A multi-target gas micro-sensor based on optical metamaterials

Abstract

The present invention relates to a multi-target gas micro-sensor based on optical metamaterials. The sensor includes a light source module and a signal detection module; the light source module includes a thermal radiation source and an optical modulator. The thermal radiation source is a highly directional thermal radiation device based on metamaterials, which can provide stable and high-efficiency radiation output within the mid-infrared wavelength regulation range, and uses temperature regulation to make the radiation center wavelength match the gas characteristic wavelength; the signal detection module includes a detection gas chamber and a signal detection end; the thermal radiation source is equipped with a feedback detection circuit for monitoring or stabilizing the mid-infrared output of the thermal radiation light source. Compared with sensors using quantum cascade lasers, this sensor has a low manufacturing cost; compared with non-dispersive infrared (NDIR) gas sensors based on traditional thermal light sources, it has the characteristics of low noise and high sensitivity, and can achieve high-precision detection of multiple target gases.

Description

Technical Field

[0001] The present invention relates to a multi-target gas micro-sensor based on optical metamaterials, which is applicable to the field of gas sensing. Background Art

[0002] With the continuous and rapid economic development, while the construction of many industries is booming, some potential safety hazards are also brought. In some practical applications, detecting some specific gases can effectively prevent and avoid the occurrence of safety accidents in industrial construction. Therefore, gas sensors have very wide applications in the fields of environmental monitoring, automotive industry, biotechnology, etc. With the development of science and technology, the performance requirements for sensors are constantly increasing, which promotes the development of various gas detection technologies based on different detection principles.

[0003] Gas detection methods are extremely widely used. Along with the continuous development of absorption spectroscopy gas sensors, absorption spectroscopy gas sensors based on different detection technologies have gradually developed. So far, the absorption spectroscopy technologies mainly include: direct absorption spectroscopy (DAS), wavelength modulation spectroscopy (WMS), cavity enhanced absorption spectroscopy (CEAS), photoacoustic and photothermal spectroscopy (PAS / PTS), etc.

[0004] Among them, photoacoustic spectroscopy measures the gas concentration by detecting the thermal perturbation signal generated after the gas absorbs the light signal or the acoustic wave signal generated by the periodic thermal signal. This type of sensing system usually has high compactness. In the initial stage of the development of photoacoustic spectroscopy, it was restricted by the detection sensitivity of the microphone and the power of the light source, and the development was relatively slow. With the emergence of various high-sensitivity microphones, photoacoustic cells, and high-power laser light sources, photoacoustic spectroscopy has developed rapidly, and its detection sensitivity has been greatly improved.

[0005] In recent years, quartz tuning forks have been widely used in photoacoustic spectroscopy. The photoacoustic spectroscopy based on quartz tuning forks is called quartz enhanced photoacoustic spectroscopy (QEPAS). In QEPAS, after the gas absorbs the radiation, periodic thermal expansion occurs, and then acoustic waves are generated. The quartz tuning fork is used to measure the acoustic wave signal.

[0006] Compared with traditional photoacoustic spectroscopy sensors, sensors based on quartz tuning forks benefit from the structure and material properties of quartz tuning forks, and have advantages such as high quality factor (the quality factor Q-factor of commercial ~ 32kHz quartz tuning forks > 10000 under standard atmospheric pressure), low cost, strong anti-interference ability, wide spectral response range, and small volume of the detection system, and have developed rapidly in recent years.

[0007] At present, the quantum cascade laser is a narrowband light source that is widely used for gas characteristic fingerprint spectrum detection in the mid-infrared band range. Generally speaking, as a mid-infrared light source, the quantum cascade laser has a narrow emission bandwidth and can also perform intensity modulation, but its drawback is that its price is too expensive. The non-dispersive infrared (NDIR) gas sensor based on traditional thermal light sources has low cost but insufficient sensitivity. Summary of the Invention

[0008] The present invention aims at the problems that the quantum cascade laser is expensive for mid-infrared band gas detection technology, while the non-dispersive infrared (NDIR) gas sensor based on traditional thermal light sources has low cost but insufficient sensitivity, and proposes a multi-target gas micro-sensor based on optical metamaterials. The present invention uses a thermal radiation source, which has low cost and wide spectral coverage. After being combined with a suitable narrowband filter, it can realize the detection of multiple target gases; and uses an optical modulator to generate harmonic signals, which has the characteristics of low noise and high sensitivity.

[0009] To solve the above problems, the technical solution proposed by the present invention is as follows:

[0010] A multi-target gas micro-sensor based on optical metamaterials, comprising a light source module and a signal detection module; the light source module includes a thermal radiation source and an optical modulator; the signal detection module includes a detection gas chamber and a signal detection end; a feedback detection circuit is connected between the thermal radiation source and the signal detection end for monitoring or stabilizing the mid-infrared output of the thermal radiation source;

[0011] The thermal radiation source is a high-directivity thermal radiation device based on metamaterials, which is used to provide stable and high-efficiency radiation output in the mid-infrared wavelength range, and the radiation center wavelength is matched with the gas characteristic wavelength by temperature regulation;

[0012] The optical modulator is used to generate harmonic signals;

[0013] The detection gas chamber is used to fill the gas to be measured;

[0014] The signal detection end is used to detect the photoelectric conversion signal or the photoacoustic conversion signal.

[0015] The high-directivity thermal radiation device described is a thermal radiation source in the mid-infrared band, corresponding to multiple narrow-band detections to save power consumption, or a thermal radiation device with a relatively narrow bandwidth but high efficiency to increase the blackbody radiation output power at the absorption wavelength of the gas to be measured.

[0016] The optical modulator described uses a metasurface microelectromechanical system modulator.

[0017] The volume of the detection gas chamber described does not exceed 2 cm 3 , and by introducing a porous structure material or a multi-reflection, multi-channel structure, a long optical path is realized to enhance the gas sensing ability.

[0018] The signal detection end described is a mid-infrared thermocouple, a pyroelectric detector or a detector array, integrated with a narrow-band filter at the absorption wavelength of the gas to be measured, for detecting the photoelectric conversion signal; or a quartz tuning fork is used as a broadband acoustic sensor for detecting the photoacoustic conversion signal.

[0019] When the signal detection end described uses a quartz tuning fork as a photoacoustic conversion detector, a narrow-band filter at the absorption wavelength of the gas to be measured is connected behind the high-directivity thermal radiation device, so that a photoacoustic signal corresponding to the frequency of the optical modulator is generated after filtering.

[0020] A microstructure lens is provided between the optical modulator and the detection gas chamber, which is used to compress the radiation light spot of the thermal radiation source, so that the radiation light hits between the two fingers of the quartz tuning fork in the detection gas chamber.

[0021] The multi-target gases described include CO 2 , CH 4 , NO, SO 2 , NH 3 , CO, NO 2 , SF 6 .

[0022] Advantages of the present invention:

[0023] Compared with the sensor using a quantum cascade laser, the present invention has a low manufacturing cost; compared with the non-dispersive infrared (NDIR) gas sensor based on a traditional thermal light source, it has the characteristics of low noise and high sensitivity, and can achieve high-precision detection of multiple target gases.

[0024] The optical metamaterial described in the present invention refers to a sub-wavelength structure or microstructure material with special properties. A low-cost thermal radiation source based on optical metamaterials can be used as a mid-infrared light source and has great potential in practical applications. In addition, at present, multi-gas analysis is generally completed by multiple instruments, and each instrument separately detects one gas, which is costly and occupies a large space. Therefore, the low-cost miniaturized multi-gas analysis sensor described in the present invention has broad application prospects. Description of the Drawings

[0025] Figure 1 It is a schematic structural diagram of a multi - target gas micro - sensor based on optical metamaterials.

[0026] Figure 2 It is a schematic structural diagram of a narrow - band and highly directional infrared radiation device.

[0027] Figure 3 It is the radiation spectrum of the highly directional narrow - band radiator varying with the angle obtained by simulation.

[0028] Figure 4 It is a side view of the MEMS modulator.

[0029] Figure 5 It is a side view of the MEMS modulator.

[0030] In the figure, there are light source module 1, signal detection module 2, thermal radiation source 3, optical modulator 4, detection gas chamber 5, signal detection end 6, feedback detection circuit 7, silicon substrate of the radiation source 8, tungsten substrate of the radiation source 9, concentric - ring micro - structure of the radiation source 10, upper metasurface layer of the modulator 11, middle support layer of the modulator 12, modulator electrode 13, lower metasurface layer of the modulator 14, and lower substrate of the modulator 15. Detailed Embodiment

[0031] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs.

[0033] As Figure 1 shown, a multi - target gas micro - sensor based on optical metamaterials includes a light source module 1 and a signal detection module 2; the light source module 1 includes a thermal radiation source 3 and an optical modulator 4; the signal detection module 2 includes a detection gas chamber 5 and a signal detection end 6; a feedback detection circuit 7 is connected between the thermal radiation source 3 and the signal detection end 6 for monitoring or stabilizing the mid - infrared output of the thermal radiation source 3.

[0034] The thermal radiation source 3 is a highly directional thermal radiation device based on metamaterials, which is used to provide stable and high - efficiency radiation output in the mid - infrared wavelength range, and the radiation center wavelength is matched with the gas characteristic wavelength by temperature regulation.

[0035] The optical modulator 4 is used to generate harmonic signals.

[0036] The detection gas chamber 5 described above is used to fill the gas to be measured.

[0037] The signal detection end 6 described above is used to detect the photoelectric conversion signal or the photoacoustic conversion signal.

[0038] The highly directive thermal radiation device is a thermal radiation source covering a relatively wide spectrum in the mid-infrared band, corresponding to multiple narrow-band detections to save power consumption, or a thermal radiation device with a relatively narrow bandwidth but high efficiency to increase the blackbody radiation output power corresponding to the absorption wavelength of the gas to be measured.

[0039] As Figure 2 shown, it is a narrow-band highly directive infrared radiation device, which includes a lower substrate 8, a middle substrate 9 and an upper concentric ring microstructure 10. The lower substrate 8 is made of silicon, and the middle substrate 9 is made of tungsten and used as a stable high-temperature resistant heating material. An electrode is led out at each end of the heating material. By applying a voltage to the electrode, the current can be converted into heat energy through metal heating. A concentric ring structure 10 made of tungsten material is fabricated on the substrate 9, and these structures can radiate (absorb) infrared light of specific wavelengths. By designing the upper concentric ring structure 10, the radiation characteristics of the device can be conveniently adjusted to achieve characteristics such as narrow band and high directivity.

[0040] As Figure 3 shown, we simulated the radiation spectrum of the highly directive narrow-band radiator varying with the angle, and it can be seen that the device only has a high emissivity in the vertical direction, and the emissivity drops rapidly when deviating from the vertical direction; in addition, the radiation wavelength and radiation intensity of the above structure can be changed by changing the spacing of the concentric ring structure 10, so as to match the gas actually detected.

[0041] At a certain temperature and wavelength, the power spectrum of thermal radiation can be expressed as:

[0042]

[0043] where

[0044]

[0045] is the blackbody radiation power spectrum, λ and T represent the radiation wavelength and the blackbody temperature respectively, h, c, k represent the constant Planck's constant, the speed of light in vacuum and the Boltzmann constant respectively, and the radiation power only depends on the wavelength and the temperature of the blackbody, is the emissivity of the object, θ is the radiation angle.

[0046] The thermal radiation of natural objects is usually broadband natural light, that is, it has the characteristics of broadband, non-directional, non-polarized and incoherent. According to Kirchhoff's law, under the condition of thermodynamic equilibrium, the emissivity of any object satisfies the following relationship with its absorptivity:

[0047] .

[0048] Therefore, by integrating a metasurface, the high-directivity radiation can be realized by utilizing the properties of the metasurface that can be flexibly regulated in terms of bandwidth, directivity, polarization, etc., that is, designing the metasurface so that its absorptivity (emissivity) in a specific direction θ is much greater than its absorptivity (emissivity) in other directions.

[0049] The described optical modulator adopts Figure 4 , Figure 5 the metasurface microelectromechanical system (MEMS) modulator shown in the figure. This device includes an upper metasurface layer 11, an intermediate support layer 12, an electrode 13, a lower metasurface layer 14, and a lower substrate 15.

[0050] The upper metasurface layer 11 can be composed of a metal material (such as gold, silver, aluminum, copper, etc.) or a metal-dielectric composite material (such as gold-silica, gold-magnesium oxide, etc.), and a micro-nano hole structure is etched on it. By changing the period, duty cycle, depth, etc. of the hole structure, the response wavelength, response speed, etc. of the device can be adjusted.

[0051] The intermediate support layer 12 is generally composed of a dielectric material (such as silica, magnesium oxide, silicon nitride, and polymers SU8, PMMA, etc.). Its main function is to support the upper suspended structure and perform electrical isolation.

[0052] The electrode 13 generally uses a metal material to apply an electrical signal.

[0053] The lower metasurface layer 14 is mainly used to form a resonant cavity and a planar electrode with the metasurface layer 11. Its constituent materials are common metal materials (such as gold, silver, aluminum, copper, etc.) or metal-dielectric composite materials (such as gold-silica, gold-magnesium oxide, etc.), and a micro-nano hole structure is etched on it. By changing the period, duty cycle, depth, etc. of the hole structure, the response wavelength, response speed, etc. of the device can be adjusted.

[0054] The lower substrate 15 is determined by the detected gas and is composed of materials with low loss in the detection band. Available materials include silicon, silica, calcium fluoride, magnesium fluoride, germanium, aluminum oxide, etc.

[0055] During operation, an alternating current signal is applied to the electrode 13. At this time, an electric field is generated between the upper metasurface layer 11 and the lower metasurface layer 14. The electric field drives the metasurface layer 11 to move downward, causing the emission spectrum to change, thereby modulating the incident light intensity.

[0056] The volume of the detection gas chamber is very small (not exceeding 2 cm 3 ), and by introducing porous structural materials or multi-reflection and multi-channel structures, a long optical path is achieved to enhance the gas sensing ability.

[0057] The signal detection end is a mid-infrared thermocouple, a pyroelectric detector or a detector array, integrated with a narrowband filter corresponding to the absorption wavelength of the gas to be measured, for detecting the photoelectric conversion signal; or a quartz tuning fork is used as a broadband acoustic sensor for detecting the photoacoustic conversion signal.

[0058] When the quartz tuning fork is used as the photoacoustic conversion detector at the signal detection end, a narrowband filter corresponding to the absorption wavelength of the gas to be measured is connected behind the highly directional thermal radiation device, so as to generate a photoacoustic signal corresponding to the frequency of the optical modulator after filtering.

[0059] The multi-target gases include CO 2 , CH 4 , NO, SO 2 , NH 3 , CO, NO 2 , SF 6 .

[0060] The multi-target gas microsensor adopts the gas absorption spectroscopy technology and follows the Lambert-Beer law. When electromagnetic waves of a specific wavelength pass through the gas to be measured, the attenuation of the light intensity is related to the concentration of the gas to be measured and the effective absorption path length. Among them, the Lambert law clarifies that the absorption of the light intensity by the gas to be measured is proportional to the path of the light passing through the gas to be measured, and the Beer law clarifies that the absorption amount of the gas to be measured is proportional to the concentration of the gas.

[0061] When a collimated light beam with an intensity of I 0 passes through the gas to be measured, the transmitted light intensity I after gas absorption can be expressed as:

[0062]

[0063] where v is the wave number of the electromagnetic wave, represents the absorption coefficient of the gas per unit concentration at a specific wave number, L represents the optical path length, and C represents the concentration of the substance to be measured.

[0064] The absorption coefficients of different gas molecules at specific wavenumbers can be queried through the HITRAN database. In this way, when the optical path length is known, the concentration information of the gas can be directly deduced by measuring the intensities of the incident light and the transmitted light. In a specific implementation, a thermal radiation source emits radiation light with a specific spectrum. After passing through a detection gas chamber with a certain optical path, the radiation light that is not absorbed by the gas to be measured or the interference light that is easily affected by environmental gases is filtered out by the optical filter in front of the infrared sensor. Since the gas concentration is different, the light intensity after passing through the gas is different. Thus, the concentration information of a specific gas can be retrieved (usually, a broadband light source covers multiple gas absorption peaks, and the gas absorption comes from multiple absorption peaks. Therefore, even if the optical path is known, it is difficult to directly retrieve the gas concentration. However, the change in light intensity is directly related to the gas concentration. Therefore, the relationship curve between the light intensity and the concentration of the gas to be measured can be obtained through curve fitting).

[0065] Since the absorption peaks of many gases are very weak, the signal-to-noise ratio of a simple direct absorption system is relatively poor, and the absorption signal is often submerged in the low-frequency noise. To solve the noise problem during the detection of such small absorption signals, signal modulation techniques are often used in combination. After using the signal modulation technique, instead of studying the direct absorption signal of the light intensity, the harmonic signal generated due to the nonlinear effect after the high-frequency modulation signal sweeps through the absorption peak is studied. During demodulation, only a very narrow part near the modulation frequency is selected to avoid the frequency band with high 1 / f noise and detect in the high-frequency harmonic band, thereby increasing the signal-to-noise ratio.

[0066] Signal modulation techniques include frequency modulation spectroscopy and wavelength modulation spectroscopy.

[0067] Among them, frequency modulation spectroscopy is a highly sensitive and high-resolution laser spectroscopy technique applied to weak signal detection. The spectrum uses a very high modulation frequency, and the detection signal is transferred to the high-frequency region for detection by using modulation and demodulation methods, thereby reducing the detection noise of the system. Therefore, a very high detection sensitivity can be achieved. Due to such unique advantages, the spectroscopy technique is widely used in fields such as trace gas detection.

[0068] Therefore, the multi-target gas micro-sensor reference frequency modulation spectroscopy technical solution based on optical metamaterials proposed by the present invention introduces the described optical modulator, adds high-frequency modulation to the radiation light emitted by the thermal radiation source, and after extracting the harmonic signal, the gas concentration is retrieved through the amplitude or peak-to-peak value of the harmonic signal. Since a high-frequency modulation signal is used, the concentration information of the gas is shifted to the high-frequency, greatly suppressing the system noise.

[0069] Example 1

[0070] As Figure 1As shown in the figure, the multi-target gas micro-sensor based on optical metamaterials includes a light source module 1 and a signal detection module 2. The light source module 1 includes a thermal radiation source 3 and an optical modulator 4; the signal detection module 2 includes a detection gas chamber 5 and a signal detection end 6; the thermal radiation source 3 is equipped with a feedback detection circuit 7 for monitoring or stabilizing the mid-infrared output of the thermal radiation source.

[0071] In this embodiment, the thermal radiation source 3 adopts a highly directional thermal radiation device based on metamaterials. This highly directional thermal radiation device is a thermal radiation source that covers a relatively wide spectrum in the mid-infrared band, corresponding to multiple narrow-band detections to save power consumption, or a thermal radiation device with a relatively narrow bandwidth but high efficiency to increase the blackbody radiation output power at the absorption wavelength of the gas to be measured.

[0072] In this embodiment, the thermal radiation source 3 can provide a stable and high-efficiency radiation output within the mid-infrared wavelength regulation range. For the characteristic absorption spectra of different gases, temperature regulation is applied to the thermal radiation source 3, applying different temperatures to make its radiation center wavelength match the gas characteristic wavelength, and the mid-infrared output of the thermal radiation source 3 is monitored or stabilized through the feedback detection circuit 7.

[0073] In this embodiment, the optical modulator 4 adopts a metasurface microelectromechanical system (MEMS) modulator. Using this modulator to perform specific modulation on the output light of the thermal radiation source 3, and extracting harmonic signals after demodulation to improve the recognition ability of the signal detection end 6 for the detection light carrying gas information, thereby improving the sensitivity and accuracy of gas detection for gases with dynamically changing concentrations.

[0074] In this embodiment, the volume of the detection gas chamber 5 is very small (not exceeding 2 cm 3 ) By introducing porous structural materials or multi-reflection and multi-channel structures, a long optical path is realized to enhance the gas sensing ability.

[0075] In this embodiment, the signal detection end 6 adopts a mid-infrared thermocouple / pyroelectric detector integrated with a narrow-band filter (corresponding to the absorption wavelength of the gas to be measured) for detecting the photoelectric conversion signal. Switching the narrow-band filter can screen the radiation light of the thermal radiation source 3 to make it correspond to the absorption wavelength of a certain specific gas.

[0076] In this embodiment, after changing the target measurement gas, it is necessary to change the temperature of the thermal radiation source 3 to make its radiation center wavelength match the gas characteristic wavelength, and switch the narrow-band filter to filter out other interfering lights in the radiation light. By applying different temperatures to the thermal radiation source 3 and continuously switching the narrow-band filter, the detection of multiple target gases can be realized.

[0077] The specific implementation steps of this embodiment are as follows:

[0078] ① Determine the gas to be measured, and regulate the temperature of the thermal radiation source 3 so that it outputs stable mid-infrared radiation light matching the characteristic wavelength of the gas.

[0079] ② Modulate the radiation light using the optical modulator 4 to obtain modulated light.

[0080] ③ The modulated light enters the detection gas chamber 5 and is absorbed by the gas to be measured in the chamber.

[0081] ④ The modulated light carrying the absorption information is transmitted from the other end of the detection gas chamber 5 and irradiates onto the signal detection end 6.

[0082] ⑤ After the radiation light without absorption by the gas to be measured or the interference light susceptible to environmental gases is filtered by the narrowband filter integrated in the signal detection end 6, the signal light irradiates onto the mid-infrared thermocouple / pyroelectric detector, generating an electrical signal.

[0083] ⑥ Demodulate the electrical signal, extract the harmonic signal, calibrate the strength of the signal at different concentrations, and obtain the relationship curve between the light intensity and the concentration of the gas to be measured through curve fitting, realizing the detection of the target gas concentration.

[0084] Embodiment 2

[0085] As Figure 1 shown, the multi-target gas micro-sensor based on optical metamaterials includes a light source module 1 and a signal detection module 2. The light source module 1 includes a thermal radiation source 3 and an optical modulator 4; the signal detection module 2 includes a detection gas chamber 5 and a signal detection end 6; a feedback detection circuit 7 is connected between the thermal radiation source 3 and the signal detection end 6.

[0086] In this embodiment, the thermal radiation source 3 uses a highly directional thermal radiation device based on metamaterials. This highly directional thermal radiation device is a thermal radiation source covering a relatively wide spectrum in the mid-infrared band, corresponding to multiple narrowband detections to save power consumption, or a thermal radiation device with a relatively narrow bandwidth but high efficiency to increase the blackbody radiation output power corresponding to the absorption wavelength of the gas to be measured.

[0087] In this embodiment, the thermal radiation source 3 can provide stable and high-efficiency radiation output within the mid-infrared wavelength regulation range. For the characteristic absorption spectra of different gases, temperature regulation is adopted for the thermal radiation source 3, applying different temperatures to make its radiation center wavelength match the characteristic wavelength of the gas, and monitoring or stabilizing the mid-infrared output of the thermal radiation source 3 through the feedback detection circuit 7.

[0088] In this embodiment, a narrowband filter (corresponding to the absorption wavelength of the gas to be measured) is connected behind the thermal radiation source 3, which is used to filter the radiation light without absorption by the gas to be measured or the interference light susceptible to environmental gases.

[0089] In this embodiment, the optical modulator 4 adopts a metasurface microelectromechanical system (MEMS) modulator. The modulator is used to perform specific modulation on the output light of the thermal radiation source 3, and after demodulation, the harmonic signal is extracted to improve the recognition ability of the signal detection end 6 for the detection light carrying gas information, thereby improving the sensitivity and accuracy of gas detection for dynamically changing concentrations.

[0090] In this embodiment, the detection gas chamber 5 has a small volume (not exceeding 2 cm 3 ), and by introducing a porous structure material or a multi-reflection, multi-channel structure, a long optical path is realized to enhance the gas sensing ability.

[0091] In this embodiment, the signal detection end 6 uses a quartz tuning fork as a broadband acoustic sensor for detecting the photoacoustic conversion signal.

[0092] In this embodiment, the quartz tuning fork needs to be placed in the detection gas chamber 5 for detecting the acoustic wave signal generated after the gas absorbs light energy.

[0093] In this embodiment, a microstructure lens can be added between the optical modulator 4 and the detection gas chamber 5 to compress the radiation light spot of the thermal radiation source 3, facilitating the radiation light to hit between the two fingers of the quartz tuning fork in the detection gas chamber 5.

[0094] In this embodiment, photoacoustic spectroscopy technology is used to detect gases, and the acoustic wave signal (pressure wave signal) generated after the gas absorbs light energy is used to invert the gas concentration. The generation principle of the photoacoustic signal is as follows: The light source is modulated and then irradiated on the gas molecules to be measured. After the gas molecules to be measured absorb light energy, non-radiative transitions occur. During this process, the released energy is converted into molecular internal energy, and macroscopically, it is manifested as a local temperature rise of the gas molecules. This periodic rise and fall of the local temperature causes the volume of the gas to expand and contract periodically, thereby generating a pressure wave, that is, an acoustic wave signal. The acoustic wave signal is converted into an electrical signal by an acoustic sensor, and after signal processing, it can be used to invert the gas concentration information. The photoacoustic signal S can be given by the following formula:

[0095] (5)

[0096] where C is the instrument constant, determined by the sensing system and affected by factors such as the volume and geometric shape of the photoacoustic cell (i.e., the detection gas chamber 5), the modulation frequency, the conversion efficiency of the acoustic sensor, and the system quality factor Q; P is the light source power; α is the absorption coefficient of the gas to be measured under a specific concentration per unit optical path. Common acoustic sensors include microphones, cantilever beams, and quartz tuning forks, etc. In this embodiment, a quartz tuning fork is used as the photoacoustic detector, which has the advantages of small volume, low price, and strong anti-noise ability (high Q value).

[0097] In this embodiment, the modulation frequency of the optical modulator 4 is determined by the resonance frequency of the quartz tuning fork, which is usually half of the resonance frequency of the quartz tuning fork. In this way, when the output wavelength of the light source approaches the gas absorption peak, the high-frequency sine signal will sweep across the gas absorption peak twice within one period, so the generated photoacoustic signal frequency is twice the high-frequency modulation frequency, that is, the photoacoustic signal frequency is the resonance frequency of the quartz tuning fork. The periodic photoacoustic signal pushes the quartz tuning fork to vibrate in the form of a pressure wave, and the quartz tuning fork will also enhance the photoacoustic signal through resonance. The periodically vibrating quartz tuning fork outputs a current signal at the metal electrode due to the piezoelectric effect. The current is converted into a voltage after transimpedance amplification and then output to a lock-in amplifier to extract the second harmonic signal, and then the gas concentration information is deduced.

[0098] In this embodiment, in order to confirm the modulation frequency of the optical modulator 4, it is necessary to first measure the resonance frequency of the quartz tuning fork. The measurement of the characteristic parameters of the quartz tuning fork is mainly based on the direct and inverse piezoelectric effects of the quartz crystal. The specific measurement methods can be divided into: electrical excitation measurement method, photothermal measurement method, photoacoustic measurement method, photothermal beat frequency measurement method and photoacoustic beat frequency measurement method. The electrical excitation measurement method is based on the inverse piezoelectric effect, and the rest of the methods are based on the direct piezoelectric effect. The first three methods are based on the steady-state response of the quartz tuning fork, with a long measurement period but accurate measurement results. The latter two methods are based on the transient response of the quartz tuning fork, with relatively fast measurement but usually large errors.

[0099] In this embodiment, the electrical excitation measurement method is used to determine the resonance frequency of the quartz tuning fork. When an excitation voltage (Ui) is applied to the two electrodes of the quartz tuning fork, a current (I) will flow through the quartz tuning fork. During the measurement process, a sine wave voltage excitation signal can be applied to the two electrodes of the quartz tuning fork, and by adjusting the frequency of the sine wave, the current amplitude and phase flowing through the quartz tuning fork under different frequency excitation signals can be obtained, and then the frequency response curve of the quartz tuning fork can be obtained. In addition to obtaining the resonance frequency and quality factor of the quartz tuning fork, the electrical excitation measurement method can also obtain electrical parameters such as the equivalent resistance, equivalent capacitance, equivalent inductance and stray capacitance of the quartz tuning fork. In the actual measurement process, since the current flowing through the quartz tuning fork is very weak, in order to accurately measure the current value, a transimpedance amplifier circuit and a lock-in amplifier are usually used for measurement at the back end of the quartz tuning fork.

[0100] The specific measurement steps of the quartz tuning fork parameters are as follows:

[0101] ① Set the output sine wave voltage frequency of the signal generator far from the resonance frequency of the quartz tuning fork (such as 20 kHz), measure the current passing through the quartz tuning fork. At this time, most of the current will flow through the stray capacitance, so the stray capacitance value can be approximately obtained.

[0102] ②Select a relatively large frequency range (such as 32.00 - 33.00 kHz) near the resonance frequency of the quartz tuning fork, set the step size to 5 Hz for frequency scanning, and roughly find the resonance frequency point of the quartz tuning fork.

[0103] ③Narrow the frequency range, set the step size to 0.5 Hz and scan again to accurately scan the frequency response curve of the quartz tuning fork to be measured near the resonance frequency.

[0104] ④Process the scanning results to obtain the current flowing through the RLC equivalent circuit part of the quartz tuning fork under different frequency excitations, that is, the frequency response curve. By analyzing the frequency response curve, characteristic parameters such as resonance frequency, quality factor, equivalent resistance, equivalent capacitance, and equivalent inductance can be obtained.

[0105] In this embodiment, after changing the target measurement gas, it is necessary to change the temperature of the thermal radiation source 3 so that its radiation center wavelength matches the gas characteristic wavelength, and switch the narrowband filter to filter out other interfering lights in the radiation light. By applying different temperatures to the thermal radiation source 3 and continuously switching the narrowband filter, the detection of multiple target gases can be realized.

[0106] The specific implementation steps of this embodiment are as follows:

[0107] ①Measure the resonance frequency of the quartz tuning fork.

[0108] ②Determine the gas to be measured, adjust the temperature of the thermal radiation source 3 so that it outputs stable mid-infrared radiation light matching the gas characteristic wavelength.

[0109] ③Use a narrowband filter to filter out the part of the radiation light that is not absorbed by the gas to be measured or is easily affected by ambient gases.

[0110] ④Use the optical modulator 4 to modulate the filtered radiation light, set the modulation frequency to half of the resonance frequency of the quartz tuning fork, and obtain the modulated light.

[0111] ⑤After the modulated light is focused by the microstructured lens, it enters the detection gas chamber 5, and the optical path is adjusted so that the light irradiates between the two fingers of the quartz tuning fork and is absorbed by the gas to be measured in the gas chamber.

[0112] ⑥After the gas molecules to be measured absorb light energy, non-radiative transitions occur, and the released energy is converted into molecular internal energy. The local temperature of the gas molecules periodically rises and falls, resulting in periodic expansion and contraction of the gas volume, thereby generating an acoustic wave signal.

[0113] ⑦The acoustic wave signal drives the quartz tuning fork to vibrate, and the resonance acoustic wave signal is enhanced through resonance.

[0114] ⑧The periodically vibrating quartz tuning fork generates an electrical signal due to the piezoelectric effect.

[0115] ⑨Demodulate the electrical signal, extract the harmonic signal, calibrate the signal strength at different concentrations, and obtain the relationship curve between the light intensity and the concentration of the gas to be measured through curve fitting, so as to realize the detection of the target gas concentration.

[0116] The implementation schemes described above can be further combined or replaced. Moreover, the implementation schemes are only descriptions of the preferred embodiments of the present invention, and do not limit the concept and scope of the present invention. Without departing from the design idea of the present invention, various changes and improvements made by those of ordinary skill in the art to the technical solutions of the present invention all fall within the protection scope of the present invention. The protection scope of the present invention is given by the appended claims and any equivalents thereof.

Claims

1. A multi-target gas micro-sensor based on optical metamaterials, characterized in that, it includes a light source module (1) and a signal detection module (2); the light source module (1) includes a thermal radiation source (3) and an optical modulator (4); the signal detection module (2) includes a detection gas chamber (5) and a signal detection end (6); a feedback detection circuit (7) is connected between the thermal radiation source (3) and the signal detection end (6) for monitoring or stabilizing the mid-infrared output of the thermal radiation source (3); the thermal radiation source (3) is a highly directional thermal radiation device based on metamaterials, which is used to provide stable and high-efficiency radiation output in the mid-infrared wavelength range, and the radiation center wavelength is matched with the gas characteristic wavelength by temperature regulation; the optical modulator (4) is used to generate harmonic signals; the detection gas chamber (5) is used to fill the gas to be measured; the signal detection end (6) is used to detect photoelectric conversion signals or photoacoustic conversion signals; the highly directional thermal radiation device is a thermal radiation source in the mid-infrared band, corresponding to multiple narrow-band detections to save power consumption, or a thermal radiation device with a relatively narrow bandwidth but high efficiency to increase the blackbody radiation output power at the absorption wavelength of the gas to be measured.

2. The multi-target gas micro-sensor based on optical metamaterials according to claim 1, characterized in that, the optical modulator (4) adopts a metasurface microelectromechanical system modulator.

3. The multi-target gas micro-sensor based on optical metamaterials according to claim 1, characterized in that, The volume of the detection gas chamber (5) does not exceed 2 cm 3 , and by introducing porous structural materials or multi-reflection and multi-channel structures, a long optical path is realized to enhance the gas sensing ability.

4. The multi-target gas micro-sensor based on optical metamaterials according to claim 1, characterized in that, the signal detection end (6) is a mid-infrared thermocouple, a pyroelectric detector or a detector array, integrated with a narrow-band filter corresponding to the absorption wavelength of the gas to be measured, for detecting photoelectric conversion signals; or a quartz tuning fork is adopted as a broadband acoustic sensor for detecting photoacoustic conversion signals.

5. The multi-target gas micro-sensor based on optical metamaterials according to claim 4, characterized in that, when the signal detection end (6) adopts a quartz tuning fork as a photoacoustic conversion detector, a narrow-band filter corresponding to the absorption wavelength of the gas to be measured is connected behind the highly directional thermal radiation device, so as to generate a photoacoustic signal corresponding to the frequency of the optical modulator after filtering.

6. The multi-target gas micro-sensor based on optical metamaterials according to claim 4, characterized in that, a microstructure lens is arranged between the optical modulator (4) and the detection gas chamber (5) for compressing the radiation light spot of the thermal radiation source (3), so that the radiation light hits between the two fingers of the quartz tuning fork in the detection gas chamber (5).

7. The multi-target gas micro-sensor based on optical metamaterials according to claim 1, characterized in that, The multi-target gas described above includes CO 2 , CH 4 , NO, SO 2 , NH 3 , CO, NO 2 , SF 6 .

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