Dual-wavelength THz imaging with quantum cascade lasers

We have assembled an imaging system using quantum cascade lasers at frequencies of both 3.4 and 2.3 THz. Images at the two frequencies and the resulting absorption coefficients are compared. We demonstrate imaging in both reflection and transmission. The lasers are operated in a closed-cycle refrigerator and we use a peak output power of >2.5 mW at 7-10% duty cycle and 22-40 K operating temperature. The focal spot size is approximately 300 microns for both lasers and is not diffraction-limited. The signal is detected with a single-element DTGS detector, and images are captured by scanning the sample. Applications enabled by longer wavelengths are demonstrated, as well as the determination of chemical information through imaging at two wavelengths.