Fig. 1. Layout of the experimental apparatus. Laser emission and IF signal spectra are also represented schematically. The separation of ~13 GHz between neighbouring longitudinal Fabry-Perot modes corresponds to a QCL cavity length of 3mm.

Fig 2. IF power spectrum detected at the output of the SDM with two different resolution bandwidths of the spectrum analyser. The QCL was driven in CW with a current of 1.4 A at T = 10K. The IF signal was amplified with a two-stage, 50 dB gain, 12–14 GHz low noise preamplifier (MITEQ, JSD3 and JSD2 series). Inset: QCL emission spectrum recorded with a Fourier transform infrared spectrometer (0.25 cm^-1 resolution).

Fig 3. Examples of THz images. Left: Image of a gold pattern evaporated on top of a TPX window (100 μm² pixels). Right: A razor blade imaged through a standard A4 paper sheet (200 μm² pixels). Regions of different colours correspond to different intensities detected by the mixer as a consequence of the blade surface not being perfectly flat (see text). The light concentric rings visible on the image are produced by rings patterned on the surface of the circular 1mm thick polythene spacer placed between the blade and the paper. Additional non-uniformity of the horizontal edges of the blade (see for instance the bottom edge) is due to the translation stage not regaining exactly the same position in subsequent vertical scans.