Continuous-wave coherent imaging with terahertz quantum cascade lasers using electro-optic harmonic sampling

We demonstrate a coherent imaging system based on a terahertz (THz)frequency quantum cascade laser (QCL) phase-locked to a near-infraredfs-laser comb. The phase locking enables coherent electro-optic sampling of the continuous-wave radiation emitted by the QCLthroughthe generation of a heterodyne beat-notesignal that carries the amplitude and phase of the laser field. We use this beat-note signal to demonstrate raster scan coherent imaging using a QCL emitting at 2.5THz. At this frequency, the detection noise floor of our system is of 3pW/Hz and the long-termphase stability is <3deg/hour, limited by the mechanical stability of the apparatus. a) Presentaddress: CNR-Istituto Nazionale di Ottica, Via Carrara 1, 50019 Sesto Fiorentino, Italy b) Present address: National Physical Laboratory, Dr. K. S. Krishna Marg, New Delhi-110012, India c) Electronic mail: [email protected]