Photoacoustic detection device , comprising: - a light source , configured to emit a light beam about an illumination axis; - a cavity , intended to receive a gas; - a detector , configured to detect an acoustic wave being propagated through the cavity; the device comprising a substrate , extending between a bottom face and a top face , the substrate being such that: - the cavity is formed in the substrate, between the bottom face and the top face; - a trench is formed in the substrate, delimited by the top face, the light source being inserted into the trench, on the top face, the top face thus extending between the cavity and the light source ; - the substrate comprises an illumination portion , delimiting the trench, disposed facing the light source, the illumination portion extending between: • a proximal face , formed on the top face, disposed opposite the light source; • and a distal face , formed on the top face, the distal face forming a reflector configured to reflect the light towards the cavity; such that the light beam, emitted by the light source, is propagated, through the illumination portion, between the proximal face and the distal face, before being reflected, by the distal face, towards the cavity. - a light source , configured to emit a light beam about an illumination axis; - a cavity , intended to receive a gas; - a detector , configured to detect an acoustic wave being propagated through the cavity; the device comprising a substrate , extending between a bottom face and a top face , the substrate being such that: - the cavity is formed in the substrate, between the bottom face and the top face; - a trench is formed in the substrate, delimited by the top face, the light source being inserted into the trench, on the top face, the top face thus extending between the cavity and the light source ; - the substrate comprises an illumination portion , delimiting the trench, disposed facing the light source, the illumination portion extending between: • a proximal face , formed on the top face, disposed opposite the light source; • and a distal face , formed on the top face, the distal face forming a reflector configured to reflect the light towards the cavity; - the cavity is formed in the substrate, between the bottom face and the top face; - a trench is formed in the substrate, delimited by the top face, the light source being inserted into the trench, on the top face, the top face thus extending between the cavity and the light source ; - the substrate comprises an illumination portion , delimiting the trench, disposed facing the light source, the illumination portion extending between: • a proximal face , formed on the top face, disposed opposite the light source; • and a distal face , formed on the top face, the distal face forming a reflector configured to reflect the light towards the cavity; • a proximal face , formed on the top face, disposed opposite the light source; • and a distal face , formed on the top face, the distal face forming a reflector configured to reflect the light towards the cavity; such that the light beam, emitted by the light source, is propagated, through the illumination portion, between the proximal face and the distal face, before being reflected, by the distal face, towards the cavity. <EOS>