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Abstract
Mid-infrared absorption spectroscopy is an effective method for detecting analyte fingerprints without labeling, but the inherent loss of metals in current methods is a main issue. Here, a sensing scheme was proposed that uses an all-dielectric grating metasurface and angular scanning of polarized light, and then it was verified by numerical simulation. The proposed fingerprint detection scheme could effectively couple a guided-mode resonance spectrum peak with the characteristic peak of the analyte’s phonon-polariton in the mid-infrared region, significantly enhancing the interaction between light and the analyte. The novel scheme would realize broadband enhancement to detect a variety of substances, and facilitate mid-infrared sensing and analysis of trace substances.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In biochemical detection, label-free optical detection technology has attracted research interests because it enables molecular concentrations to be measured without interference from fluorescent or other external markers [1,2]. By far, numerous biosensors based on surface plasmon resonance (SPR) have been studied. As one of the most common sensors, the SPR method reduces the complexity of the measurement device, but the sensors are not able to realize biochemical identification at the source. Such biochemical-specific optical detection is achieved by identifying biochemical molecules with absorption characteristics in the mid-infrared spectrum [3,4]. In particular, each molecule possesses its own vibration pattern of chemical bonds and corresponding extinction characteristics, which gives the material a unique absorption fingerprint [5,6].
Mid-infrared absorption spectroscopy is an effective method for detecting and identifying a variety of molecules through the different absorption spectra induced by various vibration modes [7–10]. It can detect an analyte’s fingerprint in a non-labeled way. However, due to the mismatch of scale between the wavelength of light (microns) and the size of biomolecules (nanometers), traditional infrared spectroscopy faces the challenge of low SNR (signal-to-noise ratio) when measuring a spot of biomolecules. This has prompted the development of surface-enhanced infrared absorption spectroscopy (SEIRAS), which applies the nanostructured surfaces to enhance biomolecular signals [11].
The use of plasmonic nanostructures based on SEIRAS demonstrates significant absorption enhancement in mid-IR molecular absorption spectra. However, the damping of plasma enhancement in precious metals can’t be ignored, and isolated narrow-band enhancement hinders broadband fingerprint detection [12–14]. To examine broadband absorption properties, some studies have focused on the dynamic tunability of graphene or phase-change materials [15–17]. However, the enhancement effect of these methods was not significant, so the detection data usually require further processing, such as finding the second derivative [18]. These methods also required special control of voltage or temperature because of the applied materials. These factors limited their use in practical situations. Recently, Leitis et al. reported on a promising molecular wideband SEIRAS dynamic tuning scheme that proposed an angular multiplexing-based elliptical dielectric resonator metasurface with a sawtooth array [19]. This work suggested that metasurface polarization and angle control owned great potential for broadband sensing of molecular fingerprints. However, the related models were indeed complex and the demands of manufactured instruments were excessive. Hence, it is still challenging to realize fingerprint identification of trace substances with wide spectra by a simple metasurface structure.
Here, we propose an easily fabricated all-dielectric grating structure, which optimizes lightmatter interactions through polarization and angle adjustment, coupling a spectral peak from guided-mode resonance (GMR) with the phonon pattern of biochemical molecules, thus enhancing the mid-IR fingerprint. This metasurface structural method was able to detect the different absorption characteristics of various interaction analytes, including three trace substances: B4C, the atomic compounds SiO2 and CaSO4. By extracting a series of reflectivity signals before and after coating from the analyte on the surface, the molecular absorption fingerprint was obtained. Finite-Difference Time-Domain (FDTD) method was used to conduct numerical simulation of the structure [20]. The local field on the analyte was well restricted and strengthened via employing polarized light and polarization angle processing. The potential for using mid-IR fingerprints to identify trace analytes and a novel approach for developing low-cost mid-infrared sensors was demonstrated in this study.
2. Design and methods
The design concept for the angle-multiplexer in this paper was shown in Fig. 1(a). The optical response was provided by a dielectric metasurface consisting of an array of LiCl grating periodically arranged on a calcium fluoride (CaF2) substrate, which interacted to produce resonance in reflection. The metasurface provides a monotonic relationship between the incident wavelength and incident angle. As shown in Fig. 1(b), the corresponding optical reflection around the specific resonant wavelength of each incident angle θ allowed each angular position to be uniquely associated with a specific incident wavelength over the spectral operating range. In addition, the metasurface designed in this paper provided angle-multiplexing capability and supportted an enhanced near-field electric field near the resonator, resulting in high surface sensitivity.
Fig. 1. (a) Schematic drawing of the angle-multiplexed scheme. (b) The corresponding angle-dependent reflectance spectra (m = 1). (c), (d) and (e) studied the reflection of structural parameters at different wavelength. (w/p>0.32).
The basic structure of the metamaterials was a rectangular cell with an LiCl strip grating on top of the CaF2 substrate. In this structure, all materials except the analyte layer were assumed to be light-lossless and non-magnetic (µ = µ0) [18,21,22]. The side view of the structure was shown on the right side of Fig. 1(a). The period of the structure was 4.9 µm, obtained according to Eq. (1). For further analysis, the effect of different structural parameters on the reflection performance of the structure was studied. As shown in Fig. 1(c), Fig. 1(d) and Fig. 1(e), the thickness of the substrate t1 was 1.78 µm, and the grating width w and depth t2 were 2.16 µm and 1.0 µm, respectively.
The reflections of the CaF2 substrate and LiCl grating were calculated, with a refractive index that varied with the wavelength (5-10 µm) when the incident angle was 0°, 10°, and 40°. The result was shown in Fig. 2(a) and Fig. 2(b). The change in refractive index would cause a red shift in the position of the resonant peak and a slight decrease on full width at half maximum (FWHM), but the value of the resonant peak remained a fixed value (R = 1). This result showed that the average refractive index of CaF2 and LiCl in the 5.5-10 µm band could be included into the calculation [23]. In the mid-IR range, the refractive index of LiCl and CaF2 were 1.56 and 1.35, respectively [24].
Fig. 2. (a) and (b) describe the relationship between the refractive index and wavelength of the CaF2 substrate and LiCl grating at different incident angles. (c) and (d) describe reflectance as a function of the free space wavelength λ and incident angle θ respectively (P = 4.9 µm and P = 5.7 µm). (e) The theoretical and simulation results of GMR wavelength varying with θ. (f) The reflection before and after the metasurface is coated with the analyte.
To analyze all the spectra results and field distributions when the metasurface was covered with a layer of analyte (8 nm) [25], the FDTD Solution was used to carry out a series of two-dimensional optical simulations based on the frequency-domain finite element method. In the simulation process, the X direction was set as periodic boundary conditions, and the Y direction was set as perfectly matched layer (PML) boundary conditions. In the detection process, there was a large mismatch between the thickness of the analyte and the size of the metasurface, so we hope that the incident wave could be fully located in the electric field of the ultrathin layer at any incident angle. Based on this consideration, the incidence of a transverse electric wave was more suitable to provide a non-zero electric field component and stimulating metasurface resonance, so the interaction between light and matter is would be as strong as possible [26,27]. Over a large wavelength range (6.5-10.4 µm), the monotonic linear increase in the maximum reflectivity wavelength as the incident angle θ increases from 5° to 45° was more clearly observed from the reflection spectrum shown in Fig. 2(c). This optical phenomenon could be explained by the theory of GMR [28,29], in which a light beam enters a grating waveguide at an angle and resonates at a particular wavelength. In this resonant mode, light could be captured and focused in the grating waveguide. For the incident wave to be coupled to the guided mode, the phase-matching condition should be satisfied; in the absence of a substrate, the real part of the propagation constant of the waveguide mode was determined using the following equation [29]:
where, K0 is the wave number in free space, n is the refractive index of the incident medium (in this case, the refractive index of air), m is the order of the diffraction wave, λR is the resonance wavelength, and P is the period of the grating structure. When m = 1, Eq. (1) represents the basic mode of the resonance. The order of diffraction changed the reflection angle in Fig. 2(c) within 3-6.5 µm band. There was overlap between the and covered a smaller wavelength range as a linear angular variation. The low-level mode possessed advantages over the high-level, which was not as good as the basic. The GMR wavelength (m = 1) as a function of the incident angle was shown in Fig. 2(e). The calculation based on Eq. (1) was consistent with the finite element simulation results.The proposed metasurface also implemented wideband sensing, and the detectable wideband range could be adjusted by changing the period P of the structure. The detectable wavelength ranges of θ from 0° to 65° at P = 4.9 µm and 5.7 µm were presented in Fig. 2(c) and Fig. 2(d), respectively, showing that the metasurface structure for different sensing applications could be designed and optimized. In addition, the proposed structure was simple and easy to manufacture. The CaF2 substrate could be realized by the deposition method, and the grating was made by the mask method after the LiCl layer was sputtered [30]. After the fabrication, the periodic structure couldn’t be changed, but it was able to be adjusted by angle control further.
During detection, as shown in Fig. 1(a), a layer of analytes was coated on the metasurface. Each analyte owned its unique phonon mode (fingerprint), and its complex refractive index could be expressed by the following formula:
where n is the refractive index, k is the extinction coefficient of the analyte, and for losses it is only k that is important. However, the size mismatch between the analyte and the light wavelength was large, leading the analyte to emit a weak spectral signal. The proposed metasurface could enhance the interaction between light and matter by optimizing the incident angle, so that the resonance peak by the structure was coupled with the phonon peak of the analyte, resulting in significant attenuation of the resonance line shape generated by the metasurface coated with the analyte (Fig. 2(f)). Thus, the sample on the sensor surface could be detected by measuring the change in light intensity induced by the analyte at each incident angle. The scanning angle would be adjusted according to the fingerprint spectrum range of the analyte to be detected. By combining the signal measurement from all angles, the absorption spectrum of the analyte could be recovered, enabling fingerprint identification of the analyte to be completed.3. Results and discussion
3.1 Detection of B4C
In this study, the sensing performance for B4C (boron carbide) was evaluated firstly. It was one of the three hardest materials (the others two are diamond and cubic boron nitride) and used in tank armor as well as many industrial applications. B4C also absorbed large amounts of neutrons without forming any radioactive isotopes, making it an ideal neutron absorber in nuclear power plants. Because B4C was easy manufactured with low cost, it has replaced expensive diamonds in some applications. Nondestructive mid-infrared fingerprint detection was a quick and convenient method to identify this material. The thickness of B4C was set at 8 nm in the detection process; analyte thickness can be changed according to the actual situation. Juan I. Larruquert et al. used an ellipsometer to measure the optical constants of the ion-beam sputtered B4C films in the range of 190-950 nm [31]. Based on the reststrahlen band of B4C [32], the Kramers–Kronig (KK) analysis dispersion relationship was used to calculate the refractive index n of B4C across the whole spectrum [33]:
Shiles et al. suggested that a good ζ value was within ±0.005 [35]. An evaluation parameter ζ = 9*10−4 was obtained with the n data calculated in this research. Therefore, inertia and rule tests work well within the above maximum range. Figure 3(a) presented the B4C phonon mode (the correspondence between extinction coefficient and wavelength of B4C) plotted by the above method. Figure 3(a) implied the obvious optical loss in the wavelength range from 8.7 to 9.5 µm [31], which denoted the optical phonon mode of B4C.
Fig. 3. (a) Complex refractive index of B4C. (b) Angle-dependent reflectance spectra for B4C on the metasurface. (c) and (d) respectively show the distribution of electric field and magnetic field at different incident angles at 9.09 µm wavelength on the periodically arranged LiCl grating array. (e) and (f) respectively show the distribution of electric field and magnetic field at different incident angles at 9.09 µm wavelength on the periodically arranged Ge grating array.
Next, the fingerprint detection scheme by coating the B4C analyte on the metasurface was demonstrated, as shown in Fig. 1(a). Previous work has proved the feasibility of this coating [36]. The imaginary part of the complex refractive index of B4C in Fig. 3(a) indicated a significant optical loss in the wavelength range of 8.7-9.5 µm [31], which represented the optical phonon mode of B4C. When the light source was incident at different angles, different GMR wavelengths led to different mid-infrared absorption spectra, and these spectral peaks reflected the differences in the extinction coefficient values of B4C. For example (Fig. 3(b)), when λ = 9.09 µm, the reflection peak θ = 37° corresponded to the maximum value of the extinction coefficient, and while at θ = 31°, the reflection peak increased because the resonance wavelength was far from the peak of the extinction coefficient. Compared with Fig. 1(b), the reduced reflection peak in Fig. 3(b) was caused by the extinction ability of B4C.
To provide further physical insights, the magnetic and electric field distributions at different incident angles at 9.09 µm wavelength was summarized in Fig. 3(c) and Fig. 3(d), respectively. When the incident angle gradually changed from 31° to 41°, the magnetic field θ = 37° was significantly enhanced by the GMR at this wavelength. The magnetic field was mainly confined to the LiCl grating and generated a strong electric field along its surface. Since germanium offers opportunities for improving the performance of dielectric sensors due to its higher refractive index and lower absorption losses in the mid-IR [18], we replaced the original grating material with germanium for comparison. The magnetic and electric field distributions of 9.09 µm wavelength at different incident angles were shown in Fig. 3(e) and Fig. 3(f). It was found that the use of Ge gratings couldn’t produce strong local phenomena of magnetic and electric fields at an incident angle of 37°. This effective intensity focused on the interaction between the incident light and B4C, which could be further explained by calculating the light absorption. In B4C, optical absorption was determined by the following equation [25,37]:
After establishing the fingerprint detection scheme, a comprehensive evaluation of the mid-IR fingerprint sensing of B4C was conducted. During detection, a series of reflective spectra (one for each angle) were obtained by scanning 23°−52° (in one-degree steps), as shown in Fig. 4(a). By linking all the peaks of the reflective spectra, an envelope curve (red) with a peak near 9.09 µm was obtained, which clearly represented the optical phonon mode of B4C. To compare the absorption spectra of B4C on a planar membrane structure, the absorption spectrum envelope curve was presented in Fig. 4(b). The blue curve in Fig. 4(b) represented ‘unpatterened’, which described the absorption spectrum of the analyte B4C after the analyte was directly coated on the substrate. The results showed that when the wavelength was 9.09 µm, the intensity of the absorption increased from 0.035 to 0.166. To simulate an actual situation, the same method was used to draw a series of reflection spectra corresponding to 80 nm B4C, as shown in Fig. 4(c), with a scanning angle of 8°−52°. The absorption spectrum envelope curve drawn was compared with that of 8 nm B4C, as shown in Fig. 4(d). When the wavelength was 9.09 µm, the intensity of the absorption increased from 0.166 to 0.636. Therefore, our proposed method resulted in a considerable improvement in SNR ($SNR\; = \; 10\textrm{lg}\frac{A}{\textrm{A}_{0}}$, A and A0 represented the absorption of metasurface and unpatterened surface coated with analyte, respectively), which was significant in practical measurements.
Fig. 4. (a) A series of reflectance spectra obtained by angular scanning from 23°−52°, and the corresponding envelope curve for 8 nm B4C coated on the metasurface. (b) The absorption (A = 1 - R) envelope of 8 nm B4C coated on Au surface, CaF2 substrate and grating structure was studied. (c) A series of reflectance spectra obtained by angular scanning from 8°−52°, and the corresponding envelope curve for 80 nm B4C coated on the metasurface. (d) Absorption envelope curves for 8 nm and 80 nm B4C.
In order to quantify the absorption characteristics of B4C, the absorption coefficient A(θ) was calculated for the peak reflection amplitudes R0 and RA at each incident angle before and after coating the analyte. It was expressed as:
For comparison, a standard IR reflection absorption spectroscopy (IRRAS) measurement was to coat the analyte on the gold surface, and the coating layer used the same parameters as the previous experiment [18]. The existence and spectral position of the B4C vibration band was consistent with the angle multiplexing method and IRRAS measurement results. In addition, because the resonance peak generated by the structure was coupled with the B4C phonon peak, the metasurface provided a much higher absorption signal of more than 21 times.
3.2 Detection of SiO2
To clarify the fingerprint detection scheme, the atomic compound SiO2 was detected, which was a widely used film in semiconductor device manufacturing. The fingerprint spectra of SiO2 and B4C are obviously different, so detection of the SiO2 (8 nm) fingerprint would provide a key method for identifying these two materials.
As shown in Fig. 5(a), the extinction coefficient of SiO2 reached its maximum value at a wavelength of 9.17 µm [38]. In contrast, the envelope curve of the reflectivity spectrum obtained from an incident angle scan from 20°−63° (Fig. 5(b)) generated a dip angle at the same wavelength, which corresponded to the vibration mode of the material microstructure of SiO2. The envelope spectral signal was also much stronger than the unpatterened absorption spectrum, with a peak value of 0.403 at a wavelength of 9.17 µm, which was about 9.363 times that of the unpatterened (Fig. 5(c)). The IRRAS of SiO2 was calculated in the same way and our metasurface provided a high absorption signal over 60 times. These results indicated that trace SiO2 materials could be identified by non-destructive methods using enhanced fingerprint signals.
Fig. 5. (a) Complex refractive index of SiO2. (b) A series of reflectance spectra obtained by angular scanning from 20°−63°, and the corresponding envelope curve for 8 nm SiO2 coated on the metasurface. (c) The absorption (A=1-R) envelope of SiO2 coated on Au surface, CaF2 substrate and grating structure was studied.
3.3 Detection of CaSO4
The proposed method was proved to identify B4C and atomic compounds, and it was also feasible to detect other analytes. In the final test, the molecular fingerprints of CaSO4 was evaluated, which was a food additive widely used as a hygroscopic agent or medicine. The complex refractive index of CaSO4 was shown in Fig. 6(a). Its extinction coefficient indicateed an obvious characteristic peak at a wavelength of 8.65 µm [39]. When set the thickness of CaSO4 was set at 8 nm, the electric field was effectively located and enhanced by the GMR effect. As shown in Fig. 6(b), the envelope curve could be obtained from the reflection spectrum scanned at an angle from 23°−50°. The reflectivity envelope showed a fingerprint characteristic with a spectral drop of 8.65µm. In Fig. 6(c), the absorption envelope with the unpatterened absorption spectrum was compared. The results showed that the absorption of CaSO4 molecules was lower than 5% in the wavelength range of 7.5-10 µm, and there was no obvious peak characteristic indicating the main vibration band of CaSO4 molecules. In contrast, the extracted absorption greatly enhanced the fingerprint’s characteristics: the peak of the fingerprint extracted at 8.65 µm was magnified to 0.656, nearly 21 times larger than that of the unpatterened mode. In addition, our metasurface provided the absorption signal higher than 123 times. The absorption signals were respectively 50 times and 60 times larger than that reported by Hatice Altug et al. [18,40]. This confirmed that the scheme could enhance a weak fingerprint in a wide band and be used for analyte analysis.
Fig. 6. (a) Complex refractive index of CaSO4. (b) A series of reflectance spectra obtained by angular scanning from 23°−50°, and the corresponding envelope curve for 8 nm CaSO4 coated on the metasurface. (c) The absorption (A=1-R) envelope of CaSO4 coated on Au surface, CaF2 substrate and grating structure was studied.
4. Conclusions
In conclusion, a fingerprint detection method was proposed based on the coupling of GMR spectrum peaks and phonon-polariton in the mid-IR region. The method provided effective near-field electromagnetic enhancement by controlling the incident angle of light, correlating the reflectivity signal of each incident angle with the absorption intensity of the corresponding molecular resonance. Using GMR, the fingerprint of an analyte could be extracted by measuring direct angular scanning reflectivity. Compared with other angle-scanning techniques (SPR), our method was more sensitive and easier to prepare, and it also possessed good biological specificity. This approach enabled potential opportunities for unlabeled microbiosensors as well as the possibility of novel applications for thin-film materials.
Funding
Natural Science Foundation of Fujian Province (2020J01712); Xiamen Marine and Fishery Development Special Fund (20CZB014HJ03); Innovation Fund for Young Scientists of Xiamen under Grant (3502Z20206021); Youth Talent Support Program of Jimei University (ZR2019002); the Second Youth Talent Support Program of Fujian Province (Eyas Plan of Fujian Province 2021); Science Fund for Distinguished Young Scholars of Fujian Province (2020J06025); the Fujian Provincial Department of Science and Technology (2019H0022).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
No data were generated or analyzed in the present research.
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