A terahertz wave generator

Abstract

The application discloses a terahertz wave generator, comprising: a pump light emitted by a pump source is split into a first light path and a second light path by a beam splitter prism; the first light path comprises, sequentially arranged along the propagation direction of the pump light splitting, an expansion collimation system, a blazed grating, a half-wave plate, a contraction system, a LiNbO3 crystal and a first optical path adjustment system; the blazed grating is used for realizing wavefront tilting of incident light, so that the group velocity of the pump light is equal to the phase velocity of the LiNbO3 crystal for generating terahertz waves; the second light path comprises, sequentially arranged along the propagation direction of the pump light splitting, a photoconductive antenna and a second optical path adjustment system; the emitted terahertz waves of the first light path and the emitted terahertz waves of the second light path are combined by a triangular mirror to obtain combined terahertz waves. The application can make full use of femtosecond laser energy to generate strong terahertz waves.

Description

Technical Field

[0001] This invention relates to the field of terahertz optical technology, and in particular to a terahertz wave generator based on photoconductive antenna method and optical rectification method. Background Technology

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

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

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

[0005] However, as the input power of the femtosecond laser pump source continues to increase, due to the saturation effect and field shielding effect inside the nonlinear medium, the generation efficiency of terahertz waves will no longer increase linearly with the increase of power, but will tend to saturate or even decrease, making it impossible to fully utilize the energy of the femtosecond laser pump source.

[0006] Therefore, how to provide a terahertz wave generator that can generate strong terahertz waves while making full use of femtosecond laser energy is a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0007] This invention addresses the aforementioned research status and existing problems by providing a terahertz wave generator. It utilizes optical rectification and photoconductivity to generate terahertz waves, and then enhances the generated terahertz waves by employing tilted wavefront technology and utilizing the maximum nonlinear coefficient of a nonlinear crystal. By adjusting the path difference between the two optical paths, the generated terahertz waves are ultimately converged. This invention enables the most efficient utilization of femtosecond laser energy and yields a strong terahertz wave.

[0008] The present invention first provides a terahertz wave generator, comprising a pump source emitting pump light which is split into a first optical path and a second optical path by a beam splitter prism; wherein,

[0009] The first optical path includes a beam expanding and collimating system, a blazed grating, a half-wave plate, a beam shrinking system, a LiNbO3 crystal, and a first optical path adjustment system arranged sequentially along the direction of pump light beam splitting and propagation; the blazed grating is used to achieve wavefront tilting 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;

[0010] The second optical path includes a photoconductive antenna and a second optical path adjustment system arranged sequentially along the direction of pump light beam splitting propagation;

[0011] The terahertz waves emitted from the first optical path and the terahertz waves emitted from the second optical path are combined by a triangular mirror to obtain a combined terahertz wave.

[0012] Preferably, the first optical path further includes a reflector system for reflecting the beam from the first optical path split by the beam splitter prism and incident it onto the beam expander and collimator system.

[0013] Preferably, the pump light irradiates the LiNbO3 crystal, and the frequency difference between the two frequencies in the pump light spectrum is mixed with the components in the terahertz band to output difference frequency light, i.e., light in the terahertz band.

[0014] Preferably, the formulas for calculating the incident angle α and diffraction angle β of the blazed grating are as follows:

[0015]

[0016] Where m is the reduction factor of the beam-shrinking system, λ is the center wavelength of the incident light (i.e., the pump source), γ is the wedge angle of the LiNbO3 lithium niobate crystal, and n g λ is the group refractive index of the LiNbO3 lithium niobate crystal with respect 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] Where, d eff Here, ε0 is the second-order effective nonlinear coefficient of the LiNbO3 crystal, c is the speed of light in vacuum, ε0 is the vacuum permittivity, η0 is the vacuum impedance coefficient, and n0 is the second-order effective nonlinear coefficient of the LiNbO3 crystal. g It 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 repetition frequency of the pump light, τ is the pulse width of the incident pump light, and A p It is the area of ​​the pump light, P p It is the average power of the incident pump light.

[0020] Preferably, the pump light energy of the second optical path is greater than the bandgap energy of the semiconductor in the photoconductive antenna, so as to excite the photoelectric effect and the photoconductive effect.

[0021] Preferably, the first optical path adjustment system and the second optical path adjustment system make the optical path difference between the first optical path and the second optical path a fixed value; the highest and lowest pulse energy points of the emitted terahertz waves from the first optical path and the emitted terahertz waves from the second optical path arrive at the triangular mirror simultaneously.

[0022] Preferably, both the first optical path adjustment system and the second optical path adjustment system include multiple off-axis parabolic mirrors.

[0023] Preferably, the terahertz waves emitted from the first optical path and the terahertz waves emitted from the second optical path respectively irradiate two adjacent sides of the tip of the triangular mirror, and the transmission directions of the two beams are parallel after being reflected by the triangular mirror.

[0024] Preferably, the beam reflected by the triangular mirror is collimated by an off-axis parabolic mirror to obtain a collimated terahertz beam of the target aperture.

[0025] Compared with existing technologies, it has the following advantages:

[0026] This invention proposes a novel terahertz wave generator. It utilizes optical rectification and photoconductivity to generate terahertz waves, and enhances these waves using tilted wavefront technology and the maximum nonlinear coefficient of a nonlinear crystal. The path difference between the two optical paths is adjusted using a curved mirror, and the generated terahertz wave is finally converged by a triangular mirror. This invention enables the most efficient utilization of femtosecond laser energy, producing a strong terahertz wave.

[0027] This invention has wide applications. In pharmaceuticals, automotive, and bioimaging fields, strong terahertz sources, due to their unique characteristics such as low photon energy, no absorption in nonpolar materials, and coverage of vibrational and rotational modes of both organic and inorganic macromolecules, are used in spectroscopic detection, materials analysis, and bioimaging. Furthermore, strong terahertz sources show great potential in the communications field, particularly in future communication technologies such as 6G, where their high speed, large capacity, and low latency are expected to drive revolutionary developments in communication technology. With the continuous advancement of optoelectronic and semiconductor technologies, the generation and detection technologies of strong terahertz sources are also constantly improving, opening up possibilities for their application in even more fields. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely embodiments of the present invention, and those skilled in the art can obtain other drawings based on the provided drawings without creative effort.

[0029] Figure 1A schematic diagram of a terahertz wave generator provided in an embodiment of the present invention;

[0030] Figure 2 A schematic diagram of the tilted wavefront provided for an embodiment of the present invention. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] like Figure 1 As shown, the terahertz wave generator disclosed in this embodiment of the invention includes a pump light emitted by a pump source 1, which is split into a first optical path and a second optical path by a beam splitter prism 2. After the pump light is split by the beam splitter prism 2, one path generates a terahertz wave using an optical rectification method, and the other path generates a terahertz wave using a photoconductive antenna. The first optical path includes a beam expanding and collimating system, a blazed grating 7, a half-wave plate 8, a beam shrinking system, a LiNbO3 crystal 11, and a first optical path adjustment system arranged sequentially along the direction of pump beam splitting. The blazed grating 7 is used to tilt the wavefront 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 11. The second optical path includes a photoconductive antenna 14 and a second optical path adjustment system arranged sequentially along the direction of pump beam splitting. The pump light directly hits the photoconductive antenna 14 to excite the terahertz wave. The terahertz waves emitted from the first and second optical paths are combined by a triangular mirror 18 to obtain the combined terahertz wave.

[0033] In one embodiment, the first optical path further includes a reflector system for reflecting the beam from the first optical path split by the beam splitter 2 and incident it onto the beam expander and collimator system.

[0034] In this embodiment, the reflector system includes a first plane reflector 3 and a second plane reflector 4 arranged along the optical axis. The pump light in the first optical path is first reflected twice by the first plane reflector 3 and the second plane reflector 4. The plane reflectors are used to adjust the position and direction of the incident beam. Then, the beam is expanded and collimated by the beam expansion and collimation system and then guided onto the blazed grating 7.

[0035] In one embodiment, the beam expanding and collimating system includes a beam expander 5 and a lens 6 arranged along the direction of light propagation.

[0036] In one embodiment, the two lenses form a beam-constricting system. The center of the beam emitted from the blazed grating 7 is at the focal point of lens 2 9, and the incident surface of the LiNbO3 crystal 11 is located at the focal point of lens 3 10. Lens 2 9 is a plano-convex cylindrical lens with a focal length f1 = 200 mm, and lens 3 10 is a biconvex cylindrical lens with a focal length f2 = 100 mm. The center of the blazed grating 7 is at the focal point of lens 2 9, and the incident surface of the LiNbO3 crystal is located at the focal point of lens 3 10. The scaling factor f1 / f2 = 2.

[0037] In one embodiment, pump light illuminates a LiNbO3 crystal 11. Due to nonlinear optical effects within the crystal, the frequency difference between two frequencies in the pump light spectrum, specifically the terahertz components, mix to output difference-frequency light, i.e., terahertz light. A blazed grating 7 is used to implement a tilted wavefront technique. A half-wave plate 8 then rotates the horizontally polarized pump light to vertical polarization, utilizing the maximum nonlinear coefficient of the LiNbO3 crystal. The grating is imaged onto the LiNbO3 to correct pulse broadening and spatial dispersion of the pump light pulse caused by grating angular dispersion. The pump light then enters the LiNbO3, where nonlinear effects generate terahertz waves. The frequency range of terahertz waves generated by optical rectification can generally extend from the radio frequency region to the far-infrared region, approximately between 0.1 and 3 THz, and can also be extended to higher frequency bands.

[0038] In one embodiment, such as Figure 2 As shown, collinear velocity matching is quite difficult for crystals like LiNbO3; however, this velocity matching problem can be solved using the tilted wavefront technique. The angle between the isointense surface (wavefront) and the isophase surface of the pump pulse is called the tilted wavefront angle γ. The group velocity of the pump pulse propagating in the medium is v. g Because there is an angle between the wavefront and the direction of propagation, the propagation speed v of the wavefront is... THz For v g Therefore, the propagation speed of the wavefront is less than the propagation speed of the pump light pulse. Adjusting the appropriate tilt angle can make the propagation speed of the wavefront equal to the phase velocity of the terahertz wave in the medium, thus achieving velocity matching.

[0039] The incident angle α and diffraction angle β of the blazed grating 7 can be obtained based on the grating equation and the geometric relationship of the optical path, and the calculation formula is as follows:

[0040]

[0041] Where m is the reduction factor of the beam-shrinking system, λ is the center wavelength of the incident light (i.e., the pump source), γ is the wedge angle of the LiNbO3 lithium niobate crystal, typically around 63°, and n g λ is the group refractive index of the LiNbO3 lithium niobate crystal with respect to the central wavelength λ, and d is the grating constant.

[0042] In this embodiment, the incident pump source is a horizontally polarized femtosecond laser with a power of 6W, a center wavelength of 800nm, and a frequency of 1kHz; the blazed grating is a 600-line, 800nm ​​center wavelength, first-order diffraction grating; the lithium niobate crystal is a CLN with a wedge angle of 63° and an incident surface of 10mm*10mm; the lens group has a reduction factor of m=2; for the sLiNbO3 crystal, the group refractive index at 800nm ​​is n. g = 2.264. Based on the grating equation and the geometric relationship of 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 calculated to be 77.32°. A terahertz system is then constructed based on the calculated parameters.

[0043] In one embodiment, 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:

[0044]

[0045] Where, d eff Here, ε0 is the second-order effective nonlinear coefficient of the LiNbO3 crystal, c is the speed of light in vacuum, ε0 is the vacuum permittivity, η0 is the vacuum impedance coefficient, and n0 is the second-order effective nonlinear coefficient of the LiNbO3 crystal. g It 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 repetition frequency of the pump light, τ is the pulse width of the incident pump light, and A p It is the area of ​​the pump light, P p It is the average power of the incident pump light.

[0046] From formula (3), we can see that d eff The larger the crystal, the higher the terahertz wave output power. Selecting LiNbO3 with a large nonlinear coefficient and using a half-wave plate to convert the horizontally polarized pump light into a horizontally polarized pump light is also to take advantage of the crystal phase with the largest nonlinear coefficient of LiNbO3, increase the average power of the pump light, and use a beam expander to increase the spot size, which can also improve the terahertz output power.

[0047] In one embodiment, the pump light from the second optical path irradiates the surface of a semiconductor material with photoconductive properties. The energy of the pump light from the second optical path is greater than the bandgap energy of the semiconductor in the photoconductive antenna 14, thereby exciting the photoelectric effect and the photoconductive effect. That is, laser irradiation of the photoconductive antenna will excite electrons and holes, forming an electron-hole plasma. The electrons and holes in this plasma will be accelerated by the photoelectric field and form an electric current. In the photoconductive effect, the acceleration process of electrons and holes will generate electromagnetic radiation in the terahertz wave band. This radiation can be radiated and received through a suitable antenna structure. The frequency range of the terahertz waves generated by the photoconductive antenna is approximately between 0.1 THz and 3 THz.

[0048] In one embodiment, the first optical path adjustment system and the second optical path adjustment system make the optical path difference between the first optical path and the second optical path a fixed value; the highest and lowest pulse energy points of the emitted terahertz waves from the first optical path and the emitted terahertz waves from the second optical path arrive at the triangular mirror 18 simultaneously to avoid affecting the subsequent time-domain system.

[0049] In one embodiment, both the first optical path adjustment system and the second optical path adjustment system include multiple off-axis parabolic mirrors. For example... Figure 1 As shown, it includes off-axis parabolic mirror 3 15, off-axis parabolic mirror 4 16 and off-axis parabolic mirror 5 17 arranged along the optical axis. The optical path difference between the two optical paths is made to a fixed value by adjusting the interval between the off-axis parabolic mirrors.

[0050] In one embodiment, the terahertz waves emitted from the first optical path and the terahertz waves emitted from the second optical path respectively illuminate two adjacent sides of the tip of the triangular mirror 18, and after being reflected by the triangular mirror 18, the transmission directions of the two beams are parallel.

[0051] In this embodiment, when the convergence angles of the emitted terahertz waves from the first optical path and the emitted terahertz waves from the second optical path are the same, it can be approximated as a point source emitting light from the top of the triangular mirror. As long as the off-axis parabolic reflector 619 is set in a suitable position for collimation, a collimated terahertz beam of arbitrary aperture can be obtained.

[0052] The terahertz wave generator provided in this embodiment of the invention produces strong terahertz waves. Strong terahertz sources, with their unique physical properties such as low photon energy, non-absorption in nonpolar materials, and broad coverage of macromolecular vibrational and rotational modes, exhibit enormous application potential and development prospects in multiple fields such as spectroscopic detection, materials analysis, biological imaging, communication, security inspection, environmental science, and basic scientific research.

[0053] The terahertz wave generator provided by the present invention has been described in detail above. Specific examples have been used to illustrate the principle and implementation of the present invention. The description of the above embodiments is only for the purpose of helping to understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation and application scope based on the idea of ​​the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

[0054] In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, without necessarily requiring or implying any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

Claims

1. A terahertz wave generator, characterized in that, The pump light emitted from the pump source (1) is split into a first optical path and a second optical path by a beam splitter (2); wherein, The first optical path includes a beam expanding and collimating system, a blazed grating (7), a half-wave plate (8), a beam contraction system, a LiNbO3 crystal (11), and a first optical path adjustment system arranged sequentially along the direction of pump beam propagation. The blazed grating (7) is used to achieve wavefront tilting 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 (11). The calculation formulas for the incident angle α and the diffraction angle β of the blazed grating (7) are as follows: Where m is the reduction factor of the beam-shrinking system, λ is the center wavelength of the incident light (i.e., the pump source), γ is the wedge angle of the LiNbO3 lithium niobate crystal, and n g λ is the group refractive index of the LiNbO3 lithium niobate crystal with respect to the central wavelength λ, and d is the grating constant. The second optical path includes a photoconductive antenna (14) arranged sequentially along the direction of pump light beam splitting and propagation, and a second optical path adjustment system; The terahertz waves emitted from the first optical path and the terahertz waves emitted from the second optical path are combined by the triangular mirror (18) to obtain the combined terahertz waves. The first optical path adjustment system and the second optical path adjustment system make the optical path difference between the first optical path and the second optical path a fixed value; the highest and lowest pulse energy points of the emitted terahertz wave from the first optical path and the emitted terahertz wave from the second optical path both arrive at the triangular mirror (18) at the same time.

2. The terahertz wave generator according to claim 1, characterized in that, The first optical path also includes a reflector system for reflecting the beam of the first optical path split by the beam splitter prism (2) and incident it onto the beam expansion and collimation system.

3. A terahertz wave generator according to claim 1, characterized in that, The pump light irradiates the LiNbO3 crystal (11). In the spectrum of the pump light, the frequency difference between the two frequencies in the terahertz band is mixed to output the difference frequency light, that is, the light in the terahertz band.

4. A terahertz 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: Where, d eff Here, ε0 is the second-order effective nonlinear coefficient of the LiNbO3 crystal, c is the speed of light in vacuum, ε0 is the vacuum permittivity, η0 is the vacuum impedance coefficient, and n0 is the second-order effective nonlinear coefficient of the LiNbO3 crystal. pg It 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 repetition frequency of the pump light, τ is the pulse width of the incident pump light, and A p It is the area of ​​the pump light, P p It is the average power of the incident pump light.

5. A terahertz wave generator according to claim 1, characterized in that, The pump light energy of the second optical path is greater than the bandgap energy of the semiconductor in the photoconductive antenna (14) to excite the photoelectric effect and the photoconductive effect.

6. A terahertz wave generator according to claim 1, characterized in that, Both the first optical path adjustment system and the second optical path adjustment system include multiple off-axis parabolic mirrors.

7. A terahertz wave generator according to claim 1, characterized in that, The terahertz waves emitted from the first optical path and the terahertz waves emitted from the second optical path respectively irradiate the two adjacent sides of the tip of the triangular mirror (18). After being reflected by the triangular mirror (18), the transmission directions of the two beams are parallel.

8. A terahertz wave generator according to claim 7, characterized in that, The beam reflected by the triangular mirror (18) is collimated by an off-axis parabolic mirror to obtain a collimated terahertz beam of the target aperture.

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