Dual-frequency imaging using an electrically tunable terahertz quantum cascade laser

We report dual-frequency imaging using an electrically tunable terahertz frequency quantum cascade laser (QCL). Images of powdered materials including the explosive pentaerythritol tetranitrate (PETN) were acquired at 3.05 THz and 3.24 THz in a single scan. We show that it is possible to distinguish PETN from other materials by taking the difference-transmission and difference-attenuation images. We also demonstrate difference-intensity imaging by combining amplitude modulation of the QCL bias with lock-in detection.