Sub-parts-per-billion level detection of NO^sub 2^ using room-temperature quantum cascade lasers

We report the sub-parts-per-billion-level detection of NO^sub 2^ using tunable laser-based photoacoustic spectroscopy where the laser radiation is obtained from a room-temperature continuous-wave high-power quantum cascade laser operating in an external grating cavity configuration. The continuously tunable external grating cavity quantum cascade laser produces maximum single-frequency output of [asymptotically =]300 mW tunable over [asymptotically =]350 nm centered at 6.25 µm. We demonstrate minimum detection level of [asymptotically =]0.5 parts per billion of NO^sub 2^ in the presence of humidified air. [PUBLICATION ABSTRACT]