RICHTER Ivan Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering

Approaches to Electromagnetic Simulations of Advanced SERS Substrates

Co-authors FIALA Jan, KWIECIEN Pavel, ŠTOLCOVÁ Lucie, PROŠKA Jan

Today, the techniques based on the phenomenon of surface-enhanced Raman scattering (SERS, where S can also stand for spectroscopy) has found several interesting applications. The SERS based spectroscopy enables, ultimately, single molecule detection with molecular fingerprint specificity, and, thus, competes (and even overcomes) in sensitivity with other techniques, such as fluorescence spectroscopy. Although not fully understood, prevailing explanation takes account of electromagnetic and chemical contributions to Raman scattering "amplification", with the enhancement factor spanning several orders of magnitude, when molecules under investigation are adsorbed on metal nanoparticles or more complex nanostructured metal surfaces (s.c. SERS-active substrates). Two main requirements for high-quality SERS substrates include large enhancement factors (mainly in small regions called hot spots) and homogeneity of substrates. Clearly, these two requests are difficult, or even impossible, to meet simultaneously in practice. Concerning possible morphology and design of SERS structures, there are several design strategies available, with the experimental technique, based on self-assembly approaches, belonging to the most progressive ones. Also, apart from low cost and possibility to create large surfaces, this technique additionally provides another important issue, the morphology-based tunability of optical properties of these substrates. In our department, the preparation procedure based on the bottom-up approach using self-assembly technique has been mastered and successfully applied to periodic SERS substrate preparation. In this way, series of various reproducible periodic arrays of semishells, nanobowls, and other types of substrates have been prepared, characterized, and tested under SERS measurements in recent years. These structures are indeed one of good candidates for "best" SERS-active substrates, due to a compromise between high enhancement of the local electromagnetic field (sharp features at the edges of open apertures) and good reproducibility and enhancement homogeneity (strict periodicity and repeatability over the whole structure). Concerning the complexity and rich physics of such structures, advanced numerical electromagnetic techniques are required for reliable numerical studies and predictions of structural behavior. In previous years, we have developed and mastered several such techniques, being complementary in nature. These include (aperiodic) rigorous coupled wave analysis (aRCWA), finite difference time domain techniques (FDTD), or boundary element methods (BEM). In this contribution, we present our recent studies on electromagnetic modeling of several advanced SERS substrates. We will discuss the simulation methodology, starting with a choice of various methods, in relation to definition of structures and practical aspects. These numerical studies will be beneficial, in the next stage, in connection to the experimental preparation strategies, based on the self-assembly techniques. The selected results of our modeling activities (based mainly on FDTD and RCWA techniques) in this field will be presented, together with the discussion and comparison of approaches applied, in connection to parallel experimental activities. The overall assessment of simulation possibilities will be given, in relation to our future studies. ACKNOWLEDGEMENTS: This work was supported by the Czech Science Foundation P205/13/20110S project.