Graphical wavefront control method for terahertz spin emitter

Abstract

The application belongs to the technical field of terahertz pulse source generation, and provides a patterned wave front regulation method of a terahertz spin emitter. According to the patterned design, the method arranges a plurality of sequential terahertz spin film units and a plurality of reverse sequential terahertz spin film units in the terahertz spin emitter in space to obtain a terahertz light beam with a desired shape, realize spatial phase coding of the terahertz light field, and thus complete wave front regulation. Compared with the prior art, the application has the advantages of strong operability, wide applicability, low cost and the like.

Description

Technical Field

[0001] This invention belongs to the field of terahertz pulse source generation technology, and in particular relates to a graphical wavefront modulation method for a terahertz spin emitter. Background Technology

[0002] Terahertz waves, typically defined as having a frequency range of 0.1–10 THz, are electromagnetic waves situated between microwaves and infrared light. Located in the transition band between low-energy electronics and high-energy photonics, terahertz waves are widely recognized as possessing excellent characteristics, including wide bandwidth, low photon energy, strong coherence, high signal-to-noise ratio, and good security. They are extensively used in communications, radar, security inspection, material identification, and biological detection, holding significant value for national economic and defense development. Over the past 20 years, domestic and international academic communities have conducted extensive research on the transmission mechanism of terahertz waves and their key materials and device technologies, achieving significant breakthroughs in the principles and device technologies of terahertz generation, modulation, and detection, laying a crucial foundation for the development of terahertz application technologies.

[0003] Significant progress has been made in the development of terahertz sources for time-domain spectroscopy. Among these, the method based on spin-electron thin films for generating terahertz radiation combines high electric field strength with good stability and good compatibility with pump lasers of both high and low repetition rates. This method has gained favor among researchers and become a typical approach for terahertz wave emission. Its basic principle involves using femtosecond laser pulses to excite the superdiffusion transport process of hot electrons in ferromagnetic and heavy metal materials, injecting a spin current into the heavy metal material to induce a spin-charge conversion, thereby converting the sub-picosecond spin current into a charge current and achieving terahertz radiation.

[0004] However, existing terahertz pulses often lack spatial structure, failing to meet the requirements of structured optical fields in various applications. Early methods for controlling the optical field of terahertz waves mainly relied on artificial electromagnetic microstructures such as metasurfaces to manipulate the terahertz optical field along the propagation path. However, due to the inherent size of terahertz waves, these devices cannot achieve high integration in practical systems and often suffer from significant losses, making effective control difficult. Summary of the Invention

[0005] Existing terahertz emitters generate terahertz pulses that lack spatial structure, making it difficult to achieve structured beams based on optical field manipulation in the terahertz band. This hinders cutting-edge interdisciplinary applications in fields such as advanced spectroscopy, high-resolution imaging, and high-speed communication. To address these shortcomings, this invention proposes a patterned wavefront manipulation method for terahertz spin emitters. This method, based on a patterned spin-electron thin film, generates a spatially structured terahertz pulse field, thereby achieving wavefront manipulation. It is suitable for compact terahertz spectroscopy and imaging systems for various applications.

[0006] The specific technical solution of the present invention is as follows:

[0007] A graphical wavefront modulation method for a terahertz spin emitter, based on a graphical design, spatially arranges multiple sequential terahertz spin thin film units and multiple inverse terahertz spin thin film units in the terahertz spin emitter to obtain a terahertz beam with a desired shape, thereby realizing spatial amplitude and phase encoding of the terahertz light field.

[0008] The sequential terahertz spin thin film unit has the opposite material layer stacking order to the inverse terahertz spin thin film unit.

[0009] Preferably, the sequential terahertz spin thin film unit comprises at least ferromagnetic and nonmagnetic materials stacked from bottom to top, and the reverse terahertz spin thin film unit comprises at least nonmagnetic and ferromagnetic materials stacked from bottom to top.

[0010] Preferably, the sequential terahertz spin thin film unit comprises at least non-magnetic materials, ferromagnetic materials and non-magnetic materials stacked from bottom to top.

[0011] Preferably, the sequential terahertz spin thin film unit includes at least antiferromagnetic material, ferromagnetic material and nonmagnetic material stacked from bottom to top, and the reverse terahertz spin thin film unit includes at least nonmagnetic material, ferromagnetic material and antiferromagnetic material stacked from bottom to top.

[0012] Preferably, the non-magnetic material is tungsten or platinum.

[0013] Preferably, the ferromagnetic material is cobalt iron boron or nickel iron.

[0014] Preferably, the antiferromagnetic material is iridium-manganese.

[0015] Preferably, the terahertz light field is a dual-beam, a focused terahertz beam, or a focused terahertz vortex beam.

[0016] Preferably, the sequential terahertz spin thin film unit and the reverse terahertz spin thin film unit are prepared by magnetron sputtering.

[0017] Compared with the prior art, the present invention has the following significant advantages:

[0018] 1) The patterned wavefront modulation method of the terahertz spin emitter of this invention utilizes the terahertz spin thin film, which has been extensively and deeply studied, as the emitter of terahertz radiation. The technology is relatively mature and the performance is stable and reliable. It uses the near-far field variation law in optics to generate the output deflection angle or a specific phase distribution from the phase gradient to achieve the focusing function. Through the ingenious application of the basic diffraction theory, it meets the needs of various engineering application scenarios.

[0019] 2) The patterned wavefront modulation method of the terahertz spin transmitter of the present invention is highly operable. It only requires the arrangement of terahertz spin thin film units in a patterned manner and stacked in reverse order to achieve the desired structured terahertz field emission at a relatively precise frequency.

[0020] 3) The patterned wavefront modulation method of the terahertz spin emitter of the present invention is simple to operate and can be realized using commonly used devices in engineering and existing magnetron sputtering technology and secondary photolithography technology. It does not require the use of complex and costly manufacturing processes (such as electron beam exposure) similar to various metamaterials, and overcomes the shortcomings of existing technologies such as high material requirements, complex structure, large size, excessive loss and high cost.

[0021] 4) The pump and probe optical paths required by this invention have low complexity and can be directly applied to existing unstructured terahertz transmitters. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0023] Figure 1 This is a schematic diagram of the structure of the sequential terahertz spin thin film unit in this invention;

[0024] Figure 2 This is a schematic diagram of the structure of the reverse-order terahertz spin thin film unit in this invention;

[0025] Figure 3 It is a graphic sample designed based on positive and negative stacking polarity reversal, which can realize the angular emission of terahertz waves;

[0026] Figure 4 It is a graphic sample designed based on positive and negative stacking polarity reversal, which can realize the emission and focusing of terahertz waves;

[0027] Figure 5It is a graphic sample designed based on the forward and reverse stacking polarity reversal, which can realize the emission of terahertz waves and focus.

[0028] Among them, 1-first component, 2-second component, 3-third component, 4-fused silica substrate. Detailed Implementation

[0029] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0030] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and therefore the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0031] This invention proposes a patterned wavefront manipulation method for terahertz spin emitters, which can efficiently generate terahertz radiation and control the spatial structure of the terahertz field. The terahertz spin thin film used consists of at least two layers of different materials, stacked in a specific order on the surface of a fused silica crystal to form a heterojunction. Currently, a three-layer heterojunction is a typical structure. The terahertz spin thin film is prepared using relatively mature methods such as magnetron sputtering, allowing for diverse material choices for different sample layers.

[0032] Terahertz spin thin films, such as Figure 1-2 As shown, it includes a first component 1, a second component 2, and a third component 3 stacked sequentially, with a fused silica substrate 4 on one side of the first component 1 or the third component 3. The first component 1 is made of ferromagnetic or antiferromagnetic material; the second component 2 is made of ferromagnetic material; and the third component 3 is made of nonmagnetic material.

[0033] One type of terahertz spin thin film is composed of non-magnetic / ferromagnetic / non-magnetic materials, specifically, the first component 1 is a non-magnetic material, the second component 2 is a ferromagnetic material, and the third component 3 is a non-magnetic material. This structure, when laser-pumped, can generate highly efficient terahertz radiation and offers advantages such as ultra-wide bandwidth, low cost, and polarization tunability. The ferromagnetic material can be a readily magnetized ferromagnetic material, such as cobalt-iron-boron or nickel-iron alloys; the non-magnetic material can be a non-magnetic heavy metal or other non-magnetic material, serving to deflect the spin current. Upon irradiation by the pump laser, spin-up and spin-down electrons in the ferromagnetic material are excited to above the Fermi level. Due to the different densities and mobilities of these two types of electrons, a longitudinal spin-polarized current is generated. In the non-magnetic material, the spin-polarized current is deflected under the influence of the inverse spin Hall effect. Because the spin currents flow in opposite directions and have opposite spin Hall angles in the non-magnetic materials on either side of the ferromagnetic material, the charge currents generated in the two non-magnetic material layers flow in the same direction, thus producing terahertz radiation of the same polarity. At a film thickness on the nanometer scale, the signals generated by the two layers are approximately coherently superimposed, thereby enhancing the terahertz radiation. To further increase the intensity of the generated terahertz radiation, materials with larger spin Hall angles, such as W and Pt, can be selected. These materials not only have large spin Hall angle values ​​but also opposite polarities. Therefore, due to the different spin current directions, they can generate terahertz radiation of the same polarity, which is approximately coherently superimposed at a film thickness on the nanometer scale, resulting in a higher intensity of terahertz radiation.

[0034] Another type of terahertz spin thin film is composed of antiferromagnetic / ferromagnetic / nonmagnetic materials. The first component (1) is an antiferromagnetic material, the second component (2) is a ferromagnetic material, and the third component (3) is a nonmagnetic material. This structure can generate strong terahertz radiation even without an external magnetic field and exhibits an exchange bias effect, pinning and deflecting the hysteresis loop of the heterojunction. This allows the ferromagnetic material to reach saturation magnetization without an external magnetic field, meaning it can generate efficient terahertz radiation even without an external permanent magnet. The selection of the antiferromagnetic material is similar to that of the nonmagnetic material; it should have a large spin Hall angle and be opposite in polarity to the nonmagnetic material on the other side, such as iridium-manganese and W, which can generate efficient terahertz radiation without an external magnetic field. This design of a terahertz spin thin film can promote the miniaturization and integration of transmitters, aligning with the trend towards chip-based architecture.

[0035] In the patterned wavefront modulation method of this invention, the terahertz spin thin films are stacked in both sequential and reverse order, such as... Figure 1 and Figure 2As shown, the reverse stacking method alters the direction of the spin current generated by the ferromagnetic material by reversing the stacking order of materials in the terahertz spin thin film, thereby changing the polarity of the terahertz radiation while maintaining the unchanged magnetization state of the ferromagnetic material. The peak electric fields of the terahertz signals generated by sequentially stacked and reverse-stacked terahertz spin thin films are similar, but their polarities are opposite, achieving polarity reversal. The polarity reversal designed in this invention allows the emitted terahertz polarity to correspond to the binary "0" and "1" states. Based on this, through secondary photolithography and patterned design of the terahertz spin emitter sample, the spatial structure of the "0" and "1" states can be effectively realized, thereby achieving different phases of terahertz waves emitted from different positions and realizing spatial phase encoding of the terahertz light field.

[0036] Various functions can be achieved through patterned design of terahertz spin thin films. The following examples illustrate this.

[0037] Example 1: Dual-beam emitter

[0038] like Figure 3 As shown, by alternating "0" and "1" state stripes, a terahertz spin emitter sample can be designed to encode the wavefront phase of the emitted broadband terahertz wave. The design is based on Snell's law. ,in, c The speed of light in a vacuum. For the selected terahertz wave frequency, For the phase of the terahertz wave, This refers to the relative position in space. The emission angle is defined as the deflection angle of the terahertz wave emitted from the sample. This deflection angle is constrained by the phase gradient. The rate of change of the terahertz phase with distance can be calculated based on the desired terahertz wave frequency and the emission deflection angle, allowing for the design of the period width of sequentially and inversely stacked terahertz spin thin film units. This embodiment enables the control of the terahertz wave emission angle.

[0039] Example 2: Focused Terahertz Beam Emitter

[0040] like Figure 4 As shown, the terahertz spin emitter sample in this embodiment utilizes alternating sequential and reverse-sequence terahertz spin thin film units to form numerous concentric circles, creating a structure similar to a Fresnel zone plate. This allows the sample to generate terahertz waves of different polarities that interfere with each other, ultimately achieving a focusing effect at the focal point of the optical axis. The terahertz phase distribution can be calculated based on the following formula. ,in F Focal length x i and yj This represents spatial coordinates. This embodiment enables the control of terahertz wave focusing.

[0041] Example 3: Focused Terahertz Vortex Beam Emitter

[0042] The gradient spatial phase distribution of a pure vortex beam, which is related to its azimuth angle, cannot be generated using binary phase generation. However, by introducing a focusing phase, the vortex beam can acquire a helical spatial phase distribution, such as... Figure 5 As shown in the figure. This embodiment can achieve the emission and focusing of vortex terahertz light.

[0043] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since it corresponds to the method disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section. The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A graphical wavefront modulation method for a terahertz spin emitter, characterized in that, Based on the graphical design, multiple sequential terahertz spin thin film units and multiple reverse-sequence terahertz spin thin film units in the terahertz spin emitter are spatially arranged to obtain a terahertz beam with the desired shape and realize the spatial phase encoding of the terahertz light field. The sequential terahertz spin thin film unit has the opposite material layer stacking order to the inverse terahertz spin thin film unit.

2. The graphical wavefront modulation method for a terahertz spin emitter according to claim 1, characterized in that, The sequential terahertz spin thin film unit comprises at least ferromagnetic and nonmagnetic materials stacked from bottom to top, and the reverse terahertz spin thin film unit comprises at least nonmagnetic and ferromagnetic materials stacked from bottom to top.

3. The graphical wavefront modulation method for a terahertz spin emitter according to claim 1, characterized in that, The sequential terahertz spin thin film unit includes at least non-magnetic materials, ferromagnetic materials and non-magnetic materials stacked from bottom to top. The terahertz spin thin film is composed of at least two different materials, which are stacked on the surface of fused silica crystal in a specific order to form a heterojunction.

4. The graphical wavefront modulation method for a terahertz spin emitter according to claim 1, characterized in that, The sequential terahertz spin thin film unit includes at least antiferromagnetic material, ferromagnetic material and nonmagnetic material stacked from bottom to top, and the reverse terahertz spin thin film unit includes at least nonmagnetic material, ferromagnetic material and antiferromagnetic material stacked from bottom to top.

5. The graphical wavefront modulation method for a terahertz spin emitter according to any one of claims 2-3, characterized in that, The non-magnetic material is tungsten or platinum.

6. The graphical wavefront modulation method for a terahertz spin emitter according to any one of claims 2-3, characterized in that, The ferromagnetic material is cobalt iron boron or nickel iron.

7. The graphical wavefront modulation method for a terahertz spin emitter according to claim 4, characterized in that, The antiferromagnetic material is iridium-manganese or tungsten.

8. The graphical wavefront modulation method for a terahertz spin emitter according to claim 1, characterized in that, The terahertz light field is a dual-beam, a focused terahertz beam, or a focused terahertz vortex beam.

9. The graphical wavefront modulation method for a terahertz spin emitter according to claim 1, characterized in that, Sequential terahertz spin thin film units and inverse terahertz spin thin film units were fabricated using magnetron sputtering technology.

Images