Nonlinear optical structure for phase-matched efficient second-harmonic generation efficiency

Abstract

The application discloses a kind of phase perfect matching high-efficiency second harmonic generation efficiency nonlinear optical structure, including fundamental wave resonant structure, nonlinear optical layer, second harmonic reflection structure and substrate;Fundamental wave resonant structure is used to generate electromagnetic field local enhancement at incident fundamental wavelength, and phase matching condition is realized by optimizing electric field phase distribution;Nonlinear optical layer is used for frequency conversion of fundamental wave to second harmonic, and harmonic radiation capacity and phase control are regulated;Second harmonic reflection structure is used to control the coupling-out of second harmonic;Rotating nonlinear optical layer can adjust the phase characteristics of radiated second harmonic, to realize the phase perfect matching of harmonic field;The phase perfect matching high-efficiency second harmonic generation efficiency nonlinear optical structure realizes the accurate control of phase matching condition by reasonable optical thin film stack and phase matching technology, ensures the efficient coupling-out and utilization of second harmonic.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of quantum light sources, specifically to a nonlinear optical structure with perfect phase matching and high-efficiency second-harmonic generation efficiency. BACKGROUND

[0002] With the development of laser technology and nonlinear optics, second-harmonic generation, as an important nonlinear optical phenomenon, is widely used in laser, optical communication, spectral analysis, and other fields. In the current trend of technological development, there is an increasing demand for miniaturized optical devices, and the integration of nonlinear optical devices has thus become a research hotspot. However, the miniaturization of nonlinear optical devices faces an important challenge, which is 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 unrelated to refractive index dispersion, which severely limits the efficiency of harmonic generation. In particular, in the aspect of efficient enhancement of second-harmonic emission, existing technologies still face great challenges. Therefore, understanding and manipulating the phase matching at subwavelength scale is crucial for designing compact nonlinear optical devices. SUMMARY

[0003] The purpose of the present application is to provide a nonlinear optical structure with perfect phase matching and high-efficiency second-harmonic generation efficiency. Through reasonable optical thin film stacking and phase matching technology, the present application achieves precise control of the phase matching condition, ensures the efficient coupling out and utilization of the second harmonic, and provides a new solution for research and application in related fields.

[0004] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0005] A nonlinear optical structure with perfect phase matching and high-efficiency second-harmonic generation efficiency, including a fundamental wave resonance structure, a nonlinear optical layer, a second-harmonic reflection structure, and a substrate serving as a carrier.

[0006] The fundamental wave resonance structure is used to produce electromagnetic field localization enhancement at the incident fundamental frequency wavelength, and to achieve the phase matching condition by optimizing the electric field phase distribution.

[0007] The nonlinear optical layer is used for frequency conversion of the fundamental wave to the second harmonic, and to regulate the harmonic radiation ability and phase control.

[0008] The second-harmonic reflection structure has a reflectivity ≥95% at the second-harmonic wavelength, and an absorption and reflection loss ≤20% at the fundamental frequency wavelength. The second-harmonic reflection structure is used to control the coupling out 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 phase-perfect matching high-efficiency second-harmonic generation efficiency nonlinear optical structure provided by at least one embodiment of the present disclosure, the fundamental wave resonant structure comprises a guided-mode resonant structure and a high-efficiency reflective medium or a Bragg reflector.

[0011] In the phase-perfect matching high-efficiency second-harmonic generation efficiency nonlinear optical structure provided by at least one embodiment of the present disclosure, the crystallographic non-centrosymmetric material is a crystallographic non-centrosymmetric material whose second-order nonlinear optical polarizability coefficient can be adjusted when the crystal is turned or the crystal orientation is changed.

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

[0013] In the phase-perfect matching high-efficiency second-harmonic generation efficiency nonlinear optical structure provided by at least one embodiment of the present disclosure, the second-harmonic reflection structure is a high-efficiency reflective medium or a multi-layer structure formed by alternately stacking materials with different refractive indexes.

[0014] In the phase-perfect matching high-efficiency second-harmonic generation efficiency nonlinear optical structure provided by at least one embodiment of the present disclosure, the substrate is a low-refractive-index medium material.

[0015] In the phase-perfect matching high-efficiency second-harmonic generation efficiency nonlinear optical structure provided by at least one embodiment of the present disclosure, the substrate is one of silicon dioxide, magnesium fluoride, and polydimethylsiloxane.

[0016] In the phase-perfect matching high-efficiency second-harmonic generation efficiency nonlinear optical structure provided by at least one embodiment of the present disclosure, the crystallographic non-centrosymmetric material is a single-layer or multi-layer nonlinear optical film.

[0017] In the phase-perfect matching high-efficiency second-harmonic generation efficiency nonlinear optical structure provided by at least one embodiment of the present disclosure, the crystallographic non-centrosymmetric material is a 3R phase crystal material.

[0018] The present application has the following beneficial effects: through the ingenious design of the resonant structure and the nonlinear optical film, the separation and control of the fundamental wave and the second-harmonic wave can be effectively realized, thereby providing a good structural basis for the high-efficiency generation and emission of the second-harmonic wave.

[0019] The phase matching is realized by rotating the nonlinear optical film or changing the orientation of the nonlinear optical crystal, which is simple to operate and easy to realize, without the need for complex external equipment or complicated processing technology, thereby reducing the difficulty and cost of realizing high-efficiency second-harmonic generation.

[0020] The application can greatly enhance the exit intensity of the second harmonic, significantly improve the efficiency of the second harmonic generation, provide stronger technical support for related optical applications, and has wide application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiments will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative effort.

[0022] Figure 1 is a cross-sectional schematic view of the nonlinear optical structure of the present application with perfect phase matching and high-efficiency second harmonic generation efficiency;

[0023] Figure 2 is a functional characteristic diagram of the second harmonic reflection structure and the fundamental wave resonance structure of embodiment 1;

[0024] Figure 3 is a second harmonic intensity distribution diagram in embodiment 1 without perfect phase matching;

[0025] Figure 4 is a diagram of the propagation characteristics of the fundamental electric field and harmonic radiation with respect to the thickness position in embodiment 1 without considering material dispersion;

[0026] Figure 5 is a diagram of the propagation characteristics of the fundamental electric field and harmonic radiation with respect to the thickness position in embodiment 1 considering material dispersion;

[0027] Figure 6 is a diagram of the phase change after fine phase matching in embodiment 1 without considering material dispersion, and the negative coefficient on the top represents rotating the material at the thickness position;

[0028] Figure 7 is a diagram of the phase change after fine phase matching in embodiment 1 considering material dispersion, and the negative coefficient on the top represents rotating the material at the thickness position;

[0029] Figure 8 is a diagram of the second harmonic intensity comparison after fine phase matching in embodiment 1 without considering material dispersion with the second harmonic intensity without resonance structure and without phase matching;

[0030] Figure 9 is a diagram of the second harmonic intensity comparison after fine phase matching in embodiment 1 considering material dispersion with the second harmonic intensity without resonance structure and without phase matching;

[0031] Figure 10 is a cross-sectional schematic view of the nonlinear optical structure of embodiment 2 and embodiment 3;

[0032] Figure 11 is a functional characteristic diagram of the second harmonic reflection structure and the fundamental wave resonance structure of Example 2 and Example 3;

[0033] Figure 12 is a second harmonic intensity distribution diagram in which perfect phase matching is not achieved in Example 2;

[0034] Figure 13 is a diagram of the distribution of the propagation characteristics of the fundamental electric field and harmonic radiation with the thickness position in Example 2 without considering material dispersion;

[0035] Figure 14 is a diagram of the distribution of the propagation characteristics of the fundamental electric field and harmonic radiation with the thickness position in Example 2 considering material dispersion;

[0036] Figure 15 is a second harmonic intensity distribution diagram after fine phase matching in Example 2;

[0037] Figure 16 is a second harmonic intensity distribution diagram in which perfect phase matching is not achieved in Example 3;

[0038] Figure 17 is a diagram of the distribution of the propagation characteristics of the fundamental electric field and harmonic radiation with the thickness position in Example 3 without considering material dispersion;

[0039] Figure 18 is a diagram of the distribution of the propagation characteristics of the fundamental electric field and harmonic radiation with the thickness position in Example 3 considering material dispersion;

[0040] Figure 19 is a second harmonic intensity distribution diagram after fine phase matching in Example 3.

[0041] In the drawings:

[0042] 10, second harmonic reflection structure;

[0043] 20, nonlinear optical layer;

[0044] 30, fundamental wave resonance structure;

[0045] 40, substrate. DETAILED DESCRIPTION

[0046] The technical solutions in the embodiments will be clearly and completely described below with reference to the drawings in the embodiments. Obviously, the described embodiments are only part of the embodiments, not all embodiments.

[0047] The specific design of the nonlinear optical structure provided by the application for perfect phase matching and high-efficiency second harmonic generation efficiency mainly includes three steps:

[0048] S1, selecting appropriate second harmonic reflection structure material, nonlinear optical material, fundamental wave resonance structure material and substrate material according to target second harmonic response gain band and material itself properties, designing and preparing optical structure to meet basic structure requirements and functional characteristics;

[0049] S2, optimizing the distribution of fundamental electric field by adjusting the parameters of optical structure, such as the shape, thickness, refractive index, etc., so as to reach the effective maximum value;

[0050] S3, adjusting the phase of Green function by changing the crystal orientation and spatial rotation angle of nonlinear optical film, so as to make it in phase with the square of fundamental electric field; when phase matching is achieved, the fundamental wave is incident to the optical structure, and the generated second harmonic wave is smoothly emitted under the action of the structure after conversion by the nonlinear optical film, thereby realizing high-efficiency transmission and reflection of the second harmonic wave.

[0051] Embodiment 1

[0052] With reference to Figure 1 The embodiment provides a nonlinear optical structure with high-efficiency second harmonic generation efficiency and perfect phase matching, which comprises a second harmonic reflection structure 10, a nonlinear optical layer 20, a fundamental wave resonance structure 30 and a substrate 40.

[0053] Specifically, the fundamental wave resonance structure 30 is used for producing electromagnetic field local enhancement at the incident fundamental wave length, and phase matching condition is realized by optimizing the phase distribution of electric field.

[0054] Specifically, the nonlinear optical layer 20 comprises a crystal material with second-order nonlinear effect, which is used for frequency conversion of the fundamental wave to the second harmonic wave, and controls the harmonic radiation ability and phase control.

[0055] Specifically, the second harmonic reflection structure 10 has a reflectivity ≥95% at the second harmonic wave length and an absorption and reflection loss ≤20% at the fundamental wave length, and is used for controlling the coupling-out of the second harmonic wave at the port.

[0056] Specifically, the substrate 40 plays a bearing role. The substrate is made of low-refractive-index medium material, such as silicon dioxide, magnesium fluoride and polydimethylsiloxane.

[0057] Further, the second harmonic reflection structure 10 adopts a Bragg reflector (DBR3) composed of eight layers of materials with refractive indexes 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.

[0058] The fundamental wave resonance structure 30 includes a Bragg reflector (DBR1) and a Bragg reflector (DBR2), which are respectively placed above the substrate 40 and above the nonlinear optical material, and are composed of ten layers and two layers of materials with refractive indexes of 1.45 and 2.4, respectively, each layer having a thickness of 172 nm and 104 nm, respectively; the nonlinear optical material uses a phase crystal material, the second-order nonlinear susceptibility coefficient of which is adjustable when the crystal is turned, and the thickness is 400 nm, and the second-order nonlinear susceptibility tensor is represented as:

[0059] ;

[0060] In the embodiment, the propagation of the second harmonic wave can be described by the Helmholtz equation:

[0061] ;

[0062] wherein, ε0 and εr are the vacuum and the relative dielectric constant, respectively, μr is the relative permeability, k0 is the wave vector of the fundamental wave, and P2(2) represents the second-order nonlinear polarization. In the non-pumped depletion approximation, the solution of the Helmholtz equation can be represented by a Green function as:

[0063] ;

[0064] The Green function is a dyadic tensor, which reflects the radiation ability from the source point to the field point. In order to evaluate the contribution of the sub-harmonic field generated by the point source in the nonlinear optical film to the total harmonic field, a phase matching factor is defined as the ratio of the magnitude of the cumulative vector to the cumulative magnitude of all vectors. Mathematically, it can be represented as:

[0065] ;

[0066] When all components of the complex integral function in the nonlinear optical film are in phase, the phase matching factor reaches the maximum value of 1. Therefore, by changing the thickness of the nonlinear optical film, the phase matching factor will change with a coherence length as a period.

[0067] In a multilayer film structure, the distribution of the fundamental 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 electric field propagates in each layer and undergoes reflection and transmission, forming a specific electric field distribution. Since the distribution of the fundamental electric field mainly depends on the optical thickness and refractive index of each layer, rotating the nonlinear optical film does not change the distribution of the fundamental electric field.

[0068] However, the Green 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 function is closely related to the crystal orientation of the nonlinear optical film. When the nonlinear optical film is rotated, its crystallographic symmetry changes, resulting in a change in the distribution of the Green function.

[0069] By rotating the nonlinear optical film, the phase distribution of the Green function can be adjusted to match the phase distribution of the square of the fundamental electric field. When the phases of the fundamental electric field and the Green function are consistent, the generation efficiency of the second harmonic reaches a maximum. This is because under the phase matching condition, the coherent superposition effect of the second harmonic is optimal, significantly enhancing the exit intensity of the second harmonic.

[0070] Referring to Figure 2 , DBR1 can effectively reflect the fundamental wave signal, but has high transmittance for the target second harmonic signal, ensuring the exit of the second harmonic; DBR2 and DBR3 can effectively reflect the target second harmonic signal and have low reflection loss for the fundamental wave, ensuring the basic characteristics of the structure. Referring to Figure 3 , the second harmonic effect is compared under the condition of considering the dispersion of the nonlinear optical material and not considering the dispersion of the nonlinear optical material.

[0071] Referring to Figure 4 and Figure 5 , the propagation phase of the harmonic radiation and the fundamental electric field is calculated using the Green function method without considering the material dispersion and considering the material dispersion, respectively, the phase of to changes between and is a manifestation of phase mismatch. According to the positive and negative of the phase, the nonlinear optical film can be split, and the part with negative phase is rotated by 180°. At this time, the second-order nonlinear polarization tensor of the rotated part is represented as:

[0072] ;

[0073] At this time, the phase of the square of the fundamental electric field and the Green function is positive at the same time, and the degree of phase matching is greatly increased. Referring to Figure 6 and Figure 7 , the dashed line represents the part that is adjusted for phase matching. After optimization, referring to Figure 8 and Figure 9The structure greatly improves the second harmonic conversion efficiency in the target waveband compared with the structure before adjustment and the structure without structure. Compared with the phase matching before adjustment, the second harmonic enhancement reaches an amazing 60400 and 161366 times at 925.4 nm and 1123.2 nm 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 967 nm and 1045 nm respectively. Compared with the structure without structure, the second harmonic enhancement reaches 51778 times at 925.4 nm and 1655000 times at 932.6 nm without considering and considering the dispersion respectively.

[0074] Embodiment 2

[0075] With reference to Figure 10 , the embodiment provides a phase perfectly matched nonlinear optical structure with high second harmonic generation efficiency, which comprises a second harmonic reflection structure 10, a nonlinear optical material, a fundamental wave resonance structure 30 and a substrate 40.

[0076] The second harmonic reflection structure 10 adopts a Bragg reflection mirror (DBR3) which is composed of seven layers of materials with refractive indexes 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.

[0077] The fundamental wave resonance structure 30 comprises a Bragg reflection mirror (DBR1) and a Bragg reflection mirror (DBR2), the Bragg reflection mirror (DBR1) and the Bragg reflection mirror (DBR2) are respectively placed above the substrate 40 and above the nonlinear optical material, and are composed of ten layers and three layers of materials with refractive indexes of 1.45 and 2.4 respectively, and the thickness of each layer of low refractive index medium and high refractive index medium is 172 nm and 104 nm respectively; the nonlinear optical material adopts a 3R phase crystal material, the second-order nonlinear polarization coefficient of which is adjustable when the crystal is turned, and the thickness is 400 nm.

[0078] With reference to Figure 11 , the DBR1 can effectively reflect the fundamental wave signal, but has a high transmittance for the target second harmonic signal, thereby ensuring the emission of the second harmonic; the DBR2 and the DBR3 can effectively reflect the target second harmonic signal, and have a low reflection loss for the fundamental wave, thereby ensuring the basic characteristics of the structure. With reference to Figure 12 , the second harmonic effects are compared in the cases with and without considering the dispersion of the nonlinear optical material. With reference to Figure 13 and Figure 14, the propagation phase of the harmonic radiation and the fundamental frequency electric field is calculated by using the Green function method without considering the material dispersion and considering the material dispersion, and the part with negative phase is rotated by 180°, see specific embodiment 1 for specific operation. After optimization, the second harmonic conversion efficiency of the structure in the target waveband is greatly improved compared with before adjustment and without structure (see Figure 15 ), and the highest second harmonic enhancement is 38925 times @ 936.65nm and 771885 times @ 1051nm, respectively, compared with before phase matching adjustment; if the refractive index dispersion of the nonlinear optical crystal is considered, the highest second harmonic enhancement is 584525 times @ 971nm and 2240351 times @ 885nm, respectively.

[0079] Embodiment 3

[0080] Referring to Figure 10 , the present application provides a nonlinear optical structure with perfect phase matching and high second harmonic generation efficiency, comprising: a second harmonic reflection structure 10, a nonlinear optical material, a fundamental frequency wave resonance structure 30 and a substrate 40, wherein:

[0081] The second harmonic reflection structure 10 adopts a Bragg reflector (DBR3) composed of seven layers of materials with refractive indices of 1.45 and 2.4, respectively, and the thickness of each low refractive index medium and high refractive index medium is 86nm and 52nm, respectively.

[0082] The fundamental frequency wave resonance structure 30 includes a Bragg reflector (DBR1) and a Bragg reflector (DBR2), which are placed above the substrate 40 and above the nonlinear optical material, respectively, and are composed of ten layers and three layers of materials with refractive indices of 1.45 and 2.4, respectively, and the thickness of each low refractive index medium and high refractive index medium is 172nm and 104nm, respectively; the nonlinear optical material adopts a 3R phase crystal material, the second order nonlinear polarization coefficient of which is adjustable when the crystal is turned, and the thickness is 2000nm.

[0083] Referring to Figure 11 , DBR1 can effectively reflect the fundamental frequency wave signal, but has high transmittance for 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 low reflection loss for the fundamental frequency wave, ensuring the basic characteristics of the structure. Referring to Figure 16 , the second harmonic effect is compared whether the nonlinear optical material dispersion is considered or not. Referring to Figure 17 and Figure 18 , the propagation phase of the harmonic radiation and the fundamental frequency electric field is calculated by using the Green function method without considering the material dispersion and considering the material dispersion, and the part with negative phase is rotated by 180°, see specific embodiment 1 for specific operation. Referring toFigure 19 After optimization, the second harmonic conversion efficiency of the structure in the target waveband is greatly improved compared with before adjustment and without structure. Compared with the phase matching before adjustment, the highest second harmonic enhancement reaches 59423 times at 996 nm and 69667 times at 1022 nm, respectively. If the refractive index dispersion of the nonlinear optical crystal is considered, the highest second harmonic enhancement reaches 59423 times at 894 nm and 69667 times at 941 nm, respectively.

[0084] Although the embodiments of the present application have been shown and described above, the protection scope of the present application is not limited thereto, and any changes or substitutions not through creative labor should be covered within the protection scope of the present application. Unless explicitly stated, any element, action or instruction used herein should not be interpreted as critical or essential.​​

Claims

1. A nonlinear optical structure with perfectly phase-matched high efficiency for generating second harmonics, characterized in that, include: The fundamental frequency resonant structure, the nonlinear optical layer, the second harmonic reflection structure, and the substrate that serves as the load-bearing element; The fundamental frequency resonant structure is used to generate localized electromagnetic field enhancement at the incident fundamental frequency wavelength, and to achieve phase matching conditions by optimizing the electric field phase distribution; The nonlinear optical layer is used for frequency conversion from fundamental wave to second harmonic, and to regulate harmonic radiation capability and phase modulation. The second harmonic reflection structure has a reflectivity of ≥95% at the second harmonic wavelength and an absorption and reflection loss of ≤20% at the fundamental frequency wavelength. The second harmonic reflection structure is used to control the coupling out 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; The nonlinear optical layer is a crystallographically non-centrosymmetric material, and the second-order nonlinear optical polarizability coefficient of the crystallographically non-centrosymmetric material can be adjusted when the crystal orientation is changed or the crystal orientation is altered. The crystallographically non-centrosymmetric material is a single-layer or multi-layer nonlinear optical thin film; The fundamental frequency resonant structure includes a first Bragg reflector and a second Bragg reflector, which are respectively placed above the substrate and above the nonlinear optical layer.

2. The nonlinear optical structure with perfectly matched phase and high efficiency in generating second harmonics according to claim 1, characterized in that, The fundamental frequency resonant structure includes a guided mode resonant structure and a high-efficiency reflective medium or Bragg reflector.

3. The nonlinear optical structure with perfectly matched phase and high efficiency in generating second harmonics according to claim 1, characterized in that, The crystallographically non-centrosymmetric material has a single-layer or multi-layer structure.

4. The nonlinear optical structure with perfectly matched phase and high efficiency in generating second harmonics according to claim 1, characterized in that, The second harmonic reflection structure is composed of a high-efficiency reflective medium or multiple layers of materials with different refractive indices stacked alternately.

5. The nonlinear optical structure with perfectly matched phase and high efficiency in generating second harmonics according to claim 1, characterized in that, The substrate is a low-refractive-index dielectric material.

6. The nonlinear optical structure with perfectly matched phase and high efficiency in generating second harmonics according to claim 1, characterized in that, The substrate is one of silicon dioxide, magnesium fluoride, or polydimethylsiloxane.

7. The nonlinear optical structure with perfectly matched phase and high efficiency in generating second harmonics according to claim 3, characterized in that, The crystallographically non-centrosymmetric material is a 3R phase or... Phase crystal materials.

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