The invention relates to an assembly for detecting the presence of a target based on a detection of a resonance associated to surface polaritons, such as long-range surface exciton polaritons (LRSEP). The invention relates to an assembly to be used in connection with a bio-sensor. The assembly comprising a carrier substrate (1) and a sensor layer (2) positioned on the carrier substrate. The sensor layer is of a material having a complex permittivity with an imaginary part being greater than or similar to the real part.
