一种光栅/纳米颗粒结构的双共振SERS基底

Double-Resonance SERS Substrate Based on Grating/Nanoparticle Hybrid Structure

从提高表面增强拉曼光谱(SERS)基底的增强效果出发,设计了光栅/纳米颗粒复合结构,通过有限差分时域法(FDTD)对复合结构的消光特性和光场分布进行了仿真,并对光栅的传播表面等离子体(PSP)和纳米颗粒的局域表面等离子体共振(LSPR)之间的耦合效应进行了讨论。在两种不同分布密度的金纳米颗粒阵列(间隙分别为2 nm和6 nm)的LSPR共振波长已知的前提下,基于LSPR和PSP之间的共振波长匹配原则,并令光栅衍射光PSP波矢和LSPR波矢在x轴方向上的分量一致,设计了两种周期的金光栅与相应的金纳米颗粒阵列分别匹配。光栅被入射平面波激发后产生的PSP又激发了纳米颗粒的LSPR,FDTD仿真表明,在633 nm激发下,光栅/颗粒复合基底上颗粒间隙处最大电场强度的平方比相应颗粒阵列提高一个数量级。同时,与平面波直接激发LSPR不同,通过PSP激发的LSPR复合结构中颗粒周围的电场强度在得到提升的同时,热点区域的分布更加广泛,从而可使得更多的被探测分子处于高增强区域中,这对于提高滴加于此基底上的被探测分子的整体SERS信号强度非常有利。

The grating/nanoparticle hybrid structure is designed to improve the enhancement effect of surface-enhanced Raman scattering(SERS)substrates.The extinction characteristics and the optical field distribution of the structure are simulated through the finite-difference time-domain(FDTD)method.Meanwhile,the effect of the coupling between the propagating surface plasmons(PSPs)of gratings and localized surface plasmon resonance(LSPR)of nanoparticles are discussed.Given the LSPR wavelength of two Au nanoparticle arrays(with the interparticle spacing of 2 nm and 6 nm,respectively),when the wave vector components of PSP and LSPR of diffraction gratings along the x axis are identical in view of the matching principle of resonance wavelength of LSPR and PSP,two Au gratings with different periods are designed to match two Au nanoparticle arrays separately.When the gratings are excited by plane waves,they generate PSP to excite LSPR of nanoparticles.FDTD simulations demonstrate that the square of maximum electric field intensity in the particle gaps of the grating/nanoparticle hybrid substrate is improved by one order of magnitude compared to that of the corresponding nanoparticle array under 633 nm excitation.Meanwhile,unlike the situation where LSPR is directly excited by plane waves,the electric field intensity around the particles in the hybrid structure excited in which LSPR is excited by PSP is enhanced,and the hot spot area is largely extended,and thus more molecules are in the area of high enhancement,which is highly conducive to improving the overall SERS signal intensity of those molecules dropped on the substrate.