An exercise grid includes four running strips and four crossing strips. The device includes a plurality of sample-containment features and a non-fluorescent quenching material for minimizing cross-talk between optical signals emitted from or directed toward the sample-containment features. The thickness of the dielectric thin film is lower than 50 nm, and is suitable for reducing by a third order factor at least the maximum of the square of the electric field of the electromagnetic lightwave on the metal surface and in the metal layer of the substrate as compared to the square of the electric field at the surface of a metal grating not having a dielectric thin film.