Intra-cavity absorption spectroscopy with narrow-ridge microfluidic quantum cascade lasers

We demonstrate microfluidic laser intra-cavity absorption spectroscopy with mid-infrared lambda approximate to 9 mu m quantum cascade lasers. A deep-etched narrow ridge waveguide laser is placed in a microfluidic chamber. The evanescent tails of the laser mode penetrate into a liquid on both sides of the ridge. The absorption lines of the liquid modify the laser waveguide loss, resulting in significant changes in the laser emission spectrum and the threshold current. A volume of liquid as small as similar to 10pL may, in principle, be sufficient for sensing using the proposed technique. This method, similar to the related gas-phase technique, shows promise as a sensitive means of detecting chemicals in small volumes of solutions. (c) 2007 Optical Society of America.