On-chip intermediate infrared frequency doubling polarization composite device and implementation method thereof

Abstract

The invention discloses an on-chip intermediate infrared frequency doubling polarization composite device and an implementation method thereof. According to the invention, the tellurene nanosheets are adopted to show huge second harmonic generation efficiency in an ultra-wideband region from near infrared to intermediate infrared; the intensity of the optical second harmonic signal is the strongest only when the polarization direction of the incident light is vertical to the a axis of the tellurene nanosheet, and the optical second harmonic signal is delustring when the incident light is orthogonal, so that the extinction ratio is very high; according to the invention, excellent broadband mid-infrared second harmonic generation response is shown, the problem that high-efficiency frequency multiplication conversion of mid-infrared band light is difficult to carry out in an on-chip integration environment is hopefully solved, and a new way is opened up for the development of an advanced photon technology; on the basis of obvious anisotropy of tellurium ene nanosheet-optical second harmonic signals, the polarization of the emergent optical second harmonic signals can be regulated and controlled, and the tellurium ene nanosheet-optical second harmonic signal modulator has the characteristics of multifunctionality and convenience in integration; the method is very suitable for large-scale manufacturing, and cost is saved.

Description

Technical Field

[0001] The present invention relates to a composite optical device, and in particular to an on-chip mid-infrared frequency doubling polarization composite device based on a two-dimensional Weyl semiconductor tellurene (Te) nanosheet and a realization method thereof. Background Art

[0002] Optical second harmonic generation (SHG) is a second-order nonlinear optical (NLO) process that coherently combines two photons of equal energy into a photon of double energy, usually in crystals with broken inversion symmetry. SHG plays a key role in various applications such as frequency conversion, coherent light generation, optical signal processing, and imaging. Conventional second-order NLO crystals have a three-dimensional covalent bond structure and require a large light-matter interaction volume and complex phase matching technology, which are generally not suitable for applications in small optoelectronic and all-optical devices, especially in on-chip integration. In contrast, atomically layered materials, such as transition metal chalcogenides, have become a promising alternative due to their unique van der Waals interfaces, integration capabilities without dangling bonds, and relaxed phase matching conditions due to their atomic thickness. However, thin-layer materials mean a more limited light-matter interaction distance, which limits the size of nonlinear conversion efficiency, and materials with high frequency conversion efficiency in the currently popular mid-infrared band are also very rare. In the context of on-chip mid-infrared bands, polarization technology - including polarization selection, beam splitting, and modulation - is the most basic means in operations such as light field control and signal processing. By combining these two functions while ensuring frequency doubling efficiency and polarization control capabilities, on-chip composite devices can be developed. Compared with the traditional frequency doubling polarization system of nonlinear crystal + polarizer, it can greatly improve the miniaturization and integration of devices, and will provide new possibilities in the fields of biomedicine, national defense security, gas detection, and infrared communications. Summary of the invention

[0003] In view of the problems existing in the above-mentioned prior art, the present invention proposes an on-chip mid-infrared frequency-doubling polarization composite device and its implementation method, which exhibits huge second harmonic generation (SHG) efficiency in the ultra-wideband region from near-infrared to mid-infrared. Its conversion efficiency is at least two orders of magnitude higher than that of traditional nonlinear material GaSe at the same thickness, and it also has an extremely high extinction ratio as a polarizer.

[0004] An object of the present invention is to provide an on-chip mid-infrared frequency-doubling polarization composite device.

[0005] The on-chip mid-infrared frequency-doubling polarization composite device of the present invention is used to detect the optimal polarization direction of incident light or output an optical second harmonic with a set polarization direction.

[0006] For detecting the optimal polarization direction of incident light, the on-chip mid-infrared frequency-doubling polarization composite device of the present invention comprises: a tellurene nanosheet, a substrate, a polarizer, a focusing element and a spectrometer;

[0007] The tellurene nanosheet is in sheet shape and has a nanometer-level thickness. The tellurene nanosheet has a series of parallel one-dimensional spiral atomic chain structures, and the atomic chain extends in the c-axis direction. The mutually perpendicular a-axis and c-axis are located in the tellurene nanosheet plane. The tellurene nanosheet is arranged on a substrate. For the reflection type, the substrate has a high reflectivity, and for the transmission type, the substrate has a high transmittance.

[0008] Femtosecond pulse light with a wavelength in the near-infrared to mid-infrared band is used as the excitation light, and the excitation light is incident on the tellurene nanosheet; the tellurene nanosheet does not have an inversion symmetry center, and the frequency-doubled polarizability is non-zero, which meets the conditions for generating optical second harmonics; according to density functional theory (DFT) calculations after considering spin-orbit coupling, the band structure of tellurene nano has a narrow band gap of 0.36 eV and multiple Weyl points; for the excitation light in the near-infrared to mid-infrared band, the inter-band transitions near the three Weyl points correspond to the two-photon resonant transitions in the frequency-doubled process; the Bailey connection between the valence band and the conduction band has a topological enhancement effect on the frequency-doubled polarizability; and the thickness of the tellurene nanosheet is at the nanometer level, which is much smaller than the coherent wavelength, and does not need to be restricted by the optical second harmonic phase matching, thereby generating an optical second harmonic signal;

[0009] The polarization direction of the optical second harmonic signal is always along the a-axis and has nothing to do with the polarization direction of the excitation light; however, the intensity of the optical second harmonic signal is related to the polarization direction of the incident excitation light, so the intensity of the optical second harmonic signal can be changed by adjusting the polarization direction of the excitation light.

[0010] For a set excitation light wavelength, the signal intensity of the optical second harmonic signal is related to the thickness of the tellurene nanosheet and the thickness and reflectivity or transmittance of the substrate. According to the interference effect induced by the multi-interface structure of air / tellurene nanosheet / substrate, the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest under the set substrate and at the set excitation light wavelength is obtained; the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest is related to the wavelength of the excitation light;

[0011] The excitation light first passes through a polarizer to set the polarization angle; it is then focused on the surface of the tellurene nanosheet through a focusing element to generate an optical second harmonic signal; the reflected optical second harmonic signal is separated from the fundamental frequency excitation light through a spectroscopic element and is collected by a spectrometer, or the transmitted optical second harmonic signal is collected by a spectrometer;

[0012] The polarization angle of the excitation light is adjusted by controlling the polarizer, and the intensity of the optical second harmonic signal is observed by a spectrometer. When the optical second harmonic signal is the strongest, the polarization angle of the excitation light is the optimal polarization angle of incidence.

[0013] The polarizer includes a polarizer and a half-wave plate, which can freely control the polarization direction of the excitation light.

[0014] For outputting optical second harmonics with a set polarization direction, the on-chip mid-infrared frequency-doubling polarization composite device of the present invention comprises: a tellurene nanosheet, a substrate, a polarizer, a focusing element, an analyzer and a spectrometer; wherein,

[0015] The tellurene nanosheet is a sheet with a nanometer thickness. The tellurene nanosheet has a series of parallel one-dimensional spiral atomic chain structures, and the atomic chain extends in the c-axis direction. The mutually perpendicular a-axis and c-axis are located in the tellurene nanosheet plane. The substrate has high reflectivity, and the tellurene nanosheet is arranged on the substrate. For the reflection type, the substrate has high reflectivity, and for the transmission type, the substrate has high transmittance.

[0016] Femtosecond pulse light with a wavelength in the near-infrared to mid-infrared band is used as the excitation light, and the excitation light is incident on the tellurene nanosheet; the tellurene nanosheet does not have an inversion symmetry center, and the frequency-doubled polarizability is non-zero, which meets the conditions for generating optical second harmonics; according to density functional theory (DFT) calculations after considering spin-orbit coupling, the band structure of tellurene nano has a narrow band gap of 0.36 eV and multiple Weyl points; for the excitation light in the near-infrared to mid-infrared band, the inter-band transitions near the three Weyl points correspond to the two-photon resonant transitions in the frequency-doubled process; the Bailey connection between the valence band and the conduction band has a topological enhancement effect on the frequency-doubled polarizability; and the thickness of the tellurene nanosheet is at the nanometer level, which is much smaller than the coherent wavelength, and does not need to be restricted by the optical second harmonic phase matching, thereby generating an optical second harmonic signal;

[0017] The polarization direction of the optical second harmonic signal is always along the a-axis and has nothing to do with the polarization direction of the excitation light; however, the intensity of the optical second harmonic signal is related to the polarization direction of the incident excitation light, so the intensity of the optical second harmonic signal can be changed by adjusting the polarization direction of the excitation light.

[0018] For a set excitation light wavelength, the signal intensity of the optical second harmonic signal is related to the thickness of the tellurene nanosheet and the thickness and reflectivity or transmittance of the substrate. According to the interference effect induced by the multi-interface structure of air / tellurene nanosheet / substrate, the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest under the set substrate and at the set excitation light wavelength is obtained; the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest is related to the wavelength of the excitation light;

[0019] The excitation light first passes through a polarizer and has a set polarization angle; it is focused on the surface of the tellurene nanosheet through a focusing system to generate an optical second harmonic signal; the reflected optical second harmonic signal is separated from the fundamental frequency excitation light through a beam splitter, and is collected to a spectrometer after passing through an analyzer, or the transmitted optical second harmonic signal is collected to a spectrometer after passing through an analyzer;

[0020] The polarization direction of the optical second harmonic is obtained through a polarizer and a spectrometer; for excitation light of any polarization direction passing through the polarizer, the on-chip mid-infrared frequency doubling polarization composite device can provide an optical second harmonic with a unique polarization direction, and the polarization direction of the optical second harmonic is always along the a-axis of the tellurene nanosheet; by rotating the a-axis of the tellurene nanosheet, the polarization direction of the optical second harmonic is changed, so that the optical second harmonic has a set polarization direction.

[0021] The polarizer adopts the first polarizer and the first half-wave plate; the analyzer adopts the second polarizer and the second half-wave plate; the excitation light first passes through the first polarizer to lock the polarization state, and then passes through the first half-wave plate to set the polarization angle; the optical second harmonic signal first passes through the second half-wave plate and then passes through the second polarizer; by rotating the second half-wave plate, the rotation angle of the fast axis of the second half-wave plate when the signal collected by the spectrometer is the strongest is obtained; according to the rotation angle of the fast axis of the second half-wave plate, the polarization direction of the optical second harmonic signal is obtained, and the polarization direction is twice the rotation angle of the fast axis.

[0022] The thickness of the tellurene nanosheets is 10 to 50 nm.

[0023] The pulse width of the excitation light is 100 to 250 fs, and the instantaneous light intensity is high.

[0024] The light splitting element uses a beam splitter or a dichroic mirror, which can separate the optical second harmonic from the excitation light in space.

[0025] The focusing element adopts an objective lens or a convex lens.

[0026] The spectrometer includes a filter, a monochromator and a detector. The filter removes the excitation light and only retains the second harmonic. The monochromator separates the collected signal according to the wavelength. The detector finally converts the separated second harmonic signal into an electrical signal and detects it. For the reflection type, the spectrometer and the excitation light are located on the same side of the tellurene nanosheet; for the transmission type, the spectrometer and the excitation light are located on both sides of the tellurene nanosheet.

[0027] For the reflective type, the substrate reflectivity is ≥50%; for the transmissive type, the substrate transmittance is ≥50%.

[0028] According to the density functional theory (DFT) considering spin-orbit coupling, the second-order nonlinear polarizability χ is numerically calculated from the Berry connection of the transition near the Weyl point. (2) .

[0029] Tellurene nanosheets belong to the D3 point group. The symmetry analysis of the second-order polarizability tensor shows that for the case of normal incidence, i.e., perpendicular to the surface of the tellurene nanosheet; the polarization direction of the incident excitation light is at an angle α to the a-axis, the polarization direction of the optical second harmonic signal only extends along the a-axis, and the intensity of the optical second harmonic signal is proportional to cos 4 α, that is, when α=90°, the light is extinct, and when α=0°, the optical second harmonic signal is the strongest.

[0030] According to the interference effect induced by the multi-interface structure of air / tellurene nanosheet / substrate, the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest under the corresponding excitation light wavelength is calculated; the longer the excitation light wavelength, the thicker the tellurene nanosheet when the optical second harmonic signal is the strongest. When the excitation light wavelength is 2μm, the thickness of the tellurene nanosheet is 20nm; when the excitation light wavelength is 1-1.2μm, the thickness of the tellurene nanosheet is 18nm.

[0031] Another object of the present invention is to provide a method for realizing an on-chip mid-infrared frequency doubling polarization composite device.

[0032] The method for realizing the on-chip mid-infrared frequency doubling polarization composite device of the present invention is used to detect the optimal polarization direction of incident light, and comprises the following steps:

[0033] 1) According to the interference effect induced by the multi-interface structure of air / tellurene nanosheet / substrate, the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest under a set wavelength of the excitation light and a set substrate is calculated;

[0034] 2) Device settings:

[0035] The tellurene nanosheet is in sheet shape and has a thickness set in step 1). The tellurene nanosheet has a series of parallel one-dimensional spiral atomic chain structures, and the atomic chain extends in the c-axis direction; the mutually perpendicular a-axis and c-axis are located in the tellurene nanosheet plane; the tellurene nanosheet is arranged on a substrate;

[0036] 3) Femtosecond pulse light with a wavelength in the near-infrared to mid-infrared band is used as the excitation light. The excitation light first passes through a polarizer to set the polarization angle. After being converged by a focusing element, the excitation light is incident on the tellurene nanosheet. The tellurene nanosheet does not have an inversion symmetry center, and the frequency-doubled polarizability is not 0, which meets the conditions for generating optical second harmonics. According to density functional theory (DFT) calculations after considering spin-orbit coupling, the band structure of tellurene nano has a narrow band gap of 0.36 eV and multiple Weyl points. For the excitation light in the near-infrared to mid-infrared band, the interband transitions near the three Weyl points correspond to the two-photon resonant transitions in the frequency-doubled process. The Bailey connection between the valence band and the conduction band has a topological enhancement effect on the frequency-doubled polarizability. In addition, the thickness of the tellurene nanosheet is nanometer-scale, which is much smaller than the coherent wavelength, and does not need to be restricted by the optical second harmonic phase matching, thereby generating an optical second harmonic signal.

[0037] 4) The reflected optical second harmonic signal is separated from the fundamental frequency excitation light through a spectroscopic element and is collected into a spectrometer, or the transmitted optical second harmonic signal is collected into a spectrometer;

[0038] 5) The polarization direction of the optical second harmonic signal is always along the a-axis, which is independent of the polarization direction of the excitation light; however, the intensity of the optical second harmonic signal is related to the polarization direction of the incident excitation light; by controlling the polarizer, adjusting the polarization angle of the excitation light, and adjusting the polarization direction of the excitation light, the intensity of the optical second harmonic signal can be changed;

[0039] 6) Observe the intensity of the optical second harmonic signal through a spectrometer. When the optical second harmonic signal is the strongest, the polarization angle of the excitation light at this time is the optimal polarization angle of incidence.

[0040] Wherein, in step 1), for a set wavelength of excitation light, the signal intensity of the optical second harmonic signal is related to the thickness of the tellurene nanosheet and the thickness and reflectivity or transmittance of the substrate, and according to the interference effect induced by the multi-interface structure of air / tellurene nanosheet / substrate, the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest under the set substrate and at the set wavelength of excitation light is obtained; the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest is related to the wavelength of the excitation light.

[0041] In step 2), for the reflective type, the substrate has a high reflectivity, for the transmissive type, the substrate reflectivity is ≥50%, the substrate has a high transmittance, and the substrate transmittance is ≥50%. The tellurene nanosheet has a thickness of 10-50 nm.

[0042] In step 3), the pulse width of the excitation light is 100 to 250 fs, and the instantaneous light intensity is high.

[0043] The method for realizing the on-chip mid-infrared frequency doubling polarization composite device of the present invention is used to output an optical second harmonic with a set polarization direction, comprising the following steps:

[0044] 1) According to the interference effect induced by the multi-interface structure of air / tellurene nanosheet / substrate, the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest under a set wavelength of the excitation light and a set substrate is calculated;

[0045] 2) Device settings:

[0046] The tellurene nanosheet is in sheet shape and has a thickness set in step 1). The tellurene nanosheet has a series of parallel one-dimensional spiral atomic chain structures, and the atomic chain extends in the c-axis direction; the mutually perpendicular a-axis and c-axis are located in the tellurene nanosheet plane; the tellurene nanosheet is arranged on a substrate;

[0047] 3) Femtosecond pulse light with a wavelength in the near-infrared to mid-infrared band is used as the excitation light. The excitation light first passes through a polarizer to set the polarization angle. After being converged by a focusing element, the excitation light is incident on the tellurene nanosheet. The tellurene nanosheet does not have an inversion symmetry center, and the frequency-doubled polarizability is not 0, which meets the conditions for generating optical second harmonics. According to density functional theory (DFT) calculations after considering spin-orbit coupling, the band structure of tellurene nano has a narrow band gap of 0.36 eV and multiple Weyl points. For the excitation light in the near-infrared to mid-infrared band, the interband transitions near the three Weyl points correspond to the two-photon resonant transitions in the frequency-doubled process. The Bailey connection between the valence band and the conduction band has a topological enhancement effect on the frequency-doubled polarizability. In addition, the thickness of the tellurene nanosheet is nanometer-scale, which is much smaller than the coherent wavelength, and does not need to be restricted by the optical second harmonic phase matching, thereby generating an optical second harmonic signal.

[0048] 4) The reflected optical second harmonic signal is separated from the fundamental frequency excitation light through a beam splitter, and is collected into a spectrometer after passing through an analyzer, or the transmitted optical second harmonic signal is collected into a spectrometer after passing through an analyzer;

[0049] 5) Obtain the polarization direction of the optical second harmonic through a polarizer and a spectrometer;

[0050] 6) For the excitation light of any polarization direction passing through the polarizer, the polarization direction of the optical second harmonic is always along the a-axis of the tellurene nanosheet; by rotating the a-axis of the tellurene nanosheet, the polarization direction of the optical second harmonic is changed, so that the optical second harmonic has a set polarization direction.

[0051] Wherein, in step 6), the tellurene nanosheet belongs to the D3 point group, and the second-order polarizability tensor is analyzed for symmetry. For the case of normal incidence, i.e., perpendicular to the surface of the tellurene nanosheet; the polarization direction of the incident excitation light is at an angle α to the a-axis, the polarization direction of the optical second harmonic signal only extends along the a-axis, and the intensity of the optical second harmonic signal is proportional to cos 4α, that is, when α=90°, the light is extinct, and when α=0°, the optical second harmonic signal is the strongest.

[0052] Advantages of the present invention:

[0053] The invention shows a huge second harmonic generation efficiency in the ultra-wideband region from near-infrared to mid-infrared, and its conversion efficiency is at least two orders of magnitude higher than that of the traditional nonlinear material GaSe at the same thickness; and the intensity of the optical second harmonic signal reaches the strongest only when the polarization direction of the incident light is along the a-axis of the tellurene nanosheet, and extinguishes when it is orthogonal, and has a high extinction ratio; the ultra-wideband response of the tellurene nanosheet is attributed to the topological enhancement of the double-frequency polarizability by the combined effect of three Weyl cones in the valence band and the conduction band, and its polarization characteristics are derived from the crystal structure and symmetry characteristics of the tellurene nanosheet; the invention shows an excellent broadband mid-infrared second harmonic generation response, and is very likely to solve the problem of difficulty in high-efficiency double-frequency conversion of mid-infrared band light in an on-chip integrated environment, and opens up a new path for the development of advanced photonic technology; based on the obvious anisotropy of the tellurene nanosheet-optical second harmonic signal, the invention can realize the regulation of the polarization of the output optical second harmonic signal, and has the characteristics of multifunctionality and easy integration; the core tellurene nanosheet is prepared by a hydrothermal method, which is very suitable for large-scale manufacturing and saves costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 It is a schematic diagram of Embodiment 1 of the on-chip mid-infrared frequency doubling polarization composite device of the present invention;

[0055] Figure 2 A schematic diagram of the on-chip mid-infrared frequency doubling polarization composite device of the present invention generating an optical second harmonic signal by a tellurene nanosheet;

[0056] Figure 3 This is a graph showing the frequency doubling polarizability results at different wavelengths of an embodiment of an on-chip mid-infrared frequency doubling polarization composite device of the present invention;

[0057] Figure 4 A polarization characteristic diagram of an optical second harmonic signal of an embodiment of an on-chip mid-infrared frequency-doubling polarization composite device of the present invention;

[0058] Figure 5 It is a schematic diagram of the second embodiment of the on-chip mid-infrared frequency doubling polarization composite device of the present invention. DETAILED DESCRIPTION

[0059] The present invention will be further described below through specific embodiments in conjunction with the accompanying drawings.

[0060] This embodiment is used to detect the optimal polarization direction of incident light.

[0061] The tellurene nanosheets of this embodiment are prepared by a hydrothermal method, which includes the following steps:

[0062] 1) Dissolve 3 g of polyvinylpyrrolidone (PVP, molecular weight = 58000) in 32 mL of deionized water (DL); add 92 mg of Na2TeO3 to the PVP solution while stirring continuously;

[0063] 2) Add 3.32 mL of ammonia water and 1.68 mL of hydrazine hydrate, and after 5 minutes of magnetic stirring, transfer the solution to a stainless steel autoclave lined with polytetrafluoroethylene, heat to 180°C, and maintain for 10 hours;

[0064] 3) Rinse with deionized water to remove other ions;

[0065] 4) The Te nanosheets are redispersed in ethanol and transferred to a substrate using a drop coating method. The substrate is a silicon wafer covered with 285 nm silicon dioxide.

[0066] like Figure 1 As shown, the on-chip mid-infrared frequency doubling polarization composite device of this embodiment includes: a tellurene nanosheet, a substrate, a polarizer, a focusing element, a beam splitting element and a spectrometer;

[0067] Tellurene nanosheets are sheet-shaped and nanometer-thick. They have a series of parallel one-dimensional spiral atomic chain structures, with the atomic chain extending in the c-axis direction. The mutually perpendicular a-axis and c-axis are located in the tellurene nanosheet plane. The tellurene nanosheet is located on a substrate. This structural feature can be observed by annular dark field scanning transmission electron microscopy (ADF-STEM). The sample thickness is determined to be 18nm by atomic force microscopy (AFM), and the lattice orientation can be further confirmed by Raman spectroscopy.

[0068] like Figure 2As shown, a 250kHz titanium sapphire regenerative amplifier (RegA) is used to generate an ultrafast pulse laser with a wavelength of 808nm and a pulse width of 100fs, which is then passed through a mid-infrared optical parametric amplifier (OPA) to generate a 1.2-1.56μm signal light and a 2.4-1.65μm idler light; the signal light and the idler light are then passed through a difference frequency generation optical path (DFG) to generate an excitation light of 2.5-5μm; the excitation light is normally incident on the tellurene nanosheet; the tellurene nanosheet does not have an inversion symmetry center, and the doubled frequency polarization rate is non-zero, which meets the conditions for generating optical second harmonics ; According to density functional theory (DFT) calculations after considering spin-orbit coupling, the band structure of tellurene nanosheets has a narrow band gap of 0.36 eV and multiple Weyl points; for excitation light in the near-infrared to mid-infrared bands, the interband transitions near the three Weyl points correspond to the two-photon resonant transitions in the frequency doubling process; the giant Berry connection between the valence band and the conduction band has a topological enhancement effect on the frequency doubling polarizability; and the thickness of the tellurene nanosheets is at the nanometer level, which is much smaller than the coherent wavelength, and does not need to be restricted by optical second harmonic phase matching, thereby generating optical second harmonic signals. Figure 2 Where ω is the angular frequency of the laser.

[0069] The tests were carried out at room temperature and the Figure 3 The spectral measurement results show that after normalization, tellurene nanosheets have a strong response in the mid-infrared broadband range of 1.2 to 4.86 μm. It can be seen that the device has a strong frequency doubling efficiency in the mid-infrared broadband range. This phenomenon originates from the strong Bailey connection near the Weyl point. In order to calculate the specific second-order polarizability χ (2) In this study, a four-layer material model (air / Te / SiO2 / Si) was used. After normalization, the transfer matrix method and Green's function method were used to calculate the χ (2) Extract Figure 3 Data point in the middle. (2) As the optical second harmonic photon energy rises in the range of 0.56 to 1.12 eV, it gradually decreases as the optical second harmonic photon energy gradually increases to 2.06 eV; a significant 5nm·V-1 peak is observed at 1.12 eV, and two shoulder peaks of about 3.2nm·V-1 and 2.3nm·V-1 are observed at 1.35 eV and 0.77 eV, respectively. Compared with various two-dimensional layered materials and bulk NLO crystals such as GaAs, GaSe, TaAs, etc., it is found that few materials can show such a large χ in such an ultra-wide wavelength MIR range. (2) It can be seen that this material system has great potential in the preparation of on-chip frequency doubling optical devices.

[0070] The polarization direction of the optical second harmonic signal is always along the a-axis, which is independent of the polarization direction of the excitation light; however, the intensity of the optical second harmonic signal is strongly related to the polarization direction of the incident excitation light, so the intensity of the optical second harmonic signal can be changed by adjusting the polarization direction of the excitation light; Figure 4 As shown, the polarization direction of the incident excitation light is at an angle α to the a-axis, the polarization direction of the reflected optical second harmonic signal is always along the a-axis, and the intensity of the optical second harmonic signal is proportional to cos 4 α, when α=0°, the optical second harmonic signal is the strongest, and the intensity gradually decreases as α deviates from 0°. The signal completely disappears when the incident light is orthogonal to the optical second harmonic polarization direction, that is, extinction occurs when α=90°, which is equivalent to a polarizer. When the light is completely transmitted, the optical second harmonic generation mechanism is induced by electric dipole moment, and when the light is completely extinct, the optical second harmonic generation mechanism is induced by electric quadrupole moment. There is a huge difference in the magnitude between the two, and the extinction ratio is very high. The actual extinction ratio is much higher than 100 or even reaches 1000.

[0071] For a set excitation light wavelength, the signal intensity of the optical second harmonic signal is related to the thickness of the tellurene nanosheet and the thickness and reflectivity of the substrate. Based on the interference effect induced by the air / tellurene nanosheet / substrate multi-interface structure, the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest under a set substrate and at a set excitation light wavelength is obtained; the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest is related to the wavelength of the excitation light.

[0072] The excitation light first passes through a polarizer to set the polarization angle; the incident light is focused on the surface of the tellurene nanosheet through a reflective objective with a numerical aperture (NA) of 0.5 to generate an optical second harmonic signal; the reflected optical second harmonic signal is separated from the fundamental frequency excitation light through a spectrometer and finally collected in a spectrometer; for optical second harmonic signals with wavelengths below 1 μm, a filter, a monochromator and an electrically cooled Si-CCD are used to collect the signal, while for those above 1 μm, a filter, a monochromator and an InGaAs photodetector are used to collect the signal, and a combination of a lock-in amplifier (LIA) and an optical chopper is used to analyze the signal.

[0073] The polarization angle of the excitation light is adjusted by controlling the polarizer, and the intensity of the optical second harmonic signal is observed by a spectrometer. When the optical second harmonic signal is the strongest, the polarization angle of the excitation light is the optimal polarization angle of incidence.

[0074] In this embodiment, the polarizer includes a polarizer and a half-wave plate, which can freely control the polarization direction of the excitation light; the spectroscopic element uses a beam splitter to spatially separate the doubled optical second harmonic from the fundamental frequency excitation light; the focusing element uses an objective lens; for the reflective type, the substrate reflectivity is 50%.

[0075] The spectrometer includes a filter device, a monochromator and a detector; the excitation light is removed by the filter device, and only the second harmonic is retained. The monochromator separates the collected signal according to the wavelength, and finally converts the separated second harmonic signal into an electrical signal according to the different wavelengths and detects it.

[0076] In order to explore the topological nature of the huge response of tellurene nanosheet-optical second harmonic signal, this study used density functional theory (DFT) to calculate the energy band of the material system after considering spin-orbit coupling (SOC), and numerically calculated χ (2) In relation to the Berry curvature near the Weyl point, it is found that the giant Berry connection accompanied by the interband transitions near the three Weyl points can explain the excellent optical second harmonic response of tellurene nanosheets.

[0077] In summary, the composite device of the present invention has a large bandwidth and high frequency conversion efficiency in the mid-infrared band of 1.2 to 4.86 μm, and also has a very large extinction ratio as a polarizer, which makes it have a good application prospect in the optical system of the on-chip environment.

[0078] Embodiment 2

[0079] This embodiment is used to output an optical second harmonic with a set polarization direction.

[0080] like Figure 5 As shown, the on-chip mid-infrared frequency doubling polarization composite device of this embodiment includes: a tellurene nanosheet, a substrate, a polarizer, a focusing element, a beam splitting element, an analyzer and a spectrometer; wherein,

[0081] The excitation light first passes through a polarizer and has a set polarization angle; it is then focused on the surface of the tellurene nanosheet by a focusing system to generate an optical second harmonic signal; the reflected optical second harmonic signal is separated from the fundamental frequency excitation light by a beam splitter, and is collected by a spectrometer after passing through an analyzer;

[0082] The polarization direction of the optical second harmonic is obtained through a polarizer and a spectrometer; for excitation light of any polarization direction passing through the polarizer, the on-chip mid-infrared frequency doubling polarization composite device can provide an optical second harmonic with a unique polarization direction, and the polarization direction of the optical second harmonic is always along the a-axis of the tellurene nanosheet; by rotating the a-axis of the tellurene nanosheet, the polarization direction of the optical second harmonic is changed, so that the optical second harmonic has a set polarization direction.

[0083] In this embodiment, the polarizer adopts a first polarizer and a first half-wave plate; the analyzer adopts a second polarizer and a second half-wave plate; the excitation light first passes through the first polarizer to lock the polarization state, and then passes through the first half-wave plate to set the polarization angle; the reflected optical second harmonic signal first passes through the second half-wave plate and then passes through the second polarizer; by rotating the second half-wave plate, the rotation angle of the fast axis of the second half-wave plate is obtained when the signal obtained by the spectrometer is the strongest; according to the rotation angle of the fast axis of the second half-wave plate, the polarization direction of the optical second harmonic signal is obtained, and the polarization direction is twice the rotation angle of the fast axis.

[0084] Others are the same as in Example 1

[0085] Finally, it should be noted that the purpose of publishing the embodiments is to help further understand the present invention, but those skilled in the art can understand that various substitutions and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the contents disclosed in the embodiments, and the scope of protection claimed by the present invention shall be subject to the scope defined in the claims.

Claims

1. An on-chip mid-infrared frequency-doubling polarization composite device for detecting the optimal polarization direction of incident light, characterized in that: The on-chip mid-infrared frequency-doubling polarization composite device comprises: a tellurene nanosheet, a substrate, a polarizer, a focusing element and a spectrometer; The tellurene nanosheet is in sheet shape and has a nanometer-level thickness. The tellurene nanosheet has a series of parallel one-dimensional spiral atomic chain structures, and the atomic chain extends in the c-axis direction. The mutually perpendicular a-axis and c-axis are located in the tellurene nanosheet plane. The tellurene nanosheet is arranged on a substrate. For the reflection type, the substrate has a high reflectivity, and for the transmission type, the substrate has a high transmittance. Femtosecond pulse light with a wavelength in the near-infrared to mid-infrared band is used as the excitation light, and the excitation light is incident on the tellurene nanosheet; the tellurene nanosheet does not have an inversion symmetry center, and the frequency-doubled polarizability is non-zero, which meets the conditions for generating optical second harmonics; according to density functional theory calculations after considering spin-orbit coupling, the band structure of tellurene nano has multiple Weyl points; for the excitation light in the near-infrared to mid-infrared band, the inter-band transitions near the three Weyl points correspond to the two-photon resonant transitions in the frequency-doubled process; the Bailey connection between the valence band and the conduction band has a topological enhancement effect on the frequency-doubled polarizability; and the thickness of the tellurene nanosheet is at the nanometer level, which is much smaller than the coherent wavelength, and does not need to be restricted by the optical second harmonic phase matching, thereby generating an optical second harmonic signal; The polarization direction of the optical second harmonic signal is always along the a-axis and has nothing to do with the polarization direction of the excitation light; however, the intensity of the optical second harmonic signal is related to the polarization direction of the incident excitation light, so the intensity of the optical second harmonic signal can be changed by adjusting the polarization direction of the excitation light. For a set excitation light wavelength, the signal intensity of the optical second harmonic signal is related to the thickness of the tellurene nanosheet and the thickness and reflectivity or transmittance of the substrate. According to the interference effect induced by the multi-interface structure of air / tellurene nanosheet / substrate, the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest under the set substrate and at the set excitation light wavelength is obtained; the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest is related to the wavelength of the excitation light; The excitation light first passes through a polarizer to set the polarization angle; it is then focused on the surface of the tellurene nanosheet through a focusing element to generate an optical second harmonic signal; the reflected optical second harmonic signal is separated from the fundamental frequency excitation light through a spectroscopic element and is collected by a spectrometer, or the transmitted optical second harmonic signal is collected by a spectrometer; The polarization angle of the excitation light is adjusted by controlling the polarizer, and the intensity of the optical second harmonic signal is observed by a spectrometer. When the optical second harmonic signal is the strongest, the polarization angle of the excitation light is the optimal polarization angle of incidence.

2. The on-chip mid-infrared frequency doubling polarization composite device according to claim 1, characterized in that: The polarizer includes a polarizing plate and a half-wave plate.

3. An on-chip mid-infrared frequency-doubling polarization composite device for outputting an optical second harmonic with a set polarization direction, characterized in that: The on-chip mid-infrared frequency-doubling polarization composite device comprises: a tellurene nanosheet, a substrate, a polarizer, a focusing element, an analyzer and a spectrometer; wherein, The tellurene nanosheet is a sheet with a nanometer thickness. The tellurene nanosheet has a series of parallel one-dimensional spiral atomic chain structures, and the atomic chain extends in the c-axis direction. The mutually perpendicular a-axis and c-axis are located in the tellurene nanosheet plane. The substrate has high reflectivity, and the tellurene nanosheet is arranged on the substrate. For the reflection type, the substrate has high reflectivity, and for the transmission type, the substrate has high transmittance. Femtosecond pulse light with a wavelength in the near-infrared to mid-infrared band is used as the excitation light, and the excitation light is incident on the tellurene nanosheet; the tellurene nanosheet does not have an inversion symmetry center, and the frequency-doubled polarizability is non-zero, which meets the conditions for generating optical second harmonics; according to density functional theory calculations after considering spin-orbit coupling, the band structure of tellurene nano has multiple Weyl points; for the excitation light in the near-infrared to mid-infrared band, the inter-band transitions near the three Weyl points correspond to the two-photon resonant transitions in the frequency-doubled process; the Bailey connection between the valence band and the conduction band has a topological enhancement effect on the frequency-doubled polarizability; and the thickness of the tellurene nanosheet is at the nanometer level, which is much smaller than the coherent wavelength, and does not need to be restricted by the optical second harmonic phase matching, thereby generating an optical second harmonic signal; The polarization direction of the optical second harmonic signal is always along the a-axis and has nothing to do with the polarization direction of the excitation light; however, the intensity of the optical second harmonic signal is related to the polarization direction of the incident excitation light, so the intensity of the optical second harmonic signal can be changed by adjusting the polarization direction of the excitation light. For a set excitation light wavelength, the signal intensity of the optical second harmonic signal is related to the thickness of the tellurene nanosheet and the thickness and reflectivity or transmittance of the substrate. According to the interference effect induced by the multi-interface structure of air / tellurene nanosheet / substrate, the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest under the set substrate and at the set excitation light wavelength is obtained; the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest is related to the wavelength of the excitation light; The excitation light first passes through a polarizer and has a set polarization angle; it is focused on the surface of the tellurene nanosheet through a focusing system to generate an optical second harmonic signal; the reflected optical second harmonic signal is separated from the fundamental frequency excitation light through a beam splitter, and is collected to a spectrometer after passing through an analyzer, or the transmitted optical second harmonic signal is collected to a spectrometer after passing through an analyzer; The polarization direction of the optical second harmonic is obtained through a polarizer and a spectrometer; for excitation light of any polarization direction passing through the polarizer, the on-chip mid-infrared frequency doubling polarization composite device can provide an optical second harmonic with a unique polarization direction, and the polarization direction of the optical second harmonic is always along the a-axis of the tellurene nanosheet; by rotating the a-axis of the tellurene nanosheet, the polarization direction of the optical second harmonic is changed, so that the optical second harmonic has a set polarization direction.

4. The on-chip mid-infrared frequency doubling polarization composite device according to claim 3, characterized in that: The polarizer adopts a first polarizer and a first half-wave plate; the analyzer adopts a second polarizer and a second half-wave plate; the excitation light first passes through the first polarizer to lock the polarization state, and then passes through the first half-wave plate to set the polarization angle; the optical second harmonic signal first passes through the second half-wave plate and then passes through the second polarizer; by rotating the second half-wave plate, the rotation angle of the fast axis of the second half-wave plate when the signal collected by the spectrometer is the strongest is obtained; according to the rotation angle of the fast axis of the second half-wave plate, the polarization direction of the optical second harmonic signal is obtained.

5. The on-chip mid-infrared frequency-doubling polarization composite device according to claim 1 or 3, characterized in that: The spectrometer includes a filter device, a monochromator and a detector; the filter device removes the excitation light and only retains the second harmonic, the monochromator separates the collected signal according to the wavelength, and the detector finally converts the separated second harmonic signal into an electrical signal for detection.

6. The on-chip mid-infrared frequency-doubling polarization composite device according to claim 1 or 3, characterized in that: The tellurene nanosheet has a thickness of 10 to 50 nm.

7. A method for realizing an on-chip mid-infrared frequency doubling polarization composite device as claimed in claim 1, characterized in that: The implementation method comprises the following steps: 1) According to the interference effect induced by the multi-interface structure of air / tellurene nanosheet / substrate, the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest under a set wavelength of the excitation light and a set substrate is calculated; 2) Device settings: The tellurene nanosheet is in sheet shape and has a thickness set in step 1). The tellurene nanosheet has a series of parallel one-dimensional spiral atomic chain structures, and the atomic chain extends in the c-axis direction; the mutually perpendicular a-axis and c-axis are located in the tellurene nanosheet plane; the tellurene nanosheet is arranged on a substrate; 3) Femtosecond pulse light with a wavelength in the near-infrared to mid-infrared band is used as the excitation light. The excitation light first passes through a polarizer to set the polarization angle. After being converged by a focusing element, the excitation light is incident on the tellurene nanosheet. The tellurene nanosheet does not have an inversion symmetry center, and the frequency-doubled polarizability is not zero, which meets the conditions for generating optical second harmonics. According to density functional theory calculations after considering spin-orbit coupling, the band structure of tellurene nanosheets has multiple Weyl points. For the excitation light in the near-infrared to mid-infrared band, the inter-band transitions near the three Weyl points correspond to the two-photon resonant transitions in the frequency-doubled process. The Bailey connection between the valence band and the conduction band has a topological enhancement effect on the frequency-doubled polarizability. In addition, the thickness of the tellurene nanosheet is at the nanometer level, which is much smaller than the coherent wavelength, and does not need to be restricted by the optical second harmonic phase matching, thereby generating an optical second harmonic signal. 4) The reflected optical second harmonic signal is separated from the fundamental frequency excitation light through a spectrometer and collected by a spectrometer. Or the transmitted optical second harmonic signal is collected to a spectrometer; 5) The polarization direction of the optical second harmonic signal is always along the a-axis, which is independent of the polarization direction of the excitation light; however, the intensity of the optical second harmonic signal is related to the polarization direction of the incident excitation light; by controlling the polarizer, adjusting the polarization angle of the excitation light, and adjusting the polarization direction of the excitation light, the intensity of the optical second harmonic signal can be changed; 6) Observe the intensity of the optical second harmonic signal through a spectrometer. When the optical second harmonic signal is the strongest, the polarization angle of the excitation light at this time is the optimal polarization angle of incidence.

8. A method for realizing an on-chip mid-infrared frequency doubling polarization composite device as claimed in claim 3, characterized in that: The implementation method comprises the following steps: 1) According to the interference effect induced by the multi-interface structure of air / tellurene nanosheet / substrate, the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest under a set wavelength of the excitation light and a set substrate is calculated; 2) Device settings: The tellurene nanosheet is in sheet shape and has a thickness set in step 1). The tellurene nanosheet has a series of parallel one-dimensional spiral atomic chain structures, and the atomic chain extends in the c-axis direction; the mutually perpendicular a-axis and c-axis are located in the tellurene nanosheet plane; the tellurene nanosheet is arranged on a substrate; 3) Femtosecond pulse light with a wavelength in the near-infrared to mid-infrared band is used as the excitation light. The excitation light first passes through a polarizer to set the polarization angle. After being converged by a focusing element, the excitation light is incident on the tellurene nanosheet. The tellurene nanosheet does not have an inversion symmetry center, and the frequency-doubled polarizability is not zero, which meets the conditions for generating optical second harmonics. According to density functional theory calculations after considering spin-orbit coupling, the band structure of tellurene nanosheets has multiple Weyl points. For the excitation light in the near-infrared to mid-infrared band, the inter-band transitions near the three Weyl points correspond to the two-photon resonant transitions in the frequency-doubled process. The Bailey connection between the valence band and the conduction band has a topological enhancement effect on the frequency-doubled polarizability. In addition, the thickness of the tellurene nanosheet is at the nanometer level, which is much smaller than the coherent wavelength, and does not need to be restricted by the optical second harmonic phase matching, thereby generating an optical second harmonic signal. 4) The reflected optical second harmonic signal is separated from the fundamental frequency excitation light by the beam splitter and passes through the polarizer. is collected into a spectrometer, or the transmitted optical second harmonic signal passes through an analyzer and is collected into a spectrometer; 5) Obtain the polarization direction of the optical second harmonic through a polarizer and a spectrometer; 6) For the excitation light of any polarization direction passing through the polarizer, the polarization direction of the optical second harmonic is always along the a-axis of the tellurene nanosheet; by rotating the a-axis of the tellurene nanosheet, the polarization direction of the optical second harmonic is changed, so that the optical second harmonic has a set polarization direction.

9. The implementation method according to claim 7 or 8, characterized in that: In step 1), for a set wavelength of excitation light, the signal intensity of the optical second harmonic signal is related to the thickness of the tellurene nanosheet and the thickness and reflectivity or transmittance of the substrate. According to the interference effect induced by the multi-interface structure of air / tellurene nanosheet / substrate, the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest under the set substrate and at the set wavelength of excitation light is obtained; the thickness of the tellurene nanosheet when the optical second harmonic signal is the strongest is related to the wavelength of the excitation light.

10. The implementation method according to claim 8, characterized in that: In step 6), the polarization direction of the incident excitation light is at an angle α to the a-axis, the polarization direction of the optical second harmonic signal only extends along the a-axis, and the intensity of the optical second harmonic signal is proportional to cos 4 α.

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