Time-resolved spectral characteristics of external-cavity quantum cascade lasers and their application to stand-off detection of explosives

We present the time-resolved spectral analysis of the settling process in a pulsed external-cavity quantum-cascade laser (EC-QC laser) emitting in the wavelength range around 7.4 mu m. Due to mode competition between the two cavities formed by the laser-chip on one hand and the external-cavity on the other hand, a time-dependent spectral emission on a ns time-scale can be observed. Depending on the spectral characteristic of the external cavity with respect to the chip gain curve time-delays of the external cavity emission of 15-35 ns within the 100 ns drive pulse can be observed. Regarding the application, we discuss the results in the context of using the EC-QC laser system together with a high-performance IR-camera for stand-off detection of surface-contaminations with explosives such as TNT. The detected densities can be as small as 10 mu g/cm(2) corresponding to traces left by a fingerprint.