A polarization-adjustable multi-cycle terahertz pulse generation method

Abstract

The application discloses a polarization-adjustable multi-period terahertz pulse generation method, and relates to the technical field of terahertz, and comprises the following steps: femtosecond laser emitted by a femtosecond laser is expanded into a large light spot and is collimated after passing through a concave lens and a first off-axis parabolic mirror, and irradiates on a terahertz source film in a terahertz pulse source, so that a terahertz pulse train with a certain time delay is generated, and then the terahertz pulse train is focused through a second off-axis parabolic mirror, and a multi-period terahertz pulse with spatial coincidence is formed at a focal point. The terahertz source film is distributed along the steps through the insulating stepped substrate, and the polarization state of the multi-period terahertz pulse can be changed by changing the magnetic field applied to the terahertz source film through an external magnetic field, and the center frequency of the multi-period terahertz pulse can be changed by changing the step height; and the application has the advantages of simple structure, convenient operation, polarization adjustment, and large frequency adjustment range.

Description

Technical Field

[0001] This invention relates to the field of terahertz technology, and more specifically to a method for generating multi-cycle terahertz pulses with adjustable polarization. Background Technology

[0002] Terahertz (THz) waves are electromagnetic waves with frequencies ranging from 0.1 THz to 10 THz, falling between millimeter waves and infrared light, with a corresponding timescale of picoseconds. Terahertz waves have broad application prospects, including terahertz radar and communication, spectroscopy and imaging, and security detection. Furthermore, many condensed matter physics phenomena, cosmic microwave background radiation, and biological macromolecules exhibit characteristic frequencies in the terahertz band or characteristic timescales in the picosecond range; therefore, terahertz spectroscopy has become a powerful tool for scientific research.

[0003] Terahertz waves are classified into continuous waves and pulsed waves. Continuous waves are generally generated by electronic devices such as Schottky diode frequency doublers, quantum cascade lasers, and resonant tunneling diodes; pulsed waves are generally generated by the interaction of femtosecond lasers with matter, primarily through photoconductive antennas, electro-optic crystals, ferromagnetic heterojunction thin films, air, and water. Terahertz pulses are further classified into sub-periodic, single-periodic, and multi-periodic types based on the number of oscillation periods of their time-domain electric field. The direct interaction of femtosecond lasers with the aforementioned matter typically produces single-period terahertz pulses.

[0004] There are currently a few main methods for generating multi-period terahertz pulses: (1) Femtosecond laser is incident on a periodically polarized lithium niobate crystal and interacts with it to generate the pulse, but this method has low generation efficiency and is difficult to process; (2) Chirped pulse beat frequency technology, which involves delaying two chirped laser pulses in time to form a pulse train with time intensity modulation, and then applying it to a periodically polarized lithium niobate crystal or a regular lithium niobate crystal, but its operation is complex and its generation efficiency is low; (3) Laser beam splitting technology transforms a single femtosecond laser into a laser pulse train with a certain time delay but not separated in the transverse space, and then irradiates an electro-optic crystal to generate multi-period terahertz pulses, but its operation is complex.

[0005] Due to the limitations of terahertz crystals, the above methods result in narrow terahertz frequencies across multiple periods, with a frequency adjustment range of less than 3 THz. Furthermore, the polarization of all of these methods is not adjustable.

[0006] Therefore, it is of great significance to develop a multi-period terahertz pulse generation method that is simple to operate, has a wide range of adjustable frequencies, and adjustable polarization. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention aims to provide a polarization-tunable multi-cycle terahertz pulse generation method. This method utilizes a stepped insulating substrate to distribute terahertz source films along the steps. By applying an external magnetic field, the polarization state of the multi-cycle terahertz pulse can be altered by changing the magnetic field applied to the terahertz source films. Furthermore, the center frequency of the multi-cycle terahertz pulse can be changed by altering the step height. The method is simple in structure, convenient to operate, polarization-tunable, and has a wide frequency adjustment range.

[0008] This invention is achieved through the following technical solution:

[0009] A method for generating polarization-tunable multi-cycle terahertz pulses includes the following steps:

[0010] A femtosecond laser, a concave lens, a first off-axis parabolic mirror, a terahertz pulse source, and a second off-axis parabolic mirror are respectively set up, and a magnetic field is applied to the terahertz pulse source by an external magnetic field.

[0011] The femtosecond laser is activated to emit laser light into the concave lens, and the light rays are diverged by the concave lens into the reflecting surface of the first off-axis parabolic mirror for collimation.

[0012] Subsequently, the first off-axis parabolic mirror reflects the parallel light through the reflecting surface and irradiates the stepped surface of the terahertz pulse source perpendicularly; wherein, the terahertz pulse source includes an insulating stepped substrate, one side of the insulating stepped substrate is a stepped surface with multiple steps, and a terahertz source film is deposited on the width surface of each step, and the parallel light irradiates the terahertz source film on the width surface perpendicularly.

[0013] Parallel light passes through a terahertz source thin film to generate a terahertz pulse train with a certain time delay, and then enters the reflecting surface of the second off-axis parabolic mirror. The light is focused by the reflecting surface of the second off-axis parabolic mirror to finally obtain a multi-cycle terahertz pulse.

[0014] Compared to existing technologies, which suffer from narrow frequency range, limited frequency adjustment range, and non-adjustable polarization in multi-cycle terahertz pulses, this invention provides a polarization-tunable multi-cycle terahertz pulse generation method. This method utilizes a stepped insulating substrate to distribute terahertz source films along the steps. By applying an external magnetic field, the polarization state of the multi-cycle terahertz pulse can be altered by changing the magnetic field applied to the terahertz source films. Furthermore, the center frequency of the multi-cycle terahertz pulse can be changed by altering the step height. This method is simple in structure, easy to operate, polarization-tunable, and has a wide frequency adjustment range. The specific design includes a femtosecond laser, a concave lens, a first off-axis parabolic mirror, a terahertz pulse source, a second off-axis parabolic mirror, and an external magnetic field. The region of diverging light from the concave lens is located within the reflecting surface of the first off-axis parabolic mirror, and the parallel light reflected by the first off-axis parabolic mirror is perpendicular to the incident light from the femtosecond laser. The terahertz pulse source includes an insulating stepped substrate, the sides of which are stepped surfaces formed by multiple steps. The parallel light reflected by the first off-axis parabolic mirror is located within the stepped surface. A terahertz source thin film is deposited on the width surface of each step, and a terahertz source thin film can also be deposited on the height surface to convert the incident parallel light into a terahertz pulse train. The incident range of the terahertz pulse train is located within the reflecting surface of the second off-axis parabolic mirror, so that it is focused by the second off-axis parabolic mirror to form a multi-period terahertz pulse. In this invention, an external magnetic field can be applied to the terahertz pulse source. By changing the magnetic field, the polarization state of the multi-cycle terahertz pulse can be altered. Furthermore, by changing the step height, the center frequency of the multi-cycle terahertz pulse can be changed, and the adjustment range is relatively large. The specific steps are as follows: The femtosecond laser emitted from the femtosecond laser passes through a concave lens and a first off-axis parabolic mirror, where it is expanded into a large spot and collimated. This spot then irradiates the terahertz source film in the terahertz pulse source, generating a terahertz pulse train with a certain time delay. This train is then focused by a second off-axis parabolic mirror, forming spatially overlapping multi-cycle terahertz pulses at the focal point.

[0015] For further optimization, the insulating stepped substrate is made of one of the following materials: high-resistivity silicon, Al2O3, MgO, SiO2, TiO2, SrTiO3, PET, PEN, ZrO2, ZnO, LaAlO3, GaN, GGG, diamond, and silicon carbide.

[0016] In a further optimization, the insulating stepped substrate has a step width of 2-1000 μm, a step height of 10-100 μm, and a step number of 10-10000. Changing the step height alters the center frequency of the multi-cycle terahertz pulse; while changing the step width and the number of cycles alters the number of cycles of the multi-cycle terahertz pulse.

[0017] In a further optimization, the terahertz source thin film includes, from bottom to top, [high refractive index dielectric thin film / low refractive index dielectric thin film]n, a first non-magnetic nanofilm, a magnetic nanofilm, and a second non-magnetic nanofilm;

[0018] Wherein, [high refractive index dielectric film / low refractive index dielectric film]n represents n periods of alternating high refractive index dielectric films and low refractive index dielectric films; n is an integer from 4 to 10;

[0019] The spin Hall angles of the first and second non-magnetic nanofilms have opposite signs. In this scheme, by selecting the thickness and number of periods of the high-refractive-index and low-refractive-index dielectric films, the energy of the femtosecond laser is absorbed by the first, magnetic, and second non-magnetic nanofilms, thereby maximizing the generation of terahertz waves. Since the terahertz spectrum generated by the ferromagnetic heterojunction nanofilm is greater than 10 THz, meaning that the first, magnetic, and second non-magnetic nanofilms have an intrinsic ability to generate waves greater than 10 THz, the center frequency adjustment range of the multi-cycle terahertz is greater than 10 THz and can be adjusted within this range. Furthermore, the thickness range of the high-refractive-index and low-refractive-index dielectric films is 50-200 nm.

[0020] For further optimization, the high refractive index dielectric film is one of TiO2, Ta2O5, ZnS, ZnSe and HfO2.

[0021] For further optimization, the low refractive index dielectric film is made of one of SiO2, YbF3 and MgF2.

[0022] For further optimization, the magnetic nanofilm is made of ferromagnetic or subferromagnetic material;

[0023] The ferromagnetic material is a single component of Fe, Co, or Ni or an alloy thereof, or an alloy in which B, Si, etc. are added to a single component of Fe, Co, or Ni or an alloy thereof, such as CoFeB or Fe3Si.

[0024] The ferrimagnet is one of YIG, Fe3O4, GdFeCo, GdCo5, DyCo5, TbFe2, and BaFe12O19.

[0025] For further optimization, both the first and second non-magnetic nanofilms can be made of one of the following: non-magnetic metal films, topological insulator films, Weyl semimetal films, and two-dimensional materials.

[0026] For further optimization, the non-magnetic metal thin film is selected from one of Pt, W, Pd, Ta, Bi, Cr, Ir, IrMn, PtMn, PdMn, FeMn, AuPt, AuW, CuBi, CuIr and CuPb;

[0027] The topological insulator film is one of Bi2Se3, Bi2Te3, Bi2Se2Te(BST), Bi2Te2Se, Sn-dopedBi2Te2Se, BiSbTeSe, (BixSb1-x)2Te3, and α-Sn;

[0028] The Weyl semimetallic thin film is one of TaAs, TaP, NbAs, NbP, WTe2, MoTe2 and ZrSiS;

[0029] The two-dimensional material is either graphene or MoS2.

[0030] In a further optimization, the thicknesses of the first non-magnetic nanofilm, the magnetic nanofilm, and the second non-magnetic nanofilm are all between 0.1 and 10 nanometers.

[0031] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0032] This invention provides a polarization-tunable multi-cycle terahertz pulse generation method. A terahertz source film is distributed along a stepped insulating substrate. By applying an external magnetic field, the polarization state of the multi-cycle terahertz pulse can be changed by altering the magnetic field applied to the terahertz source film. The center frequency of the multi-cycle terahertz pulse can be changed by altering the step height, and the number of cycles can be changed by altering the step width and the number of cycles. The method is simple in structure, easy to operate, polarization-tunable, and has a wide frequency adjustment range. Attached Figure Description

[0033] To more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be considered as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort. In the drawings:

[0034] Figure 1 This is a schematic diagram showing the overall distribution of the components provided by the present invention;

[0035] Figure 2 The structural diagram of the terahertz pulse source provided by the present invention;

[0036] Figure 3A schematic diagram of multi-cycle terahertz pulses at different step heights provided by the present invention;

[0037] Figure 4 Provided by the present invention Figure 3 A schematic diagram of the spectrum corresponding to each multi-cycle terahertz pulse.

[0038] The attached diagram shows the markings and corresponding component names:

[0039] 1- Femtosecond laser, 2- Concave lens, 3- First off-axis parabolic mirror, 4- Terahertz pulse source, 5- Second off-axis parabolic mirror, 6- Terahertz pulse train, 7- Multi-cycle terahertz pulse, 8- Insulating stepped substrate, 9- Terahertz source thin film, 10- External magnetic field. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0041] Example: This example provides a polarization-tunable multi-cycle terahertz pulse generation method, such as... Figures 1-4 As shown, it includes the following steps:

[0042] A femtosecond laser 1, a concave lens 2, a first off-axis parabolic mirror 3, a terahertz pulse source 4, and a second off-axis parabolic mirror 5 are respectively set up, and a magnetic field is applied to the terahertz pulse source 4 through an external magnetic field 10.

[0043] The femtosecond laser 1 is activated to emit laser light into the concave lens 2, and the light rays are diverged by the concave lens 2 into the reflecting surface of the first off-axis parabolic mirror 3 for collimation.

[0044] Subsequently, the first off-axis parabolic mirror 3 reflects the parallel light through the reflecting surface and irradiates the stepped surface of the terahertz pulse source 4 perpendicularly; wherein, the terahertz pulse source 4 includes an insulating stepped substrate 8, one side of the insulating stepped substrate 8 is a stepped surface with multiple steps, and a terahertz source film 9 is laid on the width surface of each step, and the parallel light irradiates the terahertz source film 9 on the width surface perpendicularly.

[0045] Parallel light passes through the terahertz source thin film 9 to generate a terahertz pulse train 6 with a certain time delay, and enters the reflecting surface of the second off-axis parabolic mirror 5. The reflecting surface of the second off-axis parabolic mirror 5 focuses the light to finally obtain a multi-cycle terahertz pulse 7.

[0046] Compared to existing technologies, which suffer from narrow frequency range, limited frequency adjustment range, and non-adjustable polarization in multi-cycle terahertz pulses, this invention provides a polarization-tunable multi-cycle terahertz pulse generation method. This method utilizes a stepped insulating substrate 8 to distribute a terahertz source film 9 along the steps. By applying an external magnetic field 10, the polarization state of the multi-cycle terahertz pulse 7 can be altered by changing the magnetic field applied to the terahertz source film 9. Furthermore, the center frequency of the multi-cycle terahertz pulse 7 can be changed by altering the step height. This method is simple in structure, convenient to operate, polarization-tunable, and has a wide frequency adjustment range. The specific scheme includes a femtosecond laser 1, a concave lens 2, a first off-axis parabolic mirror 3, a terahertz pulse source 4, a second off-axis parabolic mirror 5, and an external magnetic field 10. The diverging light region of the concave lens 2 is located within the reflecting surface of the first off-axis parabolic mirror 3, and the parallel light reflected by the first off-axis parabolic mirror 3 is perpendicular to the incident light of the femtosecond laser 1. The terahertz pulse source 4 includes an insulating stepped substrate 8, the side of which is a stepped surface formed by multiple steps. The parallel light reflected by the first off-axis parabolic mirror 3 is located within the range of the stepped surface. A terahertz source thin film 9 is laid on the width surface of each step, and a terahertz source thin film 9 can also be laid on the height surface to convert the incident parallel light into a terahertz pulse train 6. The incident range of the terahertz pulse train 6 is located within the reflecting surface range of the second off-axis parabolic mirror 5, so that it is focused by the second off-axis parabolic mirror 5 to form a multi-period terahertz pulse 7. In this invention, an external magnetic field 10 can apply a magnetic field to the terahertz pulse source 4. By changing the magnetic field, the polarization state of the multi-cycle terahertz pulse 7 can be changed. Furthermore, by changing the step height, the center frequency of the multi-cycle terahertz pulse 7 can be changed, and the adjustment range is relatively large. The specific steps in this invention are as follows: The femtosecond laser emitted from the femtosecond laser 1 is expanded into a large spot and collimated after passing through the concave lens 2 and the first off-axis parabolic mirror. It then irradiates the terahertz source thin film 9 in the terahertz pulse source 4, generating a terahertz pulse train 6 with a certain time delay. This train is then focused by the second off-axis parabolic mirror, forming a spatially overlapping multi-cycle terahertz pulse 7 at the focal point.

[0047] In some possible embodiments, the insulating stepped substrate 8 is made of one of the following materials: high-resistivity silicon, Al2O3, MgO, SiO2, TiO2, SrTiO3, PET, PEN, ZrO2, ZnO, LaAlO3, GaN, GGG, diamond, and silicon carbide.

[0048] In some possible embodiments, the insulating stepped substrate 8 has a step width of 2-1000 μm, a step height of 10-100 μm, and a step number of 10-10000. Changing the step height alters the center frequency of the multi-cycle terahertz pulse 7; while changing the step width and the number of cycles alters the number of cycles of the multi-cycle terahertz pulse 7.

[0049] In some possible embodiments, the terahertz source thin film 9 includes, from bottom to top, [high refractive index dielectric film / low refractive index dielectric film]n, a first non-magnetic nanofilm, a magnetic nanofilm, and a second non-magnetic nanofilm;

[0050] Wherein, [high refractive index dielectric film / low refractive index dielectric film]n represents n periods of alternating high refractive index dielectric films and low refractive index dielectric films; n is an integer from 4 to 10;

[0051] The spin Hall angles of the first and second non-magnetic nanofilms have opposite signs. In this scheme, by selecting the thickness and number of periods of the high-refractive-index and low-refractive-index dielectric films, the energy of the femtosecond laser is absorbed by the first, magnetic, and second non-magnetic nanofilms, thereby maximizing the generation of terahertz waves. Since the terahertz spectrum generated by the ferromagnetic heterojunction nanofilm is greater than 10 THz, meaning that the first, magnetic, and second non-magnetic nanofilms have an intrinsic ability to generate waves greater than 10 THz, the center frequency adjustment range of the multi-cycle terahertz is greater than 10 THz and can be adjusted within this range. Furthermore, the thickness range of the high-refractive-index and low-refractive-index dielectric films is 50-200 nm.

[0052] In some possible embodiments, the high refractive index dielectric film is one of TiO2, Ta2O5, ZnS, ZnSe and HfO2.

[0053] In some possible embodiments, the low refractive index dielectric film is one of SiO2, YbF3 and MgF2.

[0054] In some possible embodiments, the magnetic nanofilm is ferromagnetic or subferromagnetic;

[0055] The ferromagnetic material is a single component of Fe, Co, or Ni or an alloy thereof, or an alloy in which B, Si, etc. are added to a single component of Fe, Co, or Ni or an alloy thereof, such as CoFeB or Fe3Si.

[0056] The ferrimagnet is one of YIG, Fe3O4, GdFeCo, GdCo5, DyCo5, TbFe2, and BaFe12O19.

[0057] In some possible embodiments, both the first and second nonmagnetic nanofilms can be made of one of the following: nonmagnetic metal films, topological insulator films, Weyl semimetal films, and two-dimensional materials.

[0058] In some possible embodiments, the non-magnetic metal thin film is one of Pt, W, Pd, Ta, Bi, Cr, Ir, IrMn, PtMn, PdMn, FeMn, AuPt, AuW, CuBi, CuIr and CuPb;

[0059] The topological insulator film is one of Bi2Se3, Bi2Te3, Bi2Se2Te(BST), Bi2Te2Se, Sn-dopedBi2Te2Se, BiSbTeSe, (BixSb1-x)2Te3, and α-Sn;

[0060] The Weyl semimetallic thin film is one of TaAs, TaP, NbAs, NbP, WTe2, MoTe2 and ZrSiS;

[0061] The two-dimensional material is either graphene or MoS2.

[0062] In some possible embodiments, the thicknesses of the first non-magnetic nanofilm, the magnetic nanofilm, and the second non-magnetic nanofilm are all between 0.1 and 10 nanometers.

[0063] In the above scheme, the femtosecond laser emitted from the femtosecond laser 1, after passing through the concave lens 2 and the first off-axis parabolic mirror, is expanded into a large spot and collimated, illuminating the terahertz source thin film 9 in the terahertz pulse source 4, generating a terahertz pulse train 6 with a certain time delay. This train is then focused by the second off-axis parabolic mirror, forming a multi-period terahertz pulse 7 with spatial overlap at the focal point. Experimental results are as follows... Figure 3 and Figure 4 As shown, by setting different step heights, the center frequency of the multi-cycle terahertz pulse 7 can be changed, and the adjustment range of the center frequency is relatively large.

[0064] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for generating multi-period terahertz pulses with adjustable polarization, characterized in that, Includes the following steps: A femtosecond laser (1), a concave lens (2), a first off-axis parabolic mirror (3), a terahertz pulse source (4) and a second off-axis parabolic mirror (5) are respectively set up, and a magnetic field is applied to the terahertz pulse source (4) by an external magnetic field (10); The femtosecond laser (1) is activated to emit laser light into the concave lens (2), and the light is diverged by the concave lens (2) into the reflecting surface of the first off-axis parabolic mirror (3) for collimation. Subsequently, the first off-axis parabolic mirror (3) reflects the parallel light through the reflecting surface and irradiates the stepped surface of the terahertz pulse source (4) perpendicularly; wherein, the terahertz pulse source (4) includes an insulating stepped substrate (8), one side of the insulating stepped substrate (8) is a stepped surface with multiple steps, and a terahertz source film (9) is laid on the width surface of each step, and the parallel light irradiates the terahertz source film (9) on the width surface perpendicularly. Parallel light passes through the terahertz source thin film (9) to generate a terahertz pulse train (6) with time delay, and enters the reflecting surface of the second off-axis parabolic mirror (5). The light is focused by the reflecting surface of the second off-axis parabolic mirror (5) to finally obtain a multi-cycle terahertz pulse (7). The terahertz source thin film (9) includes, from bottom to top, [high refractive index medium film / low refractive index medium film]n, a first non-magnetic nanofilm, a magnetic nanofilm, and a second non-magnetic nanofilm; [high refractive index medium film / low refractive index medium film]n represents the high refractive index medium film and the low refractive index medium film arranged alternately for n periods; n is an integer from 4 to 10; the spin Hall angles of the first non-magnetic nanofilm and the second non-magnetic nanofilm have opposite signs.

2. The polarization-tunable multi-period terahertz pulse generation method according to claim 1, characterized in that, The insulating stepped substrate (8) is made of one of the following materials: high-resistivity silicon, Al2O3, MgO, SiO2, TiO2, SrTiO3, PET, PEN, ZrO2, ZnO, LaAlO3, GaN, GGG, diamond, and silicon carbide.

3. The polarization-tunable multi-period terahertz pulse generation method according to claim 1, characterized in that, In the insulating stepped substrate (8), the width of the step is 2-1000μm, the height is 10-100μm, and the number of steps is 10-10000.

4. The polarization-tunable multi-period terahertz pulse generation method according to claim 1, characterized in that, The high refractive index dielectric film is one of TiO2, Ta2O5, ZnS, ZnSe and HfO2.

5. The polarization-tunable multi-period terahertz pulse generation method according to claim 1, characterized in that, The low-refractive-index dielectric thin film is one of SiO2, YbF3 and MgF2.

6. The polarization-tunable multi-period terahertz pulse generation method according to claim 1, characterized in that, The magnetic nanofilm is ferromagnetic or subferromagnetic; The ferromagnetic material is a single component of Fe, Co, or Ni or an alloy thereof, or an alloy in which B or Si is added to a single component of Fe, Co, or Ni or an alloy thereof. The ferrimagnets used are YIG, Fe3O4, GdFeCo, GdCo5, DyCo5, TbFe2, and BaFe. 12 O 19 One of them.

7. The polarization-tunable multi-period terahertz pulse generation method according to claim 1, characterized in that, Both the first and second nonmagnetic nanofilms are made of one of the following: nonmagnetic metal film, topological insulator film, and Weyl semimetal film.

8. The polarization-tunable multi-period terahertz pulse generation method according to claim 7, characterized in that, The non-magnetic metal thin film is one of Pt, W, Pd, Ta, Bi, Cr, Ir, IrMn, PtMn, PdMn, FeMn, AuPt, AuW, CuBi, CuIr and CuPb; The topological insulator film is one of Bi2Se3, Bi2Te3, Bi2Se2Te, Bi2Te2Se, Sn-doped Bi2Te2Se, BiSbTeSe, (BixSb1-x)2Te3, and α-Sn; The Weyl semimetal thin film is one of TaAs, TaP, NbAs, NbP, WTe2, MoTe2, and ZrSiS.

9. The method for generating a polarization-tunable multi-period terahertz pulse according to claim 1, characterized in that, Both the first and second nonmagnetic nanofilms are made of two-dimensional materials.

10. A method for generating polarization-tunable multi-period terahertz pulses according to claim 9, characterized in that, The two-dimensional material is either graphene or MoS2.

11. The method for generating polarization-tunable multi-period terahertz pulses according to claim 1, characterized in that, The thicknesses of the first non-magnetic nanofilm, the magnetic nanofilm, and the second non-magnetic nanofilm are all between 0.1 and 10 nanometers.

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