Refractive Index Response of Square and Hexagonal Gold Nanoparticle Arrays in a Kretschmann LSPR Sensor

Abstract

Localized surface plasmon resonance (LSPR) of close-packed gold nanoparticle (AuNP) arrays in the Kretschmann configuration offers an attractive platform for optical sensing owing to its high sensitivity, label-free operation, and real-time response. Both near-field enhancement and far-field optical responses are sensitive to variations in the organization of nanoscale AuNPs and their interparticle spacing. In this work, we systematically investigate the far-field spectra and sensing behavior of square- and hexagonally packed AuNPs with diameters ranging from 10 to 60 nm and interstitial spacings from 5 to 30 nm, using three-dimensional finite element modeling (FEM). To the best of our knowledge, this parameter range and coverage density constitute the first systematic exploration of array-based sensing under the Kretschmann configuration. Compared with square arrays, hexagonally packed AuNPs generate a higher density of interstitial hot spots along two interparticle axes under p-polarized incidence, corresponding to up to ~ 3-fold enhancement in near-field intensity. The increased electromagnetic coupling results in a ~ 70% increase in refractive index (RI) sensitivity for a given AuNP diameter and interparticle gap. Notably, the sensitivity enhancement with decreasing gap distance deviates from a monotonic trend in hexagonal arrays, challenging the conventional intuition that ‘the smaller the gap, the better’. This finding suggests the existence of an optimal gap window, which holds important guiding significance for process tolerance design and parameter selection in practical devices. An optimized AuNP array achieves the calculated RI sensitivity 363 nm/RIU with a figure of merit (FOM) 9.6 RIU^− 1.