A Dual-Band Carbon Dioxide Sensor Based on Metal–TiO 2 –Metal Metasurface Covered by Functional Material

: Highly sensitive and integrated optical multi-band CO 2 sensors are significant at the shortwave infrared (SWIR) region and still lack research. A compact CO 2 sensor composed of a Au-disk/TiO 2 - cylinder/Au-film metasurface coated by polyhexamethylene biguanide (PHMB) film, functioning at multi-band resonances as well as having high sensitivity to gas concentrations, is presented. It can be employed as a dual-band narrowband absorber, producing two strongly resonant modes at the SWIR region under a reﬂection-type framework of linearly polarized incidence. Moreover, the metasurface sensor possesses high refractive index sensitivity of 109.25 pm/ppm at around 1040 nm and 42.57 pm/ppm at around 1330 nm in the range of 200–600 ppm, which is suitable for detecting atmospheric CO 2 . Furthermore, the numerical results show that the sensitivity increases with a thicker PHMB ﬁlm and optimizes at a thickness above 600 nm. The physical mechanism reveals that the higher order mode exhibits more extended near-ﬁeld energy than the lower order mode, resulting in more sensitivity towards the surroundings. The design and results of our investigation show high-quality CO 2 sensing performance which functions at dual spectrum bands in the SWIR region and is promising for integrated photonic applications.


Introduction
Gas detection has received widespread attention in environmental protection and climate change with the rapid pace of global urbanization and industrialization [1]. Carbon dioxide (CO 2 ), as one of fundamental greenhouse gases and essential components in industrial process, has been a critical concern over the last few decades [2]. Much effort has been carried out to realize specific CO 2 detection, and two major approaches, electrochemical methods [3] and nondispersive infrared (NDIR) technique [4], have been extensively adopted. Although electrochemical sensors are economical in the market and suitable for downsizing in portable applications, poor selectivity, hysteresis and harsh reaction conditions make them insufficient in highly sensitive, real-time, or special situations. Current NDIR gas sensors promise fast responses, but the centimeter-or millimeter-long optical interaction path to achieve high signal-to-noise ratio limits device integration, and the selectivity is still inadequate for complex gas detection in different spectral regions [5]. In particular, CO 2 absorption bands in the shortwave infrared (SWIR) spectral range from approximately 0.9 µm to 2.5 µm are sensitive down to the lowermost layers of the atmosphere, which tend to be affected by fluxes emitted from point sources [6,7]. Thus, SWIR gas sensors are essential in atmospheric monitoring, such as extensively used satellite and airborne hyperspectral imaging detection, which is indispensable to the estimation of natural phenomena and human activities. Metasurfaces are a kind of artificial electromagnetic material with subwavelength scale character, which occupy numerous electromagnetic effects and are regarded as promising frequency-selective platforms for highly integrated, functionalized, and CMOS-compatible optical sensors [8,9]. Although metasurface emitters with high-quality-factor are achieved, the NDIR application scenario limits the miniaturization possibility of CO 2 sensors, and enhanced absorption characteristics with high selectivity have not been reported [10]. Several plasmonic metasurfaces covered by gas-sensitive polymer materials, such as polyethylenimine, have demonstrated prominent electromagnetic resonances and selectively sensing features in the mid-infrared/visible spectrum region by interacting with the physically captured CO 2 molecules from their surroundings [11][12][13]. However, the sensing performance of this material is susceptible to environmental humidity, resulting in additional preprocessing procedures to deal with this polymer. For the SWIR probe, similar gasselective-trapping polymer layers have been employed in fiber Bragg grating sensing installations to realize high chemical stability utilizing volume dilation of polyether sulfone in CO 2 exposure [14]. Nevertheless, the same humidity vulnerability exists, and the sensor is inadaptable for low concentration CO 2 detection in atmospheric environments.
Compared with other functional polymer materials, polyhexamethylene biguanide (PHMB) is advantageous to refractive index sensing, which is insensitive to humidity variation and can reversibly bind to CO 2 gas molecules via a simple alkali-acid interaction [15]. Theoretical work has been conducted to verify the sensing performance of this material covering grating slot waveguides [16]. In addition, experimental results exploiting microbubbles at fiber end have validated high CO 2 response selectivity of this material by sensing different types of gases (e.g., nitrogen, hydrogen, and argon) [17]. Unfortunately, these optical waveguide sensors under transmission type are not conducive to integration along the light path direction, and more importantly, the sensitivities are all below several tens of pm/ppm. A silicon disk metasurface based on reflection-type CO 2 detection downsizes the optical distance, whereas low sensitivity with only a single band of fundamental electromagnetic resonance is reported in the SWIR regime, which is not conducive to multi-frequency selected detection and is vulnerable to environmental disturbances without sufficient system stability [18,19]. Apart from these structures, metal-insulator-metal (MIM) metasurfaces are alternative platforms for supporting multiple resonant frequencies and combining functional gas-sensitive material [20]. The CMOS compatibility make them less difficult for mass production in contrast to nanostructures with 2D material, which generally need dedicated chemical vapor deposition growth and extra transfer procedures [21]. Therefore, highly sensitive and integrated optical sensors based on metasurface devices for multi-band CO 2 detection at the SWIR region are significant, but still lack research.
In this work, a compact CO 2 sensor composed of a metal-insulator-metal metasurface absorber coated with PHMB film with high sensitivity to gas concentration at dual resonant bands in the SWIR range is proposed. The insulator layer of this MIM structure is chosen to be TiO 2 , which has a high refractive index at the visible/SWIR region compared to the common metal oxide interlayers such as Al 2 O 3 or SiO 2 [22]. The metasurface is formed by a Au-disk/TiO 2 -cylinder/Au-film array on a silica substrate. This configuration offers the possibility to be employed as a dual-band narrowband absorber, producing two strongly resonant modes at around 858.56 nm and 1254.35 nm in the near-infrared band. Moreover, the metasurface possesses high refractive index sensitivity, indicating 109.25 pm/ppm at 1040 nm and 42.57 pm/ppm at~1330 nm when coated with the functional host material PHMB for CO 2 concentration detection. Furthermore, the numerical results show that the sensitivity increases with a thicker PHMB film and optimizes with a PHMB thickness above 600 nm. The design and results of our investigation show high-quality CO 2 sensing performance which functions at dual channel in the SWIR region and occupies competitive sensitivity, which is promising for integrated photonic and CMOS-compatible gas-sensing applications.

Design and Simulation Method
Metasurfaces can be described by complex electric permittivity and magnetic permeability. By regulating the effective impedance determined by metasurface design parameters, it is conceivable to obtain specific electromagnetic characteristics exhibiting tremendous merits over conventional predecessors due to its compact size and extraordinary features. Our design is inspired by the MIM resonators supporting diversified waveguide modes when the transverse dimension along the polarization direction changes [23,24]. Such modes are tightly concentrated so that the evanescent tail is short and strongly overlaps with the subwavelength scale molecules which are bounded around the structural localized volume. Figure 1 shows a schematic diagram of the proposed metasurface carbon dioxide gas sensor with periodical Au-disk/TiO 2 -cylinder/Au-film cells, which define the three layers of the resonator. Instead of a continuous insulator film to restrict the near fields inside the interlayer of the resonator [23], a discrete insulator layer (TiO 2 -cylinder) is employed to expand the interaction area between the evanescent fields and the gas molecules to be detected. Accounting for the practical application, the unit cell was located on the top of the SiO 2 substrate with a refractive index of 1.45. The thicknesses of the Au disks and TiO 2 nanocylinders are represented by H 1 and H 2 , and their radius is r. The thickness of the Au layer is represented with D. P x and P y represent the period length of a unit cell in the xand y-directions, respectively. The geometric parameters are fixed at P x = P y = 650 nm, r = 270 nm, H 1 = 25 nm, H 2 = 40 nm.
PHMB thickness above 600 nm. The design and results of our investigation show highquality CO2 sensing performance which functions at dual channel in the SWIR region and occupies competitive sensitivity, which is promising for integrated photonic and CMOScompatible gas-sensing applications.

Design and Simulation Method
Metasurfaces can be described by complex electric permittivity and magnetic permeability. By regulating the effective impedance determined by metasurface design parameters, it is conceivable to obtain specific electromagnetic characteristics exhibiting tremendous merits over conventional predecessors due to its compact size and extraordinary features. Our design is inspired by the MIM resonators supporting diversified waveguide modes when the transverse dimension along the polarization direction changes [23,24]. Such modes are tightly concentrated so that the evanescent tail is short and strongly overlaps with the subwavelength scale molecules which are bounded around the structural localized volume. Figure 1 shows a schematic diagram of the proposed metasurface carbon dioxide gas sensor with periodical Au-disk/TiO2-cylinder/Au-film cells, which define the three layers of the resonator. Instead of a continuous insulator film to restrict the near fields inside the interlayer of the resonator [23], a discrete insulator layer (TiO2-cylinder) is employed to expand the interaction area between the evanescent fields and the gas molecules to be detected. Accounting for the practical application, the unit cell was located on the top of the SiO2 substrate with a refractive index of 1.45. The thicknesses of the Au disks and TiO2 nanocylinders are represented by H1 and H2, and their radius is r. The thickness of the Au layer is represented with D. Px and Py represent the period length of a unit cell in the xand y-directions, respectively. The geometric parameters are fixed at Px = Py = 650 nm, r = 270 nm, H1 = 25 nm, H2 = 40 nm. A plane wave polarized in the y direction at normal incidence is exploited to investigate the optical properties of the symmetric structure under random linear polarized incidence as shown in Figure 1. The Au film is modeled as a perfect electric conductor (PEC) to prevent the electromagnetic wave from penetrating in the near-infrared range, and the transmission is nearly null. The optical spectrum and the electromagnetic field features are calculated by three-dimensional (3D) full-wave simulations utilizing the finite element method (FEM). In the FEM simulations, the effect of weak transmission and calculated absorbance (A) is considered using both reflectance (R) and transmittance (T) from the following relationship: A = 1 − R − T. To reduce computational memory, we calculate a single unit-cell by setting a periodic boundary condition on the lateral sides of the design. The proposed metasurface configuration is positioned between the source port and the output port. Perfectly matched layers (PMLs) are applied in the inner boundaries to diminish the electromagnetic wave propagating in the direction perpendicular to the port A plane wave polarized in the y direction at normal incidence is exploited to investigate the optical properties of the symmetric structure under random linear polarized incidence as shown in Figure 1. The Au film is modeled as a perfect electric conductor (PEC) to prevent the electromagnetic wave from penetrating in the near-infrared range, and the transmission is nearly null. The optical spectrum and the electromagnetic field features are calculated by three-dimensional (3D) full-wave simulations utilizing the finite element method (FEM). In the FEM simulations, the effect of weak transmission and calculated absorbance (A) is considered using both reflectance (R) and transmittance (T) from the following relationship: A = 1 − R − T. To reduce computational memory, we calculate a single unit-cell by setting a periodic boundary condition on the lateral sides of the design. The proposed metasurface configuration is positioned between the source port and the output port. Perfectly matched layers (PMLs) are applied in the inner boundaries to diminish the electromagnetic wave propagating in the direction perpendicular to the port boundary. The electric permittivity and magnetic permeability parameters of Au and TiO 2 are referenced from [25,26].

Absorption Properties and Resonance Characterization
The transmission, reflection and absorption spectra of the proposed metamaterial have been calculated from 800 nm to 1400 nm in the near-infrared region shown as Figure 2. Two resonances, mode 1 and mode 2, with the central wavelengths λ 1 = 1254.35 nm and λ 2 = 858.56 nm are excited. The max absorption intensities are I 1 = 88.1% and I 2 = 84.5% when the surrounding medium is air. The transmission is nearly zero, which means the absorption spectra of the proposed metasurface can be obtained by simply measuring the light reflection in the experiments. From the FEM model, it is noted that the reflection dip intensity and the spectral peak position depend on the period and the thickness of the top metal and the insulator layer. Although the intrinsic dual-mode absorption spectra of the metasurface are not the strongest, the structure parameters are chosen to obtain intense resonances at the SWIR region when coated with CO 2 -sensitive layer, which is discussed in the sensing performance section.
Photonics 2022, 9, x FOR PEER REVIEW 4 of 10 boundary. The electric permittivity and magnetic permeability parameters of Au and TiO2 are referenced from [25,26].

Absorption Properties and Resonance Characterization
The transmission, reflection and absorption spectra of the proposed metamaterial have been calculated from 800 nm to 1400 nm in the near-infrared region shown as Figure  2. Two resonances, mode 1 and mode 2, with the central wavelengths λ1= 1254.35 nm and λ2 = 858.56 nm are excited. The max absorption intensities are I1 = 88.1% and I2 = 84.5% when the surrounding medium is air. The transmission is nearly zero, which means the absorption spectra of the proposed metasurface can be obtained by simply measuring the light reflection in the experiments. From the FEM model, it is noted that the reflection dip intensity and the spectral peak position depend on the period and the thickness of the top metal and the insulator layer. Although the intrinsic dual-mode absorption spectra of the metasurface are not the strongest, the structure parameters are chosen to obtain intense resonances at the SWIR region when coated with CO2-sensitive layer, which is discussed in the sensing performance section. To explain the physical mechanism more clearly for the two absorption peaks, the electric (|E|) and magnetic (|H|) field distributions of mode 1 and mode 2 in different planes are calculated as shown in Figure 3. The operation of the MIM resonator can be understood using a Fabry-Perot model [24], in which the slow gap plasmons excited along the polarized y-direction are reflected at either end of the top metal feature. Different from the fundamental plasmonic mode in MIM metasurface with only a single "hot spot" in the magnetic field distribution [24,27], the two modes demonstrate two symmetric higherorder waveguide modes in the SWIR band.
Within the Fabry-Perot picture, the longer wavelength resonance (mode 1) is described as a standing wave pattern along the y-direction created by the bouncing between the nanoparticle facets at y = −R and y = R, whose electric (Figure 3a,b) and magnetic (Figure 3e,f) field distribution cross-sections are represented. The white arrows in Figure 3a,e manifest the corresponding directions of |E| and |H| field distributions in the x-y plane at the insulator interior, indicating that the electric charge oscillation of the excited localized surface plasmon resonance (LSPR) are restricted by the incident polarization and the circular outline of the structure. It is noteworthy that most near-field energy is confined in the intermediate dielectric TiO2 layer, as depicted in Figure 3b,f.
Compared with mode 1, mode 2, shown in the lower row of Figure 3, is considered to be a higher-order mode of LSPR between the topmost Au disk and the reflector layer, To explain the physical mechanism more clearly for the two absorption peaks, the electric (|E|) and magnetic (|H|) field distributions of mode 1 and mode 2 in different planes are calculated as shown in Figure 3. The operation of the MIM resonator can be understood using a Fabry-Perot model [24], in which the slow gap plasmons excited along the polarized y-direction are reflected at either end of the top metal feature. Different from the fundamental plasmonic mode in MIM metasurface with only a single "hot spot" in the magnetic field distribution [24,27], the two modes demonstrate two symmetric higher-order waveguide modes in the SWIR band.
Within the Fabry-Perot picture, the longer wavelength resonance (mode 1) is described as a standing wave pattern along the y-direction created by the bouncing between the nanoparticle facets at y = −R and y = R, whose electric (Figure 3a,b) and magnetic (Figure 3e,f) field distribution cross-sections are represented. The white arrows in Figure 3a,e manifest the corresponding directions of |E| and |H| field distributions in the x-y plane at the insulator interior, indicating that the electric charge oscillation of the excited localized surface plasmon resonance (LSPR) are restricted by the incident polarization and the circular outline of the structure. It is noteworthy that most near-field energy is confined in the intermediate dielectric TiO 2 layer, as depicted in Figure 3b,f.
Compared with mode 1, mode 2, shown in the lower row of Figure 3, is considered to be a higher-order mode of LSPR between the topmost Au disk and the reflector layer, owing to the shorter wavelength resonance and denser electric charge distributions. According to the data in Figure 3d,h, more light energy in mode 2 penetrates into the ambient medium and couples with adjacent nano-disks revealed by the brighter color map distributed in the Photonics 2022, 9, 855 5 of 10 top Au and the surroundings when compared to Figure 3b,f of mode 1, respectively. That means the higher-order mode (mode 2) is a comparatively extended SPR mode, whereas the lower-order mode (mode 1) is a more confined mode, which exhibits relatively higher absorption (I 1 > I 2 ) by the structure in the SWIR spectra region. In this paper, the sensing performance resulting from the different resonance/absorption mechanisms of two intrinsic resonant modes in the proposed metasurface absorber is discussed.
Photonics 2022, 9, x FOR PEER REVIEW 5 of 10 owing to the shorter wavelength resonance and denser electric charge distributions. According to the data in Figure 3d,h, more light energy in mode 2 penetrates into the ambient medium and couples with adjacent nano-disks revealed by the brighter color map distributed in the top Au and the surroundings when compared to Figure 3b,f of mode 1, respectively. That means the higher-order mode (mode 2) is a comparatively extended SPR mode, whereas the lower-order mode (mode 1) is a more confined mode, which exhibits relatively higher absorption (I1 > I2) by the structure in the SWIR spectra region. In this paper, the sensing performance resulting from the different resonance/absorption mechanisms of two intrinsic resonant modes in the proposed metasurface absorber is discussed.

Sensing Performance
The influence of the surrounding medium refractive index (RI) on the absorption properties of the proposed metasurface and its sensing performance is further investigated. To explore the possibility of gas sensing using the proposed metasurface, the refractive index change range surrounding the metasurface is set from 1.48 to 1.54 considering the RI variation of PHMB polymer when sensing CO2 gas concentration in this simulation [28]. The sensitivity is linearly fitted to 2.024 × 10 −4 RIU/ppm in the detection range of 200-600 ppm according to this previous study. Figure 4a presents the central reflection wavelength of the two resonant modes when the ambient refractive index changes considering R = 1 − A and reflection is feasible to be detected in practice. Mode 1 demonstrates that the reflection/absorption peaks possess red-shift from 1324.93 nm to 1337.55 nm as pointed out in Figure 4b, and mode 2 shows that the reflection/absorption peaks possess red-shift from 1027.54 nm to 1060.01 nm. The absorption intensity of mode 1 reaches up to 0.988, close to a perfect absorber and nearly remains constant when the RI of PHMB varies. The absorption intensity of mode 2 reaches 0.925 when the refractive index of PHMB is 1.48, and indicates a slight decrease with larger RI of PHMB. The variations in resonant wavelength are linearly fitted as a function of medium refractive index in Figure 4a, demonstrating high refractive index sensitivity of

Sensing Performance
The influence of the surrounding medium refractive index (RI) on the absorption properties of the proposed metasurface and its sensing performance is further investigated. To explore the possibility of gas sensing using the proposed metasurface, the refractive index change range surrounding the metasurface is set from 1.48 to 1.54 considering the RI variation of PHMB polymer when sensing CO 2 gas concentration in this simulation [28]. The sensitivity is linearly fitted to 2.024 × 10 −4 RIU/ppm in the detection range of 200-600 ppm according to this previous study. Figure 4a presents the central reflection wavelength of the two resonant modes when the ambient refractive index changes considering R = 1 − A and reflection is feasible to be detected in practice. Mode 1 demonstrates that the reflection/absorption peaks possess red-shift from 1324.93 nm to 1337.55 nm as pointed out in Figure 4b where ∆λ is the center wavelength change in resonance modes and ∆c is the corresponding CO 2 gas concentration change which results in PHMB refractive index change impacting the resonances. According to Equation (1), the sensitivity of CO 2 gas sensing for mode 1 and mode 2 is calculated to be S 1 = 42.57 pm/ppm and S 2 = 109.25 pm/ppm, respectively.
Photonics 2022, 9, x FOR PEER REVIEW 6 of 10 541.16 nm/RIU for mode 1 and 210.35 nm/RIU for mode 2. The CO2 concentration sensitivity of the proposed structure combined with PHMB gas-sensitive materials can be expressed as: where Δλ is the center wavelength change in resonance modes and Δc is the corresponding CO2 gas concentration change which results in PHMB refractive index change impacting the resonances. According to Equation (1), the sensitivity of CO2 gas sensing for mode 1 and mode 2 is calculated to be S1 = 42.57 pm/ppm and S2 = 109.25 pm/ppm, respectively. Therefore, when exposed in a gas-sensitive medium of high RI, the proposed metasurface absorber demonstrates superior spectra performance (absorptivity) in the SWIR range, with the higher-order mode (mode 2, extended LSPR mode) being more sensitive towards the surroundings than the lower-order mode (mode 1, confined mode). Furthermore, to highlight the excellent performance of the proposed structure, a detailed comparison with other optical RI-based CO2 gas sensors is summarized in Table 1. It is worth noting that the two resonant modes, which can be simultaneously used for CO2 gas sensing, are not revealed in other reports. In addition, the features of high sensitivity in the SWIR region under compact reflection-type with humidity insensitive gas gathering layer make the proposed sensing system conspicuous.  Therefore, when exposed in a gas-sensitive medium of high RI, the proposed metasurface absorber demonstrates superior spectra performance (absorptivity) in the SWIR range, with the higher-order mode (mode 2, extended LSPR mode) being more sensitive towards the surroundings than the lower-order mode (mode 1, confined mode). Furthermore, to highlight the excellent performance of the proposed structure, a detailed comparison with other optical RI-based CO 2 gas sensors is summarized in Table 1. It is worth noting that the two resonant modes, which can be simultaneously used for CO 2 gas sensing, are not revealed in other reports. In addition, the features of high sensitivity in the SWIR region under compact reflection-type with humidity insensitive gas gathering layer make the proposed sensing system conspicuous.  Finally, to improve application feasibility, the effect of PHMB gas-sensitive materials thickness (D) on CO 2 gas sensing is calculated as shown in Figure 5. It indicates that the sensing sensitivity increases as D increases, but the increase rate of sensitivity slows down as the thickness increases. When D is greater than 600 nm, increasing D for sensitivity rise does not help significantly. Moreover, when D = 900 nm, the sensitivity of CO 2 gas sensing is close to the situation that the metasurface is completely immersed in the PHMB environment discussed in Figure 4 above. Finally, to improve application feasibility, the effect of PHMB gas-sensitive materials thickness (D) on CO2 gas sensing is calculated as shown in Figure 5. It indicates that the sensing sensitivity increases as D increases, but the increase rate of sensitivity slows down as the thickness increases. When D is greater than 600 nm, increasing D for sensitivity rise does not help significantly. Moreover, when D = 900 nm, the sensitivity of CO2 gas sensing is close to the situation that the metasurface is completely immersed in the PHMB environment discussed in Figure 4 above.  Figure 6. When the metasurface is covered by a PHMB layer, the light energy of the extended mode (mode 2, Figure 6h,i,k,l) spreads to the high refractive index gas-trapping layer, resulting in sufficient interaction with the surroundings. Nevertheless, most near fields of the confined mode (mode 1, Figure 6b,c,e,f) are still concentrated tightly inside the resonator. Accordingly, the RI sensitivity of mode 2 is much higher than mode 1.
When the thickness of PHMB is thin (e.g., D = 200 nm), the near-field energy also permeates into the ambient air above PHMB (Figure 6b,c,h,i)). When D is increased (e.g., D = 900 nm), more energy is gathered inside the PHMB layer issuing in increased RI sensitivity, which is evident by Figure 6k,l,e,f. Moreover, the increased effect of PHMB thickness on sensitivity is more pronounced in mode 2 than mode 1 as shown in Figure 5, because of the more dispersive field distributions of mode 2 revealed by the contrast of Figure 6k,e. In practice, the thickness of PHMB influences the response time to gather CO2 gas for adequate reaction [28,29]. Therefore, the optimum thickness of PHMB gas-sensitive materials needs to be determined in experiments by measuring the relationship between PHMB thickness and response time.  Figure 6. When the metasurface is covered by a PHMB layer, the light energy of the extended mode (mode 2, Figure 6h,i,k,l) spreads to the high refractive index gas-trapping layer, resulting in sufficient interaction with the surroundings. Nevertheless, most near fields of the confined mode (mode 1, Figure 6b,c,e,f) are still concentrated tightly inside the resonator. Accordingly, the RI sensitivity of mode 2 is much higher than mode 1.
When the thickness of PHMB is thin (e.g., D = 200 nm), the near-field energy also permeates into the ambient air above PHMB (Figure 6b,c,h,i)). When D is increased (e.g., D = 900 nm), more energy is gathered inside the PHMB layer issuing in increased RI sensitivity, which is evident by Figure 6k,l,e,f. Moreover, the increased effect of PHMB thickness on sensitivity is more pronounced in mode 2 than mode 1 as shown in Figure 5, because of the more dispersive field distributions of mode 2 revealed by the contrast of Figure 6k,e. In practice, the thickness of PHMB influences the response time to gather CO 2 gas for adequate reaction [28,29]. Therefore, the optimum thickness of PHMB gas-sensitive materials needs to be determined in experiments by measuring the relationship between PHMB thickness and response time.

Discussion
The numerically simulated results of this proposed sensor rely on the previously measured data of the functional PHMB material with experiments conducted in the SWIR region. Therefore, sensing performances presented here focus on the similar spectra band as well as the same detection range of 200-600 ppm which is critical and suitable for atmospheric CO2 detection. More experiment data of PHMB features relating to CO2 concentration is essential for different working band and larger detection range. In addition, the sensing performance also depends on the exact MIM metasurface parameters, which are affected by the fabrication quality and accuracy. It is worth highlighting that this classical MIM structure can be feasibly realized by nanofabrication methods [31]. After deposition of the lower layer gold film onto the substrate, photoresist can be spin-coated and exposed to an electron-beam to be nanopatterned. After development, the interlayer TiO2 and the top Au film can be deposited onto the patterned photoresist. Following that, the MIM metasurface can be obtained after lift-off to remove the resist layer. Finally, the proposed sensor can be realized by spin-coating PHMB onto this metasurface.

Conclusions
In summary, this paper has presented a compact CO2 sensor composed of a metalinsulator-metal metasurface coated by PHMB film functioning at multi-band resonances as well as having high sensitivity to gas concentration. The metasurface is formed by a Au-disk/TiO2-cylinder/Au-film array on a silica substrate. It can be employed as a dualband narrowband absorber, producing two strongly resonant modes at the SWIR region under a reflection-type framework of linearly polarized incidence. Moreover, the metasurface sensor possesses high refractive index sensitivity of 109.25 pm/ppm at around 1040 nm and 42.57 pm/ppm at 1330 nm. Furthermore, the numerical results show that the sensitivity increases with a thicker PHMB film and optimizes with a PHMB thickness above 600 nm. The physical mechanism of sensing performance differences between the lower order mode (the confined mode 1) and the higher order mode (the extended mode 2) have been discussed in detail. The design and results of our investigation show competitive dual-band CO2 sensing performance, providing significant guidance for SWIR integrated photonic sensing applications.

Discussion
The numerically simulated results of this proposed sensor rely on the previously measured data of the functional PHMB material with experiments conducted in the SWIR region. Therefore, sensing performances presented here focus on the similar spectra band as well as the same detection range of 200-600 ppm which is critical and suitable for atmospheric CO 2 detection. More experiment data of PHMB features relating to CO 2 concentration is essential for different working band and larger detection range. In addition, the sensing performance also depends on the exact MIM metasurface parameters, which are affected by the fabrication quality and accuracy. It is worth highlighting that this classical MIM structure can be feasibly realized by nanofabrication methods [31]. After deposition of the lower layer gold film onto the substrate, photoresist can be spin-coated and exposed to an electron-beam to be nanopatterned. After development, the interlayer TiO 2 and the top Au film can be deposited onto the patterned photoresist. Following that, the MIM metasurface can be obtained after lift-off to remove the resist layer. Finally, the proposed sensor can be realized by spin-coating PHMB onto this metasurface.

Conclusions
In summary, this paper has presented a compact CO 2 sensor composed of a metalinsulator-metal metasurface coated by PHMB film functioning at multi-band resonances as well as having high sensitivity to gas concentration. The metasurface is formed by a Au-disk/TiO 2 -cylinder/Au-film array on a silica substrate. It can be employed as a dual-band narrowband absorber, producing two strongly resonant modes at the SWIR region under a reflection-type framework of linearly polarized incidence. Moreover, the metasurface sensor possesses high refractive index sensitivity of 109.25 pm/ppm at around 1040 nm and 42.57 pm/ppm at 1330 nm. Furthermore, the numerical results show that the sensitivity increases with a thicker PHMB film and optimizes with a PHMB thickness above 600 nm. The physical mechanism of sensing performance differences between the lower order mode (the confined mode 1) and the higher order mode (the extended mode 2) have been discussed in detail. The design and results of our investigation show competitive dual-band CO 2 sensing performance, providing significant guidance for SWIR integrated photonic sensing applications.