Effect of Metallic Nanoparticles on Improving the Detection Capacity of a Micro-SERS Sensor Created by the Hybrid Waveguide of Metallic Slots and Dielectric Strips

The enhancement factor (EF) of surface-enhanced Raman scattering (SERS) mainly depends on the electrical field intensity of surface plasmons in the place of Raman-active molecules. Because of this dependence, the Raman detection sensitivity is much higher with molecules in a small metallic gap than near a single metallic surface because of the intense local electric field resulting from the interaction between metallic objects. In this study, we investigate the SERS detection capacity improved by metallic nanoparticles in a micro-SERS sensor made of a metallic slot and a dielectric strip using the three-dimensional finite-difference time domain method. We calculated the field and charge distributions in the metallic sphere–slot junction to discuss the electromagnetic interaction between the in-sphere localized surface plasmon and the in-slot surface plasmon polariton. After that, the EF dependence of the sensor on the in-slot particle’s position, size, shape, and number is demonstrated and discussed to show the strategy of optimizing the SERS detection capacity. It follows the rule that a strong enhancement always appears in a small metallic gap due to the strong field confinement. We show that the averaging SERS enhancement factor around the particle can be increased by 10⁵ times, compared to the averaging EF in the slot without metallic nanoparticles that is reported in our previous work, reaching 10⁶ (all factors in this study are obtained by the fourth power of the division of the local plasmonic field E_Loc to the maximum electric value of the incident light E_Inc(max)) and at some single points, we have a factor as high as 10¹⁰, which is enough to detect a single molecule. With metallic nanoparticles, the micro-SERS sensor can be developed into a highly sensitive tool for the portable and stable Raman detection of molecules or markers in pharmacology, biology, etc.