In order to reduce the exposure of a detector surface 180 of a photo-multiplier 160 to stray charged particles, an off-axis structure is interposed between the resonant structure and the detector surface of the photo-multiplier. By providing the off-axis structure with at least one reflective surface, photons are reflected toward the detector surface of the photo-multiplier while at the same time absorbing stray charged particles. Stray particles may be absorbed by the reflective surface or by any other part of the off-axis structure. The off-axis structure may additionally be provided with an electrical bias and/or an absorbing coating for absorbing stray charged particles.