Fig. 1. Schematic of the boundary-value problem solved for the surface-plasmonic sensor based on the grating-coupled configuration. The CTF is symbolically represented by a single row of nanocolumns, each of which is modeled as a string of electrically small ellipsoids with semi-axes in the ratio 1:γ_b:γ_τ.

Fig. 2. (a) Real and (b) imaginary parts of q/k₀ of the SPP wave propagating along the x axis as functions of the refractive index n_L of the infiltrating fluid computed using solutions of the canonical boundary-value problem, whereas χ_v = 15 deg, γ = 30 deg, and ε_m = −15.4 + 0.4i, see Sections 3.1 and 3.2 for other relevant parameters.

Fig. 3. Absorptance A_p as a function of incidence angle θ for L_c∈{1000, 2000, 3000, 4000} nm and L = 500 nm in the grating-coupled configuration. Whereas (a) n_L = 1, (b) n_L = 1.27, (c) n_L = 1.37, (d) n_L = 1.43, and (e), (f) n_L = 1.70, see Sections 3.1 and 3.3 for other relevant parameters. A downward arrow identifies the excitation of the SPP wave as a Floquet harmonic of order n, which is indicated alongside the arrow.

Fig. 4. Absorptance A_p as a function of incidence angle θ when (a) n_L ∈ [1.00, 1.20], (b) n_L ∈ [1.21, 1.29], (c) n_L ∈ [1.30, 1.39], and (d) n_L ∈ [1.40, 1.50]. Whereas L_c = 3000 nm and L = 500 nm, see Sections 3.1 and 3.3 for other relevant parameters. The horizontal arrows show the direction of the shift of peaks representing the excitation of the SPP wave.

Fig. 5. Sensitivity S as a function of the refractive index n_L of the infiltrating fluid. The sensitivity, given by Eq. (10), was computed from the absorptance plots like the ones given in Fig. 4 with L_c = 3000 nm and L = 500 nm. Doublet excitation is possible for some ranges of n_L in Fig. 5(c). The predicted sensitivity was computed using the solutions of the canonical problem in Re(q) = k₀ sin θ_p + 2nπ/L to find predicted θ_p as a function of n_L. All parameters were kept the same as for Fig. 4.

Fig. 6. The angular location θ_p of an absorptance peak indicating the excitation of the SPP wave as a function of the refractive index n_L ∈ [0.3, 2.5] of the infiltrating fluid. All parameters are the same as for Fig. 4. Triple excitation of the SPP wave occurs in the blue-shaded regions, double excitation in the gray-shaded regions, and single excitation in the green-shaded regions.