A multilayer grating-based evanescent wave sensing technique

Chemical and biological sensors based on evanescent wave sensing techniques like surface plasmon resonance are continually improving in sensitivity and becoming important tools for pharmaceutical development, clinical diagnostics, environmental and food quality testing, etc. This paper presents results from the multilayer grating resonance sensor which is based on a multilayer dielectric waveguide deposited on a diffraction grating. The substrate and thin films are fabricated using mass production technology employed for compact discs. The resonant mode is excited from substrate-incident light coupled by the grating into the waveguide, eliminating the need for prisms and simplifying multichannel experiments. The narrow resonant angle (0.2° FWHM) is measured in the reflected zero-order. The evanescent field has a decay length of ∼100 nm, which is much more tightly confined than that of surface plasmon resonance and increases the surface sensitivity relative to the bulk sensitivity. With a 0.4 s integration time, a change in bulk index of refraction at the surface of the waveguide of <2×10−7 can be detected. The surface density detection limit (3σ) is <1 pg/mm2, which is sufficient to measure surface binding of molecules with molecular weight <250 Da.