Supercontinuum extreme ultraviolet coherent light source generation device based on ultra-intense laser induction

Abstract

The present invention discloses a supercontinuum extreme ultraviolet coherent light source generating device based on ultra-intense laser induction, comprising an intense laser source, a gas pool and a focusing mirror, wherein the intense laser source is used to output a few-cycle intense laser pulse; the few-cycle intense laser pulse contains no more than three light cycles, the pulse width is between 5-7fs, and the single pulse energy is above 500mJ. The gas pool is filled with gas and is arranged in a vacuum chamber; the few-cycle intense laser pulse is incident on the gas pool after passing through the focusing mirror to excite the gas in the gas pool to generate supercontinuum extreme ultraviolet coherent light. The technical solution provided by the present invention generates supercontinuum extreme ultraviolet radiation with a bandwidth of more than twenty electron volts through the excitation and decoration effect of the strong laser field in the gas pool, realizes the strong field attosecond control of the radiation dynamics of foreign atoms, and can simultaneously generate spatially separated coherent attosecond light pulses, solving the light source problem of attosecond pulse interference experiments.

Description

Technical Field

[0001] The invention relates to the field of stimulated radiation, and in particular to a supercontinuum extreme ultraviolet coherent light source generating device based on ultra-strong laser induction. Background Art

[0002] Coherent supercontinuum radiation in the extreme ultraviolet (XUV) range is essential for synthesizing attosecond light pulses and probing transient atomic structures.

[0003] Coherent supercontinuum radiation not only provides a light source for time-resolved spectroscopy, frequency comb technology, nonlinear microscopy and waveform synthesis, but is also important for synthesizing isolated attosecond pulses. Coherent supercontinuum radiation originally refers to the phenomenon that the spectrum of ultrashort light pulses is sharply broadened when propagating in a medium, which is usually caused by third-order nonlinear processes such as self-phase modulation, self-focusing, stimulated Raman scattering and self-steepening. Existing technologies can generate attosecond and zettasecond supercontinuum spectra using higher-order and non-perturbative nonlinear optical responses. However, in the extreme ultraviolet spectral region, there are still technical barriers to generating ultra-wide bandwidth coherent continuous radiation.

[0004] From the perspective of quantum mechanics, photons are generated by transition radiation between atomic states, and the frequency of the radiated photons corresponds to the atomic transition energy level. When the upper state of the atom is a continuous state, a continuous spectrum will be radiated. For example, high harmonic generation (HHG) caused by strong field ionization leads to attosecond pulse radiation. When continuous radiation between bound states is involved, it is understood as the transition of atoms between modified states of bound states, involving nonlinear responses. In particular, if the pulse width of the driving light pulse only includes a few light cycles or even a single light cycle, there are simultaneous modified states of multiple photon modes and the interaction between these modified states, which makes the photon modification map very complicated. Summary of the invention

[0005] Based on this, the technical solution provided by the present invention generates supercontinuum radiation with a bandwidth of more than twenty electron volts through the ionization effect of a strong laser field in a gas pool. The radiation wavelength range is between 35nm-73nm, realizing strong-field attosecond control of the radiation dynamics of external atoms. Combined with HHG, separate coherent attosecond light pulses can be generated simultaneously, solving the light source problem of attosecond pulse interference experiments.

[0006] In order to achieve the above-mentioned object, the present invention provides a supercontinuum extreme ultraviolet coherent light source generating device based on ultra-intense laser induction, comprising an intense laser source, a gas pool and a focusing mirror. The intense laser source is used to output a few-cycle intense laser pulse; the number of light cycles contained in the few-cycle intense laser pulse is not more than three, the pulse width of the few-cycle intense laser pulse is between 5-7fs, and the single pulse energy of the few-cycle intense laser pulse is above 500mJ. The gas pool is filled with gas, and the gas pool is arranged in a vacuum chamber. The few-cycle intense laser pulse is incident on the gas pool after passing through the focusing mirror to excite the gas in the gas pool to generate supercontinuum extreme ultraviolet coherent light.

[0007] Preferably, the gas in the gas pool is helium, and the gas pressure of the helium is set to 60 mbar.

[0008] Preferably, the central wavelength of the few-cycle intense laser pulse is between 700-800 nm, and each frequency component in the few-cycle intense laser pulse is dispersion-free.

[0009] Preferably, the gas pool comprises a glass tube with one end closed, and a through hole is provided on the wall of the glass tube for emitting the generated supercontinuum extreme ultraviolet coherent light.

[0010] Preferably, the inner diameter of the glass tube is 1.75 mm, and the diameter of the through hole is 100 μm.

[0011] Furthermore, the device also includes a filter, which includes a metal film with a thickness of 200 nm and is used to filter out the few-cycle strong laser pulses emitted from the gas pool.

[0012] Furthermore, the gas pool is placed between the focus of the few-cycle strong laser pulse and the focusing mirror, and the vertical distance between the focus and the axis of the gas pool is between 5-15 mm.

[0013] Furthermore, it is characterized in that the focal length of the focusing mirror is set to 500 mm; the focusing mirror is reflective focusing.

[0014] Furthermore, it is characterized in that the vacuum degree of the vacuum chamber is between 10-8 and 10-5 mbar.

[0015] Furthermore, it is characterized in that the wavelength range of the supercontinuum extreme ultraviolet coherent light is between 35 nm and 73 nm.

[0016] The beneficial effects achieved by the present invention through the above technical solution are:

[0017] (1) The measured extreme ultraviolet supercontinuum (XSC) spectral range covers the excited and autoionized states of most atoms and molecules, and can be an ideal multi-spectral probe for ultrafast electron dynamics in materials.

[0018] (2) By increasing the intensity of the driving laser, the bandwidth of the XSC is further broadened, generating ultrashort isolated XUV pulses. The XSC is emitted at a large divergence angle and is spatially separated from the accompanying HHG. The technical solution provided by the present invention can generate two synchronous attosecond light sources at the same time, thereby providing a reliable coherent light source for attosecond interferometry experiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG1 is a schematic diagram of the structure of a device for generating a supercontinuum extreme ultraviolet coherent light source according to an embodiment of the present invention.

[0020] FIG. 2 is a frequency spatial profile of the spectrum radiated by helium atoms in a gas pool using a few-cycle strong laser pulse according to an embodiment of the present invention.

[0021] FIG. 3 is a graph showing XSC yields under different laser intensity conditions according to an embodiment of the present invention.

[0022] FIG. 4 is a spectrum diagram of the transition from dark to bright state under different laser intensities according to an embodiment of the present invention.

[0023] FIG. 5 is a schematic diagram of the spatial distribution of extreme ultraviolet radiation driven by a strong laser pulse according to an embodiment of the present invention.

[0024] The symbols in the figure represent:

[0025] 1. Focusing mirror; 2. Vacuum cavity; 3. Intense laser pulse with few cycles; 4. Grating image plane; 5. Filter; 6. Gas pool;

[0026] 7. Focus; 8. Flat-field grating; 31. Extreme ultraviolet free induction decay (XFID); 32. Supercontinuum extreme ultraviolet

[0027] (XSC); 33. Higher harmonics (HHG). DETAILED DESCRIPTION

[0028] In order to make the purpose, technical scheme and advantages of the present invention more clear, the specific implementation of the present invention is further described in detail below in conjunction with examples. It should be understood that the embodiments described herein are only used to explain the present invention, but are not used to limit the scope of the present invention.

[0029] Please refer to Figure 1, which is a schematic diagram of the structure of the device for generating a supercontinuum extreme ultraviolet coherent light source according to an embodiment of the present invention. As can be seen from the figure, the device for generating a supercontinuum extreme ultraviolet coherent light source according to an embodiment of the present invention comprises a strong laser source, a gas pool 6 and a focusing mirror 1, wherein the strong laser source is used to output a few-cycle strong laser pulse 3; the number of light cycles contained in the few-cycle strong laser pulse 3 is less than three, and each frequency component in the few-cycle strong laser pulse 3 is dispersion-free; the gas pool 6 is filled with gas, and the gas pool 6 is arranged in a vacuum chamber 2; the focusing mirror 1 is used to focus the few-cycle strong laser pulse 3 in the vacuum chamber 2; wherein the few-cycle strong laser pulse 3 is incident on the gas pool 6 after being focused by the focusing mirror 1, so as to excite the gas in the gas pool 6 to generate supercontinuum extreme ultraviolet coherent light, and the wavelength range of the supercontinuum extreme ultraviolet coherent light is between 35 nm and 73 nm.

[0030] In an embodiment of the present invention, the gas pool 6 is filled with helium, and the gas pressure of the helium is 60 mbar. In one or other embodiments of the present invention, the gas pool 6 may also be filled with neon, krypton or a mixed gas. In one or other embodiments of the present invention, the gas pressure in the gas pool 6 may be other low pressure values ​​around 60 mbar.

[0031] In an embodiment of the present invention, the gas pool 6 includes a glass tube, the inner diameter of the glass tube is 1.75 mm, and the wall thickness of the glass tube is about 0.2 mm. Since the glass material has a strong absorption of extreme ultraviolet light, in an embodiment of the present invention, a through hole with a diameter of 100 μm is also provided on the wall of the glass tube for emitting the generated extreme ultraviolet light. At the same time, the gas in the gas pool 6 will overflow the gas pool 6 through the through hole, which not only destroys the gas pressure stability of the gas pool 6, but also affects the vacuum degree of the vacuum chamber 2. In one or other embodiments of the present invention, the glass tube of the gas pool 6 is externally connected to a gas pump for timely replenishing the gas pressure of the gas pool 6, and the vacuum chamber 2 is externally connected to a vacuum pump to maintain the vacuum degree in the vacuum chamber 2 between 10-8 and 10-5 mbar.

[0032] In an embodiment of the present invention, the device for generating a supercontinuum extreme ultraviolet coherent light source further includes a filter 5, the filter 5 is arranged in the vacuum chamber 2, and the filter 5 includes a metal aluminum film with a thickness of 200nm, which is used to filter out the few-cycle strong laser pulses 3. In one or other embodiments of the present invention, the metal aluminum film of the filter 5 is arranged on a bracket, and the bracket is movable. When the optical path is debugged, the bracket moves to make the metal aluminum film arranged on the bracket deviate from the optical path. When generating extreme ultraviolet light, the bracket moves to make the metal aluminum film enter the optical path to filter out the few-cycle strong laser pulses 3.

[0033] In particular, in an embodiment of the present invention, after the few-cycle intense laser pulse 3 passes through the focusing mirror 1, it is incident on the gas pool 6, and the focus 7 is set in the vacuum chamber 2 and outside the gas pool 6. Since the relative position of the focus 7 and the interaction point determines the spatial distribution of the laser light intensity, it affects the density spatial distribution of the neutral atoms-ions-electrons at the interaction point, and this density spatial distribution does not change with the refractive index, so that the supercontinuum extreme ultraviolet light generated in different places is coherent and constructive. That is, the spectrum of the radiated supercontinuum extreme ultraviolet light has the best phase matching, which is conducive to obtaining a higher intensity supercontinuum extreme ultraviolet light source. Preferably, in an embodiment of the present invention, the gas pool is placed before the focus of the few-cycle intense laser pulse 3, and the distance between the focus 7 and the gas pool 6 is 10 mm. In one or other embodiments of the present invention, the distance between the focus 7 and the gas pool 6 is 10 mm and can be set between 5 mm and 15 mm.

[0034] In the embodiment of the present invention, a reflective focusing mirror with a focal length of 500 mm is used. In one or other embodiments of the present invention, the focusing mirror 1 may be a transmissive focusing mirror. In one or other embodiments of the present invention, the focal length may be other values.

[0035] In one or other embodiments of the present invention, under the condition of modification of the few-cycle strong laser pulse 3, the atomic bound state is coherently populated, thereby radiating a coherent supercontinuum extreme ultraviolet spectrum. This process is often overlooked in HHG 33, so it has not been discovered for a long time. For helium atoms, the minimum excitation energy is as high as 21.2 eV, but it can still be seen that the atomic excited state is coherently filled. This is because after the few-cycle strong laser pulse 3 is turned off, the quantum coherence generated by its excitation makes the helium atom a huge dipole, and through a coupled resonance similar to the nuclear XUV free induction decay, the helium atom radiates coherently at the resonant frequency. However, when interacting with the few-cycle strong laser pulse 3, the above-mentioned resonant radiation of the atom is modulated by the transient AC Stark shift, so that the dark state that does not participate in the dipole radiation in the absence of the few-cycle strong laser pulse 3 can be modified to a bright state, thereby generating supercontinuum coherent radiation. The technical solution provided by the embodiment of the present invention not only enhances the radiation intensity of the supercontinuum extreme ultraviolet, but also provides a new way to detect the atomic dark state.

[0036] In one or other embodiments of the present invention, a concave mirror with a pulse duration of 6fs is used to focus a few-cycle intense laser pulse 3 into a 60mbar helium chamber using an f=500 mm concave mirror. In one or other embodiments of the present invention, the few-cycle intense laser pulse 3 is a near-infrared laser with a central wavelength range of 700-800nm. In one or other embodiments of the present invention, the pulse width of the few-cycle intense laser pulse 3 can be between 5fs-7fs. In one or other embodiments of the present invention, the single pulse energy of the few-cycle intense laser pulse 3 is preferably above 500mJ. In an embodiment of the present invention, the peak intensity of the few-cycle intense laser pulse 3 is preferably more than 2.1 PW / cm2, and, in one or other embodiments of the present invention, the peak intensity of the few-cycle intense laser pulse 3 can be reduced by a combination of a broadband half-wave plate and a broadband polarizer, thereby obtaining few-cycle intense laser pulses 3 of different intensities to observe the generation law of supercontinuum extreme ultraviolet light driven at different intensities.

[0037] In particular, in order to characterize the supercontinuum extreme ultraviolet coherent light source generated by the device of the embodiment of the present invention, one or other embodiments of the present invention use a flat-field grating 8, a grating image plane 4 and a high-resolution XUV spectrometer to perform spectral frequency domain and spatial analysis on the radiation light. Please refer to Figure 2, which is a spectral frequency spatial profile of the helium atoms in the gas pool 6 irradiated by the few-cycle strong laser pulse 3 in the embodiment of the present invention. Among them, the intensity of the few-cycle strong laser pulse 3 used is 0.75 PW / cm2, and the three different XUV radiations obtained are: (1) HHG 33, the divergence angle is very small, and the optical comb frequency interval is twice the center frequency of the few-cycle strong laser pulse 3; (2) extreme ultraviolet free induction decay (XUV free induction decay, XFID) radiation, the divergence angle is large, the spectral line presents a linear structure, and the line width is narrow and sharp overall. The central energy of these radiations is related to the electric dipole transitions from various 1snp states to the ground state, where n (n>2) is the principal quantum number; (3) XSC 32 has a characteristic divergent distribution centered at ±8 mrad and a half-maximum full width less than 5 mrad, covering a wide spectral range and even exceeding the upper limit detection value of XUV spectrometers under ultra-intense laser conditions.

[0038] Please refer to Figure 3, which is a graph of XSC yield under different laser intensity conditions of an embodiment of the present invention. In the figure, the horizontal axis is photon energy (electron volts) and the vertical axis is photon yield (arbitrary units). The laser intensity range investigated is between 0.1-2.0 PW / cm2. As shown in the figure, when the laser intensity is 0.11 PW / cm2, only XFID 31 radiates, and it is two separate radiation series. One of the series corresponds to the radiation transition above the thirteenth harmonic. , another series corresponds to radiative transitions, where n ranges from 5 to 9. In addition, it can be seen from the figure that the XFID 31 radiation from the 3p and 4p states is not very obvious at lower laser intensities. As the laser intensity increases, a continuous spectrum above the helium ionization potential gradually appears, and its cutoff frequency is linearly proportional to the laser intensity. Then, as the laser intensity increases, the field modification of the bound state produces a continuous emission below the threshold. Under greater laser intensity conditions, the lower energy continuous spectrum extends to the spectrum below the first excited state. As the laser intensity increases further, the resonant structure in the XFID 31 radiation spectrum continues to shrink and disappear, and eventually evolves into a supercontinuum spectrum covering the energy range of 17-35eV. In particular, the additional emission peak that appears between 1s2p and the threshold has a peak frequency that moves linearly with the laser intensity.

[0039] In addition to the bright state 1snp associated with XFID 31, the dark state is also important for XSC 32 emission. According to the selection rule, only the 1snp state can directly radiate through a single-photon ground state transition, and the 1sns and 1snd states are not radiative. Therefore, the 1sns and 1snd states are called dark states. However, driven by a strong laser pulse, the 1sns and 1snd states can still remain in a filled state and can be modified with additional photons to form a Floquet state. Please refer to Figure 4, which is a dark to bright state transition spectrum under different laser intensities of an embodiment of the present invention. It can be seen from the figure that the dark states 3s, 2s and 2p are transformed into bright states and radiate outward through emission modification, and the radiation peak position varies linearly with the intensity of the excitation laser. The dotted 2s and 3s dark states in the figure lead to and The radiation is called dark to bright state emission (DTB). In particular, because these dark states are coupled to other states, the energy undergoes an additional Stark shift, which is linearly related to the laser intensity. Therefore, in the figure, the central energy of each DTB radiation shows a linear dependence on the laser intensity.

[0040] In the process of interaction between the few-cycle intense laser pulse 3 and the atom, HHG 33, XFID 31 and XSC 32 are all generated. However, the three types of radiation are separated from each other in space and time. In the time domain, HHG 33 is generated in the few-cycle intense laser pulse 3, followed by XSC 32 at the tail of the few-cycle intense laser pulse 3, and XFID 31 is radiated after the few-cycle intense laser pulse 3. In particular, please refer to Figure 5, which is a schematic diagram of the spatial distribution of extreme ultraviolet radiation driven by an intense laser pulse according to an embodiment of the present invention. As can be seen from the figure, the discrete HHG 33 is emitted at a divergence angle close to zero, and the XFID 31 presents a linear emission almost reaching the range of ± 20mrad, corresponding to the 11-21 harmonic XSC 32, and the emission frequency covers a wide spectrum of 17-35eV.

[0041] In summary, the embodiment of the present invention uses a strong few-cycle strong laser pulse 3 to drive the helium atomic gas pool 6, uses a high-resolution XUV spectrometer to perform spectral spatial analysis on the emission spectrum, and provides model calculations and reasonable explanations for three different radiations. The device of the embodiment of the present invention can provide a stable light source for attosecond interferometry.

[0042] Finally, it should be noted that the above specific implementation methods are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A supercontinuum extreme ultraviolet coherent light source generation device based on ultra-intense laser induction, characterized in that: It includes a strong laser source, a gas pool and a focusing lens. The strong laser source is used to output a few-cycle strong laser pulse; the few-cycle strong laser pulse is a near-infrared laser; the number of light cycles contained in the few-cycle strong laser pulse is no more than three, the pulse width of the few-cycle strong laser pulse is between 5-7fs, and the single pulse energy of the few-cycle strong laser pulse is above 500mJ; The gas pool is filled with gas, and the gas pool is arranged in a vacuum chamber; Wherein, the gas pool is placed between the focus of the few-cycle strong laser pulse and the focusing mirror, and the vertical distance between the focus and the axis of the gas pool is between 5-15 mm; The few-cycle strong laser pulse is incident on the gas pool after passing through the focusing mirror to excite the gas in the gas pool to generate supercontinuum extreme ultraviolet coherent light.

2. The supercontinuum extreme ultraviolet coherent light source generating device according to claim 1, characterized in that: The gas in the gas pool is helium, and the gas pressure of the helium is set to 60 mbar.

3. The supercontinuum extreme ultraviolet coherent light source generating device according to claim 1, characterized in that: The central wavelength of the few-cycle intense laser pulse is between 700-800nm, and each frequency component in the few-cycle intense laser pulse has no dispersion.

4. The supercontinuum extreme ultraviolet coherent light source generating device according to claim 1, characterized in that: The gas pool comprises a glass tube with one end closed, and a through hole is arranged on the tube wall of the glass tube for emitting the generated supercontinuum extreme ultraviolet coherent light.

5. The supercontinuum extreme ultraviolet coherent light source generating device according to claim 4, characterized in that: The inner diameter of the glass tube is 1.75 mm, and the diameter of the through hole is 100 μm.

6. The supercontinuum extreme ultraviolet coherent light source generating device according to claim 1, characterized in that: The device also includes a filter, which includes a metal aluminum film with a thickness of 200 nm and is used to filter out the few-cycle strong laser pulses emitted from the gas pool.

7. The supercontinuum extreme ultraviolet coherent light source generating device according to claim 1, characterized in that: The focal length of the focusing mirror is set to 500 mm; the focusing mirror is reflective focusing.

8. The supercontinuum extreme ultraviolet coherent light source generating device according to claim 1, characterized in that: The vacuum degree of the vacuum chamber is 10 -8 Up to 10 -5 mbar.

9. The supercontinuum extreme ultraviolet coherent light source generating device according to claim 1, characterized in that: The wavelength range of the supercontinuum extreme ultraviolet coherent light is between 35nm and 73nm.

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