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

Abstract

The present invention discloses an on-chip mid-infrared frequency-doubling polarization composite device and its implementation method. The present invention uses tellurene nanosheets to demonstrate significant second harmonic generation efficiency in an ultra-wideband region from near-infrared to mid-infrared. Furthermore, the intensity of the optical second harmonic signal reaches its highest intensity only when the incident light polarization direction is perpendicular to the a-axis of the tellurene nanosheet, and is extinguished when it is orthogonal, resulting in a high extinction ratio. The present invention exhibits excellent broadband mid-infrared second harmonic generation response, promising a solution to the difficulty in efficiently frequency-doubling mid-infrared light in on-chip integrated environments, opening up new avenues for the development of advanced photonic technologies. Based on the significant anisotropy of the tellurene nanosheet-optical second harmonic signal, the present invention is able to regulate the polarization of the emitted optical second harmonic signal, exhibiting versatility and ease of integration. The present invention is highly suitable for large-scale manufacturing and saves costs.

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 two-dimensional Weyl semiconductor tellurene (Te) nanosheets 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 single photon of double energy, typically occurring in crystals with broken inversion symmetry. SHG plays a key role in various applications, including 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 techniques, making them generally unsuitable for applications in miniaturized optoelectronic and all-optical devices, especially on-chip integration. In contrast, atomically layered materials, such as transition metal chalcogenides, offer 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 layers of materials result in a more limited light-matter interaction distance, limiting the magnitude of nonlinear conversion efficiency. Furthermore, materials with high frequency-doubled conversion efficiency in the currently sought-after mid-infrared band are extremely scarce. In the context of on-chip mid-infrared bands, polarization technology—including polarization selection, beam splitting, and modulation—is the most fundamental means in operations such as light field manipulation and signal processing. Combining these two functions while ensuring frequency doubling efficiency and polarization control capabilities allows the development of on-chip composite devices. Compared with traditional frequency doubling and polarization systems consisting of nonlinear crystals and polarizers, this can greatly improve the miniaturization and integration of devices, providing new possibilities in fields such as biomedicine, national defense and security, gas detection, and infrared communications. Summary of the Invention

[0003] To address the problems existing in the above-mentioned existing technologies, 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 materials 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] Tellurene nanosheets are sheet-shaped and nanometer-thick. They have a series of parallel one-dimensional spiral atomic chains, with the atomic chains extending along the c-axis. The mutually perpendicular a-axis and c-axis are located within the tellurene nanosheet plane. The tellurene nanosheet is placed on a substrate. For reflective mode, the substrate has high reflectivity, while for transmissive mode, the substrate has 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 symmetric 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 nanosheets has a narrow band gap of 0.36eV ​​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 frequency process. The Berry 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 coherence wavelength, and does not need to be restricted by the optical second harmonic phase matching, thereby generating optical second harmonic signals.

[0009] The polarization direction of the optical second harmonic signal is always along the a-axis and 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. Therefore, 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. 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 strongest under a set substrate and at a set excitation light wavelength is determined. The thickness of the tellurene nanosheet when the optical second harmonic signal is strongest is related to the wavelength of the excitation light.

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

[0012] By controlling the polarizer, the polarization angle of the excitation light is adjusted, and the intensity of the optical second harmonic signal is observed 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.

[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] Tellurene nanosheets are nanometer-thick sheets with a series of parallel one-dimensional spiral atomic chains extending along the c-axis. The a-axis and c-axis, which are perpendicular to each other, are located within the plane of the tellurene nanosheet. The tellurene nanosheets are arranged on a substrate with high reflectivity. For reflective mode, the substrate has high reflectivity, while for transmissive mode, 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 symmetric 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 nanosheets 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 frequency process. The Berry 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 coherence 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 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. Therefore, 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. 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 strongest under a set substrate and at a set excitation light wavelength is determined. The thickness of the tellurene nanosheet when the optical second harmonic signal is strongest is related to the wavelength of the excitation light.

[0019] The excitation light first passes through a polarizer, with a set polarization angle; it is then focused on the surface of the tellurene nanosheet by a focusing system, generating an optical second harmonic signal; the reflected optical second harmonic signal is separated from the fundamental frequency excitation light by a spectrometer, and is collected by a spectrometer after passing through an analyzer, or the transmitted optical second harmonic signal is collected by 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 uses a first polarizer and a first half-wave plate; the analyzer uses 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, 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 spatially separate the optical second harmonic and the excitation light.

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

[0026] The spectrometer consists of a filter, a monochromator, and a detector. The filter removes the excitation light, retaining only the second harmonic. The monochromator then separates the collected signal by wavelength, and the detector converts the separated second harmonic signal into an electrical signal for detection. For reflective analysis, the spectrometer and excitation light are located on the same side of the tellurene nanosheet; for transmissive analysis, the spectrometer and excitation light are located on opposite 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, that is, 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] Based on the interference effect induced by the multi-interface structure of air / tellurene nanosheet / substrate, the thickness of the tellurene nanosheet at the peak of the optical second harmonic generation (SHG) signal at the corresponding excitation wavelength was calculated. The longer the excitation wavelength, the thicker the tellurene nanosheet at this peak. For an excitation wavelength of 2 μm, the tellurene nanosheet thickness was 20 nm; for excitation wavelengths of 1 to 1.2 μm, the tellurene nanosheet thickness was 18 nm.

[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, comprising the following steps:

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

[0034] 2) Device settings:

[0035] The tellurene nanosheet is in sheet form and has a thickness set in step 1). The tellurene nanosheet has 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 within the tellurene nanosheet plane. The tellurene nanosheet is disposed 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 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 band, the interband transitions near the three Weyl points correspond to the two-photon resonant transitions in the frequency-doubled frequency process; the Berry 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;

[0037] 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 by a spectrometer;

[0038] 5) The polarization direction of the optical second harmonic signal is always along the a-axis and 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] In step 1), 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. 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 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 strongest is related to the wavelength of the excitation light.

[0041] In step 2), for the reflective mode, the substrate has a high reflectivity, and for the transmissive mode, 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 an on-chip mid-infrared frequency-doubling polarization composite device of the present invention, which is used to output an optical second harmonic with a set polarization direction, comprises the following steps:

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

[0045] 2) Device settings:

[0046] The tellurene nanosheet is in sheet form and has a thickness set in step 1). The tellurene nanosheet has 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 within the tellurene nanosheet plane. The tellurene nanosheet is disposed 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 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 band, the interband transitions near the three Weyl points correspond to the two-photon resonant transitions in the frequency-doubled frequency process; the Berry 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;

[0048] 4) The reflected optical second harmonic signal is separated from the fundamental frequency excitation light by a spectroscopic element, passes through an analyzer, and is collected into a spectrometer, 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 an analyzer and a spectrometer;

[0050] 6) For 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 symmetry analysis of the second-order polarizability tensor is performed. For the case of normal incidence, that is, 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 present invention exhibits enormous second harmonic generation efficiency in the ultra-wideband region from near-infrared to mid-infrared, with a conversion efficiency at least two orders of magnitude higher than that of traditional nonlinear materials such as GaSe at the same thickness. Furthermore, the intensity of the optical second harmonic signal reaches its maximum only when the incident light is polarized along the a-axis of the tellurene nanosheet, and is extinguished when the incident light is orthogonal, resulting in a high extinction ratio. The ultra-wideband response of the tellurene nanosheet is attributed to the topological enhancement of the doubled-frequency polarizability by the combined effect of three Weyl cones in the valence and conduction bands, while its polarization characteristics are derived from the crystal structure and symmetry of the tellurene nanosheet. The present invention exhibits excellent broadband mid-infrared second harmonic generation response, and is promising for resolving the difficulty in efficiently converting mid-infrared light into doubled-frequency light in an on-chip integrated environment, thus opening up a new path for the development of advanced photonic technologies. Based on the significant anisotropy of the tellurene nanosheet-optical second harmonic signal, the present invention can achieve regulation of the polarization of the output optical second harmonic signal, and has the characteristics of versatility and ease of integration. The core tellurene nanosheet is prepared using a hydrothermal method, which is very suitable for large-scale manufacturing and saves costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 Schematic diagram of Example 1 of the on-chip mid-infrared frequency-doubling polarization composite device of the present invention;

[0055] Figure 2 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 for an embodiment of the on-chip mid-infrared frequency-doubling polarization composite device of the present invention;

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

[0058] Figure 5 This is a schematic diagram of Example 2 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 = 58,000) 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. After 5 minutes of magnetic stirring, transfer the solution to a stainless steel autoclave lined with polytetrafluoroethylene and heat to 180°C for 10 hours.

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

[0065] 4) The Te nanosheets were redispersed in ethanol and transferred to a substrate using a drop coating method. The substrate was 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 spectrometer, and a spectrometer;

[0067] Tellurene nanosheets are sheet-like and nanometer-thick, with a series of parallel one-dimensional spiral atomic chains extending along the c-axis. The mutually perpendicular a- and c-axes lie within the plane of the tellurene nanosheets, which are located on a substrate. This structural feature can be observed using annular dark-field scanning transmission electron microscopy (ADF-STEM). Atomic force microscopy (AFM) confirmed the sample thickness to be 18 nm, and Raman spectroscopy further confirmed the lattice orientation.

[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. The signal light of 1.2-1.56μm and the idler light of 2.4-1.65μm are generated through the mid-infrared optical parametric amplifier (OPA). The signal light and the idler light are then passed through the difference frequency generation (DFG) to generate the excitation light of 2.5-5μm. The excitation light is incident on the tellurene nanosheet. The tellurene nanosheet does not have an inversion symmetry center, and the frequency doubling 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 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 the tellurene nanosheets have a strong response in the mid-infrared broadband range of 1.2 to 4.86 μm after normalization. It can be seen that the device has a strong frequency doubling efficiency in the mid-infrared broadband range. This phenomenon is due to 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 The 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 then gradually decreases as the optical second harmonic photon energy gradually increases to 2.06 eV; a significant 5 nm·V-1 peak is observed at 1.12 eV, and two shoulder peaks of approximately 3.2 nm·V-1 and 2.3 nm·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, and TaAs, it is found that few materials can exhibit 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 and has nothing to do with 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, and 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 When α is 0°, the optical second harmonic signal is the strongest, and the intensity gradually decreases as α deviates from 0°. When the incident light is orthogonal to the polarization direction of the optical second harmonic, the signal completely disappears, that is, extinction occurs when α is 90°, which is equivalent to a polarizer. When the light is completely transmitted, the generation mechanism of the optical second harmonic is induced by the electric dipole moment, and when the light is completely extinct, the generation mechanism of the optical second harmonic is induced by the 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 strongest under a set substrate and at a set excitation light wavelength is calculated; the thickness of the tellurene nanosheet when the optical second harmonic signal is 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 via a reflective objective with a numerical aperture (NA) of 0.5, generating an optical second harmonic signal; the reflected optical second harmonic signal is separated from the fundamental frequency excitation light by a spectrometer and ultimately collected by a spectrometer; for optical second harmonic signals with wavelengths below 1μm, the signal is collected using a filter, monochromator, and electrically cooled Si-CCD, while for wavelengths above 1μm, the signal is collected using a filter, monochromator, and InGaAs photodetector, and analyzed using a combination of a lock-in amplifier (LIA) and an optical chopper.

[0073] By controlling the polarizer, the polarization angle of the excitation light is adjusted, and the intensity of the optical second harmonic signal is observed 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.

[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 and 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, a monochromator, and a detector. The filter removes the excitation light, retaining only the second harmonic. The monochromator separates the collected signal according to wavelength. Depending on the wavelength, an indium gallium arsenide (InGaAs) detector or a Si-CCD is used to convert the separated second harmonic signal into an electrical signal for detection.

[0076] In order to explore the topological nature of the huge response of tellurene nanosheet-optical second harmonic generation 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 was 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 good application prospects in optical systems in on-chip environments.

[0078] Example 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 spectrometer, an analyzer, and a spectrometer; wherein,

[0081] The excitation light first passes through a polarizer, with a set polarization angle. It is then focused on the surface of the tellurene nanosheet by a focusing system, generating an optical second harmonic signal. The reflected optical second harmonic signal is separated from the fundamental frequency excitation light by a spectrometer, and then collected by a polarizer and a spectrometer.

[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 uses a first polarizer and a first half-wave plate; the analyzer uses 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 when the signal obtained 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.

[0084] Other aspects are the same as those of Example 1

[0085] Finally, it should be noted that the purpose of disclosing the embodiments is to facilitate a further understanding of the present invention. However, those skilled in the art will appreciate 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; the scope of protection claimed by the present invention shall be determined by 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; Tellurene nanosheets are sheet-shaped and nanometer-thick. They have a series of parallel one-dimensional spiral atomic chains, with the atomic chains extending along the c-axis. The mutually perpendicular a-axis and c-axis are located within the tellurene nanosheet plane. The tellurene nanosheet is placed on a substrate. For reflective mode, the substrate has high reflectivity, while for transmissive mode, 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 symmetric 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 nanosheets has 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 frequency process. The Berry 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 coherence 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 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. Therefore, 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. 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 strongest under a set substrate and at a set excitation light wavelength is determined. The thickness of the tellurene nanosheet when the optical second harmonic signal is strongest is related to the wavelength of the excitation light. The excitation light first passes through a polarizer to set the polarization angle; then it is focused on the surface of the tellurene nanosheet by a focusing element, generating an optical second harmonic signal; the reflected optical second harmonic signal is separated from the fundamental frequency excitation light by a spectrometer and collected by a spectrometer, or the transmitted optical second harmonic signal is collected by a spectrometer; By controlling the polarizer, the polarization angle of the excitation light is adjusted, and the intensity of the optical second harmonic signal is observed 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.

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, Tellurene nanosheets are nanometer-thick sheets with a series of parallel one-dimensional spiral atomic chains extending along the c-axis. The a-axis and c-axis, which are perpendicular to each other, are located within the plane of the tellurene nanosheet. The tellurene nanosheets are arranged on a substrate with high reflectivity. For reflective mode, the substrate has high reflectivity, while for transmissive mode, 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 symmetric 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 nanosheets has 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 frequency process. The Berry 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 coherence 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 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. Therefore, 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. 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 strongest under a set substrate and at a set excitation light wavelength is determined. The thickness of the tellurene nanosheet when the optical second harmonic signal is strongest is related to the wavelength of the excitation light. The excitation light first passes through a polarizer, with a set polarization angle; it is then focused on the surface of the tellurene nanosheet by a focusing system, generating an optical second harmonic signal; the reflected optical second harmonic signal is separated from the fundamental frequency excitation light by a spectrometer, and is collected by a spectrometer after passing through an analyzer, or the transmitted optical second harmonic signal is collected by 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 uses a first polarizer and a first half-wave plate; the analyzer uses 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 is obtained when the signal collected 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.

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 retains only the second harmonic, the monochromator separates the collected signal according to 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 the on-chip mid-infrared frequency-doubling polarization composite device according to claim 1, characterized in that: The implementation method comprises the following steps: 1) Based on the interference effect induced by the air / tellurene nanosheet / substrate multi-interface structure, the thickness of the tellurene nanosheet is calculated when the optical second harmonic signal is strongest under a set excitation light wavelength and a set substrate; 2) Device settings: The tellurene nanosheet is in sheet form and has a thickness set in step 1). The tellurene nanosheet has 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 within the tellurene nanosheet plane. The tellurene nanosheet is disposed 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 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 nanosheets has multiple Weyl points; for 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 frequency process; the Berry 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; 4) The reflected optical second harmonic signal is separated from the fundamental frequency excitation light by a spectrometer and collected by a spectrometer. Alternatively, the transmitted optical second harmonic signal is collected into a spectrometer; 5) The polarization direction of the optical second harmonic signal is always along the a-axis and 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) Based on the interference effect induced by the air / tellurene nanosheet / substrate multi-interface structure, the thickness of the tellurene nanosheet is calculated when the optical second harmonic signal is strongest under a set excitation light wavelength and a set substrate; 2) Device settings: The tellurene nanosheet is in sheet form and has a thickness set in step 1). The tellurene nanosheet has 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 within the tellurene nanosheet plane. The tellurene nanosheet is disposed 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 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 nanosheets has multiple Weyl points; for 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 frequency process; the Berry 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; 4) The reflected optical second harmonic signal is separated from the fundamental frequency excitation light by the spectrometer and then passes through the analyzer. The optical second harmonic signal 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 an analyzer and a spectrometer; 6) For 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 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. 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 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 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 is only along the a-axis, and the intensity of the optical second harmonic signal is proportional to cos 4 α.

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