Spectral Characteristics of Near-Infrared Surface Plasmon Resonance

Abstract

Intrinsic properties of surface plasmons (SPs) excited with Kretschmann configuration were analyzed as a function of wavelength, including the propagation length, the penetration depth, the Goos–Hanchen (GH) shift, and the field enhancement. The calculated results indicate that there exists a critical thickness (tcr) of the gold layer and that the maximum GH shift occurring exactly at the SP resonance wavelength (λR) rapidly varies from positive to negative with changing of the gold layer thickness from t tcr. The maximum field enhancement happens not at λR but at a wavelength smaller than λR due to the phase retardation between the transmitted and reflected light. Simulations also reveal that a broadband collimated near-infrared beam can simultaneously excite two SPs with different responses to a refractive index (RI) change: the shorter-wavelength SP able to make a small redshift and the longer-wavelength SP capable of yielding a large blueshift. Only the shorter-wavelength SP was experimentally observed and its RI sensitivity was measured to increase from 3,539 nm/RIU at λR = 707.6 nm to 57,143 nm/RIU at λR = 1,398 nm. The SP at λR = 1,013 nm moved to λR = 1,029 nm in response to the saturation adsorption of bovine serum albumin, and the corresponding surface coverage was determined to be Γ = 1.565 ng/mm2 based on a quasilinear dependence of Γ on the resonance wavelength shift (∆λR) deduced theoretically. Butyrylcholinesterase adsorption from a dilute solution of 10 nM protein in phosphate buffer solution leads to a redshift of ∆λR = 10 nm, corresponding to Γ ≈ 0.97 ng/mm2.