Method and device for realizing space-polarization state programmable wide-spectrum terahertz radiation

Abstract

The invention relates to the technical field of ultrafast nonlinear optics and spintronics, in particular to a space-polarization state programmable wide-spectrum terahertz radiation implementation method and device, and the method comprises the following steps: S1, controlling the magnetic domain orientation of a multilayer heterojunction film through a magnetic field, writing a magnetic domain structure on the multilayer heterojunction thin film through a laser heating direct writing technology; and S2, exciting the programmed multilayer heterojunction thin film through femtosecond laser pulses to generate terahertz radiation with a complex space-polarization state. Compared with the prior art, the defect that in the prior art, when the space-polarization state of a far field is regulated and controlled, a new device always needs to be manufactured again and cannot be reused or universally used is overcome, the magnetic domain structure of the multi-layer heterojunction thin film can be flexibly rewritten according to requirements, and then space-polarization distribution of far-field terahertz radiation is regulated and controlled.

Description

Technical Field

[0001] The invention relates to the technical fields of ultrafast nonlinear optics and spin electronics, and in particular to a method and device for realizing spatial-polarization state programmable wide-spectrum terahertz radiation. Background Art

[0002] The terahertz radiation spectrum is between the infrared spectrum and the microwave spectrum, covering frequencies from 0.1 to 10 terahertz. Terahertz spectroscopy uses a wide-spectrum terahertz light source to detect material properties. Terahertz light sources have many unique advantages and characteristics in material detection: First, the binding energy of many macromolecules is in the 0.1 to 15 THz band, and terahertz spectroscopy can analyze the composition of materials in the biological, chemical and medical fields; second, terahertz spectroscopy can measure the high-frequency conductivity of electrons in condensed materials; third, the propagation of terahertz light in materials is very sensitive to the conductivity of the materials. Materials such as clothing, paper, plastic, ceramics and semiconductor silicon wafers are transparent to terahertz light, which also makes terahertz light a useful light source for non-destructive imaging; fourth, terahertz light is also a potential carrier light source for high-speed wireless communication technology.

[0003] The regulation of terahertz polarization and spatial-polarization distribution can bring new opportunities for the application of terahertz band. For example, circularly polarized terahertz light can play a huge role in the detection of magnetic materials and chiral macromolecules because it carries photon angular momentum; for another example, terahertz radiation of vector light field can carry higher-dimensional information, thus providing a larger communication bandwidth in the next generation of 6G communication.

[0004] Due to the long wavelength, the spatial-polarization control of broadband terahertz radiation has always been very difficult, and there is a lack of suitable materials. Existing technologies include the ability to generate and control circularly polarized terahertz light sources after a linearly polarized terahertz light source passes through a quarter-wave plate composed of a Fresnel rhomb array. On the other hand, the wavefront and polarization of terahertz radiation can also be controlled by metasurface design. In addition, as proposed in Chinese patent CN110518439B, a broadband chiral terahertz emission source and emission method are used, in which the intensity and chirality of terahertz are generated by changing the direction of an external magnetic field and adjusting the polarization of the excitation light; as proposed in Chinese patent CN115133378A, a circularly polarized terahertz radiation device based on a wire grid structure spintronic device is used, in which the polarization direction of spin-polarized electrons is changed by changing the direction of the external magnetic field, thereby changing the distribution of transient current in the wire grid, and realizing the control of ellipticity and chirality. However, for each far-field spatial-polarization control requirement, the above methods require the processing and manufacture of different optical devices, so they are very inconvenient in application. Summary of the invention

[0005] The purpose of the present invention is to provide a method and device for realizing spatial-polarization state programmable wide-spectrum terahertz radiation in order to solve at least one of the above problems, so as to solve the defect that in the prior art, when regulating the spatial-polarization state of the far field, new devices always need to be remade and cannot be reused or used universally, and the magnetic domain structure of the multilayer heterojunction film can be flexibly rewritten according to needs, thereby realizing the regulation of the spatial-polarization distribution of the far-field terahertz radiation.

[0006] The purpose of the present invention is achieved through the following technical solutions:

[0007] The first aspect of the present invention discloses a method for realizing spatial-polarization state programmable wide-spectrum terahertz radiation, comprising the following steps:

[0008] S1: Control the magnetic domain orientation of the multilayer heterojunction film by magnetic field, and write the magnetic domain structure on the multilayer heterojunction film by laser heating direct writing technology;

[0009] S2: Producing terahertz radiation with complex spatial-polarization states by exciting programmed multilayer heterojunction films with femtosecond laser pulses;

[0010] The multilayer heterojunction film comprises an antiferromagnetic medium layer, a ferromagnetic medium layer and a non-magnetic metal layer stacked in sequence;

[0011] The material of the antiferromagnetic medium layer is antiferromagnetic material;

[0012] The material of the ferromagnetic dielectric layer is ferromagnetic metal, ferromagnetic Heusler alloy or ferrimagnetic metal;

[0013] The material of the non-magnetic metal layer is a metal with high spin-charge conversion efficiency;

[0014] The antiferromagnetic medium layer, the ferromagnetic medium layer and the non-magnetic metal layer are all of nanometer thickness.

[0015] Preferably, the intensity of the magnetic field is greater than 1000 Oersteds, and the energy per unit area of ​​the excitation laser in the femtosecond laser pulse is less than the energy per unit area of ​​the heating laser in the laser heating direct writing technology.

[0016] Preferably,

[0017] The excitation laser has a duration of 1fs-1ps, a wavelength of 200nm-2μm, and an energy density of no less than 0.1nJ / cm2 applied to the multilayer heterojunction film. 2 ;

[0018] The heating laser has a duration of 100fs-100ns, a wavelength of 200nm-2μm, and a power density of 100W / cm2 applied to the multilayer heterojunction film. 2 -100,000 W / cm2 .

[0019] Preferably, the antiferromagnetic material includes IrMn and FeMn; the ferromagnetic metal includes Fe 50 Co 50 , Fe, Co, Ni, CoFeB, CoFe and FeNi(Py); the ferrimagnetic metals include FeGd and CoGd; the metals with high spin-charge conversion efficiency are materials with inverse spin Hall effect, including Pt, Ta, Pd, W and Ir.

[0020] Preferably, the multilayer heterojunction thin film is produced by thin film growth technology.

[0021] Preferably, the multilayer heterojunction thin film further comprises a substrate for support;

[0022] The substrate is bonded to the antiferromagnetic medium layer;

[0023] The materials of the substrate include silicon dioxide, magnesium oxide, glass, diamond, sapphire, organic glass, silicon wafer and flexible substrate;

[0024] The thickness of the substrate is 50 μm-1 cm.

[0025] The second aspect of the present invention discloses a device for implementing any of the above methods, comprising a writing system and an excitation system;

[0026] The writing system comprises a heating laser, a power regulating component, a mechanical shutter, a focusing lens, a vector magnet and a displacement stage; the power regulating component, the mechanical shutter, the focusing lens and the multi-layer heterojunction film are sequentially arranged on the optical path of the heating laser, so that the heating laser is focused on the multi-layer heterojunction film; the vector magnet is arranged on the side of the multi-layer heterojunction film, and the magnetic field direction has a 360° adjustment range in the plane where the multi-layer heterojunction film is located; the displacement stage carries the multi-layer heterojunction film;

[0027] The excitation system comprises an excitation laser, a power regulating component and a focusing lens; the power regulating component, the focusing lens and the multi-layer heterojunction film are sequentially arranged on the optical path of the excitation laser so that the excitation laser is focused on the multi-layer heterojunction film.

[0028] Preferably, the power adjustment component comprises a half-wave plate and a polarizing plate arranged in sequence;

[0029] The power regulating component is used in the writing system to regulate the heating laser to a suitable power;

[0030] The power regulating component is used in the excitation system to regulate the energy per unit area of ​​the excitation laser to be lower than that of the heating laser.

[0031] Preferably, the shutter time of the mechanical shutter is less than 10 ms.

[0032] Preferably, the translation stage performs two-dimensional horizontal motion, with a displacement accuracy of less than 1 μm and a stroke of more than 5 mm.

[0033] In combination with the device, the implementation method is specifically as follows:

[0034] Step S1 specifically includes the following sub-steps:

[0035] 1) Design the magnetic domain distribution structure on the multilayer heterojunction film according to the spatial and polarization requirements of the far-field target terahertz field.

[0036] 2) Establish the heating laser optical path: Place a half-wave plate, polarizer, mechanical shutter, and focusing lens at the outlet of the heating laser in sequence. The incident laser passes through the above components in sequence and is focused on the multi-layer heterojunction film. Adjust the laser to the appropriate power by adjusting the half-wave plate and polarizer combination.

[0037] 3) By moving the two-dimensional translation stage within the plane of the multilayer heterojunction film, the focus of the laser spot is moved to the area on the multilayer heterojunction film where the designed magnetic domain state needs to be written.

[0038] 4) Turn on the vector magnet and apply a magnetic field in the plane of the multilayer heterojunction film. The direction of the magnetic field points to the direction in which the magnetization intensity needs to be written, and the magnetic field intensity is greater than 1000 Oersted.

[0039] 5) Open the mechanical shutter and expose for a long enough time to fully rewrite the magnetization intensity direction of the laser spot heating area. Close the mechanical shutter and turn off the external magnetic field.

[0040] 6) Use a two-dimensional mechanical translation stage to move the multilayer heterojunction film in the plane so that the laser spot is aligned with the next area where the magnetic susceptibility needs to be written. Repeat steps 4)-5) until the magnetic domain structure of the entire multilayer heterojunction film is written. The width of the written magnetic structure is more than 10 microns and less than 100 microns.

[0041] 7) Turn off the laser and turn off the vector magnet.

[0042] Step S2 specifically includes the following sub-steps:

[0043] 1) Establish a femtosecond laser excitation optical path: Place a half-wave plate, a polarizer, and a focusing lens at the exit of the excitation laser in sequence. The incident femtosecond laser passes through the above components in sequence and focuses on the multi-layer heterojunction film that has completed magnetic writing programming. The femtosecond laser spot must be aligned with and cover the magnetic domain pattern written in the first step. The coverage range is within the range of 10 microns to 1 cm.

[0044] 2) The half-wave plate and polarizer combination are adjusted so that the power per unit area of ​​the femtosecond laser exciting the multilayer heterojunction film is lower than the laser energy used to write the magnetic susceptibility pattern in the first step.

[0045] 3) Use femtosecond laser pulses to excite broadband terahertz radiation and send it into a far-field terahertz spatial-polarization detection system.

[0046] The working principle of the present invention is:

[0047] The present invention is achieved through two steps:

[0048] Step 1: Based on the spatial-polarization requirements of the far-field terahertz field, the laser heating-assisted magnetic writing technology is used to write the microscopic spatial distribution of the in-plane magnetic domains in the ferromagnetic medium layer in the multilayer heterojunction film and control the magnetic domain orientation, thereby completing the programming and writing of the magnetic domain pattern of the multilayer heterojunction film;

[0049] Step 2: Apply femtosecond laser pulses to excite the programmed multilayer heterojunction film. The spin-polarized current in the ferromagnetic medium layer in the multilayer heterojunction film is excited by the femtosecond laser, and then converted into a planar transient charge flow through the non-magnetic metal layer, thereby generating terahertz radiation. The localized terahertz radiation is polarized perpendicular to the direction of the magnetization intensity written in the area, thereby generating the desired spatial-polarization distribution of wide-spectrum terahertz radiation in the far field.

[0050] If the spatial-polarization characteristics of the terahertz radiation need to be changed, it can be achieved by simply rewriting a new spatial magnetic domain distribution in the multilayer heterojunction film through step one.

[0051] Compared with the prior art, the present invention has the following beneficial effects:

[0052] The present invention utilizes the magnetic exchange polarization effect generated by the heterojunction of antiferromagnetic and ferromagnetic materials, and the property that the effect can be rewritten locally at the microscopic level through the heat-assisted magnetic writing process, to realize programming control of the magnetic domain orientation and microscopic distribution in the ferromagnetic film layer. Thus, programming control of the far-field spatial-polarization distribution of the wide-spectrum terahertz radiation generated by femtosecond laser excitation is realized.

[0053] 1) The present invention can flexibly rewrite the spatial-polarization distribution of far-field terahertz radiation according to demand, without the need to re-grow or process spatial-polarization control components for terahertz frequencies.

[0054] 2) The present invention can arbitrarily rewrite the polarization state of localized terahertz radiation with high spatial resolution, achieving more flexible terahertz spatial-polarization control than existing methods.

[0055] 3) The present invention can generate wide-spectrum coherent far-field terahertz radiation by exciting a multilayer heterojunction thin film with a femtosecond laser pulse. In one implementation of the present invention, wide-spectrum terahertz radiation covering a range of 0 to 5 terahertz can be generated by excitation with a femtosecond laser pulse with a pulse width of 22 femtoseconds and a wavelength of 1030 nanometers. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 Schematic diagram of the method for realizing spatial-polarization state programmable broadband terahertz radiation;

[0057] Figure 2 It is a schematic diagram of the structure of the writing system and the writing process of the local magnetic domain structure;

[0058] Figure 3 The schematic diagram of the structure of the excitation system and the excitation process;

[0059] Figure 4 A microscopic photograph of writing microscopic magnetic domains in a multilayer heterojunction film using heat-assisted magnetic writing technology;

[0060] Figure 5 is a graph showing the relationship between the spatial resolution of the written magnetic domain and the heating laser power in the heat-assisted magnetic writing process, wherein the heating laser emitting the heating laser is an excitation laser light source with a pulse width of 100 femtoseconds, a central wavelength of 1030 nanometers, and a repetition frequency of 36 megahertz;

[0061] Figure 6 This is the local hysteresis loop result obtained by rewriting the magnetic domain orientation of the multilayer heterojunction film using the heat-assisted magnetic writing process, where H ex is the magnetic exchange bias field of the film;

[0062] Figure 7 For the corresponding Figure 6 Terahertz time-domain waveforms of multilayer heterojunction films radiated under zero-field conditions under different heat-assisted magnetic writing conditions;

[0063] Figure 8 The spectrum diagram of the terahertz field radiated by the multilayer heterojunction film in the present invention, and the corresponding excitation laser conditions are 22fs, 1030nm femtosecond laser pulses;

[0064] Fig. 9 A schematic diagram of a radial emission magnetic domain pattern written by a heat-assisted magnetic writing process in Example 1;

[0065] Fig.10 This is a diagram showing the amplitude and phase distribution results of the terahertz radiation emitted by the programmed multilayer heterojunction film excited by a femtosecond laser pulse in the far field in Example 1. The measured polarization direction is along the y direction in space, and the terahertz frequency is 0.293 terahertz.

[0066] Fig.11 This is a diagram showing the amplitude and phase distribution results of the terahertz radiation emitted by the programmed multilayer heterojunction film excited by a femtosecond laser pulse in the far field in Example 1. The measured polarization direction is along the spatial x direction, and the terahertz frequency is 0.293 terahertz.

[0067] Fig.12 This is a diagram showing the amplitude and phase distribution results of the terahertz radiation emitted by the programmed multilayer heterojunction film excited by a femtosecond laser pulse in the far field in Example 1. The measured polarization direction is along the y direction in space, and the terahertz frequency is 0.674 terahertz.

[0068] Fig.13 This is a diagram showing the amplitude and phase distribution results of the terahertz radiation emitted by the programmed multilayer heterojunction film excited by a femtosecond laser pulse in the far field in Example 1. The measured polarization direction is along the spatial x direction, and the terahertz frequency is 0.674 terahertz.

[0069] Fig.14 A schematic diagram of an angularly distributed magnetic domain pattern written by a heat-assisted magnetic writing process in Example 2;

[0070] Fig.15 This is a diagram showing the amplitude and phase distribution results of the terahertz radiation emitted by the programmed multilayer heterojunction film excited by a femtosecond laser pulse in the far field in Example 2. The measured polarization direction is along the y direction in space, and the terahertz frequency is 0.293 terahertz.

[0071] Fig.16 This is a diagram showing the amplitude and phase distribution results of the terahertz radiation emitted by the programmed multilayer heterojunction film excited by a femtosecond laser pulse in the far field in Example 2. The measured polarization direction is along the spatial x direction, and the terahertz frequency is 0.293 terahertz.

[0072] Fig.17 This is a diagram showing the amplitude and phase distribution results of the terahertz radiation emitted by the programmed multilayer heterojunction film excited by a femtosecond laser pulse in the far field in Example 2. The measured polarization direction is along the y direction in space, and the terahertz frequency is 0.469 terahertz.

[0073] Fig.18 This is a diagram showing the amplitude and phase distribution results of the terahertz radiation emitted by the programmed multilayer heterojunction film excited by a femtosecond laser pulse in the far field in Example 2. The measured polarization direction is along the spatial x direction, and the terahertz frequency is 0.469 terahertz.

[0074] Fig.19 A schematic diagram of a strip-shaped magnetic domain distribution pattern written by a heat-assisted magnetic writing process in Example 3;

[0075] Fig. 20The spatial distribution of the terahertz radiation emitted by the programmed multilayer heterojunction film excited by femtosecond laser pulses in the far field in Example 3, the left-handed component (LCP) is emitted towards the emission angle direction, while the right-handed component (RCP) is directed toward direction;

[0076] Fig.21 The temporal structure of the terahertz radiation emitted by the programmed multilayer heterojunction film excited by femtosecond laser pulses at different emission angles in Example 3;

[0077] In the figure: 1-multilayer heterojunction film; 11-substrate; 12-antiferromagnetic medium layer; 13-ferromagnetic medium layer; 14-non-magnetic metal layer; 2-heating laser; 3-half-wave plate; 4-polarizer; 5-mechanical shutter; 6-focusing lens; 7-vector magnet; 8-excitation laser. DETAILED DESCRIPTION

[0078] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0079] Figure 1 The schematic diagram of the spatial-polarization state programmable wide-spectrum terahertz radiation realization method is shown, which is mainly realized through two steps: Step 1, according to the spatial-polarization requirements of the far-field terahertz field, the laser heating assisted magnetic writing technology is used to write the microscopic spatial distribution of the magnetic domain in the plane in the ferromagnetic layer of the multilayer heterojunction film 1 and control the magnetic domain orientation, so as to complete the programming and writing of the magnetic domain pattern of the multilayer heterojunction film 1; Step 2, femtosecond laser pulses are applied to excite the programmed multilayer heterojunction film 1 to generate the required spatial-polarization distribution of wide-spectrum terahertz radiation in the far field. If the spatial-polarization characteristics of the terahertz radiation need to be changed, it is only necessary to re-write the new spatial magnetic domain distribution in the multilayer heterojunction film 1 through step 1.

[0080] Among them, the multilayer heterojunction film 1 is as Figure 1 , Figure 3As shown, the substrate 11, the antiferromagnetic dielectric layer 12, the ferromagnetic dielectric layer 13 and the non-magnetic metal layer 14 are stacked in sequence. The magnetic exchange bias effect generated by the antiferromagnetic dielectric layer 12 and the ferromagnetic dielectric layer 13 is utilized to write the magnetic domain structure of the micron-scale spatial scale and control the magnetic domain orientation through the laser heating direct writing technology, so as to realize arbitrary programming and regulation of the direction and spatial distribution of the microscopic magnetic domains in the ferromagnetic dielectric layer 13. Furthermore, the programmed multilayer heterojunction film 1 is excited by a femtosecond laser pulse, and a spin polarized current is generated in the ferromagnetic dielectric layer 13, which is converted into a transient in-plane current flowing perpendicular to the microscopic magnetization direction through the non-magnetic metal layer 14, thereby generating a wide-spectrum coherent terahertz pulse with an arbitrarily programmable spatial-polarization state in the far field.

[0081] More specifically, step one includes the following sub-steps:

[0082] 1) Designing the magnetic domain distribution structure on the multilayer heterojunction film 1 according to the spatial and polarization requirements of the far-field target terahertz field.

[0083] 2) Establishing a heating laser optical path: A half-wave plate 3, a polarizing plate 4, a mechanical shutter 5, and a focusing lens 6 are sequentially placed at the outlet of the heating laser 2. The incident laser passes through the above components sequentially and is focused on the multilayer heterojunction film 1. The laser is adjusted to a suitable power by adjusting the combination of the half-wave plate 3 and the polarizing plate 4.

[0084] 3) By moving the two-dimensional translation stage within the plane of the multilayer heterojunction film 1, the focus of the laser spot is moved to the area on the multilayer heterojunction film 1 where the designed magnetic domain state needs to be written.

[0085] 4) Turn on the vector magnet 7 and apply a magnetic field in the plane of the multilayer heterojunction film 1. The direction of the magnetic field points to the direction where the magnetization intensity needs to be written, and the magnetic field intensity is greater than 1000 Oersted.

[0086] 5) Open the mechanical shutter 5 and expose for a long enough time to fully rewrite the magnetization intensity direction of the laser spot heating area. Close the mechanical shutter 5 to cool the laser heating area. Turn off the external magnetic field.

[0087] 6) Use a two-dimensional mechanical translation stage to move the multilayer heterojunction film 1 in a plane so that the laser spot is aligned with the next area where the magnetic susceptibility needs to be written. Repeat steps 4)-5) until the magnetic domain structure of the entire multilayer heterojunction film 1 is written.

[0088] 7) Turn off the laser and the vector magnet 7.

[0089] Step 2 specifically includes the following sub-steps:

[0090] 1) Establish a femtosecond laser excitation optical path: Place a half-wave plate 3, a polarizer 4, and a focusing lens 6 at the exit of the femtosecond laser in sequence, and the incident femtosecond laser passes through the above components in sequence and focuses on the multilayer heterojunction film 1 that has completed magnetic writing programming. The femtosecond laser spot must be aligned with and cover the magnetic domain pattern written in the first step.

[0091] 2) The half-wave plate 3 and the polarizer 4 are adjusted so that the power per unit area of ​​the femtosecond laser exciting the multilayer heterojunction film 1 is lower than the laser energy used to write the magnetic susceptibility pattern in the first step.

[0092] 3) Use femtosecond laser pulses to excite broadband terahertz radiation and send it into a far-field terahertz spatial-polarization detection system.

[0093] Further integration Figure 2 and Figure 3 , shows a device for implementing the above method, which consists of two parts: a writing system and an excitation system.

[0094] like Figure 2 As shown, the writing system specifically includes a heating laser 2, a half-wave plate 3, a polarizing plate 4, a mechanical shutter 5 and a focusing lens 6. The half-wave plate 3, the polarizing plate 4, the mechanical shutter 5, the focusing lens 6 and the multilayer heterojunction film 1 are arranged in sequence on the optical path of the heating laser 2. Among them, the half-wave plate 3 and the polarizing plate 4 are used to adjust the output power of the heating laser 2 in combination; the mechanical shutter 5 is used to control the on and off of the optical path, and its driving time is less than 10 milliseconds; the focusing lens 6 focuses the laser on the multilayer heterojunction film 1. The width of the magnetic structure written by the writing system is more than 10 microns and less than 100 microns. In addition, the writing system also includes a vector magnet 7 for controlling the direction of the magnetic domain of the multilayer heterojunction film 1 and a two-dimensional mechanical displacement stage for adjusting the horizontal position of the multilayer heterojunction film 1 on a plane; the vector magnet 7 can apply a magnetic field greater than 1000 Oersteds, and the direction of the magnetic field is adjustable 360 ​​degrees in the film plane; the displacement accuracy of the two-dimensional mechanical displacement stage is less than 1 micron and the stroke is greater than 5 mm. The vector magnet 7 can be realized by an electromagnet, or by a permanent magnet through an external mechanical rotation angle; in the following embodiments, an electromagnet is preferably used to simplify the overall structure.

[0095] like Figure 3 As shown, the excitation system specifically includes an excitation laser 8, a half-wave plate 3, a polarizer 4 and a focusing lens 6, wherein the half-wave plate 3, the polarizer 4, the focusing lens 6 and the multilayer heterojunction film 1 are sequentially arranged on the optical path of the excitation laser 8. Among them, the half-wave plate 3 and the polarizer 4 are used to adjust the unit area power of the excitation laser 8 in combination to make it lower than the unit area power of the heating laser 2, and the focusing lens 6 focuses the laser on the multilayer heterojunction film 1 and keeps the formed femtosecond laser spot aligned and covers the magnetic domain pattern of the written part. The coverage range of the excitation system is within the range of 10 microns to 1 cm.

[0096] Example 1

[0097] This embodiment provides a wide-spectrum terahertz radiation device that generates angular polarization in the far field.

[0098] The structure of the magnetic domain writing device is as follows Figure 2 A combination of a half-wave plate 3 and a polarizing plate 4 is placed at the outlet of the heating laser 2 to adjust the output power of the heating laser 2, and then the laser is focused on the surface of the multilayer heterojunction film 1, with a spot diameter of about 40 microns.

[0099] In this embodiment, the multilayer heterojunction film 1 is composed of an antiferromagnetic medium layer 12, a ferromagnetic medium layer 13 and a non-magnetic metal layer 14 stacked on each other with a nanometer thickness, which are prepared on a substrate 11 by a thin film growth technology (which can be achieved by conventional methods in the art). The structure of the film is as follows: Figure 1 and 3 In this embodiment, the antiferromagnetic dielectric layer 12 is IrMn with a thickness of 6 nanometers, and the ferromagnetic dielectric layer 13 is Fe with a thickness of 3 nanometers. 50 Co 50 , the non-magnetic metal layer 14 is 2 nanometers thick Pt, and the substrate 11 is 500 microns thick Al2O3. Other types of materials can also form the multilayer heterojunction film 1, and are consistent with the implementation method of this embodiment. For example, in other embodiments, the antiferromagnetic dielectric layer 12 can also be made of FeMn, and the ferromagnetic dielectric layer 13 can be made of ferromagnetic materials including Fe 50 Co 50 , Fe, Co, Ni, CoFeB, CoFe, FeNi(Py), ferromagnetic Heusler alloys, ferrimagnetic materials include FeGd, CoGd; the non-magnetic metal layer 14 can select materials whose conversion mechanism is the inverse spin Hall effect including Pt, Ta, Pd, W, etc.; the substrate 11 can select materials such as silicon dioxide, magnesium oxide, glass, diamond, sapphire, organic glass, silicon wafer and flexible substrate 11.

[0100] In this embodiment, the heating laser 2 uses a femtosecond laser light source with a pulse width of 100 femtoseconds, a central wavelength of 1030 nanometers, and a repetition frequency of 36 megahertz. Other types of laser light sources can also be used as the heating laser 2, and are consistent with the implementation method of this embodiment. In other embodiments, the heating laser 2 uses a pulse time length that meets the range of 100 femtoseconds to 100 nanoseconds, a light wavelength that has a range of 200 nanometers to 2 micrometers, and a power of 0.1 watts to 100 watts.

[0101] like Figure 4 As shown, under the conditions of the heating laser 2 selected in this embodiment, the spatial resolution of the written magnetic domain can be changed by controlling the laser irradiation power in combination with the half-wave plate 3 and the polarizer 4. Figure 5 The relationship between the spatial resolution of magnetic domain writing and the laser irradiation power is summarized. Under the conditions of the heating laser 2 selected in this embodiment, when the power of the heating laser 2 is less than 0.4 watts, no magnetic domain can be written into the multilayer heterojunction film 1, and when the power of the heating laser 2 is higher than 1.6 watts, the multilayer heterojunction film 1 is permanently damaged by the laser. Therefore, the laser power range that can be used to write magnetic domains is between 0.4 watts and 1.6 watts, and the spatial resolution of the written magnetic domain can be adjusted by the power of the heating laser 2.

[0102] like Figure 6 and 7 As shown, by regulating the external magnetic field (H w ) direction, it is possible to control the direction of the magnetic domains in the heated area of ​​the multilayer heterojunction film 1, so that when the femtosecond laser pulse is excited, the polarization direction of the terahertz radiation in the area is controlled. Taking the writing magnetic field angle θ = 0, 180 degrees, 90 degrees and 270 degrees as an example, Figure 6 The hysteresis loop in the film proves that the laser heating magnetic writing technique can change the magnetic exchange bias field H in the local area of ​​the film. ex direction, so that the magnetic domains in the laser-heated area under zero magnetic field point in the opposite direction. Correspondingly, Figure 7 As shown in Figure 1, when a femtosecond laser pulse is used to excite the above-mentioned different writing magnetic field angle regions, linearly polarized terahertz pulses with different polarization directions are generated. The polarization direction of these pulses is always perpendicular to the writing magnetic field direction (for example, when the writing magnetic field angle θ = 0, the magnetic field points to the y direction, and the emitted terahertz light field is polarized along E x direction, etc.), and the intensity and time structure of the pulses are almost the same.

[0103] like Figure 8 As shown, this embodiment generates wide-spectrum terahertz radiation, covering a spectrum width of 0-5 terahertz.

[0104] like Fig. 9 As shown, in this embodiment, the laser heating magnetic writing step is first performed to write radially divergent magnetic domain directions at a spatial resolution of 40 microns in the ferromagnetic layer of the multilayer heterojunction film 1. The total size of the writing area is 8 mm×8 mm.

[0105] In this embodiment, the excitation laser 8 light source with a pulse width of 50 femtoseconds, a central wavelength of 780 nanometers, and a repetition frequency of 1 kHz is used to excite and generate terahertz radiation. Other types of femtosecond laser light sources can also be used to excite and generate terahertz radiation, and are consistent with the implementation method of this embodiment. In other embodiments, the excitation laser 8 is selected to have a time length in the range of 1 femtosecond to 1 picosecond, a wavelength in the range of 200 nanometers to 2 micrometers, and an energy density of at least 0.1 nJ / cm2 applied to the multilayer heterojunction structure. 2 .

[0106] Femtosecond laser excitation produces terahertz devices such as Figure 3 As shown. We place a half-wave plate 3 and a polarizer 4 combination at the exit of the excitation laser 8 to adjust the output power of the laser, and then use a focusing lens 6 to focus the laser beam on the multilayer heterojunction film 1 where the magnetic domain has been written. The excitation process is completed without an external magnetic field, and the energy per unit area of ​​the excitation light should be lower than the energy per unit area of ​​the heating laser used for magnetic domain writing. The terahertz pulse generated by the excitation is sent to the far-field terahertz space-polarization detection system.

[0107] Fig.10 and 11 The terahertz radiation emitted by the multilayer heterojunction film 1 with radially divergent magnetic domains written in this embodiment is measured by a spatial-polarization resolution detection system. The terahertz frequency is selected to be 0.293 terahertz. Fig.10 shows that the measured polarization direction is along the spatial y-axis, and Fig.11 The polarization direction is shown along the x-axis. The results show that the amplitude of the terahertz spot shows a minimum along the polarization direction, and there is a phase difference of π on both sides of the center of the spot in the vertical polarization direction. These results show that the far-field terahertz radiation generated by the device has angular polarization characteristics.

[0108] Fig.12 and 13 The same measurement is shown, but for a THz frequency of 0.674 THz. The result is consistent with the 0.29 THz measurement result, proving that the device written in this embodiment has a wide spectrum of angular polarization characteristics.

[0109] Example 2

[0110] This embodiment provides a wide-spectrum terahertz radiation device that generates radial polarization in the far field.

[0111] like Fig.14 As shown, the magnetic domain pattern written in the multilayer heterojunction film 1 is an angularly rotated distribution. In this embodiment, the method of writing the magnetic domain pattern in the multilayer heterojunction film 1 is consistent with that in Example 1. In this embodiment, the method of exciting the broad spectrum terahertz radiation from the programmed multilayer heterojunction film 1 is also consistent with that in Example 1.

[0112] Fig.15 and 16 The terahertz radiation emitted by the multilayer heterojunction film 1 with the angular orientation of the magnetic domains written in this embodiment is measured by a spatially-polarization-resolved detection system. The terahertz frequency is selected to be 0.29 terahertz. Fig.15 shows that the measured polarization direction is along the spatial y-axis, and Fig.16The polarization direction is shown along the x-axis. The results show that the amplitude of the terahertz spot shows a maximum value along the polarization direction, and there is a phase difference of π on both sides of the center of the spot along the polarization direction. These results show that the far-field terahertz radiation generated by the device has radial polarization characteristics.

[0113] Fig.17 and 18 The same measurement is shown, but for a THz frequency of 0.469 THz. The result is consistent with the 0.29 THz measurement result, proving that the writing device of this embodiment has a wide spectrum of radial polarization characteristics.

[0114] Example 3

[0115] This embodiment provides a device for generating spatially separated left-handed and right-handed terahertz radiation in the far field.

[0116] like Fig.19 As shown, the magnetic domain pattern written in the multilayer heterojunction film 1 is a strip-shaped rotating magnetic domain distribution in the y-axis direction. In this embodiment, the method of writing the magnetic domain pattern in the multilayer heterojunction film 1 is consistent with that in Example 1. In this embodiment, the excitation laser 8 light source with a pulse width of 22 femtoseconds, a central wavelength of 1030 nanometers, and a repetition frequency of 10 kHz is used to excite and generate terahertz radiation. Other types of femtosecond laser light sources can also be used to excite and generate terahertz radiation, and are consistent with the implementation method of this embodiment. In other embodiments, the excitation laser 8 is selected to meet the requirements of having a time length in the range of 1 femtosecond to 1 picosecond, a wavelength in the range of 200 nanometers to 2 microns, and an energy density of at least 0.1nJ / cm applied to the multilayer heterojunction structure. 2 .

[0117] Femtosecond laser excitation produces terahertz devices such as Figure 3 As shown. We place a half-wave plate 3 and a polarizer 4 combination at the exit of the excitation laser 8 to adjust the output power of the laser, and then use a focusing lens 6 to focus the laser beam on the multilayer heterojunction film 1 where the magnetic domain has been written. The excitation process is completed without an external magnetic field, and the energy per unit area of ​​the excitation light should be lower than the energy per unit area of ​​the heating laser used for magnetic domain writing. The terahertz pulse generated by the excitation is sent to the far-field terahertz space-polarization detection system.

[0118] Fig. 20 The distribution of far-field left-handed (LCP) and right-handed (RCP) terahertz radiation in space is shown. The left-handed circularly polarized terahertz radiation is emitted to the negative emission angle direction. The right-handed circularly polarized terahertz radiation is emitted to the positive emission angle direction. Fig.21 Shows the launch angle Degree and The terahertz time-domain waveform of different degrees proves that terahertz radiation of different chirality is emitted at different angles.

[0119] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for realizing spatial-polarization state programmable broadband terahertz radiation, characterized in that: The steps include: S1: controlling the magnetic domain orientation of the multilayer heterojunction film (1) by means of a magnetic field, and writing a magnetic domain structure on the multilayer heterojunction film (1) by means of a laser heating direct writing technique; S2: Exciting the programmed multilayer heterojunction film (1) by femtosecond laser pulses to generate terahertz radiation with spatial-polarization states; The multilayer heterojunction film (1) comprises an antiferromagnetic medium layer (12), a ferromagnetic medium layer (13) and a non-magnetic metal layer (14) stacked in sequence; The material of the antiferromagnetic medium layer (12) is an antiferromagnetic material; The material of the ferromagnetic medium layer (13) is ferromagnetic metal, ferromagnetic Heusler alloy or ferrimagnetic metal; The material of the non-magnetic metal layer (14) is a metal with high spin-charge conversion efficiency; The antiferromagnetic medium layer (12), the ferromagnetic medium layer (13) and the non-magnetic metal layer (14) all have a nanometer-level thickness.

2. The method for realizing spatial-polarization state programmable wide-spectrum terahertz radiation according to claim 1, characterized in that: The intensity of the magnetic field is greater than 1000 Oersteds, and the energy per unit area of ​​the excitation laser in the femtosecond laser pulse is smaller than the energy per unit area of ​​the heating laser in the laser heating direct writing technology.

3. The method for realizing spatial-polarization state programmable wide-spectrum terahertz radiation according to claim 2, characterized in that: The excitation laser has a duration of 1 fs-1 ps, a wavelength of 200 nm-2 μm, and an energy density applied to the multilayer heterojunction film (1) of not less than 0.1 nJ / cm 2 ; The heating laser has a duration of 100 fs-100 ns, a wavelength of 200 nm-2 μm, and a power density of 100 W / cm2 applied to the multilayer heterojunction film (1). 2 -100,000 W / cm 2 .

4. The method for realizing spatial-polarization state programmable wide-spectrum terahertz radiation according to claim 1, characterized in that: The antiferromagnetic material includes IrMn and FeMn; the ferromagnetic metal includes Fe 50 Co 50 , Fe, Co, Ni, CoFeB, CoFe and FeNi(Py); the ferrimagnetic metals include FeGd and CoGd; the metals with high spin-charge conversion efficiency are materials with inverse spin Hall effect, including Pt, Ta, Pd, W and Ir.

5. The method for realizing spatial-polarization state programmable wide-spectrum terahertz radiation according to claim 1, characterized in that: The multilayer heterojunction thin film (1) is produced by thin film growth technology.

6. The method for realizing spatial-polarization state programmable wide-spectrum terahertz radiation according to claim 5, characterized in that: The multilayer heterojunction thin film (1) further comprises a substrate (11) for support; The substrate (11) is bonded to the antiferromagnetic medium layer (12); The material of the substrate (11) includes silicon dioxide, magnesium oxide, glass, diamond, sapphire, organic glass, silicon wafer and flexible substrate.

7. A device for implementing the method according to any one of claims 1 to 6, characterized in that: Including writing system and excitation system; The writing system comprises a heating laser (2), a power adjustment component, a mechanical shutter (5), a focusing lens (6), a vector magnet (7) and a displacement stage; the power adjustment component, the mechanical shutter (5), the focusing lens (6) and the multilayer heterojunction film (1) are sequentially arranged on the optical path of the heating laser (2) so that the heating laser is focused on the multilayer heterojunction film (1); the vector magnet (7) is arranged on the side of the multilayer heterojunction film (1), and the direction of the magnetic field has an adjustment range of 360° within the plane where the multilayer heterojunction film (1) is located; the displacement stage carries the multilayer heterojunction film (1); The excitation system comprises an excitation laser (8), a power adjustment component and a focusing lens (6); the power adjustment component, the focusing lens (6) and the multi-layer heterojunction film (1) are sequentially arranged on the optical path of the excitation laser (8) so that the excitation laser is focused on the multi-layer heterojunction film (1).

8. The device for spatially-polarization-state programmable broadband terahertz radiation according to claim 7, characterized in that: The power adjustment component comprises a half-wave plate (3) and a polarizing plate (4) arranged in sequence; The power regulating component is used in the writing system to regulate the heating laser to a suitable power; The power regulating component is used in the excitation system to regulate the energy per unit area of ​​the excitation laser to be lower than that of the heating laser.

9. The device for spatially-polarization-state programmable broadband terahertz radiation according to claim 7, characterized in that: The shutter time of the mechanical shutter (5) is less than 10 ms.

10. The device for spatially-polarization-state programmable broadband terahertz radiation according to claim 7, characterized in that: The displacement stage performs two-dimensional horizontal motion, with a displacement accuracy of less than 1 μm and a travel distance of more than 5 mm.

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