Continuous-wave terahertz imaging using diode lasers

We demonstrate an all-optoelectronic continuous-wave terahertz (cw-THz) imaging system using technology based on low-cost and compact diode lasers. THz radiation is generated by photomixing two near-infrared lasers (830 nm) in a photoconductive emitter and is tunable from 0.1-2 THz. Images are captured in reflection geometry, and a phase-sensitive photoconductive detection scheme is used, which operates at room temperature. We have optimised the growth and annealing of low-temperature gallium arsenide (LT-GaAs), and achieved state-of-the-art material with 100 fs carrier trapping lifetimes. The photomixers load resonant antennas, which efficiently couple out monochromatic THz radiation. Images are captured in the time-domain using a real-time rapid scan delay line capable of data acquisition at 15 Hz, with both amplitude and phase information available. There are a number of advantages in using continuous-wave imaging systems, compared to the more established pulsed technologies. In particular, the combination of diode lasers and photoconductive detection demonstrates, for the first time, an imaging system that is compact, robust, genuinely turn-key and of low cost. Such a system would be well suited for routine THz imaging in both medical and non-medical applications.