A method and apparatus for second harmonic generation

Abstract

The application discloses a second harmonic generation method, and belongs to the field of nonlinear optics and nanophotonics. The application comprises the following steps: constructing a periodic nanostructure array by using a thin film lithium niobate, and configuring the array to support an optical mode with an equivalent wave vector close to zero at a frequency-doubled wavelength; introducing two coherent fundamental frequency lights propagating in opposite directions into the periodic nanostructure array, and making the two coherent fundamental frequency lights have equal size and opposite direction in-plane wave vector components; spontaneously generating second-order nonlinear polarization of the two coherent fundamental frequency lights in the periodic nanostructure array to excite the frequency-doubled light with a wave vector close to zero, and realizing the second harmonic generation based on the momentum conservation. Through the synergistic effect of the Bloch mode momentum compensation mechanism and the zero refractive index effect, and in combination with the strong field localization caused by the quasi-continuous domain bound state, the second harmonic conversion efficiency under the nanometer scale is significantly improved without the traditional refractive index matching or periodic polarization structure.

Description

Technical Field

[0001] This invention relates to the fields of nonlinear optics and nanophotonics, specifically to a method and apparatus for achieving efficient second harmonic generation (SHG) by combining zero refractive index characteristics with quasi-BIC modes. Background Technology

[0002] Second harmonic generation, as an important second-order nonlinear optical effect, plays an indispensable role in many cutting-edge fields. In the field of integrated photonics, on-chip optical frequency conversion is a key technology for realizing cross-band signal interconnection and wavelength routing. The efficient conversion of fundamental frequency light (approximately 1550nm) in the communication band into frequency-doubled light (approximately 775nm) in the visible or near-ultraviolet bands is of great significance for constructing chip-level optical interconnects, photonic routing, and optical computing systems.

[0003] Lithium niobate (LiNbO3) is valued for its excellent second-order nonlinear coefficients (such as its large nonlinear optical coefficient d). 33 (approximately 27-34 pm / V), which is widely used in frequency conversion in nonlinear optics.

[0004] Wavelength conversion in nonlinear materials is crucial for both classical and quantum optics. Traditional methods typically utilize periodically polarized lithium niobate (PPLN) waveguides to achieve quasi-phase matching (QPM) through the artificial design of ferroelectric domain structures, or by adjusting the waveguide dimensions to make the equivalent refractive indices of different modes equal at different wavelengths.

[0005] In recent years, zero-index metamaterials have attracted widespread attention due to their infinite spatial wavelength and uniform spatial phase distribution, and have been explored for solving phase matching problems in integrated photonics.

[0006] Meanwhile, the development of bound states in the optical continuum (BICs) has made it possible to effectively confine the optical field within the radiation continuum region, and radiation loss can be greatly suppressed through specific geometric symmetry protection.

[0007] In traditional bulk or thin-film lithium niobate devices, the efficiency of second harmonic generation is severely limited by phase mismatch and localization in weak light fields. Traditional phase matching methods (such as birefringence matching and quasi-phase matching) are usually bulky and have stringent process requirements, making it extremely difficult to meet the current demands for high-efficiency conversion in on-chip integration and extremely small sizes. Summary of the Invention

[0008] The present invention aims to at least partially solve one of the technical problems existing in the prior art.

[0009] Therefore, the purpose of this invention is to provide a method and apparatus for generating second harmonics that combines the phase-matching advantage of zero-index with the field-enhancing advantage of quasi-BIC, in order to solve the bottleneck problem of phase mismatch and weak light field localization limiting the efficiency of second harmonic conversion in traditional nonlinear crystals. This enables efficient second harmonic generation at the nanoscale, and is particularly suitable for efficiently converting fundamental light in the approximately 1550nm band into frequency-doubled light in the approximately 775nm band.

[0010] To achieve the above objectives, the present invention adopts the following technical solution:

[0011] The first aspect of this invention provides a method for generating a second harmonic, comprising:

[0012] A periodic nanostructure array is constructed using thin-film lithium niobate, and the periodic nanostructure array is configured to support an optical mode with an equivalent wave vector close to zero at the frequency harmonic wavelength.

[0013] Two coherent fundamental frequency beams propagating in opposite directions are introduced into the periodic nanostructure array, and the two fundamental frequency beams are made to have in-plane wave vector components of equal magnitude and opposite direction.

[0014] The two fundamental frequency beams spontaneously generate second-order nonlinear polarization in the periodic nanostructure array to excite harmonic light with a wave vector close to zero, thereby realizing the generation of second harmonics based on momentum conservation.

[0015] In some embodiments, the periodic nanostructure array includes a plurality of structural units arranged along the light propagation direction, the structural units having structural symmetry broken to support quasi-continuous bound state modes with high quality factor.

[0016] A second aspect of the present invention provides a second harmonic generation device, comprising:

[0017] A periodic nanostructure array based on thin-film lithium niobate is configured to support an optical mode with an equivalent wave vector close to zero at the frequency harmonic wavelength.

[0018] The input ends are located at both ends of the periodic nanostructure array and are used to introduce two coherent fundamental frequency beams propagating in opposite directions into the periodic nanostructure array, and to make the two fundamental frequency beams have in-plane wave vector components of equal magnitude and opposite direction.

[0019] The output ends are located at both ends of the periodic nanostructure array and are used to extract frequency-harmonic light with a wave vector close to zero, which is excited by the spontaneous second-order nonlinear polarization generated by the two fundamental frequency beams in the periodic nanostructure array.

[0020] In some embodiments, the periodic nanostructure array includes a plurality of structural units arranged along the light propagation direction, the structural units having structural symmetry broken to support quasi-continuous bound state modes with high quality factor.

[0021] In some embodiments, the structural unit includes a central spine, and the symmetry structure is disrupted by air-column perforation on the opposite side walls of the central spine.

[0022] In some embodiments, the air column has an elliptical cross-section, and after being hollowed out, regions that are recessed into the interior of the central spine are formed on the two side walls of the central spine.

[0023] In some embodiments, the periodic nanostructure array is configured to support an optical mode with an equivalent wave vector close to zero at the frequency harmonic wavelength by adjusting geometric parameters, wherein the geometric parameters to be adjusted include the period and geometric dimensions of the central ridge, and the geometric dimensions of the air column along and perpendicular to the light propagation direction.

[0024] In some embodiments, the central ridge has a period of 300nm-500nm, a structural thickness of 500nm-900nm, a top width of 150nm-300nm, a bottom width of 500nm-1350nm, and a sidewall tilt angle of 60°-70°; the air column has a geometric dimension of 300nm-500nm along the light propagation direction and a geometric dimension of 100nm-300nm perpendicular to the light propagation direction.

[0025] In some embodiments, the periodic nanostructure array is formed by fully or partially etching the thin film lithium niobate; when partially etched, a non-zero thickness lithium niobate plate layer is left at the bottom of the periodic nanostructure array.

[0026] The thin-film lithium niobate has a Z-cut or X-cut crystal orientation structure.

[0027] In some embodiments, the input end employs a grating coupling structure or an evanescent field coupling structure to excite a guided mode or Bloch mode having a preset in-plane wave vector component; the two fundamental frequency beams have a fixed phase relationship.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] This invention synergistically applies the phase-matching mechanism of zero-refractive-index metawaveguides and the strong-field localization effect of quasi-continuous domain bound states (quasi-BIC) to a thin-film lithium niobate platform, proposing a second-harmonic generation method based on opposing Bloch mode momentum cancellation. Specifically, by designing a fishbone-shaped periodic nanostructure array with broken structural symmetry, an optical mode with an equivalent refractive index close to zero (equivalent wave vector approaching zero) is achieved at the harmonic wavelength. Simultaneously, two coherent fundamental frequency beams propagating in opposite directions are introduced, ensuring their in-plane wave vectors are equal in magnitude and opposite in direction. This automatically satisfies the momentum conservation condition during nonlinear polarization, eliminating the need for traditional periodic polarization (PPLN) or birefringence phase matching. Furthermore, utilizing the high-quality factor characteristics of the quasi-BIC mode significantly enhances the local field strength of the fundamental and harmonic beams, thereby substantially improving the second-harmonic conversion efficiency at the nanoscale. This invention offers advantages such as compact structure, large process tolerance, and no need for complex crystal modification, providing a novel technical path for on-chip integrated nonlinear frequency conversion. Attached Figure Description

[0030] Figure 1 A three-dimensional schematic diagram of a lithium niobate zero-refractive-index metawaveguide composed of multiple structural units, provided for an embodiment of the present invention;

[0031] Figure 2 A three-dimensional schematic diagram of the fishbone-shaped lithium niobate metawaveguide unit structure provided in an embodiment of the present invention;

[0032] Figure 3 A three-dimensional top view of the central spine of the fishbone-shaped lithium niobate metawaveguide unit structure provided in an embodiment of the present invention;

[0033] Figure 4 A planar geometric projection of the central spine of the fishbone-shaped lithium niobate metawaveguide unit structure provided in an embodiment of the present invention (showing the central rectangle and the hollowed-out areas on both sides cut by ellipses).

[0034] Figure 5 This is a schematic diagram of the electric field (Ez) distribution of a fishbone-shaped lithium niobate metawaveguide provided in an embodiment of the present invention, wherein (a) is the dipole mode and (b) is the monopole mode;

[0035] In the picture:

[0036] 100. Lithium niobate zero-refractive-index meta-waveguide; 110. Structural unit; 111. Central spine; 111a. First sidewall; 111b. Second sidewall; 112. Air column; 112a. First region; 112b. Second region; 113. Lithium niobate planar layer; A. First end; B. Second end.

[0037] 200. Substrate. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this application clearer, the application will be described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for explaining this application and are not intended to limit this application.

[0039] Conversely, this application covers any alternatives, modifications, equivalent methods, and schemes made within the spirit and scope of this application as defined by the claims. Furthermore, to provide the public with a better understanding of this application, certain specific details are described in detail below. However, this application can be fully understood by those skilled in the art even without these detailed descriptions.

[0040] In the description of this application, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the foundation or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0041] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0042] In this application, unless otherwise expressly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature being directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature being directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0043] The first aspect of this invention provides a method for generating a highly efficient second harmonic based on lithium niobate on insulator (LNOI), comprising:

[0044] Step S1: Construct a periodic nanostructure array using thin-film lithium niobate to form a lithium niobate zero-refractive-index metawaveguide, and configure the periodic nanostructure array to support an optical mode with an equivalent wave vector close to zero at the harmonic wavelength; wherein, the periodic nanostructure array includes several structural units arranged along the light propagation direction, and each structural unit has structural symmetry broken to support a quasi-continuous bound state mode (quasi-BIC) with a high quality factor.

[0045] Step S2: Introduce two coherent fundamental frequency beams propagating in opposite directions into the constructed periodic nanostructure array, and make the two fundamental frequency beams have in-plane wave vector components of equal magnitude and opposite direction.

[0046] Step S3: Two fundamental frequency beams spontaneously generate second-order nonlinear polarization in a periodic nanostructure array to excite harmonic light with a wave vector close to zero, thereby realizing the generation of second harmonics based on momentum conservation.

[0047] See Figure 1 A second aspect embodiment of the present invention provides a second harmonic generation device, comprising:

[0048] Substrate 200 serves as a support structure;

[0049] A lithium niobate zero-refractive-index metawaveguide 100 is formed on a substrate 200. The lithium niobate zero-refractive-index metawaveguide 100 includes a periodic nanostructure array configured to support an optical mode with an equivalent wave vector close to zero at the frequency harmonic wavelength.

[0050] In this embodiment, the lithium niobate zero-refractive-index metawaveguide is composed of an initial total thickness of H. total The thin film of lithium niobate is etched using micro-nano processing. To accommodate processing tolerances and further optimize optical mode constraints, the etching process is preferably partial etching, i.e., the etching depth (ridge height) is h.etch Thus, a layer with a thickness of h is retained below the lithium niobate zero-refractive-index metawaveguide. slab The lithium niobate planar layer satisfies H total = h etch + h slab ;

[0051] In this embodiment, the total thickness H of the thin-film lithium niobate is... total The range is 500nm-900nm, with an etching depth h. etch The range is 400nm–800nm, and the remaining plate layer thickness h slab The range is 50nm–250nm;

[0052] Input terminal (this input terminal is in) Figure 1 (Not shown in the diagram), arranged at both ends of the periodic nanostructure array, are used to introduce two coherent fundamental frequency beams propagating in opposite directions into the periodic nanostructure array, and to give the two fundamental frequency beams opposite in-plane wave vector components.

[0053] Output terminal (this output terminal is at) Figure 1 (Not shown in the diagram), arranged at both ends of a periodic nanostructure array, are used to extract frequency-doubled light with a wave vector close to zero excited by the second-order nonlinear polarization generated by two fundamental frequency beams in the periodic nanostructure array, thereby realizing the generation of second harmonics based on momentum conservation.

[0054] It should be noted that although this embodiment uses a linear metamorphic waveguide as an example to describe the technical solution of the present invention in detail, the topological shape and arrangement path of the periodic nanostructure array described in the present invention are not limited to a straight line. Those skilled in the art can arrange the above-mentioned fishbone-shaped unit structure along curves, rings, disks, or other arbitrary topological paths according to the layout requirements of actual photonic integrated chips, thereby constructing structures such as curved waveguides, micro-ring resonators, and micro-disk resonators.

[0055] For ease of description, an xyz coordinate system is constructed below, where the arrangement direction, width direction, and thickness (thickness can also be understood as height) direction of the periodic nanostructure array are respectively used as the x-axis, y-axis, and z-axis, and the light propagation direction is consistent with the x-axis.

[0056] In some embodiments, the substrate 200 may be made of materials such as silicon dioxide, sapphire, quartz, magnesium fluoride, silicon, or low-refractive-index polymers, with a thickness of 1 μm to 5 μm. In addition to physical support, the substrate 200 also has the following core functions: optical isolation and waveguide confinement; its refractive index is lower than that of thin-film lithium niobate; it completely confines the fundamental and harmonic frequencies within the core waveguide region through total internal reflection, preventing energy leakage to deeper layers to maintain extremely high local electric field strength; dispersion modulation and phase matching compensation; and it participates in determining the band structure of the array as a medium in the underlying environment, collaboratively assisting in achieving automatic phase matching in a zero-refractive-index environment.

[0057] In some embodiments, a periodic nanostructure array is the core of the present invention, which includes a plurality of structural units 110 arranged without gaps along the light propagation direction. Each structural unit 110 has structural symmetry broken in order to support quasi-continuous domain bound state modes with high quality factor.

[0058] Furthermore, the materials, structures, and dimensions of each structural unit 110 in the periodic nanostructure array are completely identical, see [reference]. Figure 2 , Figure 3 The diagram shows a perspective view and a top view of a single structural unit 110 in a periodic nanostructure array, the structural unit 110 including a central spine 111, the symmetry structure being broken by air column perforation on the first sidewall 111a and the second sidewall 111b opposite to the central spine 111.

[0059] In a preferred embodiment of the invention, the geometric projection of the un-hollowed-out central vertebra 111 in the yz plane is a trapezoid, with the base wider than the top. See also Figure 4 When hollowing out the first sidewalls 111a and 111b of the central spine 111, the air column 112 used has an elliptical projection in the xy plane. The major axis of this ellipse is parallel to the x-axis, and the minor axis is parallel to the y-axis. This creates symmetrically arranged arc-shaped first and second regions 112a and 112b that are recessed into the central spine 111 on the first and second sidewalls 111a and 111b. Finally, the periodic nanostructure array formed by the above-mentioned multiple structural units has a fishbone shape in the xy plane.

[0060] Furthermore, the periodic nanostructure array is configured to support an optical mode with an equivalent wave vector close to zero at the frequency harmonic wavelength by adjusting the geometric parameters. The geometric parameters to be adjusted include the period and geometric dimensions of the central spine 111, as well as the dimensions of the air column 112 along the light propagation direction and perpendicular to the light propagation direction.

[0061] Specifically, the process of geometric parameter control is as follows: a physical model for characterizing the meta-waveguide structure is constructed using the optical simulation software COMSOL Multiphysics. The model includes geometric configuration, material properties (lithium niobate waveguide and silicon dioxide substrate), excitation conditions (working wavelengths are set to 1550 nm and 775 nm, respectively), and electromagnetic boundary conditions (periodic boundary in the x-direction and scattering boundary in the y and z directions).

[0062] The period and height of each structural unit were initialized, and then their width, major axis, and minor axis of the ellipse were adjusted sequentially. The excitation wavelength was set to around 775 nm, and simulations were used to induce a Dirac cone-like dispersion in the model at the center of the Brillouin zone (the origin of wave vector space), corresponding to the simultaneous excitation of electric monopole and magnetic dipole modes (see...). Figure 5 Based on this, the parameters such as the major axis, minor axis, width, and height of the ellipse are further scanned to extract the quality factor (Q value) of the electric monopole eigenmode that constitutes the Dirac cone-like structure. The dependency relationship between the Q value and the above geometric parameters is established, and finally a set of parameters is selected so that the Q value of the electric monopole mode reaches a local maximum.

[0063] By setting the excitation wavelength to around 1550nm and adjusting the wave vector of the fundamental frequency light, the relationship between the fundamental mode quality factor (Q value) and the wave vector can be obtained. A wave vector can be selected to maximize the Q value of the fundamental mode.

[0064] After obtaining the optimal geometric parameters based on the above steps, when the excitation wavelength is 775nm, both the electric monopole and the magnetic dipole can achieve degeneracy and have high Q values; when the excitation wavelength is 1550nm, the Q value of the fundamental mode is also high.

[0065] After the above optimization, the central spine 111 has a period of 300nm-500nm, a structural thickness of 500nm-900nm, a top width of 150nm-300nm, a bottom width of 500nm-1350nm, and a side wall tilt angle of 60°-70°; the elliptical air column 112 has a dimension of 300nm-500nm along the light propagation direction and a dimension of 100nm-300nm perpendicular to the light propagation direction.

[0066] In some embodiments, the periodic nanostructure array is provided with a first input terminal and a second input terminal, as well as a first output terminal and a second output terminal. The first input terminal and the first output terminal are respectively connected to a first end A and a second end B of the periodic nanostructure array, and the second input terminal and the second output terminal are respectively connected to a second end B and a first end A of the periodic nanostructure array. The two input terminals can employ a grating coupling structure or an evanescent field coupling structure to guide two coherent fundamental frequency beams with opposite propagation directions into the periodic nanostructure array. The two output terminals can employ directional couplers to extract the frequency-doubled light from the periodic nanostructure array.

[0067] The technical solution of the present invention will be further described in detail below with reference to a specific embodiment of the present invention and its physical mechanism.

[0068] A 2 μm thick silicon dioxide (SiO2) substrate 200 is used as the support substrate, and a Z-cut lithium niobate (LiNbO3) thin film is disposed on the substrate 200 as a nonlinear functional layer. Periodically arranged fishbone-shaped structural units 110 are constructed on the lithium niobate thin film to form a zero-refractive-index metawaveguide 100.

[0069] The specific form of the fishbone-shaped structural unit 110 is as follows: it includes a central spine 111 and symmetrically distributed recessed regions on both sides and a lithium niobate plate layer 113. The main geometric parameters include: a period length of 372 nm, a thickness of 752 nm, a waveguide top width of 224 nm, and the major and minor axes of the hollow ellipse are 194 nm and 87 nm, respectively. The sidewall tilt angle (the angle between the sidewall and the horizontal direction) is 62°, and the thickness of the lithium niobate plate layer is 100 nm.

[0070] The physical mechanism of this embodiment is as follows:

[0071] Regarding phase matching: Momentum conservation in the nonlinear process is achieved by constructing an anti-propagating fundamental frequency light excitation method. Specifically, two coherent fundamental frequency light beams (wavelength approximately 1550 nm) are introduced at both ends of a periodic nanostructure array, each possessing opposite in-plane wave vector components along the propagation direction of the structure. , Let be the in-plane wave vector component of the first fundamental frequency light. (This refers to the in-plane wave vector component of the second fundamental frequency light). The fundamental frequency light is injected through an integrated grating coupling structure or an evanescent field coupling structure, thereby exciting a guided mode with a preset Bloch wave vector. The periodic nanostructure array, through geometric parameter tuning, achieves an effective refractive index close to zero (n0) at the frequency doubling wavelength (approximately 775 nm). eff The physical property of ≈0 corresponds to the wave vector approaching zero (i.e., the plane wave vector component of frequency-doubled light). Thus, the condition of conservation of momentum is satisfied. This enables an intrinsic phase-matching mechanism based on the cancellation of opposing Bloch modulus momentum.

[0072] In terms of strong-field localization: By introducing structural symmetry breaking (such as adjusting the lengths of the major and minor axes of the hollowed-out ellipse), quasi-continuous bound states (quasi-BIC) with extremely high quality factors are excited in the metamorphic waveguide. This mode possesses both high quality factor (Q-factor) and strong-field localization characteristics, which can significantly enhance the electric field intensity of fundamental and frequency-harmonic light in the lithium niobate metamorphic waveguide, thereby improving the nonlinear polarization intensity and enhancing the second harmonic generation efficiency.

[0073] This embodiment uses Z-cut thin-film lithium niobate (TFLN) as the nonlinear functional layer, and combines it with a wet etching process to fabricate a periodic nanostructure array, obtaining a lithium niobate zero-refractive-index metawaveguide. The specific fabrication steps are as follows:

[0074] 1) Material Selection and Wafer Preparation. A Z-cut lithium niobate thin-film wafer (LNOI) was selected as the substrate. Its structure, from top to bottom, consists of: an 852 nm thick Z-cut lithium niobate thin film, a 2 μm thick silicon dioxide (SiO2) insulating layer, and a 525 μm thick silicon substrate. The Z-cut crystal orientation was chosen based on the anisotropic characteristics of wet etching: compared to X-cut wafers, Z-cut TFLNs exhibit a more symmetrical cross-sectional morphology and lower sidewall roughness after wet etching, which is beneficial for achieving high-quality optical modes.

[0075] 2) Hard mask fabrication. A 100 nm thick SiO2 hard mask layer was grown on the Z-cut TFLN surface using plasma-enhanced chemical vapor deposition (PECVD). This mask layer exhibits extremely high selectivity (etch selectivity ratio > 10:1) during subsequent wet etching, effectively protecting the lithium niobate film in non-etched areas.

[0076] 3) Electron beam lithography (EBL) patterning. Electron beam lithography resist (ZEP 520A) is spin-coated onto a SiO2 mask layer, and a pattern of a fishbone-shaped periodic nanostructure array is written using an electron beam lithography system (EBPG 5000plus or equivalent equipment). The geometric parameters of this fishbone-shaped periodic nanostructure array are designed as follows: period length: 372nm, mask bottom width (corresponding to the bottom width of the spine): 1024nm, major axis of the hollow ellipse: 194nm, minor axis: 87nm.

[0077] 4) Mask Pattern Transfer. Inductively Coupled Plasma-Reactive Ion Etching (ICP-RIE) is used to transfer the photoresist pattern to the SiO2 hard mask layer. Fluorine-based gases (such as a CHF3 / CF4 mixture) are selected as the etching gas. After etching, residual photoresist is removed using oxygen plasma or an organic solvent.

[0078] 5) Wet etching to prepare the fishbone-shaped structure. Using patterned SiO2 as a hard mask, a mixed solvent of hydrogen peroxide-ammonium hydroxide-water (H2O2:NH4OH:H2O = 2:2:1) was used to wet-etch the Z-cut TFLN at 85°C. After wet etching, the SiO2 hard mask was removed using a buffered silicon oxide etchant (BOE, 6:1).

[0079] 6) Annealing treatment. Place the sample obtained in step 5) in a tube furnace and anneal it at 250°C in air for 2 hours to repair crystal damage that may be introduced during the ion implantation slicing process and wet etching, and reduce waveguide propagation loss.

[0080] 7) End-face cleavage and optical coupling. The wafer is cleaved along the yz-symmetry plane of the Z-cut TFLN to obtain a smooth end face for fiber-to-chip coupling. This cleaved surface can form a Fabry-Perot resonator, facilitating subsequent optical characterization. Thus, the lithium niobate zero-refractive-index metawaveguide of this embodiment is obtained.

[0081] The optical performance of the fabricated lithium niobate zero-refractive-index metawaveguide was characterized and optimized. Specifically, the optical transmission spectrum of the fabricated fishbone-shaped metawaveguide was measured using a tunable continuous-wave laser (wavelength range 1520-1640 nm). The zero-refractive-index phase-matching condition and the excitation effect of the quasi-BIC mode were verified by monitoring the conversion efficiency and resonance quality factor of the frequency-doubled light.

[0082] The zero refractive index characteristic of the fabricated metawaveguide at the frequency-harmonic wavelength (approximately 775 nm) was experimentally verified. To achieve quantitative characterization of the equivalent refractive index of the metawaveguide, a measurement method based on on-chip opposing interferometry was adopted, the specific implementation of which is as follows:

[0083] 1) Construct an on-chip optical path structure for interferometry. Input coupling structures are set at both ends of the fabricated metamorphic waveguide. These coupling structures can be grating couplers or end-face coupling structures, which are used to achieve efficient injection of external light sources into the metamorphic waveguide.

[0084] 2) Select a continuous wave laser source with adjustable operating wavelength (covering the 750nm–800nm ​​band) and adjust it to the target polarization state (preferably TM polarization) using a polarization controller. Use a beam splitter to split the incident light into two paths, and use an optical path matching structure to ensure that the two paths have a stable phase relationship, thereby forming two coherent frequency-doubled beams.

[0085] 3) The two coherent frequency-doubled beams are coupled into the fabricated metamorphic waveguide from both ends, creating electromagnetic field distributions propagating in opposite directions within the waveguide. A high numerical aperture microscope is used to collect the scattered light from the metamorphic waveguide surface, and a CCD or CMOS camera is used to record the spatial distribution image of the interference fringes. Furthermore, by performing a spatial Fourier transform on the interference pattern or directly measuring the fringe spacing, the equivalent refractive index at the corresponding wavelength can be extracted.

[0086] 4) By scanning the incident light wavelength (e.g., gradually adjusting it within the range of 750 nm to 800 nm) and recording the changes in the interference fringe period corresponding to different wavelengths, the relationship between the equivalent refractive index and wavelength can be obtained. When the working wavelength approaches the zero refractive index point, the interference fringe period increases significantly, tending towards infinity, and the corresponding spatial electric field distribution tends to be uniform, thus indicating that the n-wavelength condition is satisfied at this wavelength. eff The zero refractive index condition is approximately 0.

[0087] Through the synergistic effect of the above-mentioned structure and physical mechanism, the present invention achieves:

[0088] 1) Phase matching is achieved by canceling the momentum of the opposing Bloch mode at the fundamental frequency and the zero momentum corresponding to the zero refractive index at the harmonic frequency;

[0089] 2) Improve nonlinear conversion efficiency based on the finite impedance corresponding to zero-refractive-index waveguides;

[0090] 3) Based on the quasi-BIC mode, strong field localization and energy enhancement of fundamental and frequency-doubled light are achieved;

[0091] This enables efficient second harmonic frequency conversion in nanoscale structures.

[0092] Compared with existing second harmonic generation devices and methods, this invention has the following significant advantages: First, phase matching is achieved through on-chip reverse collision and momentum automatic cancellation mechanism of frequency doubling zero refractive index, eliminating the dependence of traditional nonlinear devices on complex crystal modification processes such as periodic polarization (PPLN), and significantly improving the manufacturing tolerance of the device; Second, symmetry breaking is introduced into the metastructure to excite quasi-continuous domain bound state (quasi-BIC) mode with high quality factor (High-Q), and the strong resonant field local amplification effect is used to improve the nonlinear frequency conversion efficiency.

Claims

1. A method for generating second harmonics, characterized in that, include: A periodic nanostructure array was constructed using thin-film lithium niobate, and the periodic nanostructure array was configured to support an optical mode with an equivalent wave vector close to zero at the frequency harmonic wavelength. Two coherent fundamental frequency beams propagating in opposite directions are introduced into the periodic nanostructure array, and the two fundamental frequency beams are made to have in-plane wave vector components of equal magnitude and opposite direction. The two fundamental frequency beams spontaneously generate second-order nonlinear polarization in the periodic nanostructure array to excite harmonic light with a wave vector close to zero, thereby realizing the generation of second harmonics based on momentum conservation.

2. The method for generating second harmonics according to claim 1, characterized in that, The periodic nanostructure array includes several structural units arranged along the direction of light propagation. These structural units have broken structural symmetry to support quasi-continuous bound state modes with high quality factor.

3. A second harmonic generation device, characterized in that, include: A periodic nanostructure array based on thin-film lithium niobate is configured to support an optical mode with an equivalent wave vector close to zero at the frequency harmonic wavelength. The input ends are located at both ends of the periodic nanostructure array and are used to introduce two coherent fundamental frequency beams propagating in opposite directions into the periodic nanostructure array, and to make the two fundamental frequency beams have in-plane wave vector components of equal magnitude and opposite direction. The output ends are located at both ends of the periodic nanostructure array and are used to extract frequency-harmonic light with a wave vector close to zero, which is excited by the spontaneous second-order nonlinear polarization generated by the two fundamental frequency beams in the periodic nanostructure array.

4. The second harmonic generation device according to claim 3, characterized in that, The periodic nanostructure array includes several structural units arranged along the direction of light propagation. These structural units have broken structural symmetry to support quasi-continuous bound state modes with high quality factor.

5. The second harmonic generation device according to claim 4, characterized in that, The structural unit includes a central spine, and the symmetry structure is disrupted by perforating the opposite side walls of the central spine with air columns.

6. The second harmonic generation device according to claim 5, characterized in that, The air column has an elliptical cross-section, and after being hollowed out, it forms regions that are recessed into the interior of the central spine on both sides of the central spine.

7. The second harmonic generation device according to claim 5, characterized in that, The periodic nanostructure array is configured to support an optical mode with an equivalent wave vector close to zero at the frequency harmonic wavelength by adjusting geometric parameters. The geometric parameters to be adjusted include the period and geometric dimensions of the central spine, as well as the geometric dimensions of the air column along and perpendicular to the light propagation direction.

8. The second harmonic generation device according to claim 5, characterized in that, The central ridge has a period of 300nm-500nm, a structural thickness of 500nm-900nm, a top width of 150nm-300nm, a bottom width of 500nm-1350nm, and a sidewall tilt angle of 60°-70°; the air column has a geometric dimension of 300nm-500nm along the light propagation direction and a geometric dimension of 100nm-300nm perpendicular to the light propagation direction.

9. The second harmonic generation device according to claim 3, characterized in that, The periodic nanostructure array is formed by completely or partially etching the thin film lithium niobate; when partially etched, a non-zero thickness lithium niobate plate layer is left at the bottom of the periodic nanostructure array. The thin-film lithium niobate has a Z-cut or X-cut crystal orientation structure.

10. The second harmonic generation device according to claim 3, characterized in that, The input end employs a grating coupling structure or an evanescent field coupling structure to excite guided modes or Bloch modes with preset in-plane wave vector components; the two fundamental frequency beams have a fixed phase relationship.

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