Photoacoustic device for detecting gas comprising: - a first photoacoustic cavity (11, 11-1) having a side wall extending between a first end and a second end and having an inner surface and an outer surface ; - a mirror arranged on at least one portion of the outer surface of the side wall of the first photoacoustic cavity; - a light source (S) suitable for emitting a light radiation supplying an excitation energy to a gas contained in the first photoacoustic cavity , the light radiation being modulated at a pulse ω , the light source (S) being coupled to the first end of the first photoacoustic cavity , the first photoacoustic cavity being made of a material transparent to the radiation of the light source (S); - a first microphone (M, M1) coupled to the side wall of the first photoacoustic cavity , the device being characterised in that : - the material transparent to the light radiation of the light source (S) is selected from silica, silicon, germanium, indium phosphide or aluminium nitride; - the side wall of the first photoacoustic cavity has a thickness e12 which is chosen as a function of the depth of penetration δ of a thermal wave coming from the mirror into said transparent material, in such a way that such a thermal wave is attenuated before arriving within the first photoacoustic cavity , the depth of penetration δ being defined by: δ = 2 α / ω with α the thermal diffusivity of the transparent material and ω the pulse of the light source (S); - the outer surface of the side wall of the first photoacoustic cavity (11, 11-1) is of parallelepiped shape and the mirror is arranged on the two horizontal faces (12b1, 12b3) of the outer surface of the side wall of the first photoacoustic cavity (11, 11-1) and a second mirror (T15) is arranged parallel, at a distance equal to the thickness e12, to the two vertical faces (12b2, 12b4) of the outer surface of the side wall of the first photoacoustic cavity (11, 11-1). - a first photoacoustic cavity (11, 11-1) having a side wall extending between a first end and a second end and having an inner surface and an outer surface ; - a mirror arranged on at least one portion of the outer surface of the side wall of the first photoacoustic cavity; - a light source (S) suitable for emitting a light radiation supplying an excitation energy to a gas contained in the first photoacoustic cavity , the light radiation being modulated at a pulse ω , the light source (S) being coupled to the first end of the first photoacoustic cavity , the first photoacoustic cavity being made of a material transparent to the radiation of the light source (S); - a first microphone (M, M1) coupled to the side wall of the first photoacoustic cavity , - the material transparent to the light radiation of the light source (S) is selected from silica, silicon, germanium, indium phosphide or aluminium nitride; - the side wall of the first photoacoustic cavity has a thickness e12 which is chosen as a function of the depth of penetration δ of a thermal wave coming from the mirror into said transparent material, in such a way that such a thermal wave is attenuated before arriving within the first photoacoustic cavity , the depth of penetration δ being defined by: δ = 2 α / ω with α the thermal diffusivity of the transparent material and ω the pulse of the light source (S); - the outer surface of the side wall of the first photoacoustic cavity (11, 11-1) is of parallelepiped shape and the mirror is arranged on the two horizontal faces (12b1, 12b3) of the outer surface of the side wall of the first photoacoustic cavity (11, 11-1) and a second mirror (T15) is arranged parallel, at a distance equal to the thickness e12, to the two vertical faces (12b2, 12b4) of the outer surface of the side wall of the first photoacoustic cavity (11, 11-1). <EOS>