A sensor element (101) for detecting an analyte in a fluid sample by optical probing comprises a translucent membrane (110) with a front side (111) defining a sensor surface for contacting a fluid sample, and a back side (112) facing away from the front side (111). The sensor element (101) has a reflective layer (114) at the front side (111) of the translucent membrane (110). An optical input port (120) is connected to the back side (112) of the translucent membrane (110) for feeding probing light to the translucent membrane (110) through the back side (112). An optical output port (130) is connected to the back side (112) of the translucent membrane (110) for collecting an optical response from the translucent membrane (110) through the backside (112). The sensor element further comprises an optical constriction element (140) with a screen element (141) arranged between the input port (120) and the output port (130), thereby optically separating the output port (130) from the input port (140). The constriction element (140) further comprises a reflective element (142) arranged at the backside (112) of the translucent membrane (110). The reflective element (142) faces towards the reflective layer (114) at a vertical distance D therefrom, thereby defining a probing region (116). Light travelling from the input port (120) to the output port (130) is confined to pass through the probing region (116), wherein at least the probing region (116) comprises pores (118) extending from respective openings at the sensor surface into the translucent membrane (110). A further aspect relates to sensor assembly (101) comprising a sensor element (101) integrated with a sample chamber (102).L'invention concerne un élément capteur (101) permettant de détecter un analyte dans un échantillon de fluide par sondage optique, lequel élément capteur comprend une membrane translucide (110) ayant un côté avant (111) délimitant une surface de capteur pour entrer en contact avec un échantillon d
