Double-Band Metasurface Infrared Optics for Integrated Multichannel Spectral Sensors

Simultaneously identifying the composition of mixtures and determining their concentration levels has aroused much attention in recent years to meet the large demands of compact and low-cost infrared spectral sensors to replace their conventional bulky counterparts with costly separated infrared components. In this work, we propose a design method for multichannel metasurface infrared array optics covering two bands middle-wave infrared (MWIR) and long-wave infrared (LWIR) for construction of integrated infrared gas sensors. The use of metasurface microstructures with the same height realizes the effective filtering and focusing of electromagnetic waves at multiple wavelengths in both infrared bands, which is beneficial to decrease the manufacturing costs. Four-channel metasurface lens arrays and filter arrays with operating wavelengths of 3.5, 9.7, 10.6, and $3.9 \mu \text{m}$ corresponding to the infrared absorption peaks of three gases of ethanol, ammonia, ethylene, and one reference wavelength, respectively, were designed and produced by photolithographic patterning and dry etching processes on silicon substrates. The multichannel metasurface infrared optics have the advantages of easy fabrication and integration with wideband infrared detector array to form a compact infrared spectral detector, which meets the requirements of small volume and low cost to advanced infrared spectral sensors for applications in portable smart sensing systems and the Internet of Things.


I. INTRODUCTION
I NFRARED absorption spectrum detection technology is a powerful method to analyze the compositions of an object by using the interaction between the object to be measured and the incident electromagnetic waves [1], [2], [3]. When the  object is irradiated by infrared electromagnetic waves, certain frequencies of the electromagnetic waves will be absorbed by the bond vibrations in the molecules of the object, resulting in reduction of the intensity of electromagnetic waves in the corresponding spectral region [4], [5]. Thus, by measuring the absorption spectrum strength of the molecules of the object, an effective analysis of the compositions of the object can be realized. Because infrared absorption spectrum detection technology can quickly and nondestructively detect the compositions of object, it is widely used in environmental monitoring, food safety control, industrial production detection, air quality detection, component analysis, and other aspects [6], [7], [8].
For the infrared absorption spectrum detection system, the spectrum generating and infrared energy collection units are vital to achieve functions of effective filtering and detecting specific wavelengths with sufficient energy. One of the traditional detection systems is mainly composed of three separated optical components: a beam splitter (prism or grating) that promotes the separation of electromagnetic waves of different wavelengths, a focusing lens used to focus electromagnetic waves on the detector, and a detector that realizes the conversion of light intensity to an electrical signal [9], [10], [11]. Another common spectral detection system is based on filter wheel for several discrete wavelengths and a broadband single detector [3], [5]. However, these systems all have cumbersome separated optical components, inevitably leading to a complex spectral detection system with a relatively large volume and cost. Nowadays, compact spectral detection systems have been developed by combining electronically controlled tunable filters for near continuous wavelengths searching, while only one broadband single infrared detector each used [12], [13], [14]. However, the tunable spectral detectors heavily relied on the characteristic of tunable filters and are time-consuming for finishing the whole spectrum traversal.
An efficient approach to solve the above problems is to develop integrated chips to realize on-chip integrated spectral detectors. For the existing multielement, line-array, or 2-D array detector with broadband-sensitive pixels, a convenient way is to integrate diverse filter arrays, including graded filter arrays [15], 2-D filter arrays [16], and tunable filter arrays [17]. The traditional technique to realize high-Q narrowband filter array is a Fabry-Perot (FP) cavity resonator based on multibeam thin-film interference principle, and wavelength selection is realized by adjusting the length of the cavity to select the resonant wavelength [18]. However, due to the complex of fabrication of multiple-cavity lengths, it is a challenge to implement the FP filter array with various cavity lengths. Recently, the realization of bandpass or narrowband filter arrays based on the resonant effect of metal and dielectric micro-nano structures has received more and more attention. In the spectrum from near infrared (NIR) to long-wave infrared (LWIR) band or even THz band, the filtering characteristics based on guided mode resonance [19], extraordinary optical transmission (EOT) from metallic hole array [20], metamaterials [21], and metasurface structures [22] have been investigated. As for multiple-gas sensing applications, there have been a lot of reports on filter array design for operation on one or limited infrared band [13]. However, the implementation of narrow spectrum filter arrays covering medium-and LWIR dual-band range is still challenging.
On the other hand, there is always an area competition inside a pixel of a multichannel, linear array, or 2-D array detector, resulting in the fill factor of a pixel less than 100% [23], [24], [25]. In other words, the infrared radiation energy falling on each pixel after being collected by an objective lens cannot fully be received by the infrared-sensitive zone in a pixel, due to the fact that the infrared-sensitive zone cannot cover the whole pixel. In fact, for a bolometer-like thermal detector pixel, the thermal isolation leg occupies a certain planar area, while for a photon detector pixel, the metal electrode and the readout circuit occupy a portion of the area of pixel [26]. Moreover, there is a trend to use a smaller sensitive unit to achieve a shorter response time to increase the detection velocity [27]. Indeed, refractive or diffractive microlens array have been extensively used in various detector array with pixel sizes from a few to several tens micrometers to improve the fill factor regardless of the waveband [28], [29]. However, in a small-scale array detector with a relatively large pixel size of several hundred micrometers, it is a challenge to fabricate a matching microlens array and achieve an accurate integration [30]. This is because the traditional thermal reflow and etching transfer techniques for forming large diameter (hundreds of micrometers) refractive microlens with high fidelity are difficult to achieve, while the fabrication process of diffraction microlenses is complex and it is difficult to reach high optical efficiency.
The emergence of metasurface provides alternative solutions to broadband or multispectral flat optics [31], [32], [33], [34]. The metasurface structure realizes the effective control of the electromagnetic wavefront through the subwavelength microstructure array having resonance. However, existing metasurfaces are often used to realize the construction of metasurface lens arrays in a narrow wavelength range and the multichannel metasurface lens array with different central working wavelengths can be constructed by using different microstructure arrays [35], while wideband metasurface lens arrays often require the use of microstructure arrays of different heights [36], but these requirements will inevitably need more steps of fabrication processes and thus more costs in the production of wideband metasurface lens arrays [37].
In this work, we propose an integrated multichannel optical chip that can work at middle-wave infrared (MWIR) and longwave infrared (LWIR) bands. This chip consists of a metasurface microlens array and a metallic hole filter array on the top and bottom sides of a silicon substrate, respectively. The multichannel optical chip is designed to achieve two optical functions sequentially: one is for selecting several wavelengths with proper bandwidths and another is for controlling their wavefronts to concentrate radiation energies on the corresponding detector units. We first present a generic design principle of metasurface microlens and metallic hole array filter and then introduce some simulation and experimental results for a four-channel optical chip as an example to demonstrate the validity of this design method. The four-channel optical chip is for detection of three kinds of gases, i.e., ethanol, ammonia, and ethylene having absorption wavelengths of 3.5, 9.7, and 10.6 µm and a nonabsorption wavelength of 3.9 µm (as a reference wavelength). We also propose a scheme to integrate the optical chip with a detector chip having multiple sensitive elements to form an integrated multichannel infrared detector. This study is believed to promote the development of integrated infrared optics and multispectral infrared sensors for compact and low-cost applications.

A. Design Procedure
In order to realize the miniaturization and integration of the traditional gas detection optical system for multigas sensing application, we proposed a design scheme of an integrated infrared spectrum detector. Its goal is to simultaneously achieve effective detection of electromagnetic waves corresponding to the various infrared absorption peaks of the multiple gas species, such as three gases of ethanol, ammonia, and ethylene for example here. Based on the constructed multichannel metasurface lens array, we designed an integrated infrared spectrum detector having two kinds of chips, as shown in Fig. 1. This detector is mainly composed of three functional parts: a surface plasmon filter array and a multichannel metasurface lens array on the upper and lower sides of the optics chip, respectively, and an infrared detector array on the detection chip. For the designed integrated infrared spectral detector, it must first be able to precisely select the infrared absorption wavelength and reference wavelength via four filters for these three gases (ethanol 3.5 µm, ammonia 9.7 µm, and ethylene 10.6 µm) and the reference wavelength (3.9 µm). Second, the selected wavelength can be focused on the positions of the corresponding detection units, thereby effectively increasing the energy density of the electromagnetic wave absorbed by each wideband detection unit with a small sensitive area, thus increasing the spectral detection sensitivity. The small sensitive areas have the advantages of quicker response as well as low crosstalk among the sensitive units. In order to realize the required application functions, we carried out structural designs and simulation experiments on the surface plasmon filter array, the metasurface focusing array, and the infrared detector.
First, a metasurface array is designed by constructing a fixed-height microcylinders array of high refractive index dielectric materials in a low refractive index environment, and each microcylinder unit is a resonant structure with a low quality factor, which can be regarded as a truncated waveguide, which was described in detail in [22]. The microcylinder array forming the metasurface of the high-contrast medium has low resonance characteristics and can be used to construct a wideband multichannel metasurface lens array. The phase of the electromagnetic wave transmitted through the microcylinder can be expressed as Among them, ϕ 0 is the initial phase of the electromagnetic wave, h is the height of the miniature cylinder, n eff is the effective refractive index of the cylinder, and k 0 is the wavenumber of the electromagnetic wave under vacuum. The effective refractive index n eff is related to the refractive index n s of the microcylinder and the environmental material refractive index n i , so its range is limited to n i ≤ n eff ≤ n s . The effective refractive index n eff can be manipulated by adjusting the radius r of the cylinder. The phase difference of electromagnetic waves under different radii r is expressed as where n 1 eff and n 2 eff are the effective refractive index of cylindrical arrays with radii r 1 and r 2 , respectively. According to the limiting conditions of effective refractive index, the maximum phase difference of the outgoing electromagnetic wave is In order to achieve complete manipulation of electromagnetic waves, cylindrical structures with different radii must be able to achieve the range of 0-2π for electromagnetic wavefront phase control, which means that the height of the cylinder should be Since the working wavelengths of the multichannel metasurface lens array are set as 3.5, 9.7, 10.6, and 3.9 µm, covering MWLR and LWIR bands, the minimum height of the minicylinder should be determined according to the maximum wavelength 10.6 µm. When the microcylinder medium and the environment medium are silicon (n Si = 3.42) and air (n air = 1), respectively, the height of the microcylinder must satisfy h ≥ 4.38 µm. Also, the value is large compared to the wavelength of the electromagnetic wave required to operate in the mid-infrared band when λ = 3.5 µm, for which h ≥ 1.42 µm. In order to realize the fabrication of metasurface lens, which can adjust multiple wavelengths simultaneously on the same silicon chip, the height of the microcylinder for constructing the metasurface structure is set to h = 5 µm.
For LWIR waves (9.7 and 10.6 µm), the wavelengths are greater than the height of the microcylinder structure, so the microcylinder can choose the same height as the short wavelength. Fig. 2(a)-(e) shows the transmission and phase distribution diagram of the microcylindrical array in the wavelength range of 9-12, 3.5, and 3.9 µm when the height of the cylinder is h = 5.0 µm, the period is P = 6.0 µm, and the radius is r = 0.5-2.5 µm. It can be seen from Fig. 2(b) that there is a low transmission zone due to resonance, but when the wavelength is small, the low transmission zone covers a smaller range. Also, the microcylindrical array can achieve a larger range of phase manipulation for electromagnetic waves with smaller wavelengths. In the mid-infrared bands of 3.5 and 3.9 µm, the transmission diagram, phase diagram, and radius relationship diagram are shown in Fig. 2(c)-(e), respectively. For λ = 3.5 µm, the more appropriate period P = 3.55 µm is determined by the scanning program, and for λ = 3.9 µm, a more appropriate period is P = 4 µm. In Fig. 2(f) and (g), the transmission and phase change curves of the microcylinder at wavelengths of λ = 9.7 µm and λ = 10.6 µm under different cylindrical radius conditions are drawn. When the radius of the cylinder is r = 0.725-2.0 µm and r = 0.5-2.275 µm, the phase change corresponding to the wavelength λ = 9.7 µm and λ = 10.6 µm can all roughly cover 0-2π. In Fig. 2(h), the phase and transmission curves of the microcylindrical structure with period P = 3.55 µm, height h = 5.0 µm, and radius r = 0.7-0.93 µm at wavelength λ = 3.5 µm are shown and Fig. 2(i) presents the phase and transmission variation curves of the microcylindrical structure with period P = 4.0 µm, height h = 5.0 µm, and radius r = 0.5-1.5 µm at wavelength λ = 3.9 µm. The phase distribution on the xz plane required to realize the focusing function can be calculated by the following formula: where x and z represent the position parameters of the microcylinders, f represents the focal length size of the lens, and λ is the wavelength parameter corresponding to the lens. The focal length is set to f = 500 µm. For electromagnetic waves of different wavelengths, the phase distribution to achieve the same focusing function is different. After each lens size and corresponding focal length are established, the position of each microstructure is arranged according to (5) and the radius of each microcylinder is set one by one depending on the relationship between radius and phase.
A single lens of a multichannel metasurface lens is composed of multiple cylinders and substrates of different sizes, as shown in Fig. 3(a). These cylinders are made of silicon of the same height but with different radii and periods. As shown in Fig. 3(b), when λ = 10.6 µm, the phase distribution of a square aperture of 500 × 500 µm with different cylindrical radii on the xz plane can be calculated. At the same time, the xz plane phase distribution diagram of the corresponding outgoing electromagnetic wave at λ = 9.7 µm is shown in Fig. 3(c). The corresponding outgoing electromagnetic wave xz plane phase distribution diagrams of other wavelengths of 3.5 and 3.9 µm are shown in the Supporting Fig. 1.
We calculated the intensity distributions of the transmitted electromagnetic waves with wavelengths of 3.5, 3.9, 9.7, and 10.6 µm, as shown in Fig. 3(d). The x values shown in the horizontal axis are not the actual locations of lens array, but just for convenient comparison. The results show that after the electromagnetic wave enters the designed metasurface structure vertically, its phase is effectively controlled, and the metasurface lens achieves a better focusing function. By observing the focal position of electromagnetic waves of various wavelengths, it can be found that the focal length of the metasurface focusing lens is approximately the same, i.e., 500 µm. In order to further analyze the focusing performance of the designed metasurface lens, the normalized intensity distribution curve of the focal spot along the x-direction section is drawn, as shown in Fig. 3(e). The focal spot profiles at the focal plane corresponding to the wavelengths of 3.5, 3.9, 9.7, and 10.6 µm are all close to the diffraction limit, i.e., 3.9, 4.3, 10.3, and 11.3 µm, respectively, which are nearly the same as values calculated by full-width at half-maxima (FWHM) = 0.514 λ /NA, where numerical aperture NA = 0.45.
Second, an infrared filter array is designed based on the EOT of metal hole array, which originated in the excitation of surface plasmon resonances (SPRs) [34], [38], [39]. The SPRs are known for their ability to enhance light intensity in the visible light to NIR range. Nanostructured metal films have been widely used to build red, green, blue (RGB) color filters in the visible spectrum [40], [41]. Also, metal microhole arrays have also been proposed to enhance the transmission of mid-infrared light [42], [43] and even terahertz radiation [44], [45]. Transmission peaks at specific wavelengths can be achieved by properly designing the metal structure, providing a new approach to realize multispectral infrared detectors. Filtering based on EOT has the advantages of relatively simple structure, high integration ability, and adjustable filtering wavelength, so it has attracted extensive attention from researchers [32], [45] to achieve multicolor filters as well as infrared spectral filters. Compared with the design method of fitting multichannel filters by algorithm [20], [46], the fourchannel detector proposed in this article has a more accurate design structure and brief usage method.
The schematic of the designed metal hole-type surface plasmon filter is shown in Fig. 4(a), which is formed by constructing a periodic array of holes on the silver film on the surface of silicon substrate. The period of the metal hole is P s , the radius of the hole is r s , and the thickness of the metal film is h s . By adjusting the structural parameters of the metal holes, the effective manipulation of the SPR characteristics can be achieved. When the incident electromagnetic waves are perpendicular to the metal cavity structure, a surface plasmon wave is generated on the upper surface of the metal film and then transmitted to the lower surface of the metal film through the coupling effect of the metal cavity. When the structural parameters are set as hole period P s = 3.84 µm, hole radius r s = 1.0 µm, and metal film thickness h s = 1.1 µm, the transmission spectrum curves in the wavelength range of 3-15 µm are shown in Fig. 4(b)-(e). It can be seen that there are three abnormal transmission peaks near the wavelength of 3.9, 9.3, and 13.2 µm, indicating that the abnormal transmission phenomenon caused by surface plasmon in these three transmission peaks can realize the enhancement of transmission. Among them, the enhancement effect of electromagnetic wave transmission near 3.9 µm is the most significant (up to 0.79), while the enhancement effect of electromagnetic wave transmission near the other two wavelengths is relatively weak. This phenomenon may be caused by the fact that electromagnetic waves at 3.9 µm can be efficiently converted into surface plasmon waves and can be coupled to the lower surface of the metal through the hole structure to effectively enhance the transmitted electromagnetic waves. The wavelength of SPRs can be calculated as follows: where ε m and ε d are the permittivity of the metal film and the substrate, respectively, so ε m = ε Ag and ε d = ε Si ; integers i and j are the resonance orders. The wavelength of the SPR generated by the metal hole structure is mainly related to the period of the metal hole in addition to the dielectric constant of the metal film and the substrate. When the initial metal hole period is P s = 3.84 µm, the approximate wavelength value of the SPR under different mode orders can be calculated by the formula. According to calculations, when the mode order is i = 1 and j = 0 or i = 0 and j = 1, λ spp = 13.1 µm; when i = 1 and j = 1, λ spp = 9.3 µm; when i = 3 and j = 0 or i = 0 and j = 3, λ spp = 4.4 µm. These three values are very close to the center wavelengths of the three transmission peaks. Therefore, the transmission peaks with wavelengths of 13.2, 9.3, and 3.9 µm correspond to the modes of (1, 0) or (0, 1), (1, 1), and (3, 0) or (0, 3) orders, respectively. In order to facilitate the preparation of the device, the same height is selected for MWIR and LWIR simulation, and to reveal the influence of height of metal film on the filtering performance, we choose the thickness h s of the metal film to be 0.8, 1.1, and 1.4 µm as typical values for simulation. As shown in Fig. 4(b), when λ = 3.5 µm, the parameters are set to P s = 3.44 µm and r s = 0.9, and the variables for controlling h s are 0.8, 1.1, and 1.4 µm. Finally, as shown in Fig. 4(f), we have selected four most suitable parameter combinations, among which the transmissions of the required four wavelengths of electromagnetic waves are all over 0.7 and the FWHM of the transmission peaks are also as low as 0.1 µm, which satisfies the design requirements. In order to realize the detection of infrared radiation after being focused by the metasurface lens, we design an array of infrared detectors. As shown in Fig. 4(g), it is a schematic of a four-unit infrared detector, which is composed of two same units for the MWIR band and other two same units for the LWIR band. Thus, the detector can achieve four-channel effective detection of electromagnetic waves with wavelengths of 3.5, 3.9, 9.7, and 10.6 µm when the corresponding filters are employed. Since the spot size of the metasurface focusing lens is positively correlated with the infrared wavelengths, in order to achieve high-sensitivity energy detection, the sizes of the units are also designed to correspond to the focal spot size, in which a small size of 15 × 15 µm is for mid-wave infrared and a large size of 25 × 25 µm is for long-wave infrared. The principal diagram of the detector unit structure is shown in Fig. 4(h). This structure is mainly composed of two function layers: one is a photothermal conversion layer, which is a thinfilm multilayer of metal-dielectric-metal (MDM) composite and another is a thermoelectric conversion layer, including a temperature-sensitive resistive film VO x and two-terminal metal electrodes, and between these function layers is a silicon dioxide or silicon nitride insulating layer. As thermalsensitive detection units, they were all thermally isolated from the silicon substrate with micrometer-thickness SiO 2 layers or air cavities to decrease the thermal conductivity of the detector. When infrared radiation is incident on the detection unit, the photothermal conversion layer can effectively achieve high-efficiency absorption of infrared radiation at the designed wavelength band, which promotes the temperature increase in the layer. Due to the thermal diffusion effect, the temperature of the vanadium oxide material in the thermoelectric conversion layer increases subsequently. In this process, the resistivity of the vanadium oxide material will be converted from a high-resistance value to a low-resistance value, and an external readout circuit will eventually output the signal of the resistance change. To achieve high absorption of the multichannel detector in the two infrared bands, we designed two sets of structural parameters based on the FP cavity composed of the MDM composite. The thickness of the upper metal layer titanium h a1 and the bottom metal layer aluminum h a3 remain unchanged, i.e., 10 and 100 nm, respectively, while the thickness of the germanium dielectric layer is different, that is, 235 nm for the MWIR band and 820 nm for the LWIR band. This difference of Ge layer thickness is consistent with the resonance conditions of the FP cavity. As shown in Fig. 4(i), it is the absorption spectra of MDM three-layer film structure with two thicknesses of Ge layer for normal incident infrared radiation. When the thickness of the germanium dielectric layer is h a2 = 820 nm, an obvious absorption peak appears in the LWIR band, and the absorption efficiencies at the wavelengths of 9.7 and 10.6 µm reach 91.5% and 91.9%, respectively. When the germanium dielectric layer h a2 = 235 nm, an absorption peak appears in the MWIR band, and the absorption efficiencies at the wavelengths of 3.5 and 3.9 µm reach 84.8% and 87.8%, respectively.

B. Device Characterization
We have carried out experiments for fabrication of the designed multichannel metasurface infrared detector. The specific processing flowchart of multichannel metasurface lens is shown in the Supporting Fig. 2 since the structure of the multichannel metasurface lens has a minimum radius of 0.5 µm for the microcylindrical array, and a projection lithography machine was used to pattern the fine microstructures with higher yield than electron beam lithography.
In order to examine the light-gathering effect of the multichannel metasurface array, a measurement of LWIR image based on infrared thermal emission microscope (Optotherm IS640) was carried out and its schematic is shown in Fig. 5(a).
The metasurface lens array being tested was prepared for working at a wavelength of 10.6 µm and their SEM image is shown in Fig. 5(b) with an enlarged image shown in Fig. 5(c). It can be seen that the microcylinders of different radii are periodically arranged to form a metasurface structure. From the oblique view as shown in Supporting Fig. 3, it can be seen that the sidewall of each cylindrical structure has a high verticality, and their surfaces show good morphological characteristics. Also, the structure of each cylindrical structure is almost perfect, which can effectively realize the control of multiple infrared light. Fig. 5(d) shows the energy density distribution of the metasurface lens at the focal plane detected by the infrared imaging detector and strong focal spot appeared in the center of focal plane with low side lobs. This focusing phenomenon can also be clearly seen in the energy density distribution curve of the cross-focus section in Fig. 5(e). Compared with the simulation results, the FWHM of the normalized energy density distribution curve obtained by the experiment is about 100 µm, which shows obvious broadening. We analyze that the reason is that the light source used in the simulation is a single-wavelength plane wave that is incident normally, and its wavelength is consistent with the center working wavelength of the metasurface lens. In the experiment, the infrared wave radiated by the hot plate has a certain divergence angle, and the narrowband filter has a bandwidth of 190 nm, so the infrared wave incident on the metasurface lens is not strictly a vertical incident monochromatic wave, and this results in increased focal spot radius. Similarly, combined with projection ultraviolet lithography and electron beam evaporation coating process, we have carried out experimental preparation on the designed surface plasmon filter array. Fig. 6(a) shows the SEM image of the metal hole-based surface plasmon filter. The enlarged SEM image of the four sections of metal holes forming the filter array is shown in Fig. 6(b). The diameter of the metal holes of each structure basically matches the diameter obtained from the simulation, which verifies the accuracy of the manufacturing process. Fig. 6(c) and (d) plots the transmission spectra of the metal hole array in the MWIR and LWIR bands, respectively. It can be seen that there are two transmission peaks caused by SPR roughly appearing at each band, i.e., the center wavelengths of 3.52 and 3.69 µm at MWIR and 9.97 and 10.62 µm at LWIR. In the figure, the transmission peaks of metal holes with different structures obtained by simulation calculation are marked at the same time. It can be seen that the transmission peak positions achieved by the measurement and simulation are roughly the same. The slight deviation of the center wavelength is due to the fact that the infrared radiation generated by the infrared source used in the test process is not strictly normal incident plane wave, and it also leads to the broadening of the FWHM of the transmission peak obtained in the measurement. For the infrared detectors, we used ultraviolet lithography and film coating experimentally prepare the designed infrared detector array. Fig. 6(e) shows the overall microscopic metallographic image of the infrared detector array, in which the middle four-square zones are the functional zones of the detector, and the metal pad arrays are arranged on two lateral sides. With a microinfrared spectrometer (Nicolet iN10), we performed an absorption spectrum measurement of the thin-film absorption multilayer (titanium metal layer h a1 = 10 nm, germanium dielectric layer h a2 = 235 nm, and bottom metal layer h a3 = 100 nm) in the square zones, which is shown in Fig. 6(f). It can be seen from Fig. 6 that there is an obvious absorption peak in the wavelength range of 3-12 µm, with a center wavelength of 4.892 µm, which verifies the absorption function of the MDM three-layer film structure in a specific wavelength region. However, there is a large difference compared with the central absorption wavelengths obtained by the simulation. To analyze the reason, we think that it is mainly caused by the insufficient thickness of the Ge film during the preparation process. Therefore, it is necessary to precisely control the required film thickness to achieve high-efficiency absorption of the specific wavelengths.

III. DISCUSSION AND CONCLUSION
In order to construct a high-performance integrated infrared spectroscopy detection system, we propose a design scheme that can realize a multichannel metasurface lens array covering the medium and long-wave infrared bands. It is demonstrated that microcylindrical arrays with different radii can achieve effective control of electromagnetic wavefront over the two bands. By selecting appropriate parameters, metasurface structures with the same height are designed and fabricated to achieve focusing functions for all the infrared absorption peak wavelengths of ethanol, ammonia, and ethylene. At the same time, optical performance measurements were carried out on a metasurface lens with a center wavelength of 10.6 µm. Based on the constructed multichannel metasurface lens array, the design scheme of the integrated infrared spectrum detector is given. The integrated infrared spectrum detector consists of three functional parts: a surface plasmon filter array, a multichannel metasurface lens array, and an infrared detector array. Its goal is to simultaneously achieve effective detection of several gases, including ethanol, ammonia, and ethylene. In order to realize the required functions, the simulation design and experimental preparation of each component were carried out. The designed four groups of metal hole-type surface plasmon arrays with different structural parameters can effectively filter electromagnetic waves of the required wavelengths, and their transmissions exceed 70%. The infrared detector array composed of the photothermal conversion zone and the thermoelectric conversion zone can be achieved by controlling the thickness of the intermediate dielectric layer to effectively control the center wavelength of the FP cavity-type absorber. Preliminary preparations of the designed filter and detector array were carried out through the micro/nano processing technology combining photolithography and coating. Finally, it successfully solved the difficulties of narrowband filtering, wavefront manipulation, and high-efficiency absorption of infrared radiation in the medium-and long-wave infrared bands, laying a technical ground work for the development of integrated multicomponent infrared gas sensors.