This dataset contains experimental data relating to that presented in 'Apertureless near-field terahertz imaging using the self-mixing effect in a quantum cascade laser' Fig2.csv contains (position, voltage) data presented in Fig.2 of the above paper. Position relates to the arbitary position of the sample in the z-axis. The separation d=0 is taken as being at z=-3.04280E+01 mm. Fig3.csv contains (separation, voltage) data presented in Fig.3 of the above paper. Separation has been calculated from the recorded position of the sample and the PZT stage in the z-axis. Voltage columns correspond to different positions of the PZT stage from Delta_z=0 um to Delta_z=18 um. Fig4a.csv contains (x position, y position, voltage) data presented in Fig.4 of the above paper. Positions relate to the arbitary position of the sample in the x-axis and y-axis. Fig4b.csv contains (position, voltage) data presented in Fig.4b of the above paper. Voltage data is shown for ten adjacent rows of pixels traversing the edge of the gold-coated region along the x direction. Fig4c.csv contains (position, voltage) data presented in Fig.4b of the above paper. Voltage data is shown for ten adjacent rows of pixels traversing the edge of the gold-coated region along the y direction.