Terahertz wave generator

Abstract

The terahertz wave generator is characterized in that pump light emitted by a pump source is split into a first light path and a second light path through a beam splitter prism; the first light path comprises a beam expanding and collimating system, a blazed grating, a half-wave plate, a beam shrinking system, a LiNbO3 crystal and a first light path adjusting system which are sequentially arranged along the beam splitting propagation direction of the pump 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 second optical path comprises a photoconductive antenna and a second optical path adjusting system which are sequentially arranged along the beam splitting propagation direction of the pump light; and the emergent terahertz wave of the first light path and the emergent terahertz wave of the second light path are subjected to terahertz wave beam combination through a triangular mirror to obtain the combined terahertz wave. According to the invention, femtosecond laser energy can be fully utilized, and strong terahertz waves can be generated.

Description

Technical Field

[0001] The present invention relates to the field of terahertz optical technology, and in particular to a terahertz wave generator based on a photoconductive antenna method and an optical rectification method. 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] However, when 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 efficiency of terahertz wave generation will no longer increase linearly with the increase of power, but will tend to saturation 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 and fully utilize the energy of femtosecond laser is a technical problem that needs to be solved urgently by those skilled in the art. Summary of the invention

[0007] In view of the above research status and existing problems, the present invention provides a terahertz wave generator, which generates terahertz waves by using optical rectification method and photoconductivity method respectively, and enhances the generated terahertz waves by using tilted wavefront technology and the maximum nonlinear coefficient of nonlinear crystal, adjusts the optical path difference of the two paths, and finally converges to generate terahertz waves. The present invention can make the most of the femtosecond laser energy and obtain stronger terahertz waves.

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

[0009] The first optical path includes a beam expansion and collimation system, a blazed grating, a half-wave plate, a beam reduction system, a LiNbO3 crystal and a first optical path adjustment system arranged in sequence along the propagation direction of the pump light beam; 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;

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

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

[0012] Preferably, the first optical path further comprises a reflector system for reflecting the light beam of the first optical path split by the beam splitting prism and making it incident on the beam expansion and collimation system.

[0013] 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.

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

[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 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.

[0020] Preferably, the pump light energy of the second optical path is greater than the band gap energy of the semiconductor in the photoconductive antenna to stimulate 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 point and the lowest point of the pulse energy of the outgoing terahertz wave of the first optical path and the outgoing terahertz wave of the second optical path both arrive at the triangular mirror at the same time.

[0022] Preferably, the first optical path adjustment system and the second optical path adjustment system both include a plurality of off-axis parabolic reflectors.

[0023] Preferably, the outgoing terahertz wave of the first optical path and the outgoing terahertz wave of the second optical path are respectively irradiated on two adjacent side surfaces of the tip of the triangular mirror, and the transmission directions of the two light beams are parallel after being reflected by the triangular mirror.

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

[0025] Compared with the prior art, it has the following beneficial effects:

[0026] The present invention proposes a novel terahertz wave generator, which generates terahertz waves by using optical rectification and photoconductivity respectively, and enhances the generated terahertz waves by using tilted wavefront technology and the maximum nonlinear coefficient of nonlinear crystals, and adjusts the optical path difference of the two paths by using a curved reflector, and finally converges the generated terahertz waves by using a triangular mirror. The present invention can make the most of the femtosecond laser energy and generate stronger terahertz waves.

[0027] The present invention has a wide range of applications. In the fields of pharmaceuticals, automobiles, and bio-imaging, strong terahertz sources are used for spectral detection, material analysis, and bio-imaging due to their unique characteristics such as low photon energy, no absorption in non-polar materials, and covering the vibration and rotation modes of organic and inorganic macromolecules. In addition, strong terahertz sources also show great potential in the field of communications, especially in future communication technologies such as 6G, where their high speed, large capacity, and low latency are expected to promote the revolutionary development of communication technology. With the continuous advancement of optoelectronics and semiconductor technologies, the generation and detection technology of strong terahertz sources is also constantly improving, providing the possibility for its application in more fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] 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.

[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 a tilted wavefront provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0031] 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.

[0032] like Figure 1 As shown, the terahertz wave generator disclosed in the embodiment of the present invention includes a pump source 1 that emits pump light that is split into a first light path and a second light path by a beam splitter prism 2; the pump light is split by the beam splitter prism 2, one path generates terahertz waves by using an optical rectification method, and the other path generates terahertz waves by using a photoconductive antenna. Among them, the first optical path includes a beam expansion and collimation system, a blazed grating 7, a half-wave plate 8, a beam reduction system, a LiNbO3 crystal 11 and a first optical path adjustment system which are sequentially arranged along the propagation direction of the pump light beam; the blazed grating 5 is used to realize the 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 11; the second optical path includes a photoconductive antenna 14 and a second optical path adjustment system which are sequentially arranged along the propagation direction of the pump light beam, and the pump light directly hits the photoconductive antenna 14 to excite the terahertz wave; the outgoing terahertz wave of the first optical path and the outgoing terahertz wave of the second optical path 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 light beam of the first optical path split by the beam splitting prism 2 and making it incident on the beam expansion and collimation system.

[0034] In this embodiment, the reflector system includes a plane reflector 1 3 and a plane reflector 2 4 arranged along the optical axis. The pump light in the first optical path first undergoes secondary reflection by the plane reflector 1 3 and the plane reflector 2 4. The plane mirrors are used to adjust the position and direction of the incident light beam, and then the beam is expanded and collimated by the beam expansion and collimation system, and finally guided to the blazed grating 7.

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

[0036] In one embodiment, two lenses form a beam reduction system, the center of the outgoing light beam of the blazed grating 5 is at the focus of the second lens 9, and the incident surface of the LiNbO3 crystal 11 is located at the focus of the third lens 10. The second lens 9 is a plano-convex cylindrical lens with a focal length of f1=200mm, and the third lens 10 is a biconvex cylindrical lens with a focal length of f2=100mm. The center of the blazed grating 5 is at the focus of the second lens 9, and the incident surface of the LiNbO3 crystal is located at the focus of the third lens 10, and the zoom factor f1 / f2=2 times.

[0037] In one embodiment, the pump light irradiates the LiNbO3 crystal 11. Due to the existence of nonlinear optical effects in the crystal, the frequency difference between the two frequencies in the spectrum of the pump light is mixed in the terahertz band component, and the difference frequency light, i.e., the light in the terahertz band, is output. The blazed grating 7 is used to implement the tilted wavefront technology, and then the horizontally polarized pump light is rotated to vertical polarization through the half-wave plate 8, thereby utilizing the maximum nonlinear coefficient of the LiNbO3 crystal. The grating is imaged into LiNbO3 to repair the pulse broadening and spatial dispersion of the pump light pulse caused by the grating role dispersion, and then the pump light enters LiNbO3 to produce nonlinear effects and generate terahertz waves. The frequency band range of terahertz waves generated by optical rectification can generally extend from the radio frequency region to the far infrared region, that is, roughly between 0.1 and 3 THz, and can also be extended to higher frequency bands.

[0038] 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.

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

[0040]

[0041] 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.

[0042] 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.

[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] 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.

[0046] 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.

[0047] In one embodiment, the pump light of the second optical path is irradiated onto the surface of a semiconductor material having photoconductive properties, and the pump light energy of the second optical path is greater than the band gap energy of the semiconductor in the photoconductive antenna 14 to stimulate the photoelectric effect and the photoconductive effect, that is, the laser irradiation of the photoconductive antenna will excite electrons and holes and form an electron-hole plasma, and the electrons and holes in this plasma will be accelerated by the photoelectric field and form a current. In the photoconductive effect, the acceleration process of electrons and holes will generate electromagnetic radiation in the terahertz wave frequency band, which can be radiated and received by an appropriate antenna structure. The frequency band range of the terahertz wave generated by the photoconductive antenna is roughly between 0.1THz and 3THz.

[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 point and the lowest point of the pulse energy of the outgoing terahertz wave of the first optical path and the outgoing terahertz wave of the second optical path both arrive at the triangular mirror 18 at the same time to avoid affecting the subsequent time domain system.

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

[0050] In one embodiment, the outgoing terahertz wave of the first optical path and the outgoing terahertz wave of the second optical path are respectively irradiated on two adjacent side surfaces of the tip of the triangular mirror 18 , and after being reflected by the triangular mirror 18 , the transmission directions of the two light beams are parallel.

[0051] In this embodiment, when the convergence angles of the outgoing terahertz wave of the first optical path and the outgoing terahertz wave of the second optical path are the same, they can be approximately regarded as a point light source emitting light from the top of the triangular mirror. By simply setting an off-axis parabolic reflector 19 at a suitable position for collimation, a collimated terahertz beam of arbitrary aperture can be obtained.

[0052] The terahertz wave generator provided by the embodiment of the present invention generates strong terahertz waves. Strong terahertz sources, with their unique physical properties, such as low photon energy, non-absorption in non-polar materials, and wide coverage of macromolecular vibration and rotation modes, have shown great application potential and development prospects in many fields such as spectral detection, material analysis, biological imaging, communications, security inspection, environmental science, and basic scientific research.

[0053] The terahertz 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.

[0054] 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 wave generator, characterized in that: The method comprises: the pump light emitted by the pump source (1) is split into a first light path and a second light path by a beam splitting prism (2); wherein: The first optical path comprises a beam expansion and collimation system, a blazed grating (7), a half-wave plate (8), a beam reduction system, a LiNbO3 crystal (11) and a first optical path adjustment system which are sequentially arranged along the propagation direction of the pump light beam splitting; 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 (11); The second optical path comprises a photoconductive antenna (14) and a second optical path adjustment system which are sequentially arranged along the propagation direction of the pump light beam; The outgoing terahertz wave of the first optical path and the outgoing terahertz wave of the second optical path are combined by a triangular mirror (18) to obtain a combined terahertz wave.

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

3. The terahertz wave generator according to claim 1, characterized in that: The pump light irradiates the LiNbO3 crystal (11), 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.

4. The terahertz 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 (7) 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.

5. The 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: 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.

6. The terahertz wave generator according to claim 1, characterized in that: The pump light energy of the second optical path is greater than the band gap energy of the semiconductor in the photoconductive antenna (14) so ​​as to stimulate the photoelectric effect and the photoconductive effect.

7. The terahertz wave generator according to claim 1, characterized in that: 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 point and the lowest point of the pulse energy of the outgoing terahertz wave of the first optical path and the outgoing terahertz wave of the second optical path both arrive at the triangular mirror (18) at the same time.

8. The terahertz wave generator according to claim 1, characterized in that: The first optical path adjustment system and the second optical path adjustment system both include a plurality of off-axis parabolic reflectors.

9. The terahertz wave generator according to claim 1, characterized in that: The outgoing terahertz wave of the first optical path and the outgoing terahertz wave of the second optical path are respectively irradiated onto two adjacent side surfaces of the tip of the triangular mirror (18), and after being reflected by the triangular mirror (18), the transmission directions of the two light beams are parallel.

10. The terahertz wave generator according to claim 9, characterized in that: The light beam reflected by the triangular mirror (18) is collimated by an off-axis parabolic reflector to obtain a collimated terahertz beam of a target aperture.

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