Polarization-adjustable multi-period terahertz pulse generation method

Abstract

The invention discloses a polarization-adjustable multi-cycle terahertz pulse generation method, which relates to the technical field of terahertz, and comprises the following steps: after femtosecond laser emitted by a femtosecond laser passes through a concave lens and a first off-axis parabolic mirror, the femtosecond laser is expanded into a large light spot and collimated, and the large light spot irradiates a terahertz source film in a terahertz pulse source; and a terahertz pulse string with a certain time delay is generated and is focused by the second off-axis parabolic mirror, and a multi-cycle terahertz pulse with space coincidence is formed at the focus. According to the invention, the terahertz source films are distributed along the steps through the step-shaped insulating step substrate, and the polarization state of the multi-period terahertz pulse can be changed by changing the magnetic field applied to the terahertz source films and changing the center frequency of the multi-period terahertz pulse by changing the height of the steps through externally applying the magnetic field; the device is simple in structure, convenient to operate, adjustable in polarization and large in frequency adjustment range.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of terahertz, in particular to a polarization-adjustable multi-cycle terahertz pulse generation method. BACKGROUND

[0002] Terahertz (THz) waves are electromagnetic waves with frequencies from 0.1 THz to 10 THz, between millimeter waves and infrared light, corresponding to a time scale of picoseconds. Terahertz waves have wide application prospects, including terahertz radar and communication, spectroscopy and imaging, security detection, etc.; at the same time, many condensed matter physics phenomena, cosmic background radiation, biological macromolecules, etc. have characteristic frequencies in the terahertz wave band or characteristic time scales in picoseconds, so terahertz spectroscopy has become a powerful means for scientific research.

[0003] Terahertz waves can be divided into continuous waves and pulse waves. Continuous waves are generally generated by Schottky diode frequency doublers, quantum cascade lasers, resonant tunneling diodes, etc.; pulse waves are generally generated by the interaction of femtosecond lasers with matter, mainly photoconductive antennas, electro-optic crystals, ferromagnetic heterojunction films, air, water, etc. Terahertz pulses, according to the number of oscillation periods of their time-domain electric field, can be divided into sub-cycle, single-cycle and multi-cycle. The single-cycle terahertz pulse is generally generated by the direct interaction of femtosecond laser with the matter described above.

[0004] There are currently only a few methods to generate multi-cycle terahertz pulses: (1) femtosecond laser is incident on and interacts with a periodically poled lithium niobate crystal, but this method has low efficiency and the crystal is difficult to process; (2) chirped pulse beating technology, i.e. introducing a delay in time between two chirped laser pulses to form a pulse train with intensity modulation in time, and then acting it on a periodically poled lithium niobate crystal or a common lithium niobate crystal, but the operation is complex and the generation efficiency is low; (3) laser splitting technology changes a single femtosecond laser into a laser pulse train with a certain time delay but not separated in transverse space, and then irradiates the electro-optic crystal to generate multi-cycle terahertz pulses, but the operation is complex.

[0005] The above methods have narrow multi-cycle terahertz frequency and a frequency adjustment range of less than 3 THz due to the terahertz crystal. Moreover, the polarization of the above methods is not adjustable.

[0006] Therefore, it is of great significance to develop a multi-cycle terahertz pulse generation method with simple operation, large frequency adjustment range and adjustable polarization. SUMMARY

[0007] The application aims at providing a polarization-adjustable multi-period terahertz pulse generation method, and the method comprises the following steps: arranging a femtosecond laser, a concave lens, a first off-axis parabolic mirror, a terahertz pulse source and a second off-axis parabolic mirror, and applying a magnetic field to the terahertz pulse source through an external magnetic field.

[0008] The application is achieved by the following technical solutions:

[0009] A polarization-adjustable multi-period terahertz pulse generation method comprises 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 arranged respectively, and a magnetic field is applied to the terahertz pulse source through an external magnetic field.

[0011] The femtosecond laser is started, laser is emitted to the concave lens, and the laser is collimated in the reflecting surface of the first off-axis parabolic mirror by the concave lens.

[0012] Then, the reflected parallel light is vertically irradiated on the stepped surface of the terahertz pulse source by the reflecting surface of the first off-axis parabolic mirror, wherein the terahertz pulse source comprises an insulating stepped substrate, one surface of the insulating stepped substrate is a stepped surface with multiple steps, a terahertz source film is arranged on the width surface of each step, and the parallel light is vertically irradiated on the terahertz source film on the width surface.

[0013] The parallel light generates a terahertz pulse train with a certain time delay after passing through the terahertz source film, and enters the reflecting surface of the second off-axis parabolic mirror, and is focused by the reflecting surface of the second off-axis parabolic mirror to finally obtain a multi-period terahertz pulse.

[0014] The present application provides a kind of polarization adjustable multi-period terahertz pulse generation method relative to prior art, multi-period terahertz frequency is narrow, frequency regulation range is small and polarization is not adjustable, by insulating stepped substrate with ladder shape, to along the ladder distribution terahertz source film, again by external magnetic field, i.e.it can be changed by changing the magnetic field applied to terahertz source film, the polarization state of multi-period terahertz pulse, by changing the height of ladder, the center frequency of multi-period terahertz pulse can be changed;It is simple in structure, convenient to operate, polarization adjustable, and has wide frequency regulation range.The specific scheme includes femtosecond laser, concave lens, first off-axis parabolic mirror, terahertz pulse source, second off-axis parabolic mirror and external magnetic field, wherein the area of divergent light rays of the concave lens is located in 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 of the femtosecond laser;The terahertz pulse source includes an insulating stepped substrate, the side surface of the insulating stepped substrate is a stepped surface formed by a plurality of steps, and the parallel light reflected by the first off-axis parabolic mirror is located within the range of the stepped surface;A terahertz source film is laid on the width surface of each step, and a terahertz source film can also be laid on the height surface to convert the incident parallel light into a terahertz pulse train;The incidence range of the terahertz pulse train is located within the reflecting surface range of the second off-axis parabolic mirror to focus to form a multi-period terahertz pulse by the second off-axis parabolic mirror;The external magnetic field can apply a magnetic field to the terahertz pulse source, and by changing the magnetic field, the polarization state of the multi-period terahertz pulse can be changed;By changing the height of the ladder, the center frequency of the multi-period terahertz pulse can be changed, and the adjustment range is large.In the present application, the specific steps are as follows: the femtosecond laser emitted by the femtosecond laser is expanded into a large spot and collimated after passing through the concave lens and the first off-axis parabolic mirror, and irradiates on the terahertz source film in the terahertz pulse source to generate a terahertz pulse train with a certain time delay, and then focuses by the second off-axis parabolic mirror to form a spatially coincident multi-period terahertz pulse at the focal point.

[0015] Further optimization, the material of the insulating stepped substrate is one of high-resistance silicon, Al2O3, MgO, SiO2, TiO2, SrTiO3, PET, PEN, ZrO2, ZnO, LaAlO3, GaN, GGG, diamond and silicon carbide.

[0016] Further optimization, in the insulating stepped substrate, the width of the ladder is 2-1000 μm, the height is 10-100 μm, and the number of ladders is 10-10000. Among them, by changing the height of the ladder, the center frequency of the multi-period terahertz pulse can be changed;And by changing the width and period number of the ladder, the period number of the multi-period terahertz pulse can be changed.

[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] Further optimization, the non-magnetic metal thin film adopts one of Pt, W, Pd, Ta, Bi, Cr, Ir, IrMn, PtMn, PdMn, FeMn, AuPt, AuW, CuBi, CuIr and CuPb;

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

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

[0029] The two-dimensional material adopts one of graphene and MoS2.

[0030] Further optimization, the thickness of the first non-magnetic nanometer film, the magnetic nanometer film and the second non-magnetic nanometer film is between 0.1-10 nanometers.

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

[0032] The present application provides a kind of polarization adjustable multi-cycle terahertz pulse generation method, by insulating stepped substrate of ladder shape, to along the ladder distribution terahertz source film, again by external magnetic field, i.e. by changing the magnetic field applied on terahertz source film, the polarization state of multi-cycle terahertz pulse is changed, by changing the ladder height, the center frequency of multi-cycle terahertz pulse can be changed;By changing the ladder width and the number of cycles, the number of cycles of multi-cycle terahertz pulse can be changed;It is simple in structure, convenient to operate, polarization adjustable, and has a large frequency adjustment range. BRIEF DESCRIPTION OF DRAWINGS

[0033] In order to more clearly illustrate the technical scheme of the exemplary embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be considered as limiting the scope. For those skilled in the art, other related drawings can also be obtained without creative labor. In the drawings:

[0034] Figure 1 The overall distribution diagram of the components provided by the present application is shown in the drawings;

[0035] Figure 2 The structure diagram of the terahertz pulse source provided by the present application is shown in the drawings;

[0036] Figure 3A multi-cycle terahertz pulse schematic diagram at different step heights provided by the present application is shown in the figure.

[0037] Figure 4 A multi-cycle terahertz pulse schematic diagram at different step heights provided by the present application is shown in the figure. Figure 3 A multi-cycle terahertz pulse schematic diagram at different step heights provided by the present application is shown in the figure.

[0038] Markings in the drawings 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 film, 10-applied magnetic field. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical scheme and advantages of the present application clearer, further detailed description of the present application is made below in combination with examples and drawings, the illustrative embodiments of the present application and the description thereof are only used to explain the present application, and do not limit the present application.

[0041] Embodiment: The present embodiment provides a polarization-adjustable multi-cycle terahertz pulse generation method, as shown in the figure, comprising the following steps: Figures 1-4

[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 arranged, and a magnetic field is applied to the terahertz pulse source 4 by an applied magnetic field 10;

[0043] The femtosecond laser 1 is started, laser is emitted to the concave lens 2, and the light rays are diverged by the concave lens 2 to be collimated in the reflecting surface of the first off-axis parabolic mirror 3;

[0044] Then the reflected parallel light is vertically irradiated on the stepped surface of the terahertz pulse source 4 by the reflecting surface of the first off-axis parabolic mirror 3; wherein the terahertz pulse source 4 comprises 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 each step width surface, and the parallel light is vertically irradiated on the terahertz source film 9 on the step width surface;

[0045] The parallel light generates a terahertz pulse train 6 with a certain time delay after passing through the terahertz source film 9, and enters the reflecting surface of the second off-axis parabolic mirror 5, and is focused by the reflecting surface of the second off-axis parabolic mirror 5 to finally obtain a multi-cycle terahertz pulse 7.

[0046] ​The present application provides a kind of polarization adjustable multi-period terahertz pulse 7 generation method relative to the problems of narrow frequency, small frequency adjustment range and polarization unadjustable in prior art, by insulating stepped substrate 8 of ladder shape, to along the ladder distribution terahertz source film 9, again by external magnetic field 10, i.e.it can be changed by changing the magnetic field applied to terahertz source film 9, the polarization state of multi-period terahertz pulse 7, by changing the height of ladder, the center frequency of multi-period terahertz pulse 7 can be changed;It is simple in structure, convenient to operate, polarization adjustable, and has large frequency adjustment range.Specific scheme includes femtosecond laser 1, concave lens 2, first off-axis parabolic mirror 3, terahertz pulse source 4, second off-axis parabolic mirror 5 and external magnetic field 10, wherein the area of divergent light rays of concave lens 2 is located in the reflecting surface of first off-axis parabolic mirror 3, and the parallel light reflected by first off-axis parabolic mirror 3 and the incident light of femtosecond laser 1 are perpendicular to each other;Terahertz pulse source 4 includes insulating stepped substrate 8, the side of insulating stepped substrate 8 is ladder face formed by multiple ladders, and the parallel light reflected by first off-axis parabolic mirror 3 is located in the range of ladder face;Terahertz source film 9 is laid on the width surface of each ladder, and terahertz source film 9 can also be laid on the height surface to change the incident parallel light into terahertz pulse train 6;The incidence range of terahertz pulse train 6 is located in the reflecting surface range of second off-axis parabolic mirror 5 to focus and form multi-period terahertz pulse 7 by second off-axis parabolic mirror 5.Wherein, external magnetic field 10 can apply magnetic field to terahertz pulse source 4, and by changing the magnetic field, the polarization state of multi-period terahertz pulse 7 can be changed;And by changing the height of ladder, the center frequency of multi-period terahertz pulse 7 can be changed, and the adjustment range is large.In the present application, the specific steps are as follows: the femtosecond laser emitted by femtosecond laser 1 is expanded into a large spot and collimated after passing through concave lens 2 and first off-axis parabolic mirror, and irradiates on terahertz source film 9 in terahertz pulse source 4 to generate terahertz pulse train 6 with a certain time delay, and then focuses on the focal point of second off-axis parabolic mirror to form spatially coincident multi-period terahertz pulse 7.

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

[0048] In some possible embodiments, the width of the ladder in the insulating stepped substrate 8 is 2-1000 μm, the height is 10-100 μm, and the number of ladders is 10-10000. Wherein, by changing the height of the ladder, the center frequency of the multi-period terahertz pulse 7 can be changed;And by changing the width and the number of ladders, the number of periods of the multi-period terahertz pulse 7 can be changed.

[0049] In some possible embodiments, the terahertz source film 9 comprises, 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.

[0050] wherein [high refractive index medium film / low refractive index medium film]n represents that the high refractive index medium film and the low refractive index medium film are arranged alternately for n periods; n is an integer in the range of 4-10.

[0051] The spin Hall angles of the first non-magnetic nanofilm and the second non-magnetic nanofilm are opposite. In this scheme, by selecting the thickness and the number of periods of the high refractive index medium film and the low refractive index medium film, the energy of the femtosecond laser is absorbed by the first non-magnetic nanofilm, the magnetic nanofilm, and the second non-magnetic nanofilm, thereby maximizing the generation of terahertz. Since the terahertz frequency spectrum range generated by the ferromagnetic heterojunction nanofilm is greater than 10 THz, that is, the first non-magnetic nanofilm, the magnetic nanofilm, and the second non-magnetic nanofilm have the intrinsic ability to generate waves greater than 10 THz, the central frequency adjustment range of the multi-period terahertz is greater than 10 THz, and is adjusted within the range. In addition, the thickness of the high refractive index medium film and the low refractive index medium film ranges from 50 nm to 200 nm.

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

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

[0054] In some possible embodiments, the magnetic nanofilm adopts ferromagnetic or ferrimagnetic.

[0055] The ferromagnetic adopts a single component or an alloy thereof in Fe, Co, and Ni, or an alloy of B, Si, etc. in a single component or an alloy thereof in Fe, Co, and Ni, such as CoFeB and Fe3Si.

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

[0057] In some possible embodiments, the first non-magnetic nanofilm and the second non-magnetic nanofilm can each adopt one of a non-magnetic metal film, a topological insulator film, a Weyl semimetal film, and a two-dimensional material.

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

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

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

[0061] The two-dimensional material adopts one of graphene and MoS2.

[0062] In some possible embodiments, the thickness of the first non-magnetic nanometer thin film, the magnetic nanometer thin film and the second non-magnetic nanometer thin film is between 0.1 and 10 nanometers.

[0063] In the above scheme, the femtosecond laser emitted by the femtosecond laser 1 is expanded into a large spot and collimated after the concave lens 2 and the first off-axis parabolic mirror, and irradiates the terahertz source thin film 9 in the terahertz pulse source 4 to generate a terahertz pulse train 6 with a certain time delay, and then is focused by the second off-axis parabolic mirror to form a multi-period terahertz pulse 7 with spatial coincidence at the focal point. The experimental results are shown in FIGS. 2 and 3. Figure 3 and Figure 4 As shown in FIGS. 2 and 3, by setting different step heights, the center frequency of the multi-period terahertz pulse 7 can be changed, and the adjustment range of the center frequency is large.

[0064] The above specific embodiments further illustrate the purpose, technical solutions and advantages of the present application. It should be understood that the above description is only a specific embodiment of the present application and is not used to limit the protection scope of the present application. Any modification, equivalent replacement, improvement, etc. within the spirit and principle of the present application should be included in the protection scope of the present application.

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 a certain 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).

2. The polarization-tunable multi-cycle 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 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; 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; The spin Hall angles of the first and second nonmagnetic nanofilms have opposite signs.

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

6. The polarization-tunable multi-period terahertz pulse generation method according to claim 4, characterized in that, The low-refractive-index dielectric thin film is one of SiO2, YbF3 and MgF2.

7. The polarization-tunable multi-period terahertz pulse generation method according to claim 4, 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 ferrimagnet is one of YIG, Fe3O4, GdFeCo, GdCo5, DyCo5, TbFe2, and BaFe12O19.

8. The polarization-tunable multi-period terahertz pulse generation method according to claim 4, characterized in that, Both the first and second non-magnetic nanofilms can be made from one of the following: non-magnetic metal films, topological insulator films, Weyl semi-metal films, and two-dimensional materials.

9. A method for generating polarization-tunable multi-period terahertz pulses according to claim 8, 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(BST), Bi2Te2Se, Sn-dopedBi2Te2Se, BiSbTeSe, (BixSb1-x)2Te3, and α-Sn; The Weyl semimetallic thin film is one of TaAs, TaP, NbAs, NbP, WTe2, MoTe2 and ZrSiS; The two-dimensional material is either graphene or MoS2.

10. A method for generating polarization-tunable multi-period terahertz pulses according to claim 4, 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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