银纳米线-三角片耦合结构发射光的偏振依赖特性

Polarization Dependence Between Excitation and Emission in Nanowire-Triangular Plate Coupled Structures

银纳米线可以承载传播的表面等离激元,纳米片可以产生局域的表面等离激元,二者形成的耦合结构不但可以将传播光场耦合为局域增强光场,还可以调控光场的偏振态等性质,为纳米光调控提供新的自由度。本团队构建了银纳米线-三角片耦合结构,并发现耦合结构的发射偏振与纳米线-三角片的耦合方式有关:当三角片与纳米线之间是“线”接触耦合时,耦合结构的发射偏振随着激发偏振的旋转而旋转;当二者是“点”接触耦合时,无论激发偏振如何变化,发射偏振角度几乎保持160°不变。进一步,利用时域有限差分法验证了出射偏振对入射偏振的依赖特性。通过计算自由电流密度体积分揭示了纳米线中传播的表面等离激元模式与银纳米线-三角片耦合模式的转化机制,以及不同表面等离激元模式的叠加对发射偏振的调控。这些发现为纳米尺度上的光调控以及在纳米尺度上构建纳米光子器件提供了更多灵活性。

Objective Surface plasmon polaritons(SPPs)supported by metallic nanostructures can confine electromagnetic fields at the nanoscale and overcome the diffraction limit.Compared with conventional electronic devices,integrated subwavelength optical devices with high data transmission rates and wide bandwidths provide a promising platform for the development of nanophotonics.In recent decades,various plasmon-based optical elements have been developed,including waveguides,resonators,nanoantennas,multiplexers,and switchers.Polarization-controllable optical elements are important in chip-to-chip interconnects,sensors,and quantum cryptography.Chemically synthesized nanowires have been reported to function as polarization-controllable plasmonic waveguides,where the emission of different SPPs modes relies on the shape of the wire end.Additionally,optical properties sensitive to incident polarization have been observed in coupled nanostructures composed of wires or nanoparticles.For example,the near-field enhancement and far-field emission polarization of coupled junctions can be modulated via incident polarization,through which polarization-dependent routers,modulators,and logic gates have been achieved.Hence,further optimization of coupled waveguides can provide additional flexibility for controlling light polarization at the nanoscale.In this study,we investigated the correlations between the emission and incident polarization of a nanowire-triangular plate coupled structure.The results indicate that the polarization dependence can be modulated by the wire-triangular plate coupling type.The response of the emission polarization to the incident polarization is verified using the finite-difference time-domain(FDTD)method.In theory,by calculating the volume integration of the free current density,the conversion of propagating SPP modes and wire-triangular plate coupled mode can be revealed.The emission mechanism of the coupled structures can be elucidated based on the superposition of different SPP modes.The findings provide a new degree of freedom for manipulating light polarization at the nanoscale,and are beneficial to the development of plasmonic nanophotonics.Methods In this study,we fabricated silver nanowire-triangular plate coupled structures using a typical chemical method.A dilute ethanol suspension comprising nanowires and triangular plates was drop-cast onto an indium tin oxide-coated glass slide,where the coupled structures were self-assembled after drying under ambient conditions.Scanning electron microscopy was used to distinguish the different coupled structures,as shown in Figs.2(a)and 3(a).Subsequently,the samples were immersed promptly in an indexmatched oil with n=1.518 to protect the samples against oxidization or sulfuration in air.Next,optical measurements were performed,where He-Ne laser with a wavelength of 633 nm was focused onto the nanowire end through an oil objective.The response of the emission polarization to the incident polarization was obtained by rotating the half-wave plate and polarizer.The experimental data were verified via simulation using the FDTD method,and the emission mechanism was determined by calculating the volume integration of the free current density.Results and Discussion The experimental results show different polarization correlations between the excitation and emission of the nanowire-triangular plate coupled structure.The emission polarization follows the direction of the excitation polarization.Meanwhile,the emission polarization is maintained at 160°,regardless of the incident polarization.In the simulation,we recorded the local near-field distribution and obtained the corresponding far-field emission using the FDTD method.When the triangular platenanowire coupling exhibits a“line”connection,the emission polarization increases monotonically with the incident polarization.However,when the coupling type exhibits a“point”connection,the emission polarization remains at 160°,regardless of the incident light.These experimental and simulation results show that the emission behaviors can be effectively modulated by the wire-triangular plate coupling type.To elucidate the modulation mechanism,we analyzed the plasmon modes excited in the nanowire-triangular plate junction.The three modes contribute to the emission from the junction:hybridized wire modes m=0 and m=1 and the nanowiretriangular plate coupling mode P(Fig.5).Based on antenna radiation theory and the volume integration of the free current density,we calculate the emission contributed by the triangular plate and nanowire.The results show that the light emitted from the triangular plate is always polarized at 160°,regardless of the incident polarization.It can be understood by the fact that the orientation of induced dipole oscillation is 160°.In addition,the emission polarization from the wire part is the same as the incident polarization.Hence,when the triangular plate-nanowire exhibits“wire”coupling,the junction serves as a Fabry-Perot cavity and the wire modes dominate the emission polarization rotating with the incident polarization.When the triangular plate-nanowire exhibits a“point”connection,the near-field is mainly localized on the triangular plate,in which the emitted light dominated by coupling mode P with polarization remains in the 160°orientation.Conclusions We fabricated silver nanowire-triangular plate coupled structures via wet chemical synthesis and investigated their polarization correlations between incident light and emission.The results showed that the incidence-emission polarization correlation depended on the coupling type of the nanowire-triangular plate.When the triangular plate-nanowire exhibited a“line”connection,a Fabry-Perot cavity was formed in the junction.The polarization of the emitted light followed with the excitation polarization.However,for the“point”coupling type,the emission polarization remained at 160°,regardless of the incident polarization.The emission mechanism can be interpreted by the superposition of propagating SPP modes and the localized mode of the coupled nanowire-triangular plate structure.Hence,the emission polarization at the junction can be manipulated by the nanowire-triangular plate coupling type.These findings can be used to develop more sophisticated polarization-controllable devices and are beneficial to the development of integrated nanophotonics.