Imaging apparatus and method

1. A system for investigating a sample, the system comprising a detector having non-linear current voltage characteristics and being configured to mix two radiation signals having frequencies in the range from 25 GHz to 100 THz, one of the signals being a local oscillator signal and the other signal being a sample signal carrying information about the sample being investigated, the system further comprising a quantum cascade laser for providing at least the local oscillator signal, wherein said quantum cascade laser provides a source signal which is transmitted by or reflected from said sample in order to produce said sample signal and the local oscillator signal.

2. A system according to claim 1 , configured such that said local oscillator signal is separated from said source signal so that said local oscillator signal is not transmitted by or reflected from said sample.

3. A system according to claim 2 , wherein said local oscillator signal is transmitted from one facet of the quantum cascade laser and the source signal is transmitted from a different facet of the quantum cascade laser.

4. A system for investigating a sample, the system comprising a detector having non-linear current voltage characteristics and being configured to mix two radiation signals having frequencies in the range from 25 GHz to 100 THz, one of the signals being a local oscillator signal and the other signal being a sample signal carrying information about the sample being investigated, the system further comprising a quantum cascade laser for providing at least the local oscillator signal, further comprising an independent source to produce said source signal.

5. A system for investigating a sample, the system comprising a detector having non-linear current voltage characteristics and being configured to mix two radiation signals having frequencies in the range from 25 GHz to 100 THz, one of the signals being a local oscillator signal and the other signal being a sample signal carrying information about the sample being investigated, the system further comprising a quantum cascade laser for providing at least the local oscillator signal, wherein the system is configured such that the sample signal is produced by the sample itself or arises from natural background radiation being transmitted by or reflected from the sample.

6. A system for investigating a sample, the system comprising a detector having non-linear current voltage characteristics and being configured to mix two radiation signals having frequencies in the range from 25 GHz to 100 THz, one of the signals being a local oscillator signal and the other signal being a sample signal carrying information about the sample being investigated, the system further comprising a quantum cascade laser for providing at least the local oscillator signal, wherein the detector is configured as a homodyne detector and mixes a sample signal and a local oscillator signal having the same frequencies.

7. A system for investigating a sample, the system comprising a detector having non-linear current voltage characteristics and being configured to mix two radiation signals having frequencies in the range from 25 GHz to 100 THz, one of the signals being a local oscillator signal and the other signal being a sample signal carrying information about the sample being investigated, the system further comprising a quantum cascade laser for providing at least the local oscillator signal, wherein the local oscillator signal is also transmitted by or reflected from the sample.

8. A system for investigating a sample, the system comprising a detector having non-linear current voltage characteristics and being configured to mix two radiation signals having frequencies in the range from 25 GHz to 100 THz, one of the signals being a local oscillator signal and the other signal being a sample signal carrying information about the sample being investigated, the system further comprising a quantum cascade laser for providing at least the local oscillator signal, wherein the apparatus further comprise phase control means in order for the detector to determine a phase dependent quantity of the radiation.

9. A system for investigating a sample, the system comprising a detector having non-linear current voltage characteristics and being configured to mix two radiation signals having frequencies in the range from 25 GHz to 100 THz, one of the signals being a local oscillator signal and the other signal being a sample signal carrying information about the sample being investigated, the system further comprising a quantum cascade laser for providing at least the local oscillator signal, wherein said information about the structure of the sample is information about the thickness of the sample, or information about the refractive index of at least part of the sample.

10. A system for investigating a sample, the system comprising a detector having non-linear current voltage characteristics and being configured to mix two radiation signals having frequencies in the range from 25 GHz to 100 THz, one of the signals being a local oscillator signal and the other signal being a sample signal carrying information about the sample being investigated, the system further comprising a quantum cascade laser for providing at least the local oscillator signal, configured as an imaging system, wherein said quantum cascade laser provides a source signal which is transmitted by or reflected from said sample in order to produce said sample signal and the local oscillator signal.

11. A system according to claim 10 , configured to output an image showing the variation of the absorption or transmission characteristic of the sample, the refractive index and/or the thickness.