Terahertz polarization wave generator and regulation and control method thereof

Abstract

The invention discloses a terahertz polarization wave generator and a regulation and control method thereof. The terahertz polarization wave generator comprises a pumping source, a beam expanding and collimating system, a reflector, a blazed grating, a half-wave plate, a beam shrinking system, a LiNbO3 crystal, a THz polarizing film, a THz quarter-wave plate and a terahertz detector which are sequentially arranged in the propagation direction of pumping light. The blazed grating is used for realizing wavefront inclination of incident light, so that the group velocity of the pump light is equal to the phase velocity of terahertz waves generated by the LiNbO3 crystal; the polarization state of the generated terahertz waves is adjusted by rotating the angles of the THz polaroid and the THz quarter-wave plate. Terahertz is generated by using an optical rectification method, the generated terahertz is enhanced by using an inclined wavefront technology, and the generated terahertz waves can be modulated into different polarization states through the THz polaroid and the THz quarter-wave plate. According to the invention, linearly polarized, elliptically polarized and circularly polarized terahertz waves can be generated.

Description

Technical Field

[0001] The present invention relates to the technical field of terahertz optics, and in particular to a terahertz polarization wave generator and a control method thereof. Background Art

[0002] Typical methods for generating terahertz based on ultrafast laser technology include photoconductive antenna method, optical rectification method and laser plasma method.

[0003] The photoconductive antenna method deposits metal electrodes on semiconductor materials, uses femtosecond laser pulses to excite photogenerated carriers, and then generates terahertz radiation under the action of a bias electric field.

[0004] The optical rectification method uses broadband ultrashort pulse laser to irradiate the second-order nonlinear crystal. Due to the existence of nonlinear optical effects in the crystal, the two frequency difference components in the pulsed laser are mixed in the terahertz band to output difference frequency light, that is, light in the terahertz band.

[0005] The laser plasma method uses high-intensity femtosecond laser pulses to induce plasma in the air, which can emit coherent radiation ranging from X-rays to terahertz wavelengths.

[0006] Among them, the optical rectification method has the advantages of wide spectral bandwidth, relatively simple experimental conditions and great application potential, but there is currently a lack of mature terahertz wave polarization control systems.

[0007] Therefore, how to provide a terahertz polarization wave generator capable of regulating terahertz polarization and a regulation method thereof is a technical problem that needs to be solved urgently by those skilled in the art. Summary of the invention

[0008] In view of the above research status and existing problems, the present invention provides a terahertz polarization wave generator and a control method thereof, which generates terahertz using the optical rectification method and adopts the tilted wavefront technology to enhance the generated terahertz. The generated terahertz wave can be modulated into different polarization states through a THz polarizer and a THz quarter wave plate. The present invention can generate linearly polarized, elliptically polarized, and circularly polarized terahertz waves.

[0009] The present invention first provides a terahertz polarization wave generator, comprising a pump source, a beam expansion and collimation system, a reflector, a blazed grating, a half-wave plate, a beam reduction system, a LiNbO3 crystal, a THz polarizer, a THz quarter-wave plate and a terahertz detector arranged in sequence according to the propagation direction of the pump light; wherein,

[0010] The blazed grating is used to achieve wavefront tilt of the incident light, so that the group velocity of the pump light is equal to the phase velocity of the terahertz wave generated by the LiNbO3 crystal;

[0011] The polarization state of the generated terahertz wave is adjusted by rotating the angles of the THz polarizer and the THz quarter wave plate.

[0012] Preferably, the pump light irradiates the LiNbO3 crystal, and in the spectrum of the pump light, the frequency difference between two frequencies is mixed in the terahertz band component to output difference frequency light, that is, light in the terahertz band.

[0013] Preferably, the calculation formulas for the incident angle α and the diffraction angle β of the blazed grating are as follows:

[0014]

[0015]

[0016] Where m is the reduction factor of the beam reduction system, λ is the central wavelength of the incident light, i.e. the pump source, γ is the wedge angle of the LiNbO3 lithium niobate crystal, and n is g is the group refractive index of LiNbO3 lithium niobate crystal corresponding to the central wavelength λ, and d is the grating constant.

[0017] Preferably, the pump light is a femtosecond pulse pump light; the terahertz wave output power and the parameters of the incident femtosecond pulse pump light satisfy the following formula:

[0018]

[0019] Among them, d eff is the second-order effective nonlinear coefficient of LiNbO3 crystal, c is the speed of light in vacuum, ε0 is the vacuum dielectric constant, η0 is the vacuum impedance coefficient, n g is the group refractive index of the pump light in the crystal, n p is the refractive index of the pump light, L is the crystal length, f is the pump light repetition frequency, τ is the incident pump light pulse width, A p is the area of ​​pumping light, P p is the average power of the incident pump light.

[0020] Preferably, a terahertz detector is also included, which is used to receive the terahertz wave generated by the pumping light irradiating the LiNbO3 crystal.

[0021] The present invention further provides a method for controlling the polarization state of the terahertz polarization wave generator, comprising the following steps:

[0022] The terahertz wave formed by the LiNbO3 crystal is vertically incident on the THz polarizer and the THz quarter wave plate in sequence;

[0023] Rotate the THz polarizer to control the generated terahertz wave into a linearly polarized terahertz wave in any direction;

[0024] Rotating the THz quarter wave plate so that the angle between the optical axis and the vibration direction of the terahertz polarized wave is 0° or 90°, thereby obtaining linearly polarized light;

[0025] Rotating the THz quarter wave plate so that the angle between the optical axis and the vibration direction of the terahertz polarized wave is 45°, thereby obtaining circularly polarized light;

[0026] When the THz quarter wave plate is rotated so that the angle between the optical axis and the vibration direction of the terahertz polarized light is an angle other than 0°, 45°, and 90°, elliptically polarized light is obtained.

[0027] Compared with the prior art, it has the following beneficial effects:

[0028] The present invention proposes a novel terahertz polarization wave generator and a control method thereof, which uses an optical rectification method to generate terahertz, and adopts a tilted wavefront technology, utilizes the maximum nonlinear coefficient of a LiNbO3 crystal, and other methods to enhance the generated terahertz. The generated terahertz can be modulated into different polarization states through a THz polarizer and a THz quarter-wave plate, and can generate linearly polarized, circularly polarized, and elliptically polarized terahertz waves, and can realize flexible control of the terahertz waves.

[0029] The present invention has unprecedented application effects. In the biomedical field, the system can utilize the low energy and high transmission characteristics of terahertz waves to achieve nondestructive detection and imaging of biological tissues, providing strong support for early diagnosis of diseases. In the field of wireless communications, the system can be applied to high-speed, high-capacity 6G communication systems, and the stability and efficiency of signal transmission can be improved by precisely controlling the polarization state of terahertz waves. In addition, in fields such as environmental monitoring, the system can also play an important role. By analyzing terahertz wave signals, it can achieve accurate measurement of environmental parameters such as atmospheric composition and surface temperature, providing a scientific basis for environmental protection and disaster warning. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the prior art descriptions are briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention, and for ordinary technicians in this field, other drawings can be obtained based on the provided drawings without creative work.

[0031] Figure 1 A schematic diagram of a terahertz polarization wave generator provided in an embodiment of the present invention;

[0032] Figure 2 A schematic diagram of a tilted wavefront provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0033] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0034] The terahertz polarization wave generator disclosed in the first aspect of an embodiment of the present invention comprises a pump source 1, a beam expansion and collimation system, a reflector 4, a blazed grating 5, a half-wave plate 6, a beam reduction system, a LiNbO3 crystal 9, a THz polarizer 10, a THz quarter-wave plate 11 and a terahertz detector 12, which are arranged in sequence according to the propagation direction of the pump light; wherein the blazed grating 5 is used to achieve wavefront tilt of the incident light so that the group velocity of the pump light is equal to the phase velocity of the terahertz wave generated by the LiNbO3 crystal 9; and the polarization state of the generated terahertz wave is adjusted by rotating the angles of the THz polarizer 10 and the THz quarter-wave plate 11.

[0035] like Figure 1 As shown, the pump light is first expanded and collimated by the beam expansion and collimation system, and then guided to the blazed grating 5 after passing through the reflector 4. The reflector 4 is used to adjust the position and direction of the incident light beam. The blazed grating 5 is used to implement the tilted wavefront technology. The horizontally polarized pump light is then rotated to vertically polarized by the half-wave plate 6, thereby utilizing the maximum nonlinear coefficient of the LiNbO3 crystal 9, and the pump light is beam-contracted by the beam contraction system, and the grating is imaged into the LiNbO3 crystal 9 to repair the pulse broadening and spatial dispersion of the pump light pulse caused by the grating angular dispersion, and then The pump light enters the LiNbO3 crystal 9 and a nonlinear effect occurs, generating a strong terahertz. The terahertz detector 12 is used to detect the generation of terahertz. By rotating the angles of the THz polarizer 10 and the THz quarter-wave plate 11, the generated terahertz wave is adjusted to different polarization states. The optical rectification method is used to generate terahertz, and the tilted wavefront technology and the maximum nonlinear coefficient of the LiNbO3 crystal are used to enhance the generated terahertz. The generated terahertz can be modulated into different polarization states through the THz polarizer and the THz quarter-wave plate.

[0036] In one embodiment, the beam expansion and collimation system includes a beam expander 2 and a first lens 3 arranged in sequence along the propagation direction of the pump light. The pump light first passes through the beam expander 2 and the first lens 3 for beam expansion and collimation, and then passes through a reflector 4 to be guided to the blazed grating 5.

[0037] In one embodiment, two lenses form a beam reduction system, the center of the light beam emitted by the blazed grating 5 is at the focus of the second lens 7, and the incident surface of the LiNbO3 crystal 9 is located at the focus of the third lens 8. The second lens 7 is a plano-convex cylindrical lens with a focal length of f1=200 mm, and the third lens 8 is a bi-convex cylindrical lens with a focal length of f2=100 mm. The center of the grating is at the focus of the second lens 7, and the incident surface of the LiNbO3 crystal is located at the focus of the third lens 8, and the zoom factor f1 / f2=2 times.

[0038] In one embodiment, a broadband ultrashort pulse laser is used to irradiate a LiNbO3 crystal 9. Due to the existence of nonlinear optical effects in the crystal, the frequency difference between two frequencies in the spectrum of the pump light is mixed in the terahertz band to output difference frequency light, i.e., light in the terahertz band.

[0039] In one embodiment, Figure 2 As shown in the figure, collinear velocity matching is difficult for a crystal such as LiNbO3. However, the tilted wavefront technology can be used to solve this velocity matching problem. The angle between the isointensity surface of the pump light pulse, i.e., the wavefront and the isophase surface, is called the tilted wavefront angle γ. The group velocity of the pump light pulse propagating in the medium is v g , because there is an angle between the wavefront and the propagation direction, the propagation speed of the wavefront v THz v g Therefore, the propagation speed of the wavefront is less than that of the pump light pulse. By adjusting the appropriate tilt angle, the propagation speed of the wavefront can be equal to the phase velocity of the terahertz wave in the medium, thus achieving speed matching.

[0040] According to the geometric relationship between the grating equation and the optical path, the incident angle α and diffraction angle β of the blazed grating 5 can be obtained. The calculation formula is as follows:

[0041]

[0042]

[0043] Where m is the reduction factor of the beam reduction system, λ is the central wavelength of the incident light, i.e. the pump source, γ is the wedge angle of the LiNbO3 lithium niobate crystal, which is generally 63°, and n g is the group refractive index of LiNbO3 lithium niobate crystal corresponding to the central wavelength λ, and d is the grating constant.

[0044] In this embodiment, the incident pump source is a horizontally polarized femtosecond laser with a power of 6W, a central wavelength of 800nm, and a frequency of 1KHz; the blazed grating is 600 lines, a central wavelength of 800nm, and a diffraction order of 1; the lithium niobate crystal uses CLN, a wedge angle of 63°, and an incident surface of 10mm*10mm; the lens group reduction factor m=2; for the sLiNbO3 crystal, the corresponding group refractive index at 800nm ​​is n g =2.264. According to the geometric relationship between the grating equation and the optical path, the incident angle α of the grating is calculated to be -29.7° and the diffraction angle β of the grating is 77.32°. The terahertz system is built according to the calculated parameters.

[0045] In one embodiment, the pump light is a femtosecond pulse pump light, and the terahertz wave output power and the parameters of the incident femtosecond pulse pump light satisfy the following formula:

[0046]

[0047] Among them, d eff is the second-order effective nonlinear coefficient of LiNbO3 crystal, c is the speed of light in vacuum, ε0 is the vacuum dielectric constant, η0 is the vacuum impedance coefficient, n g is the group refractive index of the pump light in the crystal, n p is the refractive index of the pump light, L is the crystal length, f is the pump light repetition frequency, τ is the incident pump light pulse width, A p is the area of ​​pumping light, P p is the average power of the incident pump light.

[0048] From formula (3), we can know that d eff The larger the crystal, the higher the terahertz wave output power. Selecting LiNbO3 with a larger nonlinear coefficient and using a half-wave plate to convert the horizontally polarized pump light into a horizontally polarized pump light is also to utilize the crystal phase of LiNbO3 with the largest nonlinear coefficient, increase the average power of the pump light and use a beam expander to increase the spot size, which can also increase the terahertz output power.

[0049] In one embodiment, a terahertz detector 12 is further included, which is used to receive the terahertz wave generated by the pumping light irradiating the LiNbO 3 crystal 9 .

[0050] The novel terahertz wave generator provided in the first aspect of the embodiment is a novel light source capable of generating and flexibly regulating the polarization state of terahertz waves. It has broad application prospects in the fields of scientific research, communications, and biomedicine, especially the unique advantages of circularly polarized terahertz waves in detecting magnetic substances and chiral biomolecules. It has a wide range of applications, including but not limited to non-destructive testing, biomolecule detection, wireless communication, remote sensing detection, etc., providing strong support for scientific and technological progress and industrial development.

[0051] The second aspect of the embodiment of the present invention discloses a polarization state control method according to the first aspect of the embodiment, and the following operations can be performed to complete the detection of terahertz polarization, including the following steps:

[0052] The terahertz wave formed by the LiNbO3 crystal 9 is vertically incident on the THz polarizer 10 and the THz quarter wave plate 11 in sequence;

[0053] Rotate the THz polarizer to control the generated terahertz wave into a linearly polarized terahertz wave in any direction;

[0054] Rotate the THz quarter wave plate 11 so that the angle between the optical axis (fast axis or slow axis) and the vibration direction of the terahertz polarized wave is 0° or 90°, thereby obtaining linearly polarized light;

[0055] When the THz quarter wave plate 11 is rotated so that the angle between the optical axis (fast axis or slow axis) and the vibration direction of the THz polarized wave is 45°, circularly polarized light is obtained;

[0056] When the THz quarter wave plate 11 is rotated so that the angle between the optical axis (fast axis or slow axis) and the vibration direction of the terahertz polarized light is an angle other than 0°, 45°, and 90°, elliptically polarized light is obtained.

[0057] The terahertz polarization wave generator provided by the present invention is introduced in detail above. The principle and implementation mode of the present invention are explained in this article by using specific examples. The description of the above embodiments is only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation mode and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

[0058] In this article, relational terms such as first and second, etc. are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "comprise", "include" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the statement "comprise a ..." do not exclude the presence of other identical elements in the process, method, article or device including the elements.

Claims

1. A terahertz polarization wave generator, characterized in that: The invention comprises a pump source (1), a beam expansion and collimation system, a reflector (4), a blazed grating (5), a half-wave plate (6), a beam reduction system, a LiNbO3 crystal (9), a THz polarizer (10), a THz quarter-wave plate (11) and a terahertz detector (12) which are arranged in sequence according to the propagation direction of the pump light; wherein: The blazed grating (5) is used to achieve wavefront tilt of the incident light, so that the group velocity of the pump light is equal to the phase velocity of the terahertz wave generated by the LiNbO3 crystal (9); By rotating the angles of the THz polarizer (10) and the THz quarter-wave plate (11), the polarization state of the generated terahertz wave is adjusted.

2. The terahertz polarization wave generator according to claim 1, characterized in that: The pump light irradiates the LiNbO3 crystal (9), and in the spectrum of the pump light, the frequency difference between two frequencies is mixed in the terahertz band component to output difference frequency light, that is, light in the terahertz band.

3. The terahertz polarization wave generator according to claim 1, characterized in that: The calculation formulas for the incident angle α and the diffraction angle β of the blazed grating (5) are as follows: Where m is the reduction factor of the beam reduction system, λ is the central wavelength of the incident light, i.e. the pump source, γ is the wedge angle of the LiNbO3 lithium niobate crystal, and n is g is the group refractive index of LiNbO3 lithium niobate crystal corresponding to the central wavelength λ, and d is the grating constant.

4. The terahertz polarization wave generator according to claim 1, characterized in that: The pump light is a femtosecond pulse pump light; the terahertz wave output power and the parameters of the incident femtosecond pulse pump light satisfy the following formula: Among them, d eff is the second-order effective nonlinear coefficient of LiNbO3 crystal, c is the speed of light in vacuum, ε0 is the vacuum dielectric constant, η0 is the vacuum impedance coefficient, n g is the group refractive index of the pump light in the crystal, n p is the refractive index of the pumping light, L is the crystal length, f is the repetition frequency of the pumping light, τ is the pulse width of the incident pumping light, and A p is the area of ​​pumping light, P p is the average power of the incident pump light.

5. The terahertz polarization wave generator according to claim 1, characterized in that: It also includes a terahertz detector (12) for receiving terahertz waves generated by pumping light irradiating the LiNbO3 crystal (9).

6. A method for controlling the polarization state of a terahertz polarization wave generator according to any one of claims 1 to 5, characterized in that: The steps include: The terahertz wave formed by the LiNbO3 crystal (9) is incident vertically on the THz polarizer (10) and the THz quarter-wave plate (11) in sequence; Rotate the THz polarizer to control the generated terahertz wave into a linearly polarized terahertz wave in any direction; Rotating the THz quarter wave plate (11) so that the angle between the optical axis and the vibration direction of the terahertz polarized wave is 0° or 90°, thereby obtaining linearly polarized light; Rotating the THz quarter wave plate (11) so that the angle between the optical axis and the vibration direction of the terahertz polarized wave is 45°, thereby obtaining circularly polarized light; When the THz quarter wave plate (11) is rotated so that the angle between the optical axis and the vibration direction of the terahertz polarized light is an angle other than 0°, 45° and 90°, elliptically polarized light is obtained.

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