Spin terahertz emitter with adjustable polarization without external magnetic field and manufacturing method thereof

Abstract

The application discloses a spin terahertz emitter with adjustable polarization without an external magnetic field and a manufacturing method thereof. A metal thin film layer heterostructure is deposited on a surface of a glass substrate. A ferromagnetic layer is a FeNi thin film layer, an anti-ferromagnetic layer is an IrMn3 thin film layer, and an interface between the FeNi thin film layer and the IrMn3 thin film layer is an exchange coupling interface subjected to directional magnetic field induction treatment. A surface of the metal thin film layer heterostructure is a periodic stripe array structure. The manufacturing method comprises the following steps: a magnetron sputtering process is used to sequentially deposit a ferromagnetic layer, an anti-ferromagnetic layer and a protective layer; the exchange coupling interface is formed through directional magnetic field induction treatment; and a micro-nano processing process is used for patterning treatment. Beneficial effects are as follows: under the condition of no external magnetic field, reliable spin current generation, spin-charge conversion and terahertz radiation can be realized. The stripe patterning structure realizes linear polarization, elliptical polarization and near-circular polarization output. The 'zero-field emission function' and the 'polarization control function' are integrated in the same thin film device system, and the system complexity is reduced.

Description

Technical Field

[0001] This invention pertains to the technology of electronic devices for generating electromagnetic waves, specifically terahertz transmitters. Background Technology

[0002] Terahertz waves typically refer to electromagnetic waves with frequencies ranging from 0.1 THz to 10 THz, and they hold significant promise for applications in high-speed communication, bioimaging, security detection, non-destructive testing, quantum control, and information processing. In recent years, spin-electron terahertz emitters based on femtosecond laser excitation have become an important research direction in terahertz sources due to their advantages such as ultra-wideband coverage, chip integration capability, simple fabrication, and high conversion efficiency. Existing spin-electron terahertz emitters typically employ a heterostructure of a ferromagnetic layer / strong spin-orbit coupling layer. Under femtosecond laser irradiation, an ultrafast spin current is generated in the ferromagnetic layer. After the spin current is injected into adjacent layers, it is converted into a transient charge current through the inverse spin Hall effect or the inverse Rashba-Edelstein effect, thereby emitting terahertz electromagnetic waves.

[0003] Existing devices often struggle to maintain a stable and repeatable magnetically ordered state in the absence of an external magnetic field, leading to instability in the spin current injection direction and consequently affecting the intensity, polarity, and repeatability of the terahertz radiation signal. Most current spintronic terahertz transmitters rely on an external magnetic field to set or maintain the magnetization direction of the ferromagnetic layer, thereby achieving terahertz signal polarity control and stable output. This approach has the following drawbacks: firstly, it requires the configuration of permanent magnets or electromagnets, resulting in increased system size, structural complexity, and power consumption; secondly, it hinders device miniaturization, on-chip integration, and portable applications; and thirdly, the dependence on the external field significantly limits arraying and engineering implementation.

[0004] On the other hand, in applications such as chiral detection, polarization-sensitive imaging, and quantum information encoding, terahertz sources are required not only to have stable output capabilities but also to have flexible polarization state control. Current terahertz polarization control often relies on liquid crystal devices, external polarization elements, or complex optical structures, resulting in complex systems, high costs, and a development trend inconsistent with high-integration devices. Flexible polarization state control is one of the key issues that urgently needs to be addressed in current terahertz technology. Few existing technologies can simultaneously achieve stable emission without an external field and tunable polarization output, making it difficult to meet the application requirements of functionally integrated terahertz sources.

[0005] Although existing literature has produced spin terahertz transmitters without external magnetic fields and some polarization control schemes, such as those based on synthetic antiferromagnets, exchange-biased heterostructures, or liquid crystal / metasurface integrated devices, there are still problems such as complex material systems, low structural integration, reliance on additional components for polarization control, or insufficient zero-field stability.

[0006] Therefore, there is an urgent need to provide a spin-electron terahertz emitter with a simple structure, no need for an external magnetic field, stable emission of terahertz waves, and further adjustable polarization state. Summary of the Invention

[0007] The technical problem to be solved by this invention is: how to construct a spin-electron terahertz transmitter based on a ferromagnetic / antiferromagnetic exchange-coupled heterostructure, so that it can maintain a stable magnetization orientation and achieve stable terahertz emission under the condition of no external magnetic field; at the same time, how to achieve terahertz polarization state control on the transmitter body through device structure design, so as to obtain an integrated terahertz transmitter with both zero-field operation capability and polarization tunable function.

[0008] The technical solution of the present invention is as follows: A spin terahertz transmitter with adjustable polarization without an external magnetic field, wherein a metal thin film heterostructure is deposited on the surface of a glass substrate. The metal thin film heterostructure consists of a ferromagnetic layer, an antiferromagnetic layer, and a protective layer in sequence. The ferromagnetic layer is a FeNi thin film layer, the antiferromagnetic layer is an IrMn3 thin film layer, and the interface between the FeNi thin film layer and the IrMn3 thin film layer is an exchange coupling interface induced by a directional magnetic field. The surface of the metal thin film heterostructure has a periodic stripe array structure.

[0009] The protective layer is a Ta thin film layer, and the heterostructure of the metal thin film layer is represented as FeNi(t1) / IrMn3(t2) / Ta(t3), where: t1 is the thickness of the FeNi thin film layer, t2 is the thickness of the IrMn3 thin film layer, and t3 is the thickness of the Ta thin film layer.

[0010] Preferably, t1 = 3-20 nm, t2 = 6 nm, and t3 = 3 nm.

[0011] More preferably, t1=5 nm, t2=6 nm, and t3=3 nm.

[0012] The individual stripe size of the periodic stripe structure is 20 μm × 10 mm.

[0013] Through exchange coupling, the magnetization orientation of the FeNi thin film layer can remain aligned along a predetermined direction even after the external magnetic field is removed, thereby forming a stable remanent magnetization state inside the device. Unlike existing terahertz transmitters that rely on an external magnetic field to maintain magnetization orientation, this invention achieves magnetic moment pinning of the ferromagnetic layer through exchange coupling at the FeNi / IrMn3 interface. This allows the sample to maintain a definite spin polarization orientation even without an external magnetic field, providing a stable built-in magnetic order basis for subsequent ultrafast spin current injection and terahertz radiation.

[0014] A method for fabricating a spin terahertz transmitter with tunable polarization without an external magnetic field: Step 1) Clean the glass substrate, dry it, and place it in the magnetron sputtering equipment. Set the background vacuum of the magnetron sputtering equipment to be below 5 × 10⁻⁶. -5 Pa, the working gas pressure during sputtering is 0.2 Pa, a ferromagnetic layer, an antiferromagnetic layer and a protective layer are deposited sequentially. During the deposition process, an in-plane magnetic field of 300 Oe is applied along the x-axis, so that the ferromagnetic layer is magnetized along this direction and tends to saturate during the deposition process. An exchange coupling interface is formed between the FeNi thin film layer and the IrMn3 thin film layer, resulting in a three-layer metal thin film heterostructure with an exchange coupling interface. Step 2) The three-layer metal thin film heterostructure with exchange coupling interface in Step 1) is patterned using micro-nano fabrication technology to form a periodic stripe array structure on the surface. The stripe array consists of multiple stripes with gaps between adjacent stripes and the stripes are arranged along a predetermined direction.

[0015] The micro-nano fabrication process employs photolithography or ion beam etching methods.

[0016] The stripes are arranged in parallel along the long axis, with each stripe having a width of 20 μm and a gap of 20 μm between adjacent stripes.

[0017] Preferably, the thickness of the glass substrate is 0.2 mm.

[0018] The beneficial effects of this invention are as follows: 1. This invention achieves stable pinning of the FeNi magnetic moment under conditions without an external magnetic field through FeNi / IrMn3 interface exchange coupling, enabling the device to reliably generate spin current, perform spin-charge conversion, and emit terahertz radiation even in zero field conditions, avoiding the dependence of traditional spin-electron terahertz emitters on external magnetic fields. It eliminates the need for additional permanent magnets or electromagnets, reducing system complexity, size, and power consumption, thus making it more suitable for on-chip integration, portable applications, and multi-unit array designs.

[0019] 2. This invention creates a striped pattern structure on a heterogeneous metal thin film layer, enabling the device to form different current transport and boundary charge coupling responses in two orthogonal directions. This introduces adjustable amplitude and phase differences in the output terahertz wave, achieving linear polarization, elliptical polarization, and near-circular polarization outputs, thereby improving the functional integration of the device.

[0020] 3. The present invention has a simple structure, is suitable for thin film deposition and conventional micro-nano fabrication processes, and has good fabrication feasibility and engineering implementation potential.

[0021] 4. This invention has a wide range of applications, including polarization-sensitive imaging, chiral substance detection, terahertz communication, quantum information and on-chip terahertz functional devices. Attached Figure Description

[0022] Figure 1(a) is a schematic diagram of terahertz emission from the FeNi-IrMn structure without an external magnetic field. After a femtosecond laser is incident on the FeNi-IrMn structure, the THz wave output is achieved through the SCC mechanism; (b) is the hysteresis loop of the FeNi-IrMn structure; (c) is the time-domain diagram of the terahertz wave of the FeNi-IrMn structure; (d) is the terahertz wave spectrum of the FeNi-IrMn structure. Figure 2 (a) shows the time-domain terahertz signals of the FeNi-IrMn structure under different incident directions when an external magnetic field is applied; (b) shows the time-domain terahertz signals of the FeNi-IrMn structure under different magnetic field directions when an external magnetic field is applied (inset: magnetic signal extraction); (c) shows the magnetic moment distribution of the FeNi-IrMn structure along the y-axis when no external magnetic field is applied; (d) shows the magnetic moment distribution of the FeNi-IrMn structure along the x-axis when no external magnetic field is applied; (e) shows the time-domain terahertz signal corresponding to figure (c); and (f) shows the time-domain terahertz signal corresponding to figure (d). Figure 3 (a) shows the comparison of the time-domain terahertz signals of the FeNi-IrMn structure under the conditions of an applied magnetic field and no applied magnetic field; (b) shows the functional relationship between the terahertz amplitude of the FeNi-IrMn structure and the magnetic field angle; (c) shows the relationship between the terahertz signal of the FeNi-IrMn structure and the sample azimuth angle under the condition of zero magnetic field. Figure 4 To demonstrate the effect of FeNi thickness and stacking order on terahertz emission performance, (a) shows the terahertz emission characteristics of FeNi(t) / IrMn3(6nm) heterostructures with FeNi thin film thickness of 3-20 nm; (b) shows the typical frequency domain terahertz emission signal of the FeNi(t) / IrMn3(6nm) heterostructure; (c) shows the terahertz peak-to-peak amplitude of the FeNi(t) / IrMn3(6nm) heterostructure as a function of FeNi thin film thickness; (d) shows the terahertz emission signal of IrMn3(6nm) / FeNi(t) heterostructures with FeNi thin film thickness of 1.5-8 nm; (e) shows the typical frequency domain terahertz emission signal of the IrMn3(6nm) / FeNi(t) heterostructure; and (f) shows the terahertz peak-to-peak amplitude of the IrMn3(6nm) / FeNi(t) heterostructure as a function of FeNi thin film thickness. Figure 5The following diagrams illustrate the polarization control of terahertz radiation from the FeNi-IrMn structure: (a) is a schematic diagram of the chiral terahertz radiation mechanism of the patterned FeNi-IrMn structure, which is jointly regulated by boundary charge, current dynamics, and capacitive coupling; (b) is the upper diagram showing the terahertz time-domain waveforms of the patterned metal thin film heterostructure measured along the Ex direction under different magnetic field angles φH; the lower diagram shows the corresponding waveforms measured along the Ey direction under the same conditions; (c) shows the relationship between the peak-to-peak amplitudes of the Ex and Ey components of the patterned metal thin film heterostructure and the magnetic field φH; (d) shows the phase difference between the Ex and Ey components of the patterned metal thin film heterostructure when the magnetic field angle is 100°. Detailed Implementation

[0023] This embodiment illustrates the basic structure and manufacturing method of the spin terahertz transmitter with adjustable polarization without an external magnetic field in this invention.

[0024] Basic Structure: A heterostructure of metal thin films is deposited on the surface of a glass substrate. The heterostructure consists of a ferromagnetic layer, an antiferromagnetic layer, and a protective layer. The ferromagnetic layer is a FeNi thin film, and the antiferromagnetic layer is an IrMn3 thin film. The interface between the FeNi and IrMn3 thin films is an exchange-coupled interface induced by a directional magnetic field. The surface of the heterostructure has a periodic stripe array structure. The protective layer is preferably a Ta thin film. The heterostructure is represented as FeNi(t1) / IrMn3(t2) / Ta(t3), where t1 is the thickness of the FeNi thin film, t2 is the thickness of the IrMn3 thin film, and t3 is the thickness of the Ta thin film.

[0025] Fabrication Method: Using a glass substrate (such as Corning glass) as the base material, to improve film adhesion, reduce surface contamination, and ensure the quality of subsequent film deposition, the substrate needs to be cleaned before deposition. Specifically, the substrate can be ultrasonically cleaned sequentially in acetone, anhydrous ethanol, and deionized water to remove organic contaminants and particulate impurities from the substrate surface. After cleaning, it is dried with high-purity nitrogen and placed in a clean environment for later use.

[0026] Multilayer thin film structures were fabricated on glass substrates using magnetron sputtering. The cleaned glass substrate was fixed on the sample stage of the magnetron sputtering equipment. After evacuating to a high vacuum, high-purity argon gas was introduced as the sputtering gas. The base vacuum of the magnetron sputtering equipment was below 5 × 10⁻⁶. -5The working gas pressure during sputtering is 0.2 Pa, and the functional layers are deposited sequentially. First, a FeNi thin film is deposited as a ferromagnetic layer to generate ultrafast spin-polarized charge carriers and form a longitudinal spin current under femtosecond laser excitation. Then, an IrMn3 thin film is deposited on the surface of the FeNi thin film as an antiferromagnetic layer. During the deposition process, an in-plane magnetic field of 300 Oe is applied along the x-axis to magnetize the ferromagnetic layer along this direction and make it tend to saturate. This forms an exchange coupling interface between the FeNi and IrMn3 thin films, which also undertakes the spin-charge conversion function after spin current injection. Finally, a protective layer, preferably a Ta thin film, is deposited on the outermost layer to suppress the oxidation of the thin film structure in air and improve device stability.

[0027] In a preferred embodiment, the prepared multilayer thin film structure is FeNi (5 nm) / IrMn3 (6 nm) / Ta (3 nm). To investigate the effect of ferromagnetic layer thickness on terahertz emission performance, a series of FeNi(t1) / IrMn3 (6 nm) / Ta (3 nm) structure samples can be prepared while keeping the thicknesses of the IrMn3 and Ta thin film layers constant, where t1 is the thickness of the FeNi thin film layer. Furthermore, to compare the effect of different stacking orders on device performance, IrMn3 (6 nm) / FeNi(t1) / Ta (3 nm) structure samples can also be prepared.

[0028] The key to this embodiment is that by forming a stable exchange coupling interface between the FeNi thin film layer and the IrMn3 thin film layer, the magnetization direction of the FeNi thin film layer can still maintain a stable orientation after the external magnetic field is removed. This results in a stable remanent magnetization state inside the device containing this multilayer thin film structure, enabling the device to still have a clear spin polarization direction even without an external magnetic field. This provides a structural basis for subsequent ultrafast spin current injection and terahertz radiation.

[0029] Terahertz emission performance was tested on a series of prepared samples. The tests employed a femtosecond laser-excited terahertz time-domain spectroscopy system. The preferred center wavelength of the femtosecond laser was 800 nm, the preferred pulse width was on the order of femtoseconds, and the preferred repetition frequency was on the order of kilohertz. During testing, the femtosecond laser was focused by an optical system and incident on the sample surface to excite ultrafast carrier transport processes in the sample. The terahertz time-domain waveform of the sample was acquired through electro-optic sampling or other methods suitable for terahertz time-domain detection, and the peak intensity, peak-to-peak value, or spectral intensity were extracted as characterization parameters of the terahertz emission performance.

[0030] Under femtosecond laser irradiation, the electronic system in the FeNi thin film is ultrafast excited. Differences in carrier transport behavior across different spin channels lead to the formation of non-equilibrium spin-polarized carriers, generating a longitudinal ultrafast spin current along the film normal. The spin polarization direction of this spin current is determined by the magnetization direction of the FeNi thin film. When this spin current is injected into the IrMn3 thin film, it is converted into a transient transverse charge current through the inverse spin Hall effect. This transient charge current further radiates broadband terahertz electromagnetic waves outwards.

[0031] To determine a suitable target structure for operation without an external magnetic field, systematic tests were conducted on samples with different FeNi film thicknesses and stacking orders. The test results show that the terahertz emission intensity initially increases and then decreases with varying FeNi film thickness; samples with different stacking orders also exhibit significant differences in emission performance. In the preferred embodiment, the overall terahertz emission performance of the FeNi / IrMn3 structure is superior to that of the IrMn3 / FeNi structure; more preferably, in the FeNi / IrMn3 structure, a strong terahertz emission signal is observed when the FeNi film thickness is approximately 5 nm. Therefore, the FeNi / IrMn3 structure with superior emission performance can be considered as the target structure for subsequent zero-field emission devices and polarization control devices.

[0032] The aforementioned differences primarily stem from the following factors: the thin film stacking order affects the absorption and energy deposition paths of the femtosecond laser in each layer, thus influencing the generation and injection efficiency of spin current; simultaneously, different interface formation methods lead to variations in interface quality and roughness, thereby affecting cross-interface transport efficiency; furthermore, the propagation and absorption losses of the radiated terahertz waves in the multilayer films are also related to the structural order. Therefore, optimizing the ferromagnetic layer thickness and thin film stacking order can effectively improve the terahertz emission performance of the device.

[0033] To verify that the sample still possesses stable emission capability under conditions without an external magnetic field, a zero-field test can be performed on the target sample after the external magnetic field is removed. By comparing the terahertz emission signal intensity, polarity, and repeatability of the sample under conditions with and without an external magnetic field, it can be confirmed that the FeNi / IrMn3 exchange coupling interface has a stable pinning effect on the magnetization direction of the FeNi layer, thereby enabling the device to achieve stable terahertz radiation output under zero-field conditions.

[0034] The FeNi (5 nm) / IrMn3 (6 nm) / Ta (3 nm) sample of the preferred embodiment is patterned using micro / nano fabrication processes to form a periodic stripe array structure on the surface. The micro / nano fabrication processes can employ photolithography and ion beam etching. The fabricated stripe structure is arranged parallel to a predetermined direction, with gaps between adjacent stripes, thereby constructing a distinct anisotropic transport path within the device plane.

[0035] In a preferred embodiment, the heterostructure of the metal thin film layer is processed into a stripe array with its long axis arranged along the x-direction. The size of a single stripe is preferably about 20 μm × 10 mm, and the gap between adjacent stripes is 20 μm. Of course, the present invention is not limited to this specific size, and the stripe width, stripe spacing, duty cycle, arrangement direction, and period parameters can all be adjusted according to the target frequency band and polarization control requirements.

[0036] The patterned device, under femtosecond laser excitation, still follows the aforementioned spin current excitation and spin-charge conversion mechanism: an ultrafast spin current is generated in the FeNi thin film layer, which is injected into the IrMn3 thin film layer and converted into a transient transverse charge current, further radiating terahertz electromagnetic waves. The difference lies in the fact that, since the surface of the heterostructure of the metal thin film layer has been processed into a stripe structure, this transient charge current will exhibit different transport responses in different directions within the device plane.

[0037] Along the long axis of the stripes, due to the continuous distribution of the stripes, electrons can transport relatively freely, exhibiting an approximately resistive response. However, perpendicular to the stripe direction, due to the gaps between adjacent stripes, electrons find it difficult to directly cross the stripe boundaries, easily leading to charge accumulation in the edge regions and resulting in a significant capacitive coupling effect. Since the two orthogonal directions correspond to different equivalent transport characteristics, the two orthogonal components Ex and Ey of the output terahertz wave will exhibit differences in amplitude and phase. The component perpendicular to the stripe direction can produce a phase lag compared to the component parallel to the stripe direction. By rationally designing the stripe structure parameters and adjusting the sample azimuth, magnetization direction, or stripe direction, the amplitude relationship and phase difference between the two orthogonal components Ex and Ey can be changed, thereby achieving continuous control of the output terahertz wave from linear polarization to elliptical polarization and then to near-circular polarization.

[0038] When testing the terahertz polarization performance of a patterned stripe array structure device, the terahertz electric field components in two orthogonal directions can be measured separately, and their amplitude, phase difference, and polarization trajectory can be analyzed. Test results show that, under optimized structural parameters and appropriate test conditions, the two orthogonal components can have similar amplitudes and maintain a phase difference of approximately one-quarter of a period, thus achieving near-circularly polarized terahertz wave output. Therefore, this invention not only enables stable spin terahertz emission without an external magnetic field but also allows for further terahertz polarization state manipulation on the same device platform, forming an integrated device solution with both zero-field emission and polarization manipulation functions. It eliminates the need for external permanent magnets, electromagnets, polarizers, or liquid crystal modulators, effectively reducing system complexity and improving the miniaturization, on-chip integration, and functional integration of the device.

Claims

1. A spin terahertz transmitter with tunable polarization without an external magnetic field, comprising a heterostructure of a metal thin film layer deposited on the surface of a glass substrate, characterized in that, The heterostructure of the metal thin film layer consists of a ferromagnetic layer, an antiferromagnetic layer, and a protective layer. The ferromagnetic layer is a FeNi thin film layer, and the antiferromagnetic layer is an IrMn3 thin film layer. The interface between the FeNi thin film layer and the IrMn3 thin film layer is an exchange coupling interface induced by a directional magnetic field. The surface of the heterostructure of the metal thin film layer is a periodic stripe array structure.

2. The spin terahertz transmitter with adjustable polarization without an external magnetic field according to claim 1, characterized in that, The protective layer is a Ta thin film layer, and the heterostructure of the metal thin film layer is represented as FeNi(t1) / IrMn3(t2) / Ta(t3), where: t1 is the thickness of the FeNi thin film layer, t2 is the thickness of the IrMn3 thin film layer, and t3 is the thickness of the Ta thin film layer.

3. A spin terahertz transmitter with adjustable polarization without an external magnetic field according to claim 2, characterized in that, t1=3-20 nm, t2=6 nm, t3=3 nm.

4. A spin terahertz transmitter with adjustable polarization without an external magnetic field according to claim 2, characterized in that, t1=5 nm, t2=6 nm, t3=3 nm.

5. A spin terahertz transmitter with adjustable polarization without an external magnetic field according to claim 1, characterized in that, The individual stripe size of the periodic stripe structure is 20 μm × 10 mm.

6. A method for manufacturing a spin terahertz transmitter with adjustable polarization without an external magnetic field, as described in any one of claims 1-5, characterized in that: Step 1) Clean the glass substrate, dry it, and place it in the magnetron sputtering equipment. Set the background vacuum of the magnetron sputtering equipment to be below 5 × 10⁻⁶. -5 Pa, the working gas pressure during sputtering is 0.2 Pa, a ferromagnetic layer, an antiferromagnetic layer and a protective layer are deposited sequentially. During the deposition process, an in-plane magnetic field of 300 Oe is applied along the x-axis, so that the ferromagnetic layer is magnetized along this direction and tends to saturate during the deposition process. An exchange coupling interface is formed between the FeNi thin film layer and the IrMn3 thin film layer, resulting in a three-layer metal thin film heterostructure with an exchange coupling interface. Step 2) The three-layer metal thin film heterostructure with exchange coupling interface in Step 1) is patterned using micro-nano fabrication technology to form a periodic stripe array structure on the surface. The stripe array consists of multiple stripes with gaps between adjacent stripes and the stripes are arranged along a predetermined direction.

7. The method for fabricating a spin terahertz transmitter with adjustable polarization without an external magnetic field according to claim 6, characterized in that: The micro-nano fabrication process employs photolithography or ion beam etching methods.

8. The method for fabricating a spin terahertz transmitter with adjustable polarization without an external magnetic field according to claim 6, characterized in that: The stripes are arranged in parallel along the long axis, with each stripe having a width of 20 μm and a gap of 20 μm between adjacent stripes.

9. A method for fabricating a spin terahertz transmitter with adjustable polarization without an external magnetic field according to claim 6, characterized in that: The thickness of the glass substrate is 0.2 mm.

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