Real-time terahertz imaging over a standoff distance (>25meters)

The authors demonstrate the use of a terahertz quantum cascade laser (QCL) for real-time imaging in transmission mode at a standoff distance of $25\mathrm{\text{meters}}$ . Lasing frequency was selected for optimum transmission within an atmospheric window at $\sim 4.9\mathrm{THz}$ . Coarse frequency selection was made by design of the QCL gain medium. Finer selection (to within $0.1\mathrm{THz}$ ) was made by judicious choice of laser cavity length to adjust facet losses and therefore lasing threshold bias, in order to overlap the peak frequency of the Stark-shifted gain spectrum with the atmospheric window. Images are shown using an uncooled $320\times 240$ microbolometer camera.