Microcavity structure for phase matching based on optical tunneling effect

Abstract

The application provides a microcavity structure based on optical tunneling effect for phase matching, and relates to the technical field of nonlinear optics, and comprises a substrate and a first coupling waveguide, a microcavity outer ring waveguide, a microcavity inner ring waveguide and a second coupling waveguide attached to the surface of the substrate; the microcavity inner ring waveguide is arranged inside the microcavity outer ring waveguide and has a same center, so as to form a double-ring coupled whispering gallery mode microcavity structure; the refractive index of the microcavity inner ring waveguide is greater than the refractive index of the microcavity outer ring waveguide; and the refractive index of the substrate is less than the refractive index of the first coupling waveguide, the microcavity outer ring waveguide, the microcavity inner ring waveguide and the second coupling waveguide. The application is suitable for phase matching in the case of a large optical frequency difference, so that a large mode overlapping area and interaction strength exist between different optical frequency modes, and the optical nonlinear effect in the case of a large optical frequency difference is easy to realize with high efficiency.

Description

Technical Field

[0001] This invention relates to the field of nonlinear optics, and more specifically, to a microcavity structure for phase matching based on the optical tunneling effect. Background Technology

[0002] In nonlinear optical effects, the light field can interact with the propagation medium to generate a new frequency light field. Phase matching is a crucial concept in nonlinear optics. Optical nonlinear effects are highly sensitive to the phase relationship between light fields, especially in parametric processes such as frequency doubling, difference frequency generation, parametric amplification, oscillation, and four-wave mixing, all of which require phase matching. Simply put, phase matching refers to the requirement that the wave vectors of different light fields satisfy a specific mathematical relationship in nonlinear optical effects; essentially, it reflects the requirement of momentum conservation during parametric transformation.

[0003] Whispering-gallery mode microcavities are ideal nonlinear optical platforms. Their ultra-high Q value and extremely small mode volume significantly increase the energy density of the light field in the medium, thus enabling the generation of nonlinear optical effects with relatively low power. Both second-order nonlinear three-wave mixing and third-order nonlinear four-wave mixing effects have been widely implemented in microcavities. From an application perspective, technologies based on microcavity nonlinear effects include optical frequency conversion, time measurement, aerospace engineering, astronomical calibration, coherent fiber optic communication, and non-classical light generation and ranging.

[0004] For microcavity nonlinear operations to achieve high efficiency, phase matching is also essential. The development of whispering-gallery mode microcavity technology has benefited from advancements in optical communication and semiconductor fabrication technologies; therefore, most current microcavity nonlinear applications are concentrated in the near-infrared band, where these technologies are mature. However, extending microcavity nonlinear applications to the mid-infrared, far-infrared, ultraviolet, and even terahertz bands primarily requires consideration of phase matching. Due to dispersion, the equivalent refractive index of the intracavity optical mode is a function of optical frequency; a larger optical frequency difference often implies a larger equivalent refractive index difference. For large optical frequency differences, such as when the pump light and signal light wavelengths in three-wave mixing are 1.31 μm and 8 μm respectively, traditional microcavity phase matching methods, such as mode phase matching and quasi-phase matching techniques, are difficult to implement. Therefore, developing a microcavity phase matching scheme suitable for large optical frequency conditions is crucial. Summary of the Invention

[0005] The present invention aims to solve at least one of the technical problems existing in the prior art or related technologies, and provides a microcavity structure based on optical tunneling effect for phase matching, which is suitable for phase matching in the case of large optical frequency, so that there is a large mode overlap area and interaction strength between different optical frequency modes, and it is easy to realize high-efficiency nonlinear conversion in the case of large optical frequency difference.

[0006] This invention is achieved through the following technical solution: a microcavity structure based on optical tunneling effect for phase matching, comprising: a substrate and a first coupling waveguide, a microcavity outer ring waveguide, a microcavity inner ring waveguide, and a second coupling waveguide attached to the surface of the substrate; the microcavity inner ring waveguide is disposed inside the microcavity outer ring waveguide, with their centers coinciding to form a double-ring coupled whispering-gallery mode microcavity structure; the first coupling waveguide is disposed on the upper side of the microcavity outer ring waveguide, and the second coupling waveguide is disposed inside the microcavity inner ring waveguide; the refractive index of the microcavity inner ring waveguide is greater than that of the microcavity outer ring waveguide; the refractive index of the substrate is less than that of the first coupling waveguide, the microcavity outer ring waveguide, the microcavity inner ring waveguide, and the second coupling waveguide. The refractive index of the waveguides is determined. Light waves enter the first and second coupled waveguides via spatial optical coupling, allowing the optical field within the coupled waveguides to enter and exit the microcavity structure via evanescent field coupling. The first coupled waveguide uses end-face coupling with the optical field, while the second coupled waveguide uses a coupling grating, which improves coupling efficiency for long wavelengths and reduces power requirements for those wavelengths. During this process, based on the geometric parameters and spacing between the outer and inner ring waveguides of the microcavity, long-wavelength light is allowed to tunnel from the inner ring waveguide into the outer ring waveguide, ensuring that the energy of the short-wavelength light is almost entirely distributed within the outer ring waveguide. At this point, the equivalent refractive index of the long-wavelength mode is primarily determined by the combined refractive indices of the inner and outer ring waveguide materials, while the equivalent refractive index of the short-wavelength mode is primarily determined by the single refractive index of the outer ring waveguide material. In other words, the equivalent refractive index of the long-wavelength mode is adjusted using the inner ring waveguide to ensure phase matching between the equivalent refractive indices of different wavelength modes.

[0007] In the microcavity structure based on optical tunneling effect for phase matching provided by the present invention, preferably, the first coupling waveguide is a straight waveguide made of aluminum gallium arsenide material with high aluminum content.

[0008] According to the microcavity structure based on optical tunneling effect for phase matching provided by the present invention, preferably, the coupling region of the second coupling waveguide is an arc-shaped waveguide made of aluminum gallium arsenide material with low aluminum content.

[0009] The microcavity structure based on optical tunneling effect for phase matching provided by the present invention preferably has an outer ring waveguide made of aluminum gallium arsenide material with high aluminum content.

[0010] The microcavity structure based on optical tunneling effect for phase matching provided by the present invention preferably has an inner ring waveguide made of aluminum gallium arsenide with low aluminum content.

[0011] The microcavity structure based on optical tunneling effect for phase matching provided by the present invention preferably has a substrate made of silicon dioxide, magnesium fluoride, or calcium fluoride.

[0012] According to the microcavity structure based on optical tunneling effect for phase matching provided by the present invention, preferably, the radius of the outer ring waveguide of the microcavity is 0.5mm-5mm, and the inter-ring air gap between the outer ring waveguide and the inner ring waveguide of the microcavity is 0.2μm-2μm.

[0013] According to the microcavity structure based on optical tunneling effect for phase matching provided by the present invention, preferably, the radial cross-sectional width of the inner ring waveguide of the microcavity is 1.5 μm and the height is 5 μm.

[0014] According to the microcavity structure based on optical tunneling effect for phase matching provided by the present invention, preferably, the radial cross-sectional width of the outer ring waveguide of the microcavity is 2μm and the height is 5μm.

[0015] The beneficial effects achieved by this invention include at least the following: a microcavity phase matching scheme based on optical tunneling effect is proposed. With a concentric double-ring coupling structure of different materials, not only is the energy distribution of high-frequency and low-frequency optical modes in different materials adjusted, but the equivalent refractive index of different modes is controlled, ensuring the realization of phase matching conditions. Furthermore, it makes the different optical frequency modes have a large mode overlap area and interaction strength, which makes it easy to achieve high-efficiency nonlinear conversion under large optical frequency difference conditions. Compared with the prior art, this invention has the following characteristics: (1) It can realize microcavity phase matching under large optical frequency difference conditions. Existing microcavity phase matching schemes, such as mode phase matching technology, require certain optical fields to work in ultra-high-order mode states due to the huge refractive index difference caused by dispersion under large optical frequency difference conditions. The coupling efficiency of high-order modes is very poor, the actual conversion efficiency is low, and it is almost impossible to achieve high-efficiency nonlinear parametric conversion. (2) This invention is compatible with existing on-chip microcavity semiconductor processing technology and is easy to process and fabricate with existing technology. (3) The scheme is highly flexible and can work in different wavelength ranges depending on the selected inner and outer ring materials. Existing quasi-phase matching techniques are primarily applied to ferroelectric materials, such as lithium niobate. While they theoretically have the capability to handle large optical frequency differences, the optical transparency window of ferroelectric materials is generally less than 5 μm, severely limiting their practical applicability. This proposed solution, however, is applicable to most materials and can achieve phase matching over a wide spectral range. Attached Figure Description

[0016] Figure 1A schematic diagram of a microcavity structure based on optical tunneling effect for phase matching according to an embodiment of the present invention is shown.

[0017] Figure 2 The diagram illustrates the distribution of electric field energy density of different optical modes in the radial cross-sections of the inner and outer ring waveguides according to an embodiment of the present invention, when the second-order difference-frequency conversion phase-matching relationship is met. Detailed Implementation

[0018] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0019] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. Those skilled in the art will understand the specific meaning of the above terms in this invention through specific circumstances.

[0020] Taking the second-order nonlinear difference frequency effect as an example, the wavelength λ of the signal light converted by the difference frequency effect... s The pump light λ is 8μm. p If the value is 1.31 μm, then according to the energy conservation principle in parametric conversion, the signal light frequency ω... s Pump light frequency ω p Difference frequency optical frequency ω d The following relationship must be satisfied:

[0021] ω p -ω s =ω d ;

[0022] The generated difference-frequency light has a wavelength of approximately 1.5665 μm. It can be seen that both the pump light and the difference-frequency light are located in the near-infrared band, and their wavelengths are more than 5 times that of the signal light in the long-wave mid-infrared band. It is almost impossible to achieve efficient nonlinear interaction among the three using existing microcavity phase matching schemes.

[0023] To solve the above problems, such as Figure 1As shown, this invention provides a microcavity structure for phase matching based on optical tunneling effect, comprising: a first coupling waveguide (1), a second coupling waveguide (2), an on-chip coupling grating (3), an on-chip coupling grating (4), a microcavity outer ring waveguide (5), a microcavity inner ring waveguide (6), and a substrate material (7); wherein the first coupling waveguide and the microcavity outer ring waveguide are made of aluminum gallium arsenide (Al₂O₃) with high aluminum content. x Ga 1-x The second coupling waveguide and the microcavity inner ring waveguide are made of aluminum gallium arsenide (Al₂O₃) material with low aluminum content; x Ga 1-x The substrate is made of magnesium fluoride (MgF2) or calcium fluoride (CaF2).

[0024] For aluminum gallium arsenide (Al x Ga 1-x For gallium arsenide (GaAs) thin films, they can be grown on gallium arsenide (GaAs) insulating substrates using methods such as metal-organic chemical vapor deposition (OMCVD). These thin films allow for the fabrication of highly compact on-chip integrated waveguide devices using photolithography. (Al gallium arsenide (GaAs) thin films...) x Ga 1-x AlGaAs (AlGaAs) materials have large optical transparency windows, supporting transmission wavelengths above 5 μm, and exhibit second-order nonlinearity. The aluminum content, or x-value, significantly affects its refractive index. Generally, higher aluminum content results in lower refractive index. The aluminum content of AlGaAs thin films can be controlled during growth by adjusting the feed rate. After the AlGaAs thin film is grown, a bis(p-chloromethylbenzene) (BCB) layer is bonded to a magnesium fluoride (MgF2) or calcium fluoride (CaF2) substrate. The original GaAs substrate is then removed, exposing the AlGaAs thin film to subsequent photolithography to create the desired structure.

[0025] In this embodiment, the first and second coupling waveguides serve as channels for injecting external spatial light fields into the microcavity chip. Pump light and signal light are injected from the left ends of the first and second coupling waveguides, respectively, while the generated difference-frequency light is output from the right end of the first coupling waveguide. The number of coupling waveguides is related to the number of optical bands involved in the actual nonlinear effect. Two coupling waveguides are provided in this embodiment because two optical bands participate in the nonlinear effect. The optical microcavity confines the light field within the cavity through total internal reflection. When light energy enters the outer or inner ring waveguide of the microcavity from the coupling waveguide, the light field is confined within the ring-shaped microcavity ring waveguide by total internal reflection due to the presence of low-refractive-index air on both sides of the ring waveguide. Furthermore, there is an optically denser medium on one side of the microcavity ring waveguide separated by an air gap, i.e., another ring waveguide of the microcavity. This three-layer structure of optically denser-optically rarer-optically denser medium and the generation of total internal reflection on the surfaces of the optically denser and optically rarer media constitute the basis for the optical tunneling effect. Some of the optical field energy in the microcavity ring waveguide can enter another ring waveguide through the optical tunneling effect.

[0026] However, in optical tunneling, the intensity of the tunneling light energy is closely related to the optical field frequency, i.e., the wavelength. The intensity E of the optical field transmitted from one ring of a microcavity to another ring waveguide through tunneling can be characterized by the following equation:

[0027]

[0028] In the formula, E0 is the optical field amplitude within a certain ring waveguide of the microcavity, d is the width of the air gap between the inner and outer rings, θ is the incident angle of total internal reflection, and n is defined as being related to the refractive indices n1 and n2 of the outer and inner ring materials. Tunneling from the outer ring to the inner ring can be expressed as n = n2 / n1, while tunneling from the inner ring to the outer ring can be expressed as n = n1 / n2. Therefore, with other parameters remaining essentially constant, the longer the optical wavelength, the more optical energy can enter from one ring to another through optical tunneling. The equivalent refractive index n of the optical mode... eff In other words, it can be calculated using the following formula:

[0029]

[0030] In the formula, η i n i Let ∑ be the ratio of the optical mode field energy in a certain material and the refractive index of that material, respectively. i η i =1. Therefore, by designing appropriate parameters, the energy distribution of long-wavelength signal light modes can be such that a large ratio exists in both the inner and outer ring waveguides of the microcavity, while the energy of short-wavelength pump light and difference frequency light modes is mainly distributed in the outer ring waveguide.

[0031] Figure 2This diagram illustrates the distribution of electric field energy density in different optical modes within the radial cross-sections of the inner and outer ring waveguides according to an embodiment of the present invention, when the second-order difference-frequency conversion phase-matching relationship is met. In this design, the outer ring and the first coupling waveguide are made of AlGaAs(Al) x Ga 1-x The x-value of the As material is 0.7, and the inner ring and the second coupling waveguide are made of aluminum gallium arsenide (Al). x Ga 1-x The x-value of the As material is 0.02, and the radial cross-sectional width and height of the inner and outer rings are 1.5×5μm and 2×5μm, respectively. According to the above embodiment, preferably, the outer ring radius of the microcavity is in the range of 0.5-5mm, the air gap between the inner and outer rings is preferably 0.2-2μm, and the radial cross-sectional width and height of the inner and outer rings are preferably 1.5×5μm and 2×5μm, respectively. Figure 2 To conform to the phase-matching relationship of the second-order difference-frequency conversion, the distribution of electric field energy density of different optical modes in the radial cross section of the inner and outer ring waveguides is determined. In this case, the width of the air gap layer between the inner and outer ring waveguides is approximately 0.38 μm. A darker color indicates higher energy density, and the energy density distribution of this mode is calculated through finite element simulation. In this scenario, the equivalent refractive index of the signal light mode is mainly determined by the refractive indices of two aluminum gallium arsenide materials with different aluminum content, while the pump light and difference-frequency light are mainly determined by the refractive index of the aluminum gallium arsenide material with high aluminum content, and are less affected by the refractive index of the inner ring aluminum gallium arsenide material. Thus, by introducing the high refractive index of the low-aluminum-content aluminum gallium arsenide material, the decrease in equivalent refractive index caused by the dispersion of the high-aluminum-content aluminum gallium arsenide material under large optical frequency differences is compensated, resulting in a higher equivalent refractive index n for the three modes. s n p n d Ultimately, the phase matching relationship is satisfied, that is:

[0032] n p ω p -n s ω s =n d ω d ;

[0033] Compared with the prior art, the present invention has the following characteristics:

[0034] (1) Microcavity phase matching can be achieved under large optical frequency difference conditions. Existing microcavity phase matching schemes, such as mode phase matching technology, require certain optical fields to operate in ultra-high-order modes due to the huge refractive index difference caused by dispersion under large optical frequency difference conditions. The coupling efficiency of high-order modes is very poor, the actual conversion efficiency is low, and it is almost impossible to achieve high-efficiency nonlinear parametric conversion.

[0035] (2) The present invention is compatible with existing on-chip semiconductor microcavity fabrication processes and is easy to fabricate using existing technologies.

[0036] (3) The scheme is highly flexible and can operate in different wavelength ranges depending on the selected inner and outer ring materials. Existing quasi-phase matching techniques are primarily applied to ferroelectric materials, such as lithium niobate. Although they have the theoretical capability to be applied to large optical frequency differences, the optical transparency window of ferroelectric materials is generally less than 5 μm, limiting their practical applicability. This invention, however, is applicable to most materials and can achieve phase matching over a wide spectral range.

[0037] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A microcavity structure for phase matching based on optical tunneling effect, characterized in that, include: The substrate and a first coupling waveguide, a microcavity outer ring waveguide, a microcavity inner ring waveguide, and a second coupling waveguide attached to the surface of the substrate; The inner ring waveguide of the microcavity is disposed inside the outer ring waveguide of the microcavity, with their centers coinciding, forming a double-ring coupled whispering-gallery mode microcavity structure; The refractive index of the inner ring waveguide of the microcavity is greater than that of the outer ring waveguide of the microcavity. The refractive index of the substrate is less than that of the first coupled waveguide, the outer ring waveguide of the microcavity, the inner ring waveguide of the microcavity, and the second coupled waveguide. Pump light enters the first coupled waveguide, and signal light enters the second coupled waveguide, so that the optical field in the coupled waveguide enters and exits the microcavity structure through evanescent field coupling. During this process, based on the geometric parameters and double-ring spacing of the outer and inner ring waveguides of the microcavity, the signal light tunnels from the inner ring waveguide into the outer ring waveguide, so that the optical field energy of the pump light is distributed in the outer ring waveguide.

2. The microcavity structure based on optical tunneling effect for phase matching according to claim 1, characterized in that, The first coupling waveguide is made of aluminum gallium arsenide with a high aluminum content, and the second coupling waveguide is made of aluminum gallium arsenide with a low aluminum content.

3. The microcavity structure based on optical tunneling effect for phase matching according to claim 1, characterized in that, The first coupling waveguide is a straight waveguide, and the coupling region of the second coupling waveguide is an arc-shaped waveguide.

4. The microcavity structure based on optical tunneling effect for phase matching according to claim 1, characterized in that, The outer ring waveguide of the microcavity is made of aluminum gallium arsenide with a high aluminum content, while the inner ring waveguide of the microcavity is made of aluminum gallium arsenide with a low aluminum content.

5. The microcavity structure based on optical tunneling effect for phase matching according to claim 1, characterized in that, The substrate is made of silicon dioxide, magnesium fluoride, or calcium fluoride.

6. The microcavity structure based on optical tunneling effect for phase matching according to claim 1, characterized in that, The radius of the outer ring waveguide of the microcavity is 0.5mm-5mm, and the air gap between the outer ring waveguide and the inner ring waveguide of the microcavity is 0.2μm-2μm.

7. The microcavity structure for phase matching based on optical tunneling effect according to claim 1, characterized in that, The radial cross-sectional width of the microcavity inner ring waveguide is 1.5 μm, and the height is 5 μm.

8. The microcavity structure for phase matching based on optical tunneling effect according to claim 1, characterized in that, The radial cross-sectional width of the microcavity outer ring waveguide is 2 μm, and the height is 5 μm.

Images