Polarization-adjustable antiferromagnetic terahertz radiation source

Abstract

The invention discloses a polarization-adjustable antiferromagnetic terahertz source based on a MnPt heterojunction. The polarization-adjustable antiferromagnetic terahertz source comprises a MnPt antiferromagnetic thin film layer, a light metal layer and an anti-oxidation layer, according to the invention, the femtosecond laser is used for exciting the femtosecond laser to generate the terahertz wave, the polarization direction of the generated terahertz wave is changed along with the change of the polarization direction of the femtosecond laser, that is, the polarization direction of the laser is changed by 1 degree, the polarization direction of the terahertz is changed by 2 degrees, the change of the azimuth angle of a sample is not needed, and the width of the generated terahertz spectrum exceeds 5 THz. No extra heavy metal layer needs to be added, the size is miniature, and the thickness is smaller than 40 nm. By adjusting polarization and phase difference of femtosecond laser, adjustability of a linear polarization state, an elliptical polarization state or a circular polarization state can be achieved, the structure is simple, a needed light path is simple, assistance of an external magnetic field or current is not needed, micromachining is not needed, the ultra-high external magnetic field interference resistance is achieved, the cost is reduced, and the application range is wide. And integration of the terahertz source is also facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of terahertz generation, and in particular to a polarization-adjustable antiferromagnetic terahertz radiation source. Background Art

[0002] Terahertz (THz) generally refers to electromagnetic waves with frequencies between 0.1 and 10 terahertz (THz), lying between microwaves and infrared light. This frequency band has wavelengths ranging from 0.03 to 3 millimeters, placing it somewhere between radio waves and light waves, thus exhibiting a hybrid of both. Terahertz waves possess numerous advantageous properties, and many chemical substances exhibit unique spectral "fingerprints" in the THz band, making them suitable for compositional analysis and identification. Therefore, they hold broad application prospects in fields such as biomedicine, nondestructive testing, and materials research. With the rapid development of ultrafast laser technology in recent years, research on terahertz radiation sources based on femtosecond lasers has made rapid progress, and various femtosecond laser-based terahertz radiation sources have emerged. Among them, spin terahertz sources, due to their unique terahertz generation mechanism, offer advantages such as low cost and high efficiency, making them a key development direction for future terahertz technology. Exciting a ferromagnetic heterojunction with a femtosecond laser can generate a spin current, which is converted into transient charge accumulation through the inverse spin Hall effect, leading to the emission of terahertz waves. This mechanism combines ultrafast excitation of femtosecond lasers with spin dynamics effects, resulting in a broad spectrum of terahertz radiation.

[0003] Spin terahertz radiation sources based on ferromagnetic heterojunctions rely primarily on thermal excitation of ferromagnetic thin films with femtosecond lasers. The femtosecond laser transiently heats the ferromagnetic material, causing it to undergo ultrafast demagnetization, leading to a non-equilibrium state of internal spins and the emission of spin currents into adjacent non-ferromagnetic layers. During this process, the femtosecond laser only exchanges energy with the material, so changing the laser's polarization state cannot affect the polarization state of the generated terahertz radiation. Consequently, optical manipulation of the terahertz polarization state using ferromagnetic materials is difficult. Existing ferromagnetic heterojunction terahertz radiation sources primarily rely on the inverse spin Hall effect to generate terahertz radiation, which necessitates the inclusion of heavy metal thin film layers, posing challenges to large-scale production. The need for an external magnetic field to generate terahertz radiation from ferromagnetic heterojunctions complicates device integration and introduces additional interference fields into the system. These issues pose significant challenges to the development of spin terahertz sources.

[0004] On the other hand, most of the current terahertz sources produce linear polarization, and relatively few produce circularly polarized terahertz sources. The development of terahertz polarization control devices is relatively lagging. The solutions for terahertz polarization control include the following: (1) Using a wire grid structure to generate circularly polarized terahertz. This method changes the polarization state of the emitted terahertz by etching the ferromagnetic heterojunction into wire grid stripes. This method can achieve circular polarization with a wide bandwidth, but the generation efficiency is low and an additional micromachining process is introduced, which is costly. (2) Using terahertz wave plates or metamaterials to control terahertz polarization. This method is more flexible, but it introduces additional devices. At the same time, the processing of metamaterials requires complex micro-nano processing technology, which increases the cost. (3) Utilizing the nonlinear optical rectification effect of nonlinear crystals (such as LiNbO3 and ZnTe), circularly polarized terahertz waves can be generated under optical excitation by adjusting the crystal orientation. However, the thickness of optical crystals is generally around 1 mm, and terahertz waves can only be generated in specific crystal orientations, making them difficult to integrate into complex circuits. (4) Using topological insulators such as Bi2Te3, the generated terahertz polarization state can also be changed by changing the laser polarization state, but the terahertz spectrum it produces is limited to 1 THz-2 THz, and it is necessary to change the linear polarization direction of the terahertz by rotating the sample azimuth angle, which limits the application scenarios.

[0005] (5) Currently, only NiO heterojunctions in antiferromagnetic systems can achieve terahertz polarization control by rotating laser polarization. However, the terahertz emission of NiO heterojunctions is based on the inverse spin Hall effect and must rely on a heavy metal layer. Moreover, since NiO is an oxide antiferromagnetic material, the utilization rate of photon energy is very low, and the efficiency of generating spin current is low, making it impossible to put it into practical application. Summary of the Invention

[0006] To address the shortcomings of the aforementioned existing technologies, the present invention proposes a polarization-tunable antiferromagnetic terahertz radiation source. This source generates terahertz waves by exciting them with a femtosecond laser. Adjusting the polarization and phase difference of the femtosecond laser enables adjustment of the polarization state to linear, elliptical, or circular. The present invention boasts a simple structure, can be adjusted as needed, offers robust and reliable performance, and is easily integrated with other devices, meeting various polarization adjustment requirements.

[0007] The polarization-adjustable antiferromagnetic terahertz radiation source of the present invention is an antiferromagnetic heterojunction consisting of an antiferromagnetic thin film layer, a light metal thin film layer and an anti-oxidation layer. The antiferromagnetic terahertz radiation source is formed by sequentially growing the antiferromagnetic thin film layer, the light metal thin film layer and the anti-oxidation layer from bottom to top on a substrate by magnetron sputtering.

[0008] The antiferromagnetic film is made of Mn 50 Pt 50，Abbreviated as MnPt, with a thickness of 6 nm-20 nm. The light metal film includes but is not limited to materials such as Ti and Al, with a thickness of 2 nm-10 nm. The anti-oxidation layer can be magnesium oxide, aluminum oxide, etc.

[0009] The antiferromagnetic terahertz radiation source can generate linearly polarized terahertz radiation under femtosecond laser excitation, and the polarization direction of the generated terahertz wave changes with the change of the polarization direction of the femtosecond laser, that is, the laser polarization direction changes by 1° and the terahertz polarization direction changes by 2°. This polarization control characteristic is extremely rare in antiferromagnetic materials and currently only exists in NiO. However, NiO requires an additional heavy metal layer to generate terahertz radiation. The MnPt antiferromagnetic terahertz source described in the present invention generates terahertz based on the orbital charge conversion theory, without the need for an additional heavy metal layer. It can achieve efficient terahertz emission by relying solely on cheap light metal layers such as Ti and Al. The above advantages have not yet been found in other antiferromagnetic materials. In addition, MnPt is a metal alloy collinear antiferromagnetic material, which is not affected by the external magnetic field and can effectively absorb photon energy. The terahertz generation efficiency far exceeds that of NiO.

[0010] The MnPt antiferromagnetic terahertz radiation source can control the terahertz linear polarization direction by simply changing the polarization angle of the linearly polarized laser while keeping the sample azimuth angle fixed. However, the terahertz linear polarization direction generated by topological insulators such as Bi2Te3 can only be controlled by rotating the sample azimuth angle, which makes it difficult to manipulate once integrated into a complex circuit. In addition, the terahertz spectrum width of the MnPt antiferromagnetic terahertz radiation source exceeds 5 THz, far exceeding the spectrum width of Bi2Te3.

[0011] The total thickness of an antiferromagnetic terahertz source does not exceed 40 nm, significantly thinner than nonlinear crystalline materials. MnPt can be arranged in any crystal orientation, and its terahertz generation performance is identical in all orientations, making it easy to integrate into integrated circuit chips. In contrast, nonlinear crystals such as ZnTe are only about 1 mm thick, and their terahertz emission is only strong in certain crystal orientations, with virtually no terahertz generation in other orientations.

[0012] Furthermore, the antiferromagnetic terahertz source can also achieve high-quality circularly polarized excitation through an existing dual-pump optical path system, which is used to split a femtosecond laser light source. By adjusting the optical delay lines in the two optical paths, the optical path difference between the two beams is adjusted. This adjustment ensures that the terahertz wave pulses generated by the two laser beams have a 90° phase difference. When the x- and y-polarized terahertz waves have the same amplitude and a 90° phase difference, they are combined into a high-quality circularly polarized terahertz wave. This entire process does not require the application of an external electric or magnetic field.

[0013] In addition to generating circularly polarized terahertz waves, the antiferromagnetic heterojunction terahertz source can also generate linearly polarized or elliptically polarized terahertz waves by adjusting the polarization and power of the laser.

[0014] The beneficial effects of the present invention are: 1. The terahertz radiation source of the present invention has a simple structure and low manufacturing cost. Compared with microstructures such as wire grids and metamaterials, it does not require micromachining processes such as lithography and etching, thereby simplifying the manufacturing process.

[0015] 2. The terahertz radiation source of the present invention can generate terahertz waves in a variety of polarization states, including linear polarization, elliptical polarization, or circular polarization, and the switching method is simple and flexible. Compared with materials such as Bi2Te3, there is no need to adjust the sample azimuth angle, and only the polarization direction of the linearly polarized laser needs to be adjusted to achieve this.

[0016] 3. The terahertz regulation of linear polarization, elliptical polarization or circular polarization can be achieved without the assistance of an external magnetic field, and will not be disturbed by the external magnetic field. The radiation source is small, less than 40 nm, which is conducive to integration into other systems and has extremely high stability.

[0017] 4. The terahertz radiation source of the present invention has a wide frequency bandwidth. The actual test results show that the generated terahertz spectrum range exceeds 5 THz.

[0018] 5. The terahertz radiation source of the present invention does not require the introduction of an additional heavy metal layer, which reduces production costs and is beneficial to environmental protection.

[0019] 6. The generation performance of antiferromagnetic terahertz sources in different crystal orientations is the same. These characteristics make it easy to integrate into integrated circuit chips and easy to apply. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. By referring to the drawings, the features and advantages of the present invention will be more clearly understood. The drawings are schematic and should not be understood as limiting the present invention in any way. Those skilled in the art can derive other drawings based on these drawings without inventive effort. Among them: Figure 1 Schematic diagram of the membrane structure of the present invention; Figure 2 is the terahertz signal generated under different external magnetic fields; Figure 3 is the relationship between the generated terahertz wave and the laser polarization angle; Figure 4 is the generated terahertz spectrum; Figure 5The terahertz signals of MnPt / Ti / MgO heterojunctions with different crystal orientations; Figure 6 Schematic diagram of the device for generating circular polarization; In the figure: 1-first femtosecond laser beam; 2-second femtosecond laser beam; 3-antiferromagnetic heterojunction; 4-circularly polarized terahertz wave. DETAILED DESCRIPTION In order to more clearly understand the above-mentioned objects, features and advantages of the present invention, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that the embodiments of the present invention and the features therein can be combined with each other without conflict.

[0021] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the scope of protection of the present invention is not limited to the specific embodiments disclosed below.

[0022] The antiferromagnetic terahertz radiation source of the present invention is an antiferromagnetic heterojunction 3 composed of an antiferromagnetic thin film layer, a light metal thin film layer, and an anti-oxidation layer. The antiferromagnetic terahertz radiation source is formed by sequentially growing the antiferromagnetic thin film layer, the light metal thin film layer, and the anti-oxidation layer from bottom to top on a substrate by magnetron sputtering.

[0023] The antiferromagnetic film is MnPt with a thickness of 6 nm-20 nm. The light metal film includes but is not limited to Ti, Al and other materials with a thickness of 2 nm-10 nm. The anti-oxidation layer can be magnesium oxide, aluminum oxide and the like.

[0024] The antiferromagnetic terahertz radiation source can generate linearly polarized terahertz radiation under femtosecond laser excitation, and the polarization direction of the generated terahertz wave changes with the change of the polarization direction of the femtosecond laser, that is, the laser polarization direction changes by 1° and the terahertz polarization direction changes by 2°. This polarization control characteristic is extremely rare in antiferromagnetic materials and currently only exists in NiO. However, NiO requires an additional heavy metal layer to generate terahertz radiation. The MnPt antiferromagnetic terahertz source described in the present invention generates terahertz based on the orbital charge conversion theory, without the need for an additional heavy metal layer. It can achieve efficient terahertz emission by relying solely on cheap light metal layers such as Ti and Al. The above advantages have not yet been found in other antiferromagnetic materials. In addition, MnPt is a metal alloy collinear antiferromagnetic material, which is not affected by the external magnetic field and can effectively absorb photon energy. The terahertz generation efficiency far exceeds that of NiO.

[0025] The MnPt antiferromagnetic terahertz radiation source can control the terahertz linear polarization direction by simply changing the polarization angle of the linearly polarized laser while keeping the sample azimuth angle fixed. However, the terahertz linear polarization direction generated by topological insulators such as Bi2Te3 can only be controlled by rotating the sample azimuth angle, which makes it difficult to manipulate once integrated into a complex circuit. In addition, the terahertz spectrum width of the MnPt antiferromagnetic terahertz radiation source exceeds 5 THz, far exceeding the spectrum width of Bi2Te3.

[0026] The total thickness of an antiferromagnetic terahertz source does not exceed 40 nm, significantly thinner than nonlinear crystalline materials. MnPt can be oriented in any direction, and its terahertz generation performance is identical in all orientations, making it easy to integrate into integrated circuit chips. In contrast, nonlinear crystals such as ZnTe are only about 1 mm thick, and their terahertz emission is only strong in certain orientations, with virtually no terahertz generation in other orientations.

[0027] The antiferromagnetic terahertz source can also achieve high-quality circularly polarized excitation through an existing dual-pump optical path system, used to split a femtosecond laser light source. By adjusting the optical delay lines in the two optical paths, the optical path difference between the two beams is adjusted. This adjustment ensures that the terahertz wave pulses generated by the two laser beams have a 90° phase difference. When the x- and y-polarized terahertz waves have the same amplitude and a 90° phase difference, they are combined into a high-quality circularly polarized terahertz wave. This entire process does not require the application of an external electric or magnetic field.

[0028] In addition to generating circularly polarized terahertz waves, the antiferromagnetic heterojunction terahertz source can also generate linearly polarized or elliptically polarized terahertz waves by adjusting the polarization and power of the laser.

[0029] The following is a detailed description with reference to the accompanying drawings: Figure 1 It is the basic structure of antiferromagnetic heterojunction. Femtosecond laser is incident from the substrate side, and the antiferromagnetic layer, light metal layer and anti-oxidation layer are grown on the substrate in sequence. The terahertz wave is emitted from the anti-oxidation layer side.

[0030] Figure 2 The terahertz wave signals under different magnetic fields are measured experimentally. For external magnetic fields in different directions, the terahertz signal has almost no change, indicating that the antiferromagnetic heterojunction has a strong ability to resist interference from external magnetic fields.

[0031] Figure 3The relationship between the horizontal component of the detected terahertz wave peak and the polarization of the linearly polarized laser is shown, with a variation period of 180°. The results show that the polarization direction of the generated terahertz wave changes with the polarization direction of the femtosecond laser, that is, a 1° change in the laser polarization direction results in a 2° change in the terahertz polarization direction. The azimuth angle of the sample remains unchanged throughout the entire process, demonstrating the efficient control of the polarization direction of the terahertz wave generated by the antiferromagnetic heterojunction by linear laser polarization. Figure 4 This is the terahertz spectrum diagram generated by the MnPt antiferromagnetic terahertz radiation source. Compared with Bi2Te3 and ZnTe crystals, the spectrum of MnPt is wider, exceeding 5 THz.

[0032] Figure 5 The terahertz signals generated by the MnPt / Ti / MgO antiferromagnetic terahertz sources with different crystal orientations have the same terahertz performance for different crystal orientations. This result proves that efficient terahertz emission can be achieved using light metals. In summary, the polarization-adjustable antiferromagnetic terahertz radiation source of the present invention can manipulate the polarization direction of the terahertz generated by the antiferromagnetic heterojunction by changing the linear polarization direction of the femtosecond laser. There is no need to change the sample azimuth angle, no external magnetic field or current assistance, no need to add an additional heavy metal layer, it is not limited to a specific crystal orientation, and no micro-nano processing is required. It also has extremely strong resistance to external magnetic field interference, which not only reduces costs but also facilitates the integration of terahertz sources.

[0033] Furthermore, if Figure 6 As shown, the antiferromagnetic terahertz source can also achieve high-quality circularly polarized excitation through an existing dual-pump optical system. This is achieved by splitting the femtosecond laser light source into a first femtosecond laser beam 1 and a second femtosecond laser beam 2, which are incident on an antiferromagnetic heterojunction 3, respectively. By adjusting the optical path difference between the first femtosecond laser beam 1 and the second femtosecond laser beam 2, the terahertz wave pulses generated by the two laser beams have a 90° phase difference. When the x- and y-polarized terahertz waves have the same amplitude and a 90° phase difference, they are combined into a high-quality circularly polarized terahertz wave 4. By adjusting the polarization and power of the lasers, linearly or elliptically polarized terahertz waves 4 can also be generated.

[0034] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A polarization-tunable antiferromagnetic terahertz radiation source, characterized in that: The antiferromagnetic terahertz radiation source is formed by sequentially growing the antiferromagnetic film layer, the light metal film layer and the anti-oxidation layer on the substrate from bottom to top by magnetron sputtering. The substrate used includes magnesium oxide and strontium titanate, and the crystal orientation of the substrate surface can be any crystal orientation. Terahertz waves are generated by exciting a terahertz radiation source with a femtosecond laser. By adjusting the polarization and phase difference of the femtosecond laser, the linear polarization state, elliptical polarization state or circular polarization state can be adjusted.

2. The polarization-adjustable antiferromagnetic terahertz radiation source according to claim 1, characterized in that: The antiferromagnetic film layer is made of Mn 50 Pt 50 , with a thickness of 6 nm-20 nm, the light metal film includes Ti and Al materials, with a thickness of 2 nm-10 nm; the anti-oxidation layer is magnesium oxide and aluminum oxide.

3. The polarization-adjustable antiferromagnetic terahertz radiation source according to claim 1, characterized in that: The antiferromagnetic terahertz radiation source generates terahertz radiation through femtosecond laser excitation. The generation process does not require the application of an external magnetic field. The polarization direction of the generated terahertz wave changes with the change of the polarization direction of the femtosecond laser, that is, the laser polarization direction changes by 1°, and the terahertz polarization direction changes by 2°. There is no need to change the sample azimuth angle. In addition, the rotation direction of the circularly polarized laser has no effect on the generation of terahertz.

4. The polarization-adjustable antiferromagnetic terahertz radiation source according to claim 1, characterized in that: High-quality circularly polarized terahertz emission is achieved through the existing dual-pump optical system. By controlling the polarization directions of two femtosecond laser beams, the antiferromagnetic terahertz source generates terahertz waves polarized in the x-direction and y-direction respectively. The laser power is adjusted to ensure that the amplitudes of the terahertz pulses generated by the two beams are equal; By adjusting the optical path difference, the phase difference between the two terahertz wave pulses is controlled at 90°, thereby synthesizing high-quality circularly polarized terahertz.

5. The polarization-adjustable antiferromagnetic terahertz radiation source according to claim 1, characterized in that: By adjusting the polarization and power of two femtosecond laser beams, linear polarization, elliptically polarized or other arbitrary polarized terahertz waves can be achieved.

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