Detection of single human hairs with a terahertz nonlinear quantum cascade laser

We report the demonstration of imaging of a single human hair with a terahertz quantum cascade laser (THz-QCL) source based on intracavity difference-frequency generation. A single human hair whose diameter was about 100 µm was detected using the THz-QCL source operating at 240 K, of which the THz beam had a linear polarization. The results show that the THz image of a human hair clearly depends on the polarization direction of the THz beam. The THz QCL sources that are capable of room temperature operation will be useful for detection of small foreign objects like human hairs.