CN112099286A - Optical harmonic generator and method for producing the same - Google Patents

Optical harmonic generator and method for producing the same Download PDF

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CN112099286A
CN112099286A CN202011048058.5A CN202011048058A CN112099286A CN 112099286 A CN112099286 A CN 112099286A CN 202011048058 A CN202011048058 A CN 202011048058A CN 112099286 A CN112099286 A CN 112099286A
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material layer
material film
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CN112099286B (en
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孙长征
张向秀
熊兵
王健
罗毅
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Tsinghua University
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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/37—Non-linear optics for second-harmonic generation
    • G02F1/377—Non-linear optics for second-harmonic generation in an optical waveguide structure
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/355—Non-linear optics characterised by the materials used
    • G02F1/3551—Crystals
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/365—Non-linear optics in an optical waveguide structure

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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 method for producing the same
Technical Field
The invention relates to the technical field of nonlinear optics, in particular to an optical harmonic generator utilizing nonlinear coefficients to be matched with the distribution of a second harmonic light field and a preparation method thereof.
Background
The optical second harmonic generation is a process of converting frequency up to second harmonic by utilizing the interaction of a strong coherent optical field and an optical material with second-order nonlinearity, and the optical second harmonic is an important method for generating new frequency and expanding a wave band range, and has important application in the fields of measurement, imaging and the like.
The conventional optical second harmonic generation method is to inject pump laser into a nonlinear optical crystal to generate second harmonic, and the conventional method involves large-sized devices and is difficult to integrate.
In recent years, with the development of micro-nano technology, the generation of second harmonic based on integrated optical waveguide has been widely studied. The nonlinear effect of the optical waveguide can be effectively enhanced by limiting the optical field in the small optical waveguide, and the threshold value of the nonlinear effect is reduced, so that the generation of light of other frequencies can be measured by adopting pump light with lower power. 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 (LNOI), and the like.
The second harmonic is generated by adopting the integrated optical waveguide, and the mode distribution of the fundamental wave and the second harmonic needs to be considered. The distribution of the electromagnetic field which can be transmitted in the waveguide is called as the mode of the waveguide, and the modes in the waveguide are divided into several categories, such as a space radiation mode, a substrate radiation mode, a guided mode and a surface mode, wherein the guided mode can be effectively transmitted in the waveguide, and the radiation mode is dissipated into a cladding and a substrate and cannot be transmitted. The mode of the waveguide can be derived theoretically by solving the helmholtz equation.
And solving the transverse Helmholtz equation of the waveguide under a certain electromagnetic field boundary condition to obtain a series of special solutions. There are two fundamental eigenmodes in a slab waveguide, one called the TE mode and the other called the TM mode. While other forms of electromagnetic fields in the waveguide can be expressed as fourier expansions of the two fundamental modes.
It is intuitive to define the TE and TM modes with the polarization directions of the electric and magnetic fields of light. The Electric field is chosen to be polarized only in a direction parallel to the waveguide interface, where the Electric field is perpendicular to the direction of light transmission, i.e., the polarization direction of the Electric field is Transverse, and thus this Mode is referred to as the Transverse Electric Mode (TE Mode). The selection field is only polarized in a direction parallel to the waveguide interface, where the field is perpendicular to the direction of light propagation, i.e. the polarization direction of the field is Transverse, and this Mode is therefore called the Transverse Magnetic Mode (TM Mode).
Slightly different from the slab waveguide, the two eigenmodes supported in the ridge waveguide 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 main component of the electric field of the quasi-TM mode is polarized parallel to the direction of the waveguide cross section.
One key parameter for second harmonic generation is conversion efficiency, which requires that the phase matching conditions be satisfied between interacting light waves, while requiring that the overlap integral between light waves be large. The phase matching method of the currently common integrated optical waveguide device comprises two methods: mode phase matching and quasi-phase matching. The mode phase matching is to realize the phase matching between fundamental wave and second harmonic modes by adjusting the structural parameters of the waveguide by utilizing abundant modes in the waveguide, but the mode field overlapping between the fundamental wave and the second harmonic is small due to the difference in mode spatial distribution, so that the conversion efficiency is low. The quasi-phase matching adopts a periodic optical superlattice structure to provide reciprocal lattice vector and compensate phase mismatch, and because fundamental waves and second harmonic waves adopt fundamental modes (the lowest order mode in a waveguide structure), mode field overlapping is large, efficiency is high, but a complex periodic polarization reversal process is needed, and the difficulty in preparing the device is large.
Therefore, the problems faced by the mode phase matching and the quasi-phase matching are respectively low conversion efficiency and high device preparation difficulty, so that a balance is sought between the conversion efficiency and the device preparation difficulty, and the problems to be solved are solved urgently.
Disclosure of Invention
In view of this, the present invention provides an optical harmonic generator and a method for manufacturing the same, so as to achieve a higher conversion efficiency of the second harmonic and a lower manufacturing difficulty, and to achieve an effective improvement in the generation efficiency of the second harmonic by using a simple device structure.
The technical scheme of the invention is realized as follows:
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.
Further, the material of the first material layer and the material of the second material layer are both lithium niobate.
Further, the first material layer is arranged on a substrate, and the sign of the 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.
Further, the substrate is made of lithium niobate.
Further, a silicon oxide layer is arranged between the substrate and the first material layer.
Further, 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 the second material layer is 60-90 degrees.
Further, 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 width of the surface, away from the first material layer, of the second material layer is 800nm to 1000 nm.
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.
Further, the bonding the second material film to the first material film, controlling a relative crystal orientation between the second material film and the first material film at the time of bonding, includes:
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.
Further, the material of the bulk material, the first material film, and the second material film is lithium niobate.
According to the optical harmonic generator and the preparation method thereof, the first material layer and the second material layer which have opposite signs of the second-order nonlinear coefficients and are stacked together form the waveguide core layer, and the generated second harmonic is matched with the electric field direction distribution in the waveguide core layer, so that the higher conversion efficiency of the second harmonic is realized in a simple waveguide core layer structure, the preparation difficulty of the optical harmonic generator is reduced, and the purpose of effectively improving the generation efficiency of the second harmonic by using a simple device structure is realized.
Drawings
FIG. 1A is a schematic diagram of a waveguide cross-sectional structure for one mode field distribution according to an embodiment of the present invention;
FIG. 1B is a schematic diagram of a waveguide cross-sectional structure for another mode field distribution case according to an embodiment of the present invention;
FIG. 2 is a schematic cross-sectional view of an optical harmonic generator according to an embodiment of the present invention;
FIG. 3A is a schematic cross-sectional view of a conventional waveguide structure with the same second-order nonlinear coefficients at different positions of the waveguide core;
FIG. 3B is a schematic cross-sectional view of a waveguide structure with second-order nonlinear coefficients of opposite signs at different positions of a waveguide core layer according to an embodiment of the present invention;
FIG. 3C shows the waveguide core TE00A schematic light field direction of the mode;
FIG. 3D is the waveguide core layer TE01A schematic light field direction of the mode;
FIG. 4A shows fundamental TE at different waveguide widths00Mode and second harmonic TE01A modal effective index dependence of the mode;
FIG. 4B is a graph illustrating a comparison of second harmonic generation efficiencies for different configurations;
FIG. 5 is a flow chart of a method of fabricating an optical harmonic generator in accordance with an embodiment of the present invention;
FIG. 6A is one of variations of a cross-sectional view of a device structure during fabrication using embodiments of the present invention;
FIG. 6B is a second cross-sectional view of a device structure during fabrication using the method of embodiments of the present invention;
FIG. 6C is a third variation diagram of a cross-sectional view of a device structure during a fabrication process according to an embodiment of the present invention;
FIG. 6D is a fourth illustration of a cross-sectional variation of the device structure during fabrication using embodiments of the present invention;
FIG. 6E is a fifth illustration of a cross-sectional variation of the device structure during fabrication using embodiments of the present invention;
FIG. 6F is a sixth illustration of a cross-sectional change of the device structure during fabrication using the embodiments of the present invention;
fig. 7 is a cross-sectional structure of a device obtained by the manufacturing method of the embodiment of the present invention.
In the drawings, the names of the components represented by the respective reference numerals are as follows:
1. waveguide core layer
11. A first material layer
12. Second material layer
2. Substrate
3. Cladding layer
4. Bulk material
21. Silicon oxide layer
11', first material film
12' of a second material
5. Mask and method for manufacturing the same
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and examples.
For second harmonic generation, the expression for normalized frequency conversion efficiency η is:
Figure RE-GDA0002772343980000051
wherein P is the optical power, L is the waveguide length, n is the effective refractive index,0c is the speed of light in vacuum, ω is the angular frequency of the fundamental, 2 ω is the angular frequency of the second harmonic, and A is the normalized mode area, expressed as
Figure RE-GDA0002772343980000052
Wherein E is a normalized electric field and has an expression of
Figure RE-GDA0002772343980000053
Figure RE-GDA0002772343980000054
Is an effective second-order nonlinear coefficient expressed by
Figure RE-GDA0002772343980000055
Wherein, χ(2)Is the second order nonlinear coefficient of the waveguide core layer material.
Wherein, the expression is normalized mode field overlap factor
Figure RE-GDA0002772343980000056
The integral interval of the numerator is the area where the second-order nonlinear material is located, and the integral interval of the denominator is the whole area including the cladding, the waveguide core layer and the substrate.
Wherein the content of the first and second substances,
sinc(ΔkL/2)＝sin(ΔkL/2)/(ΔkL/2)
where Δ k is a phase mismatch factor expressed as
Δk＝2ω(nω-n2ω)。
From the formula (1), the conversion efficiency η and the effective second-order nonlinear coefficient
Figure RE-GDA0002772343980000057
Is proportional to the square of the phase mismatch factor ak, sinc2And (4) functional relation. Therefore, on the premise of satisfying the phase matching condition, the second harmonic generation efficiency can be effectively improved by increasing the effective second-order nonlinear coefficient, namely, by increasing
Figure RE-GDA0002772343980000058
Eta can be effectively improved.
It can be seen from the effective expression of the second order nonlinear coefficient 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 conventional waveguide structure, the second-order nonlinear coefficients of the waveguide core layer are uniformly distributed, and the conversion efficiency depends on the mode overlapping factor. However, for the phase matching method of the mode phase matching integrated optical waveguide device, the fundamental wave is the fundamental mode, and the second harmonic wave is the high-order mode, and due to the difference in spatial distribution of different modes, the mode overlap factor is very small, and the efficiency is low, especially when the spatial symmetry of the optical field distribution of the fundamental wave mode and the second harmonic wave mode is different, the overlap factor is almost zero, and it is difficult to realize the generation of the second harmonic wave with high efficiency.
In order to solve the problem, the embodiment of the invention provides a distribution design structure of second-order nonlinear coefficients of a waveguide core layer, so that the signs of the second-order nonlinear coefficients at different positions of the waveguide core layer are matched with the direction of a second harmonic light field, thereby overlapping effective second-order nonlinear coefficients at different positions and improving the conversion efficiency. The principle of the waveguide cross-sectional structure designed for two different mode field distributions is shown in fig. 1A and 1B, respectively.
As shown in fig. 1A, a waveguide core layer 1 is formed on a substrate 2, and a clad layer 3 is provided on the outer periphery of the waveguide core layer 1. Wherein, the waveguide core layer 1 has a multilayer stack structure, and the second-order nonlinear coefficient χ of each layer(2)Matching the direction of the second harmonic light field at the location of the layer. As shown in fig. 1B, a waveguide core layer 1 is formed on a substrate 2, and a clad layer 3 is provided on the outer periphery of the waveguide core layer 1. Unlike fig. 1A, the multilayer structure in the waveguide core layer 1 is juxtaposed on the substrate 2, each layer is in contact with the substrate 2, and the second-order nonlinear coefficients χ of the layers are different(2)Matching the direction of the second harmonic light field at the location of the layer. Wherein the propagation direction of the fundamental wave is perpendicular to the view surfaces of fig. 1A and 1B.
Based on the waveguide cross-sectional structure principle, the embodiment of the invention provides an optical harmonic generator, as shown in fig. 2, which includes a first material layer 11 and a second material layer 12. Wherein the second material layer 12 is laminated on the first material layer 11, andsecond order non-linear coefficient chi of two material layers 12(2)And the second-order nonlinear coefficient χ of the first material layer 11(2)The signs of (A) and (B) are opposite. The waveguide core layer 1 is formed by the first material layer 11 and the second material layer 12. Wherein the propagation direction of the fundamental wave is perpendicular to the view surface of fig. 2. In an alternative embodiment, the material of the first material layer 11 and the material of the second material layer 12 are both lithium niobate (LiNbO)3)。
With continued reference to FIG. 2, in an alternative embodiment, the first material layer 11 is disposed on a substrate 2, and the second-order nonlinear coefficient χ of the substrate 2(2)And the second-order nonlinear coefficient χ of the first material layer 11(2)The symbols are the same, and the second material layer 12 is stacked on the side of the first material layer 11 far away from the substrate 2. In an alternative embodiment, the material of the substrate 2 is lithium niobate.
In addition, 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.
The following provides a specific physical explanation for realizing high-efficiency second harmonic generation by using a device structure with second-order nonlinear coefficients matched with second-order harmonic optical field distribution according to the embodiment of the invention.
For isotropic crystals, the second-order nonlinear coefficients in all directions are the same in magnitude, and no special requirements are imposed on the waveguide direction, the light propagation direction and the light polarization. For anisotropic crystals, such as lithium niobate in the embodiments of the present invention, the magnitudes of the second-order nonlinear coefficients in different directions are different, and it is necessary to align the principal components of the fundamental wave and the second-order harmonic optical field with the direction in which the second-order nonlinear coefficient component is largest. For lithium niobate crystals, the maximum component of the second-order nonlinear coefficient is
Figure RE-GDA0002772343980000061
Therefore, based on the crystal coordinate system of lithium niobate, the propagation direction of light is selected as the y direction of the crystal, the waveguide cross section corresponds to the xOz plane (i.e., the cross section shown in fig. 2), and if the x-cut (i.e., the upper direction in the view of fig. 2, 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 thin film is used, the waveguide cross section is based on the x-cut lithium niobate thin filmThe waves and the second harmonic should be selected to be TE mode, and the main component of TE mode corresponds to the z direction of the crystal (the width direction of the waveguide core layer 1 in the view of FIG. 2); if z-cut, i.e., the chip vertical direction is the z direction of the crystal) is used, the TM mode should be selected for the fundamental and second harmonics, and the principal component of the TM mode corresponds to the z direction of the crystal.
When the fundamental wave is TE00Mode (or TM)00Mode), second harmonic is TE01Mode (or TM)01Mode), due to the opposite spatial symmetry of the two mode fields, in particular, as shown in fig. 3C, TE is present in the waveguide core layer00Mode (or TM)00Mode) has the same optical field direction, as shown in fig. 3D, TE01Mode (or TM)01Mode) the light fields of the upper and lower portions are in opposite directions. FIG. 3A shows a cross-sectional structure of the waveguide core layer having the same second-order nonlinear coefficient at different positions in the conventional waveguide structure in which the second-order nonlinear coefficients χ are different at different positions in the waveguide core layer 1(2)Similarly, the mode field overlaps of the upper and lower parts of the fundamental wave and the second harmonic are cancelled out, resulting in a small effective second-order nonlinear coefficient, as can be seen from the above expressions of the mode field overlap factor and the effective second-order nonlinear coefficient (i.e., formula (2) and formula (3)).
TE due to second harmonic01Mode (or TM)01Mode), and further, based on the fact that the signs of the second-order nonlinear coefficients of the upper and lower portions of the waveguide core layer 1 are designed to be opposite in the optical harmonic generator according to the embodiment of the present invention, as shown in fig. 3B, the second-order nonlinear polarization amounts of the upper and lower portions are set to be opposite
Figure RE-GDA0002772343980000071
In the opposite direction, wherein,
Figure RE-GDA0002772343980000072
in the optical harmonic generator of the embodiment of the invention shown in FIGS. 2 and 3B, the upper and lower portions (i.e., the second material layer 12 and the first material layer 11) of the waveguide core layer 1 are respectively twoSub-harmonic TE01Mode (or TM)01Mode) corresponds to the electric field direction, the effective second-order nonlinear coefficient at this time is
Figure RE-GDA0002772343980000073
At this time, the effective second-order nonlinear coefficients of the upper and lower portions (i.e., the second material layer 12 and the first material layer 11) of the waveguide core layer 1 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 of the embodiment of the invention is as follows.
Firstly, phase matching design is carried out, a Finite Element Method (FEM) is adopted to calculate a thin film lithium niobate waveguide structure, and mode field distribution and a corresponding mode effective refractive index can be obtained. In the embodiment of the invention, the cladding 3 adopts a fully-etched thin-film lithium niobate structure of an air cladding. For the partially etched structure (as shown in fig. 7), the high-order mode has large radiation loss and cannot form a guided mode, while the contact surface between the waveguide core layer and the cladding layer of the fully etched structure (as shown in fig. 2) is larger, and the refractive index difference can be formed in a larger area, so that better mode limitation can be provided, and the number of effective guided modes is large.
For different lithium niobate film thicknesses, the condition of meeting phase matching can be determined by scanning the waveguide width, namely when the fundamental wave TE00TE at mode and second harmonic01The mode effective refractive index of the mode is the same and satisfies the phase matching condition, as shown in fig. 4A. Specially, the second harmonic TE and the boundary of the signs of the second-order nonlinear coefficients01The boundaries in the mode electric field direction are the same as shown in fig. 3B and 3D.
The normalized second harmonic generation efficiency can be calculated from formula (1) and formula (4). By calculating the second harmonic generation efficiency of the phase matching structure under different lithium niobate film thicknesses, it can be found that the conversion efficiency is firstly increased and then reduced along with the film thickness, wherein a maximum value exists. The reason is that when the film thickness is too small, the mode confinement effect is poor, and the mode diffusion is severe, and when the film thickness is too large, the mode area increases with the increase in the waveguide size, so that there is a minimum value of the mode area corresponding to the maximum value of the second harmonic generation efficiency.
The research result shows that when the thickness of the lithium niobate thin film is 450nm and the width is 950nm, the phase matching condition is satisfied, as shown in FIG. 4A, and the conversion efficiency is maximum, and the corresponding efficiency is 13000%/W/cm2The corresponding mode field distributions are shown in fig. 3C and 3D. It can be seen that the second harmonic TE01The division line in the direction of the mode electric field corresponds substantially to the centre line of the waveguide core. Meanwhile, by comparing the generation efficiency of the second harmonic waves with different structures, as shown in fig. 4B, it can be seen that the conversion efficiency of the optical harmonic generator adopting the antisymmetric second-order nonlinear structure in the embodiment of the present invention is far higher than that of the conventional waveguide structure and higher than that of the Periodically Poled Lithium Niobate (PPLN) waveguide structure. In fig. 4B, the five-pointed star is located at the position where the second harmonic generation efficiency of the optical harmonic generator according to the embodiment of the present invention is the highest.
With continued reference to fig. 2, based on the above research results, in an alternative embodiment, the included angle between the sidewalls of the first material layer 11 and the second material layer 12 (i.e., the sidewalls of the waveguide core layer 1) and the bottom plane of the first material layer 11 on the side away from the second material layer 12 (i.e., the bottom plane of the first material layer 11 adjacent to the substrate 2) (i.e., the sidewall inclination angle of the waveguide core layer 1) is 60 ° to 90 °, specifically, for example, 60 °, 65 °, 70 °, 75 °, 80 °, 85 °, 90 °, preferably 90 °, and 75 ° is currently achievable in the art.
In an alternative embodiment, the thickness of the first material layer 11 is 200nm to 300nm, the thickness of the second material layer 12 is 200nm to 300nm, the total thickness of the first material layer 11 and the second material layer 12 (i.e. the thickness of 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 (i.e. the surface of the top of the second material layer 12 extending along the waveguide core layer 1 in fig. 2) is 800nm to 1000 nm. Wherein the direction of the width is perpendicular to the propagation direction of the fundamental wave. In a preferred embodiment, the first material layer 11 and the second material layer 12 are of the same thickness. In an alternative embodiment, the thickness of the first material layer 11 is, for example, 200nm, 250nm, 300nm, and the thickness of the second material layer 12 is, for example, 200nm, 250nm, 300nm, and preferably, the thickness of the first material layer 11 is 250nm and the thickness of the second material layer 12 is 250 nm. In the present description, 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, and in an alternative embodiment, the width of the waveguide core layer 1 is, for example, 800nm, 850nm, 900nm, 950nm, 1000nm, and preferably, the width of the waveguide core layer 1 is 910 nm.
In a preferred embodiment, when lithium niobate material is adopted, the inclination angle of the side wall of the waveguide core layer 1 is 75 degrees, the thickness of the first material layer 11 is 250nm, the thickness of the second material layer 12 is 250nm, and the width of the waveguide is 910nm, the corresponding second harmonic conversion efficiency can reach 9500%/W/cm2。
The embodiment of the present invention further provides a method for manufacturing an optical harmonic generator, as shown in fig. 5, including:
step 1, providing a bulk material and a wafer with a first material film;
step 2, bombarding the bulk material to form a second material film which has a set thickness and can be peeled from the bulk of the bulk material in the bulk material;
step 3, bonding 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 when bonding, so that the sign of the second-order nonlinear coefficient of the second material film is opposite to the sign of the second-order nonlinear coefficient of the first material film;
step 4, stripping the second material film from the body material;
and 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 first material film includes a substrate and the first material film on the substrate.
In an alternative 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.
In an alternative embodiment, the bonding the second material film to the first material film in step 3, and controlling the relative crystal orientation between the second material film and the first material film during the bonding specifically includes:
disposing a surface of the second material film opposite 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 body material plane by 180 degrees by taking the surface of the second material film as a reference plane;
the surface of the second material film is bonded to the surface of the first material film.
The specific process of the method for manufacturing the optical harmonic generator according to the embodiment of the present invention is further described below.
As shown in FIG. 6A, high-energy helium ions (He) of a specific energy are used+) Bombarding the lithium niobate material. In fig. 6A, the dashed line indicates the bombardment depth of the high-energy helium ions, and at the dashed line position, the molecular structure of the bulk material 4 is damaged due to the bombardment of the high-energy helium ions, so that the lithium niobate thin film with a corresponding thickness can be peeled off from the bulk material 4 in cooperation with the subsequent process. In fig. 6A and subsequent figures, the arrows in the bulk material 4 and in the chip indicate the directions corresponding to the maximum components of the second-order nonlinear coefficients of the material.
As shown in fig. 6B, the bulk material 4 is aligned with the crystal direction of the first material film 11' of the lithium niobate material on the chip. Wherein the chip comprises a substrate 2 and a first material film 11 'on the substrate 2, a silicon oxide layer 21 being provided between the first material film 11' and the substrate 2. The chip can be a commercial chip provided by a third party, and can also be prepared by the chip.
As shown in fig. 6C, the bulk material 4 is rotated 180 ° in-plane and then bonded to a chip.
After bonding is complete, a second material film 12' of lithium niobate material of a specified thickness (determined by the depth of bombardment of the energetic helium ions, i.e., by the energy of the energetic helium ions) is stripped from the bulk material 4 using hydrofluoric acid (HF), as shown in fig. 6D.
A mask is prepared on the second material film 12' using electron beam exposure as shown in fig. 6E.
Finally, the second material film 12 'and the first material film 11' are etched by a Reactive Ion Etching (RIE) method to form the waveguide core layer 1 of the thin film lithium niobate material and remove the mask 5, so as to obtain the antisymmetric second-order nonlinear second-order optical harmonic generator based on the thin film lithium niobate, as shown in fig. 6F.
Compared with the traditional preparation process of the waveguide core layer, the preparation method provided by the embodiment of the invention only adds one step of bonding process, so that the process complexity is greatly reduced and the realization is easy compared with the traditional waveguide core layer which adopts a periodic optical superlattice structure to provide quasi-phase matching of inverted lattice vectors.
Fig. 7 shows a cross-sectional structure of an optical harmonic generator manufactured by the manufacturing method of the embodiment of the present invention, and the following table shows parameter values of respective portions in fig. 7.
Parameter(s) wt he h+ h- θ
Value of 910nm 450nm 250nm 250nm 75°
According to the optical harmonic generator and the preparation method thereof, the first material layer and the second material layer which have opposite signs of the second-order nonlinear coefficients and are stacked together are utilized to form the waveguide core layer, and the generated second harmonic is matched with the electric field direction distribution in the waveguide core layer, so that the higher conversion efficiency of the second harmonic is realized in a simple waveguide core layer structure, the preparation difficulty of the optical harmonic generator is reduced, and the purpose of effectively improving the generation efficiency of the second harmonic by utilizing a simple device structure is realized.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.

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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Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2003066499A (en) * 2001-08-30 2003-03-05 Murata Mfg Co Ltd Optical device and method of manufacturing the same
CN102338966A (en) * 2011-09-09 2012-02-01 上海交通大学 Polarization-independent quasi-phase-matching frequency multiplier and manufacturing method thereof
US20130230270A1 (en) * 2010-11-10 2013-09-05 Rolf Tjalle Horn Waveguide sandwich source of polarization entangled photons
CN104678494A (en) * 2013-11-28 2015-06-03 日本碍子株式会社 Optical waveguide devices
CN110749552A (en) * 2019-12-02 2020-02-04 南开大学 System and method for determining second-order nonlinear polarizability of material
CN111129932A (en) * 2019-12-12 2020-05-08 广东晶启激光科技有限公司 Device for generating broadband supercontinuum laser and manufacturing method of crystal in device

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2003066499A (en) * 2001-08-30 2003-03-05 Murata Mfg Co Ltd Optical device and method of manufacturing the same
US20130230270A1 (en) * 2010-11-10 2013-09-05 Rolf Tjalle Horn Waveguide sandwich source of polarization entangled photons
CN102338966A (en) * 2011-09-09 2012-02-01 上海交通大学 Polarization-independent quasi-phase-matching frequency multiplier and manufacturing method thereof
CN104678494A (en) * 2013-11-28 2015-06-03 日本碍子株式会社 Optical waveguide devices
CN110749552A (en) * 2019-12-02 2020-02-04 南开大学 System and method for determining second-order nonlinear polarizability of material
CN111129932A (en) * 2019-12-12 2020-05-08 广东晶启激光科技有限公司 Device for generating broadband supercontinuum laser and manufacturing method of crystal in device

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