Femtosecond laser preparation method of porous silicon-based integrated optical waveguide gas sensor

Abstract

The invention discloses a femtosecond laser preparation method of a porous silicon-based integrated optical waveguide gas sensor, and belongs to the technical field of micro-nano optoelectronic devices and gas sensing. Comprising an oxidized porous silicon film; the three-dimensional waveguide structure is positioned in the oxidized porous silicon film and is formed by arranging positive refractive index densified track units; the fluorescent probe molecules are embedded into the waveguide; to-be-detected gas can interact with the fluorescent probe molecules so as to change the luminescence characteristic of the waveguide; two ends of the three-dimensional waveguide structure are respectively coupled with a section of optical fiber as a light input end and a light output end. A fluorescence probe and an optical waveguide are integrated in an oxidized porous silicon film through a femtosecond laser induced collapse densification technology, and a large-specific-surface-area nanopore structure and an evanescent field enhanced waveguide structure are utilized to realize full-optical-process high-sensitivity and ultrafast-response real-time optical detection of gas to be detected at normal temperature.

Description

Technical Field

[0001] The present invention belongs to the technical field of micro-nano optoelectronic devices and gas sensors, and in particular relates to a femtosecond laser preparation method for a porous silicon-based integrated optical waveguide gas sensor. Background Art

[0002] Traditional gas sensors primarily rely on metal oxide semiconductor (MOS) heterojunction electrochemical sensing or optical interferometry, which suffer from slow response, complex fabrication processes, high operating temperatures, and difficulty in on-chip integration. In recent years, porous silicon has been widely used for gas sensing due to its high surface area and optical activity. However, due to the complex spatial optical path and packaging process, miniaturization and large-scale manufacturing are difficult. Summary of the Invention

[0003] To address the above problems, the present invention proposes a femtosecond laser preparation method for a porous silicon-based integrated optical waveguide gas sensor. Fluorescent probes and optical waveguides are integrated into oxidized porous silicon (OPSi) films through femtosecond laser induced collapse densification (FLICD) technology. By utilizing a large specific surface area nanopore structure and an evanescent field enhanced waveguide structure, real-time optical detection of the gas to be measured with high sensitivity and ultrafast response in the full optical process at room temperature is achieved.

[0004] The technical solution adopted in the present invention is as follows:

[0005] In a first aspect, the present invention provides a porous silicon-based integrated optical waveguide gas sensor, comprising:

[0006] Oxidized porous silicon films;

[0007] A three-dimensional waveguide structure formed by an arrangement of positive refractive index densified track units located inside an oxidized porous silicon film;

[0008] and, fluorescent probe molecules embedded inside the waveguide;

[0009] The gas to be measured can interact with the fluorescent probe molecules to change the luminescence characteristics of the waveguide; the two ends of the three-dimensional waveguide structure are respectively coupled with a section of optical fiber as the light input end and the light output end.

[0010] As a preferred embodiment of the present invention, the fluorescent probe molecule includes any one of rhodamine 6G, diaminorhodamine derivatives, coumarin or fluorescein.

[0011] As a preference of the present invention, the gas to be measured is ethanol gas, atmospheric pollution gas or toxic gas.

[0012] As a preferred embodiment of the present invention, the detection performance of the gas sensor at room temperature and pressure meets the following requirements: response time ≤ 1 second, recovery time ≤ 5 seconds, detection limit ≤ 10 ppm, dynamic range ≥ 10 4 ppm.

[0013] In a second aspect, the present invention provides a femtosecond laser preparation method for a porous silicon-based integrated optical waveguide gas sensor, comprising the following steps:

[0014] (1) preparing an oxidized porous silicon film on a silicon substrate, wherein the oxidized porous silicon film has a porosity of 5% to 60% and a pore size of 2 nm to 50 nm;

[0015] (2) Immersing the plasma-cleaned oxidized porous silicon film in a 1-10 mM probe solution to achieve probe molecule penetration into the oxidized porous silicon film; plasma cleaning of the oxidized porous silicon film can make the porous structure surface have high wettability;

[0016] (3) Using high-repetition-rate femtosecond laser direct writing technology, the local nanoporous structure is collapsed and densified inside the oxidized porous silicon film doped with probe molecules, while the probe molecules are simultaneously encapsulated to form a positive refractive index densified track unit with embedded probe molecules;

[0017] The plurality of track units are arranged in a preset three-dimensional structure to form a waveguide core layer, and the untreated area in the porous silicon film is used as a cladding layer to prepare a sensor chip;

[0018] (4) A section of optical fiber is coupled to both ends of the waveguide structure of the sensor chip as the light input end and the light output end, respectively, to complete the chip packaging and obtain a porous silicon-based integrated optical waveguide gas sensor.

[0019] As a preferred embodiment of the present invention, the repetition frequency of the femtosecond laser direct writing technology is greater than 500 kHz, the pulse width is less than 1 ps, and the pulse energy is 40-60 nJ.

[0020] As a preferred embodiment of the present invention, the focusing of the femtosecond laser is achieved by an objective lens with a numerical aperture NA ≥ 0.80, and the scanning speed is 50 μm-20 mm / s.

[0021] As a preference of the present invention, the cross-sectional lateral dimension of the densified track is 200-500 nm (corresponding to the short axis of the elliptical cross section), and the longitudinal dimension is 800-1600 nm (corresponding to the long axis of the elliptical cross section).

[0022] As a preference of the present invention, the spacing between the densified tracks is 0.6-1 μm.

[0023] In a third aspect, the present invention proposes an application of a porous silicon-based integrated optical waveguide gas sensor doped with rhodamine 6G probe molecules in detecting ethanol gas concentration. The porous silicon-based integrated optical waveguide gas sensor doped with rhodamine 6G probe molecules is prepared by the above-mentioned femtosecond laser preparation method. The probe solution is a rhodamine 6G solution. The detection performance of the gas sensor at room temperature and pressure meets the following requirements: response time ≤ 620ms, detection limit as low as 5.7ppm, dynamic range ≥ 10 4 ppm.

[0024] The beneficial effects of the present invention are:

[0025] The present invention utilizes femtosecond laser-induced local collapse densification (FLICD) technology to induce nanopore collapse and densification inside the OPSi film to form a positive refractive index modified track, while encapsulating fluorescent probe molecules through the nanopore collapse; by densely arranging multiple densification tracks, a waveguide region with sensing function is formed; the porous structure of the OPSi film has a high specific surface area, allowing for the rapid capture and release of gas molecules, while the waveguide design supports efficient interaction between the evanescent field and the gas molecules to be measured, thereby enhancing detection sensitivity and response speed.

[0026] The gas sensor fabricated by this invention utilizes an in-situ fluorescent probe integration and dynamic response mechanism. Probe molecules are embedded in OPSi nanopores. Gas molecules penetrate the porous structure and interact with the probe molecules, causing the waveguide's luminescence properties (luminescence intensity or wavelength) to change, thereby modulating the optical signal. This sensor integrates an integrated optical path and sensing design, based on a high-surface-area nanopore structure and an evanescent field-enhanced waveguide structure, achieving a sensitivity of 1.73% light intensity / 100 ppm.

[0027] The sensor waveguide of the present invention has a customizable cross-sectional shape, supports efficient fiber-chip-fiber integrated packaging (coupling loss ≤ 0.5dB / surface), and avoids complex spatial optical paths and packaging processes by connecting the optical fiber to the laser and spectrometer, thus achieving full-function integration of "excitation-transmission-detection". It supports real-time dynamic monitoring and operation at ambient temperature, achieving sub-second response (620ms) and fast recovery (3.2s), a detection limit ≤ 10ppm, and a dynamic range ≥ 10 4 ppm; the probe molecules contained in the functional waveguide have good stability. During 60 minutes of continuous operation, the standard deviation of the luminescence intensity fluctuation is only ±1.15%. Chip size ≤1cm 2 , compatible with silicon photonics manufacturing, and can be expanded to simultaneous detection of multiple gases. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Schematic diagram of the sensing test system for ethanol gas sensor.

[0029] Figure 2 This is the ethanol sensor prepared in Example 1: (a) actual image; (b) photo of the sensing waveguide under 405nm laser excitation.

[0030] Figure 3 : is the normalized response spectrum of the ethanol sensor in Example 1 at 0-7400 ppm ethanol gas concentration.

[0031] Figure 4 is the normalized luminescence intensity corresponding to the ethanol sensor in Example 1 at 0-17000 ppm ethanol gas concentration.

[0032] Figure 5 Response and recovery time curves of the ethanol sensor in Example 1 when testing 5000 ppm ethanol gas concentration.

[0033] Figure 6 is the normalized intensity corresponding to the ethanol sensor in Example 1 in 15 0-2600-8000 ppm ethanol gas concentration cycle tests.

[0034] Figure 7 : This is the curve of the normalized intensity change over time of the ethanol sensor in Example 1 under 60 minutes of continuous operation. DETAILED DESCRIPTION

[0035] The present invention will be further described in detail below with reference to the accompanying drawings and examples, but the specific implementation manner of the present invention is not limited thereto.

[0036] The present invention proposes a femtosecond laser preparation method for a porous silicon-based integrated optical waveguide gas sensor, comprising the following steps:

[0037] (1) preparing an oxidized porous silicon film on a silicon substrate, wherein the oxidized porous silicon film has a porosity of 5% to 60% and a pore size of 2 nm to 50 nm;

[0038] (2) immersing the plasma-cleaned oxidized porous silicon film in a 1-10 mM probe solution to allow the probe molecules to penetrate into the oxidized porous silicon film;

[0039] (3) Using high-repetition-rate femtosecond laser direct writing technology, the local nanoporous structure is collapsed and densified inside the oxidized porous silicon film doped with probe molecules, while the probe molecules are simultaneously encapsulated to form a positive refractive index densified track unit with embedded probe molecules;

[0040] The plurality of track units are arranged in a preset three-dimensional structure to form a waveguide core layer, and the untreated area in the porous silicon film is used as a cladding layer to prepare a sensor chip;

[0041] In this step, the fluorescent probe is fixed to the waveguide region through femtosecond laser-induced nanopore collapse technology, and the probe molecules react with the gas to be measured to achieve luminescence signal modulation;

[0042] (4) A section of optical fiber is coupled to both ends of the waveguide structure of the sensor chip as the light input end and the light output end, respectively, to complete the chip packaging and obtain a porous silicon-based integrated optical waveguide gas sensor.

[0043] When using femtosecond laser direct writing technology, the wavelength is typically selected from 343nm, 515nm, 800nm, or 1030nm, with a repetition rate greater than 500kHz and a pulse width less than 1ps. The femtosecond laser is focused through an objective lens with a numerical aperture NA ≥ 0.80, and the scanning speed is 50μm-20mm / s. The resulting densified track units have a spacing of 0.6-1μm, and the track cross-section has a lateral dimension of 200-500nm and a longitudinal dimension of 800-1600nm.

[0044] Example 1: Ethanol sensor based on rhodamine 6G probe

[0045] In this embodiment, the porous film is immersed in a 5 mM Rhodamine 6G organic solution to achieve the embedding of the Rhodamine 6G probe molecules.

[0046] Set the femtosecond laser direct writing technical parameters as follows:

[0047] Laser parameters: central wavelength 1030nm, pulse width 214fs, repetition rate 1MHz, pulse energy 55nJ, linearly polarized light output;

[0048] Objective lens parameters: magnification 100×, numerical aperture NA = 0.8;

[0049] Scanning speed: 5mm / s;

[0050] Track spacing: 1 μm.

[0051] Preparation method of ethanol sensor:

[0052] 1) A porous silicon film was prepared on a silicon substrate by electrochemical etching, and the porous silicon film was heat-treated at 1000° C. in an oxygen atmosphere for 20 minutes to obtain an OPSi film (porosity 17%, pore size 10 nm, thickness 35 μm);

[0053] 2) After plasma cleaning, the OPSi film was immersed in a 5 mM rhodamine 6G-ethanol solution for 10 minutes;

[0054] 3) Using the femtosecond laser direct writing technique described above, a three-dimensional waveguide is directly written within the OPSi film, causing the local nanoporous structure to collapse and densify while simultaneously encapsulating Rhodamine 6G probe molecules to form a positive refractive index densified track unit embedded with the Rhodamine 6G probe molecules; multiple track units are arranged in a predetermined circular cross-sectional structure to form a waveguide core layer, and the untreated area within the porous silicon film serves as a cladding layer, thereby preparing a sensor chip;

[0055] 4) After cutting the chip prepared by laser direct writing, the two ends are connected to the standard single-mode optical fiber and fixed with optical curing glue to complete the packaging of the sensor.

[0056] The sensing principle of the ethanol sensor: Rhodamine 6G molecules are embedded in the nanopores as probe molecules. Ethanol gas molecules are captured by the porous structure and induce the probe molecules to transform from monomers to J / H dimers or even polymers, resulting in luminescence intensity attenuation and spectral red shift (593→600nm). By calibrating the dynamic response process, the sensing detection of ethanol gas concentration is achieved.

[0057] The prepared ethanol sensor is as follows Figure 2 As shown, (a) is a real picture, and (b) is a photo of the sensor waveguide under 405nm laser excitation. Under 405nm laser excitation, the sensor waveguide emits yellow-green light.

[0058] Example 2 Performance Test

[0059] use Figure 1 The sensor testing system calibrates and tests ethanol sensors. An air compressor provides a gas source, which is connected to a drying and filtering device for gas purification. Flow controllers 1 and 2 precisely regulate the flow rate. One gas path connects to a container for ethanol solution, forming a mixed gas and ethanol solution supply system to ensure stable input. The mixed gas enters a test chamber, which contains the ethanol sensor chip (i.e., the "sensor chip") prepared in Example 1. The input optical fiber is directly connected to a 405nm laser to provide a 405nm laser source; the output optical fiber is connected to a spectrometer for analyzing the sensor response signal. The right side also includes a commercial ethanol sensor for calibration. The entire system uses a computer to precisely control ethanol concentration and record data. The various components (e.g., the flow controller, spectrometer, and laser) work together to calibrate and test the ethanol sensor.

[0060] like Figure 3 As shown, when the ethanol concentration changes in the range of 0-7400 ppm, the luminescence intensity of the waveguide decreases as the concentration increases, and the luminescence wavelength red-shifts.

[0061] like Figure 4As shown, the peak intensity of the waveguide luminescence spectrum decreases continuously with the increase of ethanol concentration in the range of 0-17000 ppm, and is not saturated at a concentration of 17000 ppm, showing a detection upper limit greater than 17000 ppm.

[0062] like Figure 5 As shown, the sensor exhibited a response time of 620 milliseconds and a recovery time of 3.2 seconds when responding to 5000 ppm concentration of ethanol gas.

[0063] like Figure 6 As shown in Figure 3, the sensor remained stable during 15 consecutive cycles of testing, demonstrating good repeatability.

[0064] like Figure 7 As shown in the figure, the sensor signal remains stable after 60 minutes of continuous operation, showing good stability.

[0065] Example 3 Nitric oxide sensor based on diaminorhodamine derivative probe

[0066] In this embodiment, the porous film is immersed in a 5 mM organic solution of a diaminorhodamine derivative to achieve the embedding of the diaminorhodamine derivative probe molecules.

[0067] The femtosecond laser direct writing technical parameters were set the same as in Example 1, and the preparation process was the same as in Example 1 to obtain a nitric oxide sensor based on a diaminorhodamine derivative probe.

[0068] Sensing principle: Diaminorhodamine derivatives are embedded in OPSi nanopores as probe molecules. The rhodamine part provides fluorescence properties, while the diamino group serves as a capture site for NO. Under aerobic conditions, the diamino group reacts with nitric oxide to form a triazole compound, resulting in an enhanced fluorescence signal. By calibrating the dynamic response process, the sensing detection of nitric oxide gas concentration is achieved.

[0069] Example 4 Hydrogen sulfide sensor based on coumarin probe

[0070] In this embodiment, the porous film is immersed in a 10 mM coumarin organic solution to achieve embedding of coumarin probe molecules.

[0071] The femtosecond laser direct writing technical parameters were set the same as in Example 1, and the preparation process was the same as in Example 1 to obtain a coumarin probe-based H2S sensor.

[0072] Sensing Principle: Using a coumarin derivative as a fluorescent group, an electrophilic reaction site (such as a benzopyrylium cation) is introduced. Hydrogen sulfide specifically attacks the benzopyrylium group of the probe, triggering a nucleophilic addition reaction that breaks the p-conjugated structure and inhibits photoinduced electron transfer, thereby activating the fluorescence properties of the coumarin. By calibrating the dynamic response process, the sensing detection of hydrogen sulfide is achieved.

[0073] Example 5 Carbon monoxide sensor based on fluorescein probe

[0074] In this embodiment, the porous film is immersed in a 10 mM fluorescein organic solution to achieve the embedding of fluorescein probe molecules.

[0075] The femtosecond laser direct writing technical parameters were set the same as in Example 1, and the preparation process was the same as in Example 1 to obtain a carbon monoxide sensor based on a fluorescein probe.

[0076] Sensing Principle: The probe molecule contains an electrophilic center (such as an onium salt or electron-deficient aromatic ring). Carbon monoxide acts as a nucleophile to attack this site, triggering a chemical reaction that changes the charge distribution within the molecule, inhibiting photoinduced electron transfer or disrupting the π-conjugated system, resulting in a significant change in the fluorescence signal. By calibrating the dynamic response process, carbon monoxide sensing detection is achieved.

[0077] The present invention's room-temperature operation, real-time rapid response, wide dynamic range, high sensitivity, and compatibility with silicon photonics manufacturing give it unique advantages in industrial safety (chemical plant leak monitoring), smart agriculture (fermentation process monitoring), and wearable health devices (alcohol breath analysis). Combined with its modular probe design, it can quickly adapt to emerging sensing needs (such as greenhouse gas monitoring), promoting the industrialization and upgrading of optical gas sensing technology.

[0078] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A porous silicon-based integrated optical waveguide gas sensor, characterized in that: include: Oxidized porous silicon films; A three-dimensional waveguide structure formed by an arrangement of positive refractive index densified track units located inside an oxidized porous silicon film; and, fluorescent probe molecules embedded inside the waveguide; The gas to be measured can interact with the fluorescent probe molecules to change the luminescence characteristics of the waveguide; the two ends of the three-dimensional waveguide structure are respectively coupled with a section of optical fiber as the light input end and the light output end.

2. The porous silicon-based integrated optical waveguide gas sensor according to claim 1, characterized in that: The fluorescent probe molecule includes any one of rhodamine 6G, diaminorhodamine derivatives, coumarin or fluorescein.

3. The porous silicon-based integrated optical waveguide gas sensor according to claim 1, characterized in that: The gas to be tested is ethanol gas, atmospheric pollutant gas or toxic gas.

4. The porous silicon-based integrated optical waveguide gas sensor according to claim 1, characterized in that: The detection performance of the gas sensor at room temperature and pressure meets the following requirements: response time ≤ 1 second, recovery time ≤ 5 seconds, detection limit ≤ 10ppm, dynamic range ≥ 10 4 ppm.

5. A femtosecond laser preparation method for a porous silicon-based integrated optical waveguide gas sensor, characterized in that: The following steps are involved: (1) preparing an oxidized porous silicon film on a silicon substrate, wherein the oxidized porous silicon film has a porosity of 5% to 60% and a pore size of 2 nm to 50 nm; (2) immersing the plasma-cleaned oxidized porous silicon film in a 1-10 mM probe solution to allow the probe molecules to penetrate into the oxidized porous silicon film; (3) Using high-repetition-rate femtosecond laser direct writing technology, the local nanoporous structure is collapsed and densified inside the oxidized porous silicon film doped with probe molecules, while the probe molecules are simultaneously encapsulated to form a positive refractive index densified track unit with embedded probe molecules; The plurality of track units are arranged in a preset three-dimensional structure to form a waveguide core layer, and the untreated area in the porous silicon film is used as a cladding layer to prepare a sensor chip; (4) A section of optical fiber is coupled to both ends of the waveguide structure of the sensor chip as the light input end and the light output end, respectively, to complete the chip packaging and obtain a porous silicon-based integrated optical waveguide gas sensor.

6. The femtosecond laser preparation method of a porous silicon-based integrated optical waveguide gas sensor according to claim 5, characterized in that: The repetition frequency of femtosecond laser direct writing technology is greater than 500kHz, the pulse width is less than 1ps, and the pulse energy is 40-60nJ.

7. The femtosecond laser preparation method of a porous silicon-based integrated optical waveguide gas sensor according to claim 5, characterized in that: The focusing of the femtosecond laser is achieved through an objective lens with a numerical aperture NA ≥ 0.80, and the scanning speed is 50um-20mm / s.

8. The femtosecond laser preparation method of a porous silicon-based integrated optical waveguide gas sensor according to claim 5, characterized in that: The cross-sectional dimensions of the densified tracks are 200-500 nm in lateral dimensions and 800-1600 nm in longitudinal dimensions.

9. The femtosecond laser preparation method for a porous silicon-based integrated optical waveguide gas sensor according to claim 5, characterized in that: The spacing between the densified tracks is 0.6–1 μm.

10. Application of a porous silicon-based integrated optical waveguide gas sensor doped with rhodamine 6G probe molecules in detecting ethanol gas concentration, characterized in that: The porous silicon-based integrated optical waveguide gas sensor doped with rhodamine 6G probe molecules is prepared by the femtosecond laser preparation method according to any one of claims 5 to 9, and the probe solution is a rhodamine 6G solution.

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