CN112510469A - Polarization tunable terahertz radiation source based on spin emission and linearly polarized light current - Google Patents
Polarization tunable terahertz radiation source based on spin emission and linearly polarized light current Download PDFInfo
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Abstract
The invention discloses a polarization tunable terahertz radiation source based on spin emission and linear polarized light current, wherein when femtosecond laser is incident to a ferromagnetic metal-topological insulator heterojunction film, two surfaces of the topological insulator film are simultaneously excited to generate spin charge conversion and linear polarized light current effect, photocurrent component terahertz waves and spin charge conversion component terahertz waves of line polarization are respectively radiated, and terahertz waves contributing to the linear polarized light current and the spin charge conversion are combined and regulated on a nano heterojunction sample to obtain a high-quality circularly polarized terahertz wave radiation source. The invention can not only generate high-quality circularly polarized terahertz radiation, but also realize the generation of terahertz waves in any polarization state, thereby laying the foundation for the next generation of integrated novel terahertz radiation source.
Description
Technical Field
The invention belongs to the technical field of terahertz wave generation, and particularly relates to a polarization tunable terahertz radiation source based on spin emission and linearly polarized light current.
Background
The terahertz wave is an electromagnetic wave with a frequency band of 0.1-30 THz, and the frequency range of the terahertz wave is between the upper edge of microwave millimeter wave and the lower edge of far infrared ray, because the traditional electronic means and the classical photonic means are difficult to well process the electromagnetic wave with the frequency band, the terahertz wave becomes the last section of electromagnetic wave band which is known and recognized by human beings. Terahertz waves, which are known to have excellent characteristics, have the advantages of penetrability, low energy, broadband property, uniqueness and the like, and have wide application prospects in the fields of wireless communication, biomedical treatment, condensed material research and the like.
In recent years, terahertz radiation sources based on femtosecond laser technology have been developed rapidly, and various terahertz radiation sources based on miniaturization, high reliability and low cost of femtosecond laser are developed successively. In many terahertz radiation sources based on femtosecond lasers, generating terahertz waves by utilizing corresponding spintronics phenomena and principles is a method with great potential, and the method is very hopeful to meet the continuously proposed severe requirements of the terahertz technology which is developing day by day and promote the development of the terahertz technology. However, the typical spin-emission terahertz radiation source known up to now uses a strong spin-orbit coupling material to convert spin current into charge current, and uses the charge current of ultrafast oscillation to generate terahertz waves. However, the method which only relies on spin orbit coupling limits the movement of spin reversal electrons to two opposite directions in principle, and the polarity enables terahertz generated by a terahertz radiation source which is purely based on spin emission to be only linearly polarized, so that the purpose of polarization adjustability cannot be achieved. However, in the current research, the chiral tunable terahertz wave group carries complete spatial dimension information, and has a potentially wide application prospect in aspects of condensed physical and biochemical dynamic process exploration, short-distance high-frequency communication and the like. Therefore, how to generate the circularly polarized terahertz wave by using spin emission is still a great problem. Meanwhile, after the circularly polarized terahertz wave is obtained, how to further realize accurate and random regulation and control of the polarization state and the chirality of the circularly polarized wave is of great significance to the application of the terahertz technology to various fields. Unfortunately, only a few reports of realizing circular polarization terahertz wave radiation based on spin emission exist at present, and most reports are trial researches, and the performance of the terahertz wave radiation far cannot reach indexes required by practical application. Therefore, the existing research results are necessary to be combined to explore how to realize high-quality generation of circularly polarized terahertz waves on the basis of spin emission and arbitrary regulation and control of polarization states and chiralities.
Topological Insulators (TI) are an extremely interesting two-dimensional material. Due to strong spin-orbit coupling in the topological insulator, the surface of the topological insulator can be subjected to energy band inversion to form a linear Dirac cone and intersect to form a Dirac point near the Fermi level. This causes the bulk state to exhibit an insulating state, but the surface to exhibit a metallic state with energy momentum linear dispersion relation. Due to this unique property, TI is considered to have a broad and significant application prospect in the terahertz field. The invention combines the topological insulator and the ferromagnetic metal to form a topological insulator-ferromagnetic metal heterojunction, comprehensively utilizes two effects of spin emission and linearly polarized light current, and realizes efficient and stable circular polarization terahertz radiation. Meanwhile, the characteristics of spin charge conversion and linearly polarized light current are utilized, and the polarization state can be further regulated and controlled by regulating the external magnetic field and the polarization angle of the pump laser. The scheme of the broadband circular polarization terahertz radiation source is simple in structure, the emergent terahertz waves meet broadband phase difference conditions, the performance is firm and reliable, miniaturization can be achieved, corresponding requirements can be met, and the broadband circular polarization terahertz radiation source has great market potential.
The existing methods for generating circularly polarized terahertz waves mainly comprise the following steps: (1) terahertz wave generation is realized based on the metamaterial; (2) a terahertz radiation source based on a nonlinear crystal difference frequency effect; (3) a terahertz radiation source based on a photoconductive antenna; (4) terahertz radiation source based directly on topological insulator. However, these several solutions have their own drawbacks:
(1) the generation of the circularly polarized terahertz wave is realized by designing a special metamaterial, and the method has high flexibility in design, and can adopt transmission emission and reflection. However, like quarter-glass, the metamaterial also faces the challenge of low efficiency of circular polarization generation, and if the metamaterial is generated by transmission, the working bandwidth of the metamaterial is narrow; if a high bandwidth is required, the device must be reflective, which makes the experiment difficult.
(2) The difference frequency effect of the nonlinear crystal is an extremely effective method for obtaining a high-frequency terahertz radiation source, and at present, a common terahertz radiation source based on the nonlinear crystal can generate extremely strong terahertz radiation with the central frequency covering 15-30THz and the narrow-band tunable peak electric field reaching 100 MV/cm. However, the frequency of such radiation sources is generally high, and it is difficult to generate terahertz radiation with a central spectrum below 15 THz. Because the frequency band below 15THz is regarded as the most applicable terahertz frequency band in the condensed state system, the phonon vibration frequency of a plurality of condensed state systems just falls into the frequency band, and the defect of overhigh central frequency spectrum limits the relevant application of the terahertz radiation source in the fields of condensed state physics and the like.
(3) The terahertz radiation source of the photoconductive antenna is a relatively mature and widely applied terahertz radiation source at present, and the reported result can realize that the peak electric field is close to 1MV/cm after focusing, and completely meets the requirement of a strong-field terahertz radiation source on the peak field intensity. However, the biggest disadvantage of such radiation source is that the radiation source is easily saturated, it is difficult to further increase the radiation electric field, and meanwhile, a simple photoconductive antenna can only generate linearly polarized terahertz radiation, and the purpose of polarization tuning cannot be achieved.
(4) Terahertz waves are generated directly by using a topological insulator. Terahertz radiation sources which can realize tunable polarization by directly utilizing topological insulators have been reported, and circular polarization generation is mainly realized by utilizing a circular photo-galvanic effect on the surface of the topological insulator. However, the terahertz source device based on the topological insulator is extremely complex and unstable in regulation and control amount, and the radiation efficiency is also very low. Even at this level of distance from laboratory applications, there is a long way to get away from commercial implementations.
Disclosure of Invention
In order to solve the defects of the prior art, the invention designs and realizes a polarization tunable terahertz radiation source based on spin emission and linearly polarized light current by utilizing a heterojunction film obtained by growing a ferromagnetic metal and a topological insulator and utilizing the spin emission on the surface of a heterojunction layer of the film and the linearly polarized light current radiation effect on the surface of a non-heterojunction layer, and the specific technical scheme of the invention is as follows:
a polarization tunable terahertz radiation source based on spin emission and linearly polarized light current is characterized by comprising a pump laser light source, a topological insulator ferromagnetic metal heterojunction nano-film, a sample frame, a pair of magnets, a terahertz wave collection and electro-optical detection system, wherein,
the pump laser light source is positioned at the foremost end of the radiation source and used for providing femtosecond laser pulses;
sequentially growing a topological insulator film and a ferromagnetic metal film on an alumina substrate from bottom to top in a molecular beam epitaxy mode to form a topological insulator ferromagnetic metal heterojunction nano film; the aluminum oxide substrate on which the topological insulator ferromagnetic metal heterojunction nano-film grows is fixed on the sample holder; the pair of magnets are positioned at the periphery of the sample holder and used for providing externally-applied magnetic fields which are distributed in parallel and can rotate in the direction for the topological insulator ferromagnetic metal heterojunction nano film;
the terahertz wave collecting and electro-optical detecting device is positioned behind the sample holder and is used for collecting and detecting chiral terahertz waves generated by the topological insulator ferromagnetic metal heterojunction nano-film;
femtosecond laser output by the pump laser source is irradiated into the topological insulator ferromagnetic metal heterojunction nano film, and a spin charge conversion effect and a linearly polarized light current effect are simultaneously excited on two surfaces of the topological insulator ferromagnetic metal heterojunction nano film, wherein the two effects can simultaneously radiate a polarized terahertz wave component; the spin charge conversion effect is generated on one side of a heterojunction formed by the topological insulator film and the ferromagnetic metal film, the linearly polarized light current effect is generated on the other side of the topological insulator film, the two effects can be isolated by the insulated body state of the topological insulator, and the linear polarization directions of the two terahertz wave components can be independently modulated; in addition, the two effects have different relaxation times, so that the two terahertz wave components have a phase difference of 90 degrees;
the polarization state directions of the two terahertz wave components are perpendicular to each other by adjusting the polarization included angle of the two terahertz wave components, the amplitudes of the two terahertz wave components are the same by controlling the thickness of the topological insulator film or controlling the external magnetic field intensity, and the direct radiation of the circularly polarized terahertz waves is finally realized by utilizing the phase difference caused by different relaxation times.
Further, the polarization states of the two beams of terahertz wave components are perpendicular to each other by adjusting the direction of the magnetic field on the periphery of the sample holder, or the polarization states of the two beams of terahertz wave components are perpendicular to each other by adjusting the polarization angle of femtosecond laser output by the pump laser light source.
Furthermore, the amplitude of the two terahertz wave components is the same by controlling the thickness range of the topological insulator film to be 5-20nm and controlling the external magnetic field intensity range to be 5-9 mT.
Further, the amplitude of the two terahertz wave components is the same by controlling the thickness of the topological insulator film to be 10nm or controlling the external magnetic field strength to be 7.5 mT.
Further, the topological insulator has a composition of Bi2Te3。
Furthermore, the included angle between the emergent laser polarization plane of the pump laser light source and the horizontal plane can realize the random rotation of the pump laser polarization plane by means of an additional half wave plate.
Further, the topological insulator thin film in the topological insulator ferromagnetic metal heterojunction thin film has a thickness ranging from 5nm to 20nm, and the ferromagnetic metal thin film has a thickness ranging from 2nm to 6 nm.
Further, the thickness of the topological insulator thin film in the topological insulator ferromagnetic metal heterojunction thin film is 10nm, and the thickness of the ferromagnetic metal thin film is 3 nm.
Further, the pump laser light source is a femtosecond laser.
The invention has the beneficial effects that:
1. the terahertz radiation source is simple in structure and low in implementation cost, does not need a complex and high-cost manufacturing process similar to various metamaterials, and overcomes the defects of high material requirement, complex structure and high cost of various existing schemes.
2. The terahertz radiation source has diversified polarization modulation modes, and can meet the requirements of different aspects according to different conditions. The modulation mode of the terahertz radiation source can be divided into a magnetic field modulation mode and an optical modulation mode. The magnetic field mode can realize the regulation and control of chirality and ellipticity very simply through adjusting the direction of the external magnetic field, and has the advantages of simplicity, portability, compactness and the like. The light polarization state regulation mode has the characteristics that in the mode, the polarization state of the generated terahertz waves can be modulated at an ultra-high speed in a sub-picosecond order by the polarization angle of the pump laser, and the modulation amount is only the incident linear polarization light polarization angle.
3. The terahertz radiation source has wide frequency band, and the ellipticity of circularly polarized terahertz waves radiated by the system can reach an ideal set value in wide main energy frequency domain distribution in actual tests.
4. The terahertz radiation source is relatively simple in implementation principle, low in material saving cost, firm and stable in performance, and can be made into a portable plug-in unit.
Drawings
In order to illustrate embodiments of the present invention or technical solutions in the prior art more clearly, the drawings which are needed in the embodiments will be briefly described below, so that the features and advantages of the present invention can be understood more clearly by referring to the drawings, which are schematic and should not be construed as limiting the present invention in any way, and for a person skilled in the art, other drawings can be obtained on the basis of these drawings without any inventive effort. Wherein:
FIG. 1 is a block diagram of the apparatus of the present invention;
FIG. 2 is a schematic diagram of the present invention for generating circularly polarized terahertz radiation;
FIG. 3 is a schematic diagram showing the distribution details of each region of the Fe-TI sample after being excited by the pump.
The reference numbers illustrate:
1-femtosecond laser source; 2-Fe-TI nano-film; 21-topological insulator thin film; 22-ferromagnetic metal thin film; 3-a silicon wafer; 4-off-axis parabolic mirror; 5-terahertz radiation propagation path; 6-terahertz polarizing film; 7-ZnTe; 8-1/4 slide; a 9-Wollaston prism WP; 10-an electro-optical sampler; 11-detection laser; 12-a magnet; 13-terahertz wave collection and electro-optical detection system.
Detailed Description
In order that the above objects, features and advantages of the present invention can be more clearly understood, a more particular description of the invention will be rendered by reference to the appended drawings. It should be noted that the embodiments of the present invention and features of the embodiments may be combined with each other without conflict.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, however, the present invention may be practiced in other ways than those specifically described herein, and therefore the scope of the present invention is not limited by the specific embodiments disclosed below.
The invention designs and realizes a high-efficiency, compact, stable and low-cost broadband circularly-polarized terahertz radiation source by utilizing a heterojunction film obtained by growing a topological insulator film and a ferromagnetic metal film and utilizing spin emission on the surface of a heterojunction layer of the film and linearly polarized light current radiation effect on the surface of a non-heterojunction layer. Meanwhile, the terahertz radiation source can further realize the arbitrary tuning of the polarization of the emergent circularly polarized terahertz wave by adjusting an external magnetic field or adjusting the polarization angle of the pump light.
Specifically, as shown in fig. 2-3, a polarization tunable terahertz radiation source based on spin emission and linearly polarized light current comprises a pump laser light source, a topological insulator ferromagnetic metal heterojunction nano-film, a sample holder, a pair of magnets, a terahertz wave collection and electro-optical detection system, wherein,
the pump laser light source is positioned at the most front end of the radiation source and used for providing femtosecond laser pulses;
sequentially growing a topological insulator film and a ferromagnetic metal film on an alumina substrate from bottom to top in a molecular beam epitaxy mode to form a topological insulator ferromagnetic metal heterojunction nano film; the alumina substrate on which the topological insulator ferromagnetic metal heterojunction nano film grows is fixed on the sample frame; the pair of magnets are positioned at the periphery of the sample holder and used for providing external magnetic fields which are distributed in parallel and can rotate in the direction for the topological insulator ferromagnetic metal heterojunction nano film;
the terahertz wave collecting and electro-optical detecting device is positioned behind the sample rack and is used for collecting and detecting chiral terahertz waves generated by the topological insulator ferromagnetic metal heterojunction nano-film;
femtosecond laser output by a pump laser source is irradiated into the topological insulator ferromagnetic metal heterojunction nano film, and a spin charge conversion effect and a linearly polarized light current effect are simultaneously excited on two surfaces of the topological insulator ferromagnetic metal heterojunction nano film, wherein the two effects can simultaneously radiate a polarized terahertz wave component; the spin charge conversion effect is generated on one side of a heterojunction formed by the topological insulator film and the ferromagnetic metal film, the linearly polarized light current effect is generated on the other side of the topological insulator film, the two effects can be isolated by the insulated body state of the topological insulator, and the linear polarization directions of the two terahertz wave components can be independently modulated; in addition, the two effects have different relaxation times, so that the two terahertz wave components have a phase difference of 90 degrees;
the polarization state directions of the two terahertz wave components are perpendicular to each other by adjusting the polarization included angle of the two terahertz wave components, the amplitudes of the two terahertz wave components are the same by controlling the thickness of the topological insulator film or controlling the intensity of the external magnetic field, and the direct radiation of the circularly polarized terahertz wave is finally realized by utilizing the phase difference caused by different relaxation times.
In some embodiments, the polarization states of the two terahertz wave components are perpendicular to each other by adjusting the direction of the magnetic field at the periphery of the sample holder, or the polarization states of the two terahertz wave components are perpendicular to each other by adjusting the polarization angle of the femtosecond laser output by the pump laser light source.
In some embodiments, the topological insulator has a composition of Bi2Te3。
In some embodiments, the angle between the exit laser polarization plane of the pump laser light source and the horizontal plane can realize the arbitrary rotation of the pump laser polarization plane by means of an additional half-wave plate.
In some embodiments, the topological insulator thin film in the topological insulator ferromagnetic metal heterojunction thin film has a thickness in a range of 5nm to 20nm, and the ferromagnetic metal thin film has a thickness in a range of 2nm to 6 nm.
Preferably, the thickness of the topological insulator thin film in the topological insulator ferromagnetic metal heterojunction thin film is 10nm, and the thickness of the ferromagnetic metal thin film is 3 nm.
In some embodiments, the pump laser light source is a femtosecond laser.
The invention is further described below with reference to the accompanying drawings and examples, it being understood that the examples described below are intended to facilitate the understanding of the invention, and are not intended to limit it in any way.
As shown in figure 1, a polarization tunable terahertz radiation source based on spin emission and linearly polarized light current adopts a femtosecond laser source 1 of a commercial titanium sapphire laser oscillator with the pulse width of 100fs, a Fe-TI nano film 2 formed by a topological insulator film 21 and a ferromagnetic metal film 22 is sequentially grown on an alumina substrate from bottom to top in a high-quality manner in a molecular beam epitaxy manner, and a terahertz wave collecting and electro-optical detecting system 13 comprises a silicon wafer 3, an off-axis parabolic mirror 4, a terahertz radiation propagation path 5, a terahertz polarizing plate 6, ZnTe 7, 1/4 slide 8, a Wollaston prism WP9, an electro-optical sampler 10 and a detection laser 11.
According to the invention, the self photocurrent effect of a topological insulator is combined with the spin charge conversion effect to realize chiral terahertz emission, and firstly, the generation process of two terahertz wave components is as follows:
the topological insulator side of the Fe-TI nano film 2 is placed facing the femtosecond laser light source 1 to ensure that the non-heterojunction region side of the pure topological insulator is firstly excited by femtosecond laser pumping. When the converged femtosecond pump laser is firstly pumped to a non-heterojunction region of the Fe-TI nano film 2, the pump light can cause ultrafast redistribution of the electron density of Te-Bi atomic chemical bonds on the surface of the topological insulator, the ultrafast distribution process of the electron density can cause generation of ultrafast migration photocurrent, and the oscillated ultrafast photocurrent can radiate out a line-polarized terahertz wave pulse, namely a linearly polarized light current terahertz component. Meanwhile, the component is derived from ultrafast redistribution of chemical bond electron density of Te-Bi atoms of the topological insulator, and the strict hexagonal periodic change rule in the crystal of the topological insulator leads the polarization state of the terahertz component of the linearly polarized light current to have a strict two-fold change relation with the incident terahertz component of the pump laser. Almost simultaneously with the photocurrent effect, the femtosecond laser continuously propagating forward starts pumping and exciting the heterojunction region of the Fe-TI nano-film 2 and excites the electron pump in the nano-ferromagnetic metal film above the fermi level, because the density and mobility of the electrons with upward spin and the electrons with downward spin are different, when the electrons diffuse from the ferromagnetic metal (such as Fe) to the surface of the topological insulator, a transverse spin-polarized current is firstly generated, when the spin-polarized current diffuses to the heterojunction interface formed by the ferromagnetic metal and the surface region of the topological insulator, a longitudinal in-plane charge flow is generated due to the inverse spin hall effect, and the longitudinal oscillating in-plane charge flow contributes to the terahertz wave spin charge conversion component generating a linear polarization state.
For convenience of understanding and analysis, the processes in specific analysis are logically sequential, and actually, because the Fe-TI nano film 2 is only a few nanometers, the temporal sequence does not exist, the processes can be understood as being simultaneously excited at a moment, two effects of spin charge conversion and linearly polarized light current can be simultaneously excited and generated on two surfaces of the Fe-TI nano film 2, wherein the surface of one side of the topological insulator film, which is attached to the ferromagnetic metal film to form a heterojunction, is subjected to spin charge conversion, the other surface of the topological insulator film is subjected to linearly polarized light current effect, the two effects are isolated by the body state insulated by the topological insulator, and the polarized terahertz wave components are respectively radiated; the two linearly polarized terahertz wave components are used for generating circularly polarized terahertz waves, which needs to meet the following requirements: (1) the polarization directions of the two terahertz wave components are mutually vertical; (2) two terahertz wave components possess
A phase difference of (n is an integer); (3) the amplitudes of the two terahertz wave components are the same.
For the first condition, the two terahertz wave components contributed by the two effects generated by the pumping excitation of the Fe-TI nano-film 2 are both linearly polarized, and the linear polarization directions can be independently modulated. For the spin charge conversion effect, the linear polarization terahertz wave component is always vertical to the direction of an external magnetic field, and the polarization angle of the spin charge conversion linear polarization terahertz wave component can be controlled by changing the direction of the magnetic field; for the linearly polarized light current effect, the linearly polarized terahertz wave component and the polarization angle of the incident linearly polarized pump light have a determined relationship, and the rotation of the polarization angle of the linearly polarized terahertz wave component contributed by the linearly polarized light current effect can be realized by controlling the polarization angle of the incident linearly polarized light.
In practical use, the polarization angle of incident light can be changed by fixing an external magnetic field, so that the polarization of the two terahertz wave components is vertical, or the polarization angle of incident light can be fixed, so that the direction of the external magnetic field can be changed, thereby achieving the same purpose.
For the second condition, because the relaxation time of the spin charge conversion effect and the relaxation time of the linearly polarized light current effect are different, the phase difference is introduced by the relaxation time difference, and finally the two beams of terahertz waves have
(n is an integer) phase difference.
For the third condition, the same amplitude of the two terahertz wave components can be realized by controlling the thickness of the topological insulator film or controlling the external magnetic field intensity. Because the intensity of the terahertz wave component of the spin charge conversion effect has a direct relation with the intensity of the external magnetic field, the intensity of the external magnetic field can be adjusted near 7.5mT (milliTesla) to realize the adjustment of the amplitude of the terahertz wave component of the spin charge conversion; the pump laser firstly irradiates through a region where a heterojunction is not formed on the topological insulator and then reaches the heterojunction region of the sample, the region where the heterojunction is not formed on the topological insulator of the topological insulator has an attenuation effect on the intensity of the pump light, the thicker the region is, the smaller the light intensity reaching the heterojunction region is, and the smaller the amplitude of the spin charge conversion terahertz wave component is, so that the same amplitude of the two terahertz wave components can be realized by controlling the thickness of the nano thin film topological insulator region.
In some embodiments, the same amplitude of the two terahertz wave components is realized by controlling the thickness of the topological insulator film to be in a range of 5-20nm or controlling the external magnetic field intensity to be in a range of 5-9 mT.
Preferably, the thickness of the topological insulator film is controlled to be 10nm, and the external magnetic field intensity is controlled to be 7.5mT, so that the amplitudes of the two terahertz wave components are the same.
In summary, the control of the polarization included angle of the two linearly polarized terahertz wave components can be realized by adjusting the direction of the external magnetic field or the polarization direction of the pump light, and the Fe-TI heterojunction film 2 can radiate and generate circularly polarized terahertz waves efficiently by utilizing the phase difference caused by the relaxation time difference and further control the ellipticity and chirality of the emergent circularly polarized terahertz waves.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (9)
1. A polarization tunable terahertz radiation source based on spin emission and linearly polarized light current is characterized by comprising a pump laser light source, a topological insulator ferromagnetic metal heterojunction nano-film, a sample frame, a pair of magnets, a terahertz wave collection and electro-optical detection system, wherein,
the pump laser light source is positioned at the foremost end of the radiation source and used for providing femtosecond laser pulses;
sequentially growing a topological insulator film and a ferromagnetic metal film on an alumina substrate from bottom to top in a molecular beam epitaxy mode to form a topological insulator ferromagnetic metal heterojunction nano film; the aluminum oxide substrate on which the topological insulator ferromagnetic metal heterojunction nano-film grows is fixed on the sample holder; the pair of magnets are positioned at the periphery of the sample holder and used for providing externally-applied magnetic fields which are distributed in parallel and can rotate in the direction for the topological insulator ferromagnetic metal heterojunction nano film;
the terahertz wave collecting and electro-optical detecting device is positioned behind the sample holder and is used for collecting and detecting chiral terahertz waves generated by the topological insulator ferromagnetic metal heterojunction nano-film;
femtosecond laser output by the pump laser source is irradiated into the topological insulator ferromagnetic metal heterojunction nano film, and a spin charge conversion effect and a linearly polarized light current effect are simultaneously excited on two surfaces of the topological insulator ferromagnetic metal heterojunction nano film, wherein the two effects can simultaneously radiate a polarized terahertz wave component; the spin charge conversion effect is generated on one side of a heterojunction formed by the topological insulator film and the ferromagnetic metal film, the linearly polarized light current effect is generated on the other side of the topological insulator film, the two effects can be isolated by the insulated body state of the topological insulator, and the linear polarization directions of the two terahertz wave components can be independently modulated; in addition, the two effects have different relaxation times, so that the two terahertz wave components have a phase difference of 90 degrees;
the polarization state directions of the two terahertz wave components are perpendicular to each other by adjusting the polarization included angle of the two terahertz wave components, the amplitudes of the two terahertz wave components are the same by controlling the thickness of the topological insulator film or controlling the external magnetic field intensity, and the direct radiation of the circularly polarized terahertz waves is finally realized by utilizing the phase difference caused by different relaxation times.
2. The polarization tunable terahertz radiation source based on spin emission and linearly polarized light current as claimed in claim 1, wherein the polarization states of the two terahertz wave components are perpendicular to each other by adjusting the magnetic field direction at the periphery of the sample holder, or the polarization states of the two terahertz wave components are perpendicular to each other by adjusting the polarization angle of the femtosecond laser output by the pump laser light source.
3. The polarization tunable terahertz radiation source based on spin emission and linearly polarized light current as claimed in claim 1 or 2, is characterized in that the amplitude of two terahertz wave components is the same by controlling the thickness of the topological insulator film to be 5-20nm and controlling the external magnetic field intensity to be 5-9 mT.
4. The polarization tunable terahertz radiation source based on spin emission and linearly polarized light current as claimed in claim 1 or 2, is characterized in that the same amplitude of the two terahertz wave components is realized by controlling the thickness of the topological insulator film to be 10nm or controlling the external magnetic field strength to be 7.5 mT.
5. The polarization tunable terahertz radiation source based on spin emission and linearly polarized light current as claimed in claim 1 or 2, wherein the topological insulator is Bi2Te3。
6. The polarization tunable terahertz radiation source based on spin emission and linearly polarized light current as claimed in claim 1 or 2, wherein an included angle between an emergent laser polarization plane of the pump laser light source and a horizontal plane can realize arbitrary rotation of the pump laser polarization plane by means of an additional half wave plate.
7. The polarization tunable terahertz radiation source based on spin emission and linearly polarized light current as claimed in claim 1 or 2, wherein the thickness of the topological insulator thin film in the topological insulator ferromagnetic metal heterojunction thin film is 5nm-20nm, and the thickness of the ferromagnetic metal thin film is 2nm-6 nm.
8. The polarization tunable terahertz radiation source based on spin emission and linearly polarized light current as claimed in claim 1 or 2, wherein the thickness of the topological insulator thin film in the topological insulator ferromagnetic metal heterojunction thin film is 10nm, and the thickness of the ferromagnetic metal thin film is 3 nm.
9. The polarization tunable terahertz radiation source based on spin emission and linearly polarized light current of any one of claims 1-7, wherein the pump laser light source is a femtosecond laser.
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