CN112099286A - Optical harmonic generator and preparation method thereof - Google Patents

Abstract

The invention discloses an optical harmonic generator and a preparation method thereof, wherein the optical harmonic generator mainly comprises: a first material layer; and the second material layer is stacked on the first material layer, and the sign of the second-order nonlinear coefficient of the second material layer is opposite to that of the second-order nonlinear coefficient of the first material layer. The invention utilizes the first material layer and the second material layer which have opposite signs of the second-order nonlinear coefficients and are stacked together to form the waveguide core layer, and matches the electric field direction distribution of the generated second harmonic in the waveguide core layer, thereby realizing higher conversion efficiency of the second harmonic in a simple waveguide core layer structure, reducing the preparation difficulty of an optical harmonic generator and realizing the purpose of effectively improving the generation efficiency of the second harmonic by utilizing a simple device structure.

Description

Optical harmonic generator and preparation method thereof

technical field

The invention relates to the technical field of nonlinear optics, in particular to an optical harmonic generator using nonlinear coefficients to match the second harmonic optical field distribution and a preparation method thereof.

Background technique

Optical second harmonic generation is the process of up-converting the frequency to the second harmonic using the interaction of a strong coherent optical field and an optical material with second-order nonlinearity. It has important applications in measurement, imaging and other fields.

The traditional optical second harmonic generation method is to inject the pump laser into the nonlinear optical crystal to generate the second harmonic. The devices involved in this traditional method are bulky and difficult to integrate.

In recent years, with the development of micro-nano technology, second harmonic generation based on integrated optical waveguides has been widely studied. By confining the optical field in a small optical waveguide, the nonlinear effect of the optical waveguide can be effectively enhanced, and the threshold of the nonlinear effect can be lowered, so that the generation of light at other frequencies can be measured by using a lower power pump light. High-efficiency second harmonic generation has been achieved on integrated platforms such as aluminum nitride (AlN), gallium nitride (GaN), gallium arsenide (GaAs), thin-film Lithium Niobate on Insulator (LNOI), etc. .

Using an integrated optical waveguide to generate the second harmonic requires consideration of the mode distribution of the fundamental wave and the second harmonic. The distribution of the electromagnetic field that can be transmitted in the waveguide is called the mode of the waveguide. The modes in the waveguide are divided into several categories, such as space radiation mode, substrate radiation mode, guided mode and surface mode, among which the guided mode can be effectively transmitted in the waveguide, Radiation modes are dissipated into the cladding and substrate and cannot be transported. Theoretically, the modes of the waveguide can be obtained by solving the Helmholtz equation.

A series of special solutions can be obtained by solving the transverse Helmholtz equation of the waveguide under certain electromagnetic field boundary conditions. There are two fundamental eigenmodes in slab waveguides, one is called TE mode and the other is called TM mode. The other forms of electromagnetic fields in the waveguide can be expressed by Fourier expansion of these two fundamental modes.

It is intuitive to define the TE mode and the TM mode by the polarization directions of the electric and magnetic fields of light. The electric field is only selected to be polarized in the direction parallel to the waveguide interface. At this time, the electric field is perpendicular to the transmission direction of the light, that is, the polarization direction of the electric field is lateral. Therefore, this mode is called Transverse Electric Mode, or TE mode. . The magnetic field is only selected to be polarized in the direction parallel to the waveguide interface. At this time, the magnetic field is perpendicular to the transmission direction of the light, that is, the polarization direction of the magnetic field is transverse. Therefore, this mode is called Transverse Magnetic Mode, or TM mode. .

Slightly different from slab waveguides, the two eigenmodes supported in ridge waveguides are quasi-TE mode and quasi-TM mode, where the main component of the electric field of the quasi-TE mode is polarized parallel to the direction of the waveguide cross-section, and the electric field of the quasi-TM mode is polarized parallel to the direction of the waveguide cross-section. The principal component of is polarized parallel to the direction of the waveguide cross section.

A key parameter of the second harmonic generation is the conversion efficiency, which requires that the interacting light waves meet the phase matching condition, and at the same time, the overlap integral between the light waves is required to be large. There are two commonly used phase matching methods for integrated optical waveguide devices: mode phase matching and quasi-phase matching. Among them, the mode phase matching is to use the abundant modes in the waveguide to realize the phase matching between the fundamental wave and the second harmonic mode by adjusting the structural parameters of the waveguide. The mode field overlap is small, resulting in lower conversion efficiency. Quasi-phase matching uses a periodic optical superlattice structure to provide an inverted lattice vector to compensate for phase mismatch. Since the fundamental mode (the lowest order mode in the waveguide structure) is used for both the fundamental wave and the second harmonic, the mode field overlap is large and the efficiency is low. However, the complex periodic polarization inversion process is required, and the fabrication of the device is difficult.

It can be seen that the problems faced by mode phase matching and quasi-phase matching are low conversion efficiency and device fabrication difficulty, respectively. Therefore, a balance should be sought between conversion efficiency and device fabrication difficulty, which has both high conversion efficiency and high device fabrication difficulty. With low preparation difficulty, it has become an urgent problem to be solved.

SUMMARY OF THE INVENTION

In view of this, the present invention provides an optical harmonic generator and a preparation method thereof, so as to achieve higher conversion efficiency of the second harmonic and lower preparation difficulty, and to effectively improve the second harmonic by using a simple device structure production efficiency.

The technical scheme of the present invention is realized as follows:

An optical harmonic generator comprising:

the first material layer;

A second material layer, the second material layer is disposed on the first material layer, and the sign of the second-order nonlinear coefficient of the second material layer is the same as the sign of the second-order nonlinear coefficient of the first material layer on the contrary.

Further, the materials of the first material layer and the second material layer are both lithium niobate.

Further, the first material layer is disposed on a substrate, and the sign of the second-order nonlinear coefficient of the substrate is the same as the sign of the second-order nonlinear coefficient of the first material layer;

The second material layer is disposed on a side of the first material layer away from the substrate.

Further, the material of the substrate is lithium niobate.

Further, a silicon oxide layer is provided between the substrate and the first material layer.

Further, the included angle between the sidewalls of the first material layer and the second material layer and the bottom plane of the first material layer on the side away from the second material layer is 60° to 90°.

Further, the thickness of the first material layer is 200nm to 300nm, the thickness of the second material layer is 200nm to 300nm, and the total thickness of the first material layer and the second material layer is 400nm to 600nm, so The width of the surface of the second material layer away from the first material layer is 800 nm to 1000 nm.

A preparation method of an optical harmonic generator, comprising:

providing a bulk material and a wafer having a film of the first material;

bombarding the bulk material such that a second material film having a set thickness and peelable from the body of the bulk material is formed in the bulk material;

Bonding the second material film to the first material film, and controlling the relative crystal orientation between the second material film and the first material film during bonding, so that the second material film The sign of the second-order nonlinear coefficient of is opposite to the sign of the second-order nonlinear coefficient of the first material film;

peeling the second material film from the bulk material;

The second material film and the first material film are etched to form a second material layer and a first material layer.

Further, the bonding of the second material film to the first material film, and controlling the relative crystal direction between the second material film and the first material film during bonding, includes:

setting the surface of the second material film opposite to the surface of the first material film, and aligning the crystal direction of the second material film with the crystal direction of the first material film;

Taking the surface of the second material film as a reference plane, rotating the body material in-plane by 180°;

The surface of the second material film is bonded to the surface of the first material film.

Further, materials of the bulk material, the first material film and the second material film are all lithium niobate.

It can be seen from the above solution that the optical harmonic generator and the preparation method thereof of the present invention utilize the opposite signs of the second-order nonlinear coefficients, and the first material layer and the second material layer stacked together constitute a waveguide core layer , combined with the distribution of the electric field direction of the generated second harmonic in the waveguide core layer, a higher conversion efficiency of the second harmonic is achieved in the simple waveguide core layer structure, and the fabrication difficulty of the optical harmonic generator is reduced. , to achieve the purpose of effectively improving the generation efficiency of the second harmonic using a simple device structure.

Description of drawings

FIG. 1A is a schematic diagram of a cross-sectional structure principle of a waveguide for a mode field distribution situation according to an embodiment of the present invention;

FIG. 1B is a schematic schematic diagram of a waveguide cross-sectional structure for another mode field distribution situation according to an embodiment of the present invention;

2 is a schematic cross-sectional structure diagram of an optical harmonic generator according to an embodiment of the present invention;

3A is a schematic diagram of a cross-sectional structure with the same second-order nonlinear coefficient at different positions of a waveguide core layer in a conventional waveguide structure;

3B is a schematic diagram of a cross-sectional structure with opposite signs of second-order nonlinear coefficients at different positions of a waveguide core layer in a waveguide structure according to an embodiment of the present invention;

3C is a schematic diagram of the optical field direction of the TE ₀₀ mode of the waveguide core layer;

FIG. 3D is a schematic diagram of the optical field direction of the TE ₀₁ mode of the waveguide core layer;

Fig. 4A is the mode effective refractive index relationship curve of the fundamental wave TE ₀₀ mode and the second harmonic TE ₀₁ mode under different waveguide widths;

4B is a schematic diagram of a curve comparing the second harmonic generation efficiencies of different structures;

5 is a flowchart of a method for preparing an optical harmonic generator according to an embodiment of the present invention;

6A is one of the cross-sectional changes of the device structure in the process of adopting the preparation method of the embodiment of the present invention;

FIG. 6B is the second change diagram of the cross-sectional view of the device structure in the process of adopting the preparation method according to the embodiment of the present invention;

FIG. 6C is the third change diagram of the cross-sectional view of the device structure in the process of using the preparation method of the embodiment of the present invention;

6D is a fourth view of a change in the cross-sectional view of the device structure in the process of using the preparation method according to the embodiment of the present invention;

6E is a fifth view of a change in the cross-sectional view of the device structure in the process of using the preparation method according to the embodiment of the present invention;

FIG. 6F is the sixth schematic cross-sectional change diagram of the device structure in the process of adopting the preparation method according to the embodiment of the present invention;

FIG. 7 is a cross-sectional structure of a device obtained by using the preparation method of the embodiment of the present invention.

In the accompanying drawings, the names of the components represented by each number are as follows:

1. Waveguide core layer

11. The first material layer

12. The second material layer

2. Substrate

3. Cladding

4. Body material

21. Silicon oxide layer

11', the first material film

12', the second material film

5. Mask

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be described in further detail below with reference to the accompanying drawings and examples.

For second harmonic generation, the expression for the normalized frequency conversion efficiency η is:

where P is the optical power, L is the waveguide length, n is the effective refractive index, _ε0 is the vacuum permittivity, c is the speed of light in vacuum, ω is the angular frequency of the fundamental wave, and 2ω is the angle of the second harmonic frequency, A is the normalized mode area, and its expression is

where E is the normalized electric field, and its expression is

为有效的二阶非线性系数，其表达式为

is an effective second-order nonlinear coefficient whose expression is

Among them, χ ⁽²⁾ is the second-order nonlinear coefficient of the waveguide core material.

where Γ is the normalized mode field overlap factor, which is expressed as

Wherein, the integration interval of the numerator is the region where the second-order nonlinear material is located, and the integration interval of the denominator is the entire region including the cladding, the waveguide core layer and the substrate.

in,

sinc(ΔkL/2)=sin(ΔkL/2)/(ΔkL/2)

Among them, Δk is the phase mismatch factor, and its expression is

Δk=2ω(n _ω −n _2ω ).

的平方成正比，与相位失配因子Δk呈sinc²函数关系。因此，在满足相位匹配条件的前提下，通过增大有效的二阶非线性系数，可以有效提高二次谐波产生效率，即通过增大

可以有效提高η。It can be seen from formula (1) that the conversion efficiency η is related to the effective second-order nonlinear coefficient

It is proportional to the square of , and has a sinc ² function relationship with the phase mismatch factor Δk. Therefore, on the premise of satisfying the phase matching conditions, by increasing the effective second-order nonlinear coefficient, the generation efficiency of the second harmonic can be effectively improved, that is, by increasing the effective second-order nonlinear coefficient

can effectively increase η.

From the expression of the effective second-order nonlinear coefficient, it can be seen that it is not only related to the spatial distribution of the fundamental and second harmonic mode fields, but also to the distribution of the second-order nonlinear coefficient. In the traditional waveguide structure, the second-order nonlinear coefficient of the waveguide core layer is uniformly distributed, and the conversion efficiency depends on the mode overlap factor. However, for the phase matching method of the integrated optical waveguide device with mode phase matching, the fundamental wave is the fundamental mode, and the second harmonic is the higher-order mode. Due to the difference in the spatial distribution of different modes, the mode overlap factor is small and the efficiency is low. Especially when the spatial symmetry of the optical field distribution of the fundamental mode and the second harmonic mode is different, the overlap factor is almost zero, and it is difficult to achieve high-efficiency second harmonic generation at this time.

In response to this problem, an embodiment of the present invention proposes a distribution design structure for the second-order nonlinear coefficients of the waveguide core layer, so that the signs of the second-order nonlinear coefficients at different positions of the waveguide core layer are related to the second harmonic optical field. Therefore, the effective second-order nonlinear coefficients at different positions are superimposed, and the conversion efficiency is improved. The waveguide cross-sectional structure principles designed for two different mode field distributions are shown in Fig. 1A and Fig. 1B, respectively.

As shown in FIG. 1A , a waveguide core layer 1 is formed on a substrate 2 , and a cladding layer 3 is provided on the outer periphery of the waveguide core layer 1 . The waveguide core layer 1 is a multilayer stack structure, and the second-order nonlinear coefficient χ ⁽²⁾ of each layer matches the direction of the second harmonic light field at the position of the layer. As shown in FIG. 1B , a waveguide core layer 1 is formed on a substrate 2 , and a cladding layer 3 is provided on the outer periphery of the waveguide core layer 1 . 1A, the multilayer structures in the waveguide core layer 1 are arranged side by side on the substrate 2, each layer is in contact with the substrate 2, and the second-order nonlinear coefficient χ ⁽²⁾ of each layer is related to the The direction of the second harmonic light field at the location of the layer is matched. Here, the propagation direction of the fundamental wave is perpendicular to the view surfaces of FIGS. 1A and 1B .

Based on the above structure principle of the waveguide cross section, an embodiment of the present invention provides an optical harmonic generator, as shown in FIG. 2 , which includes a first material layer 11 and a second material layer 12 . The second material layer 12 is stacked on the first material layer 11 , and the sign of the second-order nonlinear coefficient χ ⁽²⁾ of the second material layer 12 is the same as the second-order nonlinear coefficient χ (2) of the first material layer 11 ^. sign is opposite. The waveguide core layer 1 is composed of the first material layer 11 and the second material layer 12 together. Among them, the propagation direction of the fundamental wave is perpendicular to the view surface of FIG. 2 . In an optional embodiment, the materials of the first material layer 11 and the second material layer 12 are both lithium niobate (LiNbO ₃ ).

Continuing to refer to FIG. 2 , in an optional embodiment, the first material layer 11 is disposed on a substrate 2 , and the sign of the second-order nonlinear coefficient χ ⁽²⁾ of the substrate 2 is the same as that of the first material layer 11 . The sign of the second-order nonlinear coefficient χ ⁽²⁾ is the same, and the second material layer 12 is stacked on the side of the first material layer 11 away from the substrate 2 . In an alternative embodiment, the material of the substrate 2 is lithium niobate.

Additionally, in an alternative embodiment, a silicon oxide layer (not shown in FIG. 2 ) is provided between the substrate 2 and the first material layer 11 .

Hereinafter, a specific physical explanation for realizing high-efficiency second harmonic generation by using a device structure in which the second-order nonlinear coefficient matches the second-harmonic optical field distribution by using the embodiment of the present invention is given.

因此，基于铌酸锂的晶体坐标系，光的传播方向选为晶体的y方向，波导横截面对应xOz平面(即图2所示的横截面)，若采用x 切(即图2视图中的上方，垂直于芯片表面的方向，垂直于波导芯层1的第二材料层12的上表面方向)铌酸锂薄膜，则基波和二次谐波应当选择TE模，此时TE模的主分量对应晶体的z方向(图2视图中波导芯层1的宽度方向)；若采用z 切即芯片垂直方向为晶体的z方向)铌酸锂薄膜，则基波和二次谐波应当选择TM 模，此时TM模的主分量对应晶体的z方向。For isotropic crystals, the second-order nonlinear coefficients in all directions are the same, and there are no special requirements for the waveguide direction, light propagation direction and light polarization. For anisotropic crystals, such as lithium niobate in the embodiment of the present invention, the magnitudes of the second-order nonlinear coefficients in different directions are different. The direction in which the linear coefficient component is largest. For lithium niobate crystals, the largest component of the second-order nonlinear coefficient is

Therefore, based on the crystal coordinate system of lithium niobate, the propagation direction of light is selected as the y direction of the crystal, and the cross section of the waveguide corresponds to the xOz plane (ie the cross section shown in Figure 2). Above, the direction perpendicular to the chip surface, and the direction perpendicular to the upper surface of the second material layer 12 of the waveguide core layer 1) lithium niobate film, then the fundamental wave and the second harmonic should select the TE mode. At this time, the main TE mode The component corresponds to the z direction of the crystal (the width direction of the waveguide core layer 1 in the view of Figure 2); if the z-cut is used, that is, the vertical direction of the chip is the z direction of the crystal) lithium niobate film, the fundamental wave and the second harmonic should choose TM mode, the principal component of the TM mode corresponds to the z-direction of the crystal.

When the fundamental wave is the TE ₀₀ mode (or TM ₀₀ mode) and the second harmonic is the TE ₀₁ mode (or TM ₀₁ mode), due to the opposite spatial symmetry of the mode fields of the two, specifically, as shown in Figure 3C, In the waveguide core layer, the optical field directions of the upper and lower parts of the TE ₀₀ mode (or TM ₀₀ mode) are the same. As shown in Figure 3D, the optical field directions of the upper and lower parts of the TE ₀₁ mode (or TM ₀₁ mode) are opposite. FIG. 3A shows a cross-sectional structure with the same second-order nonlinear coefficient at different positions of the waveguide core layer in the conventional waveguide structure. In the conventional waveguide structure, the second-order nonlinear coefficient χ ⁽²⁾ at different positions of the waveguide core layer 1 is the same. , so the upper and lower part of the fundamental wave and the second harmonic mode field overlap, resulting in a very small effective second-order nonlinear coefficient. , formula (3)) can also be seen.

的方向相反，其中，Since the electric field directions of the upper and lower parts of the second harmonic TE ₀₁ mode (or TM ₀₁ mode) are opposite, and based on this, in the optical harmonic generator of the embodiment of the present invention, the second-order upper and lower parts of the waveguide core layer 1 are divided into two parts. The sign of the nonlinear coefficient is designed to be opposite, as shown in Fig. 3B, the second-order nonlinear polarization of the upper and lower parts

in the opposite direction, where,

The upper and lower parts of the waveguide core layer 1 (ie the second material layer 12 and the first material layer 11 ) in the optical harmonic generator according to the embodiment of the present invention shown in FIGS. 2 and 3B are respectively connected to the second harmonic TE ₀₁ mode. (or TM ₀₁ mode) corresponding to the electric field direction, the effective second-order nonlinear coefficient at this time is

At this time, the effective second-order nonlinear coefficients of the upper and lower parts of the waveguide core layer 1 (ie, the second material layer 12 and the first material layer 11 ) are superimposed, so that the conversion efficiency can be improved.

Taking the second harmonic generation of the x-tangential thin-film lithium niobate waveguide structure as an example, the specific design process of the optical harmonic generator according to the embodiment of the present invention is as follows.

Firstly, the phase matching design is carried out, and the thin-film lithium niobate waveguide structure is calculated by the finite element method (FEM), and the mode field distribution and the corresponding mode effective refractive index can be obtained. In the embodiment of the present invention, the cladding layer 3 adopts the fully etched thin-film lithium niobate structure of the air cladding layer. For the partially etched structure (as shown in Figure 7), the higher-order modes have large radiation loss and cannot form guided modes, while for the fully etched structure (as shown in Figure 2), the contact surface between the waveguide core and the cladding is more large, the refractive index difference can be formed in a larger area, so it can provide better mode confinement, and the number of effective guided modes is large.

For different thicknesses of lithium niobate films, the conditions for satisfying the phase matching can be determined by scanning the width of the waveguide, that is, when the mode effective refractive index of the fundamental TE ₀₀ mode and the TE ₀₁ mode at the second harmonic are the same, the phase is satisfied. matching conditions, as shown in Figure 4A. Specially, the dividing line of the sign of the second-order nonlinear coefficient is the same as the dividing line of the electric field direction of the second harmonic TE ₀₁ mode, as shown in FIG. 3B and FIG. 3D .

The normalized second harmonic generation efficiency can be calculated from Equation (1) and Equation (4). By calculating the second harmonic generation efficiency of the phase-matching structure under different LiNbO3 film thicknesses, it can be found that the conversion efficiency first increases and then decreases with the film thickness, and there is a maximum value. The reason is that when the film thickness is too small, the mode confinement effect is poor, and the mode diffusion is serious, and when the film thickness is too large, the mode area will increase with the increase of the waveguide size, so there is a minimum value of the mode area, corresponding to The maximum value of the second harmonic generation efficiency.

The research results show that when the thickness of the lithium niobate film is 450nm and the width is 950nm, the phase matching conditions are met, as shown in Figure 4A, and the conversion efficiency is the largest, the corresponding efficiency is 13000%/W/cm ² , and the corresponding mode field distribution is as follows Figure 3C, Figure 3D. It can be seen that the dividing line of the electric field direction of the second harmonic TE ₀₁ mode basically corresponds to the center line of the waveguide core layer. At the same time, by comparing the second harmonic generation efficiency of different structures, as shown in FIG. 4B , it can be seen that the conversion efficiency of the optical harmonic generator using the antisymmetric second-order nonlinear structure in the embodiment of the present invention is far higher Compared with the traditional waveguide structure, and higher than the periodically polarized lithium niobate (PPLN) waveguide structure. In FIG. 4B , the position of the five-pointed star is the position where the second harmonic generation efficiency of the optical harmonic generator according to the embodiment of the present invention is the highest.

Continuing to refer to FIG. 2 , based on the above research results, in an optional embodiment, the sidewalls of the first material layer 11 and the second material layer 12 (ie, the sidewalls of the waveguide core layer 1 ) are far away from the first material layer 11 The angle between the bottom plane on one side of the second material layer 12 (that is, the bottom plane adjacent to the first material layer 11 and the substrate 2 ) (that is, the inclination angle of the sidewall of the waveguide core layer 1 ) is 60° to 90°, Specifically, for example, 60°, 65°, 70°, 75°, 80°, 85°, 90°, preferably 90°, and 75° currently achievable in technology.

In an optional embodiment, the thickness of the first material layer 11 is 200 nm to 300 nm, the thickness of the second material layer 12 is 200 nm to 300 nm, and the total thickness of the first material layer 11 and the second material layer 12 (ie, the waveguide core layer) 1) is 400nm to 600nm, and the width of the second material layer 12 away from the surface of the first material layer 11 (that is, the surface where the top of the second material layer 12 extends along the waveguide core layer 1 shown in FIG. 2) is 800nm to 1000nm. Among them, the direction of the width is perpendicular to the propagation direction of the fundamental wave. In a preferred embodiment, the thicknesses of the first material layer 11 and the second material layer 12 are the same. In an optional embodiment, the thickness of the first material layer 11 is, for example, 200 nm, 250 nm, and 300 nm, and the thickness of the second material layer 12 is, for example, 200 nm, 250 nm, and 300 nm. The thickness of the material layer 12 is 250 nm. In the description of the present invention, the width of the surface of the second material layer 12 away from the first material layer 11 is the width of the waveguide core layer 1 . 950nm, 1000nm, preferably, the width of the waveguide core layer 1 is 910nm.

In a preferred embodiment, when lithium niobate material is used, the inclination angle of the sidewall of the waveguide core layer 1 is 75°, the thickness of the first material layer 11 is 250 nm, the thickness of the second material layer 12 is 250 nm, and the waveguide width is 910 nm. , the corresponding second harmonic conversion efficiency can reach 9500%/W/cm ² .

An embodiment of the present invention also provides a method for preparing an optical harmonic generator, as shown in FIG. 5 , including:

Step 1, providing a bulk material and a wafer with a film of the first material;

Step 2, bombarding the bulk material, so that a second material film having a set thickness and peelable from the body of the bulk material is formed in the bulk material;

Step 3. Bond the second material film to the first material film, and control the relative crystal direction between the second material film and the first material film during bonding, so that the second-order nonlinear coefficient of the second material film is The sign is opposite to the sign of the second-order nonlinear coefficient of the first material film;

Step 4, peel off the second material film from the bulk material;

Step 5, etching the second material film and the first material film to form a second material layer and a first material layer.

In an alternative embodiment, a wafer having a film of the first material includes a substrate and a film of the first material on the substrate.

In an optional embodiment, the bulk material, the first material film and the second material film are all lithium niobate, and the substrate of the wafer is also lithium niobate.

Wherein, in an optional embodiment, the second material film is bonded to the first material film in step 3, and the relative crystal orientation between the second material film and the first material film is controlled during bonding, specifically including:

The surface of the second material film is opposite to the surface of the first material film, and the crystal direction of the second material film is aligned with the crystal direction of the first material film;

Taking the surface of the second material film as the reference plane, the body material is rotated in-plane by 180°;

The surface of the second material film is bonded to the surface of the first material film.

The specific process of the preparation method of the optical harmonic generator according to the embodiment of the present invention will be further described below.

As shown in Figure 6A, the lithium niobate bulk material is bombarded with high-energy helium ions (He ⁺ ) of a specific energy. As shown in FIG. 6A , the dotted line represents the bombardment depth of high-energy helium ions. At the position of the dotted line, the molecular structure of bulk material 4 is destroyed due to the bombardment of high-energy helium ions, and then with the subsequent process, the lithium niobate film of corresponding thickness can be changed from Body material 4 peeled off. In FIG. 6A and subsequent figures, the arrows located in the bulk material 4 and located in the chip indicate the directions corresponding to the largest components of the second-order nonlinear coefficients of the materials.

As shown in Fig. 6B, the bulk material 4 is aligned with the crystal direction of the first material film 11' of the on-chip lithium niobate material. The chip includes a substrate 2 and a first material film 11' located on the substrate 2, and a silicon oxide layer 21 is provided between the first material film 11' and the substrate 2. Among them, the chip can be a commercial chip provided by a third party, or can be prepared by itself.

As shown in Figure 6C, the bulk material 4 is rotated in-plane by 180° and then bonded to the chip.

After the bonding is completed, the second material film 12 ′ of the lithium niobate material with a specific thickness (determined by the bombardment depth of the high-energy helium ions, that is, determined by the energy of the high-energy helium ions) is peeled off from the bulk material 4 by using hydrofluoric acid (HF). , as shown in Figure 6D.

A mask is prepared by electron beam exposure on the second material film 12', as shown in Fig. 6E.

Finally, the second material film 12' and the first material film 11' are etched by the reactive ion etching (RIE) method to form the waveguide core layer 1 of thin film lithium niobate material and remove the mask 5 to obtain a thin film niobium-based waveguide core layer 1. A second-order optical harmonic generator for the antisymmetric second-order nonlinearity of lithium oxide is shown in Figure 6F.

Compared with the traditional preparation process of the waveguide core layer, the preparation method provided by the embodiment of the present invention only adds one step of bonding process, so compared with the traditional method that adopts the periodic optical superlattice structure to provide the quasi-phase matching of the inverted lattice vector. The waveguide core layer greatly reduces the complexity of the process and is easy to implement.

FIG. 7 shows the cross-sectional structure of the optical harmonic generator manufactured by the manufacturing method of the embodiment of the present invention, and the following table shows the parameter values of each part in FIG. 7 .

parameter w_t h_e h₊ h_- θ value 910nm 450nm 250nm 250nm 75°

In the optical harmonic generator and the manufacturing method thereof according to the embodiments of the present invention, the signs of the second-order nonlinear coefficients are opposite to each other, and the first material layer and the second material layer stacked together constitute a waveguide core layer, and the generated The distribution of the electric field direction of the second harmonic in the waveguide core layer achieves a higher conversion efficiency of the second harmonic in the simple waveguide core layer structure, and reduces the preparation difficulty of the optical harmonic generator. The purpose of the device structure is to effectively improve the generation efficiency of the second harmonic.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the present invention. within the scope of protection.

Claims (10)

1. An optical harmonic generator comprising:

a first material layer;

and a second material layer which is laminated on the first material layer and has a second-order nonlinear coefficient having a sign opposite to that of the first material layer.

2. The optical harmonic generator of claim 1 wherein:

the first material layer and the second material layer are both made of lithium niobate.

3. The optical harmonic generator of claim 2 wherein:

the first material layer is arranged on a substrate, and the sign of a second-order nonlinear coefficient of the substrate is the same as that of the first material layer;

the second material layer is stacked on one side, far away from the substrate, of the first material layer.

4. The optical harmonic generator of claim 3 wherein:

the substrate is made of lithium niobate.

5. The optical harmonic generator of claim 4 wherein:

and a silicon oxide layer is arranged between the substrate and the first material layer.

6. The optical harmonic generator of claim 3 wherein:

the included angle between the side wall of the first material layer and the second material layer and the bottom plane of the first material layer far away from one side of the second material layer is 60-90 degrees.

7. The optical harmonic generator of any one of claims 1 to 6 wherein:

the thickness of the first material layer is 200nm to 300nm, the thickness of the second material layer is 200nm to 300nm, the total thickness of the first material layer and the second material layer is 400nm to 600nm, and the surface width of the second material layer far away from the first material layer is 800nm to 1000 nm.

8. A method of making an optical harmonic generator comprising:

providing a bulk material and a wafer having a first material film;

bombarding the bulk material such that a second material film having a set thickness and being peelable from a body of the bulk material is formed in the bulk material;

bonding the second material film to the first material film, and controlling a relative crystal direction between the second material film and the first material film at the time of bonding such that a sign of a second order nonlinear coefficient of the second material film is opposite to a sign of a second order nonlinear coefficient of the first material film;

peeling the second material film from the bulk material;

and etching the second material film and the first material film to form a second material layer and a first material layer.

9. The method of claim 8, wherein bonding the second material film to the first material film while controlling a relative crystal orientation between the second material film and the first material film comprises:

disposing a surface of the second material film to be opposed to a surface of the first material film, and aligning a crystal direction of the second material film with a crystal direction of the first material film;

rotating the bulk material in-plane by 180 ° with the surface of the second material film as a reference plane;

bonding a surface of the second material film with a surface of the first material film.

10. The production method of an optical harmonic generator according to claim 8 or 9, characterized in that:

the material of the bulk material, the first material film, and the second material film is lithium niobate.

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