Infrared absorption spectroscopy is a powerful technique that is used for label-free and non-destructive identification^[1–3]. Due to the small absorption cross section of the molecules in the mid-infrared, it is difficult to extract the vibration of tiny molecules using Fourier transform infrared spectroscopy^[4–6]. Surface-enhanced infrared absorption spectroscopy (SEIRAS) can amplify molecular vibrations through localized electric fields, improving sensitivity and resolution in molecular analysis^[7–9]. However, the amplification of the infrared signal is constrained by several factors, including the coupling mode of the sensor, the sensing band, and the coupling efficiency between the molecules and the resonances. These sensor properties struggle to independently adjust in a traditional antenna array sensor^[10–12].

Infrared metasurfaces with artificial subwavelength structures offer the flexible dimension to independently adjust sensor properties and can become the next generation of sensors for SEIRAS^[13–15]. The temporal coupled model theory (TCMT), which well explains the coupling between plasmon resonances and molecular vibrations, is widely used as the analytical framework for sensor design^[16]. The resonances of the metasurfaces are determined by the radiative damping ${\gamma }_e$, absorptive damping ${\gamma }_o$ of the metasurface, and the center frequency ${\omega }_0$. The ratio of ${\gamma }_e$ to ${\gamma }_o$ divides the metasurfaces into three coupled modes: under-coupled (UC, ${\gamma }_e/{\gamma }_o<1$), critical coupled (CC, ${\gamma }_e/{\gamma }_o=1$), and over-coupled (OC, ${\gamma }_e/{\gamma }_o>1$). Due to the inherent loss of the materials and the high-quality factor of the perfect absorber^[17,18], most metasurface sensors are designed as UC and CC metasurfaces^[3,19]. However, the spectra of the UC and CC metasurfaces are shown as the electromagnetically induced transparency (EIT) with a narrow bandwidth, causing the nonlinear amplification of signals and limited sensing bands^[20]. In contrast, the OC metasurface with broadband resonance is the preferred design for SEIRAS due to the electromagnetically induced absorption (EIA) during the coupling^[15,21].

Although the absorptance spectrum in the far field was analyzed by the TCMT to identify the coupling mode of the metasurface, the potential physical mechanism and near-field properties of the sensor were often ignored^[2,13]. This oversight has led to ongoing debates about the analysis methods and design schemes for the OC metasurface sensors. The absorptance of the sensors is influenced by multiple near-field properties, including the extinction properties of the sensor, the vibrations of targeting molecules, and the distribution of the electromagnetic field [Fig. 1(A)]. Specifically, the intrinsic coupling mode of the resonator is governed by the extinction properties of the metasurface, which are defined by the Poynting vector of the scattering field and the power loss density of the scatter^[22–24]. The UC and OC modes lead to the nonlinear amplification of the infrared signals represented as the EIT and EIA, respectively^[16,25]. Furthermore, the central frequency and bandwidth of the metasurface resonance need to match with the infrared vibration of the targeting molecule. The wide bandwidth of the OC metasurfaces has good detuning properties for achieving broadband SEIRAS^[15,21]. Finally, the light–material coupling efficiency is manifested as the spatial-temporal coincidence between the electromagnetic field of the sensor and the molecular vibration^[12,26]. When the molecules are overlapped with the localized electric field, the light–matter interaction exhibits an exponential enhancement, amplifying the infrared vibrations of the molecules^[27–29]. Therefore, the advantage of a modular design can independently modify the multiple sensing properties in the OC metasurfaces to meet the customized design requirements.

Herein, we proposed the modularized strategy in a hybrid plasmonic metasurface to independently adjust the sensor properties. The coupling mode, sensing bandwidth, and coupling efficiency could be independently modified by adjusting the vertical, horizontal, and hybrid dielectric layers of the metasurface. By combining analyses of the optical properties in both the near-field and far-field scenarios, we demonstrated how factors, such as the degree of OC, the frequency detuning, and the spatial overlap between the electromagnetic field and molecular, influence the sensing capabilities of OC metasurfaces.

The numerical simulations were performed using the commercial software COMSOL Multiphysics. The periodic structure was simplified as a two-dimensional optical structure to optimize computational efficiency. The periodic port was set at the top and bottom of the periodic unit, while perfectly matched layer boundary conditions were applied. The periodic boundary was set at each side of the unit. To ensure accurate calculation, a fine mesh size of 2 nm was used at the metal interface. The refractive index (RI) and extinction coefficient of the Si and Ag films in the mid-infrared were consistent with those in our previous study^[14]. The incident plane was perpendicular to the channel direction of the grating [Fig. 1(B)]. The incident light was vertical on the metasurface. When the incident angle was small ($\theta <15$ deg), the impact of the oblique incidence on signal amplification was limited (Fig. S1, Supplement 1).

The numerical molecular absorption is defined as a Drude–Lorentz dispersion model as follows: $${\epsilon }_r={\epsilon }_{\infty }+\sum _{i=1}^2\frac{f_i*w_p^2}{{\omega }_i^2-{\omega }^2+i\omega {\gamma }_i},$$(1)where ${\epsilon }_{\infty }$ is the isotropic high-frequency dielectric constant (set as 2.07), $w_p$ is the plasma frequency (set as 3.7673 × 10¹³ rad/s), ${\omega }_i$ is the central frequency of the absorption, ${\gamma }_i$ is the damping rate of the absorption ($60{\mathrm{cm}}^{-1}$), and $f_i$ is the oscillation strength of the absorption.

The extinction properties and absorptance were obtained by calculating the scattering field of the periodic units. The background was formed by calculating the light vertically incident on air, on a multilayered dielectric layer, and on metal reflective layers with the $x$-axis polarization. The Ag grating was added at the top of the dielectric layers. The above electromagnetic field without grating was used as the background electromagnetic field to calculate the scattering field of the grating. The calculated band was covered $3000–700{\mathrm{cm}}^{-1}$.

The scattering cross section, the absorption cross section, and the extinction cross section of the metasurface were calculated through the scattering electromagnetic field. The scattering cross section is defined as $$C_{\mathrm{sca}}=\frac{1}{I_0}\iint \mathit{n}·{\mathit{P}}_{\mathrm{sca}}\mathrm{d}S,$$(2)where $\mathit{n}$ is the normal vector pointing from the integration boundary to the outside, $I_0$ is the intensity of the incident plane wave, and ${\mathit{P}}_{\text{sca}}$ is the Poynting vector of the scattered electromagnetic field. The perform curve integration was on a closed surface of the scatterer, and the integral domain was a closed surface that wrapped around a three-layer structure. The absorption of cross section is defined as $$C_{\mathrm{abs}}=\frac{1}{I_0}\iiint Q_h\mathrm{d}V,$$(3)where $Q_h$ is the power loss density of the scatterer. Perform integration on the volume of the scatterer, and the extinction cross section is defined as $$C_{\mathrm{ext}}=C_{\mathrm{sca}}+C_{\mathrm{abs}}.$$(4)

The TCMT was used to describe the light–matter interaction in the plasma molecule coupling system^[13]. It was mainly used for analyzing and fitting the infrared absorption spectra of the resonantly coupled molecules and extracting the radiation damping ${\gamma }_e$, absorption damping ${\gamma }_o$, and center frequency of resonance ${\omega }_0$. The linear fitting function in the MATLAB software was used to fit the absorption spectrum, and the formula is as follows: $$A_0(\omega )=1-{|1-\frac{2*{\gamma }_e}{(\omega -{\omega }_0)*j+({\gamma }_e+{\gamma }_o)}|}^2.$$(5)When the resonator was coupled with a single absorption of a chemical bond vibration, the radiation damping ${\gamma }_e$, absorption damping ${\gamma }_o$, and central frequency of resonance ${\omega }_0$ remained unchanged. The fitting formula changes as follows: $$A_{\mathrm{cou}}(\omega )=1-{|1-\frac{2*({\gamma }_e+\mathrm{\Delta }{\gamma }_e)}{(\omega -{\omega }_0-\mathrm{\Delta }\omega )*j+({\gamma }_e+\mathrm{\Delta }{\gamma }_e+{\gamma }_o+\mathrm{\Delta }{\gamma }_o)}|}^2,$$(6)where $\mathrm{\Delta }\omega$ is the red shift of the resonance caused by refractive index variation. The variation of radiation damping $\mathrm{\Delta }{\gamma }_e$ and absorption damping $\mathrm{\Delta }{\gamma }_o$ were produced when the molecular absorption changed the radiation damping and absorption damping of the resonance. The enhanced signal of the SEIRAS is defined as $$\mathrm{\Delta }A(\omega )=A_{\mathrm{cou}}(\omega )-A_0(\omega ).$$(7)When the molecule was over-coupled with the resonator, the EIA effect would be observed with a positive $\mathrm{\Delta }A(\omega )$. The EIT effect would be observed with a negative $\mathrm{\Delta }A(\omega )$ when the molecule is UC with the resonator.

The metal-dielectric-metal (MDM)-based metasurface was widely studied as an ideal metasurface sensor for SEIRAS [Fig. 1(B)], due to its strong localized electric field and versatile impedance adjustment methods^[30–33]. The top metal structure was the silver grating defined with the grating width ($\mathit{w}$), period ($P$), and height = 45 nm. Since the silver film of the bottom was optically opaque ($H_{\mathrm{Ag}}>200\mathrm{nm}$), the relationship between absorbance $A$ and reflectance $R$ could be simplified as $A=1-R$. The middle dielectric layer ($H$) was replaced with the hybrid dielectric layers composed of a fixed ratio ($H_1:H_2:H_1$). The significant difference in the refractive index between the upper and middle layers provided additional opportunities for the modification of the optical field. Moreover, for the transparent dielectric layer in the mid-infrared, aluminum oxide (${\mathrm{Al}}_2{\mathrm{O}}_3$) and silicon (Si) were selected as the dielectric layers to reduce the impact of intrinsic losses on the sensor performance.

We designed four distinct sensors, incorporating the following hybrid dielectric layers: pure ${\mathrm{Al}}_2{\mathrm{O}}_3$, ${\mathrm{Al}}_2{\mathrm{O}}_3\text{-}\mathrm{Si}\text{-}{\mathrm{Al}}_2{\mathrm{O}}_3$ (ASA), $\mathrm{Si}\text{-}{\mathrm{Al}}_2{\mathrm{O}}_3\text{-}\mathrm{Si}$ (SAS), and pure Si, to reveal the modification of hybrid dielectrics on the resonance frequency and the electromagnetic field. The absorptance of the metasurfaces with both $X$- and $Y$-polarizations revealed that the sensors not only supported Fabry–Pérot resonances in both polarization states but also enabled the localized surface plasmon resonances exclusively in the $X$-polarization [Fig. 1(C)]. Our previous study indicated that the absorptance with the $X$-polarization exhibited considerable potential for SEIRAS, which was the primary focus of this paper^[14]. Notably, as the equivalent refractive index of the dielectric layer was increased, the surface plasmon resonances were red shifted from $1600$ to $700{\mathrm{cm}}^{-1}$. This phenomenon offered an additional mechanism for detuning, thereby enhancing the tunability of the sensor design.

Moreover, the hybrid dielectric layers could modularly control the electric intensity and Poynting vector of the fundamental resonance, which were displayed above the central wavenumber of each resonance, respectively [Fig. 1(C)]. The electric field of the pure dielectric layer (${\mathrm{Al}}_2{\mathrm{O}}_3$ and Si) metasurfaces was primarily confined to the lateral tips of the grating and the gap between the adjacent silver films. The anti-parallel currents were induced at the edge of the grating, generating the magnetic dipole. The energy flux was mainly confined in the intermediate layer. Only a small portion of the energy leaks at the edge of the grating, allowing for interaction with target molecules. When the dielectric layer was replaced with the ASA dielectric layer with the same thickness, the electric field intensity decreased in the middle Si layer. The Poynting vectors showed the energy in the Si film was repelled toward the grating and reflective layers, promoting the overlap between the strong electric fields and the surface molecules. Conversely, the metasurface with the SAS dielectric layer could concentrate the energy into the ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer, effectively acting like a fiber. It would reduce the coupling efficiency between the resonances and the molecules. The results indicated that the hybrid plasmonic metasurface could modify the spatiotemporal coupling efficiency between the molecules and the resonances, without generating additional interference on the resonance of the metasurface.

The parameter space of the sensor ($H$, $\mathit{w}$, $P$, $H_1$, $H_2$) had a synchronous effect on the spectral properties (Fig. S2, Supplement 1). Identifying geometric parameters that can linearly and independently control the coupling mode of the metasurface was crucial for the modularized design of the OC sensor. The absorptance of the metasurface was linearly fitted by TCMT to obtain ${\gamma }_e$, ${\gamma }_o$, and ${\omega }_0$. ${\gamma }_e$ and ${\gamma }_o$, along with the coupling mode of the resonance determined by ${\gamma }_e/{\gamma }_o$, were illustrated in Figs. 2(A)–2(D). ${\gamma }_e$ increased with the overall thickness of the dielectric layer $H$ [Fig. 2(A)], while ${\gamma }_o$ exhibited a decreasing trend. The resultant circulating currents between the silver films yielded a magnetic dipole response that was associated with a reduction in ${\gamma }_e$ and an increment in ${\gamma }_o$^[13,34]. ${\gamma }_e/{\gamma }_o$ was linearly increased with $H$, tuning the UC metasurface to the OC one. The OC metasurface could be achieved when the thickness of the dielectric layer exceeded 150 nm. $\mathit{w}$ could alter ${\gamma }_e$ to the maximum lever when $\mathit{w}$ was up to 900 nm [Fig. 2(B)]. The nonlinear modification on ${\gamma }_e/{\gamma }_o$ indicated that $w$ was not suitable for the modulation of the coupling mode. The period of the metasurface had a limited modification on ${\gamma }_e$ and ${\gamma }_e/{\gamma }_o$. Enlarging the period ($P$) could expand the gap between gratings, which was beneficial for manufacturing the structure [Fig. 2(C)]. Although the ASA and SAS hybrid dielectric layers had an opposing regulation on ${\gamma }_e$ and ${\gamma }_e/{\gamma }_o$, their underlying physical mechanisms remained consistent [Fig. 2(D)]. As the equivalent refractive index of the dielectric layer increased, the magnetic dipole responded with a reduction in ${\gamma }_e$ and ${\gamma }_o$^[34]. These results suggested that the vertical configuration could linearly adjust the coupling properties of the system. Therefore, $H$ was set as 450 nm to achieve the OC sensor.

The central frequency of the resonance was another crucial factor for the modularized design of the metasurfaces. The effect of the parameter space on the resonant frequency ${\omega }_0$ was demonstrated in Fig. 2(E). While $H$ and $P$ exhibit limited capabilities to detune ${\omega }_0$, $\mathit{w}$ (green line) could be linearly tuned on a large scale through the local plasma field at the edges of the grating, making it a primary method for spectral tuning. Additionally, the incorporation of the ASA and SAS dielectric layers provided a supplementary mean for fine tuning. These findings demonstrated that both the lateral and vertical configurations of the metasurface independently control the resonant frequency and coupling modes to achieve the modularized design.

The spectral resonances of the metasurfaces in the far field were analyzed using the universal model. Only ${\gamma }_e$, ${\gamma }_o$, and ${\omega }_0$ were used to reveal the resonance properties within a specific frequency band [Fig. 3(A), up]. This approach inevitably overlooked the frequency resolution of the resonance, especially in the broadband over-coupled resonance. According to previous research^[13], ${\gamma }_e$ and ${\gamma }_o$ were related to the scattering cross section $C_{\mathrm{sca}}$ and the absorption cross section $C_{\mathrm{abs}}$ of the metasurface, respectively. $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ corresponded to ${\gamma }_e/{\gamma }_o$, which was used to evaluate the potential coupling modes of the resonance. These extinction parameters in the near field provided a comprehensive frequency landscape of the metasurface [Fig. 3(A), down].

The parameters that influence the extinction properties were as follows: thickening the dielectric layer resulted in the reduction of $C_{\mathrm{abs}}$ and an increment of $C_{\mathrm{sca}}$ at the resonance [Fig. 3(B)]. The heatmap of $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ indicated that the resonator in the broadband mid-infrared was gradually turned to the OC one. In particular, the pixels around the center frequency of the resonance (black dotted line) indicated that the potential coupling mode was gradually changed from the UC to the OC. $C_{\mathrm{sca}}$ and $C_{\mathrm{abs}}$ were simultaneously increased and red-shifted with $\mathit{w}$ [Fig. 3(C)]. $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ indicated that the metasurface behaved as an OC resonator in the high-frequency band and as a UC resonator in the low-frequency band. $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ around the center frequency of the resonance was increased to the maximum when the grating width was 900 nm. As the grating width grew to 2700 nm, the metasurface functioned as an OC resonator throughout the entire mid-infrared range. $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ around the center frequency of the resonance had the same trend with the TCMT analysis, and the trend was consistent with the OC resonator designed by Paggi^[21]. The period had no effect on the extinction properties of the metasurface [Fig. 3(D)].

As the Si was wrapped by the ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer, $C_{\mathrm{sca}}$ and $C_{\mathrm{abs}}$ of the metasurface around the center frequency of the resonance were enhanced and blue-shifted in the ASA metasurface. The synchronized change in $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ could fine-tune the metasurfaces and transform the coupling mode toward an OC mode [Fig. 3(E)]. In contrast, as the ${\mathrm{Al}}_2{\mathrm{O}}_3$ was wrapped by the Si layer, $C_{\mathrm{sca}}$ and $C_{\mathrm{abs}}$ around the center frequency of the resonance were decreased and red-shifted in the SAS metasurfaces. The synchronized blue shift of $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ provided a dual means to fine-tune the metasurfaces and transform the coupling mode toward a UC mode [Fig. 3(F)]. These results were consistent with the spectral absorptance obtained through the TCMT fitting analysis and presented a better frequency resolution in a specific physical landscape.

The potential use of an OC metasurface for broadband SEIRA was quantitatively analyzed using a resonator with the following specifications: $H=450\mathrm{nm}$, $\mathit{w}=1500\mathrm{nm}$, $P=3000\mathrm{nm}$, and pure ${\mathrm{Al}}_2{\mathrm{O}}_3$. The numerical molecule layer was defined using a single Drude–Lorentz dispersion model, which was uniformly distributed above the metasurface with 50 nm. According to Lambert Beer’s law, the maximum absorptance of the molecular layer was calculated to be 0.0344 ($A_0$) (Fig. S3A, Supplement 1). The coupling absorptance with detuning was illustrated as the line shapes of the EIA, indicating the OC mode between the molecules and metasurface (Fig. S3B, Supplement 1). The increment of absorptance ($\mathrm{\Delta }A$) was calculated as the difference between the absorptance of the metasurface before and after interacting with the molecules. Additionally, the magnification of absorptance ($\mathrm{\Delta }A/A_0$) was plotted as the red line, illustrating a Lorentzian enhancement as a function of detuning. The sensor band, exhibiting a magnification greater than 10, ranged from $2000$ to $1000{\mathrm{cm}}^{-1}$, which encompasses the most vibrational bands associated with the chemical bonds [Fig. 4(A)].

The resonances before and after molecular coupling were treated as two independent resonators to assess the impact of molecular coupling on the sensor properties in both the far field and near field. The variation of spectral losses, $\mathrm{\Delta }{\gamma }_e$ and $\mathrm{\Delta }{\gamma }_o$, were considered as the additional disturbances on the original ${\gamma }_e$ and ${\gamma }_o$ obtained by fitting Eq. (6). At zero detuning, $\mathrm{\Delta }{\gamma }_e$ and $\mathrm{\Delta }{\gamma }_o$ exhibited distinct reductions and increments, respectively, which resulted in the attenuation in $\mathrm{\Delta }{\gamma }_e/{\gamma }_o$ (Fig. S3C, Supplement 1). This phenomenon facilitated the transformation of the OC resonances into the CC resonances at the molecular vibration frequency, ultimately leading to enhanced absorptance in the vibrational band of the molecules. However, with the TCMT analysis, it was difficult to accurately obtain $\mathrm{\Delta }{\gamma }_e$ and $\mathrm{\Delta }{\gamma }_o$ during the significant detuning.

The extinction properties, as a function of wavenumber, further illustrated the above phenomena. When the original $C_{\mathrm{sca}}$ was maintained at a high level, molecular coupling resulted in a decrease in $C_{\mathrm{sca}}$ [Fig. 4(B)]. Conversely, when the original $C_{\mathrm{sca}}$ was low, molecular coupling led to an increase in $C_{\mathrm{sca}}$. The variation of $C_{\mathrm{sca}}$ ($\mathrm{\Delta }{C}_{\mathrm{sca}}$) after coupling depended on the original $C_{\mathrm{sca}}$ (Fig. S3D, Supplement 1). Meanwhile, $C_{\mathrm{abs}}$ of the coupling system showed that a Lorentz-type enhancement appeared at the vibrational band of the molecule [Fig. 4(C)], indicating that the resonance had the potential to amplify the inherent loss of the molecules. The enhancement of $C_{\mathrm{sca}}$ ($\mathrm{\Delta }{C}_{\mathrm{sca}}$) showed that maximum attenuation occurred at zero detuning, and the $\mathrm{\Delta }{C}_{\mathrm{sca}}$ with detuning exhibited a similar trend to that of the metasurface resonance (Fig. S3E, Supplement 1). The amplified molecular losses were transmitted as spectral characteristics through the scattering properties of the metasurface, ultimately manifesting as the EIA in the absorptance spectrum. Figure 4(D) showed that $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ of the metasurface was decreased within a distinct molecular vibration band. The reduction would further transform the OC spectrum toward the CC one characterized with a larger absorptance. Moreover, the larger $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ of the metasurface corresponded to a more pronounced indentation in $\mathrm{\Delta }{C}_{\mathrm{sca}}/C_{\mathrm{abs}}$ (Fig. S3F, Supplement 1). These results provided a higher frequency resolution in radiation properties, and systematically demonstrated the physical mechanism of EIA on OC metasurfaces.

The OC level of the resonator was defined by ${\gamma }_e/{\gamma }_o$ and $C_{\mathrm{sca}}/C_{\mathrm{abs}}$. The metasurfaces ($\mathit{w}=1500\mathrm{nm}$, $P=3000\mathrm{nm}$) with different thicknesses of the ${\mathrm{Al}}_2{\mathrm{O}}_3$ layers (150, 210, 300, and 450 nm) were studied to investigate the influence of the OC levels on the signal amplification (Fig. S4, Supplement 1). The absorptance of the metasurface and the absorptance amplifications ($\mathrm{\Delta }A/A_0$) with the detuned signals are shown in Fig. 5(A), respectively. The absorptance amplification of the low-level OC metasurface ($1<{\gamma }_e/{\gamma }_o<6$, red and orange lines) could not achieve the maximum at zero detuning. The transition from the OC resonance toward the CC resonance was caused by the reduction of ${\gamma }_e$ and an increment of ${\gamma }_o$ during molecular coupling, which resulted in the reduction of magnification at zero detuning. As the metasurface was designed as a deeper OC resonator ($6<{\gamma }_e/{\gamma }_o<11$, green and blue lines), the amplification with detuning exhibited a Lorentzian distribution that aligned with the absorptance spectrum. When the level of coupling increased further (${\gamma }_e/{\gamma }_o>11$), the magnification at zero detuning reached the maximum. Notably, the deeper OC resonators offered a broader bandwidth for molecular sensing applications.

The influence of the OC level on the signal amplification at zero detuning was further illustrated through the extinction properties [Figs. 5(B)–5(E)]. It was observed that $C_{\mathrm{sca}}$ of the metasurface with varying OC levels exhibited the red shift after molecular coupling (blue line), which was attributed to the refractive index of the molecule layers^[35]. Additionally, the variation of the second derivative (SD) $C_{\mathrm{sca}}$ further indicated that the molecular vibration produced the same Lorentz-type decay in different OC-level metasurfaces, aligning with the observed intrinsic molecular vibrations (Fig. S6A, Supplement 1). Therefore, infrared transparent dielectric materials should be prioritized for the design of OC metasurfaces in order to achieve larger $C_{\mathrm{sca}}$^[36].

In contrast, the molecular coupling had a different effect on $C_{\mathrm{abs}}$ of the metasurface with varying OC levels (green line). $C_{\mathrm{abs}}$ of the low-level OC metasurface showed a hump-like enhancement consistent with its absorptance amplification [Figs. 5(B) and 5(C)]. As the metasurface trended to a deeper OC one, $C_{\mathrm{abs}}$ displayed a Lorentzian enhancement centered around the resonance central frequency [Figs. 5(D) and 5(E)]. The enhanced $C_{\mathrm{sca}}$ ($\mathrm{\Delta }{C}_{\mathrm{sca}}$) in the deep OC system had the same Lorentzian trend as the frequency of the molecular intrinsic vibrations, which was beneficial for extracting amplified signals from the resonant background (Fig. S6B, Supplement 1). Therefore, the coupling mode of the resonator and the enhanced $C_{\mathrm{abs}}$ would jointly determine the signal amplification.

The above factors would further determine $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ of the metasurface after coupling. The larger reduction of $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ at zero detuning could be observed in the deeper OC metasurface [Figs. 5(B)–5(E), red line]. The SD $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ of the metasurface further demonstrated that the molecules with the same volume produced the varying reduction of $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ in different OC level systems, leading to different absorptance amplifications in the far field (Fig. S6C, Supplement 1). Above all, the amplification of the absorptance at zero detuning was dependent on the larger $C_{\mathrm{sca}}$ of the sensor itself and the enhanced $C_{\mathrm{abs}}$ during the coupling process. The results indicated that the extinction properties provided a more accurate representation for investigating the physical mechanism of the SEIRAS compared to the spectral analysis.

To further visualize the impact of the coupling efficiency on signal amplification (Fig. S5, Supplement 1), the spatial overlap between the electric field and the molecules was modified through the hybrid dielectric layer. The total thickness of the dielectric layer was fixed at 450 nm to maintain the OC state. As the Si layer was wrapped by the ${\mathrm{Al}}_2{\mathrm{O}}_3$ within the ASA metasurface, ${\gamma }_e/{\gamma }_o$ of the spectrum gradually increased [Fig. 6(A)]. The signal amplification with spectral detuning demonstrated that the sensing band was expanded with ${\gamma }_e/{\gamma }_o$. Notably, the maximum amplification of the metasurface remained consistent at zero detuning, indicating that ${\gamma }_e/{\gamma }_o$ was not the sole factor determining the magnification of the absorptance.

Meanwhile, the extinction properties of the metasurfaces at zero detuning exhibited a blue shift as the ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer thickened [Figs. 6(B)–6(D)]. $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ was approximate to ${\gamma }_e/{\gamma }_o$, defining the OC level and sensing bandwidth of the metasurface. The observed decline in $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ was driven by the reduction of $C_{\mathrm{sca}}$ and the increment of $C_{\mathrm{abs}}$ during molecular coupling. Furthermore, the variation of SD $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ showed that a similar reduction of $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ was achieved as the Si layer was wrapped by the ${\mathrm{Al}}_2{\mathrm{O}}_3$, resulting in the equivalent level of signal amplification [Fig. 6(E)]. The maximum magnification obtained at zero detuning was not only related to the inherent properties of the sensor in the near field ($C_{\mathrm{sca}}/C_{\mathrm{abs}}$) and far field (${\gamma }_e/{\gamma }_o$) but also related to the coupling efficiency between the sensor and the molecule. As the sandwich silicon layer increased, the localized electric field energy in the hybrid dielectric layer was repelled toward the upper part of the metasurface and the reflective layer. The promoted spatial overlap between the molecules and the local field further improved the coupling efficiency, allowing the maximum magnification of the three sensors to remain at a consistent level.

As the Si film increased in the SAS dielectric layer, ${\gamma }_e/{\gamma }_o$ of the spectrum gradually decreased and red-shifted [Fig. 6(F)]. The amplified absorptance demonstrated that both the sensing bandwidth and the magnitude of the signal were diminished alongside the decreased ${\gamma }_e/{\gamma }_o$. Meanwhile, the extinction properties at zero detuning showed the red shift of $C_{\mathrm{sca}}$ and $C_{\mathrm{abs}}$ with the thicker Si film [Figs. 6(G)–6(I)]. The variation trend of $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ mirrored that of ${\gamma }_e/{\gamma }_o$, defining the change of mode and range in the OC metasurface. The variation SD of $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ at zero detuning showed that the decline of $C_{\mathrm{sca}}/C_{\mathrm{abs}}$ was decreased with the thickness of the Si layer [Fig. 6(J)]. The intensity of the electromagnetic field on the metasurfaces was gradually concentrated in the ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer and acted like an optical fiber. Consequently, the localized electromagnetic field further reduced the coupling efficiency between the sensors and the molecules, which ultimately led to a reduction in signal amplification.

Interestedly, although the ASA metasurface with the 150 nm ${\mathrm{Al}}_2{\mathrm{O}}_3$ film exhibited the same ${\gamma }_e/{\gamma }_o$ with the SAS metasurface with the 50 nm Si film, the signal amplification of the former was greater than that of the latter. This discrepancy can be attributed to the spatial overlap between the local optical field and the molecules, which was altered through the hybrid dielectric layer and subsequently impacted the signal amplification factor. Consequently, the ASA metasurface emerged as a promising OC resonator for SEIRAS when compared to the SAS metasurface. The results indicated that both the intrinsic coupling mode of the resonator and the coupling efficiency played crucial roles in determining the final signal amplification. Furthermore, the extinction properties of the metasurface with molecular coupling provided a clearer perspective on the amplification performance of the sensors compared to spectral analysis based on the TCMT.

In this paper, the modular-designed OC metasurface could independently modify the coupling state, the resonance frequency, and the coupling efficiency between electromagnetic fields and the sensing molecules by adjusting the vertical and horizontal structures and the hybrid dielectric layer of the metasurface, respectively. The underlying physical mechanisms of the modular design have been systematically demonstrated through the combined analysis of the near-field and far-field features of the metasurfaces. The combined analysis also demonstrated the physical landscape of the infrared signal amplification in the OC metasurface, and the potential of deep OC ASA metasurfaces was achieved for the broadband SEIRAS. These results systematically highlighted the advantages of the modular design in customized sensors and underscored the necessity of near-field analysis in the sensor design. This approach significantly reduces the complexity of the design process and paves the way for the application of machine learning techniques in auxiliary design for future studies.