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 device (10) arranged to generate re) driving laser pulses (2), and a converter material source device (20) arranged to provide a droplet-shaped converter material, which is capable of generating X-rays (1) by non-linear frequency conversion in response to an irradiation with the driving laser pulses (2), wherein the excitation laser device (10) is arranged for a focused irradiation of the droplet-shaped converter material and the converter material source device (20) is configured to provide superfluid Helium droplets (3), which provide the converter material. Furthermore, a method for generating X-rays (1) is described, wherein superfluid Helium droplets (3) are utilized as a converter material.

Claims

1. An X-ray laser apparatus, being configured to generate X-rays, comprising an excitation laser device arranged to generate driving laser pulses, and a converter material source device arranged to provide a droplet-shaped converter material, which is capable of generating X-rays by non-linear frequency conversion in response to an irradiation with the driving laser pulses, wherein the excitation laser device is arranged for a focused irradiation of the droplet-shaped converter material, and the converter material source device is configured to provide superfluid Helium droplets, which provide the droplet-shaped converter material.

2. The X-ray laser apparatus according to claim 1, wherein the converter material source device is configured to provide the superfluid Helium droplets with at least one of (a) droplet diameters in a range of 10 nm to 10 μm and (b) an atomic density of at least 10.sup.23 atoms per cm.sup.3 in the superfluid Helium droplets.

3. The X-ray laser apparatus according to claim 1, wherein the converter material source device, comprises a nozzle device, a pressure device, a cooling device and a Helium reservoir wherein the cooling device is arranged to cool the nozzle device to a temperature in a range from 6 K to 300 K, the pressure device: is configured for applying the Helium to the nozzle device with a pressure in a range from 100 mbar to 100 bar, and the nozzle device comprises a nozzle which opens into a space with a pressure lower than 10.sup.−2 mbar and is configured to generate the superfluid Helium droplets by jet expansion.

4. The X-ray laser apparatus according to claim 1, wherein the converter material source device is configured to provide the superfluid Helium droplets as a continuous droplet flow or as a pulsed beam of droplet groups.

5. The X-ray laser apparatus according to claim 1, wherein at least one of the excitation laser device and the converter material source device is provided with a positioning device so that the superfluid Helium droplets and the driving laser pulses can be positioned relative to other.

6. The X-ray laser apparatus according to claim 1, wherein the excitation laser device is configured to generate the driving laser pulses with at least one of (a) repetition rate in a range from 10 Hz to 100 MHz, (b) a pulse duration in a range from 1 fs to 5 ps, (c) a wavelength in a range from 200 nm to 20 μm, and (d) a focus intensity in the droplet-shaped converter material greater than 10.sup.13 W/cm.sup.2.

7. The X-ray laser apparatus according to claim 1, wherein the excitation laser device is configured to generate the driving laser pulses with a beam profile having a predominantly flat intensity distribution.

8. The X-ray laser apparatus according to claim 1, further comprising a focusing device which is configured to focus the X-rays.

9. A method for generating X-rays, comprising the steps of generating driving laser pulses with an excitation laser device, providing a droplet-shaped converter material with a converter material source device, and focused irradiation of the droplet-shaped converter material with the driving laser pulses, wherein the X-rays are generated by non-linear frequency conversion, wherein the droplet-shaped converter material comprises superfluid Helium droplets.

10. The method according to claim 9, wherein the superfluid Helium droplets have at least one of (a) droplet diameters in a range of 10 nm to 10 μm and (b) an atomic density of at least 10.sup.23 atoms per cm.sup.3.

11. The method according to claim 9, wherein the excitation laser device has at least one of (a) repetition rate in a range from 10 Hz to 100 MHz, (b) a pulse duration in a range from 1 fs to 1 ps, (c) a wavelength in a range from 200 nm to 20 μm, and (d) a focus intensity in the droplet-shaped converter material greater than 10.sup.13 W/cm.sup.2.

12. The method according to claim 9, wherein the driving laser pulses during irradiation of the superfluid Helium droplets have a beam profile with a predominantly flat intensity distribution

13. The method according to claim 9, wherein the X-rays are generated in a spectral range between 10 eV and 2000 eV photon energy.

14. The method according to claim 9, wherein the excitation laser device and the converter material source device are operated synchronously.

15. A method for generating X-rays, comprising the steps of generating driving laser pulses with an excitation laser device, providing a droplet-shaped converter material with a converter material source device, and focussed irradiation of the droplet-shaped converter material with the driving laser pulses wherein the X-rays are generated by non-linear frequency conversion using the X-ray laser apparatus according to claim 1, and the droplet-shape converter material comprises superfluid Helium droplets.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Further details and advantages of the invention are described in the following with reference to the attached drawings, which schematically show in:

[0048] FIG. 1: an overview of the X-ray laser apparatus according to an embodiment of the invention;

[0049] FIG. 2: a nozzle device of the converter material source device included in the X-ray laser apparatus of FIG. 1; and

[0050] FIGS. 3 and 4: illustrations of generating X-rays by the non-linear frequency conversion of driving laser pulses.

PREFERRED EMBODIMENTS OF THE INVENTION

[0051] FIG. 1 shows the main components of an embodiment of an inventive X-ray laser apparatus 100, including an excitation laser device 10, a converter material source device 20, a positioning device 30, a focusing device 40, a vacuum chamber 50, and a control device 60, like a control computer unit.

[0052] In the vacuum chamber 50, the X-rays 1 are created by focussed irradiation of superfluid Helium droplets 3 with driving laser pulses 2 in a target interaction area 4. The vacuum chamber 50 comprises a schematically shown chamber wall 51 and a chamber window 52 transmitting the driving laser pulses 2, and it is connected with pumping device, like a turbo-molecular pump or similar device, to achieve vacuum conditions, and with control devices (not shown). The vacuum chamber 50 preferably allows for pressure of equal to or below <10-2 mbar, thus supporting a high transmission of the generated beam X-rays 1. The chamber window 52 should have a high transmission for the wavelength of the driving laser pulses 2. It is preferably attached to the chamber wall 51 in a way that the vacuum inside is maintained and the light source beam can be guided inside the vacuum chamber 50.

[0053] Furthermore, the vacuum chamber 50 may include an application area 5 being configured for an interaction of the generated X-ray beam 1 and having experimental measurement equipment and/or instruments that are operated in vacuum. In the application area 5, the X-rays 1 are ap-plied, e. g. for lithography, material processing or imaging tasks. Alternatively, the application area 5 can be arranged separately from the vacuum chamber 50 in an evacuated space connected with the vacuum chamber 50 via evacuated X-ray optics.

[0054] The excitation laser device 10 includes a laser source 11 and a focusing element 12. The laser source 11 includes a laser oscillator and optically non-linear components, like an amplifier, an optical-parametric device, a difference-frequency generation device, a sum-frequency generation device and/or a nonlinear spectral broadening device. The laser oscillator and the optically non-linear components are configured for creating coherent optical driving laser pulses 2 in a spectral range between ultraviolet (UV) to infrared (IR). The laser source wavelength can emit a fixed or tunable wavelength (centre wavelength of the driving laser pulses). The excitation laser device 10 comprises e. g. a laser source of the type Supernova OPCPA or Supernova DFG (manufactured by Class 5 Photonics GmbH, Germany).

[0055] The focusing element 12 can be at least one of at least one lens and at least one mirror, e. g. of parabolic, elliptical and/or spherical shape, or a free-form focusing element. The focusing element 12 is adapted for transmission of the wavelength emitted by the laser source 10. With the focusing element 12, driving laser pulses 2 are created with a focal spot in the target interaction area 4 with an intensity preferably larger than 10.sup.13 W/cm.sup.2. The focusing element 12 can be placed inside or outside the vacuum chamber 50, i. e. it can be swapped with the chamber window 52, or the focusing element 12 simultaneously may provide the chamber window. Alternatively, the focusing element 12 can be omitted if the focussing function is fulfilled by an output component of the laser source 10.

[0056] The converter material source device 20 comprises a nozzle device 21, which is illustrated with further details in FIG. 2, and a schematically illustrated arrangement of a pressure device 22, a cooling device 23, e. g. cryostat, and a Helium reservoir 24, like a gas bottle with an adjustable valve. Depending on the operation conditions, the converter material source device 20 provides a fluid droplet source or cluster source, i. e. it creates liquid helium droplets 3 or liquid helium clusters, in particular a continuous or pulsed beam of cryogenically cooled liquid helium droplets or atomic Helium clusters. Additionally, pressure and temperature sensors (not shown) are provided, which are coupled with the control device 60 for controlling the operation of the converter material source device 20, in particular for stabilizing the nozzle temperature of the nozzle device 21.

[0057] As schematically shown in FIG. 2, the nozzle device 21 comprises 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 a section of the cooling device 23, i. e. it has the temperature set at the cooling device 23. Both of the cold head 25 and the nozzle holder 26 are preferably made of copper or a material with similar heat conductivity so that the temperature measured at the cold head 25 of the cooling device 23 corresponds to the nozzle temperature. The nozzle holder 26 and cold head 25 are sealed with indium. The nozzle filter 29 between the cold head 25 and the nozzle holder 26 is a sinter filter (filter made of a porous sinter material to protect the nozzle from contamination. The nozzle 27 is a perforated nozzle plate with a nozzle diameter between 5 μm and 20 μm, and it is pressed by means of the nozzle cap 28 against the nozzle holder 26 and again sealed with indium. High-purity Helium gas is expanded into the vacuum in the vacuum chamber 50 under high pressure<100 bar and cryogenic temperatures>6 K.

[0058] The superfluid condition of the Helium droplets 3 is set by controlling pressure and temperature of the Helium at the nozzle 27 just before the expansion onto the vacuum, using the control device 60. Particular pressure and temperature settings for creating the superfluid state of Helium are obtained from tests or available reference tables (phase diagram). The average droplet size and/or the creation of clusters can be controlled by pressure and temperature control as well (see [10]).

[0059] The positioning device 30 comprises an xyz-positioner being adapted for moving the converter material source device 20, in particular the nozzle device 21 thereof, with μm or down to nm steps in all spatial directions relative to the excitation laser device 10. The positioning device 30 allows for precise positioning of the target area 4 with respect to the focus of the incoming driving laser pulses 2, in order to optimize the higher-harmonic generation conversion yield. Alternatively or additionally, an optical component of the excitation laser device 10 can be provided with a positioning device (not shown) for adjusting the position of the focal spot of the driving laser pulses 2 in the target interaction area 4.

[0060] The focusing device 40 comprises e. g. a lens or a mirror of parabolic, elliptical or spherical shape, or a free-form focusing element optimized for the characteristics of the generated X-rays 1, e. g. extreme ultraviolet to soft x-ray spectral range. It can be configured to focus or collimate or guide the generated X-rays 1 to a vacuum beam line for an experimental apparatus in the application area 5.

[0061] In operation of the X-ray laser apparatus 100, the beam of driving laser pulses 2 from the ultra-short coherent laser source 11 is focused with the focusing element 12 and guided through the optical chamber window 52, with focus in the target interaction area 4. The converter material target is produced by the converter material source device 20, producing a dense macroscopic sequence of superfluid Helium droplets or clusters. The focused driving laser pulses 2 are con-verted to the generated beam of X-rays 1 by the interaction with the superfluid Helium droplets or clusters by the HHG process, further illustrated in FIGS. 3 and 4. The generated beam of X-ray 1 has a spectral range that extends far into the extreme ultraviolet and soft x-ray regime. The generated beam can be focused by the focusing device 40 to the application area 5. Depending on the operation of the laser source 11 with fixed or tuneable wavelength, the X-ray laser apparatus 100 provides a pulsed beam of X-rays 1 with fixed or tunable wavelength.

[0062] FIG. 3 illustrates the interaction of the light field 2A of the driving laser pulses with single atoms 3′ (FIG. 3A, prior art) in comparison with superfluid Helium droplets 3 (FIG. 3B, invention). With the diameter of single atoms 3′ of about 10.sup.−10 m (FIG. 3A), the probability of interacting with the light field is essentially smaller than with the diameter of Helium droplets 3 of about 10.sup.−6 m. Furthermore, since the recombination cross section of the Helium droplets 3—characterized by the cross-sectional area thereof-is extremely large compared to the individual atom 3′ in the gas phase, the conversion efficiency per ionization event and thus the X-ray pulse energy E.sub.HHG is significantly increased.

[0063] Additionally, a high conversion efficiency is achieved as the Helium droplets provide an optically relatively thin medium with negligible propagation effects (losses). Thus, it is compensated for the dispersion of the electronic wave packet 3A in the ionization continuum, which is unavoidable due to the use of a long-wavelength driving laser pulses. The reduced recombination efficiency due to the divergence of the wave packet is compensated by the large increase in the recombination area of the droplet compared to the atomic cross section. This drastically increases the yield of X-ray light.

[0064] FIG. 4 shows details of the non-linear frequency conversion, including the 3 steps of ionization of electron wave packets 3A in the light field 2A of the driving laser pulses, e. g. MIR laser pulses, so that the electron wave packets 3A leave the atomic potential 3B of Helium atoms in the droplet 3 (FIG. 4A), propagation and energy accumulation of the electron wave packets 3A in the light field 2A of the driving laser pulses (FIG. 4B) and recombination of the emitted electron wave packet 3A in the field of the driving laser pulses with the release of an X-ray photon 1A (FIG. 4C). The strong light field 2A of the driving laser pulses, e. g. in the mid-infrared, leads to a “bending” of the atomic potential 3B. The medium (liquid droplet 3) is ionized and the emitted electrons are accelerated in the laser field 2A. X-ray pulses 1A are created by recombining high-energy electrons with the atomic nuclei.

[0065] The features of the invention disclosed in the above description, the drawings and the claims can be of significance individually, in combination or sub-combination for the implementation of the invention in its different embodiments.