The invention relates to an apparatus for probing a sample (4) comprising a light source (1) for emitting an illuminating light beam, a birefringent element (3) for splitting the illuminating light beam into two sheared beams, a reflective element (6) for reflecting the two sheared beams, wherein the apparatus is configured such that the reflected beams propagate through the birefringent element (3) for recombining the reflected beams, and a detector (7) for detecting the recombined beam, wherein the sample (4) is arrangeable in the optical path of the sheared beams or at the backside of a reflective surface in the optical path of the sheared beams, the reflective surface exhibiting a surface plasmon resonance or a localized surface plasmon resonance.