One example discloses an interferometer device (100), including: a first side (102), having a first reflectivity (104); a second side (106), having a second reflectivity (108); a cavity (110) disposed between the first and second sides, and having an opening (112) configured to receive a substance (114), such as carbon dioxide; an electromagnetic source input region (116) configured to receive an electromagnetic signal(118); and an electromagnetic detector output region (122) configured to output the electromagnetic signal (124) modulated in response to the substance in the cavity. Another example discloses a method including: fabricating a first side, having a first reflectivity; fabricating a second side, having a second reflectivity and positioned with respect to the first side so as to form a cavity; fabricating an opening configured to receive a substance; fabricating an electromagnetic source input region configured to receive an electromagnetic signal; and fabricating an electromagnetic detector output region configured to output the electromagnetic signal modulated in response to the substance in the cavity. Preferably, the first side, the second side and the cavity form a Fabry-Perot etalon.