Abstract:
An apparatus and method for imaging a sample, the apparatus including a source for irradiating a sample with a beam of substantially continuous electromagnetic radiation having a frequency in the range 25 GHz to 100 THz; means for subdividing an area of the sample which is to be imaged into a two dimensional array of pixels; means for detecting radiation from each pixel wherein the detector is configured to detect a phase dependent quantity of the detected radiation which is measured relative to the radiation which irradiates the sample.

Description:
BACK GROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of imaging apparatus and methods. More specifically, the present invention relates to imaging using frequencies in the range overlapping the infrared and microwave parts of the spectrum. This frequency range encompasses the so-called Terahertz (THz) frequency range and is often referred to as Terahertz radiation. 
     2. Description of Related Art 
     Recently, there has been considerable interest in THz pulse imaging (TPI) which is showing promising results for both medical and non-medical use. THz radiation is non ionising radiation. Therefore, it is believed to be medically safer than well established x-ray techniques. The lower power levels used (nW to μW) also suggest that heating effects are not problematic, as may be the case with microwaves for example. 
     THz pulse imaging uses a plurality of frequencies within a single pulse in order to probe the frequency dependent absorption characteristics of the sample under test. Pulsed sources suffer from the drawback that they are expensive and also it is difficult to efficiently transmit pulses down optical waveguides etc. The complexity of the transmitted and reflected pulses, in lossless and in particular lossy mediums, also renders interpretation of the pulsed data difficult. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention addresses the above problems and, in a first aspect, provides an apparatus for imaging a sample, the apparatus comprising:
         a source for irradiating a sample with a beam of substantially continuous electromagnetic radiation having a frequency in the range from 25 GHz to 100 THz;   means for subdividing an area of the sample which is to be imaged into a two dimensional array of pixels,   means for detecting radiation from each pixel wherein the detector is configured to detect a phase dependent quantity of the detected radiation which is measured relative to the radiation which irradiates the sample.       

     The term substantially continuous is hereinafter taken to mean that the radiation source outputs radiation for all or most of the time, if the output of or from the source is gated such that the flow of radiation from the source is periodically interrupted, the length of the interruptions will be shorter than the length of time over which the source is continuously producing radiation. 
     A single or plurality of frequencies in the range from 25 GHz to 100 THz is used. Preferably, the frequency is in the range from 50 GHz to 84 THz, more preferably 100 GHz to 20 THz. 
     The present invention uses a single frequency or a plurality of discrete frequencies through the sample at any one time. Information concerning the internal structure of the sample can be determined from radiation of a single frequency as variations in the phase of the radiation as it passes through the sample will allow structural information such as the width of the sample and compositional information about the sample to be obtained. 
     The use of just a single frequency through the sample at any one time means that relatively inexpensive single frequency dedicated sources may be used. 
     The frequency of the radiation incident on the sample can be varied by known methods in order to obtain information about the frequency dependent characteristics of the sample. 
     Alternatively, the radiation incident on the sample can comprise two or more discrete frequencies. These frequencies are preferably selected to probe different materials or components in the sample. 
     The detector is preferably configured to detect a phase dependent quantity of each frequency component relative to the radiation which irradiates the sample. 
     This means that broadband incoherent or short coherence length radiation may also be used as random variations in the phase between the different frequency components do not matter since the phrase change for each frequency component is measured. 
     It is difficult to produce an efficient and powerful source for THz radiation as there is no good naturally occurring source of such radiation. Previously, there have been two main methods for generating THz radiation. The first has been to use a solid state radiation source such as a Gunn diode, molecular laser, free electron laser, cascade laser etc. The second has been to convert commonly available radiation such a radiation in the visible or near IR range, lower frequency microwaves into THz regime using a frequency conversion member. 
     The frequency conversion member could be an optically non-linear material which is configured to emit a beam of emitted radiation in response to irradiation by two input beams, or a photoconductive antenna which upon application of an electric field is configured to emit a beam of emitted radiation in response to irradiation by two input beams. The emitted beam has a frequency which is equal to the difference of the two input beams. In these examples, the input beams will generally have a frequency which is in the visible or near IR frequency range. 
     Preferably, two beams of input radiation will be supplied by two continuous wave (CW) sources. Such continuous wave sources may be two near-infrared/visible lasers. Three or more continuous wave sources may also be used to generate an emitted beam having two or more frequencies. Alternatively, a single source running in multi mode, i.e. outputting two or more wavelengths at the same time, could also be used. A broadband source could also be used. 
     Alternatively, the optically non-linear member could be configured to emit a beam of emitted radiation in response to irradiation by an input beam, the emitted radiation having a frequency which is a harmonic of the frequency of the input radiation. The input beam could have a frequency in the low frequency microwave range. 
     The detector measures a change in phase dependent quantity of the radiation, this might be a direct measurement of the phase itself, or a measurement of the electric field which is transmitted through or reflected from the sample, the amplitude of which will be phase dependent etc. 
     In order for the detector to be able to detect the phase dependent quantity with respect to the radiation which irradiates the sample, the detector needs to have some way of knowing information about the phase of the radiation which irradiates the sample. A convenient way to achieve this is for the detector to receive a probe beam which has a phase related to that of the radiation which is used to irradiate the sample. 
     The probe beam could be obtained by splitting the one or more of the input beams or it could be provided by splitting the Terahertz beam used to irradiate the sample. The detector could directly detect the probe beam or the probe beam could be combined with the radiation which has been transmitted through or reflected from the sample before detection. This combining of the two beams could be achieved by using a mixing component. 
     As previously mentioned, broadband incoherent radiation could also be used. A broadband source generates radiation having a plurality of different frequencies. Unlike pulsed laser sources, phase relationship between the different frequency components. Thus, there is a random phase relationship between the different frequency components in a broadband source. If part of this broadband beam is also used as the probe beam then the fact that the beam is incoherent is of no consequence, since only the phase difference for each frequency component is measured. 
     In order to detect the phase dependent quantity, the apparatus further preferably comprises a phase control means, which can be used to control the phase of the probe beam or the beam of radiation which irradiates the sample. The phase control means may be provided by an optical delay line which varies the length of the path of the probe beam with respect to the length of the path of the irradiating radiation. Of course, the length of the path of the irradiating radiation could be varied with respect to the length of the path of the probe beam to achieve the same result. 
     The length of the path of the probe beam can be varied during the imaging process to obtain information relating to the phase of the detected radiation. The path length of the probe beam could also be oscillated or dithered about a point. The oscillation period or ‘dithering’ period could be used for lock-in detection by the detector. 
     Once the THz is emitted from the sample, detection is required. A particularly useful detection technique is to use Electro-Optic Sampling (EOS) which uses the AC Pockels effect. The detector may comprise a photoconductive antenna. 
     It is also possible to combine the beam which has been reflected from or transmitted by the sample with another beam of radiation which has substantially the same wavelength or which differs in frequency by at most 10 GHz. Such combined radiation can be detected using a bolometer, Schottky diode etc. 
     Possible materials which posses good non linear characteristics for any of the above mechanisms are GaAs or Si based semiconductors. More preferably, a crystalline structure is used. The following are further examples of possible materials: 
     NH 4 H 2 PO 4 , ADP, KH 2 PO 4 , KH 2 ASO 4 , Quartz, AlPO 4 , ZnO, CdS, GaP, BaTiO 3 , LiTaO 3 , LiNbO 3 , Te, Se, ZnTe, ZnSe, Ba 2 NaNb 5 O 15 , AgAsS 3 , proustite, CdSe, CdGeAs 2 , AgGaSe 2 , AgSbS 3 , ZnS, organic crystals such as DAST (4-N-methylstilbazolium). 
     The apparatus is used to image an area of the sample. An area of the sample can be imaged in a number of different ways. For example, the sample can be moved with respect to the beam or the beam with respect to the sample. Alternatively, the sample could be illuminated with a wide beam or a plurality of beams from different sources. The detector could also be configured in a similar manner. The detector could comprises a CCD camera which will allow a large area of the sample to be examined at once. 
     The above description has been mainly concerned with generating and detecting THz radiation using non-linear materials. However, there are other methods. A particularly useful detection method is to combine the THz radiation which is emitted from the sample with another beam of THz radiation. This radiation can then be passed through a non-linear member which allows the difference of the radiation, which may typically be in the GHz range to be detected. 1 GHz is in the microwave range and detectors for such radiation are well known in the art. 
     The present invention can use a small number of single frequency sources in order to generate the THz radiation. Therefore, it is possible to construct a highly efficient THz probe where the probe is located remote from the source of input radiation. For example, if the source of input radiation is two visible wavelength CW lasers, two fibre optic cables which are each optimised to the frequency of the relevant CW laser can be used to carry the input radiation to a probe. The probe may be for example an endoscope which can be inserted into the human body or a surface probe for skin or teeth, or other non-medical items. The purpose of the probe may be either to collect local spectral or other diagnostic information, or alternatively it may be run in an imaging mode by being dragged across the surface or having the surface dragged across it. The THz radiation can then be generated within the endoscope by using a frequency conversion member 
     The THz radiation can then be detected in the same manner as previously described. The probe or reference beam can be split from one of the CW laser inputs. The reference beam with a rotated polarisation can then be transmitted down a polarisation preserving optical fibre back to analysis equipment. Alternatively, photoconductive emitters and detectors may be placed on the end of the fibre, in which case electrical power may have to be supplied by additional wires. 
     A broadband source may also be used to provide the radiation. It is difficult to send a plurality of frequencies down a fibre as a pulse since the high peak power of the pulse can given rise to non-linear effects what may destroy the pulse. Broadband radiation provides a continuous lower level and hence does not suffer from this problem. Also, a single multimode CW source may also be used. 
     In a second aspect, the present invention provides a method of imaging a sample, the method comprising the steps of irradiating a sample with substantially continuous radiation with a frequency in the range form 25 GHz to 100 THz; subdividing an area of the sample which is to be imaged into a two dimensional array of pixels; detecting radiation from each pixel, wherein the detector is configured to detect a phase dependent quantity of the detected radiation which is measured relative to the radiation which irradiates the sample. 
     In a third aspect, the present invention provides an apparatus for investigating a sample, the apparatus comprising means for generating a beam of substantially continuous electromagnetic source radiation having a frequency in the range 25 GHz to 100 THz; means for moving the sample relative to the beam to scan the beam over the sample; means for detecting the radiation transmitted by or reflected from the sample; wherein the means for detecting includes means for detecting a change in a phase dependent quantity of the transmitted or reflected radiation relative to the source radiation. 
     In a fourth aspect, the present invention provides an apparatus for investigating a sample, the apparatus comprising means for generating a beam of substantially continuous electromagnetic source radiation having a frequency in the range 25 GHz to 100 THz; means for moving the sample relative to the beam to scan the beam over the sample; means for detecting the radiation transmitted by or reflected from the sample; wherein the means for detecting includes means for comparing a phase dependent quantity of the transmitted or reflected radiation with that of the source radiation. 
     In a fifth aspect, the present invention provides an apparatus for investigating a sample, the apparatus comprising means for generating a beam of substantially continuous electromagnetic source radiation having at least two frequency components in the range from 25 GHz to 100 THz; means for detecting radiation transmitted by or reflected from the sample; wherein the means for detecting includes means for detecting a change in a phase dependent quantity of each frequency component of the transmitted or reflected radiation relative to the source radiation. 
     In a sixth aspect, the present invention provides an apparatus for investigating a sample, the apparatus comprising means for generating a beam of substantially continuous electromagnetic source radiation having at least two frequency components in the range from 25 GHz to 100 THz; means for detecting radiation transmitted by or reflected from the sample; wherein the means for detecting includes means for comparing a phase dependent quantity of each frequency component of the transmitted or reflected radiation with that of the source radiation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       Thus, the present invention can be used for both imaging a sample and also studying the spectra of a sample at a point. 
       The present invention will now be described with reference to the following non-limiting preferred embodiments in which: 
         FIG. 1  shows a schematic imaging system in accordance with an embodiment of the present invention; 
         FIG. 2  shows a variation of the imaging system of  FIG. 1 ; 
         FIG. 3  is a schematic generator which may be used in either of the imaging systems of  FIGS. 1 and 2 ; 
         FIG. 4  shows a generator which may be used in either of the imaging systems of  FIGS. 1 and 2 ; 
         FIG. 5  shows a generator which may be used with either of the imaging systems of  FIGS. 1 and 2 ; 
         FIG. 6  shows a generator which may be used with either of the imaging system of  FIGS. 1 and 2 ; 
         FIG. 7  shows the generator of  FIG. 6  in more detail; 
         FIGS. 8   a ,  8   b  and  8   c  show further variations on the generators of  FIGS. 6 and 7 ; 
         FIG. 9  shows a variation on the generator of  FIG. 6 ; 
         FIGS. 10 ,  10   a  and  10   b  show a detector which may be used with either of the imaging systems of  FIG. 1  or  2 ; 
         FIG. 11  shows a detector which may be used in accordance with either of the imaging systems of  FIG. 1  or  2 ; 
         FIG. 12  shows an imaging system in accordance with an embodiment of the present invention, using diode lasers to generate the imaging radiation; 
         FIG. 13  shows an imaging system in accordance with an embodiment of the present invention using an electro-optic detection technique; 
         FIG. 14  shows an imaging system in accordance with an embodiment of the present invention using diode lasers and mixing elements; 
         FIG. 15  shows an imaging system n accordance with an embodiment of the present invention using a photoconductive antenna as a detector; 
         FIG. 16  shows an imaging system in accordance with an embodiment of the present invention using photoconducting antenna in both the generator and the detector; 
         FIG. 17  shows an imaging system in accordance with an embodiment of the present invention using a frequency multiplier; 
         FIG. 18  shows an imaging system in accordance with an embodiment of the present invention, using a laser source which can directly output radiation in the desired frequency range; 
         FIG. 19  shows a variation of the imaging system of  FIG. 18  using an optical mixer; 
         FIG. 20  shows a dual frequency imaging system in accordance with an embodiment of the present invention; 
         FIG. 21  shows an imaging probe in accordance with an embodiment of the present invention; 
         FIG. 22  shows a further detail of the imaging system of  FIG. 21 ; and 
         FIG. 23  shows an apparatus in accordance with an embodiment of the present invention using a broadband source. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the imaging system of  FIG. 1 , radiation is generated from THz generator  1 . THz generator  1 , generates terahertz radiation with a single frequency in the range from 0.025 THz to 100 THz. (Details of THz generator  1  will be described with reference to  FIGS. 3 to 9 .) The THz radiation emitted from the generator  1  irradiates sample  3 . 
     Sample  3  is located on a stage (not shown), the stage is capable of moving sample  3  through the beam of radiation emitted from generator  1  in the x and y directions. The x and y directions being taken as two orthogonal directions which are substantially perpendicular to the path of the incident irradiating radiation from the source  1 . 
     Sample  3  will both transmit and reflect radiation. In the specific example of  FIG. 1 , the sample is only shown to transmit radiation and only transmitted radiation will be detected. However, reflection measurements are possible. 
     The transmitted radiation is detected by detector  5 . (Examples of the types of detector which may be used will be described with reference to  FIGS. 10 and 11 . Further variations on the imaging system and detector will also be described with reference to  FIGS. 12 to 22 ). 
     The detector  5  is used to detect both the amplitude and phase of the radiation emitted from the sample  3 . In order to do this, there is a phase coupling/control means  7  provided between the detector (or an input to the detector) and the generator  1  or an input/output from generator  1 . This phase control/coupling means will either provide the detector with a parameter corresponding to a phase input which can be varied relative to the source beam or it will vary the phase of the source beam with respect to a probe beam which will be supplied to an input of the detector. 
     Typically, a beam, a ‘probe beam’ with a known phase relationship to that of the imaging radiation is fed into the phase coupling/control means  7 . The phase coupling control means will typically comprise a variable optical path line which will allow the path length of the probe beam to be varied. 
     In many cases, the probe beam will be combined with the THz radiation which is transmitted through the sample  3 . One particularly popular way is to use electro-optic sampling (EOS). This type of detector will be described in more detail with respect to  FIG. 10 . 
     An explanation of how the phase and amplitude of the transmitted radiation is detected will be described for use with EOS detection. However, it will be apparent to those skilled in the art that this type of analysis could be performed for any type of detector. 
     In this type of detector, the THz beam and the probe beam co-linearly propagate through a detection member. The transmitted THz electric field passes through this member and will be referred to as E THz (t). The intensity of the probe beam is I probe (t). The transmitted radiation from the sample  3  passes through the detection member and modulates the probe beam. The emitted probe beam intensity can be written:
 
ΔI eo (t)αI probe (t)E THz (t).
 
 I   probe ( t )= I   0   opt   [A+ cos(ω THz   t−φ   p )]
 
     Where I 0   opt  is the maximum intensity of the probe beam, A is a constant, ω THz  is the frequency of the THz radiation and φ p  is the phase of the probe beam.
 
 E   THz ( t )= E   THz  cos(ω t−φ   THz ).
 
     Where E THz  varies as I 0   opt ; and φ THz  is the phase of the THz radiation. 
     Hence
 
ΔI eo αI opt E THz  cos(φ THz −φ p )  (1)
 
     E THz  and φ THz  will depend on the sample. Therefore, by varying φ THz −φ p , it is possible to determine E THz  and  φTHz . It should be noted that either φ p  or φ THz  can be varied. The change in φ THz  due to the sample will be a constant for a fixed frequency. 
     Further, varying the quantity φ THz −φ p  allows the time of flight of the THz pulse through the sample to be determined. 
     The phase of the Terahertz beam, φ THz =ω.n t .d t /c and the phase of the probe beam is φ p =ω THz n p d p /c 
     Where n THz  and n p  are the refractive index (or indices) associated with the path lengths of THz and probe, respectively. d t  and d p  are the path lengths associated with the THz and probe, respectively. 
     Δφ=φ THz −φ p  may be measured using photoconductive, EOS or other detection techniques where the detector has phase knowledge of the generated THz. In the case of photoconductive of EOS techniques, these detection techniques may applied to coherently generated THz, and may be used to deduce the width and refractive index of the medium. This is because in the most general case, Δφ=φ THz −φp may be written as
 
Δφ=φ THz −φ p =ω THz   /c ( d   t   n   t   −d   p   n   p )
 
which obtains explicit expression for the refractive index or indices n t  and path lengths (thickness) d t  of the sample  3 .
 
     The cosine dependence of Eq. (1) implies that as one of the path lengths (say d p ) is changed, a maximum in the measured signal occurs whenever
 
2π i=αφ=ω   THz   /c ( d   t   n   t   −d   p   n   p )
 
     where i is an integer denoting the i th  oscillation, and
 
 d   t   n t =ic/f   THz   +d   p   n   p   , f   THz =ω THz /2π.
 
     Because f THz =(f 1 −f 2 ) is known accurately from the optical/near-IR frequencies, or by conventional calibration means in the case of electronic sources such as Gunn diodes, and d p  (determined by the delay in the probe beam) and n p  (typically=1 for free space) are accurately known, it is possible to determine d t  and n t  of the object under study at each pixel in the image. 
     By moving the sample through the THz beam, or alternatively scanning the beam across the sample, it is possible to build up refractive index or thickness image of the object. It is also possible to build up transmission or absorption images of the sample using information from the detected E THz . 
     This may be done in transmission, reflection, or a combination of the two. For the case of the refractive index, panchromatic images are additionally possible (in addition to the monochromatic image described above) by tuning ω THz  to different values at each pixel. Where the THz radiation is produced by converting the frequency of one or more input beams in radiation within the THz range, it is possible to sweep the frequency of one or more of the input beams. The emitted THz radiation may be tuned, for example, by varying the frequency of one of the near IR/visible diodes if photoconductive or difference frequency generation means are utilised in generation, or alternatively by voltage tuning or cavity tuning of electronic devices such as Gunn diodes are utilised. 
     There are a variety of ways to obtain an image of sample  3 : 
     1) Monochromatic transmission/absorption: 
     The delay of the probe beam (d p ), which is essentially one way of sweeping the phase of the detector relative to the source, may be swept at each pixel, and the measured peak amplitude may be plotted at each pixel as the object is rastered through the beam (or the beam through the object). Alternatively, the absorption coefficient may be extracted from the ratio of the peak amplitude to that of a reference e.g. free space and then plotted for each pixel. 
     2) Panchromatic transmission/absorption: As for 1) except, it is performed for a variety of different THz frequencies ω THz  Individual monochromatic images may be compared, ratioed, subtracted, added etc. Alternatively the transmission or absorption at each pixel may be integrated over a range of measurement each at different ω THz . 
     3) Thickness of the image. The probe delay d p  as explained with reference to 1) above may be swept at each pixel and the product d t n t  may be extracted from suitable manipulation of the above equations. d t  so obtained at each pixel using pre-determined n t  can be plotted across the sample to build up a thickness (tomographic) image. 
     4) Refractive index image: Manipulation of the above equations measuring phase, n t  can be plotted using a fixed d t . Monochromatic (at single ω THz ) and/or panchromatic (over a multitude of ω THz  analogus to point 2) above) images may be used. 
     5) Alternatively, a fixed delay d p  can be used. If d p  and n p  are fixed as well as ω THz  the sample can be rastered through the beam (or vice versa). All variations in the image produced are either due to changes in the thickness of the object, the refractive index of the object or due to changes in absorption of the object. 
       FIG. 2  shows a further variation on the imaging system of  FIG. 1 . As for  FIG. 1 , the imaging system comprises a generator  1  which irradiates a sample  3 . Radiation which is transmitted or reflected by the sample  3  is then detected by detector  5 , to output signal  6 . The detector  5  is configured to be able to detect a phase dependent quantity of the detected radiation via phase coupling/control means  7  which serves to input a signal into the detector concerning the phase of the radiation emitted from the generator. 
     In this example, the sample  3  remains fixed and the incident radiation beam is swept in the x and y direction with respect to the sample. A beam sweeping stage  11  is positioned between the generator  1  and the sample  3 , this serves to ‘raster’ the incident radiation across the surface of the sample. A beam detection stage  13  is located between the sample  3  and the detector  5 . The beam detection stages sweeps detection optics used to detect radiation transmitted through the sample  3  with the beam irradiating the sample  3 . Usually, the beam sweeping stage  11  and the beam detection stage  13  will be swept together using the same stepper motor to ensure that both stages move together. In some instances such as if the detector is based on CCD or Terahertz imaging arrays of mixers, it may not be necessary to have stage  13 . 
       FIG. 3  shows a so-called χ(2) method for producing THz radiation. The generator  1  in both of  FIGS. 1 and 2  could work using this principle. 
     Typically, the Polarisation of a medium can be written as:
 
PαχE
 
     Where χ is the polarisability of the medium and E is the incident electric field. In reality the polarization should be written as: 
     PαχE+χ (2) E 2 +χ (3) E 3  etc. In many materials, the higher order terms such as χ (2)  will be negligible, but in some materials and especially non-centrosymmetric crystals, they will be significant. 
     A large χ (2)  can manifest itself in a number of ways. If such a crystal is irradiated with a single frequency then the second harmonic of the frequency can be emitted by the crystal. If the crystal is irradiated by the different frequencies ω 1  and ω 2 , radiation having a frequency which is the difference or the sum of these frequencies is outputted. Which will depend on the configuration and properties of the crystals. 
       FIG. 3  shows such an arrangement. The electrons in the non-linear material which will be referred to as the ‘frequency conversion member’  15  can be thought of as being on springs. As the frequency conversion member  15  is irradiated with visible or infra red radiation ω 1  and ω 2 , the electrons vibrate to emit radiation with a THz frequency, the THz radiation ω THz =ω 1 −ω 2 . 
     Typically, such frequency conversion member will have phase matching means in order to keep the transmitted THz signal and the incident radiation in phase as they pass through the frequency conversion member. Such phase matching can be achieved by providing the frequency conversion member with a variation in its refractive index configured to keep the two signals in phase (at all points) as they pass through the frequency conversion member. 
       FIG. 4  shows a THz generator using a frequency conversion member as described above. The radiation used to generate the THz radiation via frequency conversion member  15 . Radiation is supplied to frequency conversion member  15  from Ti:Sapphire crystals  17   a  and  17   b . Ti:Sapphire crystal  17   a  emits radiation with a frequency of ω 1  (the first pump beam) in response to radiation with laser driving beam  19  and Ti:Sapphire crystal  17   b  emits radiation with a frequency ω 2  (the second pump beam) in response to irradiation with pump beam  19 . In order to provide efficient lasing, it is desirable to continually reflect the first and second pump beams onto Ti:Sapphire crystals  17   a  and  17   b . Therefore, the lasing crystals  17   a  and  17   b  are typically provided within a lasing cavity. 
     The driving beam  19  is directed onto crystals  17   a  and  17   b  using mirrors M 1  and M 2 . The driving beam  19  can pass through mirror M 3  and onto lasing crystals  17   a  and  17   b . The driving beam  19  which is not absorbed by crystals  17   a  and  17   b , is emitted through mirror M 4 . Mirror M 4  serves to reflect any radiation with frequencies ω 1  and ω 2  back onto the lasing crystals  17   a  and  17   b . This radiation is then reflected via mirror M 3  onto mirror M 5  and onto output coupler  21 . Output coupler  21  serves to reflect radiation with the frequencies ω 1  and ω 2  onto the frequency conversion member  15  to produce ω THz =ω 1 −ω 2 . The pump beams are focused onto frequency conversion member  15  via lens L 1 . Any radiation which is transmitted through the frequency conversion member  15  is reflected back through the frequency conversion member  15  by mirror  6 . This radiation then impinges on output coupler  21 . 
     Output coupler  21  transmits radiation with the frequency ω THz , but it reflects light with the frequencies ω 1  and ω 2  back onto mirror M 5 , which in turn reflects the radiation back onto the lasing crystals  17   a  and  17   b  via mirror M 3 . In other words, the lasing crystals  17   a ,  17   b  and the frequency conversion member  15  are all located within the same lasing cavity defined by mirror M 6 , the output coupler and mirrors M 5 , M 3  and M 4 . Radiation with frequencies ω 1  and ω 2  are constantly reflected within this cavity to efficiently generate the pump beams and the THz beam. 
     Other types of generator may also be used.  FIG. 5  illustrates a so-called photoconductive emitter. The emitter comprises a member  23  comprising a semiconductor such as low temperature GaAs, GaAs, Si on Sapphire etc. The semiconductor member has a pair of electrodes  25   a  and  25   b  located on its surface. The electrodes  25   a  and  25   b  are connected to a power supply such that a field can be generated between the two electrodes  25   a  and  25   b.    
     The simplest electrode arrangement is show in  FIG. 5 . However, the electrodes may be triangular and arranged in a bow-tie shape, a so-called bow-tie antenna or they may be interdigitated electrodes at the centre of a bow tie or spiral antenna. Alternatively, such designs may be incorporated into transmission lines on the chip. 
     The semiconductor member is irradiated by two pump beams with frequencies ω 1  and ω 2 . The pump beams impinge on the semiconductor member  23  on the part of its surface between the electrodes  25   a  and  25   b , i.e. where the field is applied. The beating of the two visible or near-infrared lasers in the non-linear region of the semiconductor member between the two electrodes  25   a  and  25   b  results in the emission of THz radiation from the semiconductor member  23 . The semiconductor member  23  is provided with a lens  27 , which may be of a hemispherical or other design, on its surface which is opposite to that of the surface with the electrodes, to allow the emission of a beam of THz radiation. 
       FIG. 6  shows a further type of generator. This is a so-called cascade laser which directly generates the THz radiation from the application of a bias i.e. there is no need to supply a pump-beam. The cascade laser uses three coupled quantum wells  31 ,  33  and  35  interposed between an emitter  37  and a collector  39 . Possible layer structures for the laser will be discussed with reference to  FIG. 9 . 
       FIG. 6  shows a conduction band of a cascade laser, the three quantum wells  31 ,  33  and  35  are coupled such that the excited energy levels extend across the three quantum wells. Three excited energy levels  41 ,  42  and  43  are populated and/or depopulated during the emission process. The emitter  37  comprises an emitter contact  45  separated from an injector quantum well  47  by emitter energy barrier  49 . An electron from the emitter contact  45  tunnels through barrier  49  into injector quantum well  47 . 
     The laser is configured such that the confined energy level in the injector quantum well  47  aligns with the highest energy level  41  of the triple quantum well arrangement  31 ,  33  and  35 . This results in the electron in the injector quantum well  47  resonantly tunnelling into highest energy level  41  of the triple quantum well system  31 ,  33  and  35 . The electron in this energy level relaxes into the second energy level  42 . During this process, it emits a photon with a wavelength in the THz range, in other words a THz photon. The electron which is now in the second level  42  will either be swept into the collector  39  through collector barrier layer  51 , or it will relax further into the lowest energy level  43  of the quantum well structure, emitting an phonon and then tunnel through collector barrier  51  into the collector contact  39 . 
     In practice, the laser will contain a plurality of triple quantum well structures as shown in  FIG. 7 . Here, there are two triple quantum well structures  31   a ,  33   a ,  35   a  and  31   b ,  33   b ,  35   b . As explained in relation of  FIG. 6 , an electron is injected into triple quantum well region  31   a ,  33   a ,  35   a  and a THz photon is emitted due to the electron relaxing from the highest energy level  41   a  and the middle energy level  42   a . The electron will then relax via a phonon process into the lowest energy level  43   a.    
     In the laser of  FIG. 6 , the electron tunnels into the collector via collector energy barrier  51 . In  FIG. 7 , once the electron is in the lowest energy level  43   a , it tunnels through energy barrier  61  into the second injector quantum well  63 . Once in this well, the electron tunnels through energy barrier  65  into the highest level  41   b  of a second triple quantum well system  31   b ,  33   b ,  35   b  where the process is repeated. Once the electron reaches the lowest level  43   b  in this second quantum well structure, the electron tunnels through energy barrier  67  into third injector quantum well (not shown) and so on. Typically, there will be about 30 triple quantum well structures. 
     In  FIGS. 6 and 7  the lasing region of the laser or the ‘active region’ is formed by a triple quantum well structure. However, it is possible to also fabricate a lasing region which has four or more quantum wells. This is shown in  FIG. 8   a . Here, the lasing region comprises 6 quantum wells. Providing that the wells are configured such that the difference in energy between two of the levels is such that this transition gives rise to emission of a THz photon then any number of quantum wells can be used. Once the electrons exit the active region  71  they tunnel into injector region  73  which serves to inject the electrons into second active region  75  for the process to begin again. 
     In  FIG. 8   a , electrons in the active region both emit THz and relax back into their lowest energy state. However, it is possible for this lower energy transition to be achieved by in the injector region as shown in  FIG. 8   b . Here, the electrons are only allowed to make a single transition in the active region  71 . Using the reference numerals of  FIG. 6 , they are only allowed to tunnel from the highest level  41  to the middle layer  42 . The electrons then tunnel into the injector region and relax from the middle level  42  into the lower level  43  ready for injection into the second active region  75  within the injector  73 . 
     In all of the previous examples, the electrons in the injector have resonantly tunnelled into the highest energy level of the active region i.e. the energy of the carrier in the injector quantum well has been aligned with that of the highest energy of the lasing region. However, the electron could relax from a higher energy level in the injector into the highest energy level of the active region as shown in  FIG. 8   c.    
       FIG. 9  shows a further variation on the cascade laser of the  FIGS. 6 to 8 . In the above, the lasing region comprises three or more quantum wells and the all of the electron transitions have been intra-band transitions and specifically conduction band transitions. 
       FIG. 9  shows a cascade laser where the lasing region  91  is formed by two semiconductors which exhibit a type-II heterojunction. Initially, looking at the lasing region,  91 , the region has a first semiconductor layer  93  located adjacent a second semiconductor layer  95 . Possibly, a thin semiconductor barrier layer could be located between the first and second semiconductor layers. The first excited level  97  of the conduction band  93   a  in the first semiconductor layer  93  is located above a level  99  of the valence band  95   b  of the second semiconductor layer  95 . The energy separation between conduction band level  97  and valence band level  99  is such that an electron relaxing from the upper level  97  to the lower level  99  causes the emission of a THz photon. 
     The other regions of the device remain essentially similar to those described with reference to  FIG. 9 , the electron is injected into level  97  from injector layer  101  which is separated from the lasing region by injector tunnel barrier  103 . Once the electron exits level  99  it tunnels through the injector region  105  which in this example is a digitally graded super lattice. 
     A typical layer structure for example  9  would have the lasing region being formed from InAs and GaSb. The barrier layers could be formed from AlSb and the injector  101  could be n + InAs. The superlattice  105  is formed from InAs/AlSb. 
       FIGS. 10 and 11  show typical detectors which can be used with the imaging systems of  FIGS. 1 and 2 . 
       FIG. 10  illustrates a possible detection mechanism which utilises the physical phenomenom known as the AC Pockels effect. The detector comprises a detection member  111 . The transmitted THz radiation  113  from the sample  3  ( FIG. 1 ) is detected by passing a visible beam or ‘probe beam’  115  through the detection member  111  with the THz beam  113 . The THz beam  113  modulates the birefringence of the detection crystal  111  as the AC Pockels effect gives:
 χ 0 E 0 +χ (2) E 0 E THz             n o +Δn(E THz )
     Prior to entry into the detection member  111 , the THz beam  113  and the probe beam  115  are polarised.  FIG. 10   a  shows the situation where there is no THz beam. Here, the probe beam passes unaffected through the detection crystal  111 . It is then passed into quarter wave plate  117 . This serves to circularly polarise the emitted radiation as shown in  FIG. 10   a . The circularly polarised light is then fed through Wollaston prism  119  which divides the polarization of the light onto two orthogonal components. These two orthogonal components are then directed onto balanced photodiode assembly  121 . The balanced photodiode assembly comprises two photo diodes  123 , 125  to respectively detect each of the orthogonal components from the Wollaston prism  119 . The output of the photodiodes  123  and  125  are linked together such that the balanced photodiode assembly  121  only outputs an electrical signal if there is a difference between the readings of the two photodiodes  123 ,  125 . 
     In the case of  FIG. 10   a , there is no difference between the two signals as there is no THz beam present.  FIG. 10   b  shows the case where there is a THz beam  113 . The THz beam  113  serves to make the radiation exiting the detection member  111  slightly elliptically polarised. This change in the polarization still remains after the radiation is passed through quarter waveplate  117 . Extracting the orthogonal components of this radiation using prism  119  causes a different signal to be measured at the two photodiodes  123 , 125  and hence balanced photodiode assembly  121  outputs a signal corresponding to the strength of the THz field  113 . 
       FIG. 11  shows a further example of a detector which may be used with the imaging systems of  FIGS. 1 and 2 . This type of detector is known as a photoconductive detector and comprises a detection member which may be, for example, GaAs, Si on Sapphire etc. The THz radiation is incident on the back surface of the detection member  131 . The radiation is collected by lens  133  which may be hemispherical or have another shape. On the opposing side of the detection member  131  is located a pair of electrodes  135  and  137 . The region between these two electrodes  135  and  137  is illuminated by radiation of the visible or near infrared range. As the detector needs to know information about the phase of the radiation emitted from the generator  1  (see  FIG. 1 ), then this radiation preferably carries such information. Typically, the THz radiation which is used to image the sample will be described from this radiation. The near-infrared/visible radiation illuminates the surface of the detector between the electrodes  135  and  137 . The Terahertz radiation which is collected by lens  133  induces a photocurrent through the region between the electrodes  135  and  137  which is being illuminated by the visible/infrared radiation. The current which can be detected by the electrodes is proportional to the strength of the THz field. 
     The electrode  135 ,  137  may be of a simple diode formation embedded in a transmission line. Alternatively, they may be triangular and arranged in the shape of a bow-tie to from a so-called bow-tie antenna. They may also be interdigitated electrodes at the centre of a bow-tie or spiral antenna. 
       FIG. 12  shows a variation on the imaging system of  FIGS. 1 and 2 . To avoid unnecessary repetition, like reference numerals will be used to denote like features. 
     The THz generator  1  comprises two laser diodes  201 ,  203  which are configured to emit radiation with frequencies ω 1  and ω 2  respectively. The radiation emitted from both laser diodes  201  and  203  is combined using beam splitter/combiner  205 . The combined radiation which contains both frequencies ω 1  and ω 2  is then directed into THz source  207  for emitting THz radiation. The THz radiation is produced with a frequency of ω 1 −ω 2  and THz source  207  can use the difference frequency generation methods described with reference to  FIGS. 3 to 5 . 
     The beams emitted from laser diodes  201 ,  203  are taken as the probe beam  209  using beam splitter  205 . This probe beam will be used to give the detector information about the phase of the radiation which is emitted from the THz source  1 . The probe beam is fed into optical delay line  211  which is used as the phase coupling/control means explained with reference to  FIG. 1 . 
     In the optical delay line, the probe beam  209  is reflected off cube mirror  213  which is used to reflect the light through 180° and onto mirror  215  which in turn reflects the probe beam  209  into the detector  5  via the mirror  217 . 
     Cube mirror  213  is moveable such that the path length of the probe beam can be varied as described with reference to  FIG. 1 . The probe beam is then directed into THz detector  5  which can be a detector as described with reference to with of  FIGS. 11 and 12 . 
     The sample and imaging apparatus  3  are configured such that either the sample can be moved with respect to the beam or the beam can be moved with respect to a stationary sample or both. 
     Improvements in the signal to noise ratio and hence acquisition times can be made by various modulation schemes. For example, dithering or oscillating of the mirror  213  will cause sinusoidal variations in the d p  that can be detected using standard lock-in techniques. This is essentially a frequency modulation of the THz waveform as it is plotted out versus d p . Similarly, it is possible to modulate the amplitude or frequencies of the sources outputting the radiation ω 1  and ω 2  to affect the amplitude and/or frequency modulation. This again results in noise suppression. 
       FIG. 13  shows a variation on the imaging system of  FIG. 12 . To avoid unnecessary repetition, like features will be denoted with like numerals. The generator  1 , the sample and imaging apparatus and the optical delay line  211  are identical to that described with reference to  FIG. 12 . Prior to the probe beam being reflected from mirror  217 , the beam is passed through compensator  219  to ensure the probe beam is polarised parallel to the THz beam  232 . After reflection from mirror  217 , the probe beam  209  is reflected onto beam combiner  221 . Beam combiner  221  will typically be a mirror to reflect the probe beam  209  and having an aperture which can transmit the THz radiation  223  coming from the sample  3 . 
     The combined probe  209  and Terahertz  223  beams are then directed onto detection member  111  which is identical to the member described with reference to  FIG. 10 . After the radiation has passed through the detection member, it is passed through the same optical and electrical elements described with relation to  FIG. 10 . The analysis of the data for this type of system where the phase coupling is achieved via an optical delay line and where the detector uses free space electro-optic sampling is set out in detail with relation to  FIG. 1 . 
       FIG. 14  shows a slight variation on the imaging systems of  FIGS. 12 and 13 . As in  FIGS. 12 and 13 , radiation with frequencies ω 1  and ω 2  are produced respectively by laser diodes  201  and  203 . The source comprises a χ (2)  frequency conversion member as explained with reference to  FIG. 3 . The source different from that of the  FIGS. 12 and 13  as in this example, laser diode  201  has a variable frequency output and the emitted frequency can be chosen by applying a suitable bias to the diode. Also, it possible to sweep the frequency of the laser diode  203 . 
     The THz beam which is transmitted through the sample impinges on the back of detection member  111  which is located at about 45° to the path of the transmitted THz beam  223 . The detection member is also located at 45° to the path of the probe beam  209 . The detection member  111  is provided with a reflective coating which is configured to reflect probe beam  209  such that the probe beam and the THz beam are combined within the detection member  111 . The remaining optics have already been described in detail with reference to  FIG. 3 . 
       FIG. 15  shows a variation on the imaging system of  FIG. 12 . The source, sample/imaging apparatus and optical delay line are the same. However, the detector here is a photoconductive antenna which has been described with reference to  FIG. 11 . 
       FIG. 16  shows a variation on the imaging system of  FIG. 15 . Here, a photoconductive antenna is used to generate the THz radiation. This is described in detail with reference to  FIG. 5 . As described with reference to  FIG. 14 , the frequency of the first laser diode  201  can be varied with the application of a bias. 
       FIG. 17  shows a further variation on the imaging system of  FIGS. 1 and 2 . This system follows the same basic design system of  FIGS. 1 and 2 . The source is a harmonic source which emits radiation with frequency below that of the THz range. The emitted frequency is such that doubling or tripling etc of the frequency will give radiation with a frequency in that of the THz regime. The radiation emitted from low frequency oscillator is divided. One signal is fed into optical delay line  211  (as described with reference to  FIG. 12 ), the other signal is fed into harmonic generator  233 , which generates a plurality of harmonics for the frequency. The harmonic generator may be a Schottky diode or an optically non-linear crystal. The radiation emitted by harmonic generator  233  is then fed into harmonic filter  235  which selects the desired harmonic in the THz range. The radiation is then directed onto sample  3 . The sample can be rastered with respect to the beam of incident radiation or the beam can be moved with respect to the sample. Once the radiation has been transmitted through the sample  3 , it is directed into harmonic detector  5  where it is recombined with the probe beam  209 . The harmonic mixer can be a Schottky diode which will output a signal corresponding to the strength of the detected THz field. 
       FIG. 18  shows a further variation on the imaging systems of  FIGS. 1 and 2 . Here, the THz can be generated by using THz source which does not used the method of converting the frequency of an input beam, instead, the source directly outputs THz radiation in response to an input parameter such as a bias applied across the source. Typical sources are Gunn diodes, Molecular gas lasers, cascade lasers, backward wave oscillators and free electron lasers. A beam of THz radiation is outputted from this direct THz source  241  onto THz beam splitter  243  which splits the beam into probe beam  209  which is fed into optical delay line  245  and the imaging radiation is directed onto the sample  3 . Optical delay line  245  comprise two mirrors  247  and  249 , the probe beam  209  is directed onto mirror  247  and then onto mirror  249 . The separation between the two mirrors can be varied so that the path length of the probe beam can be varied as required. 
     The probe beam  209  is then combined with the radiation which is transmitted through sample  3  using beam combiner  251 . The output of beam combiner  251  is then fed into bolometer  253  which outputs a current which is related to the detected THz field. 
       FIG. 19  shows a variation on the imaging system of  FIG. 18 . Here, the beam combiner  251  is replaced with a THz mixer  255  which can be a Schottky diode, bolometer, semiconductor-insulator-semiconductor diode and outputs a current which is related to the strength of the detected THz field. 
       FIG. 20  shows a further possible variation on the imaging systems of  FIGS. 1 and 2 . Here, the sample is illuminated with two frequencies in the THz range. The THz generator is based on the generator described with reference to  FIGS. 3 and 4 . There are three laser diodes,  301 ,  303  and  305 . The first laser diode  301  emits radiation with a frequency ω 1  into beam splitter  307 . Beam splitter  307  directs part of the beam into beam combiner  309  where it combines with radiation of a frequency ω 2  emitted from the second diode. The other part of the beam is directed towards combiner  311 , where it is combined in beam combiner  311  with radiation from the third diode  305  having a frequency ω 3 . 
     Radiation from beam combiner  309  is directed into beam splitter  313  which in turn splits the beam into an input for the phase control means  7  and an input for the THz source  317 . 
     Radiation from beam combiner  311  is directed into beam splitter  315  where it is split into an input for the phase control means  7  and an input to the THz source  317 . The THz source is configured to output beams in the THz range with frequencies ω 1 −ω 2  and ω 1 −ω 3 . These two beams travel through the sample  3 . Typically, the two THz frequencies ω 1 −ω 2  and ω 1 −ω 3  will be chosen such that they can be used to probe different materials which make up the sample  3 . 
     The two transmitted THz beams are combined with the two reference beams as previously described. The detector can be any type of detector which has been previously described for the use of one THz beam. The different frequency components can be split by Fourier transforming the signal obtained due to the detected radiation. 
     One major disadvantage with the use of pulsed radiation is that it is very difficult to transmit the pulses along waveguides/optical fibres and the like due to substantial losses. The use of CW radiation overcomes this problem. Hence, it is possible to make a small probe which can be used to detect the response of a system to THz radiation as a large part of the THz generator and the detector can be located remote from the probe. 
       FIG. 21  shows such a system. The imaging system is largely based on the system of  FIG. 12 . Therefore to avoid unnecessary repetition like numerals will be used to denote like features. As in  FIG. 12 , radiation from laser diodes  201  and  203  are combined using beam splitter/coupler  205 . Part of this combined radiation is sent to fibre optic coupler  351  which directs the radiation into fibre optic cable  353  which carries the radiation to THz source  355  which generates the THz radiation to irradiate sample  3 . THz source and imaging optics  3  are remote from the laser diodes  201 ,  203  in probe head  357 . 
     The other part of the beam from beam/splitter combiner  205  is directed into optical delay line  211  which is the same as that described with reference to  FIG. 12 . However, mirror  215  directs the probe beam  209  into fibre optic coupler  359  which in turn direct the radiation into fibre optic cable  361  where it is carried towards THz detector part  363 . Terahertz detector part  363  combines the radiation transmitted through sample  3  with that of the probe beam. It serves to convert the THz radiation into some form which it can be transmitter back to the system box  365  for analysis. 
       FIG. 22  shows an imaging system similar to that of  FIG. 21 , but having an EOS based detection system, of the type described with reference to  FIG. 3 . Here, the detection member  111  is housed remote from the box system. The probe beam with the rotated polarisation is then fed back to the signal box using polarization preserving fibre  367 . The radiation leaves fibre  367  and is directed onto quarter waveplate using fibre optic coupler  369 . The remainder of the detection is then the same as described with relation to  FIG. 13  and will not be repeated here. 
       FIG. 23  shows a system which can be used for imaging or investigating a sample using THz radiation. The system is similar to that described with reference to  FIG. 12 . Therefore, to avoid unnecessarily repetition, like reference numerals will be used to denote like features. 
     The imaging system of  FIG. 12  used two laser diodes  201 ,  203  which are configured to emit radiation with the frequencies of ω 1  and ω 2 . The apparatus of  FIG. 23  uses a single broadband source  401  to generate radiation which is directed into THz source  207 . THz source  207  is a difference frequency source which can use the difference frequency generation methods described with reference to  FIGS. 3 to 5 . 
     The broadband laser  401  emits radiation having a plurality of frequencies. THz source  207  then emits THz radiation having a plurality of frequencies, each of the plurality of frequencies corresponding to a difference between two of the frequencies from the broadband source  401 . 
     Examples of widely available broadband sources are “superluminence LEDs” or amplified spontaneous emission light sources based on Er-doped fibre amplifiers. Both of these types of sources generate broadband, low-coherence light centred around 1550 nm wavelengths. Typical bandwidths are from 20 to 50 nm corresponding to 2 to 5 THz. 
     Specifically “Newport” sell one such system under their part number PTS-BBS, as do “ILX Lightwave” under their part number MPS-8033APE. Another example of a source is E-tek who sell a broadband source working at 980 nm, part number BLS980. 
     In the same manner as described with reference to  FIG. 12 , the beam from the broadband source  401  is divided using beamsplitter  205  which generates a reference beam which is supplied to the THz detector  5 . The broadband wave source only has a short-coherent length and can be essentially thought of as being incoherent. There is no definite phase relationship between the frequencies i.e. the laser modes are all independent of each other. Thus, there is a random phase at each frequency. However, as part of the broadband laser source beam is used as the probe beam, the random phase relationship between different frequencies does not matter because the detection method only measures the phase difference between the THz pump and probe beam. Thus, it is possible to determine the actual phase change for each frequency component. 
     As the above apparatus illustrates a system where THz power is delivered in a continuous manner as opposed to a pulsed manner, this system is also advantageous for delivering radiation down optical fibres. Therefore, this type of broadband source can be used in the fibre delivery system detailed in  FIGS. 21 and 22 . 
     The above system can be used for imaging or it can be used to obtain information about a sample at a point. 
     Any of the previously described detection mechanisms can be used with the broadband source  401  described with reference to  FIG. 23 .