GB2409026A

GB2409026A - A system or a receiver investigating samples using Quantum Cascade Laser and heterodyne detections

Info

Publication number: GB2409026A
Application number: GB0328541A
Authority: GB
Inventors: Stefano Barbieri; Donald Dominic Arnone
Original assignee: TeraView Ltd
Current assignee: TeraView Ltd
Priority date: 2003-12-09
Filing date: 2003-12-09
Publication date: 2005-06-15
Anticipated expiration: 2023-12-09
Also published as: EP1697704A1; WO2005057146A1; GB0328541D0; GB2409026B

Abstract

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).

Description

An Investigation System, a Receiver and a Method of Investigating a Sample

The present invention is concerned generally with the field of systems which investigate samples using radiation in the TeraHertz regime. More specifically, the present invention is concerned with such systems which use heterodyne detection.

The Terahertz frequency range is generally considered to be the range from 25GHz to 1 00THz, particularly the range from 50GHz to 84THz, more particularly the range from GHz to 50 THz and especially the range from 1 00GHz to 20THz.

There has been much interest in using THz radiation to look at a wide variety of samples using a range of methods. THz radiation penetrates most dry, non-metallic and non-polar objects like plastics, paper, cardboard and non-polar organic substances.

Therefore, THz radiation can be used instead of X-rays to look inside boxes, cases, etc. THz photons are lower energy than those of X-rays and are non-ionising. Therefore, the health risks of using THz radiation are expected to be vastly reduced compared to those using conventional X-rays.

Heterodyne detection is a well established detection technique where two signals of different frequencies are mixed together so that they "beat" against each other. The resulting signal contains frequencies from the original two signals and its amplitude is modulated at the difference or "beat" frequency.

To perform heterodyne detection it is therefore necessary to provide two signals to the detector of differing frequencies. One of these signals, will generally pass through or be reflected from a sample under investigation, the other signal is a local oscillator for the detector.

The inventors of the present invention have realised that heterodyne detection principles may be applied to TeraHertz investigative systems to produce good results. These results may be significantly enhanced if a quantum cascade laser is used as the source of the local oscillator signal.

Quantum cascade lasers were developed in 1994 by researchers at AT&T Bell Labs. QC lasers are a type of laser formed by a plurality of layers of different materials. In other words, the conduction band is made up of a number of sub-bands. In these lasers, electrons are 'pumped" to an excited state, but when they fall back to their ground state, the electrons effectively cascade down an energy staircase formed by the different subbands. At each step a photon of light is emitted. Therefore, instead of each electron emitting a single photon when falling to their normal state, as occurs with standard lasers, a number of photons are emitted. The amount of energy emitted and hence the wavelength for each photon can be controlled through the thickness of the layers. The radiation frequency is determined by the energy spacings of the sub-bands.

Although QCLs were developed which operated in the infra red frequency range, a terahertz QCL proved more difficult since it required thicker layers. Fabricating a device with thicker layers is not a problem per se, however, such devices did not lase since difficulties were encountered in recycling electrons within the device and guiding the photons out of the device. Kohler et al, Nature 417, 156 (2002) and S. Barbieri, J. Alton, S. S. Dhillon, H. E. Beere, M. Evans, E. H. Linfield, A. G. Davies, D. A. Ritchie, R. Kohler, A. Tredicucci, and F. Beltram, J. Quantum Electron. 39, 586 (2003) reported Terahertz emission from a Quantum cascade laser (QCL).

GB 2 359 716 mentions briefly the use of a QCL and a mixer in an example of an imaging system using CW radiation. The output from the QCL is divided by a beam splitter to form two beams. A first beam is transmitted through the sample to a mixer.

A second beam is passed through a phase control means so that the phase of the second beam reaching the mixer can be varied with respect to the first beam. Contrary to heterodyne detection, in this case the two beams provided to the detector have the same frequency. Therefore the difference frequency signal, or beat signal, is constant in time (dc). To distinguish it from heterodyne detection, this technique is called "homodyne" detection. Both heterodyne and homodyne detection are phase sensitive techniques.

These detection techniques are distinguished from direct detection which uses a single beam. Direct detection is not phase-sensitive.

Heterodyne detection has advantages over homodyne and direct detection. As the beat signal is constant in time, a higher noise level affects homodyne and direct detection.

Also, the sensitivity of the heterodyne technique can be increased by increasing the amplitude of the local oscillator signal up to the saturation point of the mixer or detector.

In a first aspect, the present invention provides a system for investigating a sample, the system comprising a detector having nonlinear current voltage characteristics and being configured to mix two radiation signals having frequencies in the range from 25GHz to I OOTHz, 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, the detector being configured to perform heterodyne detection.

In the case of heterodyne detection it is not required for the system to comprise means to control the phase of the sample signal relative to the local oscillator signal. For example, in the case where what is measured is only the amplitude of the heterodyne signal. This signal oscillates in time at the difference frequency, therefore its amplitude is independent from the phase difference between the sample signal and the local oscillator signal.

Thus, in a second aspect, the present invention provides a system for investigating a sample, the system comprising a detector having nonlinear current voltage characteristics and being configured to mix two radiation signals having frequencies in the range from 25GHz to I OOTHz, 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 path length of the sample signal relative to the local oscillator signal remains fixed during investigation of the sample.

In the system of the second aspect of the present invention, the detector may be configured for heterodyne detection, homodyne detection or a combination of the two.

The above systems may be configured so that one source is used to produce both the first and second beams. In the case of heterodyne detection this is possible because QCLs have a multimode emission spectrum. Thus emission is concentrated at several narrow lines (longitudinal modes) separated by a frequency which is dictated by the length of the laser cavity.

Thus, two longitudinal modes of the QCL can be used to produce the first and second beams.

Using one QCL to produce both beams is advantageous because the heterodyne signal will be far more stable because any temperature or current fluctuations in the laser produces almost the same effects on the amplitude and frequency of both modes.

Further only one QCL provides a cheaper and simpler system.

Both beams from the QCL may be emitter collinearly and may be transmitted through and/or reflected by a sample under investigation.

Alternatively, the two beams may be divided, for example by collecting the beams from different facets of the laser or by using a beam splitter or the like. Thus, the first beam is transmitted through and/or reflected by a sample under investigation and the second beam is provided to the detector without interacting with the sample.

Two separate sources may be used instead of one source, where a first source is configured to provide the first beam and a second source is configured to provide the second beam, the system being configured such that said first beam is transmitted through and/or reflected by a sample under investigation and the second beam is provided to the detector without interacting with the sample. The first source may be a QCL laser or another coherent or even incoherent source of THz radiation.

The detector is a non-linear element and is preferably a Schottky diode. Schottky diodes perform best in the frequency range up to 40 GHz. Therefore, preferably, the frequency difference between the first and second beams is 10 MHz to 40 GHz. The Schottky diode used can be either produced using planar technology, or exploit a "whisker"-type of contact.

The system may be configured as a scanning system, for example imaging or it may be used to take a measurement of a sample at a fixed point.

In the above preferred embodiments, the sample signal is produced by providing a signal source. However, the above system may also be used for so-called passive imaging. In this case, the radiation to be detected or sample signal is generated either by natural Terahertz light from the sample itself, or by the reflection/transmission of natural (or other light) off of the medium. The sample can be any object capable of emitting, reflecting, or transmitting THz radiation. Practically any object is a source of heat which is emitted in the form of electromagnetic radiation, also called "blackbody" radiation". The spectrum of such radiation covers all possible frequencies, from the visible to the far infrared, or THz region. Therefore virtually any object is a source of THz radiation, with an intensity depending on its temperature.

Thus, the system may be 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.

In a third aspect, the present invention provides a system for investigating a sample, the system comprising a detector having nonlinear current voltage characteristics and being configured to mix two radiation signals having frequencies in the range from 25GHz to 1 OOTHz, 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.

The detector may be configured as a heterodyne or homodyne detector.

In any of the above systems, the local oscillator source is preferably a continuous wave source, but may be a pulsed source.

The receiver may be used independently of the above systems. Thus, in a fourth aspect, the present invention provides a receiver for an investigative system, said receiver comprising a detector configured to perform heterodyne detection having non-linear current voltage characteristics and being configured to mix two radiation signals having frequencies in the range from 25GHz to lOOTHz, the system further comprising a quantum cascade laser for providing at least one signal to said detector.

In a fifth aspect, the present invention provides a method of investigating a sample, said method comprising: providing a local oscillator signal to a detector from a quantum cascade laser, said detector having non-linear current voltage characteristics; and receiving said local oscillator signal at said detector with a sample signal received from a sample under test, wherein said first and second frequencies are both in the range from 25GHz to I OOTHz and said detector is configured to perform heterodyne detection by mixing two radiation signals having different frequencies in this range.

In a sixth aspect, the present invention provides a method of investigating a sample, said method comprising: providing a local oscillator signal to a detector from a quantum cascade laser, said detector having non-linear current voltage characteristics; and receiving said local oscillator signal at said detector with a sample signal received from a sample under test, wherein said first and second frequencies are both in the range from 25GHz to 1 OOTHz and said detector is configured to mix two radiation signals having frequencies in this range and the path length of the sample signal relative to the local oscillator signal remains fixed during investigation of the sample.

In a seventh aspect, the present invention provides a method of investigating a sample, said method comprising: providing a local oscillator signal to a detector from a quantum cascade laser, said detector having non-linear current voltage characteristics; and receiving said local oscillator signal at said detector with a sample signal received from a sample under test, wherein said first and second frequencies are both in the range from 25GHz to 1 OOTHz and said detector is configured to mix two radiation signals having frequencies in this range and the sample signal is produced by the sample itself or arises from natural background radiation being transmitted by or reflected from the sample. receiver for an investigative system, said receiver comprising a detector configured for heterodyne detection and to receive two beams of radiation having different frequencies in the range from 25GHz to lOOTHz, the receiver further comprising a quantum cascade laser for providing at least one of said beams to act as a local oscillator for said detector.

The present invention will now be described with reference to the following non- limiting embodiments in which: Figure 1 is a system in accordance with a first embodiment of the present invention; Figure 2 is a schematic of a system in accordance with a second embodiment of the present invention where the sample is investigated using reflection; Figure 3 is a schematic of a system in accordance with a third embodiment of the present invention using a single quantum cascade laser (QCL); Figure 4 is a system in accordance with a further embodiment of the present invention using a single QCL configured to direct one beam to the detector and a further beam to interact with the sample; Figure 5 is a variation on the system of figure 4, configured for transmissions measurements; Figure 6 is a plot of the intensity of a signal received from a heterodyne detector against frequency; Figure 7 is a plot of the intensity against frequency of the different frequency signal measured at different temperatures of the laser; Figure 8 is a plot of emission from a blackbody source at 360K; and Figure 9a is a system in accordance with a further embodiment of the present invention where the sample emits a blackbody spectrum, figure 9b is a schematic plot of the emission from the sample, figure 9c is a schematic plot showing the emission from the sample and the local oscillator and figure 9d is a schematic plot of the output of the detector.

Figure I illustrates a basic imaging system in accordance with an embodiment of the present invention. The system comprises a first radiation source 1 which outputs a first beam of radiation 3 in the range from 25GHz to 1 OOTHz. The beam is directed through sample 5 and is transmitted by the sample 5 to mixer 7. Mixer 7 may be any non-linear component and in this particular example is a Schottky diode.

Second quantum cascade laser 9 outputs a second beam of radiation I I which is directed towards diode 7. Second radiation beam 11 acts as a local oscillator signal for mixer 7.

The non-linear I-V characteristic of the mixer 7 can be expressed in terms of a Taylor series around the point V=VO. This is shown in equation (1).

I(V) I(V ) + (d V) d V + 2 (d V2) d V + 3! (d V3 d V + (1) When an electromagnetic wave hits the mixer 7, the oscillating electric field produces an additional voltage dV(t) = Aexp(ia' I) . By substituting this term into equation (1), all of the terms in the expansion giving a nonzero time average are those containing even powers of dV. The other terms simply average to zero since dV is constantly oscillating around zero. The contribution of all the even powers of dV produce a change in current with respect to its value I(Vo) in the absence of radiation. This change in current is manifested as a DC signal which is typically referred to as the direct detection signal. Usually, the term Ed 2 dominates and therefore we will ignore the higher order terms. Thus, the direct detection signal is given by equation 2.

dI=I(V)-I(Vo)=tdv2) dV (dV2) A (2) where A is the amplitude of the incoming radiation. ALO will be used to refer to the amplitude of the local oscillator signal and AS will be used to refer to the amplitude of the second beam.

In heterodyne detection, two beams of radiation with different frequencies cot and con impinges on the mixer 7. The beam with the frequency co' will be referred to as the local oscillator (LO) signal and will be referred to as SOLO The beam with frequency ce2 is the signal wave which is to be detected and will be referred to as co in the following

description.

0) = COLO + dco, where dce lies typically in the range from 1 OOMHz up to a few GHz.

When both waves hit the mixer 7: (dV2 v (Ago exp[ia)Ot] + As exp[i(a'LO + dm)t])2 = d I (A,2o exp[i2a);0t] + As2 exp[i2(wL0 + day)] + AsAo exp[i(20 + d0))t] + AsAo exp[i(do))t]) lo The first three terms produce oscillation in the current dI(t) at frequencies that are far too high to be handled by the electronics at the output of the mixer 7. Therefore, the only term which is left is the term oscillating at dot. This is heterodyne signal.

Therefore, a signal is produced with a current which oscillates at a frequency dco and an amplitude AsALo. Thus, the signal is linearly dependent on the amplitude of the signal As which has been transmitted by sample 5.

The homodyne signal is produced in the same way as for the heterodyne, the only difference being the fact that the signal wave and the local oscillator have the same frequency. Therefore the homodyne signal is constant in time.

It is assumed that the amplitude of the local oscillator signal 11 remains fixed.

In the specific example of Figure 1, the local oscillator 9 is provided by a quantum cascade laser. For example, of the type described in S. Barbieri, J. Alton, S. S. Dhillon, H. E. Beere, M. Evans, E. H. Linfield, A. G. Davies, D. A. Ritchie, R. Kohler, A. Tredicucci, and F. Beltram, J. Quantum Electron. 39, 586 (2003). The signal source 1 is also a quantum cascade laser of the same type. However, it may be provided by any coherent source or by an incoherent source, for example, a hot filament lamp.

In the example of Figure 1, the signal source is a quantum cascade laser 1. The temperature and current of the QCL 1 can be changed and in this way the frequency CO2 can be tuned to continuously probe the absorption line of the gas to be detected.

Figure l schematically illustrates a transmission measurement. Figure 2 illustrates a reflection measurement. To avoid unnecessary repetition, like reference numerals will be used to denote like features. In this example, the configuration of the local oscillator source 9 and the mixer 7 is the same as that described for Figure 1. Signal source l emits beam of radiation 3 which then passes through beam splitter/combiner 13 towards sample 5. Sample 5 can again be any sample. The reflected Terahertz radiation from the sample is then reflected by beam splitter/combiner 13 and directed towards mixer 7 where heterodyne detection is performed in the same manner as described with reference to Figure 1.

Figure 3 shows a further variation on the systems of Figures I and 2. Here, there is a single QCL laser 21 which outputs at least two co-linear beams 23 having differing frequencies. The co-linear beams 23 are then passed through sample 25 and are received at mixer 27. The mixer 27 will then perform heterodyne detection of the received co-linear beams 23 as described with reference to Figure I. This arrangement is possible because the emission spectrum of a quantum cascade laser is naturally multimode. This means that the emission is concentrated in several narrow lines (longitudinal modes) separated by a frequency that is dictated by the length of the laser cavity and the group refractive index at the emission frequency.

Since the frequency difference Ace between two single modes is inversely proportional to the length of the ridge of the quantum cascade laser, Aco can be changed as required.

In particular, Ao, can be brought into the GHz range where the Schottky mixer can operate.

The heterodyne signal is then produced by the interaction of two single longitudinal modes generated by a single laser.

This constitutes a great advantage with respect of the configurations of Figures I and 2.

The heterodyne signal is more stable since any temperature or current fluctuation in the laser produces the same effects on the amplitude and frequency of both modes. It is also advantageous in that only one device is needed instead of two.

In the arrangement of Figure 3, both modes travel through sample 25. It the sample is a gas with a narrow emission spectrum, then it is likely that only one of the modes will be effected as it passes through the sample. However, if the sample has a broad emission spectrum, the both the local oscillator signal and the sample signal will be attenuated.

Although a transmission arrangement is shown, the system may also operate in a reflection mode of the type described with reference to Figure 2. For example, the single quantum cascade laser 21 could be placed in the position of quantum cascade laser I of Figure 2 and the separate local oscillator 9 could be removed.

Figure 4 shows a further variation on the system of Figure 3. Here, a single quantum cascade laser 31 is provided and it is configured so that one beam of radiation (beam one) 33 is directed towards mirror 35 and reflected onto mixer 37.

A second beam (beam two) 39 is emitted from the opposing side of QCL laser 31 to beam one 33. The second beam 39 is then reflected off mirror 41 which then directs the radiation through beam splitter 43 onto sample 45. Radiation is then reflected from sample 45 onto beam splitter 43 and is reflected towards mixer 37. At mixer 37, heterodyne detection is performed using beam one and beam two.

Figure 5 is a variation of the system shown in figure 4, but configured for transmission as opposed to reflection. To avoid unnecessary repetition, like reference numerals have been used to denote like features. Quantum cascade laser 31 is configured as for figure 4, with beam one 33 directed towards mirror 35 and reflected onto mixer 37.

Beam two 39, is emitted from the opposing facet of QCL 31 to beam one and is reflected off mirror 41, then off mirror 51, through sample 45. The transmitted beam two 39 is then focuses by lens 53 onto mixer 37. At mixer 37, heterodyne detection is performed using beam one and beam two.

The above configurations are achieved because the output from one facet of the laser is used to probe the medium while the output from the other facet goes directly to the mixer as the local oscillator. In this configuration, the local oscillator signal does not interact with the medium to be probed and its full power can always be exploited.

Figure 6 is a plot of the output from a Schottky diode in intensity against frequency when mixing two 88 micron quantum cascade lasers.

Figure 7 is a similar plot to Figure 6 showing intensity against frequency for a Schottky mixer. The results are taken for five different temperatures of one laser, 25K, 30K, 35K, 40K and 45K. The peaks seen are due to the modes of the different lasers beating.

As the spectre of both lasers are multimode, the beating of different single longitudinal modes is seen. The peaks 101, 103 and 105 arise from when one laser is at 25K. The peaks at 107 and 109 are seen when one of the lasers is at 30K. The peaks at 111, 113 and 115 are seen when the temperature of one laser is changed from 35K, 40K to 45K respectively.

Figure 8, is a plot of intensity in arbitrary units against frequency for a blackbody emitter at 360K. Superimposed onto its particular blackbody spectrum, which is basically set by its average temperature, a sample can also have some other spectral features that are inherent to its microscopic composition. These features manifest themselves in absorption or other optical quantities and affect the blackbody radiation, or natural light, emitted by the object via Planck's law for blackbody radiation.

Figure 9a is a schematic of a system in accordance with an embodiment of the present invention. Sample 61 is a black body emitter, the sample may be any sample which generates radiation in the frequency range from 25GHz to l OOTHz itself or which can reflect or transmit THz radiation from background radiation which may be provided naturally or otherwise. The radiation outputted from sample 61 is shown in schematic plot 9b.

The radiation from sample 61 impinges on mixer 63. Quantum cascade laser 65 acts as a local oscillator for mixer 63 and outputs a beam of THz radiation having a frequency of O2. Figure 9c shows schematically a plot of Intensity against frequency showing both the output from the sample 61 and QCL 65. In this particular example, QCL 65 is shown emitting at one frequency CO2, however, QCL 65 may be configured to emit in multimode.

In figure 9c, it can be seem that since the QCL frequency CO2, lies within the range of frequencies emitted by sample 61, detection may be performed in homodyne or heterodyne modes.

The output from the QCL 65 and the sample 61 are mixed at mixer 63 and the radiation is detected as previously described. Figure 9d schematically illustrates the output 01-2 of mixer 63.

Claims

CLAIMS: 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 25GHz to I OOTHz, 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, the detector being configured to perform heterodyne detection.
2. 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 25GHz to 1 OOTHz, 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 path length of the sample signal relative to the local oscillator signal remains fixed during investigation of the sample.
3. A system according to claim 2, wherein said detector is configured to perform heterodyne detection.
4. A system according to any preceding claim, 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.
5. A system according to claim 4, wherein quantum cascade laser emits a source signal having a different frequency to the local oscillator signal.
6. A system according to either of claims 4 or 5, 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.
7. A system according to claim 6, 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.
8. A system according to any of claims I to 3, further comprising an independent source to produce a source signal, said source signal being transmitted by or reflected from said sample in order to produce said sample signal
9. A system according to any of claims I to 3, 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.
10. A system according to claim 2, wherein the detector is configured as a homodyne detector and mixes a sample signal and a local oscillator signal having the same frequencies.
11. A system according to any preceding claim, wherein the sample signal is a polychromatic signal.
12. A system according to any preceding claim, wherein the local oscillator signal is also transmitted by or reflected from the sample.
13. A system according to any preceding claim, configured as an imaging system.
14. A system according to any preceding claim, configured for taking spectra of said sample.
15. 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 25GHz to I OOTHz, 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.
16. A receiver for an investigative system, said receiver comprising a detector configured to perform heterodyne detection having non-linear current voltage characteristics and being configured to mix two radiation signals having frequencies in the range from 25GHz to I OOTHz, the system further comprising a quantum cascade laser for providing at least one signal to said detector.
17. A method of investigating a sample, said method comprising: providing a local oscillator signal to a detector from a quantum cascade laser, said detector having non-linear current voltage characteristics; and receiving said local oscillator signal at said detector with a sample signal received from a sample under test, wherein said first and second frequencies are both in the range from 25GHz to 1 OOTHz and said detector is configured to perform heterodyne detection by mixing two radiation signals having different frequencies in this range.
18. A method of investigating a sample, said method comprising: providing a local oscillator signal to a detector from a quantum cascade laser, said detector having non-linear current voltage characteristics; and receiving said local oscillator signal at said detector with a sample signal received from a sample under test, wherein said first and second frequencies are both in the range from 25GHz to 1 OOTHz and said detector is configured to mix two radiation signals having frequencies in this range and the path length of the sample signal relative to the local oscillator signal remains fixed during investigation of the sample.
19. A method of investigating a sample, said method comprising: providing a local oscillator signal to a detector from a quantum cascade laser, said detector having non-linear current voltage characteristics; and receiving said local oscillator signal at said detector with a sample signal received from a sample under test, wherein said first and second frequencies are both in the range from 25GHz to I OOTHz and said detector is configured to mix two radiation signals having frequencies in this range and the sample signal is produced by the sample itself or arises from natural background radiation being transmitted by or reflected from the sample.
20. A system as substantially hereinbefore described with reference to any of the accompanying figures.
21. A receiver as substantially hereinbefore described with reference to any of the accompanying figures.
22. A method as substantially hereinbefore described with reference to any of the accompanying figures.