用于聚合物传感的表面增强红外银天线阵列

Surface Enhanced Infrared Silver Antenna Arrays for Polymer Sensing

红外光谱技术因分子吸收截面较小限制了其灵敏度,采用光学性质相似且价格更低的银阵列替代金阵列,在节约成本的同时,利用表面增强红外吸收效应,以提高微量分析物的检测灵敏度。分别设计了微米尺度的中空十字形与六边形天线阵列结构,使用时域有限差分算法进行数值仿真,研究了天线尺寸对超表面光学性能的影响。利用紫外曝光技术,以银作为沉积金属,实现低成本、大尺寸天线阵列的制备。利用傅里叶红外光谱仪测量超表面基底,在400~800 cm^(-1)的中红外波段实现了表面增强吸收,十字形天线与六边形天线结构的消光系数最高分别可达20%与24.5%。为了评估超表面的传感性能,分别在硅基底和六边形结构的超表面基底上涂覆聚甲基丙烯酸甲酯,六边形天线结构在483.14 cm^(-1)处实现了2.85倍的增强吸收,增强因子为1995。

Infrared spectroscopy is widely studied and applied due to its label-free and accurate molecular identification capabilities.However,the small absorption cross-section of the molecule limits its sensitivity.To overcome this problem,the surface-enhanced absorption effect of metal nanoantennas is used to increase the sensitivity when detecting trace target molecules.In the design optimization of metal nanoantenna arrays,silver(Ag)is often used as an alternative to gold(Au)due to its similar properties and lower price.Although much progress has been made in the study of antenna structures,these structures are generally smaller in size,generally less than 100 nm.Relying on advanced technology,the preparation cost is expensive.In this paper,Ag and ultraviolet exposure techniques are used to realize low-cost,largearea metasurface design.First,to study the impact of nanorod size and guide experiments,hollow cross-shaped antenna arrays and hexagonal antenna arrays metasurface are simulated with the help of FDTD software.The simulation results show that the hollow cross-shaped nanoantennas has an extinction peak at 547.7 cm^(-1),and its extinction coefficient is 64.8%.The extinction coefficient of the hexagonal nanoantennas at 448.3 cm^(-1) is 85.5%.Compared with hollow cross-shaped nanoantennas,hexagonal nanoantennas have a redshift in resonant wavelength and a larger extinction coefficient.The parameters of length and width of the hollow cross-shaped and hexagonal antennas are swept separately.For hollow cross-shaped nanoantennas,with the increase of length,the formant is significantly redshifted,and with the increase of width,the formant is slightly blue-shifted.For hexagonal nanoantennas,with the increase of length,the formant is significantly redshifted,and with the increase of width,the formant is slightly redshifted.The influence of length on the position of the formant is much greater than the effect of width.The electric field enhancement of the two antenna structures at 547.7 cm^(-1) and 448.3 cm^(-1) is 576 and 1335,respectively.Hot spots are distributed near the edge of the antenna element,especially at the tip.This indicates that the electric field is significantly enhanced around the edge of the metal antennas.Then,the two optimized patterns of metasurface are prepared.To characterize their shape,the substrates are observed with scanning electron microscope.The result shows that the tip of the deposited metal is relatively rounded and not strictly rectangular,which will lead to a decrease in the electric field enhancement of the tip.The Ag nanorod unit is larger in size(error less than 1μm)relative to the theoretically designed nanorod cell.Compared with the size of the nanorod unit with the hollow crossshaped structure,the nanorod cell size of the hexagonal structure is smaller.Antenna size is related to the resolution of the lithography machine,the shape of the structure and other factors.To measure the position and shape of the extinction peaks,the transmission spectra of the prepared substrates are measured with a Fourier transform infrared spectrometer.The results show that the hollow cross-shaped antenna substrate has an extinction peak at 525 cm^(-1),and the extinction coefficient is about 20%.The extinction peak of the hexagonal antenna is located at 474.4 cm^(-1),and its extinction coefficient is about 24.5%.The extinction coefficient of both structures is smaller than that of the two structures in the simulation,which is caused by the excessive deviation in the transmittance of the silicon used in the experiment and simulation.Compared with the simulation of the extinction peak of the hollow cross-shaped antenna,the extinction peak shows a red-shift of 22 cm^(-1).And the extinction peak of the hexagonal antenna shows a blue-shift of 26 cm^(-1) compared with the simulation result.This is due to the fact that the size of the antenna element structure in the prepared substrate deviates from the theoretical design.To verify the performance of designed silver antenna arrays for polymer sensing,Polymethyl Methacrylate(PMMA)was chosen as the analyte.The enhanced absorption effect of hexagonal Ag-Surface-Enhanced Infrared Absorption(Ag-SEIRA)substrate is studied.Both the silicon and hexagonal Ag-SEIRA substrates coated with PMMA have extinction peaks at an absorption peak of PMMA(483.14 cm^(-1)).The extinction coefficients are 3.15%and 8.99%,respectively.The hexagonal antenna arrays achieve 2.85-fold enhancement in PMMA absorption with an enhancement factor of 1995.