Optical Properties and Preparation of Asymmetric Silver Nanorods
Objective Surface plasmons have attracted significant attention due to their broad applications in nanophotonics,biology,and spectroscopy.Among these,polarization-sensitive nanostructures are especially critical in biology and communications.Asymmetric structures,in particular,offer an effective means to achieve polarization-sensitive responses.While considerable research has focused on nanostructures,individual asymmetric nanostructures have not been studied as extensively.In this study,we investigate the optical properties of single asymmetric silver nanopillars,examining their polarization-dependent characteristics through both simulation and experimental methods.In addition,we introduce a double-layer asymmetric silver nanopillar structure,expanding the potential applications and functionality of these nanostructures.Methods We utilize the finite-difference time-domain(FDTD)method to simulate the transmission spectra of asymmetric silver nanopillar structures,evaluating the influence of various parameters on their optical properties.Specifically,we analyze the effects of variations in height,diameter,and tilt angles on the optical behavior of these nanopillars with results shown in Fig.5.Mode field analysis is conducted to explore the surface plasmon modes excited by these structures,as shown in Fig.4.The asymmetric silver nanopillar structures are fabricated on quartz substrates using magnetron sputtering for silver deposition,followed by ion beam etching.Submicron microsphere spin-coating is utilized as a masking technique,with details provided in Fig.7.Transmission spectroscopy is then employed to analyze the relationship between the optical properties and geometric parameters of the fabricated nanopillars,as shown in Fig.11.This analysis further investigates the potential application in environmental refractive index change sensing.Results and Discussions We provide details of the simulated transmission spectra of asymmetric silver nanopillar structures under varying periodic conditions,as shown in Fig.2.As the periodicity increases,the wavelength of the transmission trough shifts towards the red,attributed to surface lattice resonances(SLRs)induced by the ordered,large-area array structure of the silver nanopillars.Alterations in the array's periodicity also influence the SLR troughs.Figure 5(c)shows the transmission spectra of asymmetric silver nanopillars under different polarization angles α,with corresponding plan views shown in Fig.6(c).As the polarization angle increases,the transmission troughs at 478,658,and 900 nm exhibit redshifts,demonstrating pronounced polarization selectivity.The electric field at the 731 nm transmission trough results from the coupling between the surface plasmon resonance(SPR)mode generated by diffraction at the silver-air interface and the SLR surface plasmon between the asymmetric silver nanopillars.Notably,the enhanced electric field at the 900 nm trough primarily occurs at the top of the asymmetric silver nanopillars at the air interface,where the incident light excites SPR.In contrast,at 1300 nm,the enhanced electric field primarily occurs at the bottom of the asymmetric silver nanopillars at the interface with the substrate,due to SPR excited by the incident light at the silver/silica interface.When the diameter is increased to 600 and 700 nm,no shifts in the resonance wavelengths at 900 and 1300 nm are observed.Experimentally fabricated single-layer tilted silver nanopillars,with a spacing exceeding 1000 nm,demonstrate that as the polarization angle of incident light increases from 0° to 90°,the resonance wavelength of the transmission spectrum at 678 nm shifts to 762 nm,and the intensity of the transmission spectrum correspondingly increases,indicating the high sensitivity of the asymmetric silver nanopillar structure to changes in the polarization angle of the incident light.In addition,the refractive index sensitivity of the structure at the 778 nm wavelength is measured to be 258 nm/RIU.We also simulate a double-layer asymmetric silver nanopillar structure,as shown in Fig.13.The simulations reveal that as the tilt angle δ between the layers of silver nanopillars increases,the response to polarized light becomes more sensitive,with the resonance wavelength shifting further towards longer wavelengths under different polarization angles of incident light.Compared to the single-layer structure,the double-layer asymmetric silver nanopillar structure exhibits a more sensitive response to changes in the polarization angle of incident light,further affirming the advantages of asymmetric nanostructures in enhancing localized near-field effects and reducing plasmonic resonance loss.These characteristics have significant implications for applications in optical sensing,optical communications,and polarization modulation.Conclusions We outline the fabrication process of asymmetric silver nanopillar structures and employ numerical simulations using the FDTD method to explore the surface plasmon modes and optical properties they excite.Single-layer asymmetric silver nanopillar structures are fabricated using magnetron sputtering for silver deposition and ion beam etching.Optical spectroscopy measurements demonstrate that these structures are sensitive to the polarization of incident light and changes in environmental refractive indices,making them suitable for monitoring variations in environmental refractive indices.They also show substantial potential for the rapid on-site detection of biochemical substances.In addition,a double-layer asymmetric silver nanopillar structure is proposed.Simulation studies confirm that a larger angle between the tilt directions of the double-layer nanopillars leads to a more significant redshift of the resonance wavelength as the polarization angle of the incident light increases,highlighting the enhanced sensitivity of these nanostructures to polarized light.The proposed double-layer asymmetric silver nanopillar structure is characterized by low cost,simplicity in fabrication,and high repeatability,making it especially suitable for optical polarization control and promising for high-sensitivity detection of biomolecules.
surface plasmon resonanceasymmetric silver nanorodrefractive index sensing
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