A refractive index sensor and a method of probing

Abstract

The application provides a refractive index sensor, comprising a reflective bottom layer and a medium layer arranged above the reflective bottom layer; further comprising a plurality of groups of metal structures, each group of the metal structures comprising a first metal ring uniformly attached to the upper surface of the medium layer and a second metal ring embedded in the interior of the medium layer; the first metal ring is provided with a first opening, and the second metal ring is provided with a second opening; the first metal ring is arranged in a two-dimensional array on the upper surface of the medium layer, and the second metal ring is arranged in a two-dimensional array in the interior of the medium layer. The application realizes the enhancement of the sensing effect of the environmental refractive index, and by adjusting the relative angle difference between the second opening and the first opening, high-sensitivity sensing can be realized at different frequencies, thereby realizing multi-band detection of the refractive index. The refractive index detection method based on the sensor setting can not only realize high-sensitivity sensing and more accurate detection, but also realize multi-band detection of the refractive index.

Description

Technical Field

[0001] This invention belongs to the field of optical measurement technology, and more specifically relates to a refractive index sensor and detection method. Background Technology

[0002] A refractive index sensor is a sensor that uses changes in the refractive index of a measured medium to measure the properties of the material and the environment. In recent years, with the continuous development of optical and microelectronic technologies, refractive index sensors have seen significant improvements in both application and performance. The main development directions for refractive index sensors include new materials, micro / nano fabrication, MEMS technology, and fiber optic sensors.

[0003] Non-Hermitian metasurfaces offer new ideas and methods for the development of refractive index sensors. A non-Hermitian metasurface is a novel artificial microstructure with the ability to overcome traditional refractive index limitations. It consists of multilayered metallic nanopatterns and can modulate the phase and amplitude of electromagnetic waves. The singularity points, or EP points, present in non-Hermitian metasurfaces allow for strong absorption or reflection of light. When external substances enter the metasurface's range, they alter the internal light field distribution, causing changes in the light field at the singularity points and thus affecting the optical properties of the non-Hermitian metasurface. By accurately measuring the changes in reflected light intensity caused by these changes, high-precision and high-sensitivity detection of the refractive index of external substances can be achieved. Non-Hermitian metasurface singularity point refractive index sensing has broad application prospects in material detection and sensing fields. Its applications in chemistry, biomedicine, and environmental monitoring include detecting heavy metal ions in water, diagnosing diabetes, and monitoring atmospheric gas concentrations. Compared with traditional refractive index sensing technology, non-Hermitian metasurface singularity refractive index sensing has higher detection sensitivity and selectivity, enabling rapid and accurate detection of trace substances.

[0004] Currently, sensor setups based on non-Hermitian metasurfaces suffer from several drawbacks due to suboptimal parameter control. Firstly, they exhibit poor robustness. Robustness refers to a system's or structure's ability to withstand changes, disturbances, or damage from internal and external environments. A robust system or structure can maintain its normal operating state or performance even under certain levels of interference or damage. Conversely, poor sensor robustness can lead to unstable sensing, difficulty in ensuring continuous sensing, and increased susceptibility to damage, resulting in higher maintenance costs. Secondly, multi-wavelength refractive index detection is not possible. Refractive index variations can differ in different materials, and using only a single wavelength for measurement reduces accuracy. Some materials exhibit significant refractive index changes only at specific wavelengths, showing little change at others. Using only a single wavelength makes accurate refractive index measurement difficult for these materials, limiting its application. Furthermore, single-wavelength measurement is highly susceptible to interference from other optical parameters, such as scattering and absorption, which can cause errors in the refractive index measurement, thus affecting accuracy. Finally, using only a single wavelength means multiple measurements of the same sample are required to obtain more accurate results, increasing measurement time and reducing efficiency.

[0005] In summary, the structural parameters of the current refractive index sensor need to be optimized. Summary of the Invention

[0006] Based on the aforementioned shortcomings and deficiencies in the prior art, one of the objectives of this invention is to at least solve one or more of the aforementioned problems in the prior art. In other words, one of the objectives of this invention is to provide a refractive index sensor and detection method that meets one or more of the aforementioned requirements.

[0007] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a refractive index sensor, including a reflective substrate and a dielectric layer disposed above the reflective substrate; it also includes multiple sets of metal structures, each set of metal structures including a first metal ring uniformly attached to the upper surface of the dielectric layer and a second metal ring embedded inside the dielectric layer; the first metal ring is provided with a first opening and the second metal ring is provided with a second opening; the first metal ring is arranged in a two-dimensional array on the upper surface of the dielectric layer and the second metal ring is arranged in a two-dimensional array inside the dielectric layer.

[0009] The above technical solution incorporates a first metal ring and a second metal ring, enabling strong resonance of the electromagnetic field between them. Furthermore, the reflective sub-layer ensures strong absorption and reflection of electric field components in different directions along the first and second openings. A second-order non-Hermitian system is constructed, utilizing the singularities and their characteristics present in non-Hermitian metasurfaces. These singularities exhibit greater sensitivity to external perturbations. Based on this sensor structure, the characteristic surface formed by the system parameters at the singularity is degenerate at a single point. By applying an external perturbation, this degeneracy point undergoes nonlinear splitting. By constructing a functional relationship between the perturbation and the nonlinear splitting, refractive index sensing can be achieved.

[0010] As a preferred embodiment, the first opening is set within the range of [15.3μm, 16.5μm]; the second opening is 33μm.

[0011] The above technical solution involves left-handedly polarized light (LCP) and right-handedly circularly polarized light (RCP). The first and second openings selectively target LCP and RCP. Experimental data shows that when the first opening is within the range of [15.3 μm, 16.5 μm] and the second opening is set to 33 μm, the second-order non-Hermitian system more easily reaches the singularity. By adjusting other parameters, the second-order non-Hermitian system reaches the singularity, where it can achieve strong absorption or reflection of light. When external materials enter the metasurface, they alter the light field distribution within the metasurface, causing changes in the light field at the singularity, thus affecting the optical properties of the metasurface. By accurately measuring the change in reflected light intensity caused by this change, high-precision and high-sensitivity detection of the refractive index of external materials can be achieved.

[0012] As a preferred embodiment, the distance between the first metal ring and the second metal ring is set in the range of [19μm, 31μm].

[0013] Through the above technical solution, when the distance between the first metal ring and the second metal ring is set within the range of [19μm, 31μm], the second-order non-Hermitian system more easily reaches the singularity. By coordinating with the setting of other parameters, the second-order non-Hermitian system is at the singularity, where it can achieve strong absorption or reflection of light. When external matter enters the metasurface, it changes the light field distribution inside the metasurface, causing changes in the light field at the singularity, thereby affecting the optical properties of the metasurface. By accurately measuring the change in the intensity of reflected light caused by this change, high-precision and high-sensitivity detection of the refractive index of external matter can be achieved.

[0014] As a preferred embodiment, the dielectric layer is made of polyimide material.

[0015] With the above technical solution, light propagates in the sensor, and the system will generate losses, which is the key to building a non-Hermitian system.

[0016] As a preferred embodiment, the length of the two-dimensional array in both the horizontal and vertical directions is in the range of [140μm, 180μm]; the number of metal structures in the horizontal direction of the two-dimensional array is in the range of [6, 8]; and the number of metal structures in the vertical direction of the two-dimensional array is in the range of [6, 8].

[0017] As a preferred embodiment, the second opening has a relative angular difference with the first opening.

[0018] With the above technical solution, when the second-order non-Hermitian system reaches the singular point, the refractive index can be detected in multiple bands by adjusting the relative angle difference between the second opening and the first opening. This structural setting enhances the refractive index discrimination capability, expands the application field, and improves measurement accuracy and stability by using information from multiple bands for calculation.

[0019] As a preferred embodiment, the relative angle difference between the first opening and the second opening in multiple metal structures is equal.

[0020] As a preferred embodiment, the relative angle difference is 60°.

[0021] Using the above technical solution, when the second-order non-Hermitian system reaches the singularity, the refractive index can be detected at 0.513 THz by adjusting the relative angle difference between the second opening and the first opening to 60°.

[0022] As a preferred embodiment, the relative angle difference is 30°.

[0023] Using the above technical solution, when the second-order non-Hermitian system reaches the singularity, the refractive index can be detected at 0.522 THz by adjusting the relative angle difference between the second opening and the first opening to 60°.

[0024] Secondly, the present invention provides a method for refractive index detection, comprising the following steps:

[0025] S1. In any of the above schemes, a refractive index sensor is subjected to left-handed circularly polarized light and right-handed circularly polarized light, so that the sensor is in the EP point state.

[0026] S2. Apply an external perturbation to the sensor in the EP point state to obtain a reflection spectrum, and calculate the refractive index based on the reflection spectrum.

[0027] The above technical solutions not only enable high-sensitivity sensing for more accurate detection, but also allow for multi-band refractive index detection.

[0028] Compared with the prior art, the beneficial effects of this invention are:

[0029] This invention, through the aforementioned structural parameter settings, constructs a second-order non-Hermitian system capable of reaching multiple singularities. This enhances the sensing effect of environmental refractive index, and by adjusting the relative angle difference between the second and first openings, high-sensitivity sensing can be achieved at different frequencies, thereby enabling multi-band refractive index detection. This invention's refractive index detection method based on sensor settings not only achieves high-sensitivity sensing for more accurate detection but also enables multi-band refractive index detection.

[0030] Further or more detailed beneficial effects will be described in conjunction with specific embodiments in the detailed implementation. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 This is a schematic diagram of the structure of a refractive index sensor according to the present invention.

[0033] Figure 2 This is a schematic diagram of the structure of a refractive index sensor according to the present invention.

[0034] Figure 3 This is a schematic diagram of the structure of a refractive index sensor according to the present invention.

[0035] Figure 4 This is a schematic diagram of the structure of a refractive index sensor according to the present invention.

[0036] Figure 5 This is a schematic diagram of the structure with a relative angle difference of 60° as described in this invention.

[0037] Figure 6 This is a schematic diagram of the structure with a relative angle difference of 60° as described in this invention.

[0038] Figure 7 This is the reflectance spectrum of the relative angle difference of 60° as described in this invention.

[0039] Figure 8This is a degenerate diagram of the real and imaginary parts of the eigenvalues ​​with a relative angle difference of 60° as described in this invention.

[0040] Figure 9 This is a degenerate diagram of the real and imaginary parts of the eigenvalues ​​with a relative angle difference of 60° as described in this invention.

[0041] Figure 10 This is a graph showing the relationship between the eigenvalue frequency splitting with a relative angle difference of 60° as described in this invention and the external refractive index.

[0042] Figure 11 The Re(ω) with a relative angle difference of 60° as described in this invention -1 -ω1) as a function of external refractive index.

[0043] Figure 12 This is a schematic diagram of the structure with a relative angle difference of 30° as described in this invention.

[0044] Figure 13 This is the reflectance spectrum of the relative angle difference of 60° as described in this invention.

[0045] Figure 14 This is a degenerate diagram of the real and imaginary parts of the eigenvalues ​​with a relative angle difference of 60° as described in this invention.

[0046] Figure 15 This is a degenerate diagram of the real and imaginary parts of the eigenvalues ​​with a relative angle difference of 60° as described in this invention.

[0047] Figure 16 This is a flowchart of a refractive index detection method according to the present invention.

[0048] Icon labels:

[0049] 100. Reflective substrate;

[0050] 200. Dielectric layer;

[0051] 300. Metal structure; 310. First metal ring; 311. First opening; 320. Second metal ring; 321. Second opening. Detailed Implementation

[0052] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings.

[0053] In the following description, several embodiments of this application are provided. Different embodiments can be substituted or combined. Therefore, this application can also be considered to include all possible combinations of the same and / or different embodiments described. Thus, if one embodiment includes features A, B, and C, and another embodiment includes features B and D, then this application should also be considered to include embodiments containing one or more other possible combinations of A, B, C, and D, even if such embodiments are not explicitly described in the following text.

[0054] The following description provides examples and does not limit the scope, applicability, or examples set forth in the claims. Changes may be made to the function and arrangement of the described elements without departing from the scope of this application. Various processes or components may be appropriately omitted, substituted, or added to the examples. For example, the described methods may be performed in a different order than described, and various steps may be added, omitted, or combined. Furthermore, features described with respect to some examples may be combined into other examples.

[0055] To facilitate a better understanding of the embodiments of this application, its application scenario will be explained before providing a detailed explanation of the specific implementation methods.

[0056] Example 1:

[0057] like Figure 1-2 As shown, this embodiment provides a refractive index sensor, including a reflective substrate 100, a dielectric layer 200 disposed above the reflective substrate 100, and multiple metal structures 300. The dielectric layer 200 is made of polyimide, and both the reflective substrate 100 and the metal structures 300 are made of aluminum or a material with a refractive index similar to aluminum.

[0058] Each group of metal structures 300 includes a first metal ring 310 uniformly attached to the upper surface of the dielectric layer 200 and a second metal ring 320 embedded inside the dielectric layer 200. The first metal ring 310 has a first opening 311, and the second metal ring 320 has a second opening 321. Both the first metal ring 310 and the second metal ring 320 are configured as open-ended resonant rings. The first metal rings 310 are arranged in a two-dimensional array on the upper surface of the dielectric layer 200, and the second metal rings 320 are arranged in a two-dimensional array inside the dielectric layer 200. The length p of the two-dimensional array in both the horizontal and vertical directions is [140μm, 180μm]. The number of metal structures 300 in the horizontal direction of the two-dimensional array ranges from [6, 8], and the number of metal structures 300 in the vertical direction of the two-dimensional array ranges from [6, 8].

[0059] like Figure 3-4As shown, in this embodiment, the setting range g1 of the first opening is [15.3μm, 16.5μm], and the setting range g2 of the second opening is 33μm. The setting range of the distance between the first metal ring and the second metal ring is [19μm, 31μm]. r1 is 25μm, r2 is 45μm, the width w of the first metal ring and the second metal ring is 11.2μm, and the thickness h1 of the first metal ring and the second metal ring is 0.2μm. The thickness h2 of the dielectric layer is [60μm, 70μm], and the thickness of the reflective substrate is equal to the thickness h1 of the first metal ring and the second metal ring, both being 0.2μm.

[0060] like Figure 5-6 As shown, in this embodiment, there is a relative angle difference between the second opening and the first opening. In multiple metal structures, the relative angle difference between the first opening and the second opening is equal, and the relative angle difference is 60°.

[0061] like Figure 7-11 As shown, the sensor configured through the above operations can detect the refractive index at 0.513 THz.

[0062] This embodiment constructs a second-order non-Hermitian system and, through the specific parameter settings described above, positions the system at a singular point. Applying external perturbations at this singular point makes its numerical changes more sensitive. The specific implementation principle is as follows:

[0063] The non-Hermitian singularity effect refers to the phenomenon where, in a non-Hermitian system formed by the coupling of energy modes with different gain and loss, multiple originally distinct eigenvalues ​​and eigenvectors become degenerate at a singularity. This singularity occurs at the phase transition point between parity-time (PT) symmetry and PT-symmetry-broken states in the non-Hermitian system. Adjusting the mode loss and coupling strength of the non-Hermitian system can bring it to this singularity. At the non-Hermitian singularity, the eigenstates of the metasurface system are degenerate; that is, under circular polarization, the eigenstates with degenerate reflection matrices are a single circularly polarized state, i.e., left-handed circularly polarized light (LCP) or right-handed circularly polarized light (RCP). When the incident light is in an eigenstate, the system does not produce cross-polarization components; that is, no LCP component is produced under RCP incident light, and vice versa.

[0064] According to electromagnetic field theory, the bimetallic split-ring resonator structure interacts strongly with electromagnetic waves. Changing the structural parameters can adjust the phase difference and amplitude of the emitted light. When the reflectivity is the same in the y-polarization and x-polarization directions, and the phase difference is 180°, LCP light incident on the system emits RCP light, and RCP light incident on the system emits LCP light, effectively closing the co-polarization channel of the circularly polarized reflection matrix. Due to the different mechanisms of action, these phenomena can be combined to achieve a circular polarization filtering effect—when the system's eigenstate is RCP polarization, both the cross-polarization and co-polarization channels of the RCP incident light are closed, resulting in no reflection of RCP light incident on the system, while only pure RCP light is emitted when LCP light incident on the system. When the system's eigenstate is LCP polarization, the system exhibits the opposite filtering effect. The system's eigenstate can be obtained by mirroring the embedded split-ring resonator. These characteristics give it significant potential application value in remote sensing, astronomy, polarization imaging, medical diagnosis, and refractive index sensing.

[0065] In metasurface structure design, split-ring resonators are commonly used circularly polarized response elements. Split-ring resonators are constructed from metallic materials, and the electromagnetic field resonates strongly between the two split-rings. Considering the effect of the reflective layer, adjusting the structural parameters can cause electric field components in different directions to be strongly absorbed and reflected along the opening direction of the resonator rings. Under specific structural parameters, changing the rotation angle of the embedded split-ring resonators allows selection of the desired circularly polarized chiral response.

[0066] Based on the chiral EP response of circularly polarized light, starting from the definition of PT symmetry, PT symmetric systems with balanced gain and loss are widely studied systems. Sensors operating near the EP point are considered superior to those operating far from the EP point because applying external perturbations near the EP point makes their numerical changes more sensitive. This enhanced sensitivity is attributed to the n-root topology of the characteristic surface formed by the changing system parameters, where n corresponds to the constructed n-order non-Hermitian system.

[0067] Example 2:

[0068] like Figure 12 As shown, this embodiment provides a refractive index sensor. The structure of this embodiment is basically the same as that of the above embodiments, except that the relative angle difference in this embodiment is 30°.

[0069] like Figure 13-15 As shown, the sensor configured through the above operations can detect the refractive index at 0.522 THz.

[0070] Example 3:

[0071] like Figure 16 As shown, the present invention provides a refractive index detection method, including the following steps:

[0072] S1. In any of the above schemes, a refractive index sensor is subjected to left-handed circularly polarized light and right-handed circularly polarized light, so that the sensor is in the EP point state.

[0073] S2. Apply an external perturbation to the sensor in the EP point state to obtain a reflection spectrum, and calculate the refractive index based on the reflection spectrum.

[0074] This invention relates to a refractive index detection method based on sensor settings. Based on a constructed second-order non-Hermitian system, at its second-order EP point, the characteristic surface formed by the system parameters is degenerate at a single point. By applying an external perturbation, this degeneracy point will undergo nonlinear splitting. By constructing a functional relationship between the perturbation and the nonlinear splitting, and by analyzing the relationship between the intrinsic frequency splitting and the perturbation, a highly sensitive and chiral-response refractive index sensing detection method is obtained in the terahertz (THz) band.

[0075] The specific principle for analyzing the relationship between intrinsic frequency splitting and perturbations is as follows:

[0076] The above is expressed using coupled-mode theory, which is widely used to describe how two resonant cavities achieve ideal effects through near-field coupling. In the time domain, considering a sinusoidal time-varying field, the resonant modes of the resonant cavity are expressed by dynamic equations as follows:

[0077]

[0078]

[0079] a and b are the resonance modes of the open-loop resonator. The mode amplitudes a and b in the above formula obey the coupling differential equation, where g is the gain loss coefficient and k is the coupling coefficient.

[0080]

[0081] Will Substituting into the above formula, we can write it in the form of an eigenvalue equation, H0V. n =ω n V n

[0082] Matrix H0 is:

[0083]

[0084] Applying an external perturbation to this second-order Hamiltonian, the Hamiltonian can be written as

[0085]

[0086] Solving this eigenvalue equation, its corresponding characteristic equation and characteristic frequencies are:

[0087]

[0088]

[0089] This shows that the intrinsic frequency splitting is related to the square root of the perturbation.

[0090] It should be noted that, for the sake of simplicity, the foregoing method embodiments are all described as a series of actions. However, those skilled in the art should understand that this application is not limited to the described order of actions, as some steps may be performed in other orders or simultaneously according to this application. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and modules involved are not necessarily essential to this application.

[0091] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0092] The foregoing description is merely an exemplary embodiment of this disclosure and should not be construed as limiting the scope of this disclosure. Any equivalent changes and modifications made in accordance with the teachings of this disclosure shall still fall within the scope of this disclosure. Those skilled in the art will readily conceive of embodiments of this disclosure upon considering the specification and practicing the disclosure herein. This application is intended to cover any variations, uses, or adaptations of this disclosure that follow the general principles of this disclosure and include common knowledge or customary techniques in the art not described herein. The specification and embodiments are to be considered exemplary only, and the scope and spirit of this disclosure are defined by the claims.

Claims

1. A refractive index sensor, characterized in that: It includes a reflective substrate (100) and a dielectric layer (200) disposed above the reflective substrate (100); It also includes multiple sets of metal structures (300), each set of metal structures (300) including a first metal ring (310) uniformly attached to the upper surface of the dielectric layer (200) and a second metal ring (320) embedded inside the dielectric layer (200); The first metal ring (310) is provided with a first opening (311), and the second metal ring (320) is provided with a second opening (321); The first metal ring (310) is arranged in a two-dimensional array on the upper surface of the dielectric layer (200), and the second metal ring (320) is arranged in a two-dimensional array inside the dielectric layer (200); The second opening (321) has a relative angular difference with the first opening (311); The first opening (311) is set within the range of [15.3μm, 16.5μm]; The second opening (321) is set to 33 μm; The distance between the first metal ring (310) and the second metal ring (320) is set in the range of [19μm, 31μm]; The relative angle difference between the first opening (311) and the second opening (321) in the multiple metal structures (300) is equal.

2. A refractive index sensor according to claim 1, characterized in that: The dielectric layer (200) is made of polyimide material.

3. A refractive index sensor according to claim 1, characterized in that: The length of the two-dimensional array ranges from [140μm, 180μm] in both the horizontal and vertical directions; The number of metal structures (300) in the horizontal direction of the two-dimensional array ranges from [6,8], and the number of metal structures (300) in the vertical direction of the two-dimensional array ranges from [6,8].

4. A refractive index sensor according to claim 1, characterized in that: The relative angle difference is 60°.

5. A refractive index sensor according to claim 1, characterized in that: The relative angle difference is 30°.

6. A method for detecting refractive index, characterized in that, Including the following steps: S1. A refractive index sensor according to any one of claims 1-5 is subjected to left-handed circularly polarized light and right-handed circularly polarized light, so that the sensor is in the EP point state; S2. Apply an external perturbation to the sensor in the EP point state to obtain a reflection spectrum, and calculate the refractive index based on the reflection spectrum.

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