the first general aspect of the present invention or an embodiment thereof.

[0017] According to the present invention, pulsed-laser-driven coherent X-ray radiation is generated by nonlinear frequency conversion. The nonlinear frequency conversion is based on electron recombination in superfluid helium droplets. In particular, the nonlinear frequency conversion is represented by a process in which electrons are separated from helium atoms by irradiation with a driving laser pulse, the electrons are accelerated in the optical field of the driving laser pulse, and the electrons are recombined with atomic helium nuclei. X-rays are generated in a wavelength range covering extreme ultraviolet and soft X-rays, as described below. As used herein, the term "X-rays" generally refers to pulsed X-ray radiation (or X-ray beams) from irradiated helium droplets. Due to the consistency of the generation process, X-rays are also referred to as X-ray laser pulses.

[0018] The present invention's use of superfluid helium as a conversion material for X-ray generation offers the following advantages over conventional techniques: First, superfluid helium in droplet form is an optically thin material, especially in the droplet jet from the nozzle-jet expansion, and the number of electrons in superfluid helium is relatively small compared to, for example, metallic droplets, so propagation and absorption effects in the conversion material are small. This allows for a longer usable conversion section length. Second, superfluid helium droplets can be used in a gas cell with a length of only 10 s. 20 particles / cm 3 extremely high local atomic densities (e.g., 10 23 particles / cm 3 ) thus increasing the particle density in the conversion material. Third, the recombination cross section (cross-section) of superfluid helium droplets is extremely large compared to single atoms in the gas phase, as will be explained in more detail below with reference to Figures 3 and 4. As an advantageous result, the conversion efficiency per ionization event can be increased by more than 100 times, in contrast to the low conversion efficiency of conventional processes. Furthermore, superfluid helium droplets make it possible to compensate for the dispersion of electron wave packets in the ionization continuum, which is unavoidable, especially when using long-wavelength emission excitation laser devices.

[0019] A further substantial advantage is the ability to generate X-rays with photon energies within the spectral range of approximately 280 eV to 530 eV (the so-called "water window"). This spectral range is characterized by the high transparency of water, but carbon, nitrogen, and other elements important in molecular biology are strongly absorbed there. Therefore, the present invention has advantageous imaging applications for investigating biological function principles in a location- and element-specific manner, particularly in natural aqueous environments with high contrast.

[0020] The term "superfluid helium" refers to liquid helium in a superfluid state, i.e., helium having a physical state condition that makes it a Bose quantum liquid, particularly with a viscosity equal to zero. The superfluid helium used in accordance with the present invention preferably contains the isotope helium-4. Superfluid helium droplets include free-space droplets of helium, preferably generated as a series of single droplets or as a pulsed beam of droplet clusters. Superfluid helium droplets may consist exclusively of helium. Alternatively, they may consist of helium and at least one dopant (doped droplets). Advantageously, the dopant can provide nucleation centers that promote ionization and increase the X-ray conversion efficiency. Preferably, the dopant comprises a substance with an ionization potential lower than that of helium. The conversion material source device generates superfluid helium droplets in a vacuum or reduced-pressure (subatmospheric) environment.

[0021] According to a preferred embodiment of the present invention, the conversion material source device is configured to provide droplet diameters in the range of 10 nm to 10 μm and at least 10 23 atoms / cm 3 The method is configured to provide superfluid helium droplets having at least one of the parameters including atomic density of: These preferred parameter ranges have advantages in terms of efficiency of x-ray generation and x-ray power, particularly for providing high conversion efficiency in an optically relatively thin medium with negligible propagation effects (losses).

[0022] According to a further preferred embodiment of the present invention, the conversion material source device comprises a nozzle device, a pressure device, a cooling device and a helium reservoir, the cooling device being configured to cool the nozzle device to a temperature in the preferred range of 6 K to 300 K, in particular 6 K to 80 K, the pressure device being configured to apply helium to the nozzle device at a pressure in the range of 100 mbar to 100 bar, the nozzle device being configured to apply helium to the nozzle device at a pressure in the range of 100 mbar to 100 bar, -2The device comprises a nozzle opening into a space with a pressure below 100 mbar and configured to generate superfluid helium droplets by jet expansion. Advantageously, these parameter ranges have already been described in the scientific literature (see, in particular,

[10] ,

[11] and

[12] ). Furthermore, these parameter ranges allow the generation of superfluid helium with sufficient stability and uniformity. As a further advantage, available cooled expansion nozzle systems can be employed as the conversion material source device, which has a relatively simple structure for establishing the superfluid state of superfluid helium.

[0023] Preferably, the converted material source device is configured to provide superfluid helium droplets as a continuous droplet stream or as a pulsed beam of droplets, particularly preferably with an adjustable droplet density. Under extreme operating conditions, a continuous droplet stream can include a continuously generated sequence of consecutive, particularly equidistant, single droplets separated from one another. Thus, the term "continuous" refers to the operation of the converted material source device. Generating a continuous droplet stream is particularly advantageous for generating X-rays as a pulsed sequence, since the repetition rate of the X-ray laser is determined by the repetition rate of the driving laser, which can be up to 100 MHz. Pulsed droplet beams include single or consecutive packets or clouds, each containing multiple superfluid helium droplets. To generate pulsed droplet beams, the converted material source device can be configured (among other things) for single-shot or continuous operation, as required, for example, at frequencies ranging from single-shot to 500 Hz. Generating pulsed droplet beams can be particularly advantageous for obtaining high X-ray pulse energies.

[0024] In a further preferred variant of the invention, at least one of the excitation laser device and the conversion material source device can be provided with a positioning device with a superfluid helium droplet, which allows the drive laser pulse to be positioned relative to the other. Advantageously, the positioning device provides optimal mutual coordination of the laser pulse and the droplet, resulting in an increased efficiency of generating X-rays.

[0025] According to a further preferred embodiment of the present invention, the pump laser device has a repetition rate in the range of 10 Hz to 100 MHz, a pulse duration τ in the range of 1 fs to 5 ps, a wavelength in the range of 200 nm to 20 μm, and 13 W / cm 2 The method is configured to generate a drive laser pulse having at least one of parameters comprising a focal intensity within the droplet-shaped conversion material exceeding

[0026] The above-mentioned preferred repetition rate of the driving laser pulses has particular advantages for obtaining a sequence of X-ray pulses with an equally high repetition rate, particularly representing quasi-continuous X-rays, at high power. The problem of low conversion efficiency in the prior art is solved by the fact that the average HHG X-ray laser output

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[0027] Short laser pulses with only a few optical cycles within the above preferred range of pulse duration τ can be achieved with a pulse peak power

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[0028] The above-mentioned preferred wavelengths in the infrared spectrum, especially the mid-infrared laser pulse wavelengths, allow to shift the cutoff far into the X-ray range. In particular in the MIR range, the maximum X-ray photon energy

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[0029] Advantageously, the above-mentioned preferred output power of the drive laser pulses can be obtained using current high-power laser sources that provide output powers of over 100 W in the near-infrared (NIR) and over 2 W in the mid-infrared (MIR). Providing the above-mentioned preferred high focal intensities has the advantage of obtaining high powers of the generated x-rays.

[0030] According to another advantageous embodiment of the present invention, the pump laser device is configured to generate drive laser pulses having a beam profile with a predominantly flat intensity distribution in time and / or space. The drive laser pulses preferably have a predominantly flat intensity distribution during irradiation of the superfluid helium droplets. The drive laser pulses preferably have a constant intensity over a wide range of the beam profile, particularly over at least half of the beam profile. A flat intensity distribution, also known as a rectangular intensity distribution, advantageously improves optical coupling of the drive laser pulses into the helium droplets. The inventors suggest that the use of flat-top drive laser pulses can improve the efficiency of X-ray generation by at least two to four times. This idea is derived from experience in which the spatial intensity distribution of laser pulses influences frequency conversion in nonlinear crystals, particularly the efficiency of frequency doubling and tripling. Achieving a constant intensity flat-top distribution over a wide range of beam profiles significantly improves efficiency. This becomes even more relevant for higher-order nonlinear processes, such as the generation of high-order harmonics in the highly nonlinear X-ray region.

[0031] Alternatively, other beam profiles of the drive laser pulse in time and / or space can be used, such as a Gaussian beam profile, in which case the central portion of the drive laser pulse's intensity distribution primarily contributes to the frequency conversion, but with the advantage of omitting the beam profiling components of the pump laser device.

[0032] According to another preferred embodiment of the invention, if a focusing device configured to focus X-rays is provided, particular advantages for the application of X-rays are obtained, in particular with regard to spatial resolution, for example in lithography or imaging applications.

[0033] Preferably, the excitation laser device and the conversion material source device operate synchronously. Synchronous operation includes matching the repetition rate of the drive laser pulses and the helium droplet generation rate so that they have an equal or even ratio. This can improve the utilization efficiency of the superfluid helium. To provide synchronous operation, the X-ray laser device preferably includes a controller that commonly controls both the excitation laser device and the conversion material source device.

[0034] In summary, we have discovered for the first time that superfluid helium droplets, preferably with droplet sizes such as strings or clusters of droplets in the 10 nm to 10 μm diameter range, can be used as an advantageous conversion material for nonlinear frequency conversion of drive laser pulses, preferably of relatively long wavelengths (UV-IR), at repetition rates in the 10 Hz to 100 MHz range. The condensed particle beam is preferably prepared by controlled jet expansion at a defined stagnation pressure (e.g., 100 mbar to 100 bar) and gas nozzle temperature (6 to 300 K). A substantial advantage of this invention is that, based on "high-order harmonic generation (HHG)" in quantum droplets, it is possible to obtain 1000-fold higher average power X-ray lasers with repetition rates up to 100 MHz compared to conventional HHG on gas cells, liquid jets, and nanoparticles. Nonlinear frequency conversion using HHG on helium droplets offers several crucial advantages for generating ultrashort X-ray pulses on a laboratory scale. In the HHG process, coherent X-ray emission is based on a three-step model developed by Paul Corkum in the 1990s [1].

[0035] Compared to conventional techniques, a repetition rate that is, for example, 200 times higher reduces the required process time (material processing, simultaneous experiments) by up to 200 times. In many applications, a mean power that is, for example, 1000 times higher means a corresponding 1000-fold increase in efficiency. In nanostructuring processes in semiconductor and microsystems technology, the transition from EUV lithography (13.5 nm) to X-ray lithography (<4.5 nm) represents a substantial advance in the achievable information density per chip.

[0036] Further applications are possible in various fields of natural and life sciences, where understanding molecular processes is paramount. Simultaneous measurements allow for the study of the structure and function of complex molecules in reaction chains. Electronic and geometric structural changes are recorded like a movie. Ultrashort laser pulses play a key role here. They are used for selective excitation of characteristic degrees of freedom of motion (the "fingerprint region" in the mid-infrared) and element- and site-specific investigation of ionization-induced reactions (the "water window" in the X-ray range). Reaction products are detected simultaneously (concurrently) and characterized individually. Because only one ionization process can be recorded per laser pulse, a very large number of individual measurements are required to obtain sufficient data statistics. The inventive technique, using a particularly high-power femtosecond laser as the excitation laser and the high stability of X-ray generation, allows measurement times to be reduced to several hours. As a result, fairly complex experiments can be performed much more efficiently.

[0037] Further details and advantages of the invention are explained below with reference to the accompanying drawings, which are shown schematically below. [Brief explanation of the drawings]

[0038] [Figure 1] 1 is a schematic diagram of an X-ray laser device according to an embodiment of the present invention. [Figure 2] 2 shows a nozzle arrangement of a conversion material source device included in the X-ray laser device of FIG. 1; [Figure 3] FIG. 1 is a diagram illustrating the generation of X-rays by nonlinear frequency conversion of a driving laser pulse. [Figure 4] FIG. 1 is a diagram illustrating the generation of X-rays by nonlinear frequency conversion of a driving laser pulse. DETAILED DESCRIPTION OF THE INVENTION

[0039] FIG. 1 shows the main components of one embodiment of an X-ray laser device 100 of the present invention, including an excitation laser device 10, a conversion material source device 20, a positioning device 30, a focusing device 40, a vacuum chamber 50, and a control device 60, such as a control computer unit.

[0040] X-rays 1 are generated within a vacuum chamber 50 by focused irradiation of superfluid helium droplets 3 with a drive laser pulse 2 within a target interaction region 4. The vacuum chamber 50 includes a schematic chamber wall 51 and a chamber window 52 for transmitting the drive laser pulse 2, and is connected to a pumping system, such as a turbomolecular pump or similar device, and a control system (not shown) for achieving a vacuum. The vacuum chamber 50 is preferably 10 -2 The chamber window 52 allows for sub-mbar pressures and therefore supports high transmittance of the generated beam X-rays 1. The chamber window 52 should have high transmittance for the wavelength of the drive laser pulse 2. It is preferably attached to the chamber wall 50 so that an internal vacuum can be maintained and the source beam can be guided inside the vacuum chamber 51.

[0041] Furthermore, the vacuum chamber 50 may include an application area 5 with experimental measurement devices and / or equipment configured for interaction with the generated X-ray beam 1 and operated in vacuum. In the application area 5, the X-rays 1 are applied, for example, for lithography, material processing, or imaging tasks. Alternatively, the application area 5 can be located separately from the vacuum chamber 50, in a vacuum space connected to the vacuum chamber 50 via vacuum X-ray optics.

[0042] The pump laser device 10 comprises a laser source 11 and a focusing element 12. The laser source 11 comprises a laser oscillator and optically nonlinear components such as an amplifier, an optical parametric device, a difference frequency generator, a sum frequency generator, and / or a nonlinear spectral broadening device. The laser oscillator and the optically nonlinear components are configured to generate coherent optical drive laser pulses 2 in the spectral range between ultraviolet (UV) and infrared (IR). The laser source wavelength can emit a fixed or tunable wavelength (center wavelength of the drive laser pulses). The pump laser device 10 comprises, for example, a laser source of the Supernova OPCPA or Supernova DFG type (manufactured by Class 5 Photonics GmbH, Germany).

[0043] The focusing element 12 can be at least one lens and at least one mirror, e.g., a parabolic, elliptical and / or spherical, or freeform focusing element. The focusing element 12 is adapted to transmit the wavelength emitted by the laser source 10. Using the focusing element 12, the driving laser pulse 2 is preferably focused at 10 13 W / cm 2 , and has a focus within target interaction region 4 with an intensity greater than 100 . Focusing element 12 can be located inside or outside vacuum chamber 50, i.e., can replace chamber window 52, ​​or focusing element 12 can simultaneously provide the chamber window. Alternatively, focusing element 12 can be omitted if the focusing function is fulfilled by the output components of laser source 10.

[0044] 2, and a schematic arrangement of a pressure device 22, a cooling device 23, e.g., a cryostat, and a helium reservoir 24, such as a gas bottle with an adjustable valve. Depending on the operating conditions, the conversion material source device 20 provides a droplet or cluster source, i.e., generates a continuous or pulsed beam of liquid helium droplets 3 or liquid helium clusters, in particular cryogenically cooled liquid helium droplets or atomic helium clusters. Furthermore, pressure and temperature sensors (not shown) coupled to a control device 60 are provided for controlling the operation of the conversion material source device 20, in particular for stabilizing the nozzle temperature of the nozzle device 21.

[0045] As shown schematically in FIG. 2, the nozzle apparatus 21 includes a cold head 25, a nozzle holder 26, and a nozzle 27 equipped with a nozzle cap 28 and a nozzle filter 29. The cold head 25 is part of the cooling apparatus 23 and has a set temperature. Both the cold head 25 and the nozzle holder 26 are preferably made of copper or a material with similar thermal conductivity so that the temperature measured at the cold head 25 of the cooling apparatus 23 corresponds to the nozzle temperature. The nozzle holder 26 and the cold head 25 are sealed with indium. The nozzle filter 29 between the cold head 25 and the nozzle holder 26 is a sintered filter (a filter made of a porous sintered material) to protect the nozzle from contamination. The nozzle 27 is a perforated nozzle plate with a nozzle diameter of 5 μm to 20 μm, which is pressed onto the nozzle holder 26 by the nozzle cap 28 and is again sealed with indium. High purity helium gas is expanded into a vacuum in a vacuum chamber 50 under high pressure of <100 bar and cryogenic temperatures of >6K.

[0046] The superfluid conditions of the helium droplets 3 are set by using the control device 60 to control the pressure and temperature of the helium at the nozzle 27 just before it expands into a vacuum. The specific pressure and temperature settings to produce the superfluid state of helium are obtained by experimentation or from available look-up tables (phase diagrams). The average droplet size and / or cluster formation can also be controlled by pressure and temperature control (see

[10] ).

[0047] The positioning device 30 comprises an xyz positioner adapted to move the conversion material source device 20, in particular its nozzle device 21, in all spatial directions relative to the excitation laser device 10 in steps down to μm or nm. The positioning device 30 enables precise positioning of the target region 4 relative to the focus of the incident drive laser pulse 2 in order to optimize the harmonic generation conversion yield. Alternatively or additionally, the optical components of the excitation laser device 10 can be provided with a positioning device (not shown) for adjusting the position of the focus of the drive laser pulse 2 within the target interaction region 4.

[0048] The focusing device 40 comprises, for example, parabolic, elliptical or spherical lenses or mirrors, or freeform focusing elements, optimized for the characteristics of the generated X-rays 1, for example in the extreme ultraviolet to soft X-ray spectral range, and can be configured to focus, collimate or guide the generated X-rays 1 into a vacuum beamline for an experimental setup in the application area 5.

[0049] During operation of the X-ray laser device 100, a beam of drive laser pulses 2 from the ultrashort coherent laser source 11 is guided by the focusing element 12 through the optical chamber window 52 and focused at the target interaction region 4. A conversion material target is generated by the conversion material source device 20, generating a dense macroscopic sequence of superfluid helium droplets or clusters. The focused drive laser pulses 2 are further converted into a generated X-ray beam 1 by interaction with the superfluid helium droplets or clusters via the HHG process shown in FIGS. 3 and 4. The generated X-ray beam 1 has a spectral range extending into the extreme ultraviolet and soft X-ray regions. The generated beam can be focused at the application region 5 by the focusing device 40. In response to operation of the laser source 11 having a fixed or tunable wavelength, the X-ray laser device 100 provides a pulsed beam of X-rays 1 having a fixed or tunable wavelength.

[0050] FIG. 3 shows the interaction of the light field 2A of the driving laser pulse with a single atom 3′ (FIG. 3A, prior art) compared to a superfluid helium droplet 3 (FIG. 3B, present invention). -10 For a single atom 3' diameter of m (Figure 3A), the probability of interacting with the optical field is about 10 -6 m. Furthermore, the recombination cross section characterized by the recombination cross section of the helium droplet 3 is very large compared to the individual atoms 3′ in the gas phase, so the conversion efficiency per ionization event, and therefore the X-ray pulse energy E HHG increases significantly.

[0051] Furthermore, high conversion efficiency is achieved because the helium droplet provides an optically relatively thin medium with negligible propagation effects (losses). Therefore, the dispersion of the electron wave packets 3A in the ionization continuum, which is unavoidable due to the use of long-wavelength driving laser pulses, is compensated for. The decrease in recombination efficiency due to wave packet divergence is compensated for by the large increase in the recombination area of ​​the droplet compared to the atomic cross section. This significantly increases the yield of X-ray light.

[0052] Figure 4 shows the details of the nonlinear frequency conversion, which includes three steps: ionization of an electron wave packet 3A in the optical field 2A of a driving laser pulse, e.g., a mid-infrared laser pulse, which then leaves the atomic potential 3B of the helium atoms in the droplet 3 (Figure 4A); propagation and energy accumulation of the electron wave packet 3A in the optical field 2A of the driving laser pulse (Figure 4B); and recombination of the emitted electron wave packet 3A with the emission of an X-ray photon 1A in the field of the driving laser pulse (Figure 4C). The intense optical field 2A of a driving laser pulse, e.g., a mid-infrared laser pulse, results in a "bending" of the atomic potential 3B. In the laser field 2A, the medium (droplet 3) is ionized and the emitted electrons are accelerated. The X-ray pulse 1A is generated by the recombination of the high-energy electrons with atomic nuclei.

[0053] The features of the invention disclosed in the above description, in the drawings and in the claims may be important individually, in combination or in sub-combination for the implementation of the invention in its different embodiments.

Claims

1. An X-ray laser device (100) configured to generate X-rays (1), comprising: an excitation laser device (10) configured to generate a drive laser pulse (2); a conversion material source device (20) configured to provide a droplet-shaped conversion material capable of generating X-rays (1) by nonlinear frequency conversion in response to irradiation with the driving laser pulse (2); Equipped with the excitation laser device (10) is arranged for focused irradiation of the droplet-shaped conversion material, 1. An X-ray laser device (100), characterized in that the conversion material source device (20) is configured to provide superfluid helium droplets (3) that provide the conversion material.

2. The conversion material source device (20) has a droplet diameter in the range of 10 nm to 10 μm and a concentration of at least 10 ２３ atoms / cm ３ to the superfluid helium droplet (3), 2. The X-ray laser device according to claim 1.

3. the conversion material source device (20) comprises a nozzle device (21), a pressure device (22), a cooling device (23), and a helium reservoir (24) arranged to contain helium; the cooling device (23) is configured to cool the nozzle device (21) to a temperature in the range of 6K to 300K; the pressure device (22) is configured to apply the helium to the nozzle device (21) at a pressure in the range of 100 mbar to 100 bar; The nozzle device (21) is 10 －２ a nozzle (25) opening into a space at a pressure lower than 10 ...

3. The X-ray laser device according to claim 1 or 2.

4. The conversion material source device (20) is configured to provide the superfluid helium droplets (3) as a continuous droplet stream or as a pulsed beam of droplets.

4. The X-ray laser device according to claim 1.

5. At least one of the excitation laser device (10) and the conversion material source device (20) comprises a positioning device (30) having the superfluid helium droplet (3), and the drive laser pulse (2) can be positioned relative to the other.

5. The X-ray laser device according to claim 1.

6. The excitation laser device (10) has a repetition rate in the range of 10 Hz to 100 MHz, a pulse duration in the range of 1 fs to 5 ps, a wavelength in the range of 200 nm to 20 μm, and １３ W / cm ２ and generating the driving laser pulse (2) having at least one of parameters including a focal intensity within the droplet-shaped conversion material exceeding 6. The X-ray laser device according to claim 1.

7. The excitation laser device (10) is configured to generate the drive laser pulse (2) having a beam profile with a primarily flat intensity distribution.

7. The X-ray laser device according to claim 1.

8. a focusing device (40) configured to focus the X-rays (1), 8. The X-ray laser device according to claim 1.

9. A method for generating X-rays (1), comprising: generating a drive laser pulse (2) using a pump laser device (10); providing a conversion material in droplet form using a conversion material source device (20); - a step of focused irradiation of the droplet-shaped conversion material with the driving laser pulse (2), wherein the X-rays (1) are generated by nonlinear frequency conversion; Including, The conversion material comprises superfluid helium droplets (3). A method characterized by:

10. The superfluid helium droplets (3) have a droplet diameter in the range of 10 nm to 10 μm and a density of at least 10 ２３ atoms / cm ３ and at least one of the atomic densities 10. The method of claim 9.

11. The excitation laser device (10) has a repetition rate in the range of 10 Hz to 100 MHz, a pulse duration in the range of 1 fs to 1 ps, a wavelength in the range of 200 nm to 20 μm, and １３ W / cm ２ and at least one of the parameters including a focus intensity within the droplet-shaped conversion material exceeding 11. The method according to claim 9 or 10.

12. The driving laser pulse (2) during irradiation of the superfluid helium droplet (3) has a beam profile with a primarily flat intensity distribution. The method according to any one of claims 9 to 11.

13. The X-rays (1) are generated within a spectral range of photon energy from 10 eV to 2000 eV. The method according to any one of claims 9 to 12.

14. The excitation laser device (10) and the conversion material source device (20) operate synchronously. The method according to any one of claims 9 to 13.

15. The X-rays (1) are generated by the X-ray laser device according to any one of claims 1 to 8. The method according to any one of claims 9 to 14.

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