Surface enhanced Raman scattering (SERS) is a promising detection and analysis method originating from SERS substrates' huge amplification effect on Raman signal. The extensively investigated SERS substrates usually consist of metal nanoparticles that generate localized surface plasmon resonance (LSPR) under light irradiation. However, the hot spots only occur at the small gap between two adjacent nanoparticles. Since the space for the probe molecular to reside and experience the enhanced electric field are limited, the overall enhancement factor for this type of SERS substrate needs to be improved. Surface plasmon polariton (SPP) are propagating electromagnetic waves bound to the interfaces between metal and dielectrics which can be excited on a metal surface by prism coupling or grating coupling. Experimental and theoretical results show the electric field produced by coupling between SPP and LSPR is significantly higher than that purely generated by LSPR. Therefore, a type of novel and effective SERS substrate is supposed to obtain in an SPP-LSPR coupling system. The emerging research on grating/nanoparticle SPP-LSPR coupling SERS substrates is attempting to obtain high electric field enhancement by changing parameters such as grating thickness, grating duty ratio, and morphology and sizes of nanoparticles. As the strong coupling of SPP-LSPR occurs when the resonance wavelength of SPP and LSPR matches well, we think a grating/nanoparticle SERS substrate could be designed by finite difference time domain (FDTD) simulation in advance to avoid time and cost spent on material selection and parameter attempt. As concerning the gold grating/gold nanoparticle hybrid substrate applied under 633 nm excitation, the geometric parameters for gratings and nanoparticles are suggested after analyzing FDTD calculated reflectance spectra and electric field distribution beside nanoparticles. The final real Au grating/Au nanoparticle hybrid SERS substrate is obtained by combining the Au gratings prepared by electron beam lithography and Au nanoparticles from the chemical synthesis under the designed parameters. The SERS properties of prepared Au grating/Au nanoparticle substrate are measured to verify the correctness of the design idea.

Au grating/Au nanoparticle structure built in FDTD is shown in Fig. 1. Au grating periodicity is optimized to match the laser wavelength by scanning periodicity from 540 nm to 620 nm in FDTD calculated reflectance spectra. The electric field distribution of gratings/nanoparticles and nanoparticles on Si wafer is compared in Fig. 3 to show the field enhancement under SPP-LSPR coupling. Au nanoparticles are synthesized by the chemical reduction of chloroauric acid with sodium citrate. Au gratings are fabricated on Au/Cr/Si substrate by electron beam lithography. The reflectance spectrum is carried out on the spectrophotometer (Lambda950) and the morphology of nanoparticles is analyzed by transmission electron microscopy (TEM, JEM2010). The morphology of the grating and composite structures is observed by scanning electron microscopy (SEM, Zeiss Gemini 500) and atomic force microscopy (AFM, Dimension Icon).

In FDTD calculation, Au gratings with the periodicity of 580 nm have a reflectance dip at 627 nm which is close to laser wavelength of 633 nm. Thus, this grating periodicity is chosen as the optimized one to construct the grating/nanoparticles SERS substrate. The reflectance spectra of Au nanoparticles array with a diameter of 25 nm and gap of 4 nm overlap with those of Au gratings with the periodicity of 580 nm as shown in Fig. 2(b). The overlapping provides strong SPP-LSPR coupling which can be confirmed by the two reflection dips in the reflectance spectra of gratings/nanoparticles. The electric field distribution of Au grating/Au nanoparticle substrate and Au nanoparticle substrate is demonstrated in Fig. 3. The maximum electric field enhancement factor is improved by nearly one magnitude for Au gratings/Au nanoparticles compared with Au nanoparticles on Si substrate. A same color bar is set in Fig. 3 to observe and compare the electric field distribution. The space of a high electric field resulting from SPP-LSPR coupling is expanded to a broad region compared wioth that of Au nanoparticles substrate, which is just located in the small gap region between two adjacent nanoparticles. The higher electric field and broader hot spot region are extremely favorable for enhancing Raman signals of probe molecules absorbed on the SERS substrate. The average diameter for prepared Au nanoparticles is 25 nm through TEM measurement as shown in Fig. 5. The AFM image of Au gratings is shown in Fig. 6. The stripes are uniformly arranged and one periodicity is 589 nm. From the surface profile scan along the white line shown in Fig. 6(a), the height of one ridge is 33 nm. The geometrical characteristics of gratings are well agreed with those parameters in the calculation section. Au nanoparticles mainly distribute in the grating bottom observed in Fig.7(c) for the obtained Au gratings/Au nanoparticles hybrid SERS. The random distribution is not as designed in the calculation. Therefore, the reflectance dip for Au nanoparticles on the Si wafer is at 680 nm, not the same as that in the calculation of 600 nm. The overlap in reflectance spectra between Au gratings and Au nanoparticles is not as much as that in the calculation. For SERS measurement, the R6G detection concentration limits for Au grating/Au nanoparticle substrate and Au nanoparticle substrate are 10^-9 mol/L and 10^-7 mol/L respectively. The enhancement factor (EF) for grating/nanoparticle substrate and nanoparticle substrate are calculated as 1.3×10⁶ and 1.8×10⁴. The relative standard deviation (RSD) for grating/nanoparticle substrate are 12.8%, 13.9%, and 11.3% through employing Raman shift at 614 cm^-1, 1365 cm^-1, and 1512 cm^-1.

Au grating/Au nanoparticle SERS substrate adopted at 633 nm excitation is designed through FDTD simulation. The periodicity of Au gratings and the diameter of Au nanoparticles are determined by analyzing the reflectance spectra and the field enhancement factor simulated by the FDTD method to excite SPP-LSPR coupling and obtain a higher EF in Au grating/Au nanoparticle hybrid substrate. The geometrical parameters provided by FDTD simulation guide the following substrate preparation. Au grating/Au nanoparticle SERS substrate is obtained by combining the Au nanoparticles with an average diameter of 25 nm prepared by chemical reduction and Au gratings with the periodicity of 589 nm fabricated by electron beam lithography. The SERS experimental results show that the R6G detection concentration limit for Au grating/Au nanoparticle substrate and Au nanoparticles on Si wafer substrate are 10^-9mol/L and 10^-7mol/L respectively. The EF calculated from the SERS spectra for Au grating/Au nanoparticle substrate is 1.3 × 10⁶, nearly two orders of magnitude higher than the EF of Au nanoparticles on Si wafer substrate. The experimental results are in good accordance with the simulation results. Thus, the simulation method is an effective way to design the SPP-LSPR coupling SERS substrate, which provides the precise parameters of gratings and nanoparticles for researchers to prepare corresponding substrates.