Optimization of Long-Range Surface Plasmon Waveguides for Attenuation-Based Biosensing

Long-range surface plasmon Y-junctions for referenced biosensing

“…The surface sensitivity of the referenced Y-junction is similar to that of a straight waveguide [19], but is independent of the incident power P inc and the input transmittance (coupling) T C . The bulk sensitivity of the referenced Y-junction ∂(P out,F /P out,C )/∂n c is computed by assuming no adlayer on the sensing waveguide (a = 0) and approximating the differentials in Eq.…”

“…In the computations we set n c = 1.3348 (sensing fluid index equals that of CYTOP, n CYTOP ), h = 10 −4 and we use the finite element method (FEM) to compute the modal properties of the LRSPP on the sensing waveguide (see inset of Fig. 2(a)), yielding T F and α F for each n c , following [19]. The bulk sensitivity of the Y-junction as a function of the waveguide thickness t is plotted in Fig.…”

“…The surface sensitivity of the referenced Y-junction ∂(P out,F /P out,C )/∂a is computed at a sensing index of n c = 1.338 (the sensing index used for protein sensing in the experimental section), and the differentials in Eq. 15are approximated via second-order error O(h 2 ) forward finite-difference formulae [19]:…”

“…We previously demonstrated the ability of straight LRSPP waveguides to detect small changes in the bulk refractive index of solutions and the adsorption of bovine serum albumin (BSA) [13], to perform immunological blood typing of human red blood cells [14], to detect dengue antibody and antigen in infected patient blood plasma [15,16], to selectively detect gram negative or gram positive bacteria in human urine [17], and to detect leukemic markers in patient blood serum [18]. The surface sensitivity and optimization of straight waveguide biosensors were also discussed [19]. The key challenge in biosensing with straight LRSPP waveguides is achieving a stable baseline signal because the measurand is taken as the output power directly.…”

“…Such a design, implemented to operate with LRSPPs, as Au stripes fully embedded in CYTOP [21] (a fluoropolymer with a refractive index close to that of the biologically compatible fluids), was discussed and characterised in terms of its passive operation (insertion, transition, and propagation losses) in previous work [3,22]. The Y-junction biosensor should have better bulk and surface sensing detection limits than straight waveguides [13,19] because the common noise and drift in the system are eliminated. In principle, phase-dependent MZI biosensors [4,23] are better than attenuationbased biosensors, such as straight and Y-junction waveguides.…”