Humidity sensor based on tunable photonic crystal grating and preparation method thereof

Abstract

The invention discloses a humidity sensor based on a tunable photonic crystal grating and a preparation method of the humidity sensor, and belongs to the technical field of optical sensing, the sensor comprises the tunable photonic crystal grating formed by compounding a single-layer polystyrene microsphere array and humidity sensitive hydrogel, the lattice spacing of microspheres is regulated and controlled through hydrogel humidity response deformation, and the humidity sensitive hydrogel is obtained. The humidity change is converted into accurate displacement of Bragg reflection wavelength, a Czerny-Turner optical path structure is combined, spectral displacement is analyzed in real time by utilizing a linear array charge-coupled device, and high-precision detection of humidity is realized. Through a semi-embedded microsphere-hydrogel interface stress transfer structure, the sensitivity, precision and anti-interference capability of humidity detection can be improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical sensing, and in particular relates to a humidity sensor based on a tunable photonic crystal grating and a preparation method thereof. Background Art

[0002] Photonic crystal (PC)-based humidity sensors, as an emerging detection technology, have garnered widespread attention and are widely used in a variety of fields, including environmental monitoring, industrial process control, and healthcare. Photonic crystals are artificial periodic materials with photonic band gap properties. Their unique structure interacts with specific wavelengths of light, producing structural color. Based on this principle, photonic crystal humidity sensors primarily detect changes in ambient humidity by detecting variations in structural color.

[0003] However, conventional humidity detection using photonic crystal humidity sensors relies primarily on the human eye's subjective interpretation of structural color. This accuracy is significantly limited by the human eye's ability to discern color. Individuals vary in color sensitivity and discrimination, and even the same observer's color judgment can vary under different conditions (such as fatigue or changing lighting conditions). This makes it difficult to obtain accurate and consistent results when determining humidity values ​​based on structural color changes. In practical applications, even small humidity changes may be difficult for the human eye to accurately discern, affecting humidity measurement accuracy and making them unsuitable for applications requiring high humidity precision, such as precision electronic equipment manufacturing workshops and pharmaceutical storage warehouses.

[0004] Poor anti-interference capability is another prominent issue. Changes in ambient light can significantly affect the human eye's ability to perceive structural color. In strong light, the structural color may be over-illuminated, resulting in distorted color features. In low light, however, it can be difficult to clearly discern changes in structural color. Furthermore, varying viewing angles can cause structural color to appear different. When the viewing angle changes, the reflection and interference of light received by the human eye alters, leading to inaccuracies in the interpretation of structural color. These factors significantly limit the application of photonic crystal-based humidity sensors in complex, real-world environments.

[0005] To overcome these shortcomings of photonic crystal-based humidity sensors, researchers have also attempted to develop other types of humidity sensors. For example, capacitive sensors are a relatively common type. Capacitive humidity sensors offer the advantage of fast response, reacting to humidity changes in a short period of time. However, they also have some significant drawbacks. Capacitive sensors are sensitive to electromagnetic environments. In locations with strong electromagnetic interference, such as substations and industrial automation production lines, their measurement results are easily affected by interference and errors. Furthermore, capacitive sensors suffer from long-term drift. Over time, the performance of the sensor gradually changes, resulting in a decrease in the accuracy of the measurement results. Regular calibration and maintenance are required, increasing the cost and complexity of use.

[0006] In summary, existing humidity sensor technologies all have certain limitations and cannot meet the growing demand for high-precision, interference-resistant, and stable humidity detection. Therefore, developing a new type of humidity sensor that can overcome the shortcomings of existing technologies and combine high precision, interference resistance, and strong stability has become a key issue that needs to be addressed in the current humidity detection field. Summary of the Invention

[0007] In view of the above, the object of the present invention is to provide a humidity sensor based on a tunable photonic crystal grating and a preparation method thereof, integrating a tunable photonic crystal grating comprising a monolayer polystyrene microsphere array and a humidity-sensitive hydrogel into a Czerny-Turner optical path structure to achieve highly sensitive, real-time and accurate measurement of humidity.

[0008] To achieve the above-mentioned purpose, the present invention provides the following technical solutions: In a first aspect, an embodiment of the present invention provides a humidity sensor based on a tunable photonic crystal grating, comprising: a collimator, a tunable photonic crystal grating, a focusing lens, and a linear array charge-coupled device arranged in sequence along an optical path, together forming a Czerny-Turner optical path structure; The collimating lens is used to collimate the received light source into a parallel light beam; The tunable photonic crystal grating comprises a single-layer polystyrene microsphere array and a humidity-sensitive hydrogel. The single-layer polystyrene microsphere array is partially embedded in the surface of the humidity-sensitive hydrogel to form a stress transfer interface. The tunable photonic crystal grating is used to receive a parallel light beam and regulate the lattice spacing of the single-layer polystyrene microsphere array through the humidity-responsive deformation of the humidity-sensitive hydrogel to achieve a change in the grating period, thereby modulating its Bragg reflection wavelength, so that the wavelength of the reflected light beam shifts with the ambient humidity. The focusing mirror is used to focus the reflected light beam emitted by the tunable photonic crystal grating and whose wavelength has shifted with the change of ambient humidity onto the linear array charge coupled device; The linear array charge coupled device is used to analyze the Bragg reflection wavelength displacement of the tunable photonic crystal grating according to the reflected light beam, and calculate the ambient humidity value in combination with the humidity-wavelength displacement calibration curve.

[0009] Preferably, the single-layer polystyrene microsphere array is formed by a plurality of polystyrene microspheres arranged in a hexagonal close-packed manner, and the diameter of the polystyrene microspheres is 400-1000 nm. If the microsphere diameter is too small, it is difficult to control the diffraction angle, because the grating period is difficult to effectively change, the response to humidity-induced deformation is not sensitive enough, and it is difficult to produce a significant Bragg reflection wavelength shift. If the microsphere diameter is too large, the grating structure is too rough, which may affect the propagation and reflection characteristics of light in the grating, resulting in a broadening and reduced intensity of the Bragg reflection peak, affecting the precision and accuracy of humidity measurement, and is also not conducive to the miniaturization and integration of the sensor.

[0010] Preferably, the humidity-sensitive hydrogel comprises: a humidity-sensitive material: acrylamide; a crosslinker: N,N-methylenebisacrylamide; a photoinitiator: 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone; and an organic solvent: dimethyl sulfoxide. The resulting humidity-sensitive hydrogel has a thickness of 40-60 μm and effectively interacts with the polystyrene microsphere array, ensuring effective transmission of hydrogel deformation to the grating while avoiding large signal fluctuations or slow response. It also balances mechanical stability with optical performance, facilitating precise and efficient sensor operation.

[0011] Preferably, the depth of the single-layer polystyrene microsphere array embedded in the humidity-sensitive hydrogel surface is 1 / 5 to 1 / 3 of the microsphere diameter. This ensures good stress transfer between the microspheres and the hydrogel, allowing the hydrogel to deform effectively when humidity changes, driving changes in the lattice spacing of the microsphere array. This maintains the structural stability of the microsphere array, contributing to the overall stability of the sensor.

[0012] Preferably, a metal reflection enhancement layer is covered on the surface of the tunable photonic crystal grating, and the metal layer is silver or gold, and the thickness of the metal reflection enhancement layer is 30~60nm, so as to enhance the reflection efficiency of the grating to light and improve the Bragg reflection peak intensity and clarity.

[0013] In a second aspect, an embodiment of the present invention further provides a method for preparing the above-mentioned humidity sensor based on a tunable photonic crystal grating, comprising the following steps: S1, substrate pretreatment: ultrasonic cleaning of the silicon wafer and mold using acetone, isopropyl alcohol, and ultrapure water, followed by plasma treatment; S2, silicon wafer auxiliary treatment: polydimethylsiloxane and curing agent are mixed and applied to the pre-treated silicon wafer, and then cured and UV-ozone treated to complete the silicon wafer surface treatment; S3, microsphere array preparation: a polystyrene microsphere suspension is mixed with isopropyl alcohol and injected into the surface of ultrapure water to self-assemble into a monolayer film. A surface-treated silicon wafer is inserted into the water surface and tilted out to complete the monolayer film transfer, thereby obtaining a monolayer polystyrene microsphere array; S4, hydrogel synthesis: a humidity-sensitive material and a cross-linking agent are dissolved in ultrapure water to form a prepolymer solution, a solution containing a photoinitiator and an organic solvent is added, and after magnetic stirring, the solution is coated on the surface of a monolayer polystyrene microsphere array and cured by UV to form a composite structure; S5, grating peeling: peeling the composite structure from the silicon wafer surface to obtain a tunable photonic crystal grating; S6, system integration: Assemble the tunable photonic crystal grating with the collimator, focusing lens and linear array charge coupled device optical components on the mold to form a Czerny-Turner optical path structure.

[0014] Preferably, after preparing a layer of polydimethylsiloxane on the silicon wafer, it is subjected to UV-ozone treatment for 10 to 30 minutes to improve its hydrophilicity. When the microspheres are transferred, the microspheres are grown on the surface of the cured polydimethylsiloxane. In this way, after UV curing, the composite structure can be more easily peeled off from the silicon wafer surface, the operation is simpler, and a complete tunable photonic crystal grating is obtained.

[0015] Preferably, the ratio of polydimethylsiloxane to curing agent is 10:1 to 20:1, the curing temperature is 80 to 120° C., and the curing time is 0.5 to 2 hours to ensure complete curing.

[0016] Preferably, the polystyrene microsphere suspension is mixed with isopropyl alcohol and then injected into the surface of ultrapure water through a syringe pump. The needle of the syringe pump is inserted vertically into the water surface, with half of the needle tip inserted into the water surface and half remaining in the air. The injection rate is 0.05-0.25 mL / min to ensure complete curing.

[0017] Preferably, the UV curing irradiation intensity is 30-50 mW / cm², and the curing time is 30-120 s.

[0018] Preferably, the prepolymer solution is composed of acrylamide and N,N-methylenebisacrylamide, and N,N-methylenebisacrylamide accounts for 0.1% to 1.5% of the total mass of the prepolymer solution. This range can form a moderate cross-linked structure. If the proportion is too low, the cross-linking is insufficient, and the hydrogel has poor strength and is brittle; if the proportion is too high, the cross-linking is too dense, resulting in poor elasticity of the hydrogel and insensitive humidity response.

[0019] Preferably, the concentration of the mixed solution of 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone and dimethyl sulfoxide is 0.1~1g / mL, which ensures that the photoinitiator is fully dissolved and evenly distributed, which is conducive to the efficient photoinitiation reaction during the operation of the sensor.

[0020] The concentration of the polystyrene microsphere suspension is preferably 1.5-2.5 wt% to ensure that the microspheres are evenly dispersed in the solution and do not agglomerate, which facilitates the formation of a regular and orderly array structure. If the concentration is too low, the number of microspheres will be insufficient, resulting in many array defects; if the concentration is too high, the microspheres are prone to agglomeration, affecting the uniformity of the array.

[0021] In a third aspect, an embodiment of the present invention further provides a humidity detection method, which uses the above-mentioned humidity sensor based on the tunable photonic crystal grating to perform humidity detection.

[0022] Compared with the prior art, the present invention has the following beneficial effects: This invention integrates a tunable photonic crystal grating with a Czerny-Turner optical path structure. By using a humidity-sensitive hydrogel that deforms in response to changes in ambient humidity, the grating's lattice spacing and Bragg reflection wavelength are controlled. This approach offers rapid response and high sensitivity, enabling timely detection of even the smallest humidity changes, allowing for rapid and accurate humidity readings and significantly improved measurement accuracy. Compared to traditional humidity sensors, this approach eliminates the need for complex calibration and manual interpretation, offering excellent stability and robust interference resistance. It can be widely used in applications such as meteorological monitoring, industrial production, and biomedicine, providing a reliable and efficient solution for humidity measurement in various fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0024] Figure 1 1 is a schematic structural diagram of a humidity sensor based on a tunable photonic crystal grating provided in an embodiment of the present invention; Figure 2 1 is a flow chart of a method for preparing a humidity sensor based on a tunable photonic crystal grating according to an embodiment of the present invention; Figure 3 This is an SEM image of a polystyrene microsphere array provided in an embodiment of the present invention (left image: polystyrene microspheres just transferred to a PDMS membrane; right image: polystyrene microspheres coated with 60 nm silver); Figure 4 This is a physical picture of a single-layer polystyrene microsphere array provided in an embodiment of the present invention; Figure 5 1 is a schematic diagram of a Czerny-Turner optical path structure provided by an embodiment of the present invention; Figure 6Schematic diagram of a wavelength-humidity response curve provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and do not limit the scope of protection of the present invention.

[0026] The inventive concept of the present invention is: to address the problems of low accuracy and poor anti-interference performance of humidity sensors based on photonic crystals in the prior art that rely on human visual interpretation, the embodiments of the present invention provide a humidity sensor based on a tunable photonic crystal grating and a preparation method thereof. A Czerny-Turner optical path structure is composed of a collimating mirror, a tunable photonic crystal grating (containing a single-layer polystyrene microsphere array and a humidity-sensitive hydrogel), a focusing mirror, and a linear array charge-coupled device arranged in sequence along the optical path. The grating period is controlled by utilizing the humidity-responsive deformation of the humidity-sensitive hydrogel, causing the reflected wavelength to shift with humidity. The linear array charge-coupled device then analyzes the wavelength shift and calculates the humidity value, thereby achieving highly sensitive, real-time, and accurate measurement of humidity.

[0027] like Figure 1 As shown, the embodiment provides a humidity sensor based on a tunable photonic crystal grating, comprising: a collimating mirror, a tunable photonic crystal grating, a focusing mirror and a linear array charge coupled device arranged in sequence along the optical path, which together form a Czerny-Turner optical path structure.

[0028] The collimator is used to collimate the received light source into a parallel light beam.

[0029] The tunable photonic crystal grating includes a single-layer polystyrene microsphere array and a humidity-sensitive hydrogel. The single-layer polystyrene microsphere array is partially embedded in the surface of the humidity-sensitive hydrogel to form a stress transfer interface. The tunable photonic crystal grating is used to receive parallel light beams and regulate the lattice spacing of the single-layer polystyrene microsphere array through the humidity-responsive deformation of the humidity-sensitive hydrogel to achieve grating period changes, thereby modulating its Bragg reflection wavelength so that the wavelength of the reflected light beam shifts with the ambient humidity.

[0030] The focusing mirror is used to focus the reflected light beam emitted by the tunable photonic crystal grating and whose wavelength has shifted with the change of environmental humidity onto the linear array charge coupled device.

[0031] The linear array charge coupled device is used to analyze the Bragg reflection wavelength shift of the tunable photonic crystal grating according to the reflected light beam, and the ambient humidity value is calculated in combination with the humidity-wavelength shift calibration curve.

[0032] Based on the same inventive concept, Figure 2As shown, an embodiment of the present invention further provides a method for preparing a humidity sensor based on a tunable photonic crystal grating, comprising the following steps: S1, Substrate Pretreatment: Ultrasonic cleaning of the silicon wafer and mold using acetone, isopropyl alcohol, and ultrapure water, followed by plasma treatment. In the Examples, prior to preparation, ultrasonic cleaning of the beaker, culture dish, silicon wafer, and glass sheet is performed using acetone, isopropyl alcohol, and ultrapure water, followed by plasma cleaning of the ultrasonically cleaned vessels, silicon wafer, and glass sheet.

[0033] S2, silicon wafer auxiliary treatment: A mixture of polydimethylsiloxane (PDMS) and a curing agent is applied to the pretreated silicon wafer, which is then cured and treated with UV-ozone to complete the silicon wafer surface treatment. In the embodiment, PDMS is used to assist the transfer of a single-layer polystyrene microsphere array and the exfoliation of a tunable photonic crystal grating. During the PDMS synthesis process, SYLGARD184 silica gel substrate and curing agent are first weighed on a balance in a ratio of 15:1; after mixing, they are magnetically stirred for 10 minutes, then vacuumed and dried, and repeated three times; a PDMS film is prepared on the silicon wafer surface by spin coating (500 rpm, 30 seconds); heated at 100°C for 1 hour on a hot plate; removed and naturally cooled for 45 minutes to obtain a PDMS film for dry transfer, which is then treated with UV-ozone for 10 to 30 minutes to improve its hydrophilicity.

[0034] S3, preparation of microsphere array: after mixing the polystyrene microsphere suspension with isopropanol, inject it into the surface of ultrapure water to self-assemble into a single-layer film, insert the surface-treated silicon wafer into the water surface and tilt it out to complete the transfer of the single-layer film, and obtain a single-layer polystyrene microsphere array. In the embodiment, a polystyrene microsphere suspension with a concentration of 2.5wt% is mixed with isopropanol in a volume ratio of 1:1; a 1mL syringe is used to extract 1mL of the prepared polystyrene microsphere mixed solution; the syringe is then fixed to the syringe pump, and the injection completion time is set to 5 minutes; the needle of the syringe is connected to the surface of ultrapure water through a hose, half of the needle cut surface is immersed in ultrapure water, and half is exposed to the air. After refraction is completed, it self-assembles at the water-air interface to form a single-layer polystyrene microsphere film with Bragg reflection color; the PDMS film on the silicon wafer is treated with surface ozone for 20 minutes, and then the silicon wafer is tilted at 30° into ultrapure water. The polystyrene microspheres will form a single-layer polystyrene microsphere array on the PDMS film under the action of surface tension, and its scanning electron microscope image (SEM) is shown. Figure 3 Optionally, a thermal evaporation device is used to coat a layer of 60nm thick silver on the polystyrene microsphere photonic crystal to enhance the reflection effect. The SEM image of the polystyrene microsphere photonic crystal after silver coating is shown in the figure below. Figure 3 The actual picture of the single-layer polystyrene microsphere array is shown in the right figure. Figure 4As shown, the two-dimensional photonic crystal can achieve an effect similar to a diffraction grating to split natural light, and covers a wide range of wavelengths, from deep blue to red, which means that a wide range of humidity can be used to measure.

[0035] S4, hydrogel synthesis: The humidity-sensitive material and the cross-linking agent are dissolved in ultrapure water to form a prepolymer solution, a solution containing a photoinitiator and an organic solvent is added, and after magnetic stirring, it is coated on the surface of a single-layer polystyrene microsphere array, and UV curing is performed to form a composite structure. In the embodiment, in the process of preparing the hydrogel, 10wt% of the humidity-sensitive material and 0.2wt% of the cross-linking agent are first formed into a mixed solution 1 in ultrapure water, and magnetically stirred for 20 minutes; then the photoinitiator is dissolved in the organic reagent to obtain a mixed solution 2 with a mass volume ratio of 33%, and magnetically stirred for 20 minutes; the mixed solution 1 and the mixed solution 2 are mixed in a volume ratio of 50:1, and magnetically stirred for 20 minutes to ensure uniform mixing. Then, a coating machine is used to apply a 50μm thick hydrogel solution on the polystyrene microspheres, covered with a glass slide, and cured for 1 minute using a UV curing lamp with a wavelength of 365nm.

[0036] S5, Grating Stripping: The composite structure is stripped from the silicon wafer surface to obtain a tunable photonic crystal grating. In the embodiment, after curing is completed, the polystyrene microspheres containing the hydrogel are peeled off with tweezers to obtain a tunable photonic crystal grating. The polystyrene microspheres are arranged in a hexagonal close-packed manner, with a microsphere diameter of 600 nm, a hydrogel layer thickness of 50 μm, and an embedding depth of 1 / 4 of the microsphere diameter.

[0037] S6, system integration: Assemble the tunable photonic crystal grating with the collimator, focusing lens and linear array charge coupled device optical components on the mold to form a Czerny-Turner optical path structure. In this example, the resulting tunable photonic crystal grating (1.2 cm × 1.2 cm) was fixed to a custom mold (8.6 cm × 6.2 cm × 3 cm) using UV-curable adhesive along all four edges, ensuring a perpendicularity of <0.1° between the grating plane and the mold substrate. A collimator (focal length 50 mm), a focusing lens (focal length 75 mm), and a linear charge-coupled device (Hamamatsu S7031, pixel size 24 × 200 μm) were then installed in sequence according to a Czerny-Turner optical path. The spacing between these components was optimized using Zemax (tolerance ±0.2 mm) to ensure that the focal diameter of the reflected light spot on the linear charge-coupled device target surface was ≤30 μm. Five circular air holes (1.5 mm diameter, laser-drilled, with a depth-to-width ratio of 3:1) were machined in the non-optical area at the bottom of the mold to allow air to enter. The resulting humidity sensor has an overall size of 8.6 cm × 6.2 cm × 3 cm.

[0038] The principle of Czerny-Turner optical path structure is as follows Figure 5 As shown. The working principle is as follows: the sensing mechanism of the tunable photonic crystal grating is based on the Bragg wavelength shift effect regulated by humidity. Its core lies in the dynamic regulation of the photonic crystal lattice spacing through the humidity response characteristics of the hydrogel. Specifically: polystyrene microspheres form a two-dimensional photonic crystal in a hexagonal close-packed manner, and form stress coupling with the underlying hydrogel through a semi-embedded interface. This structural design enables the swelling / contraction deformation of the hydrogel to be efficiently converted into a linear change in the lattice spacing. According to the Bragg reflection law, when the ambient humidity increases, the hydrogel absorbs water molecules and swells, and the interfacial shear stress drives the expansion of the microsphere array lattice, resulting in a red shift in the reflection wavelength; conversely, when the humidity decreases, the desorption of water molecules triggers contraction, resulting in a blue shift in the wavelength. The Czerny-Turner system focuses the Bragg reflection wavelength onto the target surface of the linear array charge-coupled device through a focusing mirror, thereby converting the humidity change into a fine wavelength shift recorded by the linear array charge-coupled device.

[0039] Based on the same inventive concept, an embodiment of the present invention also provides a humidity detection method, which uses the above-mentioned humidity sensor based on tunable photonic crystal grating to detect humidity. By changing the ambient humidity, the humidity sensor is characterized, and a specific relationship between the wavelength curve and humidity is established to obtain a wavelength-humidity response curve such as Figure 6 As shown in the figure, when the relative humidity increases from 24% to 94%, the spectral peak redshifts, increasing from 448nm to 528nm; when the humidity decreases from 94% to 24%, the spectral peak blueshifts, decreasing from 528nm to 448nm. By monitoring the Bragg wavelength shift of the tunable photonic crystal grating and combining it with the calibration curve, the ambient humidity value can be quickly calculated.

[0040] In summary, experimental verification demonstrates that the present invention achieves a linear shift of the Bragg wavelength by driving the lattice spacing of polystyrene microspheres through hydrogel swelling and contraction. This ultimately replaces human visual interpretation with spectral analysis, achieving a humidity resolution of 0.1%RH. The modular design enables rapid assembly, and the sensor weighs less than 200g and is compact. Furthermore, the linearity R² is greater than 0.99 within the 24-94%RH range, enabling highly sensitive, wide-range, and high-precision humidity detection.

[0041] The specific implementation methods described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above is only the most preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A humidity sensor based on a tunable photonic crystal grating, characterized in that: include: The collimating mirror, tunable photonic crystal grating, focusing mirror and linear array charge coupled device arranged in sequence along the optical path together form the Czerny-Turner optical path structure; The collimating lens is used to collimate the received light source into a parallel light beam; The tunable photonic crystal grating comprises a single-layer polystyrene microsphere array and a humidity-sensitive hydrogel. The single-layer polystyrene microsphere array is partially embedded in the surface of the humidity-sensitive hydrogel to form a stress transfer interface. The tunable photonic crystal grating is used to receive a parallel light beam and regulate the lattice spacing of the single-layer polystyrene microsphere array through the humidity-responsive deformation of the humidity-sensitive hydrogel to achieve a change in the grating period, thereby modulating its Bragg reflection wavelength, so that the wavelength of the reflected light beam shifts with the ambient humidity. The focusing mirror is used to focus the reflected light beam emitted by the tunable photonic crystal grating and whose wavelength has shifted with the change of ambient humidity onto the linear array charge coupled device; The linear array charge coupled device is used to analyze the Bragg reflection wavelength displacement of the tunable photonic crystal grating according to the reflected light beam, and calculate the ambient humidity value in combination with the humidity-wavelength displacement calibration curve.

2. The humidity sensor based on tunable photonic crystal grating according to claim 1, characterized in that: The single-layer polystyrene microsphere array is formed by arranging a plurality of polystyrene microspheres in a hexagonal close-packed manner, and the diameter of the polystyrene microspheres is 400-1000 nm.

3. The humidity sensor based on tunable photonic crystal grating according to claim 1, characterized in that: The components of the humidity-sensitive hydrogel include: humidity-sensitive material: acrylamide; cross-linking agent: N,N-methylenebisacrylamide; photoinitiator: 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone; organic solvent: dimethyl sulfoxide; the thickness of the prepared humidity-sensitive hydrogel is 40-60 μm.

4. The humidity sensor based on tunable photonic crystal grating according to claim 1, characterized in that: The embedding depth of the monolayer polystyrene microsphere array into the humidity-sensitive hydrogel surface is 1 / 5 to 1 / 3 of the microsphere diameter.

5. The humidity sensor based on tunable photonic crystal grating according to claim 1, characterized in that: The surface of the tunable photonic crystal grating is covered with a metal reflection enhancement layer, and the thickness of the metal reflection enhancement layer is 30-60 nm.

6. A method for preparing a humidity sensor based on a tunable photonic crystal grating according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1, substrate pretreatment: ultrasonic cleaning of the silicon wafer and mold using acetone, isopropyl alcohol, and ultrapure water, followed by plasma treatment; S2, silicon wafer auxiliary treatment: polydimethylsiloxane and curing agent are mixed and applied to the pre-treated silicon wafer, and then cured and UV-ozone treated to complete the silicon wafer surface treatment; S3, microsphere array preparation: a polystyrene microsphere suspension is mixed with isopropyl alcohol and injected into the surface of ultrapure water to self-assemble into a monolayer film. A surface-treated silicon wafer is inserted into the water surface and tilted out to complete the monolayer film transfer, thereby obtaining a monolayer polystyrene microsphere array; S4, hydrogel synthesis: a humidity-sensitive material and a cross-linking agent are dissolved in ultrapure water to form a prepolymer solution, a solution containing a photoinitiator and an organic solvent is added, and after magnetic stirring, the solution is coated on the surface of a monolayer polystyrene microsphere array and cured by UV to form a composite structure; S5, grating peeling: peeling the composite structure from the silicon wafer surface to obtain a tunable photonic crystal grating; S6, system integration: Assemble the tunable photonic crystal grating with the collimator, focusing lens and linear array charge coupled device optical components on the mold to form a Czerny-Turner optical path structure.

7. The method for preparing a humidity sensor based on a tunable photonic crystal grating according to claim 6, characterized in that: The ratio of polydimethylsiloxane to curing agent is 10:1~20:1, the curing temperature is 80~120℃, and the curing time is 0.5~2h.

8. The method for preparing a humidity sensor based on a tunable photonic crystal grating according to claim 6, wherein: The polystyrene microsphere suspension was mixed with isopropyl alcohol and injected into the ultrapure water surface through a syringe pump. The needle of the syringe pump was inserted vertically into the water surface, with half of the needle tip inserted into the water surface and half remaining in the air. The injection rate was 0.05~0.25mL / min.

9. The method for preparing a humidity sensor based on a tunable photonic crystal grating according to claim 6, characterized in that: The UV curing radiation intensity is 30~50mW / cm², and the curing time is 30~120s.

10. A humidity detection method, characterized in that: Humidity detection is performed using the humidity sensor based on the tunable photonic crystal grating according to any one of claims 1 to 5.

Images