Broadband surface enhanced infrared absorption with multiple Fano resonance by metallic oblique-wire-bundle metamaterial absorbers

doi:10.21203/rs.3.rs-4935987/v1

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Broadband surface enhanced infrared absorption with multiple Fano resonance by metallic oblique-wire-bundle metamaterial absorbers

https://doi.org/10.21203/rs.3.rs-4935987/v1

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published 09 Nov, 2024

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Higher sensitivity with specific recognition of a sensor could ease the burden of sample purification or labelling procedure for specific testing and detection and there appear two methods including surface enhanced infrared absorption (SEIRA) and surface enhanced Raman scattering (SERS), promising better sensitivity and specificity, simultaneously, via detection of molecular footprints. Furthermore, researchers employ Fano resonance to further boost the detection limit of SEIRA by coupling between the two absorption bands from molecules and metamaterials. Still, the current metamaterial absorbers are almost narrow band and required specific design, only suitable for limited chemicals. Thus, in this work, we would like to design a broadband oblique-wire-bundle (OWB) metamaterial absorbers (MA) which could interact with multiple functional groups’ absorption from a sample, thus easing the burden of custom-made resonators. In experiments, indeed, our designed OWB MA developed four Fano resonance responses with three PMMAs’ functional groups and one function group from carbon dioxide. The counterpart planar MA also performed SEIRA yet without occurrence of Fano resonance as a comparison. We believe this proposed OWB MA could facilitate the development of rapid detection in the field of food safety and chemical detection.

Physical sciences/Optics and photonics/Applied optics/Optical sensors

Physical sciences/Physics/Optical physics/Nanophotonics and plasmonics

To detect ingredients from an unknown sample with higher sensitivity and specificity is crucial especially regarding food safety and chemical detection. To achieve such a goal, a detection method without labelling procedure should be developed. Till now, infrared absorption and Raman scattering provide footprints of compositions of a sample and are both applied in so many different fields such as bio-, chemical- and gas-sensors. Besides, signals from both the two methods could be further enhanced through surface modification, also known as surface enhanced infrared absorption (SEIRA) ^1–3 and surface enhanced Raman scattering (SERS)^4–6. The advance of SEIRA and SERS is quick with the development of different nanostructures⁷, integration with other materials, e.g., graphene/ graphene oxide, iron oxide, etc. ^8–13, different fabrication procedure for stronger hot spots^14–16, field enhancement from surface plasmon polariton ^17,18 and excitation of Fano resonance ^3,19. Among these different methods, one promising kind of SEIRA is to couple a narrow absorption band from molecules’ vibration mode to a broadband absorption often introduced by metamaterial for excitation of Fano resonance that revealed high quality resonance, extended interaction time due to slow group velocity and spectrum reversal for better signal to noise ratio. To test the performance of Fano resonance based SEIRA substrates, several works have been published, for example, directly printed spiky gold flakes for the coupling between localized surface plasmon absorption and function group’s absorption from protein–bovine serum albumin with absorbance difference of 8%¹⁶, double Fano resonance based on complementary plasmonic metamaterials and PMMA with a signal strength D_R equal to ~ 0.3²⁰, and also an electrically tunable graphene metal hybrid sensor with electromagnetic introduced transparency like behavior, demonstrating a multiple functional groups’ sensing ability⁹.

Still, the metamaterial introduced in the above-mentioned methods are narrow bands and only suitable for pre-determined molecular absorption bands, thus limiting their practical use in many different fields. Also, once the absorption peak frequency deviates the functional groups’ absorption frequency, the sensing abilities of the published sensor were compromised. To tackle this issue, researchers employed strategies such as multiple band metamaterial design², active tunable metamaterials²¹ and metal and thickness-dependent metamaterials²² to enrich the available detection frequency range. However, these methods are insufficient due to their drawbacks such as still limited detection frequency range, smaller hot spot area, difficult fabrication process and weaker field enhancement. Therefore, in this study, we would like to design an oblique-wire-bundle (OWB) metamaterial absorber accomplished by oblique deposition for their broad absorption band and strong hot spot with larger hot spot area.

Based on our previous work^23,24, we designed an oblique wire bundle (OWB) metamaterial absorber on a silicon substrate with a parallelogram periodicity of 2$\:\overrightarrow{x}$ and 0.5$\:\overrightarrow{x}$+$\:\overrightarrow{y}$ where |$\:\overrightarrow{x}$| and |$\:\overrightarrow{y}$| equal to 900 an and 970 nm, respectively. On top of the Si substrate, a 150-nm-thick aluminum and a seed layer of MgF₂ with a base width of 550 nm, a base length of 740 nm and a thickness of 100 nm are deposited on top of the substrate by sophisticated E-beam lithography and E-gun evaporator. Note that in experiments, the MgF₂ revealed a shape of the frustum of a rectangular pyramid due to the undercut of the resist in the lithography procedure. Then, the sample was put on a stage in an e-gun evaporator with a tilted angle of 86-degree which promised a coverage length of 1430 nm²⁵, ensuring the growth of metallic nanowires only on top of the seed layer during deposition. Here, to make a comparison, a planar metamaterial absorber with the same periodicity and dimension is fabricated. Note that the thickness of metal in the planar metamaterial absorber is reduced to 50 nm instead of 460 nm, detected thickness of aluminum when fabricating the OWB metamaterial absorber, for better yielding of the lithography process. Entire fabrication procedure of the OWB and planar metamaterial absorbers is detailed in Methods. It is worth mentioning that since the OWB metamaterial absorber possessed a broadband absorption spectrum, we expect that the OWB metamaterial absorber could reveal strong Fano resonance without a specific design in advance. Here, to make a comparison, Fig. 1 depicts the absorbance spectrum of the OWB and planar metamaterial absorber under both x- and y-polarization incidences from µ-FTIR. The OWB metamaterial absorber possessed a broad absorption band from 100 to 180 THz under x-polarization and an even wider bandwidth 111 THz (69 to 180 THz) under y-polarization incidence. On the contrary, the planar metamaterial revealed an absorption peak of 0.72 with a bandwidth of 15.6 THz under x-polarization and absorption peak of 0.6 with a bandwidth of 15.65 THz under y-polarization. It is worth pointing out that the small peak/dip shown at the frequency of 70.5 THz in the spectra of the OWB and planar metamaterial absorbers could be attributed to the stretching vibration mode of C = O from CO₂. Here, it is an absorption peak in the spectrum of the OWB metamaterial absorber but a dip for the planar absorber, which might stem from the CO₂ concentration variation for the background and device measurement. Still, due to larger hot spot areas and stronger field enhancement, the OWB metamaterial absorber enhanced the vibration mode from CO₂, thus resulting in a stronger absorption compared to the planar metamaterial absorber.

To further explore the difference between the OWB and planar metamaterial absorbers, we conducted simulation based on finite integration method with a boundary condition of unit cell along the x- and y-directions and open boundary conditions along the z-direction. Since the MgF₂ is in the shape of frustum, in simulation, its dimensions are modified with a base width and length of 550 and 740 nm and a width and a length of 492.7 and 652.7 nm for the top face. Multiple nanowires with randomly distributed positions, widths, lengths and thicknesses were generated on top of the seed layer. Indicated by the blue curves of Fig. 1, both OWB and planar metamaterial absorbers revealed similar absorption bands for both x- and y-polarization between the measured and simulated results. Moreover, from simulation, the absorption bands for x- and y-polarization incidence range start from 85 and 67 THz, respectively, and extend to 384 THz, the upper band edge of the near infrared regime; therefore, our proposed OWB metamaterial absorbers revealed bandwidths of 299 and 317 THz under the x- and y-incidences, respectively, which cover almost all the vibrational modes of chemicals for both polarizations. It is worth mentioning that we expected the OWB metamaterial would reveal stronger absorption and larger bandwidth under y-polarization incidence due to various optical paths below the metallic wire bundles. Thus, the OWB metamaterial absorber is free from re-design procedure for the detection of most chemicals. In contrast, the planar metamaterial absorber only possessed limited absorption bandwidth, thus limiting its coupling with molecular vibration modes.

To further characterize the OWB metamaterial absorber and its ability to molecule-plasmon coupled SEIRA, we spin-coated a layer of A5 PMMA on the absorbers with a rotation speed of 1000 rpm for 10 s and then 4000 rpm for 60 s. The expected thickness is around 380 to 460 nm. Here, PMMA possessed three molecular absorption modes within the targeted frequency range, including C = O stretching mode at 51.9 THz, CH₂ asymmetric stretching mode at 88.5 THz and CH₃ stretching mode at 90 THz. Meanwhile, the stretching vibration mode of C = O from CO₂ also existed. As portrayed in Fig. 2, all the four molecular vibration modes could interact with the OWB metamaterial absorber and revealed strong Fano resonance under both x- and y-polarization. Note that when PMMA was applied onto the metamaterial absorbers, the resonance frequencies of the OWB metamaterial absorber down shifted to the lower range, facilitating a stronger coupling between stretching vibrational mode of O = C = O from CO₂, indicated by the excitation of the Fano resonance. In contrast, it is difficult to observe the asymmetric line shapes from Fano resonance for the planar metamaterial absorber; instead, the superposition of absorption is observed with larger absorption values at the molecular vibrational frequencies in the spectrum.

Here, to elucidate the interaction between the metamaterial absorbers and chemicals, we conducted simulation of the OWB and planar metamaterial absorbers with PMMA and CO₂ as well. A 460-nm-thick analyte was applied onto the absorber with a dielectric constant predicted by Lorentz model listed below,

$$\:{\epsilon\:}={\epsilon\:}_{{b}_{Analytes}}+\sum\:_{j=1}^{4}\frac{{f}_{m}{\omega\:}_{0,j}^{2}}{{\omega\:}_{0,j}^{2}-{\omega\:}^{2}-i{\gamma\:}_{j}\omega\:}\cdots\:\cdots\:\cdots\:\cdots\:\cdots\:\cdots\:\cdots\:\cdots\:\cdots\:\cdots\:\cdots\:\left(1\right)$$

where $\:{\epsilon\:}_{{b}_{Analytes}}$ is the relative background permittivity of PMMA and CO₂ and is equal to 2.25, the background permittivity of PMMA, f_m resonance strength of functional absorption groups, $\:{\omega\:}_{0,j}$ the vibrational angular frequency of the functional group and $\:{\gamma\:}_{j}$ the damping frequency. Here, to infer the functional group absorption to the analyte, we included three resonance modes from PMMA, i.e., C = O stretching mode (w₀ = 51.9/g = 0.1 THz), CH₂ asymmetric stretching mode (88.5/0.7 THz) and CH₃ stretching mode (90/0.9 THz) and one resonance mode from CO₂, i.e., C = O asymmetric stretching mode (70.5/0.05 THz). All the resonance strengths are 0.001. From the blue curves of Fig. 2, the OWB metamaterial absorbers facilitated occurrence of Fano resonance for all the four vibrational modes with significant asymmetric line shapes, an indicator of Fano resonance; besides, the signals at the y-polarization incidence are stronger and observable at the 88.5 and 90 THz compared to the ones at the x-polarization incidence, another evidence of better enhancement for the y-polarization incidence. On the other hand, the planar metamaterial absorber only showed Fano resonance for the strongest vibrational mode with the smallest damping, i.e., the O = C = O stretching mode of CO₂ and showed only superposition of absorption for all the other three modes. Still, the hot spots provided by the planar metamaterial absorber could boost the magnitude of absorption when compared to PMMA absorption spectrum with an enhancement factor of around 2 to 3. In addition, the OWB metamaterial absorber revealed profound asymmetric line shapes in experiments compared the ones in simulation which could be attributed to the hot spots among nano-wire-bundles that are not included in simulation.

For the OWB absorber, under x-polarization incidence at 51.9, 70.5, 88.5 and 90.5 THz, the DR/R₀ are 3.07, 1.71, 4.79 and 7.24 times higher when we compared the experimental and simulation results. Also, for the OWB absorber, under y-polarization incidence at 51.9, 70.5, 88.5 and 90.5 THz, the DR/R₀ are 2.13, 1.33, 1.05 and 1.01 times higher when we compared the experimental and simulation results. As for the planar metamaterial absorber under x- and y-polarization incidence, the measured results show 1.47, 0.27,0.85, 0.63 times and 2, 0.49, 0.50 and 0.51 times higher DR/R₀ compared to the simulated results. The better enhancement between the measured and simulated results for the OWB metamaterial absorber could attribute to the hot spots among nanowire bundles, which is not completely considered in the simulation.

To dig out the reasons for better coupling of molecular absorption by the OWB metamaterial absorber, we recorded the absolute field distribution |E| of the two absorbers at the frequency of 126.25 and 140.75 THz as shown in Fig. 3. Note that the chosen frequency was based on the maximum absorption from the planar metamaterial absorber under x- and y-polarizations. For the x-incidence of the OWB absorber, the local highest |E| at the 126.25 THz is 6.32×10⁸ while for the y-incidence, the highest |E| at the 140.75 THz is 6.17×10⁸, which are both approximately 1.46 times larger compared to the ones of the planar absorber. On the other hand, when considering the global highest |E|, the OWB absorber revealed a value of 6.63×10⁸ and 2.16×10⁹ for the x- and y-incidence. It is worth noting that the hot spot areas from the OWB absorber are also much larger than the ones of the planar absorber.

Furthermore, we would like to discuss two different cases for examining detection ability of the proposed metamaterial absorbers. The first of the two is to reduce the thickness of the analytes, i.e., from 460 nm to 100 nm. The absorption spectra are depicted in Fig. 4(a). Again, although small, the OWB metamaterial absorber supported four asymmetric line shapes as shown in the inset of Fig. 4(a). In contrast, the planar metamaterial absorber shows the superposition behavior even for the O = C = O vibrational mode. The other is to reduce the resonance strength of the analyte from 0.001 to 0.0001 while the thickness was maintained. As illustrated in Fig. 5, the OWB metamaterial absorber supported four asymmetric line shapes as shown in the inset of Fig. 5(a). In contrast, the planar metamaterial absorber shows the superposition behavior even for the O = C = O vibrational mode.

Finally, to reinforce the detection ability of our proposed OWB metamaterial absorber, we plotted the DR with respect to different strengths of PMMA ranging from 0.1 to 0.00001, i.e., different concentrations at the three frequencies for the functional group absorption as shown in Fig. 6. Under logarithm-logarithm plot, we could observe that all the curves revealed a characteristic of exponential decay. Furthermore, due to the strongest absorption for the O = C = O, the DR was the highest for both polarizations. Also, the DR would saturate once the resonance strength increased. From Fig. 6, all the DR started to saturate at a resonance strength of 0.01 for both polarizations. Note that since the absorption is weaker for both the CH₂ asymmetric stretching modes, the smaller resonance strength (i.e., below 1🞨10^-3) was not enough to excite Fano resonance.

In this work, we have proposed an oblique-wire-bundle metamaterial absorber with broad absorption bandwidth of 299 and 317 THz under x- and y-polarization incidence, respectively, which covers almost all the infrared absorption spectra of different chemicals. Such proposed OWB metamaterial absorber alongside with the planar metamaterial absorber are tested with PMMA and CO₂ for the coupling with vibrational modes of different analytes. The OWB metamaterial absorber revealed strong interaction with analytes even with smaller thickness or weaker resonance strength; in contrast, the planar metamaterial absorber could not interact with chemicals well and showed superposition of absorption for almost all the cases. We believed that this proposed OWB metamaterial absorber could help the development of rapid detection in the field of food safety and chemical detection.

Data availability

The data sets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Fabrication

To fabricate the oblique-wire-bundle metamaterial absorber and its comparison, i.e., planar metamaterial absorber, we deposited 4-nm-thick titanium and 150-nm-thick aluminum on a silicon substrate. Then, spin coat photo resist of PMMA A5 with an initial rotation speed of 1000 rpm for 10 s and a final rotation speed of 4000 rpm for 60 s. Next, the sample was hard baked under 180 °C for 3 min. In the following, the electron beam exposure was carried out. After exposure, the sample was immersed into a solution of MIBK and IPA in a ratio of 1 to 3 for 50 s. In addition, 120-nm-thick magnesium fluoride was deposited onto the sample. In the following, to fabricate the oblique-wire-bundle metamaterial absorber, the as-fabricated sample was put into acetone for the lift-off process to remove residual photoresist; e-gun deposition was then conducted for 4-nm-thick titanium and for the growth of aluminum wire bundles on an 86-degree tilted stage. On the other hand, to fabricate the planar metamaterial absorber, 4-nm-thick titanium and 50-nm-thick aluminum were consequently deposited onto the sample. Finally, the as-fabricated sample was put into acetone for the lift-off process to remove residual photoresist.

Acknowledgement: The authors acknowledge the financial support from National Science and Technology Council (NSTC 113-2221-E-131-019) and from the Center for Plasma and Thin Film and Thin Film Technologies and Ming Chi University of Technology.

Author contributions statement: Tsung-Yu Huang conceived the project, Tsung-Yu Huang, Hsueh Shun Lee and Cheng Yu Lu designed and simulated the sample. Tsung-Yu Huang and Yu Ping Kuang measured the sample. Tsung-Yu Huang, Hsueh Shun Lee and Cheng Yu Lu co-wrote the paper. All authors discussed the results and commented on the manuscript.

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No competing interests reported.

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Broadband surface enhanced infrared absorption with multiple Fano resonance by metallic oblique-wire-bundle metamaterial absorbers

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