Non-linear optical structure with perfect phase matching and high-efficiency second harmonic generation efficiency

Abstract

The invention discloses a non-linear optical structure with perfect phase matching and high second harmonic generation efficiency. The non-linear optical structure comprises a fundamental frequency wave resonance structure, a non-linear optical layer, a second harmonic reflection structure and a substrate, the fundamental frequency wave resonance structure is used for generating electromagnetic field local enhancement at an incident fundamental frequency wavelength and realizing a phase matching condition by optimizing electric field phase distribution; the nonlinear optical layer is used for frequency conversion from fundamental frequency waves to second harmonics and regulating and controlling harmonic radiation capability and phase regulation and control; the second harmonic reflection structure is used for controlling coupling-out of second harmonics; the rotary nonlinear optical layer can adjust the phase characteristic of the radiated second harmonic so as to realize perfect phase matching of a harmonic field; according to the nonlinear optical structure with perfect phase matching and high-efficiency second harmonic generation efficiency, accurate control of phase matching conditions is realized through reasonable optical film stacking and phase matching technologies, and high-efficiency coupling-out and utilization of second harmonics are ensured.

Description

Technical Field

[0001] The invention relates to the technical field of quantum light sources, and in particular to a nonlinear optical structure with perfect phase matching and high efficiency in second harmonic generation. Background Art

[0002] With the deepening of laser technology and nonlinear optics research, second harmonic generation, as an important nonlinear optical phenomenon, is widely used in lasers, optical communications, spectral analysis and other fields. In the current technological development trend, the demand for miniaturized optical devices is increasing, and the integration of nonlinear optical devices has therefore become a research hotspot. However, the miniaturization of nonlinear optical devices faces an important challenge, namely the weakening of nonlinear effects. Traditional second harmonic generation devices mainly focus on the enhancement of the fundamental frequency electric field, while ignoring the phase mismatch problem that is unrelated to refractive index dispersion, which seriously limits the efficiency of harmonic generation. In particular, in terms of efficiently enhancing second harmonic emission, existing technologies still face great challenges. Therefore, understanding and manipulating subwavelength-scale phase matching is crucial for designing compact nonlinear optical devices. Summary of the invention

[0003] The purpose of the present invention is to provide a nonlinear optical structure with perfect phase matching and efficient second harmonic generation. Through reasonable optical film stacking and phase matching technology, precise control of phase matching conditions can be achieved to ensure efficient coupling and utilization of second harmonics, providing new solutions for research and application in related fields.

[0004] To achieve the above object, the present invention adopts the following technical solution:

[0005] A nonlinear optical structure with perfect phase matching and high efficiency of second harmonic generation, comprising a fundamental wave resonance structure, a nonlinear optical layer, a second harmonic reflection structure and a substrate having a bearing function;

[0006] The fundamental frequency wave resonant structure is used to generate a local enhancement of the electromagnetic field at the incident fundamental frequency wavelength, and to achieve a phase matching condition by optimizing the electric field phase distribution;

[0007] The nonlinear optical layer is used for frequency conversion of fundamental frequency wave to second harmonic wave, and regulating harmonic radiation capability and phase regulation;

[0008] The second harmonic reflection structure has a reflectivity of ≥95% at the second harmonic wavelength, the second harmonic reflection structure has an absorption and reflection loss of ≤20% at the fundamental wavelength, and the second harmonic reflection structure is used to control the outcoupling of the second harmonic;

[0009] Rotating the nonlinear optical layer can adjust the phase characteristics of the radiated second harmonic to achieve perfect phase matching of the harmonic field.

[0010] In the nonlinear optical structure with perfect phase matching and efficient second harmonic generation provided by at least one embodiment of the present disclosure, the fundamental wave resonance structure includes a guided mode resonance structure and a highly efficient reflection medium or a Bragg reflector.

[0011] In the nonlinear optical structure with highly efficient second harmonic generation and perfect phase matching provided by at least one embodiment of the present disclosure, the nonlinear optical material is a crystallographically non-centrosymmetric material, and the second-order nonlinear optical polarizability coefficient of the crystallographically non-centrosymmetric material is adjustable when the crystal is rotated or the crystal orientation is changed.

[0012] In the nonlinear optical structure with perfect phase matching and high efficiency second harmonic generation provided by at least one embodiment of the present disclosure, the crystallographic non-centrosymmetric material is a single layer or multilayer structure.

[0013] In the nonlinear optical structure with perfect phase matching and efficient second harmonic generation provided by at least one embodiment of the present disclosure, the second harmonic reflection structure is a highly efficient reflection medium or multiple layers of materials with different refractive indices stacked alternately.

[0014] In the nonlinear optical structure with highly efficient second harmonic generation and perfect phase matching provided by at least one embodiment of the present disclosure, the fundamental wave resonance structure is between the second harmonic reflection structure and the nonlinear optical material, and / or the fundamental wave resonance structure is between the nonlinear optical material and the substrate, so as to form a localized fundamental wave electric field in the resonant cavity.

[0015] In the nonlinear optical structure with perfect phase matching and efficient second harmonic generation provided by at least one embodiment of the present disclosure, the substrate is a low refractive index medium material.

[0016] In the nonlinear optical structure with perfect phase matching and high efficiency second harmonic generation provided by at least one embodiment of the present disclosure, the substrate is one of silicon dioxide, magnesium fluoride or polydimethylsiloxane.

[0017] In the nonlinear optical structure with perfect phase matching and high efficiency second harmonic generation provided by at least one embodiment of the present disclosure, the crystallographic non-centrosymmetric material is a single layer or a multilayer nonlinear optical film.

[0018] In the nonlinear optical structure with perfect phase matching and high efficiency second harmonic generation provided by at least one embodiment of the present disclosure, the crystallographic non-centrosymmetric material is a 3R phase crystal material.

[0019] The beneficial effects of the present invention are: through the cleverly designed resonant structure and nonlinear optical film, the fundamental frequency wave and the second harmonic can be effectively separated and controlled, providing a good structural foundation for the efficient generation and emission of the second harmonic.

[0020] Phase matching is achieved by rotating the nonlinear optical film or changing the orientation of the nonlinear optical crystal. The operation is simple and easy to implement, and does not require complex external equipment or cumbersome processing technology, which reduces the difficulty and cost of achieving efficient second harmonic generation.

[0021] It can greatly enhance the emission intensity of the second harmonic, significantly improve the efficiency of second harmonic generation, provide more powerful technical support for related optical applications, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0023] Figure 1 It is a cross-sectional schematic diagram of a nonlinear optical structure with perfect phase matching and high efficiency second harmonic generation of the present invention;

[0024] Figure 2 is a functional characteristic diagram of the second harmonic reflection structure and the fundamental frequency wave resonance structure of Example 1;

[0025] Figure 3 is a second harmonic intensity distribution diagram in Example 1 where perfect phase matching is not achieved;

[0026] Figure 4 is a distribution diagram of the propagation characteristics of the fundamental frequency electric field and harmonic radiation with thickness position when material dispersion is not considered in Example 1;

[0027] Figure 5 is a distribution diagram of the propagation characteristics of the fundamental frequency electric field and harmonic radiation with thickness position when material dispersion is considered in Example 1;

[0028] Figure 6 This is a phase change diagram after fine phase matching when material dispersion is not considered in Example 1. The negative coefficient on the top represents that the material at the thickness position is rotated;

[0029] Figure 7 This is a phase change diagram after fine phase matching when material dispersion is considered in Example 1. The negative coefficient on the top represents the rotation of the material at the thickness position;

[0030] Figure 8 This is a comparison diagram of the second harmonic intensity after fine phase matching when material dispersion is not considered in Example 1 and when there is no resonant structure and no phase matching;

[0031] Fig. 9 This is a comparison diagram of the second harmonic intensity after fine phase matching is performed when material dispersion is considered in Example 1 and when there is no resonant structure and no phase matching is performed;

[0032] Fig.10 is a cross-sectional schematic diagram of the nonlinear optical structure of Example 2 and Example 3;

[0033] Fig.11 is a functional characteristic diagram of the second harmonic reflection structure and the fundamental frequency wave resonance structure of Example 2 and Example 3;

[0034] Fig.12 is a second harmonic intensity distribution diagram in Example 2 where perfect phase matching is not achieved;

[0035] Fig.13 is a distribution diagram of the propagation characteristics of the fundamental frequency electric field and harmonic radiation with thickness position when material dispersion is not considered in Example 2;

[0036] Fig.14 is a distribution diagram of the propagation characteristics of the fundamental frequency electric field and harmonic radiation with thickness position when material dispersion is considered in Example 2;

[0037] Fig.15 is a second harmonic intensity distribution diagram after fine phase matching in Example 2;

[0038] Fig.16 is a second harmonic intensity distribution diagram in Example 3 where perfect phase matching is not achieved;

[0039] Fig.17 is a distribution diagram of the propagation characteristics of the fundamental frequency electric field and harmonic radiation with thickness position when material dispersion is not considered in Example 3;

[0040] Fig.18 is a distribution diagram of the propagation characteristics of the fundamental frequency electric field and harmonic radiation with thickness position when material dispersion is considered in Example 3;

[0041] Fig.19 This is the second harmonic intensity distribution diagram after fine phase matching in Example 3.

[0042] In the figure:

[0043] 10. Second harmonic reflection structure;

[0044] 20. Nonlinear optical layer;

[0045] 30. Fundamental frequency wave resonance structure;

[0046] 40. Substrate. DETAILED DESCRIPTION

[0047] The technical solutions in the embodiments will be described clearly and completely below in conjunction with the drawings in the embodiments. Obviously, the described embodiments are only a part of the embodiments, rather than all the embodiments.

[0048] The specific design of the nonlinear optical structure with perfect phase matching and high efficiency second harmonic generation provided by the present invention is mainly divided into three steps:

[0049] S1. Select appropriate second harmonic reflection structural materials, nonlinear optical materials, fundamental wave resonance structural materials and substrate materials according to the target second harmonic response gain band and the properties of the material itself, and design and prepare the optical structure to meet the basic structural requirements and functional characteristics;

[0050] S2. By adjusting the parameters of the optical structure, such as the shape, thickness, refractive index, etc. of each structure, the distribution of the fundamental frequency electric field is optimized to reach an effective maximum value;

[0051] S3. By changing the crystal orientation, spatial rotation angle, etc. of the nonlinear optical film, the phase of the Green's function is adjusted to make it in phase with the square of the fundamental frequency electric field; when phase matching is achieved, the fundamental frequency wave is incident on the optical structure, and after being converted by the nonlinear optical film, the generated second harmonic is smoothly emitted under the action of the structure, thereby achieving efficient transmission and reflection of the second harmonic.

[0052] Example 1

[0053] Reference Figure 1 This embodiment provides a nonlinear optical structure with perfect phase matching and high efficiency in second harmonic generation, including a second harmonic reflection structure 10, a nonlinear optical layer 20, a fundamental frequency resonance structure 30 and a substrate 40.

[0054] Specifically, the fundamental wave resonant structure 30 is used to generate a local enhancement of the electromagnetic field at the incident fundamental frequency wavelength, and to achieve a phase matching condition by optimizing the electric field phase distribution.

[0055] Specifically, the nonlinear optical layer 20 includes a crystal material having a second-order nonlinear effect, which is used for frequency conversion of a fundamental wave to a second harmonic, and for regulating harmonic radiation capability and phase regulation.

[0056] Specifically, the second harmonic reflection structure 10 has a reflectivity of ≥95% at the second harmonic wavelength and an absorption and reflection loss of ≤20% at the fundamental wavelength. The second harmonic reflection structure 10 is used to control the outcoupling of the second harmonic at the port.

[0057] Specifically, the substrate 40 plays a supporting role and is made of a low refractive index dielectric material, such as silicon dioxide, magnesium fluoride, and polydimethylsiloxane.

[0058] Furthermore, the second harmonic reflection structure 10 uses a Bragg reflector (DBR3), which is composed of eight overlapping layers of materials with refractive indices of 1.45 and 2.4, respectively, and the thickness of each layer of low refractive index medium and high refractive index medium is 86 nm and 52 nm, respectively.

[0059] The fundamental wave resonant structure 30 includes a Bragg reflector (DBR1) and a Bragg reflector (DBR2). The Bragg reflector (DBR1) and the Bragg reflector (DBR2) are placed on the substrate 40 and on the nonlinear optical material, respectively. The Bragg reflector (DBR1) and the Bragg reflector (DBR2) are composed of ten layers and two layers of materials with refractive indices of 1.45 and 2.4, respectively. The thickness of each layer of low refractive index medium and high refractive index medium is 172nm and 104nm, respectively. The nonlinear optical material adopts Phase crystal material, its second-order nonlinear polarizability coefficient is adjustable when the crystal is turned, the thickness is 400nm, and its second-order nonlinear polarizability tensor is expressed as:

[0060]

[0061] In this embodiment, the propagation of the second harmonic can be described by the Helmholtz equation:

[0062]

[0063] Among them, ∈0, ∈ r are vacuum, relative dielectric constant, μ r is the relative magnetic permeability, k0 is the wave vector of the fundamental frequency wave, represents the second-order nonlinear polarization. In the non-pump depletion approximation, the solution of the Helmholtz equation can be expressed by the Green function as:

[0064]

[0065] Green's function G(2ω; r, r') is a dyadic tensor that reflects the radiation capability from the source point to the field point. In order to evaluate the contribution of the subharmonic field generated by the point source in the nonlinear optical film to the total harmonic field, the phase matching factor Φ is defined as the ratio of the magnitude of the cumulative vector to the cumulative magnitude of all vectors. Mathematically, it can be expressed as:

[0066]

[0067] When the complex integrand in the nonlinear optical film When all components of are in phase, the phase matching factor Φ reaches a maximum value of 1. Therefore, by changing the thickness of the nonlinear optical film, the phase matching factor will vary with the coherence length as a period.

[0068] In a multilayer film structure, the distribution of the fundamental frequency electric field is mainly determined by the optical properties and structural parameters of each layer. When the incident light enters the multilayer film structure, the fundamental frequency electric field propagates in each layer and undergoes reflection and transmission, forming a specific electric field distribution. Since the distribution of the fundamental frequency electric field mainly depends on the optical thickness and refractive index of each layer, rotating the nonlinear optical film will not change the distribution of the fundamental frequency electric field.

[0069] However, the Green's function describes the radiation ability from the source point to the field point, reflecting the propagation characteristics of the second harmonic in the multilayer film structure. The distribution of the Green's function is closely related to the crystal orientation of the nonlinear optical film. When the nonlinear optical film is rotated, its crystallographic symmetry changes, causing the distribution of the Green's function to change accordingly.

[0070] By rotating the nonlinear optical film, the phase distribution of the Green's function can be adjusted to match the phase distribution of the square of the fundamental frequency electric field. When the phase of the fundamental frequency electric field and the Green's function is consistent, the efficiency of the second harmonic generation reaches the maximum value. This is because under the phase matching condition, the coherent superposition effect of the second harmonic is optimal, thereby significantly enhancing the emission intensity of the second harmonic.

[0071] Reference Figure 2 DBR1 can effectively reflect the fundamental wave signal, but has a high transmittance to the target second harmonic signal, ensuring the emission of the second harmonic; DBR2 and DBR3 can effectively reflect the target second harmonic signal, and have a low reflection loss to the fundamental wave, ensuring the basic characteristics of the structure. Figure 3 , the second harmonic effect is compared whether the dispersion of nonlinear optical materials is considered or not.

[0072] Reference Figure 4 and Figure 5 , the propagation phase of harmonic radiation and fundamental frequency electric field is calculated by using Green's function method without considering material dispersion and considering material dispersion. The phase changes between -π and π, which is a manifestation of phase mismatch. According to the positive and negative phase, the nonlinear optical film can be split, and the part with negative phase is rotated 180°. At this time, the second-order nonlinear polarizability tensor of the rotated part is expressed as:

[0073]

[0074] At this time, the square of the fundamental frequency electric field and the phase of the Green's function are both positive, and the degree of phase matching is greatly increased. Figure 6 and Figure 7 , the dotted line indicates the part where phase matching adjustment is performed. After optimization, refer to Figure 8 and Fig. 9The second harmonic conversion efficiency of the structure in the target band is greatly improved compared with that before adjustment and without structure. Compared with the phase matching before adjustment, the second harmonic enhancement reaches an astonishing 60400 and 161366 times at 925.4nm and 1123.2nm respectively; if the refractive index dispersion of the nonlinear optical crystal is considered, the second harmonic enhancement reaches a higher 891450 and 942480 times at 967nm and 1045nm respectively. Compared with the unstructured film, it reaches 51778 times @925.4nm and 1655000 times @932.6nm without and with dispersion.

[0075] Example 2

[0076] Reference Fig.10 This embodiment provides a nonlinear optical structure with perfect phase matching and high efficiency in second harmonic generation, including a second harmonic reflection structure 10, a nonlinear optical material, a fundamental frequency wave resonance structure 30 and a substrate 40.

[0077] The second harmonic reflection structure 10 uses a Bragg reflector (DBR3), which is composed of seven overlapping layers of materials with refractive indices of 1.45 and 2.4, respectively. The thickness of each layer of low refractive index medium and high refractive index medium is 86 nm and 52 nm, respectively.

[0078] The fundamental wave resonance structure 30 includes a Bragg reflector (DBR1) and a Bragg reflector (DBR2). The Bragg reflector (DBR1) and the Bragg reflector (DBR2) are placed on the substrate 40 and on the nonlinear optical material, respectively. They are composed of ten layers and three layers of materials with refractive indices of 1.45 and 2.4, respectively. The thickness of each layer of low refractive index medium and high refractive index medium is 172nm and 104nm, respectively. The nonlinear optical material adopts 3R phase crystal material. When the crystal is turned, its second-order nonlinear polarization rate coefficient is adjustable, and the thickness is 400nm.

[0079] Reference Fig.11 DBR1 can effectively reflect the fundamental wave signal, but has a high transmittance to the target second harmonic signal, ensuring the emission of the second harmonic; DBR2 and DBR3 can effectively reflect the target second harmonic signal, and have a low reflection loss to the fundamental wave, ensuring the basic characteristics of the structure. Fig.12 , the second harmonic effect is compared when the dispersion of nonlinear optical materials is considered or not. Fig.13 and Fig.14, the propagation phase of harmonic radiation and fundamental frequency electric field is calculated by Green's function method respectively without considering material dispersion and considering material dispersion, and the negative phase part is rotated by 180°. The specific operation is shown in Specific Example 1. After optimization, the second harmonic conversion efficiency of the structure in the target band is greatly improved compared with that before adjustment and without structure (refer to Fig.15 ), compared with before phase matching adjustment, the second harmonic enhancement reaches up to 38925 times @936.65nm and 771885 times @1051nm respectively; if the refractive index dispersion of the nonlinear optical crystal is considered, the second harmonic enhancement reaches up to 584525 times @971nm and 2240351 times @885nm respectively.

[0080] Example 3

[0081] Reference Fig.10 The present invention provides a nonlinear optical structure with perfect phase matching and high efficiency of second harmonic generation, comprising: a second harmonic reflection structure 10, a nonlinear optical material, a fundamental wave resonance structure 30 and a substrate 40, wherein:

[0082] The second harmonic reflection structure 10 uses a Bragg reflector (DBR3), which is composed of seven overlapping layers of materials with refractive indices of 1.45 and 2.4, respectively. The thickness of each layer of low refractive index medium and high refractive index medium is 86 nm and 52 nm, respectively.

[0083] The fundamental wave resonance structure 30 includes a Bragg reflector (DBR1) and a Bragg reflector (DBR2). The Bragg reflector (DBR1) and the Bragg reflector (DBR2) are placed on the substrate 40 and on the nonlinear optical material, respectively. They are composed of ten layers and three layers of materials with refractive indices of 1.45 and 2.4, respectively. The thickness of each layer of low refractive index medium and high refractive index medium is 172nm and 104nm, respectively. The nonlinear optical material adopts 3R phase crystal material. When the crystal is turned, its second-order nonlinear polarization rate coefficient is adjustable, and the thickness is 2000nm.

[0084] Reference Fig.11 DBR1 can effectively reflect the fundamental wave signal, but has a high transmittance to the target second harmonic signal, ensuring the emission of the second harmonic; DBR2 and DBR3 can effectively reflect the target second harmonic signal, and have a low reflection loss to the fundamental wave, ensuring the basic characteristics of the structure. Fig.16 , the second harmonic effect is compared when the dispersion of nonlinear optical materials is considered or not. Fig.17 and Fig.18 , the propagation phase of harmonic radiation and fundamental frequency electric field is calculated by using Green's function method without considering material dispersion and considering material dispersion, and the negative phase part is rotated by 180°. For specific operations, see Specific Embodiment 1. Fig.19 After optimization, the second harmonic conversion efficiency of the structure in the target band is greatly improved compared with that before adjustment and without structure. Compared with that before phase matching adjustment, the second harmonic enhancement is up to 59423 times @996nm and 69667 times @1022nm respectively. If the refractive index dispersion of nonlinear optical crystal is considered, the second harmonic enhancement is up to 1.36×10 8 times@894nm and 1.83×10 7 times@941nm.

[0085] Although the embodiments of the present application have been shown and described above, the scope of protection of the present invention is not limited thereto, and any changes or substitutions that are not conceivable through creative work should be included in the scope of protection of the present invention; unless explicitly stated, any elements, actions or instructions used in this document should not be interpreted as critical or necessary.

Claims

1. A nonlinear optical structure with perfect phase matching and high efficiency second harmonic generation, characterized in that: include: A fundamental frequency wave resonance structure, a nonlinear optical layer, a second harmonic reflection structure and a substrate having a bearing function; The fundamental frequency wave resonant structure is used to generate a local enhancement of the electromagnetic field at the incident fundamental frequency wavelength, and to achieve a phase matching condition by optimizing the electric field phase distribution; The nonlinear optical layer is used for frequency conversion of fundamental frequency wave to second harmonic wave, and regulating harmonic radiation capability and phase regulation; The second harmonic reflection structure has a reflectivity of ≥95% at the second harmonic wavelength, the second harmonic reflection structure has an absorption and reflection loss of ≤20% at the fundamental wavelength, and the second harmonic reflection structure is used to control the outcoupling of the second harmonic; Rotating the nonlinear optical layer can adjust the phase characteristics of the radiated second harmonic to achieve perfect phase matching of the harmonic field.

2. The nonlinear optical structure with perfect phase matching and high efficiency second harmonic generation according to claim 1, characterized in that: The fundamental frequency wave resonance structure includes a guided mode resonance structure and a high-efficiency reflection medium or a Bragg reflector.

3. The nonlinear optical structure with perfect phase matching and high efficiency second harmonic generation according to claim 1, characterized in that: The nonlinear optical material is a crystallographic non-centrosymmetric material. When the crystal of the crystallographic non-centrosymmetric material is turned or the crystal orientation is changed, the second-order nonlinear optical polarizability coefficient is adjustable.

4. The nonlinear optical structure with perfect phase matching and high efficiency second harmonic generation according to claim 3, characterized in that: The crystallographically non-centrosymmetric material is a single-layer or multi-layer structure.

5. The nonlinear optical structure with perfect phase matching and high efficiency second harmonic generation according to claim 1, characterized in that: The second harmonic reflection structure is a highly efficient reflection medium or a plurality of layers of materials with different refractive indices stacked alternately.

6. The nonlinear optical structure with perfect phase matching and high efficiency second harmonic generation according to claim 1, characterized in that: The fundamental frequency wave resonance structure is between the second harmonic reflection structure and the nonlinear optical material, and / or the fundamental frequency wave resonance structure is between the nonlinear optical material and the substrate, so as to form a local fundamental frequency wave electric field in the resonant cavity.

7. The nonlinear optical structure with perfect phase matching and high efficiency second harmonic generation according to claim 1, characterized in that: The substrate is a low refractive index medium material.

8. The nonlinear optical structure with perfect phase matching and high efficiency second harmonic generation according to claim 1, characterized in that: The substrate is one of silicon dioxide, magnesium fluoride or polydimethylsiloxane.

9. The nonlinear optical structure with perfect phase matching and high efficiency second harmonic generation according to claim 3, characterized in that: The crystallographic non-centrosymmetric material is a single-layer or multi-layer nonlinear optical film.

10. The nonlinear optical structure with perfect phase matching and high efficiency second harmonic generation according to claim 3, characterized in that: The crystallographic non-centrosymmetric material is a 3R phase or Phase crystalline materials.

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