Optical sensor and method for detecting refractive index of medium

Abstract

The invention relates to an optical sensor and a method for detecting the refractive index of a medium, the optical sensor comprises a plurality of metal nanoparticles, the plurality of metal nanoparticles are arranged in an a * a matrix to form a metal nanoparticle array, a is a positive integer, and the preset distance between the adjacent metal nanoparticles is P; when the medium to be measured is liquid, the optical sensor further comprises a transparent substrate, the metal nanoparticle array is arranged on one surface of the transparent substrate, and the metal nanoparticle array is used for being in contact with the liquid to be measured; when the medium to be detected is a solid, the optical sensor further comprises a transparent polymer layer, the metal nanoparticle array is completely embedded in the transparent polymer layer, and the transparent polymer layer is used for being in contact with the solid to be detected. According to the optical sensor structure and the detection method provided by the invention, the refractive indexes of liquid and solid samples can be effectively and sensitively measured, the structure is relatively simple, and the application range is wide.

Description

Technical Field

[0001] The present invention relates to the technical field of optical sensors, and in particular to an optical sensor and a method for detecting the refractive index of a detection medium. Background Art

[0002] Measuring the refractive index of a medium has important applications in fields such as chemical analysis, biosensing, environmental monitoring, and materials science. In the prior art, common refractive index sensors are mainly based on principles such as Surface Plasmon Resonance (SPR) or Fiber Bragg Grating (FBG).

[0003] Although sensors based on SPR have high sensitivity, they usually require specific coupling structures (such as prisms or gratings), the system is relatively complex, the volume is large, and they are sensitive to environmental vibrations, which limits their application in some portable or integrated scenarios. Although sensors based on FBG have a compact structure, they have cross-sensitivity to temperature changes, require an additional temperature compensation mechanism, and are inconvenient when detecting bulk solid samples.

[0004] In addition, many existing sensors are difficult to simultaneously achieve efficient and highly sensitive refractive index detection for liquid samples and solid samples in design. They often require different sensing structures or principles and lack generality. At the same time, there are still challenges in pursuing a simple structure, easy preparation, and high sensitivity.

[0005] Therefore, there is an urgent need to develop a new type of sensor with a relatively simple structure, convenient detection, and capable of achieving highly sensitive refractive index detection. Summary of the Invention

[0006] To this end, the technical problem to be solved by the present invention is to overcome the problems of complex structure, inconvenient detection, and low sensitivity of the sensors for measuring the refractive index of a medium in the prior art.

[0007] To solve the above technical problem, the present invention provides an optical sensor, including a plurality of metal nanoparticles, the plurality of metal nanoparticles are arranged in an a*a matrix to form a metal nanoparticle array, a is a positive integer, and the preset distance between adjacent metal nanoparticles is P;

[0008] When the medium to be measured is a liquid, the optical sensor further includes a transparent substrate, the metal nanoparticle array is disposed on one surface of the transparent substrate, and the metal nanoparticle array is used to contact the liquid to be measured;

[0009] When the medium to be measured is a solid, the optical sensor further includes a transparent polymer layer, and the metal nanoparticle array is completely embedded in the transparent polymer layer, and the transparent polymer layer is used to contact the solid to be measured.

[0010] In one embodiment of the present invention, the metal nanoparticles are metal nano-axisymmetric particles, and the metal nano-axisymmetric particles include any one of metal nanocolumns or metal nanohemispheres.

[0011] In one embodiment of the present invention, the radius range of the metal nanocolumns or metal nanohemispheres is 60 nm to 100 nm, the height range of the metal nanocolumns is 60 nm to 100 nm, and the difference range between the radius and height of the metal nanocolumns is 0 nm to 40 nm.

[0012] In one embodiment of the present invention, the preset distance P between adjacent metal nanoparticles ranges from 200 nm to 700 nm.

[0013] In one embodiment of the present invention, the metal materials used for the metal nanoparticles include gold, silver, aluminum, or their alloys.

[0014] In one embodiment of the present invention, the transparent substrate is any one of quartz, sapphire, silicon nitride, or a dielectric material with optical transparency.

[0015] In one embodiment of the present invention, the transparent polymer layer includes any one of polydimethylsiloxane, polytetrafluoroethylene, polymethyl methacrylate, or polyethylene terephthalate.

[0016] To solve the above technical problems, the present invention provides a method for detecting the refractive index of a medium. Using the optical sensor as described above, it includes:

[0017] Step S1: Use the optical sensor to contact the medium to be measured;

[0018] Step S2: Perform vertical light illumination from the side where the optical sensor contacts the medium to be measured and the side that does not contact the medium to be measured respectively. The side where the optical sensor contacts the medium to be measured is used as the forward or backward light illumination side, and the side where the optical sensor does not contact the medium to be measured is used as the backward or forward light illumination side, and the forward transmission spectrum of the forward light illumination side and the backward transmission spectrum of the backward light illumination side are measured respectively;

[0019] Step S3: Based on the forward transmission spectrum and the backward transmission spectrum, determine the variation relationship of the parameters characterizing the asymmetric light transmission characteristics with wavelength;

[0020] Step S4: Identify the wavelength at which a predetermined characteristic change occurs in the variation relationship of the parameters of the asymmetric light transmission characteristics with the illumination wavelength, and determine this wavelength as the cut-off wavelength λ of the first-order diffraction wave of the medium to be measured.

[0021] Step S5: According to the preset distance P between adjacent metal nanoparticles of the optical sensor and the cut-off wavelength λ of the first-order diffraction wave, calculate the refractive index n of the medium to be measured through the formula n l = λ / P l .

[0022] In an embodiment of the present invention, the parameter characterizing the asymmetric light transmission characteristics in step S3 is the difference between the forward transmission spectrum and the backward transmission spectrum, and the formula is: AS(λ) = |T f (λ) - T b (λ)|, where AS(λ) is the difference between the forward transmission spectrum and the backward transmission spectrum, T f (λ) is the forward transmission spectrum, and T b (λ) is the backward transmission spectrum.

[0023] In an embodiment of the present invention, the method for identifying the wavelength at which a predetermined characteristic change occurs in the variation relationship of the parameters of the asymmetric light transmission characteristics and determining this wavelength as the cut-off wavelength λ of the first-order diffraction wave of the medium to be measured in step S4 includes:

[0024] If the medium to be measured is a solid, at the wavelength position where the difference AS(λ) between the forward transmission spectrum and the backward transmission spectrum is equal to 0, determine this wavelength as the cut-off wavelength λ of the first-order diffraction wave of the medium to be measured;

[0025] If the medium to be measured is a liquid, at the wavelength position where the difference AS(λ) between the forward transmission spectrum and the backward transmission spectrum undergoes a mutation, determine this wavelength as the cut-off wavelength λ of the first-order diffraction wave of the medium to be measured.

[0026] In an embodiment of the present invention, the method for bringing the optical sensor into contact with the medium to be measured in step S1 includes: when the medium to be measured is a liquid, directly contacting the medium to be measured with the metal nanoparticles of the optical sensor.

[0027] In an embodiment of the present invention, the method for bringing the optical sensor into contact with the medium to be measured in step S1 includes: when the medium to be measured is a solid, fitting and contacting the medium to be measured with the surface of the transparent polymer layer of the optical sensor.

[0028] In one embodiment of the present invention, the wavelength range of the vertical light illumination of the optical sensor from one side of the metal nanoparticle array of the optical sensor and one side of the transparent substrate is 300 nm to 1500 nm; the cut-off wavelength λ ranges from 600 nm to 1300 nm.

[0029] The above technical solution of the present invention has the following advantages compared with the prior art:

[0030] The optical sensor constructed by the present invention has high sensitivity: using the steep change characteristic of the difference in asymmetric light transmission at the cut-off wavelength of the first-order diffraction wave of the medium to be measured as the sensing signal, a small refractive index change can cause a resolvable cut-off wavelength shift, thereby realizing high-sensitivity detection;

[0031] The optical sensor constructed by the present invention has a wide detection range and tunability: by reasonably designing the period P of the metal nanoparticle array and selecting a suitable transparent substrate (for liquids) or transparent polymer layer (for solids) material, the effective working wavelength (cut-off wavelength λ) of the optical sensor can cover the ultraviolet to near-infrared region, meeting the detection requirements of refractive index media in different ranges;

[0032] The present invention is applicable to the detection of both liquid samples and solid samples, with relatively wide applicability;

[0033] The structure of the present invention is relatively simple and easy to integrate: for the optical sensor for detecting the refractive index of a liquid, a metal nanoparticle array is arranged on a transparent substrate, and for the optical sensor for detecting the refractive index of a solid, the metal nanoparticle array is embedded in a transparent polymer layer. The optical sensor of the present invention can be prepared by mature micro-nano processing technologies (such as nanoimprinting, electron beam lithography, self-assembly, etc.), and is easy to miniaturize and integrate. Description of the Drawings

[0034] In order to make the content of the present invention easier to be clearly understood, the following further details the present invention according to specific embodiments of the present invention in conjunction with the drawings.

[0035] Figure 1(a) is a schematic diagram of the gold hemisphere structure of the metal nanoparticle array on the transparent substrate in the embodiment of the present invention;

[0036] Figure 1(b) is a schematic diagram of the gold cylinder structure of the metal nanoparticle array on the transparent substrate in the embodiment of the present invention;

[0037] Figure 2(a) is a schematic diagram of the optical sensor detecting the refractive index of a liquid sample in the embodiment of the present invention;

[0038] Figure 2(b) is a schematic diagram of the optical sensor detecting the refractive index of a solid sample in the embodiment of the present invention;

[0039] Figure 3 It is a schematic diagram of the transmittance of an optical sensor with different transparent substrates in the embodiments of the present invention under forward illumination and backward illumination when detecting different liquid samples.

[0040] Figure 4 It is a schematic diagram of the transmittance of an optical sensor under forward illumination and backward illumination when detecting different solid samples in the embodiments of the present invention. Detailed implementation manners

[0041] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, so that those skilled in the art can better understand the present invention and be able to implement it, but the specific embodiments cited are not intended to limit the present invention.

[0042] Embodiment 1

[0043] Referring to FIGS. 1(a) and 1(b), the present invention relates to an optical sensor, including a plurality of metal nanoparticles. The plurality of metal nanoparticles are arranged in an a*a matrix to form a metal nanoparticle array (a periodic metal nanoparticle array), where a is a positive integer, and the preset distance between adjacent metal nanoparticles is P (which can also be understood as the period P of the metal nanoparticle array);

[0044] When the medium to be measured is a liquid, the optical sensor further includes a transparent substrate. The metal nanoparticle array is disposed on one surface of the transparent substrate, and the metal nanoparticle array is used to contact the liquid to be measured;

[0045] When the medium to be measured is a solid, the optical sensor further includes a transparent polymer layer. The metal nanoparticle array is completely embedded in the transparent polymer layer (the metal nanoparticle array is flush with one side of the transparent polymer layer), and the transparent polymer layer is used to contact the solid to be measured.

[0046] Before introducing the optical sensor in detail, the diffraction principle is described below:

[0047] Diffraction is one of the most basic physical phenomena in a grating. A two-dimensional grating means having periodicity in two different directions (x and y). The diffraction orders of the grating in the x and y directions are respectively represented by a combination of integers m and n. When the grating diffracts, the grating equation can be expressed as:

[0048]

[0049] where λ is the wavelength of the incident light, n l is the background refractive index of the diffraction space, θ and are the azimuth and polar angle of the incident light, and are the azimuth and polar angle of the (m, n) order diffraction waves, respectively. Since formulas (1) and (2) have practical physical meanings, θ (m,n) The value range of P is 0° to 90°. x and P y are the periods of the grating in the x and y directions, respectively.

[0050] Assume that the period of the metal nanoparticle array in both directions is equal, that is, P = P x =P y , when the light source is incident vertically, the azimuth angle θ and polar angle of the incident light are all 0. At this time, the azimuth angle of the (m, n)-order diffraction wave can be converted according to formula (1) and formula (2):

[0051]

[0052] When θ (m,n) When the limit value of 90° is reached, the diffraction wave in the grating disappears. According to formula (3), the cutoff wavelength of the (m, n) order diffraction wave is:

[0053]

[0054] It should be noted that, since the working principle of the optical sensor of this embodiment is based on the cut-off wavelength of the first-order diffraction wave, m 2 +n 2 =1 Substituting into formula (4) we get:

[0055] λ=P×n l (5)

[0056] Formula (5) is the basic formula for optical sensors to detect the surrounding environment. Since the array period P is a fixed value, the cutoff wavelength λ of the first-order diffraction wave only depends on the background refractive index n of the diffraction space. l .

[0057] After introducing the above-mentioned first-order diffraction wave cutoff wavelength λ, the optical sensor of this embodiment is introduced in detail as follows:

[0058] The core principle of the optical sensor of this embodiment lies in the asymmetric optical response of the two sides of the optical sensor under different directions of illumination. Specifically, the optical sensor receives vertical incident light from the side in contact with the medium to be measured (forward / backward illumination side) and the side not in contact with the medium to be measured (backward / forward illumination side). When the incident light is incident, due to the difference in the refractive index of the environment in which the forward and backward illumination sides of the optical sensor are located, there are differences in the excitation mode and transmission characteristics, thus generating asymmetric light transmission. In order to obtain parameters that characterize the asymmetric light transmission characteristics, the forward transmission spectrum T of the forward illumination side is measured respectively.f and the backward transmission spectrum T of the backward illumination side b , and the forward transmission spectrum T f is not equal to the backward transmission spectrum T b in some wavelength ranges. Therefore, the characteristic of asymmetric light transmission (i.e., the difference between the forward transmission spectrum T f and the backward transmission spectrum T b |T f - T b |) or its slope (i.e., the derivative of the difference |T f - T b |) has a predetermined characteristic change at the cut-off wavelength λ corresponding to the first-order diffraction wave in the medium to be measured (i.e., the difference between the forward transmission spectrum T f and the backward transmission spectrum T b becomes 0 (corresponding to solid detection) or the slope has a sharp increase or mutation (corresponding to liquid detection)). The cut-off wavelength λ of this embodiment satisfies the diffraction relationship with the array period P and the refractive index n of the medium to be measured l : λ = P × n l .

[0059] Furthermore, in this embodiment, the transparent substrate is selected from quartz, sapphire, silicon nitride (Si3N4), or dielectric materials with similar optical transparency, and its refractive index is, for example, in the range of 1.4 to 2.2.

[0060] Furthermore, in this embodiment, the array period P ranges from 200 nm to 700 nm, preferably 550 nm.

[0061] Furthermore, in this embodiment, the metal nanoparticles include, but are not limited to, metal nanocolumns, metal nanohemispheres, or other symmetric metal nanoparticle shapes (the particle shape is axially symmetric in the x and y directions).

[0062] Furthermore, when the metal nanoparticles are metal nanocolumns, their radius is preferably 60 nm to 100 nm, and the difference between the radius and the height preferably satisfies within 0 nm to 40 nm.

[0063] Furthermore, in this embodiment, the materials of the metal nanoparticles include, but are not limited to, metals such as gold (Au), silver (Ag), aluminum (Al), or their alloys, which can excite surface plasmon resonance.

[0064] Furthermore, in this embodiment, the transparent polymer layer includes, but is not limited to, polydimethylsiloxane, polytetrafluoroethylene PTFE, polymethyl methacrylate PMMA, and polyethylene terephthalate PET.

[0065] Embodiment 2

[0066] The present invention relates to a method for detecting the refractive index of a medium, using an optical sensor as described in Embodiment 1, including:

[0067] Step S1: Bring the optical sensor into contact with the medium to be measured;

[0068] Step S2: Perform perpendicular light illumination from the side where the optical sensor is in contact with the medium to be measured and the side not in contact with the medium to be measured respectively. The side where the optical sensor is in contact with the medium to be measured is used as the forward or backward light illumination side, and the side where the optical sensor is not in contact with the medium to be measured is used as the backward or forward light illumination side, and measure the forward transmission spectrum of the forward light illumination side and the backward transmission spectrum of the backward light illumination side respectively; if one side of the optical sensor is selected as the forward light illumination side, the other side is the backward light illumination side;

[0069] Step S3: Based on the forward transmission spectrum and the backward transmission spectrum, determine the variation relationship of the parameter characterizing the asymmetric light transmission characteristic with wavelength;

[0070] Step S4: Identify the wavelength at which a predetermined characteristic change occurs in the variation relationship of the parameter of the asymmetric light transmission characteristic with the illumination wavelength, and determine this wavelength as the cutoff wavelength λ of the first-order diffraction wave of the medium to be measured;

[0071] Step S5: According to the preset distance P between adjacent metal nanoparticles of the optical sensor and the cutoff wavelength λ of the first-order diffraction wave, calculate the refractive index n of the medium to be measured through the formula n l = λ / P l .

[0072] Further, in step S3, the parameter characterizing the asymmetric light transmission characteristic is the difference between the forward transmission spectrum and the backward transmission spectrum, and the formula is: AS(λ) = |T f (λ) - T b (λ)|, where AS(λ) is the difference between the forward transmission spectrum and the backward transmission spectrum, T f (λ) is the forward transmission spectrum, and T b (λ) is the backward transmission spectrum.

[0073] Further, the method for identifying the wavelength at which a predetermined characteristic change occurs in the variation relationship of the parameter of the asymmetric light transmission characteristic with the illumination wavelength in step S4 and determining this wavelength as the cutoff wavelength λ of the first-order diffraction wave of the medium to be measured includes:

[0074] (1) If the medium to be measured is a solid, at the wavelength position where the difference AS(λ) between the forward transmission spectrum and the backward transmission spectrum is equal to 0, determine this wavelength as the cutoff wavelength λ of the first-order diffraction wave of the medium to be measured;

[0075] (2) If the medium to be measured is a liquid, at the wavelength position where the difference ΔS(λ) between the current forward transmission spectrum and the backward transmission spectrum undergoes a sudden change, determine that wavelength as the cut-off wavelength λ of the first-order diffraction wave of the medium to be measured.

[0076] Further, the method of using the optical sensor to contact the medium to be measured in step S1 includes: when the medium to be measured is a liquid, directly contacting the medium to be measured with the metal nanoparticles of the optical sensor.

[0077] Further, the method of using the optical sensor to contact the medium to be measured in step S1 includes: when the medium to be measured is a solid, fittingly contacting the medium to be measured with the surface of the transparent polymer layer of the optical sensor.

[0078] Further, the wavelength range of perpendicular light illumination on the optical sensor from one side of the metal nanoparticle array and one side of the transparent substrate is 300 nm to 1500 nm (ultraviolet light to near-infrared band); the cut-off wavelength λ ranges from 600 nm to 1300 nm.

[0079] Simulation analysis:

[0080] Figures 1(a) and 1(b) are schematic structural diagrams of a periodic metal nanoparticle array disposed on a transparent substrate. Among them, Figures 1(a) and 1(b) are gold hemisphere and gold cylinder nanoparticle arrays respectively, and the radius r of the hemisphere and the cylinder, the height h of the cylinder, and the period P of the metal nanoparticle array are marked in Figures 1(a) and 1(b) respectively. In this embodiment, the periods in the x and y directions are both set to P, so that the optical sensor does not depend on the polarization state of the incident light. This embodiment preferably uses gold (Au) as the metal material of the metal nanoparticles because the resonance wavelength of gold can be controlled within the visible light wavelength range. By changing to other metals such as silver (Ag) and aluminum (Al) and appropriately adjusting the structural parameters (the size of the metal nanoparticles and the period of the array), the sensing wavelength range can be expanded to: from ultraviolet to infrared.

[0081] As shown in Figures 1(a) and 1(b), forward light illumination (Forward in Figures 1(a) and 1(b)) is defined as light incident from the air perpendicular to the metal nanoparticle array along the -z axis direction into the transparent substrate region, and backward light illumination (Backward in Figures 1(a) and 1(b)) is defined as light incident from the transparent substrate region perpendicular to the metal nanoparticle array along the +z axis direction into the air. This embodiment selects the asymmetric subtraction (AS) to intuitively evaluate the asymmetric light transmission efficiency of the periodic metal nanoparticle array, that is, the above formula AS(λ) = |T f (λ) - T b (λ)|.

[0082] The detection principle of this embodiment for the medium to be measured is as follows: An important phenomenon is observed in the study of the asymmetric light transmission characteristics of the optical sensor: the difference in transmittance between forward illumination and backward illumination is mainly concentrated between the cut-off wavelength of the first-order diffraction wave in air and the cut-off wavelength of the first-order diffraction wave inside the transparent substrate. Therefore, based on the positions of the two cut-off wavelengths, an optical sensor based on a metal nanoparticle array can be designed in this embodiment, which is used to detect the refractive index of solid samples or liquid samples. For details, please refer to Simulation 1 and Simulation 2 below.

[0083] Simulation 1: Detection of the refractive index of a liquid sample (refer to Fig. 2(a) and Figure 3 )

[0084] As shown in Fig. 2(a), in this embodiment, the optical sensor uses sapphire (refractive index is about 1.78) as the transparent substrate, and a gold (Au) nanocylinder array with a period P = 550 nm is prepared on it. The radius of the cylinder is r = 100 nm, the height h = 100 nm, and the array period P is 550 nm.

[0085] (1) Detect the refractive index of oil (the refractive index of oil is about 1.2):

[0086] a. Immerse the optical sensor into the oil sample to be measured.

[0087] b. Measure the forward (light incident from the oil side) and backward (light incident from the sapphire substrate side) transmission spectra T f (λ) and T b (λ), as shown in Figure 3 (a).

[0088] c. Calculate the asymmetric transmission difference spectrum AS(λ) = |T f (λ) - T b (λ)|. The result is shown in Figure 3 (a), and there is a small rectangular schematic diagram of the AS(λ) spectrum in Figure 3 (a).

[0089] d. Observe the AS(λ) spectrum and find that at the wavelength λ ≈ 650 nm, the AS value shows a sharp increase (i.e., the slope of AS(λ) increases steeply).

[0090] e. Determine that this wavelength is the cut-off wavelength of the first-order diffraction wave in the oil environment to be measured.

[0091] f. According to the formula n l = λ / P, calculate the refractive index n l of the oil obtained from the actual measurement, which is 650 nm / 550 nm ≈ 1.18. This result is close to the standard refractive index of the oil, which is 1.2, verifying the effectiveness of the detection.

[0092] (2) Detect the refractive index of water (the refractive index of water is approximately 1.33):

[0093] a. Immerse the same optical sensor in the water to be measured.

[0094] b. Similarly measure the forward and backward transmission spectra T f (λ) and T b (λ), as shown in Figure 3 (c).

[0095] c. Calculate the asymmetric transmission difference spectrum AS(λ), and the result is as shown in Figure 3 (c), Figure 3 and there is a small rectangular schematic diagram of the AS(λ) spectrum in (c).

[0096] d. Observe the AS(λ) spectrum and find that at the wavelength λ≈720nm, the AS value shows a sharp increase (i.e., the slope of AS(λ) increases steeply).

[0097] e. Determine this wavelength as the cut-off wavelength of the first-order diffraction wave of the water environment to be measured.

[0098] f. Calculate the refractive index n l = 720nm / 550nm≈1.31, and this result is close to the standard refractive index of water, 1.33, verifying the effectiveness of the detection.

[0099] This embodiment also uses different transparent substrates to detect the refractive indices of oil and water: If the transparent substrate is changed to silicon nitride (Si3N4, the refractive index of silicon nitride is approximately 2.0), the parameters of the metal nanoparticle array remain unchanged. When detecting oil and water, the steep increase phenomena can also be observed in the AS(λ) spectrum at approximately 650nm and 720nm respectively (as shown in Figure 3 (b) and (d) with small rectangular schematic diagrams of the AS(λ) spectrum), proving the feasibility of using transparent substrates with different refractive indices. At the same time, it can be seen that the positions of the asymmetric light transmission cut-off wavelengths of the optical sensors with different transparent substrates (990nm for sapphire and 1110nm for silicon nitride) also change, but this does not affect the identification of the cut-off wavelength of the medium to be measured.

[0100] Simulation 2: Detection of the refractive index of solid samples (refer to Fig. 2(b) and Figure 4 )

[0101] As shown in Fig. 2(b), in this embodiment, the gold nanocylinder array in Experiment 1 (P = 550nm, the radius of the cylinder r = 100nm, height h = 100nm) is embedded in a polydimethylsiloxane (PDMS, refractive index is approximately 1.46) layer.

[0102] (1) Detect the refractive index of PC plastic (the refractive index of PC plastic is about 1.58):

[0103] a. Closely attach a PC plastic sample to the PDMS layer on the surface of the optical sensor.

[0104] b. Measure the forward (light incident from the PDMS side) and backward transmission spectra T f (λ) and T b (λ), as shown in (a) below. Figure 4 as shown in (a).

[0105] c. Analyze the asymmetric transmission characteristics ( Figure 4 The cut-off wavelength is directly marked in (a) below. The identification method is the same as in Simulation 1, that is, find the place where AS(λ) = 0, which means the forward and backward transmission spectra are equal. That is, the cut-off wavelength is located at about λ ≈ 870 nm.

[0106] d. Calculate the refractive index n l = 870 nm / 550 nm ≈ 1.58 nm, which is consistent with the refractive index of PC plastic, verifying the effectiveness of the detection.

[0107] (2) Detect the refractive index of silicon nitride (the refractive index of silicon nitride is about 2.0):

[0108] a. Closely attach a silicon nitride sample to the PDMS layer on the surface of the optical sensor.

[0109] b. Measure the transmission spectrum, as shown in (b) below. Figure 4 as shown in (b).

[0110] c. Find the place where AS(λ) = 0, and observe that the cut-off wavelength is located at about λ ≈ 1110 nm.

[0111] d. Calculate the refractive index n l = 11100 nm / 550 nm ≈ 2.02, which is close to the refractive index of silicon nitride, verifying the effectiveness of the detection.

[0112] From Figure 4 the comparison between (a) and (b) below, it can be seen that as the refractive index of the solid sample to be measured increases (from 1.58 of PC plastic to 2.0 of silicon nitride), the cut-off wavelength undergoes a significant red shift (from 870 nm to 1110 nm), which further proves the working principle of this optical sensor and shows its ability to distinguish different solid materials with high refractive indices.

[0113] The experimental results show that the optical sensor structure and detection method provided by the present invention can effectively and sensitively measure the refractive index of liquid or solid samples by utilizing the disappearance (i.e., AS(λ)=0) or steep change characteristic of the difference in asymmetric light transmission at the cut-off wavelength of the first-order diffraction wave of the medium to be measured, and the structure is relatively simple and has a wide application range.

[0114] Although the preferred embodiments of the present application have been described, those skilled in the art can make additional changes and modifications to these embodiments once they learn the basic creative concept. Therefore, the appended claims are intended to be construed to include the preferred embodiments as well as all changes and modifications falling within the scope of the present application.

[0115] Obviously, the above embodiments are merely examples given for clear illustration and are not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or variations can be made based on the above description. It is not necessary and impossible to enumerate all implementation manners here. And the obvious changes or variations derived therefrom are still within the protection scope of the present invention.

Claims

1. An optical sensor, characterized in that: It includes a number of metal nanoparticles, and a number of the metal nanoparticles are arranged in an a*a matrix to form a metal nanoparticle array, where a is a positive integer, and the preset distance between adjacent metal nanoparticles is P; When the medium to be measured is a liquid, the optical sensor further includes a transparent substrate, the metal nanoparticle array is disposed on one surface of the transparent substrate, and the metal nanoparticle array is used to contact the liquid to be measured; When the medium to be measured is a solid, the optical sensor further includes a transparent polymer layer, the metal nanoparticle array is embedded in the transparent polymer layer, and the transparent polymer layer is used to contact the solid to be measured.

2. The optical sensor according to claim 1, characterized in that: The metal nanoparticles are metal nano-axisymmetric particles, and the metal nano-axisymmetric particles include any one of metal nanocolumns or metal nanohemispheres.

3. The optical sensor according to claim 2, characterized in that: The radius range of the metal nanocolumn or the metal nanohemisphere is 60 nm to 100 nm, the height range of the metal nanocolumn is 60 nm to 100 nm, and the difference range between the radius and the height of the metal nanocolumn is 0 nm to 40 nm.

4. The optical sensor according to claim 1, wherein: The preset distance P between adjacent metal nanoparticles ranges from 200 nm to 700 nm.

5. The optical sensor according to claim 1, characterized in that: The metal material used for the metal nanoparticles includes gold, silver, aluminum or their alloys.

6. The optical sensor according to claim 1, characterized in that: The transparent substrate includes any one of quartz, sapphire, silicon nitride or a dielectric material with optical transparency.

7. The optical sensor according to claim 1, characterized in that: The transparent polymer layer includes any one of polydimethylsiloxane, polytetrafluoroethylene, polymethyl methacrylate, polyethylene terephthalate.

8. A method for detecting the refractive index of a medium, characterized in that: Using the optical sensor according to any one of claims 1-7, comprising: Step S1: Using the optical sensor to contact the medium to be measured; Step S2: Performing vertical light illumination from the side where the optical sensor contacts the medium to be measured and the side that does not contact the medium to be measured respectively. The side where the optical sensor contacts the medium to be measured is used as the forward or backward light illumination side, and the side where the optical sensor does not contact the medium to be measured is used as the backward or forward light illumination side, and the forward transmission spectrum of the forward light illumination side and the backward transmission spectrum of the backward light illumination side are measured respectively; Step S3: Based on the forward transmission spectrum and the backward transmission spectrum, determining the variation relationship of the parameter characterizing the asymmetric light transmission property with wavelength; Step S4: Identifying the wavelength at which a predetermined characteristic change appears in the variation relationship of the parameter of the asymmetric light transmission property with the illumination wavelength, and determining this wavelength as the first-order diffraction wave cut-off wavelength λ of the medium to be measured; Step S5: According to the preset distance P between adjacent metal nanoparticles of the optical sensor and the cut-off wavelength λ of the first-order diffraction wave, calculate the refractive index n of the medium to be measured through the formula n l = λ / P l .

9. The method for detecting the refractive index of a medium according to claim 8, wherein: In step S3, the parameter characterizing the asymmetric light transmission property is the difference between the forward transmission spectrum and the backward transmission spectrum, and the formula is: AS(λ) = |T f (λ) - T b (λ)|, where AS(λ) is the difference between the forward transmission spectrum and the backward transmission spectrum, T f (λ) is the forward transmission spectrum, and T b (λ) is the backward transmission spectrum.

10. The method for detecting the refractive index of a medium according to claim 9, wherein: The method in step S4 for identifying the wavelength at which a predetermined characteristic change appears in the variation relationship of the parameter of the asymmetric light transmission property with the illumination wavelength and determining this wavelength as the first-order diffraction wave cut-off wavelength λ of the medium to be measured includes: If the medium to be measured is a solid, at the wavelength position where the difference AS(λ) between the forward transmission spectrum and the backward transmission spectrum is equal to 0, determining this wavelength as the first-order diffraction wave cut-off wavelength λ of the medium to be measured; If the medium to be measured is a liquid, at the wavelength position where the difference AS(λ) between the forward transmission spectrum and the backward transmission spectrum undergoes a mutation, determining this wavelength as the first-order diffraction wave cut-off wavelength λ of the medium to be measured.

11. The method for detecting the refractive index of a medium according to claim 8, wherein: The method of using the optical sensor to contact the medium to be measured in the step S1 includes: when the medium to be measured is a liquid, directly contacting the medium to be measured with the metal nanoparticles of the optical sensor.

12. The method for detecting the refractive index of a medium according to claim 8, wherein: The method of using the optical sensor to contact the medium to be measured in the step S1 includes: when the medium to be measured is a solid, fittingly contacting the medium to be measured with the surface of the transparent polymer layer of the optical sensor.

13. The method for detecting the refractive index of a medium according to claim 8, characterized in that: The wavelength range of the vertical light irradiation is from 300 nm to 1500 nm; the cut-off wavelength λ ranges from 600 nm to 1300 nm.

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