A symmetrical surface plasmon resonance sensing structure excited by a stripe waveguide

A symmetrical metal–medium–metal surface plasmon resonance (SPR) structure was excited by a stripe waveguide. The wave-vector matching condition is adjusted by changing the metal-film spacing, which in turn allows a variation of the refractive-index testing range. This structure overcomes the range limitations encountered in the intensity SPR detection method using waveguides and makes the matching condition of wave vectors more flexible. A single-mode stripe waveguide was designed and fabricated to excite the symmetrical sensing structure based on a planar waveguide using the complementary error function as the refractive-index distribution. The stripe waveguide is analyzed more conveniently and is easily cured with optical fibers for remote online measurement. The coupling testing system of the optical fiber with the symmetrical structure excited by the stripe waveguide was constructed to facilitate the excitation of surface plasmon waves. The impacts of the metal type, the metal film thickness, and the medium thickness in the symmetrical structure were investigated to optimize the sensing structure. The resonance condition is achieved much more easily in the symmetrical than in the traditional structure, and the detection range is adjustable via the medium thickness. Online and adjustable measurements were realized simultaneously by the detection system.