Field enhancement in micron-scale metallic plasmonic gratings

Field enhancement resulting from the confinement of surface plasmons in nanoscale dimensions is commonly a key feature of plasmonic structures. However, achieving this type of field enhancement requires nanoscale fabrication, which may limit their application in low-cost devices. The primary goal of this research is to achieve hotspots with considerable field enhancement and significant long extending length without requiring nanoscale dimensions. We show that this is achievable by means of the guided-mode resonance (GMR) phenomenon existing in micron-scale plasmonic gratings which can be easily fabricated using low-cost photolithography. To demonstrate this possibility, we concentrate on a binary silver grating and investigate the variety of its GMRs and categorize them to determine those having the required characteristics in terms of field enhancement and extending length. For this purpose, one requires an accurate and fast electromagnetic solver of metallic gratings. Using a transmission-line formulation (TLF), we compute the mode fields of plasmonic gratings in a significantly shorter timescale in comparison with the finite-element method (FEM); hence, we are able to easily generate several diagrams showing the modulation depth of gratings as a function of various geometrical parameters of the proposed binary silver grating. With the help of this computational approach, we design a structure showing 1900-fold field enhancement and an extending length of up to 8 µm. Finally, we validate our numerical results using a commercial FEM solver.