A system 11 for investigating samples 5, or a receiver (63, fig. 9a) comprising: a detector 7 having non-linear voltage characteristics which is configured to mix two radiation signals having frequencies in the range from 25 GHz to 100THz. One of these signals is provided by a local oscillator 1, such as Quantum Cascade Laser (QCL), the other signal is supplied by the sample. The detector may be configured to perform a heterodyne detection. The system may be configured such that path length of the sample signal relative to the local signal remains fixed during investigation. The QCL may provide a source signal which is transmitted by or reflected from the sample. The QCL may emit two different frequencies for providing a sample and a local oscillator signals. These two signals may be emitted from different facets of the QCL (31, figures. 4-5).