Device for measuring a concentration of a predetermined gas or liquid (100; 500; 600; 700; 800; 900; 1100; 1200), comprising: - a first light source (110; 610; 710; 910), configured, to emit an excitation light beam (111; 611; 711; 911) with at least one emission peak, being centered on an excitation wavelength (λ E), wherein the excitation wavelength (λ E) corresponds to a characteristic absorption wave length of the predetermined gas or liquid; and - a so called optical excitation cavity (120; 620; 720; 920; 1120), being optically resonant at the excitation wavelength and being configured to receive at the entrance the excitation light beam, with at least one region (122; 522; 622) within an inside of the optical excitation cavity (120; 620; 720; 1120), being configured to receive a gaseous or liquid means; - wherein the device for measuring further comprises: - a second light source (130; 630; 730; 1030), being configured to emit a measuring light beam (131; 631; 732; 931; 1031) with an emission peak, being centered on a measurement wavelength (λ D); and - a movable and/or deformable mechanical element (150; 550; 650; 750; 850; 950,1150; 1250), being movable around an elastic feedback position and/or mounted elastically deformable, being placed on the optical path of the excitation light beam (120; 620; 720; 920;1120) within the optical excitation cavity and further being placed on the optical path of the measuring light beam (131; 631; 731; 931; 1031) and being configured to be moved or deformed by the excitation light beam; an wherein one of the excitation light beam and measurement light beam is configured to put into oscillation the movable and/or deformable mechanical element, wherein the device is configured such that, in use, the presence of the predetermined gas or liquid within the optical cavity modifies the oscillation of the movable and/or deformable mechanical element (150; 550; 650; 750; 850; 1150; 1250), wherein the device further comprises detection means for detecting a variation of at least one property of the measuring light beam, being induced by the oscillation of the mechanical element, and calculating means for measuring a concentration of the predetermined gas or liquid from the variation of the at least one property, being thus determined. - a first light source (110; 610; 710; 910), configured, to emit an excitation light beam (111; 611; 711; 911) with at least one emission peak, being centered on an excitation wavelength (λ E), wherein the excitation wavelength (λ E) corresponds to a characteristic absorption wave length of the predetermined gas or liquid; and - a so called optical excitation cavity (120; 620; 720; 920; 1120), being optically resonant at the excitation wavelength and being configured to receive at the entrance the excitation light beam, with at least one region (122; 522; 622) within an inside of the optical excitation cavity (120; 620; 720; 1120), being configured to receive a gaseous or liquid means; - wherein the device for measuring further comprises: - a second light source (130; 630; 730; 1030), being configured to emit a measuring light beam (131; 631; 732; 931; 1031) with an emission peak, being centered on a measurement wavelength (λ D); and - a movable and/or deformable mechanical element (150; 550; 650; 750; 850; 950,1150; 1250), being movable around an elastic feedback position and/or mounted elastically deformable, being placed on the optical path of the excitation light beam (120; 620; 720; 920;1120) within the optical excitation cavity and further being placed on the optical path of the measuring light beam (131; 631; 731; 931; 1031) and being configured to be moved or deformed by the excitation light beam; an wherein one of the excitation light beam and measurement light beam is configured to put into oscillation the movable and/or deformable mechanical element, wherein the device is configured such that, in use, the presence of the predetermined gas or liquid within the optical cavity modifies the oscillation of the movable and/or deformable mechanical element (150; 550; 650; 750; 850; 1150; 1250), wherein the device further comprises detection means for detecting a variation of at least one property of the measuring light beam, being induced by the oscillation of the mechanical element, and calculating means for measuring a concentration of the predetermined gas or liquid from the variation of the at least one property, being thus determined.