A full van der Waals spin terahertz emitter and a method for regulating the intensity and polarity of terahertz waves

Abstract

The present invention relates to a full van der Waals spin terahertz emitter and a method for regulating the intensity and polarity of terahertz waves. The full van der Waals spin terahertz emitter includes: a femtosecond laser, a nanometer thin film, and a voltage source; the femtosecond laser is used to output pump laser that penetrates the nanometer thin film to generate a first terahertz pulse; the nanometer thin film is a full van der Waals heterostructure constructed by a non-ferromagnetic layer and a ferromagnetic layer; the material of the ferromagnetic layer is a two-dimensional magnetic material; the voltage source is used to input current to the non-ferromagnetic layer, regulate the position of the Fermi level of the non-ferromagnetic layer material from the Dirac point to change the intensity of the first terahertz pulse, and generate a second terahertz pulse with the opposite polarity. The present invention also provides a method for regulating the intensity and polarity of terahertz waves. The above terahertz wave generator uses a two-dimensional ferromagnetic material to prepare the ferromagnetic layer and constructs a full van der Waals heterostructure with a non-ferromagnetic layer made of a van der Waals topological material.

Description

Technical Field

[0001] The present invention relates to the field of terahertz emission and regulation, and particularly to an efficient all-van der Waals spin terahertz emitter and a method for regulating the intensity and polarity of terahertz waves. Background Art

[0002] Terahertz (THz) waves are electromagnetic waves with frequencies ranging from 0.1 to 10 THz, which are electromagnetic waves between microwaves and infrared rays. This characteristic has technical advantages such as strong penetration ability, good safety and directivity, large bandwidth, and high time and space resolution. The development of terahertz technology has unique superiority and huge application prospects in the fields of physics, chemistry, electronic information, life science, materials science, communication radar, and national security.

[0003] Currently, the mechanism of spin terahertz emitters mainly utilizes femtosecond laser pulses to irradiate the heterostructure of ferromagnetic and non-ferromagnetic layers. Electrons in the ferromagnetic layer are excited by light and jump above the Fermi level, thus generating an unbalanced electron distribution. Most of the spin electrons (spin-up) excited by light mainly have sp-like band orbital characteristics, while a small number of balanced electrons (spin-down) have d-like band orbital characteristics. Since their velocities and lifetimes are different, the spin-up electrons will accumulate at the ferromagnetic / non-ferromagnetic interface, and finally, the spin injection into the non-ferromagnetic is realized. At the ferromagnetic / non-ferromagnetic interface, due to the inverse spin Hall effect or the inverse Rashba-Edelstein effect, the spin current is converted into a charge current on the ps time scale, and then terahertz is radiated.

[0004] Commonly used spin-electronic terahertz wave emitters are mainly heterostructures constructed by non-ferromagnetic layers composed of heavy metal materials (such as platinum, tungsten, tantalum, etc.) and ferromagnetic layer materials composed of three-dimensional materials such as cobalt-iron-boron alloys. In recent years, with the development of communication technology entering the "terahertz era", the demand for low-cost, miniaturized, and multifunctional efficient terahertz sources from all walks of life has become increasingly intense. The advent of two-dimensional materials has brought hope for solving the above problems.

[0005] Two-dimensional materials refer to materials in which electrons can only move planar in the nanoscale of two dimensions, and this nanoscale range includes 1 - 100 nm. Ferromagnetism has wide application value in technologies such as information processing and magnetic storage. Maintaining a stable ferromagnetic state in a thin film with a single-atomic-layer limit thickness can achieve advantages such as high efficiency and high integration, so two-dimensional magnetic materials have attracted extensive attention. Nowadays, the most studied two-dimensional magnetic materials with in-plane magnetic anisotropy are based on Fe 4 GeTe 2 and Fe 5 GeTe 2As the main representatives, the Curie temperatures of these materials are close to room temperature, and they can be cleaved to monolayer thickness. Meanwhile, they have good physical and chemical stability and are easy to modulate.

[0006] Currently, the best method reported for regulating the Curie temperature of two-dimensional magnetic materials is to utilize the interface engineering effect by coupling topological materials with two-dimensional magnetic materials, which can increase the Curie temperature above room temperature. In view of this, it is particularly important to construct a new efficient room-temperature all-van der Waals spin electronic terahertz wave emitter based on two-dimensional materials and other novel materials. Summary of the Invention

[0007] To solve the above technical problems, the purpose of the present invention is to provide an efficient all-van der Waals spin terahertz emitter and a method for regulating the intensity and polarity of terahertz waves.

[0008] To achieve the above purpose, the present invention provides an all-van der Waals spin terahertz emitter, which includes: a femtosecond laser, a nanometer thin film, and a voltage source, wherein:

[0009] The femtosecond laser is used to output pump laser that penetrates the nanometer thin film to generate a first terahertz pulse;

[0010] The nanometer thin film is an all-van der Waals heterostructure constructed by a non-ferromagnetic layer and a ferromagnetic layer; the material of the ferromagnetic layer is a two-dimensional magnetic material;

[0011] The voltage source is used to input current into the non-ferromagnetic layer, regulate the position of the Fermi level of the non-ferromagnetic layer material relative to the Dirac point to change the intensity of the first terahertz pulse, and generate a second terahertz pulse with opposite polarity.

[0012] According to a specific embodiment of the present invention, in the above all-van der Waals spin terahertz emitter, the ferromagnetic layer can be grown on the non-ferromagnetic layer by methods such as molecular beam epitaxy.

[0013] According to a specific embodiment of the present invention, preferably, in the above all-van der Waals spin terahertz emitter, the nanometer thin film further includes a substrate and an electrode thin film layer. Specifically, the nanometer thin film can have the following structures: the nanometer thin film includes a substrate, a ferromagnetic layer, a non-ferromagnetic layer, and an electrode thin film layer arranged in sequence; or, the nanometer thin film includes an electrode thin film layer, a substrate, a non-ferromagnetic layer, and a ferromagnetic layer arranged in sequence; or, the nanometer thin film includes a substrate, a non-ferromagnetic layer, and a ferromagnetic layer arranged in sequence, and at least two electrode thin films are provided on the substrate.

[0014] According to the specific embodiments of the present invention, preferably, in the above-mentioned all-Van der Waals spin terahertz emitter, the positive and negative electrodes of the voltage source are respectively connected to the non-ferromagnetic layer. Preferably, the positive electrode of the voltage source applies a voltage to the non-ferromagnetic layer through the electrode thin film layer, and the negative electrode of the voltage source is connected to the non-ferromagnetic layer. For example, when the nano-film includes a substrate, a ferromagnetic layer, a non-ferromagnetic layer, and an electrode thin film layer arranged in sequence, this connection method can be adopted;

[0015] Alternatively, the positive and negative electrodes of the voltage source are respectively connected to the substrate. For example, when the nano-film includes a substrate, a non-ferromagnetic layer, and a ferromagnetic layer arranged in sequence, and the substrate is provided with at least two electrode thin films, the positive and negative electrodes of the voltage source can be respectively connected to the two electrode thin films of the substrate;

[0016] Alternatively, the positive electrode of the voltage source is connected to the substrate, and the negative electrode is connected to the non-ferromagnetic layer. For example, when the nano-film includes an electrode thin film layer, a substrate, a non-ferromagnetic layer, and a ferromagnetic layer arranged in sequence, this connection method can be adopted.

[0017] According to the specific embodiments of the present invention, preferably, in the above-mentioned all-Van der Waals spin terahertz emitter, the two-dimensional ferromagnetic material is a two-dimensional Van der Waals material with magnetism at room temperature, preferably selected from Fe 4 GeTe 2 、Fe 4+x GeTe 2 、Fe 5 GeTe 2 or a combination of two or more of them; more preferably Fe 4 GeTe 2 . Fe 4 GeTe 2 has in-plane magnetic anisotropy, and its Curie temperature is lower than room temperature. However, by forming a heterojunction with a non-ferromagnetic layer made of a Van der Waals topological material, the Curie temperature of the ferromagnetic layer made of Fe 4 GeTe 2 can be raised above room temperature by using the interface engineering effect. Thus, an all-Van der Waals heterojunction structure with in-plane magnetic anisotropy is proposed to achieve terahertz emission.

[0018] According to the specific embodiments of the present invention, preferably, in the above-mentioned all-Van der Waals spin terahertz emitter, the material of the non-ferromagnetic layer is a Van der Waals topological material, preferably a strong spin-orbit coupling material. Among them, the strong spin-orbit coupling material can be selected from Bi 2 Se 3 , Bi 2 Te 3 , Bi x Sb 1-x , Sb 2 Te3 and (Bi x Sb 1-x ) 2 Te 3 or a combination of two or more thereof; preferably (Bi x Sb 1-x ) 2 Te 3 . Among them, the value of x in the Bi x Sb 1-x is about 0.9; the value range of x in the (Bi x Sb 1-x ) 2 Te 3 is about 0-1.

[0019] According to a specific embodiment of the present invention, in the above-mentioned all-Van der Waals spin terahertz emitter, the materials used for the non-ferromagnetic layer and the ferromagnetic layer are both Van der Waals topological materials. Therefore, the corresponding structure is an all-Van der Waals topological material system, and this terahertz emitter is an all-Van der Waals spin terahertz emitter. The thicknesses of the non-ferromagnetic layer and the ferromagnetic layer that make up the nanometer thin film are both at the nanometer level. Preferably, the thickness of the ferromagnetic layer is 1-10 nm, and the thickness of the non-ferromagnetic layer is 5-20 nm.

[0020] According to a specific embodiment of the present invention, preferably, in the above-mentioned all-Van der Waals spin terahertz emitter, the femtosecond laser is a femtosecond laser oscillator, a femtosecond laser amplifier, or a fiber femtosecond laser, and the pulse width of the pump laser output by the femtosecond laser is less than 1 ps.

[0021] According to a specific embodiment of the present invention, preferably, in the above-mentioned all-Van der Waals spin terahertz emitter, the material of the substrate is selected from one of sapphire, gallium arsenide with a high Miller index, strontium titanate, and lead magnesium niobate-lead titanate.

[0022] According to a specific embodiment of the present invention, in the above-mentioned all-Van der Waals spin terahertz emitter, the preparation of the nanometer thin film can be realized by methods such as molecular beam epitaxy. For example:

[0023] When the nanometer thin film is composed of a ferromagnetic layer and a non-ferromagnetic layer, the ferromagnetic layer is grown on the surface of the non-ferromagnetic layer by molecular beam epitaxy;

[0024] When the nanometer thin film includes a substrate, a ferromagnetic layer, a non-ferromagnetic layer, and an electrode thin film layer arranged in sequence, the ferromagnetic layer is grown on the surface of the substrate by molecular beam epitaxy, then the non-ferromagnetic layer is grown on the surface of the ferromagnetic layer, and then the electrode thin film layer is formed on the surface of the non-ferromagnetic layer;

[0025] When the nano-film includes an electrode film layer, a substrate, a non-ferromagnetic layer, and a ferromagnetic layer arranged in sequence, the non-ferromagnetic layer is grown on the surface of the substrate by molecular beam epitaxy, then the ferromagnetic layer is grown on the surface of the ferromagnetic layer, and then the electrode film layer is formed on the other surface of the substrate (the surface where the non-ferromagnetic layer is not grown);

[0026] When the nano-film includes a substrate, a non-ferromagnetic layer, and a ferromagnetic layer arranged in sequence, and the substrate is provided with at least two electrode films, the non-ferromagnetic layer is grown on the surface of the substrate by molecular beam epitaxy, then the ferromagnetic layer is grown on the surface of the ferromagnetic layer, and then the electrode film layer is formed in the area of the surface of the substrate where the non-ferromagnetic layer is grown and is not covered by the non-ferromagnetic layer.

[0027] The terahertz wave generator provided by the present invention uses a two-dimensional ferromagnetic material to prepare the ferromagnetic layer, and constructs a full van der Waals heterojunction structure with a non-ferromagnetic layer made of a van der Waals topological material. At present, constructing a full van der Waals heterojunction as a terahertz emitter is first proposed by the present invention. In the full van der Waals heterostructure, the two-dimensional magnetic material exists in a layered form and is stacked together by van der Waals forces, that is, intermolecular forces. The atoms within the layer are connected by chemical bonds, and still maintain novel physical and chemical properties in terms of electricity, mechanics, optics, and energy at the atomic scale thickness. Further, by combining with adjacent layers through weak van der Waals interactions, it is possible to combine atomic layers with different degrees of matching, thereby creating a wide range of van der Waals heterojunction structures, getting rid of the limitations of lattice matching and compatibility, and thus achieving advantages such as circuit miniaturization, mechanical flexibility, three-dimensional stacking high density, fast response rate, and high switching ratio performance.

[0028] The present invention also provides a method for regulating the intensity and polarity of terahertz waves, including:

[0029] Making the pump laser penetrate the nano-film to generate a first terahertz pulse, wherein the nano-film includes a full van der Waals heterostructure constructed by a ferromagnetic layer and a non-ferromagnetic layer in contact with each other, and the thicknesses of both the ferromagnetic layer and the non-ferromagnetic layer are on the nanoscale; the material of the ferromagnetic layer is a two-dimensional magnetic material;

[0030] Using a voltage source to regulate the carrier concentration of the non-ferromagnetic layer to change the Fermi level of the non-ferromagnetic layer, so that the spin direction of the ferromagnetic layer changes, and a second terahertz pulse with a polarity opposite to that of the first terahertz pulse is radiated from the non-ferromagnetic layer;

[0031] Preferably, the pump laser penetrates the nanometer thin film to generate a first terahertz pulse, and: the pump laser is incident perpendicular to the ferromagnetic layer to generate a first instantaneous spin current in the ferromagnetic layer, then the first instantaneous spin current is converted into a first charge current in the non-ferromagnetic layer, and a first terahertz pulse is radiated from the non-ferromagnetic layer; or, the pump laser is incident perpendicular to the non-ferromagnetic layer to generate a second instantaneous spin current in the ferromagnetic layer, then the second instantaneous spin current is converted into a second charge current in the non-ferromagnetic layer, and a second terahertz pulse with a polarity opposite to that of the first terahertz pulse is radiated from the non-ferromagnetic layer;

[0032] Preferably, the first instantaneous spin current is converted into a first charge current, and the second instantaneous spin current is converted into a second charge current. Further, the effect of the charge current on the spin current is related to the position of the topological material Fermi level, which is based on the spin Hall effect, Rashba effect or topological surface state.

[0033] According to a specific embodiment of the present invention, the nanometer thin film involved in the above method can adopt the same structure, material, thickness and other parameters as the nanometer thin film of the all-van der Waals spin terahertz emitter provided by the present invention.

[0034] The technical solution of the present invention uses a femtosecond laser to penetrate a nanometer thin film composed of a mutually contacting ferromagnetic layer and non-ferromagnetic layer to generate a first terahertz pulse; at the same time, a voltage source is used to change the carrier concentration of the topological material of the non-ferromagnetic layer to regulate the intensity of the first terahertz pulse, and further change the spin direction of the non-ferromagnetic layer to cause the magnetic moment of the ferromagnetic layer to undergo magnetic reversal, and a second terahertz pulse with a polarity opposite to that of the first terahertz pulse is radiated from the non-ferromagnetic layer, thereby causing a terahertz pulse with a polarity opposite to that of the initially generated terahertz pulse to be radiated from the non-ferromagnetic layer. The technical solution of the present invention can quickly change the polarity of the radiated terahertz pulse to radiate terahertz pulses with opposite polarities, and has diverse functions and simple operations, which is beneficial to the production and application of related terahertz devices. Description of the Drawings

[0035] Figure 1 Schematic structural diagram of the spin terahertz emission device according to Embodiment 2 of the present invention;

[0036] Figure 2 Schematic regulation diagram of the spin terahertz emission device according to Embodiment 3 of the present invention;

[0037] Figure 3 Schematic regulation diagram of the spin terahertz emission device according to Embodiment 4 of the present invention;

[0038] Figure 4 Schematic regulation diagram of the spin terahertz emission device according to Embodiment 5 of the present invention. Detailed Embodiments

[0039] For a clearer understanding of the technical features, objectives, and beneficial effects of the present invention, the following provides a detailed description of the technical solution of the present invention, but it should not be construed as a limitation on the scope of implementation of the present invention.

[0040] Example 1

[0041] This example provides a method for preparing a van der Waals heterojunction using molecular beam epitaxy technology, and combines material characterization means to prepare high-quality thin films. The operating steps for preparing the van der Waals heterojunction are as follows:

[0042] (1) Cleaning the substrate:

[0043] Taking (0001) sapphire as the substrate as an example, since sapphire has stable chemical properties and is not easily corroded by acids and alkalis, it is necessary to perform a cleaning treatment before growing a thin film on the substrate. The cleaning procedure is as follows:

[0044] The sapphire substrate is ultrasonically treated in acetone and isopropyl alcohol for 5 - 6 minutes each, and then ultrasonically treated in deionized water for 5 - 6 minutes to remove the organic matter on the upper surface of the substrate, and then dried with nitrogen.

[0045] (2) Temperature calibration:

[0046] The substrate temperature during growth is monitored by a thermocouple, and the temperature of the thermocouple is calibrated by a pyrometer. To ensure the temperature accuracy to the greatest extent, the temperature is slowly increased to ensure thermal equilibrium during each heating process.

[0047] (3) Growth of topological insulator:

[0048] The vacuum degree in the growth chamber is maintained at about 10 -10 Torr. There are evaporation source furnaces of bismuth (Bi), antimony (Sb), and tellurium (Te) installed in the ultra-high vacuum chamber, and thermal evaporation technology is used for evaporation. The evaporation rates of different elements are determined by a film thickness monitor (FDC) to control the flux ratio during the growth process. The cleaned sapphire substrate is quickly transferred into the ultra-high vacuum molecular beam epitaxy system, and the sapphire substrate is annealed at a high temperature for about 30 minutes, and the annealing temperature is about 600 °C - 650 °C. Subsequently, the substrate temperature is lowered to the growth temperature, and the thickness is controlled by precisely controlling the time to deposit the thin film on the substrate.

[0049] (4) Growth of two-dimensional magnetic material iron germanium telluride

[0050] There are evaporation source furnaces of iron (Fe), germanium (Ge), and tellurium (Te) installed in the ultra-high vacuum chamber, and thermal evaporation technology is used for evaporation. Growing topological insulator (Bi x Sb 1-x ) 2 Te 3Do not take it out afterwards, confirm the vacuum degree in the vacuum chamber, raise the temperature to the growth temperature of the thin film, and determine the evaporation rates of the three elements through a film thickness monitor (FDC). During the growth process, control the thin film thickness by precisely controlling the deposition time on the substrate. After the growth is completed, quickly cool the substrate to room temperature and then take it out.

[0051] The structures involved in the devices of Examples 2-5 can all be prepared by referring to the method of Example 1.

[0052] Example 2

[0053] This example provides a spin terahertz emission device, and its structure is as Figure 1 shown. The device includes: a femtosecond laser, a nano-thin film; where:

[0054] The nano-thin film includes a ferromagnetic layer and a non-ferromagnetic layer in contact with each other. Both the ferromagnetic layer and the non-ferromagnetic layer are van der Waals material thin films, and the two form a full van der Waals heterostructure;

[0055] The femtosecond laser is used to output pump laser and penetrate the nano-thin film to generate terahertz pulses.

[0056] Preferably, the material of the non-ferromagnetic layer is a topological insulator, and the specific material is (Bi x Sb 1-x ) 2 Te 3 , where x is 0.5 and the thickness is 6 nm;

[0057] The material of the ferromagnetic layer is a two-dimensional magnetic material, specifically Fe 4 GeTe 2 , and the thickness is 3-5 nm.

[0058] The nano-thin film of this example is obtained by growing the ferromagnetic layer on the surface of the non-ferromagnetic layer by molecular beam epitaxy.

[0059] Since the two-dimensional magnetic material Fe 4 GeTe 2 has in-plane magnetic anisotropy near room temperature; further, when the topological insulator is coupled with the two-dimensional magnetic material Fe 4 GeTe 2 , by using the interface engineering effect, Fe 4 GeTe 2It still has in-plane magnetic anisotropy at room temperature. At the same time, a topological insulator is a special insulator. The interior of this material is insulating, while charges can move on the surface of the material because there is a band gap at the Fermi energy in the bulk material energy band of the topological insulator. There is a spin-momentum locking effect on the surface of the topological insulator. This special property enables the topological insulator to have a strong spin-orbit coupling effect, higher than that of common heavy metal materials such as Pt, W, Ta, etc. Therefore, two-dimensional magnetic materials can achieve efficient magnetization reversal by using topological insulators. In addition, the heterostructure can be prepared in large areas by using molecular beam epitaxy technology. When the film thickness is reduced to the thickness of a single atomic layer, the device size can be reduced and the magnetic storage density can be increased.

[0060] Example 3

[0061] This embodiment provides a spin terahertz emission device, the structure of which is as Figure 2 shown. The device includes: a femtosecond laser, a nanometer thin film, and a voltage source; where:

[0062] The nanometer thin film includes a ferromagnetic layer and a non-ferromagnetic layer in contact with each other, and a corresponding sapphire substrate and an electrode thin film, which are successively the substrate, the ferromagnetic layer, the non-ferromagnetic layer, and the electrode thin film; both the ferromagnetic layer and the non-ferromagnetic layer are van der Waals material thin films;

[0063] The femtosecond laser is used to output pump laser and penetrate the nanometer thin film to generate terahertz pulses.

[0064] The voltage source is used to regulate the carrier concentration of the non-ferromagnetic layer, change the position of the Fermi level, and further change the position of the Fermi level relative to the Dirac point and the Fermi vector to regulate the strength of the terahertz pulse; or further change the spin direction generated by the non-ferromagnetic layer to generate an opposite spin current, so as to radiate a second terahertz pulse with a polarity opposite to that of the first terahertz pulse from the non-ferromagnetic layer. The principle of voltage-controlled magnetization is based on the spin-orbit coupling effect, the Rashba effect, and the topological surface state effect.

[0065] Preferably, the material of the non-ferromagnetic layer thin film is (Bi x Sb 1-x ) 2 Te 3 , where x is 0.5 and the thickness is 6 nm;

[0066] The two-dimensional magnetic material used for the ferromagnetic layer is Fe 4 GeTe 2 , and the thickness is 3 - 5 nm;

[0067] The material of the substrate is selected from sapphire and gallium arsenide with a high Miller index.

[0068] The nanometer thin film of this embodiment is prepared by molecular beam epitaxy. First, a ferromagnetic layer is grown on the surface of the substrate, then a non-ferromagnetic layer is grown on the surface of the ferromagnetic layer that is not in contact with the substrate, and finally an electrode thin film is formed on the surface of the non-ferromagnetic layer that is not in contact with the ferromagnetic layer.

[0069] The positive electrode of the voltage source is connected to the electrode thin film layer on the material, and the negative electrode of the voltage source is connected to the non-ferromagnetic layer. By applying voltages of different magnitudes to regulate the carrier concentration, or by using voltage regulation in an ionic liquid environment to greatly change the carrier concentration, the intensity and polarity of terahertz waves are regulated.

[0070] Example 4

[0071] This embodiment provides a spin terahertz emission device, the structure of which is as Figure 3 shown. The device includes: a femtosecond laser, a nanometer thin film, and a voltage source; among them:

[0072] The nanometer thin film includes a ferromagnetic layer and a non-ferromagnetic layer in contact with each other, as well as a corresponding sapphire substrate and an electrode thin film, which are, in sequence, the electrode thin film, the substrate, the non-ferromagnetic layer, and the ferromagnetic layer; both the ferromagnetic layer and the non-ferromagnetic layer are van der Waals material thin films;

[0073] The femtosecond laser is used to output pump laser and penetrate the nanometer thin film to generate terahertz pulses.

[0074] The voltage source is used to regulate the carrier concentration of the non-ferromagnetic layer, change the position of the Fermi level, and further change the position of the Fermi level relative to the Dirac point and the Fermi vector to regulate the intensity of the terahertz pulse; or further change the spin direction generated by the non-ferromagnetic layer to generate an opposite spin current, so as to radiate a second terahertz pulse with a polarity opposite to that of the first terahertz pulse from the non-ferromagnetic layer. The principle of voltage-controlled magnetization is based on the spin-orbit coupling effect, the Rashba effect, and the topological surface state effect.

[0075] Preferably, the material of the non-ferromagnetic layer thin film is (Bi x Sb 1-x ) 2 Te 3 , with a thickness of 6 nm;

[0076] The two-dimensional magnetic material used for the ferromagnetic layer is specifically Fe 4 GeTe 2 , with a thickness of 3 - 5 nm;

[0077] The substrate material is selected from strontium titanate.

[0078] The nanometer thin film of this embodiment is prepared by molecular beam epitaxy. First, a non-ferromagnetic layer is grown on the surface of the substrate, then a ferromagnetic layer is grown on the surface of the non-ferromagnetic layer that is not in contact with the substrate, and finally an electrode thin film is formed on the surface of the substrate that is not in contact with the non-ferromagnetic layer.

[0079] The positive electrode of the voltage source is connected to the electrode layer on the substrate, and the negative electrode of the voltage source is connected to the non-ferromagnetic layer. By applying voltages of different magnitudes, the carrier concentration of the non-ferromagnetic layer is further regulated.

[0080] Example 5

[0081] This example provides a spin terahertz emission device, the structure of which is as Figure 4 shown. The device includes: a femtosecond laser, a nanometer thin film, and a voltage source; wherein:

[0082] The nanometer thin film includes a ferromagnetic layer and a non-ferromagnetic layer in contact with each other, as well as a corresponding sapphire substrate and two electrode thin films, which are the substrate, the non-ferromagnetic layer, and the ferromagnetic layer in sequence. The two electrode thin films are formed in the region on one surface of the substrate where the non-ferromagnetic layer is provided and is not covered by the non-ferromagnetic layer; both the ferromagnetic layer and the non-ferromagnetic layer are van der Waals material thin films;

[0083] The femtosecond laser is used to output pump laser and penetrate the nanometer thin film to generate terahertz pulses. The voltage source is used to regulate the carrier concentration of the non-ferromagnetic layer, change the position of the Fermi level, and further change the position of the Fermi level relative to the Dirac point and the Fermi vector to regulate the intensity of the terahertz pulses; or further change the spin direction generated by the non-ferromagnetic layer to generate an opposite spin current, so as to radiate a second terahertz pulse with a polarity opposite to that of the first terahertz pulse from the non-ferromagnetic layer. The principle of voltage-controlled magnetization is based on the spin-orbit coupling effect, the Rashba effect, and the topological surface state effect.

[0084] Preferably, the material of the non-ferromagnetic layer thin film is (Bi x Sb 1-x ) 2 Te 3 , with a thickness of 6 nm;

[0085] The two-dimensional magnetic material used for the ferromagnetic layer is Fe 4 GeTe 2 , with a thickness of 3 - 5 nm;

[0086] The substrate material is selected from lead magnesium niobate-lead titanate.

[0087] The nanometer thin film of this example is prepared by molecular beam epitaxy. First, the non-ferromagnetic layer is grown on the surface of the substrate, then the ferromagnetic layer is grown on the surface of the non-ferromagnetic layer that is not in contact with the substrate, and finally two electrode thin films are formed in the region on the surface of the substrate where the non-ferromagnetic layer is grown and is not covered by the non-ferromagnetic layer.

[0088] The positive electrode and the negative electrode of the voltage source are respectively in two directions of generating charge flow. By applying voltages of different magnitudes, a built-in electric field is realized in the lead magnesium niobate-lead titanate substrate, and the reverse electric field is used to control the deterministic spin direction, and further regulate the carrier concentration of the non-ferromagnetic layer.

[0089] Based on the conventional spin terahertz structure system, the present invention adds voltage regulation of the carrier concentration, which can quickly change the intensity and polarity of the radiated terahertz pulse, so as to radiate a stronger or terahertz pulse with the opposite polarity. Moreover, the structure is simple, the operation is convenient and multifunctional. Using the mature molecular beam epitaxy technology, multi-layer nano-thin films can be quickly prepared without using the complex micro-nano processing technology for preparing large-aperture photoconductive antennas to generate terahertz pulses, which is beneficial to the production and application of related terahertz devices.

[0090] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A full van der Waals spin terahertz emitter, which comprises: a femtosecond laser, a nanometer thin film, and a voltage source, wherein: the femtosecond laser is used to output pump laser that penetrates the nanometer thin film to generate a first terahertz pulse; the nanometer thin film is a full van der Waals heterostructure constructed by a non-ferromagnetic layer and a ferromagnetic layer; the material of the ferromagnetic layer is a two-dimensional magnetic material; the voltage source is used to regulate the carrier concentration of the non-ferromagnetic layer, regulate the position of the Fermi level of the non-ferromagnetic layer material from the Dirac point to change the intensity of the first terahertz pulse, and generate a second terahertz pulse with opposite polarity; The two-dimensional magnetic material is a two-dimensional van der Waals material with magnetism at room temperature, selected from Fe 4 GeTe 2 , Fe 4+x GeTe 2 , Fe 5 GeTe 2 or a combination of two or more of them; the material of the non-ferromagnetic layer is a van der Waals topological material.

2. The full van der Waals spin terahertz emitter according to claim 1, wherein, the nanometer thin film further contains a substrate and an electrode thin film layer.

3. The full van der Waals spin terahertz emitter according to claim 2, wherein, the nanometer thin film includes a substrate, a ferromagnetic layer, a non-ferromagnetic layer, and an electrode thin film layer arranged in sequence; or, the nanometer thin film includes an electrode thin film layer, a substrate, a non-ferromagnetic layer, and a ferromagnetic layer arranged in sequence; or, the nanometer thin film includes a substrate, a non-ferromagnetic layer, and a ferromagnetic layer arranged in sequence, and at least two electrode thin films are provided on the substrate.

4. The full van der Waals spin terahertz emitter according to any one of claims 1-3, wherein: the positive and negative electrodes of the voltage source are respectively connected to the non-ferromagnetic layer; or, the positive and negative electrodes of the voltage source are respectively connected to the substrate; or, the positive electrode of the voltage source is connected to the substrate, and the negative electrode is connected to the non-ferromagnetic layer.

5. The full van der Waals spin terahertz emitter according to claim 4, wherein, when the positive and negative electrodes of the voltage source are respectively connected to the non-ferromagnetic layer, the positive electrode of the voltage source applies a voltage to the non-ferromagnetic layer through the electrode thin film layer, and the negative electrode of the voltage source is connected to the non-ferromagnetic layer.

6. The full van der Waals spin terahertz emitter according to any one of claims 1-3, wherein: The two-dimensional magnetic material is Fe 4 GeTe 2 .

7. The full van der Waals spin terahertz emitter according to any one of claims 1-3, wherein: the material of the non-ferromagnetic layer is a strong spin-orbit coupling material.

8. The full van der Waals spin terahertz emitter according to claim 7, wherein, The strong spin-orbit coupling material is selected from Bi 2 Se 3 , Bi 2 Te 3 , Bi x Sb 1-x , Sb 2 Te 3 and (Bi x Sb 1-x ) 2 Te 3 or a combination of two or more thereof.

9. The full van der Waals spin terahertz emitter according to claim 8, wherein, The strong spin-orbit coupling material is selected as (Bi x Sb 1-x ) 2 Te 3 .

10. The full van der Waals spin terahertz emitter according to claim 8, wherein, The x value in Bi x Sb 1-x is 0.

9.

11. The full van der Waals spin terahertz emitter according to claim 8 or 9, wherein, The (Bi x Sb 1-x ) 2 Te 3 where the value range of x is 0 - 1.

12. The full van der Waals spin terahertz emitter according to any one of claims 1-3, wherein: the thickness of the ferromagnetic layer is 1-10 nm, and the thickness of the non-ferromagnetic layer is 5-20 nm.

13. The full van der Waals spin terahertz emitter according to any one of claims 1-3, wherein: the femtosecond laser is a femtosecond laser oscillator, a femtosecond laser amplifier, or a fiber femtosecond laser, and the pulse width of the pump laser output by the femtosecond laser is less than 1 ps.

14. The full van der Waals spin terahertz emitter according to claim 2, wherein: The material of the substrate is selected from one of sapphire, gallium arsenide with a high Miller index, strontium titanate, and lead magnesium niobate titanate.

15. A method for regulating the intensity and polarity of terahertz waves, comprising: making a pump laser penetrate a nanometer thin film to generate a first terahertz pulse, wherein the nanometer thin film comprises a full van der Waals heterostructure constructed by a ferromagnetic layer and a non-ferromagnetic layer in contact with each other, and the thicknesses of both the ferromagnetic layer and the non-ferromagnetic layer are on the nanometer scale; the material of the ferromagnetic layer is a two-dimensional magnetic material; regulating the carrier concentration of the non-ferromagnetic layer by using a voltage source to change the Fermi level of the non-ferromagnetic layer so that the spin direction of the ferromagnetic layer changes, and radiating a second terahertz pulse with a polarity opposite to that of the first terahertz pulse from the non-ferromagnetic layer; Among them, the two-dimensional magnetic material is a two-dimensional van der Waals material with magnetism at room temperature, selected from Fe 4 GeTe 2 , Fe 4+ x GeTe 2 , Fe 5 GeTe 2 or a combination of two or more of them; the material of the non-ferromagnetic layer is a van der Waals topological material.

16. The method according to claim 15, wherein, making a pump laser penetrate a nanometer thin film to generate a first terahertz pulse, and: making the pump laser incident perpendicularly to the ferromagnetic layer to generate a first instantaneous spin current in the ferromagnetic layer, then the first instantaneous spin current is transformed into a first charge current in the non-ferromagnetic layer, and a first terahertz pulse is radiated from the non-ferromagnetic layer; or, making the pump laser incident perpendicularly to the non-ferromagnetic layer to generate a second instantaneous spin current in the ferromagnetic layer, then the second instantaneous spin current is transformed into a second charge current in the non-ferromagnetic layer, and a second terahertz pulse with a polarity opposite to that of the first terahertz pulse is radiated from the non-ferromagnetic layer.

17. The method according to claim 16, wherein, the transformation of the first instantaneous spin current into the first charge current, and the transformation of the second instantaneous spin current into the second charge current, and the effect of the charge current on the spin current are related to the position of the Fermi level of the topological material, which is based on the spin Hall effect, the Rashba effect or the topological surface state.

18. The method according to claim 15, wherein: the two-dimensional magnetic material is a strong spin-orbit coupling material.

19. The method according to claim 18, wherein, The strong spin-orbit coupling material is selected from Bi 2 Se 3 , Bi 2 Te 3 , Bi x Sb 1-x , Sb 2 Te 3 and (Bi x Sb 1-x ) 2 Te 3 or a combination of two or more thereof.

20. The method according to claim 19, wherein, The strong spin-orbit coupling material is (Bi x Sb 1-x ) 2 Te 3 .

21. The method according to claim 19, wherein, The Bi x Sb 1-x has an x value of 0.

9.

22. The method according to claim 19, wherein, The (Bi x Sb 1-x ) 2 Te 3 where the value range of x is 0 - 1.

23. The method according to claim 15, wherein, the thickness of the ferromagnetic layer is 1-10 nm, and the thickness of the non-ferromagnetic layer is 5-20 nm.

Images