All-optical ultrafast strong magnetic field generating device

Abstract

The invention provides an all-optical ultrafast strong magnetic field generation device. The device comprises an angular polarization terahertz generation module, a focusing system and a super-structure surface which are sequentially arranged along an optical path, the angular polarization terahertz generation module is used for outputting angular polarization terahertz pulses, the focusing system is used for focusing the angular polarization terahertz pulses and then enabling the angular polarization terahertz pulses to be incident to the metasurface, and the metasurface is used for exciting annular current through the angular polarization terahertz pulses. Therefore, longitudinal strong magnetic fields distributed along the optical axis direction are formed in the center and the rear area of the surface of the metasurface. The device adopts an all-optical driving mode, is simple in structure, does not need complex design and processing, and is easy to realize; the metasurface can be repeatedly used and is compatible with repetition frequency operation; the pulse width of the longitudinal strong magnetic field is in the femtosecond-picosecond magnitude and is adjustable, the magnetic field is quasi-monopole and controllable in direction, and the free space size of the magnetic field can reach the millimeter magnitude.

Description

Technical Field

[0001] This invention relates to the fields of ultrafast optics, terahertz science, and strong field physics, and specifically to an ultrafast strong magnetic field generation device based on angularly polarized terahertz pulses. Background Technology

[0002] Traditional strong magnetic fields are usually generated using magnets or current-carrying solenoids. Currently, although several technologies can achieve strong magnetic fields above 10 T, they have significant limitations: (1) DC magnets generate steady-state strong magnetic fields, which usually require harsh conditions such as liquid helium cryogenics and superconducting coils, and the highest magnetic field does not exceed 50 T; (2) Non-destructive pulsed magnets generate pulsed strong magnetic fields, with magnetic field strength not exceeding 100 T and magnetic field duration in the millisecond to sub-second range, and the magnetic field pulse interval is long and the process is complex; (3) Destructive pulsed magnets can generate magnetic fields in the kT range, but the duration is only in the microsecond range and they cannot be reused; (4) Strong laser-plasma interaction or strong laser irradiation of coil targets and other methods can drive pulsed strong magnetic fields, combined with magnetic flux compression and other technologies, which can generate magnetic fields in the kT range, but they are limited to the inside or surface of plasma and are subject to strong electromagnetic pulse interference, making it difficult to directly apply them to sample research in conventional materials science. In general, the duration of magnetic fields generated by existing strong magnetic field technologies is basically limited to the nanosecond range and is only suitable for the study of quasi-static magnetic response of materials.

[0003] Ultrafast, strong magnetic fields on the femtosecond to picosecond timescale are a core requirement for the development of next-generation ultrafast information storage devices and ultrafast magnetic manipulation of matter. Although ultrashort laser pulses contain ultrafast electric and magnetic field components, the electric dipole-photoelectric field interaction in matter is much stronger than the magnetic dipole-photomagnetic field interaction. While relativistic electron beams from large accelerators can generate picosecond pulsed strong magnetic fields, the magnetic field exists only in a few micrometers of region near the electron beam, and the accelerators are bulky and expensive, greatly limiting their usability.

[0004] Therefore, the current scarcity of ultrafast strong magnetic field sources limits the development of ultrafast science dominated by magnetic dipole interaction. There is an urgent need to develop femtosecond-picosecond strong magnetic field generating devices with simple structure, repeatability, and adjustable parameters to provide new means for research in materials science and life science. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the present invention aims to provide an ultrafast strong magnetic field generating device with simple structure, compatible with high repetition rate operation, and controllable pulse width and direction, so as to achieve stable output of high intensity longitudinal magnetic field with femtosecond to picosecond pulse width.

[0006] Before describing the technical solution of this invention, the terms used herein are defined as follows:

[0007] In this invention, the term "OAP (Off-Axis Parabolic Mirror)" refers to an off-axis parabolic mirror, which is a reflective optical element based on parabolic geometry. Its reflective surface is part of a parabola, and its optical axis is offset from the mechanical central axis of the element (i.e., "off-axis" design). Its core function is to focus parallel incident light to a single focal point, or to collimate the divergent light from a point source into parallel light, while avoiding the beam energy loss and spot distortion caused by central obstruction in traditional coaxial parabolic mirrors.

[0008] The term "Azimuthally Polarized Terahertz Pulse" in this invention refers to a transient electromagnetic pulse with a spectrum in the 0.1~10 THz band and a pulse width of 0.1~10 ps. Its electric field vector vibrates along the circumferential tangent direction (azimuth φ direction) in the plane perpendicular to the propagation axis, with zero radial electric field component and cylindrical symmetry about the propagation axis. After being focused by a high numerical aperture off-axis parabolic mirror, this pulse can form a ring focus and excite a strong longitudinal magnetic field, providing a core driving source for the generation of ultrafast strong magnetic fields and the modulation of ultrafast magnetic response of matter.

[0009] The term "Radially Polarized Terahertz Pulse" in this invention refers to a transient electromagnetic pulse with a spectrum in the 0.1~10 THz band and a pulse width of 0.1~10 ps. Its electric field vector vibrates radially (in the radial direction) in the plane perpendicular to the propagation axis, and the electric field component in the azimuth direction is zero. It has cylindrical symmetry about the propagation axis. After being focused by a high numerical aperture off-axis parabolic mirror, this pulse can form a centrally enhanced point focus. It can also be converted into an angularly polarized terahertz pulse by a 45° angled terahertz half-wave plate pair, providing a core control carrier for the generation of ultrafast strong magnetic fields and polarization-sensitive detection.

[0010] The term "ultrafast strong magnetic field" in this invention refers to a transient magnetic field with a pulse width of 1fs~10ps (rise time ≤1ps) and a peak intensity ≥1T. Its time scale can match the ultrafast dynamic processes of matter (such as electron relaxation and plasma oscillation), and its intensity can break through the magnetic field limit of conventional electromagnetic drive, so as to achieve efficient control of the motion of microscopic particles.

[0011] The term "aberration-free focusing" in this invention refers to a focusing method in which, when a beam is focused by an optical element, the focusing path differences of light rays in different aperture regions are eliminated by using an aspherical structure (such as an off-axis parabolic mirror) or a combination of multiple elements for compensation, so that all light rays are precisely converged to the same ideal focal point. This focusing method can achieve diffraction-limited spot output, and the focused spot energy is concentrated and there is no diffuse distribution along the optical axis.

[0012] The term "full width at half maximum" in this invention refers to a commonly used method of measuring the width of waveforms, pulses, spectral peaks, etc., and is used to describe the width of a peak or pulse at half its maximum height.

[0013] To achieve the above objectives, the present invention provides an all-optical ultrafast strong magnetic field generating device, the device comprising an angularly polarized terahertz generation module, a focusing system, and a metasurface arranged sequentially along the optical path; the angularly polarized terahertz generation module is used to output angularly polarized terahertz pulses, the focusing system is used to focus the angularly polarized terahertz pulses and incident them onto the metasurface, the metasurface is used to excite a ring current through the angularly polarized terahertz pulses, thereby forming a longitudinal strong magnetic field distributed along the optical axis in the center and rear region of the metasurface.

[0014] In some embodiments, the apparatus further includes an angularly polarized terahertz generation module, which includes a transit radiation generator and a polarization converter.

[0015] In some embodiments, the transit radiation generator includes a laser, a solid target, and a parabolic mirror, wherein the laser irradiates the solid target to generate transit radiation, and the parabolic mirror collects the transit radiation and forms radially polarized terahertz.

[0016] In some embodiments, the polarization converter includes a pair of terahertz half-wave plates at a 45° angle for converting the radially polarized terahertz into angularly polarized terahertz.

[0017] In some embodiments, the angularly polarized terahertz waves are distributed in a ring.

[0018] In some embodiments, the pulse width of the laser is 30~300 fs, and the focused intensity is 10. 17 ~10 21 W / cm².

[0019] In some embodiments, the solid target is a Cu thin film target.

[0020] In some embodiments, the parabolic mirror is an off-axis parabolic mirror with a reflectivity of ≥95% in the terahertz band.

[0021] In some embodiments, the metasurface is a metal structure having through holes or annular holes and located at the focal plane of the focusing system.

[0022] In some embodiments, the aperture D of the metasurface and the beam waist radius w0 of the focused angularly polarized terahertz surface satisfy the following conditions: The thickness T of the metasurface satisfies the relationship between the center wavelength λ0 of the angularly polarized terahertz wave and the surface thickness T. .

[0023] The all-optical ultrafast strong magnetic field generating device proposed in this invention can have, but is not limited to, the following beneficial effects:

[0024] (1) It adopts an all-optical drive method, which has a simple structure, requires no complex design and processing, and is easy to implement;

[0025] (2) The metasurface is reusable and compatible with repetitive frequency operation;

[0026] (3) The magnetic field pulse width is in the femtosecond to picosecond range and is adjustable; the magnetic field is quasi-monopolar and its direction is controllable; the size of the magnetic field free space can reach the millimeter level. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the embodiments will be briefly described below. Referring to the accompanying drawings will provide a clearer understanding of the features and advantages of the present invention. The drawings are illustrative and should not be construed as limiting the present invention in any way. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort. Wherein:

[0028] Figure 1 A schematic diagram of an all-optical ultrafast strong magnetic field generating device according to an embodiment of the present invention is shown.

[0029] Figure 2 A schematic diagram of the optical path layout of an angular polarization terahertz generation module according to an embodiment of the present invention is shown.

[0030] Figure 3 The spatial distribution of a magnetic field according to an embodiment of the present invention is shown.

[0031] Figure 4 The magnetic field waveform at 50 μm behind the pore under conditions of metal-free micropores according to an embodiment of the present invention is shown.

[0032] Figure 5 The electric field waveform at 50 μm behind the pore under conditions of metal-free micropores according to an embodiment of the present invention is shown.

[0033] Figure 6 A schematic diagram of the longitudinal magnetic field variation under conditions of presence and absence of metal micropores according to an embodiment of the present invention is shown.

[0034] Figure 7 The relationship between the magnetic field pulse width and the angularly polarized terahertz pulse width according to an embodiment of the present invention is shown.

[0035] Figure 8 The relationship between the longitudinal distribution of the central magnetic field and the thickness of the metasurface according to an embodiment of the present invention is shown.

[0036] Explanation of reference numerals in the attached figures:

[0037] 1. Angular polarization terahertz generation module; 2. Focusing system; 3. Metasurface; 11. Laser; 12. Solid target; 13. Parabolic mirror; 14. Terahertz half-wave plate pair; 15. Radial polarization terahertz; 16. Angular polarization terahertz. Detailed Implementation

[0038] The present invention will be further illustrated below with specific embodiments. However, it should be understood that these embodiments are merely for more detailed and specific illustration and should not be construed as limiting the present invention in any way.

[0039] This section provides a general description of the materials and testing methods used in the experiments of this invention. While many of the materials and methods of operation used to achieve the objectives of this invention are well known in the art, the invention is still described in as much detail as possible herein. It will be apparent to those skilled in the art that, unless otherwise stated in the context, the materials and methods of operation used in this invention are well known in the art.

[0040] A schematic diagram of the ultrafast strong magnetic field generating device of the present invention is shown below. Figure 1 As shown, it includes an angular polarization terahertz generation module 1, a focusing system 2, and a metasurface 3.

[0041] Specifically, such as Figure 2 As shown, the angularly polarized terahertz generation module adopts a two-stage cascaded architecture of "radiation generation-polarization conversion". In some embodiments, the angularly polarized terahertz generation module includes a transit radiation generator and a polarization converter. The transit radiation generator includes a laser 11, a solid target 12, and a parabolic mirror 13. The laser 11 irradiates the solid target 12 to generate transit radiation, and the parabolic mirror 13 collects the transit radiation and forms radially polarized terahertz 15. In some embodiments, the polarization converter includes a pair of terahertz half-wave plates 14 with an included angle of 45° to convert the radially polarized terahertz 15 into angularly polarized terahertz 16, thereby obtaining angularly polarized terahertz 16, wherein the angularly polarized terahertz 16 is ring-shaped.

[0042] Continue to refer to Figure 2 Preferably, an ultrashort pulse laser 11 with a femtosecond pulse width and high repetition rate can be used as the driving laser, which is then focused and irradiated onto the surface of a solid target (preferably a Cu thin film target) 12. When the laser 11 interacts with the solid target 12, high-energy electrons are excited on the target surface. These electrons generate a transition radiation effect when they cross the target interface, causing the generated radiation spectrum to be concentrated in the terahertz band, providing an initial radiation source for the subsequent generation of terahertz pulses. Preferably, the pulse width of the laser 11 is 30~300 fs, and the focusing intensity is 10. 17 ~10 21W / cm², which ensures the adjustable intensity of transit radiation and pulse width.

[0043] Continue to refer to Figure 2 After the transit radiation is generated, it propagates divergently and is collected and collimated by the parabolic mirror 13. Due to the inherent polarization characteristics of the transit radiation and the axisymmetric collection effect of the parabolic mirror 13, the terahertz pulse output after shaping will form a radially polarized state with "radial vibration of the electric field and ring-shaped energy distribution", that is, radially polarized ring terahertz 15. Preferably, the parabolic mirror 13 is an off-axis parabolic mirror with a reflectivity of ≥95% for the terahertz band, and its focal length is adapted to the parameters of the laser focusing system to ensure the uniformity of the ring energy distribution.

[0044] Specifically, the two terahertz half-wave plates 14 in the polarization converter are placed at a 45° angle. The first half-wave plate first modulates the phase of the radially polarized electric field component, causing the polarization direction to deflect by 45°. Then, after secondary phase modulation by the second half-wave plate, the electric field vibration direction is finally transformed to the "circumferential tangential direction," thus completing the efficient conversion from radial polarization to angular polarization, and finally outputting the required angularly polarized terahertz 16. Preferably, the angle between the two terahertz half-wave plates 14 is controlled within ±0.5° to ensure a polarization conversion efficiency greater than 90%.

[0045] Back to Figure 1 , Figure 1 The angularly polarized terahertz generation module 1, processed by the aforementioned polarization converter, stably outputs an angularly polarized terahertz pulse 16 with a ring-shaped electric field distribution. This angularly polarized terahertz pulse 16 is incident on the focusing system 2. Preferably, the focusing system 2 includes a strong focusing element such as an off-axis parabolic mirror (OAP) 13 or a TPX lens. The focusing system 2 performs aberration-free focusing on the diverging pulse output from the angularly polarized terahertz generation module 1, compressing the pulse energy from the initial millimeter-level beam waist to a micrometer-level focused spot (the electric field strength can be increased by 1-2 orders of magnitude after focusing). This significantly enhances the electric field strength and energy density incident on the metasurface 3, providing the energy basis for subsequently exciting a high-density ring current and generating a high-intensity magnetic field.

[0046] In some embodiments, the focusing element can be an off-axis parabolic mirror (OAP) 13. Due to the centerless design of the OAP, the annular electric field distribution characteristics of the angularly polarized terahertz 15 can be preserved to the greatest extent, avoiding energy loss or polarization distortion, and ensuring that the focused pulse still maintains a pure angular polarization state.

[0047] Continue to refer to Figure 1The metasurface 3 is a metallic structure capable of generating an equivalent ring current. It can be made of highly conductive metals such as copper, aluminum, or gold. The through-holes or ring holes on it constitute the core functional area of ​​the metasurface and are precisely fixed at the focal plane of the focusing system 2 to ensure that the focused high-energy angularly polarized terahertz pulse is incident perpendicularly. In this way, the high conductivity can be used to convert the incident angularly polarized terahertz electric field into a ring current, and then the magnetic effect of the current can be used to excite a strong longitudinal magnetic field, while simultaneously achieving the magnetic dominance characteristic control of "electric field attenuation - magnetic field enhancement".

[0048] Continue to refer to Figure 1 A focused, angularly polarized terahertz pulse is incident perpendicularly onto the metasurface 3 located at the focal plane. Due to the extremely high free electron density of metallic materials, the ring-shaped electric field of the angularly polarized pulse generates a Lorentz force along the circumferential tangential direction on the free electrons on the metal surface, driving the free electrons to make ring-shaped directional motion along the edge of the micropore, thereby exciting a ring current with a high transient current density on the metasurface 3. In some embodiments, the metasurface 3 can be a metallic micropore. The high conductivity of the metallic micropore ensures minimal current loss, resulting in a higher focused electric field strength, a larger ring current density, and a stronger subsequent magnetic field strength.

[0049] When the metasurface 3 is irradiated by an angularly polarized terahertz wave 15, a toroidal electric field drives a toroidal current on the metasurface 3, generating a needle-shaped ultrafast strong magnetic field distributed along the optical axis at the center. The waveform of the ultrafast strong magnetic field mainly depends on the waveform of the toroidal electric field, and the magnetic field pulse width can be changed by adjusting the pulse width of the angularly polarized terahertz wave 15. In some embodiments, the magnetic field strength and spatial distribution can be controlled by adjusting the geometric parameters of the metasurface 3, such as aperture, thickness, and shape. For example, increasing the aperture can change the magnetic field strength, and adjusting the thickness or using other toroidal hole structures can change the magnetic field strength and longitudinal distribution.

[0050] Specifically, the geometric parameters of the metasurface 3 can be determined based on the spatial distribution and center wavelength of the incident terahertz pulse (angularly polarized terahertz). When the metasurface 3 is a metallic micropore, the aperture directly determines the coupling efficiency between the annular electric field of the angularly polarized terahertz pulse and the free electrons on the metal surface, and this efficiency must be considered in conjunction with the beam waist radius of the angularly polarized terahertz pulse. The radius of the focused spot at 1 / e² intensity and the transverse field distribution (Gaussian distribution characteristics) are determined. As the aperture of the metal micropore increases, the peak intensity of the generated longitudinal magnetic field increases. When the aperture D is approximately... When the magnetic field strength reaches its maximum value, further increasing the aperture will cause the peak magnetic field strength to decrease. Therefore, the aperture... Preferred .

[0051] Specifically, the micropore thickness T affects the energy penetration depth of angularly polarized terahertz waves within the metal and the longitudinal distribution of the ring current, which needs to be determined based on the center wavelength of the angularly polarized terahertz waves. It is confirmed that the peak magnetic field strength increases with increasing thickness, especially at a thickness of approximately [thickness value missing]. The time-angle polarized terahertz energy is completely absorbed by the metal micropore, the longitudinal motion path of free electrons reaches its optimal length, the magnetic moment effect of the ring current is strongest, and the longitudinal magnetic field strength reaches its maximum value. Further increasing the micropore thickness weakens the magnetic field strength. Therefore, the optimal metal micropore thickness T is... .

[0052] This embodiment adopts Figure 1 The ultrafast strong magnetic field generator shown produces an angularly polarized terahertz wave 16 with a peak power of 0.5 TW, a pulse width of 300 fs, a center wavelength of 100 μm, and a beam waist diameter of 1 mm. This wave interacts with a metasurface (such as a copper micropore) 3 with a diameter of 0.7 mm and a thickness of 60 μm located at the focal plane of the focusing system 2. Numerical simulation results are shown below. Figure 3-5 As shown, the results indicate that a transient, elongated "needle-shaped" longitudinal magnetic field region is formed after interaction with the copper micropore, with a peak magnetic field strength of 105-110 T and a pulse width of approximately 300 fs. At 50 μm behind the pore, the magnetic field drops to one-tenth of its peak value, with a lateral dimension of approximately 680 μm and a full width at half maximum (FWHM) of approximately 50 μm. When the angularly polarized terahertz pulse propagates independently (without interaction with the copper micropore), its inherent longitudinal magnetic field peak strength is only 8-9 T, and is subject to interference from the angular electric field, resulting in weak magnetic dominance. However, when the angularly polarized terahertz pulse 16 is focused and incident on a metal micropore (taking the copper micropore as an example) and interacts with it, the metal micropore drives free electrons through a ring electric field to form a high-density ring current. The magnetic effect of this ring current increases the peak longitudinal magnetic field strength to 105-115 T (approximately 13 times stronger than without a micropore), exceeding the 100 Tesla level intensity threshold and meeting the requirements of strong-field physics research for ultra-high field strength. Furthermore, the metal micropores significantly attenuate the angular electric field of the angularly polarized terahertz 16 (the electric field strength decreases by approximately 4.6 times), while simultaneously strengthening the longitudinal magnetic field through a current-magnetic field conversion mechanism. This increases the magnetic field dominance factor (cBz / Eφ, where c is the speed of light, Bz is the longitudinal magnetic field strength, and Eφ is the angular electric field strength) from less than 10 without the metal micropores to over 100. Figure 6 As shown, this effectively suppresses the interference of electric dipole-electric field interaction on magnetic modulation, providing an ideal experimental environment for the study of ultrafast magnetic response dominated by magnetic dipole interaction.

[0053] By scanning the incident angle-polarized terahertz pulse width, it was found that the width of the generated magnetic field pulse is similar to that of the incident angle-polarized terahertz pulse width. Figure 7The near-linear correspondence shown indicates that the magnetic field pulse width can be effectively controlled by adjusting the laser pulse width. For example... Figure 8 As shown, by scanning the geometric parameters of the metal micropores, it can be shown that the magnetic field strength and spatial distribution are affected by the thickness of the metal micropores.

[0054] The ultrafast strong magnetic field generating device proposed in this application adopts an all-optical driving method, with a simple structure, no need for complex design and processing, and easy to realize; the metasurface can be reused and is compatible with repetitive frequency operation; the pulse width of the longitudinal strong magnetic field is in the femtosecond to picosecond range and is adjustable, the magnetic field is quasi-monopolar and the direction is controllable, and the free space size of the magnetic field can reach the millimeter level.

[0055] Although the invention has been described to a certain extent, it is apparent that appropriate variations can be made to the various conditions without departing from the spirit and scope of the invention. It is understood that the invention is not limited to the described embodiments, but falls within the scope of the claims, which include equivalent substitutions for each of the elements.

Claims

1. A fully optical ultrafast strong magnetic field generating device, characterized in that, The device includes an angularly polarized terahertz generation module, a focusing system, and a metasurface arranged sequentially along the optical path. The angularly polarized terahertz generation module is used to output angularly polarized terahertz pulses. The focusing system is used to focus the angularly polarized terahertz pulses and incident them onto the metasurface. The metasurface is used to excite a ring current through the angularly polarized terahertz pulses, thereby forming a strong longitudinal magnetic field distributed along the optical axis in the center and rear region of the metasurface.

2. The apparatus according to claim 1, characterized in that, It also includes an angular polarization terahertz generation module, which includes a transit radiation generator and a polarization converter.

3. The apparatus according to claim 2, characterized in that, The transit radiation generator includes a laser, a solid target, and a parabolic mirror. The laser irradiates the solid target to generate transit radiation, and the parabolic mirror collects the transit radiation and forms radially polarized terahertz.

4. The apparatus according to claim 3, characterized in that, The polarization converter includes a pair of terahertz half-wave plates at a 45° angle, used to convert the radially polarized terahertz into angularly polarized terahertz.

5. The apparatus according to claim 3, characterized in that, The angularly polarized terahertz waves are distributed in a ring.

6. The apparatus according to claim 3, characterized in that, The laser pulse width is 30~300 fs, and the focusing intensity is 10. 17 ~10 21 W / cm².

7. The apparatus according to claim 3, characterized in that, The solid target is a Cu thin film target.

8. The apparatus according to claim 1, characterized in that, The parabolic mirror is an off-axis parabolic mirror with a reflectivity of ≥95% in the terahertz band.

9. The apparatus according to claim 1, characterized in that, The metasurface is a metal structural component with through holes or annular holes, and is located at the focal plane of the focusing system.

10. The apparatus according to claim 1, characterized in that, The aperture D of the metasurface and the beam waist radius w0 of the focused angularly polarized terahertz satisfy the following conditions: The thickness T of the metasurface satisfies the relationship between the center wavelength λ0 of the angularly polarized terahertz wave. .

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