Effect Of Lspr And Pspr Coupling In Different Patterns Of Silver And Gold

Abstract

Anchu Ashok*, Arya Arackal, George Jacob, Gargi Raina Centre for Nanotechnology Research, VIT University, Vellore, India - 632014 * anchuashok@gmail.com Keywords: Surface Plasmon, Localization, Propagating surface plasmon, Coupling. Surface Plasmons (SPs) are collective electron oscillations at metal dielectric interface. They are often categorized into localized surface plasmon resonance and propagating surface plasmon resonance (PSPR). LSPR are non propagating electromagnetic waves and PSPR are propagating. In this work, we have carried out a study of Electric Field Intensity variation for different isolated and array structures of Au and Ag for corresponding plasmonic wavelength. Double plasmon resonance peak are observed, which is the combined effect of localized surface plasmonic resonance (LSPR) and propagating surface plasmonic resonance (PSPR). Shape, size and thickness of the structures affect Localized surface plasmonic resonance while as propagating surface plasmonic resonance (PSPR) is dependent on the 1D and 2D arrangements of the patterns. Depending on the sharp and confined corners, density of the hot spot in structures changes. Hence, triangular structure gives more excitation when compared with circle, square and ellipse. This work followed triangular patterns to find the coupling effect of LSPR and PSPR depend on the arrangement of patterns. Incident TM polarized plane wave of wavelength from 300 nm to 1000 nm with electric field intensity 1V/m was given for finding the enhancement of electric field in the patterns. Sharper the corners, more is the electric field enhancement. Another important factor affecting the enhancement of electric field is the grating period. If distance of two adjacent corners decreases, the coupling between the localized surface plasmons increases. This increase in coupling leads to the propagating electromagnetic wave called propagating surface plasmons (PSPs). This coupling distance depends on the propagation length of electric field from one structure to another. If the second structure is placed at a distance less than that of the propagation length there is an effective coupling of the electric field from one structure to another. This work discusses about simulation of electric field intensity for three types of patterns with the variation on their period and confinement employing Finite Element Method (FEM). Double resonance peak are noticed in all structures is due to the coupling effect of field from one structure to another and there is change in field enhancement and resonance wavelength with different patterns. Resonance wavelength due to the PSPR is at longer range and that of LSPR is at shorter range. Propagating surface plasmon resonance wavelength is same for a particular structure irrespective of number of structures in the array. Spectral separation between the LPSR peak and PSPR peak can be modified by changing the period and confinement in the structures. Silver patterns show better enhancement than gold patterns. The enhancement of Gold patterns is in near IR range and that of Silver is in visible range. Combined effect of this LSPR and PSPR give drastic field enhancement which makes these patterns as good substrate candidate for Surface Enhanced Raman Spectroscopy and biosensors.