Method and laser system for generating secondary radiation

Abstract

The invention relates to a method for generating secondary radiation, in which a target material (106) is provided in a target region (122), a pulse sequence (118b) made up of laser pulses (112) is applied to the target material (106) in the target region (122), wherein interaction of the target material (106) with the pulse sequence (118b) generates secondary radiation (108), wherein the pulse sequence (118b) comprises a prepulse (112c) and a main pulse (112b) trailing the prepulse (112c), a pulse energy of the prepulse (112c) is between 2 µJ und 200 µJ and a pulse duration (td2) of the prepulse (112c) is between 200 fs and 5 ps, a pulse energy of the main pulse (112b) is between 2 mJ and 50 mJ and a pulse duration (td) of the main pulse (112b) is between 15 fs and 300 fs and wherein a temporal pulse distance (tz2) between the prepulse (112c) and the main pulse (112b) is between 1 ps and 1 ns.

Description

[0001] Method and laser system for generating secondary radiation

[0002] The invention relates to a method and a laser system for generating secondary radiation.

[0003] From US 2018317309 A1 a method for generating EUV light is known, wherein a droplet of target material is reshaped by irradiation with a first pre-pulse laser beam, a seed plasma is generated by irradiating the reshaped droplet with a second pre-pulse laser beam and EUV light is generated by heating the seed plasma with a main pulse laser beam.

[0004] From US 2018206318 A1 a modular plasma X-ray system is known, comprising a liquid metal flow system enclosed in a low-pressure chamber, wherein the flow system contains a liquid metal and wherein a metal target irradiable by laser pulses is formed at least at one location on the liquid metal, a circulation pump within the liquid metal flow system for circulating the liquid metal, a laser pulse emitter configured to send laser pulses into the chamber via a laser window, focusing optics located between the emitter and the metal target, wherein the focusing optics guide the laser pulses so that they impinge on the metal target at a target location to form X-ray pulses, and an X-ray window positioned within the chamber and through which the X-ray pulses exit the chamber.

[0005] WO 2014044392 A1 discloses an EUV radiation generation device comprising a vacuum chamber in which a target material can be arranged at a target position to generate EUV radiation, and a beam guidance chamber for guiding a laser beam from a driver laser device toward the target position. An intermediate chamber is provided, which is arranged between the vacuum chamber and the beam guidance chamber, a first window, which seals the intermediate chamber in a gas-tight manner, for the entry of the laser beam from the beam guidance chamber, and a second window, which seals the intermediate chamber in a gas-tight manner, for the exit of the laser beam into the vacuum chamber.

[0006] The invention is based on the object of providing a method and a laser system as mentioned above, which enable the generation of secondary radiation with increased efficiency.

[0007] This object is achieved according to the invention in the method mentioned at the outset in that a target material is provided in a target area, the target material in the target area is subjected to a pulse sequence of laser pulses, secondary radiation being generated by interaction of the target material with the pulse sequence, the pulse sequence comprising a pre-pulse and a main pulse following the pre-pulse, a pulse energy of the pre-pulse being between 2 pJ and 200 pJ and a pulse duration of the pre-pulse being between 200 fs and 5 ps, a pulse energy of the main pulse being between 2 mJ and 50 mJ and a pulse duration of the main pulse being between 15 fs and 300 fs, and a temporal pulse interval between the pre-pulse and the main pulse being between 1 ps and 1 ns.

[0008] Through the interaction of the pre-pulse with the target material, nanometer-sized particles can be released from the target material. These particles, consisting of target material, are referred to herein as nanoparticles and are positioned in the area of ​​a surface of the original target material from which they were released by exposure to the pre-pulse. When the main pulse arrives at the target material, there is additional interaction between the target material and the nanoparticles. It has been shown that the preparation of the target material by means of the pre-pulse and the resulting nanoparticles improves the efficiency of the interaction of the main pulse with the target material, and in particular its absorption by the target material. This enables particularly efficient generation of secondary radiation.

[0009] The interaction of the laser pulses of the pulse sequence with the target material is, or includes, in particular, an at least partial absorption of the laser pulses by the target material. In particular, the laser pulses of the pulse sequence are each at least partially absorbed by the target material.

[0010] The term "main pulse lags behind the pre-pulse" means that the pre-pulse hits the target material before the main pulse. Thus, the pre-pulse hits the target material first, followed by the main pulse.

[0011] It can be provided that the pulse sequence comprises several pre-pulses preceding the main pulse or a pulse train consisting of several pre-pulses preceding the main pulse. The pre-pulses have the properties of the pre-pulse specified in the claims as mentioned above and / or below. In particular, the respective pre-pulses of the pulse sequence then contribute to the formation of nanoparticles or cause the formation of nanoparticles.

[0012] The secondary radiation generated by the method according to the invention is, in particular, electromagnetic radiation with a quantum energy between 0.5 keV and 100 keV, and preferably between 5 keV and 50 keV. In particular, the method according to the invention is suitable for generating electromagnetic radiation with a quantum energy in the aforementioned ranges. In particular, the generated secondary radiation is X-rays.

[0013] In particular, the laser pulses of the pulse sequence have a wavelength between 300 nm and 10 pm. Preferably, the wavelength is in a range between 330 nm and 350 nm, between 500 nm and 550 nm, between 0.8 pm and 1.2 pm, between 1.5 pm and 2.5 pm, or between 9 pm and 11 pm. In particular, all laser pulses of the pulse sequence have the same wavelength.

[0014] It can be advantageous if the pulse duration of the pre-pulse is between 800 fs and 1.5 ps. This enables effective generation of nanoparticles, which in turn allows the interaction or absorption of the main pulse by the target material to occur with particularly high efficiency.

[0015] For the same reason, it may be advantageous if the pulse energy of the pre-pulse is between 5 pJ and 100 pJ. For the same reason, it may be advantageous if the pulse spacing between the pre-pulse and the main pulse is between 10 ps and 100 ps.

[0016] In particular, the application of the pre-pulse to the target material causes the formation of nanoparticles. In particular, the nanoparticles are positioned in the area of ​​a surface and / or in a spatial region of the target material in which the pre-pulse is applied.

[0017] In particular, the surface forms a boundary surface and / or phase boundary of the target material.

[0018] In particular, the area in which the nanoparticles are positioned extends from the surface of the target material to a distance of 50 pm from the surface.

[0019] It can be advantageous if the pulse energy of the main pulse is between 5 mJ and 15 mJ, and especially between 8 mJ and 12 mJ. This allows secondary radiation, for example, in the form of X-rays, to be generated with particularly high efficiency.

[0020] For the same reason, it may be advantageous if the pulse duration of the main pulse is between 25 fs and 50 fs.

[0021] In particular, it can be provided that all laser pulses of the pulse sequence are applied to the target material at the same location and / or in the same spatial region of the target material. This results in the aforementioned increase in efficiency in the generation of secondary radiation.

[0022] In particular, the spatial region in which the laser pulses of the pulse sequence strike the target material has a maximum spatial extent, in particular maximum diameter, of at least 2.5 pm and / or at most 30 m and in particular at least 3 |jm and / or at most 15 |jm.

[0023] In particular, the laser pulses of the pulse train approach the target material at a speed that is much greater than a movement speed and / or flow speed of the target material within the target area, so that all laser pulses of the pulse train impinge on the target material at approximately the same location and / or in the same spatial area.

[0024] In particular, the target material is exposed to the main pulse in a spatial region and / or in the same spatial region in which nanoparticles were formed by the pre-pulse. In particular, the main pulse then impacts the nanoparticles formed in this spatial region and interacts with them.

[0025] It may be provided that the impact position of the respective laser pulses of the pulse sequence on the target material is adjusted so that all laser pulses of the pulse sequence hit the target material at the same location and / or in the same spatial area. For this purpose, a control device may be provided, for example.

[0026] For example, the impact position is adjusted so that the main pulse hits the indentation formed on the surface of the target material, which was formed there by the pulse train consisting of at least two pre-pulses.

[0027] It may be advantageous if a suppression and / or a vacuum and / or a gas atmosphere with a defined composition is formed in the target area.

[0028] In particular, it can be provided that the laser pulses of the pulse sequence are assigned to at least one primary laser beam, wherein the at least one primary laser beam is provided by a laser device and directed toward the target area to interact with the target material there. The target material is exposed to the laser pulses of the pulse sequence by means of the at least one primary laser beam. In particular, a single primary laser beam can be provided, to which the laser pulses of the pulse sequence are assigned. For example, this primary laser beam is then formed by the coaxial superposition of several laser beams, each of which provides one or more laser pulses of the pulse sequence.

[0029] It is also fundamentally possible to provide multiple primary laser beams directed at the target area to interact with the target material in the target area. In particular, the primary laser beams are then spaced apart from one another and / or approach the target area from different directions. One or more laser pulses of the pulse sequence are then assigned to each of the different primary laser beams.

[0030] In particular, it can be provided that the at least one primary laser beam is focused into the target area, with a focus of the primary laser beam being positioned in the target material and / or on the target material and / or in a region of the target material. The highest possible radiation intensity can be provided at the focus, which can be brought into interaction with the target material.

[0031] The focus of the at least one primary laser beam has in particular a diameter in the range of 2.5 pm to 30 pm and preferably in the range of 3 pm to 15 pm.

[0032] The target material is preferably in a liquid state. In particular, the target material is or comprises a low-melting metal. For example, the target material is or comprises gallium, indium, tin, zinc, lithium, bismuth, or lead, or an alloy comprising one or more of the aforementioned materials.

[0033] It can be advantageous if target material is continuously fed and / or conveyed into the target area. This ensures that fresh target material is continuously available in the target area, which can interact with the pulse sequence to generate secondary radiation. This allows for continuous secondary radiation generation. In particular, the target material passes through the target area as a material stream, and especially as a liquid material stream. In particular, the target material passes through the target area at a specific speed and / or conveying rate.

[0034] The material flow can be continuous, e.g. in the form of a jet, or can have interruptions, e.g. in the form of successive drops.

[0035] A flow direction of the material stream is oriented, in particular, parallel to the direction of gravity. In particular, the flow direction is oriented transversely or perpendicularly to the direction of movement of the laser pulses of the pulse sequence and / or perpendicularly to the propagation direction of at least one primary laser beam to which the laser pulses of the pulse sequence are assigned.

[0036] For example, a flow velocity of the target material in the target area oriented parallel to the flow direction is between 60 m / s and 120 m / s.

[0037] It can be advantageous if the laser pulse sequence is repeatedly re-deployed and introduced into the target area, with each newly introduced target material being exposed to a newly delivered laser pulse sequence. This allows for continuous secondary radiation generation.

[0038] In particular, the pulse sequence of laser pulses is provided anew at time intervals and, in particular, at regular time intervals.

[0039] According to the invention, the laser system mentioned at the outset comprises a laser device which is configured to provide a pulse sequence of laser pulses, wherein the pulse sequence has a pre-pulse and a main pulse following the pre-pulse, a pulse energy of the pre-pulse is between 2 pJ and 200 pJ and a pulse duration of the pre-pulse is between 200 fs and 5 ps, a pulse energy of the main pulse is between 2 mJ and 50 mJ and a pulse duration of the main pulse is between 15 fs and 300 fs, and wherein a temporal pulse interval between the pre-pulse and the main pulse is between 1 ps and 1 ns, wherein the laser system is configured to apply the pulse sequence of laser pulses to a target material in a target area, wherein secondary radiation is generated by interaction of the target material with the pulse sequence.

[0040] The laser system according to the invention has, in particular, one or more further features and / or advantages of the method according to the invention. Advantageous embodiments of the laser system have already been explained in connection with the method.

[0041] The method according to the invention can be carried out in particular by means of the laser system according to the invention. In particular, the method according to the invention is carried out by means of the laser system according to the invention.

[0042] In particular, the laser device provides at least one primary laser beam, to which the laser pulses of the pulse sequence are assigned. The at least one primary laser beam is directed at the target material located in the target area. The at least one primary laser beam contains the laser pulses of the pulse sequence with which the target material is applied.

[0043] The laser device comprises, in particular, one or more laser sources for providing the laser pulses of the pulse sequence. For example, a respective laser source provides a primary laser beam with laser pulses of a specific type and / or specific properties.

[0044] In particular, the respective primary laser beams, which are provided by different laser beam sources, are superimposed, in particular coaxially superimposed, to form a resulting primary laser beam, wherein the laser pulses of the pulse sequence are assigned to the resulting primary laser beam. In particular, the laser system comprises focusing optics for focusing the at least one primary laser beam into a focus, wherein the focus is positioned in the target area in the target material and / or on the target material and / or in a region of the target material.

[0045] In particular, the laser system may include a control device for controlling and / or regulating a beam length of the at least one primary laser beam. The control device is preferably designed to control or regulate a position of the at least one primary laser beam, and in particular its focus, within the target area and / or an impact position of the primary laser beam on the target material within the target area.

[0046] In particular, it can be provided that the laser system comprises the target area and / or the target material.

[0047] For the purposes of the present application documents, laser beam diameters and / or focus diameters are generally defined using the second moment method according to ISO 11146-3. Pulse durations are defined, in particular, by the half-width of the deconvolved autocorrelation.

[0048] In particular, the statement "at least approximately" is generally understood to mean a deviation of no more than 10%, i.e. that an actual value deviates from an ideal value by no more than 10%.

[0049] The following description of preferred embodiments, taken in conjunction with the drawings, serves to further explain the invention. They show:

[0050] Fig. 1 shows an embodiment of a laser system;

[0051] Fig. 2 shows a first example for generating secondary radiation, wherein a pulse train of pre-pulses is used to create an indentation in the target material; and Fig. 3 shows a further example for generating secondary radiation, wherein nanoparticles are generated in the area of ​​the surface of the material by means of a pre-pulse.

[0052] Identical or functionally equivalent elements are provided with the same reference numerals in all figures.

[0053] An embodiment of a laser system is shown in Fig. 1 and designated therein by 100. The laser system 100 comprises a laser device 102, by means of which at least one pulsed primary laser beam 104 is provided during operation of the laser system 100. This primary laser beam 104 is directed onto a target material 106, wherein secondary radiation 108 is generated by the interaction of the primary laser beam 104 with the target material 106.

[0054] The target material 106 is or comprises, for example, gallium, indium, tin, zinc, lithium, bismuth or alloys of these metals.

[0055] The laser device 102 is configured to provide the pulsed primary laser beam 104 with laser pulses 112 having different properties and pulse spacings. For this purpose, the laser device 102 comprises, for example, a plurality of laser sources 110, each generating pulsed laser beams with different properties. In the example shown, the respective pulsed laser beams of these laser sources 110 are coaxially superimposed to form the pulsed primary laser beam 104 emerging from the laser device 102.

[0056] The laser device 102 comprises, for example, a first laser source 110a, which provides a first pulsed primary laser beam 104a with laser pulses 112a, a second laser source 110b, which provides a second pulsed primary laser beam 104b with laser pulses 112b, and a third laser source 110c, which provides a third pulsed primary laser beam 104c with laser pulses 112c. The first primary laser beam 104a, second primary laser beam 104b, and third primary laser beam 104c emerging from the laser device 102 are coaxially superimposed in the example shown and, in particular, have the same beam path after exiting the laser device 102. In the example shown, the primary laser beam 104 is thus formed from the first primary laser beam 104a, second primary laser beam 104b and third primary laser beam 104c or comprises the first primary laser beam 104a, second primary laser beam 104b and third primary laser beam 104c.

[0057] Alternatively, it is also fundamentally possible for the different primary laser beams 104a, 104b, 104c to have different beam paths after exiting the laser device 102 and / or to be spaced apart from one another before they strike the target material 106. In this case, in particular, there is no coaxial superposition of the different primary laser beams 104a, 104b, 104c.

[0058] A respective laser source 110 comprises, for example, a seed laser 114 for generating seed laser pulses and an amplification device 116, which generates the respective laser pulses 112a, 112b, 112c of the primary laser beams 104a, 104b, 104c by amplifying the seed laser pulses (indicated at the laser source 110a in Fig. 1).

[0059] The amplification device 116 may include simple amplifiers, regenerative amplifiers, and / or multipass amplifiers. For example, the amplification device 116 may include fiber, rod, rod-type fiber, disc, slab, multislab, and / or plate amplifiers.

[0060] Alternatively, it is also possible, for example, for several or all existing laser sources 110 to be assigned a common amplification device 116. In particular, several or all laser sources 110 then use the same amplification device 116. In this case, for example, the laser pulses generated by different seed lasers 114 of the laser sources 110 are amplified by means of the same amplification device 116 to form the respective laser pulses 112a, 112b, 112c of the primary laser beams 104a, 104b, 104c. The laser device 102 is configured to decouple the laser pulses 112 provided by the different laser sources 110 with a defined temporal sequence and / or a defined temporal offset in order to apply the laser pulses 112 to the target material 106 in this temporal sequence or with this defined temporal offset.These laser pulses 112 form a pulse sequence 118 with which the target material 106 is exposed to generate secondary radiation 108.

[0061] The laser radiation associated with the laser pulses 112 has, for example, a wavelength of 10 pm, 3 pm, 515 nm or 343 nm.

[0062] To couple out the laser pulses 112 provided by the different laser sources 110 with a defined temporal sequence and / or defined temporal offset, the laser device 102 may comprise one or more optical modulators 120 and / or optical switches. For example, a modulator 120 is assigned to each of the different laser sources 110 of the laser device 102. During operation of the laser system 100, the respective modulator 120 is used to select laser pulses 112 for coupling out of the laser device 102 and / or to set the time intervals between the coupled-out laser pulses.

[0063] The optical modulator 120 can be designed, for example, as an acousto-optical modulator and / or as an electro-optical modulator.

[0064] For example, as shown in Fig. 1, the modulators 120 are each arranged after a specific laser beam source 110. It is also possible in principle for the modulators 120 to be integrated into a specific laser beam source 110 and arranged there, for example, between the seed laser 114 and the amplifier 116.

[0065] Alternatively or additionally, a specific temporal sequence and / or a specific temporal offset between the coupled-out laser pulses 112 can be realized by a defined path length difference and / or propagation time difference between the individual laser pulses 112, starting from the respective laser source 110 until reaching the target material 106. The path length difference can be realized, for example, via electronic and / or optical delay lines (not shown), wherein a delay line can be inserted into the respective beam path of one or more of the existing primary laser beams 104a, 104b, 104c. Optical delay lines can generally be designed as free-beam or fiber-based.

[0066] The laser system 100 has a target area 122 in which the target material 106 is arranged to be exposed to the primary laser beam 104 and to interact with its laser pulses 112. It is essential that target material 106 is continuously fed into the target area 122 so that fresh target material 106, which in particular has not yet been exposed to the primary laser beam 104, is always available for generating secondary radiation 108. This enables continuous generation of secondary radiation 108 during operation of the laser system 100.

[0067] In particular, the pulsed primary laser beam 104 directed onto the target material 106 is focused into a focus 123, wherein the focus 123 is arranged in the target area 122 in and / or on the target material 106. For this purpose, a focusing optics 124 can be provided, for example.

[0068] The target region 122 is understood to be a stationary region of the laser system 100 into which the target material 106 is coupled and / or into which the primary laser beam 104 is introduced in order to interact with the target material 106.

[0069] The target area 122 is preferably positioned in a fluid-tight and / or gas-tight chamber 126. In this chamber 126, for example, a negative pressure and / or a vacuum and / or a gas atmosphere with a defined composition is formed compared to the surroundings. For example, the pressure within the chamber is between 10 mbar and 500 mbar. For example, the gas arranged in the chamber 126 is or comprises hydrogen and / or helium.

[0070] To feed the target material 106 into the target area 122 and convey it through the target area 122, the laser system 100 can have a feed device 128. In particular, the feed device 128 can continuously provide target material 106, which passes through the target area 122 at a specific speed and / or conveying rate.

[0071] In particular, the target material 106 is provided by the feed device 126 as a liquid material stream that passes through the target area 122. This material stream is preferably in the form of a jet, and in particular in the form of a continuous and / or uninterrupted jet. However, the material stream can also be in the form of successive and / or spaced-apart droplets. The feed device 126 has, for example, a nozzle 128, by means of which the target material 106 is dispensed accordingly.

[0072] In the example shown in Fig. 1, the direction of gravity is oriented in the negative y-direction, so that target material 106 delivered by the feed device 126 passes through the target area 122 in the direction of gravity (ie in the negative y-direction or from top to bottom).

[0073] For example, the target material 106 passes through the target area 122 at a speed between 60 m / s and 120 m / s.

[0074] It is also fundamentally possible for the liquid material stream of the target material 106 provided by the feed device 126 to be in the form of a film formed on a suitable material surface (not shown) and passing through the target area 122. For this purpose, the feed device 126 can comprise, for example, a movable mechanism (not shown), such as a rotating wheel, a rotating drum, a rotating ball, or a moving belt, on whose surface the film is formed. Further technical details regarding the provision of target material for generating secondary radiation through interaction with a primary laser beam are described, for example, in the scientific publication "Light sources for high-volume manufacturing EUV lithography: technology, performance, and power scaling," I. Fomenkov et al., Advanced Optical Technologies 6(3): 173-186, DOI: 10.1515 / aot-2017-0029.

[0075] It can be provided that the laser system 100 has a control device 132 for controlling and / or regulating a beam length of the primary laser beam 104. This control device 132 is particularly designed to control or regulate a position of the primary laser beam 104 and in particular its focus 123 within the target area 122 and / or an impact position 134 of the primary laser beam 104 on the target material 106 within the target area 122.

[0076] For spatially shifting the primary laser beam 104, the control device 132 comprises a beam deflection device 136. This can, for example, have movable mirror elements, acousto-optical deflectors and / or electro-optical deflectors to realize the shift.

[0077] Furthermore, the control device 132 can have a detection device 138 configured to detect a spatial position of a specific feature, wherein the feature is arranged or formed on or in the region of the target material 106. For example, the feature is a geometric feature formed on the target material 106, such as an indentation (see below). To detect a specific feature, the detection device 138 can comprise a camera to detect the feature, for example, using image recognition.

[0078] The beam deflection device 136 is then configured to control and / or regulate the displacement of the primary laser beam 104 by means of the beam deflection device 136 based on the information provided by the detection device 138. For this purpose, the detection device 138 is connected to the beam deflection device 136 in a signal-effective manner. The laser system 100 functions as follows:

[0079] During operation of the laser system 100, a pulse sequence 118 is provided by the laser device 102 and interacted with target material 106 located in the target area 122 to generate secondary radiation 108.

[0080] Target material 106 is continuously conveyed into the target area 106 by means of the feed device 128, so that fresh target material 106 is always available there, which passes through the target area 122, in particular in the form of a liquid jet (in the examples shown, the target material 106 passes through the target area 122 parallel to the direction of gravity or in the negative y-direction).

[0081] It is provided that a specific spatial region of the target material 106 conveyed through the target area 122 is subjected to a defined pulse sequence 118. In this spatial region, the target material 106 interacts, in particular, with all laser pulses 112 of the pulse sequence 118. Subsequently, the laser device 102 emits, in particular, a further pulse sequence 118, which is then brought into interaction with a further spatial region of subsequently conveyed target material 106, etc. In this way, the process for generating the secondary radiation 106 can be continued continuously.

[0082] Figures 2a to 2c show a temporal sequence of laser pulses 112a, 112b impinging on the target material 106, which are assigned to a pulse sequence 118a. In the figure shown, the target material 106 flows parallel to a flow direction 140 through the target area 122.

[0083] The primary laser beam 104 comprising the laser pulses 112a, 112b, or its focus 123, strikes the target material 106 in a specific spatial region 142. This spatial region 142 is understood to be a spatial region that is stationary with respect to the target material 106, which is assigned to the target material 106 and moves with the target material 106 in the direction of flow. The pulse train 118a comprises a pulse train 144 consisting of two or more first laser pulses 112a and a further laser pulse 112b following the pulse train 144. The first laser pulses 112a are also referred to herein as pre-pulses, and the further laser pulse 112b is referred to as the main pulse of the pulse train 118a.

[0084] The focus 123 of the primary laser beam 104 has, in particular, a diameter in the range of 2.5 pm to 30 pm. An intensity of the primary laser beam 104 in the focus 123 is, in the case of the main pulse 112b, in particular between 10 16 W / cm 2 and 10 19W / cm 2 .

[0085] First laser pulses 112a are thus understood to be laser pulses 112 of a first type and / or with first pulse properties, and second laser pulses 112b are understood to be laser pulses of a second type and / or with second pulse properties. Accordingly, third laser pulses 112c are understood to be laser pulses 112 of a third type and / or with third pulse properties.

[0086] Pulse train 144 is understood to be, in particular, a "burst" of first laser pulses 112a. In particular, pulse train 144 comprises at least two, and in particular at least 20, and in particular at least 100, first laser pulses 112a.

[0087] A temporal pulse spacing ti between successive first laser pulses 112a within the pulse train 144 is between 100 ps and 100 ns, and preferably between 200 ps and 0.5 ns. In particular, the temporal pulse spacing ti between all existing adjacent first laser pulses 112a of the pulse train 144 is at least approximately the same.

[0088] A whole t g The temporal length of the pulse train 144 from first laser pulses 112a is between 1 ns and 10 ps.

[0089] The total energy of pulse train 144 is, for example, between 0.8 mJ and 1.2 mJ. The total energy of pulse train 144 is understood to be the sum of the pulse energies of all first laser pulses 112a assigned to pulse train 144. In particular, all first laser pulses 112a assigned to pulse train 144 have at least approximately the same pulse energy.

[0090] A temporal pulse interval tz between the pulse train 144 and the second laser pulse 112b is between 10 ps and 1 ps. The temporal pulse interval tz is understood to be the temporal interval between a last first laser pulse 112'a of the pulse train 144 and the second laser pulse 112b.

[0091] The second laser pulse 112b follows the pulse train 144, ie the first laser pulses 112a of the pulse train 144 first hit the target material 106 and then the second laser pulse 112b.

[0092] A pulse duration td of the second laser pulse 112b is, for example, between 25 fs and 50 fs.

[0093] A pulse energy of the second laser pulse 112b is, for example, between 8 mJ and 12 mJ.

[0094] The first laser pulses 112a are provided, for example, by means of the first laser source 110a. The first laser source 110a is then configured to provide first laser pulses 112a with the aforementioned properties. Accordingly, the second laser pulses 112b are provided, for example, by means of the second laser source 110b, which is then configured to provide second laser pulses 112b with the aforementioned properties. The described pulse sequence 118a, which comprises first and second laser pulses 112a and 112b, can be formed, for example, by means of the optical modulators 120. For this purpose, the optical modulators 120 are used, for example, as "pulse pickers" and select the laser pulses provided by the respective laser sources 110a, 110b accordingly to form the pulse sequence 118a.

[0095] Fig. 2b shows the target material 106 after interaction of several first laser pulses 112a of the pulse train 144, ie, the pulse sequence 118a has already partially interacted with the target material 106 and / or been absorbed by the target material 106. The interaction of the pulse train 144 consisting of first laser pulses 112a or

[0096] Pre-pulsing causes the formation of an indentation 146 in the target material 106, wherein this indentation is positioned in the region 142 of the target material 106 in which the interaction with the first laser pulses 112a occurred. The indentation 146 is designed, in particular, as a "cup" or "dimple." It is also possible, in principle, for the indentation 146 to be toroidal and / or annular groove-shaped.

[0097] The formation of indentations in materials due to their interaction with laser pulses and the underlying physical effects are described, for example, in the scientific publication "Review on Experimental and Theoretical Investigations of Ultra-Short Pulsed Laser Ablation of Metals with Burst Pulses" by Förster et al., Materials 2021, 14, 3331, https: / / doi.org / 10.3390 / mal4123331.

[0098] In particular, the interaction of the first laser pulses 112a of the pulse train 114 causes material removal by evaporation and / or melt expulsion.

[0099] The indentation 146 is formed on a surface 148 and / or outer side of the target material 106, which is impinged upon by the primary laser beam 104 or its laser pulses 112a, 112b. This surface 148 forms, in particular, a boundary surface of the target material 106, which, in the examples shown, is present as a liquid material stream in the form of a jet.

[0100] A depth direction 150 of the indentation 146 is oriented at least approximately parallel to the propagation direction of the primary laser beam 104 (indicated by the arrow of the primary laser beam 104) and / or at least approximately perpendicular to the flow direction 140 of the target material 106.

[0101] A maximum depth of the indentation 146 relative to the surrounding surface 148, oriented parallel to the depth direction 150, is, for example, between 5 pm and 150 pm, in particular between 10 pm and 50 pm. A maximum spatial extent and / or a maximum diameter of the indentation 146 is, for example, between 5 pm and 30 pm. Fig. 2c shows the interaction of the second laser pulse 112b or main pulse with the target material 106 at the formed indentation 146. This interaction generates secondary radiation 108, wherein the generation of secondary radiation can be particularly efficient due to the formed indentation 146. In particular, the indentation is conical and / or parabolic.

[0102] This is primarily due to improved absorption of the main pulse caused by the indentation 146, as described, for example, in the scientific publication "Enhancement of hard x-ray emission from a copper target by multiple shots of femtosecond laser pulses" by Hironaka et al., Applied Physics Letters, Volume 74, Number 12, March 22, 1999. In particular, reduced Fresnel reflection can occur, which contributes to the improved absorption.

[0103] Furthermore, the geometric shape of the indentation 146 can result in a concentration of the radiation intensity of the incident main pulse, as described, for example, in the scientific publication "Development of a bright MeV photon source with compound parabolic concentrator targets on the National Ignition Facility Radiographic Capability (NIF-ARC) laser" by Kerr et al., Phys. Plasmas 30, 013101 described.

[0104] It is fundamentally possible for the pulse sequence 118a to comprise a plurality of second laser pulses 112b to generate secondary radiation 108.

[0105] Subsequently, to continue the process, another pulse sequence 118a is generated, which is brought into interaction with a new spatial region of subsequently conveyed target material 106, etc. In this way, secondary radiation 108 is continuously generated.

[0106] In the method shown in Figs. 2a, 2b, and 2c, all laser pulses 112a, 112b of the respective pulse sequence 118a interact with the target material 106 in the same spatial region 142. In particular, the first laser pulses 112a of the pulse train 144 interact with the target material 106 in the same spatial region 142 as the second laser pulse 112b. This second laser pulse 112b interacts with the target material 106 in the spatial region 142 in which the indentation 146 is formed.

[0107] In particular, a movement speed of the laser pulses 112a, 112b in the direction of the target material 106 is much greater than its flow speed, so that all laser pulses interact with the target material 106 approximately in the same spatial region 142.

[0108] It can be provided that the impact position 134 of the primary laser beam 104 on the target material 106 is readjusted by means of the control device 132, so that it remains constant, in particular, between the formation of the indentation 146 and the impact of the second laser pulse 112b. For this purpose, for example, a spatial position of the formed indentation 146 is determined by means of the detection device 138, and based on this, the beam position of the primary laser beam 104 is adjusted by means of the beam deflection device 136.

[0109] In the example shown in Figs. 3a to 3c, the target material 106 is subjected to a pulse sequence 118b which comprises a third laser pulse 112c and a second laser pulse 112b following the third laser pulse 112c.

[0110] For example, a pulse duration td2 of the third laser pulse 112c is between 800 fs and 1.5 ps.

[0111] A pulse energy of the third laser pulse 112c is, for example, between 10 pJ and 20 pJ.

[0112] A temporal pulse interval tz2 between the third laser pulse 112c and the second laser pulse 112b is, for example, between 10 ps and 100 ps.

[0113] Third laser pulses 112c are provided, for example, by means of the third laser source 110c. The third laser source 110c is then configured to provide third laser pulses 112c with the aforementioned properties. The second laser pulse 112b has the properties mentioned above in connection with the example according to Figs. 2a to 2c. It is fundamentally possible for the pulse sequence 118b to comprise a plurality of second laser pulses 112b.

[0114] The interaction of the third laser pulse 112c with the target material 106 causes a formation of nanoparticles 152 on the surface 148 of the target material 106, wherein the nanoparticles 152 are positioned in that spatial region 142 on the surface 148 in which the target material 106 is exposed to the primary laser beam 104 and the target material 106 interacts with the third laser pulse 112c.

[0115] In this example, the third laser pulse 112c is referred to as the pre-pulse and the second laser pulse 112b as the main pulse of the pulse sequence 118b.

[0116] The formation of nanoparticles in materials through their interaction with laser pulses and the underlying physical effects are described, for example, in the above-mentioned scientific publication "Review on Experimental and Theoretical Investigations of Ultra-Short Pulsed Laser Ablation of Metals with Burst Pulses" by Förster et al. and in the scientific publication "Fs-ns double-pulse Laser Induced Breakdown Spectroscopy of copper-based alloys: Generation and elemental analysis of nanoparticles" by Guarnaccio et al., Spectrochimica Acta Part B 101 (2014) 261-268, https: described.

[0117] The average diameter of the nanoparticles 152 is between 10 nm and 100 nm.

[0118] It is also possible that a plurality of third laser pulses 112c are provided to generate the nanoparticles 152 or that a pulse train consisting of a plurality of third laser pulses 112c is provided.

[0119] Fig. 3c shows the interaction of the second laser pulse 112b with the target material 106 in the spatial region 142 of the formed nanoparticles 152, whereby this interaction generates secondary radiation 108. Due to the presence of the nanoparticles 152, secondary radiation 108 can be generated particularly efficiently using the second laser pulse 112b.

[0120] Due to the presence of the nanoparticles 152 in the area of ​​the surface 148 of the target material 106, so-called plasmonic resonances can occur, which cause a particularly good absorption of the radiation of the main pulse in the electron gas of the target material 106 and, in particular, a particularly efficient increase in the electron temperature in the electron gas.

[0121] Analogous to the example according to Figs. 2a to 2c, a movement speed of the laser pulses 112c, 112b in the direction of the target material 106 is much greater than its flow speed, so that all laser pulses interact with the target material 106 approximately in the same spatial region 142.

[0122] In this example, it can also be provided that the impact position 134 of the primary laser beam 104 on the target material 106 is readjusted by means of the control device 132, so that it remains constant, in particular between the formation of the nanoparticles 152 and the impact of the second laser pulse 112b. For this purpose, for example, a spatial position of the formed nanoparticles 152 is determined by means of the detection device 138, and based on this, the beam position of the primary laser beam 104 is adjusted by means of the beam deflection device 136.

[0123] It is possible to combine the variants described in Figs. 2a to 2c and 3a to 3c. In particular, the pulse sequence 118 is then designed such that, through its interaction with the target material 106 in a specific spatial region 142, an indentation 146 is first generated, and then nanoparticles 152 are generated in this spatial region 142. Subsequently, the secondary radiation 108 is generated through the interaction of a main pulse in this region 142. In this case, the pulse sequence 114 comprises, for example, the above-described pulse train 144 comprising first laser pulses 112a, the above-described third laser pulse 112c (cf. Fig. 1), or a pulse train comprising third laser pulses 112c and the above-described second laser pulse 112b. The pulse train 144 comprising first laser pulses 112a is arranged temporally before the third laser pulse 112c or the pulse train comprising third laser pulses 112c and the third laser pulse 112c orThe pulse train of third laser pulses 112c arrives before the second laser pulse (i.e., the pulse train 144 of first laser pulses 112a first strikes the target material 106, followed by the third laser pulse 112c or the pulse train of third laser pulses 112c, and finally the second laser pulse 112b). A time interval between the pulse train 144 of first laser pulses 112a, the third laser pulse 112c or the pulse train of third laser pulses 112c, and the second laser pulse 112b is, in particular, between 1 ps and 500 ns.

[0124] By means of the described methods, incoherent X-ray radiation can be generated particularly efficiently as secondary radiation 108.

[0125] List of reference symbols td pulse duration td2 pulse duration tg total time length ti time pulse interval tz time pulse interval tz2 time pulse interval

[0126] 102 Laser device

[0127] 104 Primary laser beam

[0128] 104a first primary laser beam

[0129] 104b second primary laser beam

[0130] 104c third primary laser beam

[0131] 106 Target material

[0132] 108 Secondary radiation

[0133] 110 Laser source

[0134] 110a first laser source

[0135] 110b second laser source

[0136] 110c third laser source

[0137] 112 laser pulses

[0138] 112a, c Laser pulse / pre-pulse

[0139] 112'a laser pulse / pre-pulse

[0140] 112b Laser pulse / main pulse

[0141] 114 seed lasers

[0142] 116 amplification device

[0143] 118 pulse sequence

[0144] 118a, b Pulse sequence

[0145] 120 optical modulator

[0146] 122 Target area

[0147] 123 Focus

[0148] 124 Focusing optics

[0149] 126 Chamber

[0150] 128 Feeding device

[0151] 130 Nozzle control device impact position

[0152] Beam deflection device Detection device Flow direction Spatial area Pulse train Indentation Surface Depth direction Nanoparticles

Claims

Patent claims 1. A method for generating secondary radiation (108), in which - a target material (106) is provided in a target area (122), - the target material (106) in the target area (122) is subjected to a pulse sequence (118b) of laser pulses (112), wherein secondary radiation (108) is generated by interaction of the target material (106) with the pulse sequence (118b), wherein - the pulse sequence (118b) comprises a pre-pulse (112c) and a main pulse (112b) following the pre-pulse (112c), - a pulse energy of the pre-pulse (112c) is between 2 pJ and 200 pJ and a pulse duration (td2) of the pre-pulse (112c) is between 200 fs and 5 ps, - a pulse energy of the main pulse (112b) is between 2 mJ and 50 mJ and a pulse duration (td) of the main pulse (112b) is between 15 fs and 300 fs, and wherein - a temporal pulse interval (tz2) between the pre-pulse (112c) and the main pulse (112b) is between 1 ps and 1 ns.

2. Method according to claim 1, characterized in that the pulse duration (td2) of the pre-pulse (112c) is between 800 fs and 1.5 ps, and / or that the pulse energy of the pre-pulse (112c) is between 5 pJ and 100 pJ.

3. Method according to one of the preceding claims, characterized in that the temporal pulse interval (tz2) between the pre-pulse (112c) and the main pulse (112b) is between 10 ps and 100 ps.

4. Method according to one of the preceding claims, characterized in that the application of the pre-pulse (112c) to the target material (106) causes a formation of nanoparticles (152), wherein the nanoparticles (152) are positioned in particular in the region of a surface (148) and / or in a spatial region (142) of the target material (106), at which or in which the application of the Target material (106) with the pre-pulse (112c).

5. Method according to one of the preceding claims, characterized in that the pulse energy of the main pulse (112b) is between 5 mJ and 15 mJ, and / or that the pulse duration (td) of the main pulse (112b) is between 25 fs and 50 fs.

6. Method according to one of the preceding claims, characterized in that the application of all laser pulses (112) of the pulse sequence (118b) to the target material (106) takes place at the same location and / or in the same spatial region (142) of the target material (106), and / or that the application of the main pulse (112b) to the target material (106) takes place in a spatial region in which nanoparticles (152) were formed by means of the pre-pulse (112c).

7. Method according to one of the preceding claims, characterized in that a suppression and / or a vacuum and / or a gas atmosphere with a defined composition is formed in the target area (122).

8. Method according to one of the preceding claims, characterized in that the laser pulses (112) of the pulse sequence (118b) are assigned to at least one primary laser beam (104), wherein the at least one primary laser beam (104) is provided by means of a laser device (102) and is directed onto the target area (122) in order to interact there with the target material (106).

9. The method according to claim 8, characterized in that the at least one primary laser beam (104) is focused into the target area (122), wherein a focus (123) of the primary laser beam (104) is positioned in the target material (106) and / or on the target material (106) and / or in a region of the target material (106).

10. Method according to one of the preceding claims, characterized in that target material (106) is continuously introduced into the target area (122) is supplied and / or conveyed.

11. The method according to claim 10, characterized in that the target material (106) passes through the target area (122) as a material flow and in particular as a liquid material flow, and / or that the target material (106) passes through the target area (122) at a specific flow velocity and / or conveying rate.

12. The method according to claim 10 or 11, characterized in that the pulse sequence (118b) of laser pulses (112) is repeatedly provided anew and introduced into the target area (122), wherein target material (106) newly introduced into the target area (122) is each exposed to a newly provided pulse sequence (122) of laser pulses (112).

13. Laser system for generating secondary radiation (108), comprising - a laser device (102) configured to provide a pulse sequence (118b) of laser pulses (112), wherein i. the pulse sequence (118b) comprises a pre-pulse (112c) and a main pulse (112b) following the pre-pulse (112c), ii. a pulse energy of the pre-pulse (112c) is between 2 pJ and 200 pJ and a pulse duration of the pre-pulse (112c) is between 200 fs and 5 ps, iii. a pulse energy of the main pulse (112b) is between 2 mJ and 50 mJ and a pulse duration of the main pulse (112b) is between 15 fs and 300 fs, and wherein iv. a temporal pulse interval (tz2) between the pre-pulse (112c) and the main pulse (112b) is between 1 ps and 1 ns, - wherein the laser system is configured to apply the pulse sequence (118b) of laser pulses (112) to a target material (106) in a target region (122), wherein secondary radiation (108) is generated by interaction of the target material (106) with the pulse sequence (118b).

14. Laser system according to claim 13, characterized in that the laser system is configured to carry out the method according to one of claims 1 to 12.

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