The Terahertz frequency range is generally considered to be the range from 25GHz to 100THz, particularly the range from 50GHz to 84THz, more particularly the range from 90 GHz to 50 THz and especially the range from 100GHz 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.

Due to the shorter wavelength of Terahertz radiation, there is a difficulty in providing focussing optics which allow the radiation to be focussed to a diffraction limited spot, Attempts to address this problem have been discussed in EP 0 828 143. Siebert et al "All Opto-Electronic CW and Terahertz Imaging for Biomedical Applications"in the Proceedings of the First International Conference on Biomedical Imaging and Sensing Applications of Terahertz Technology held on November 30th-December lSt 2001 mentions the use of hyperboloidal high-density polyethylene lenses in a TeraHertz system.

The present invention provides a new type of lens or focussing member which allows THz radiation to be focussed to a diffraction limited spot without aberration. Also, the focussing members allow radiation emanating from the focus of the member to be collimated.

In a first aspect, the present invention provides a THz investigative system comprising a focussing member, said system being configured to transmit THz radiation through said focussing member, said focussing member having a focussing surface through which radiation is transmitted, said focussing surface being ellipsoidal.

The general formula for an ellipsoid surface in cylindrical polar co-ordinates is given by: where r is the radial co-ordinate and z is the axial co-ordinate. A and B are parameters which determine the ellipsoidal surface. The z axis is the axis of revolution.

The constants A and B may be selected by using the following equations: and <BR> <BR> <BR> (n+l)<BR> <BR> B = 7Rj where C is the distance from the extrema of the ellipsoidal surface along the axis of revolution (the z-axis) to the focus of the lens, and ii is the refractive index of the lens material.

In a second aspect, the present invention provides a THz investigative system comprising a focussing member, said system being configured to transmit THz radiation through said focussing member, said focussing member having a convex focussing surface and an opposing planar surface, radiation being transmitted through both of said surface, said focussing member following the equations: y = x tan6, + d tan6o where: <BR> <BR> <BR> <BR> <BR> n0sin#0<BR> <BR> #1=sin-1<BR> n1<BR> <BR> <BR> <BR> <BR> <BR> and y is the radial coordinate measured from the axis of rotation, x is the thickness of the lens measured between the planar surface and convex surface along the rotation axis, d is the distance of the focus from the planar surface measured along the axis of rotation in a direction away from the member, no is the refractive index of the medium from the focal point to the member, ni is the refractive index of the member, n2 is the refractive index of the material on the convex side of the member, and 0o is the angle at which radiation travelling between the focus and the planar surface enters the member measured from the axis of rotation. The surface profile is given by the x and y co- ordinates, which are calculated parametrically by the parameter #1. Note, this aspect does not describe an ellipsoidal surface, but a more general aspherical surface.

In a third aspect, the present invention provides a THz investigative system comprising a focussing member, said system being configured to transmit THz radiation through said focussing member, said focussing member having a focussing surface through which radiation is transmitted, said focussing surface being convex and following the equation : (jc+J)"+y= ( + where y is the radial coordinate measured from the axis of rotation, x is the azimuth coordinate measured parallel to the rotation axis and measured from where the rotation axis intersects the focussing surface, d is the distance of the focus measured from where the rotation axis intersects the surface in a direction away from the member and n is the refractive index of the member.

In the THz investigative systems of the first to third aspects of the present invention, radiation having at least one frequency in the range from 25GHz to 100THz is passed through the focussing member. More preferably radiation in the range from 50GHz to 84THz, even more preferably the range from 90 GHz to 50 THz and especially the range from 100GHz to 20THz. The radiation may be pulsed or continuous wave radiation. It may have a single frequency or a plurality of frequencies.

In GB 2371618 from the applicant, ellipsoidal surfaces are used for reflecting radiation within the probe. The focussing members used in the THz investigation systems of the present invention use refraction through the focussing surface as opposed to reflection to focus or collimate radiation passing through the focussing member. The use of transmission optics providing more"linear"optical path geometry. Reflection optics require the optical path to be folded back on itself increasing the space required to complete the system.

Also, by using components which are inherently cylindrically symmetric, the focussing members may be formed using a CNC-lathe (one turning axis). Some of the designs disclosed in GB 2371618 required multi-axis CNC machining in order cut the complex reflecting surfaces. As one-axis CNC machining/grinding machines are fairly commonplace, the focussing members described in the THz investigative systems of the present invention may be manufactured more easily and cheaply than previous designs.

The focussing member may not be entirely cylindrically symmetric. For example, the focussing surface may be truncated at its edges in order to produce a smaller focussing member.

The above described focussing members have a convex focussing surface and generally a planar surface is provided opposing said focussing surface.

In the focussing members described with reference to the THz investigative systems of the first and second aspects of the present invention, the focus is provided on the planar surface side of the focussing member. The focus may be on the planar surface or outside the focussing member. Thus, collimated radiation impinging on the convex focussing surface is focussed to a point either on the planar surface or just outside the focussing member.

Similarly, radiation emanating from the focus will travel through the planar surface and become collimated by the focussing surface and exit the focussing member on the focussing surface side of the focussing member.

In the focussing member described with reference to the THz system of the third aspect of the present invention, the focus is provided on the convex surfacing side of the focussing member. Thus, radiation emanating from the focus passes first through the convex focussing surface, is collimated by the focussing surface and then exits the focussing member through the planar surface. Similarly, collimated radiation arriving at the planar surface passes through the focussing member and is focussed to a point on the focussing surface side of the focussing member.

For situations requiring a low numerical aperture beam characteristics, it is preferable to choose a focussing member which provides a planar face to the focus of the member, since a convex face curves away from the focus and will hence collect less radiation.

The focussing member may be made out of silicon, quartz, polyethylene or a similar material. Ceramic materials such as alumina (Al03), silicon nitride, aluminium nitride, boron nitride. Polymers such as polypropylene, polystyrene, PTFE. Semiconductors such as germanium, GaAs. Diamond. Hydrocarbons such as parafin wax.

In a preferred embodiment, the focussing member is used to focus radiation onto a sample. The focussing member has an exit surface through which radiation exits said member and impinges on said sample.

The focussing member may be configured so that the focus is provided on the exit surface and the exit surface directly abuts said sample. Alternatively, the member may be configured so that the focus lies beyond the end of the exit surface.

The cross section of the focussing member may taper towards the exit surface. This will not affect the paths of the radiation since the radiation is being gradually focussed to a point as it passes through the member towards the exit surface.

Typically, the focussing member will be a primary focussing member and the system will further comprise directing means to direct radiation onto and/or collect radiation from said primary focussing member.

The directing means may comprise a secondary focussing member having a convex surface configured to direct radiation towards said primary focussing member.

Typically, this secondary focussing member will output collimated radiation towards said primary focussing member. The collimated radiation is then focussed by the primary focussing member onto the sample.

In a preferred embodiment, the system is configured for reflection measurements. Thus, radiation is directed towards said primary focussing member which focuses radiation onto a sample. The reflected radiation from the sample is then collected by the exit surface of the primary focussing member and the radiation is reflected back through the primary focussing member. Preferably, the directing means comprises a second secondary focussing member having a convex lens configured to collect radiation reflected back through said primary focussing member. Typically, the radiation leaving said primary focussing member will be collimated.

Generally, the first and second secondary lenses are provided next to one another, both with their convex surfaces facing said primary focussing member such that radiation reflected back through said primary focussing member follows a different path to radiation entering said primary focussing member.

However, it will be appreciated by those skilled in the art, that outgoing and incoming radiation could follow the same path through the primary focussing member by using a pulse technique described in GB 2 360 842.

In a particularly preferred arrangement, the directing means comprises a beam splitter combiner configured to direct incident radiation along the same path as the reflected radiation through the primary focussing member.

Either or both of the first and second secondary lenses may be an ellipsoidal focussing member of the type described in relation to the first aspect of the present invention.

Also, the focussing members described with reference to the second and third aspects of the invention could also be used as the secondary lenses.

In a preferred embodiment, an emitter is provided at the focus of the first secondary focussing member. Typically, the focus of the second secondary focussing member will be provided on a planar surface which opposes the convex focussing surface. Thus, the emitter can be mounted to the first secondary focussing member.

There is no naturally occurring source of THz radiation. However, it is possible to directly generate THz radiation using Gunn diodes and also cascade lasers.

In order to produced THz radiation the emitter may comprise a frequency conversion member which is configured to emit radiation of the desired frequency in response to irradiation by radiation having a different frequency. Generally, the frequency conversion member is configured to emit radiation which is the difference of the frequency of two input beams. To generate THz radiation, an input beam, or pump beam may be supplied to the emitter. The pump beam will preferably comprise radiation having at least two frequencies which lie in the near infra-red part of the spectrum Photoconductive devices (see for example US 5 729 017) are preferably used to generate THz radiation, here the emitter comprises a photo-conductive material and an electrode configured to apply a bias across the photo-conductive material, the photoconductive material being configured to emit radiation of the desired frequency in response to irradiation by a pump beam of radiation having at least two frequency components with frequency different to the desired frequency.

Similarly, a receiver is preferably provided at the focus of the second secondary focussing member. Again, the receiver may be mounted to a planar surface on the second secondary focussing member. The receiver may be a photo-conducting antenna comprising a photo-conductive material and an electrode configured to measure a current flowing through the photo-conductive material, the photo-conductive material being configured to generate a current in response to irradiation by a both a probe beam comprising at least two frequency components and the beam of radiation which is to be detected.

The probe beam may have the same frequencies as the pump beam and preferably, the pump beam and probe beam originate from the same source.

The directing means and the primary focussing member are preferably provided as a probe-unit and radiation is carried to and from the probe by fibre optic cables. The directing means and primary focussing member may be provided within a probe casing or the primary focussing member may form the body of the probe and the directing means may be mounted to the primary focussing member.

It is difficult to transmit radiation in the THz regime along fibre optic cables. Thus, typically, THz radiation is generated in the probe-unit. Radiation in the near or mid- infra red range may be directed along the fibres to the emitter and detector located in the probe to form the pump and probe beams for the emitter and detector respectively. The emitter and detector both being photoconductive antennas.

Any THz radiation is detected by the detector and converted into an electrical signal which is then carried away from the probe by a wire.

Preferably, the probe comprises tertiary focussing members configured to collect radiation received in said probe from said fibre optics and direct it towards the directing means. The tertiary focussing members are there to primarily direct the pump and probe beams.

In a preferred embodiment, the focussing member comprises a first section and a second section with the first section being slidable with respect to the second section, the second section comprising the exit surface through which radiation leaves the member.

By translating the first section with respect to the second section, the position of radiation exiting the primary focussing member is scanned across the exit surface.

Preferably, the system comprises translation means so that the first section may be translated with respect to the second section in order to scan the radiation across the exit surface of the second section. More preferably, the translating means translates the directing means with the first section. Thus, the system becomes a scanning system.

Preferably, the scanning system is in the form of a probe-unit.

The second section may comprise a demountable section and a fixed section, the demountable section comprising the exit surface. This allows the demountable section to be replaced if it suffers wear and tear. As the exit surface is either placed in contact with the sample or very close to a sample, it is likely that the exit surface will be the first part of the focussing member to degrade or suffer damage.

Also, it may be desirable to have a scanning system which can be used with a range of differently sized exit surfaces/scanning areas. Thus, the fixed section may be able to cooperate with a plurality of different demountable sections each having a different exit surface size.

The above scanning probe has been described in relation to a specific type of primary focussing member. However, the above scanning probe can be used with any type of primary focussing member. Thus, in a fourth aspect, the present invention provides a system for investigating a sample, said system comprising a focussing member, a source of radiation, said focussing member comprising a first section and a second section said second section being slidable with respect to said first section and having a sample surface, the system being configured such that radiation from said source enters the first section of said focussing member and exits the focussing member through the sample surface, said system further comprising translating means such that said first section and said source may be translated with respect to the second member thus scanning the radiation across said sample surface.

The focussing members used in the THz investigative system of the second and third aspects of the invention can also be used as components in other focussing systems.

Thus, in a fifth aspect the present invention provides a focussing member comprising a convex focussing surface and an opposing planar surface, said focussing member following the equations in cylindrical coordinates: where: and y is the radial coordinate measured from the axis of rotation, x is the thickness of the lens measured between the planar surface and convex surface along the rotation axis, d is the distance of the focus from the planar surface measured along the axis of rotation in a direction away from the member, no is the refractive index of the medium from the focal point to the member, ni is the refractive index of the member, n2 is the refractive index of the material on the convex side of the member and Oo is the angle at which radiation travelling between the focus and the planar surface enters the member measured from the axis of rotation.

In a sixth aspect, the present invention provides a focussing member comprising a focussing surface, said focussing surface being convex and following the equation in cylindrical coordinates: (x+d2 + y = (d +nx) 2 where y is the radial coordinate measured from the axis of rotation, x is the azimuth coordinate measured parallel to the rotation axis and measured from where the rotation axis intersects the focussing surface, d is the distance of the focus measured from where the rotation axis intersects the surface in a direction away from the member and n is the refractive index of the member.

The present invention will now be described with reference to the following non- limiting preferred embodiments in which: Figure 1 is an ellipsoidal focussing member in accordance with an embodiment of the present invention ; Figure 2 is an aspheric focussing member of a first type in accordance with an embodiment of the present invention; Figure 3 is an aspheric focussing member of a second type in accordance with the preferred embodiment of the present invention; Figure 4 schematically illustrates a probe-unit in accordance with a preferred embodiment of the present invention having an ellipsoidal focussing member; Figure 5 is a schematic diagram of a probe-unit in accordance with a preferred embodiment of the present invention having an aspheric focussing member of the type described with reference to Figure 2; Figure 6 shows schematically a probe in accordance with a preferred embodiment of the present invention having an elongated ellipsoidal focussing member ; Figure 7 is a perspective view of the probe of figure 6; Figure 8 is a schematic of a probe-unit in accordance with a preferred embodiment of the present invention where reflected radiation through the primary focussing member follows the same path as the incident radiation; Figure 9 schematically illustrates a scanning probe in accordance with a preferred embodiment of the present invention where the focussing member has a first section and a second section which move relative to each other; Figure 10 schematically illustrates the probe of figure 9 and its connection to a controller; Figure 11 (a) is a schematic of a probe having a large exit surface and figure 11 (b) is a schematic of a probe head having a smaller exit surface; and Figure 12 is a perspective and cut-away view of the probe of figure 10.

Figure 1 schematically illustrates a focussing member in accordance with a preferred embodiment of the present invention.

Focussing member 1 has a planar surface 3 and an ellipsoidal surface 5 opposing said planar surface 3. The general formula for an ellipsoid surface in cylindrical polar co-ordinates is given by: where r is the radial co-ordinate and z is the axial co-ordinate. A and B are parameters which determine the ellipsoidal surface. The z axis is the axis of revolution.

The parameters A and B are indicated on Figure 1 and correspond to the semi-minor and semi-major radius of the ellipsoid.

If the focussing member has a refractive index n at Terahertz frequencies, the constant B is calculated as: In Figure 1, radiation is focussed at co-ordinate P. Co-ordinate P is located along the z axis of the focussing member and at a distance C from the midpoint of the opposing ellipsoidal surface 5. The co-ordinate of the focus of the Terahertz radiation is given by: z=, r=0 n Thus, this allows A to be calculated as: and Thus, the focussing member 1 may be used to either focus incoming Terahertz radiation to focus point P or produce a beam of collimated Terahertz radiation from a source position at point P.

It should be noted that the planar surface of the focusing member may not intersect point P such as in cases where an emitting or detecting member provides additional thickness to the THz path as shown in Figure 1.

Figure 2 schematically illustrates a different design of focussing member 11. Focussing member 11 has a planar surface 13 and a convex surface 15. Like the focussing member of figure 1, the focus 17 of member 11 is located on the planar surface 13 side of member 11.

The refractive index of the member 11 is nl. It is presumed that the refractive index on the planar side of member 11 is no and the refractive index on the convex surface side of member 11 is n2.

In order to simply describe the shape of member 11, it is necessary to consider two angles : Oo and Ol. Oo represents the angle of radiation entering (or exiting) the planar surface 13 measured from the rotation axis 19. The angle (il is the angle measured with respect to the symmetry axis 19 of the radiation passing through member 11.

Using Snell's law: nlsinO zOsinOO this can be rearranged to give: x, the thickness of the lens from the planar surface 13 to the convex surface 15 and y is the radial co-ordinate measured from the rotation axis 19. The maximum thickness of the lens, i. e. the thickness of the lens along rotation axis 19 is Xo, the distance from planar face 13 to focus 17 along the rotation axis is d (often referred to as the working distance).

Using eo and 01, x can be calculated to be: Finally, y can be calculated from Oo, O1 and x to be: y = xtanOl + dtanOo Thus, it is possible to describe the shape of the lens in terms of x and y parametrically, using the intermediate parameters () l and #0. The maximum lens thickness Xo and the working distance d are a design parameters chosen to fit the specific application requirements.

The above geometry can be used for situations requiring low numerical aperture beam characteristics.

Figure 3 schematically illustrates a yet further focussing member 31 in accordance with a preferred embodiment of the present invention.

The focussing member 31 has a planar side 33 and a convex side 35. The focus of the lens 37 is located on the convex surface 35 side of focussing member 31. Again, the member is cylindrically symmetric.

Two Terahertz rays 39 and 41 are schematically illustrated. In this particular example, the Terahertz rays originate from focus 37 and are passed through focussing member 31 to achieve a collimated beam. Although the direction of the Terahertz rays 39 and 41 is shown propagating away from focus 37, the same geometry would be applicable for a collimated beam of Terahertz radiation impinging on the planar surface 33 of the focussing member 31 and beam focussed to point 37.

The focus 37 is located at a distance d along the rotation axis 43 from the midpoint of convex surface 35.

The co-ordinate y is the radial co-ordinate about the rotation axis 43, the x co-ordinate is the co-ordinate parallel to the rotation axis 43 and measured from plane 45 which is perpendicular to the rotation axis 43 and located at the midpoint of convex surface 35.

Convex surface 35 may be mapped by the function x (y). This may be found from the following equation: (x+d)2+y2=(d+nx)2 This can be arranged to give: As convex surface 35 curves away from the focal point 37 as y is increased, thus the range of angles for which rays emanating from the focus are captured by the lens is limited.

Any of the focussing members described with reference to Figures 1 to 3 may be used in a probe of the type described with reference to Figure 4.

The probe-unit of Figure 4 comprises a probe body, a primary focussing member 53.

The primary focussing member 53 is a focussing member of any of the types described with reference to Figures 1 to 3 and in this particular example is an ellipsoidal focussing member of the type described with reference to Figure 1.

The probe comprises a probe casing 51. Primary focussing member 53 is provided at one end of the casing 51.

The convex ellipsoidal focussing surface 55 of the primary focussing member is provided within the probe casing 51. The primary focussing member 53 has a first region adjacent the convex surface 55 which has a fixed cross-section and has a second region integral with the first region 57 which tapers to a sample surface 61. The cross- sectional area of exit surface 61 is smaller than the cross-sectional area of the first region 57. The second region is located outside probe casing 51.

The exit surface 61 is intended to abut against an object 63 or sample under test.

Primary focussing member 53 is configured such that radiation impinging on convex surface 59 is focussed to a point either on said exit surface 61 or just beyond said exit surface 61.

First and second secondary focussing members 73 and 75 are provided adjacent one another in a direction across the width of the probe casing 51. Both secondary focussing members are ellipsoidal lenses and have their convex surfaces facing the ellipsoidal focussing surface 55 of the primary focussing member 53.

An emitter 71 of the photoconducting type is mounted at the focus of the first secondary focussing member 73. A detector 77 of the photoconducting antenna type is mounted at the focus of the second secondary focussing member 75.

A first tertiary focussing member 69 is provided to focus a pump beam onto emitter 71.

A second tertiary focussing member 81 is provided to focus a probe beam onto detector 77.

A first optical fibre 65 is provided to deliver the pump beam to the first tertiary focussing member 69. A second optical fibre 79 is provided to deliver the probe beam to the second tertiary focussing member 81. Both the first 65 and second 79 optical fibres enter the probe casing 51 through mounting block 67.

Radiation enters the probe body 51 from first optical fibre 65. First optical fibre 65, in this particular example, carries radiation in the near infrared frequency range. Thus type of radiation can be more easily transmitted down an optical fibre than THz radiation.

Optical fibre mounting block 67 is provided at the entrance to probe body 51 such that radiation will enter probe body 51 at a fixed point. Radiation exiting first optical fibre 65 is then focussed by tertiary focussing member 69 which focuses the mid infrared radiation or"pump"beam onto a Terahertz emitter 71.

The pump beam of radiation may be a pulsed radiation beam comprising a plurality of frequencies or may be a continuous wave (CW) beam of radiation. If a CW beam of radiation is used, two of more frequencies will be present in the CW radiation.

Terahertz emitter 71 is a photoconducting antenna which is preferably configured to output radiation having an output frequency which is the difference of two frequencies of the input radiation. The photoconducting antenna comprises an electrode and an electrical signal is provided across said electrode in order to effect said frequency conversion.

Other frequency conversion techniques may be used such as the use of non-linear optical members which may again be used to output the frequency having radiation which is equal to the difference of the frequencies of input radiation.

The Terahertz emitter 71 is fixed to a planar surface of first secondary focussing member 73. Secondary focussing member 73 is an ellipsoidal lens which is of the type described with reference to Figure 1. The Terahertz emitter 71 is provided at the focus of first secondary ellipsoidal lens 73. First ellipsoidal lens 73 is configured to produce a collimated beam of radiation which impinges on the convex surface 55 of focussing member 53.

Focussing member 53 then focuses the collimated radiation incident on convex surface 55 to a point which is either located on exit surface 61 or located just past exit surface 61.

Radiation is then reflected from object 63 back into focussing member 53 through exit surface 61. Collimated radiation then leaves focussing member 53 through convex surface 55 towards second secondary ellipsoidal lens 75. Second secondary ellipsoidal lens 75 is an ellipsoidal focussing member of the type described with reference to Figure 1. The convex surface faces towards the convex surface 55 of focussing member 53.

Secondary focussing member 75 collects the reflected collimated Terahertz radiation at its convex surface and focuses it to a point located on its opposing planar surface. A Terahertz receiver 77 is provided at the point. Terahertz receiver 77 is typically a frequency conversion member which serves to convert the incoming Terahertz radiation to a signal which can be carried away easily from the probe. In this particular example, Terahertz receiver 77 is a photoconducting antenna. The photoconducting antenna comprises photoconducting material and an antenna typically formed by two electrodes arranged in a so-called bow-tie configuration. However, other arrangements such as a transmission line arrangement, a straight dipole arrangement, etc. are possible.

The photoconducting antenna 77 receives the reflected Terahertz radiation through its back surface (i. e. the surface opposing the surface with the electrodes). On the front surface, it receives so-called probe radiation which, in this example, is radiation in the near infra-red regime. The probe radiation is carried to probe body 51 along secondary optical fibre 79. Secondary optical fibre 79 is again mounted in fibre mount 67 on entry into the probe. The radiation from secondary optical fibre 79 is then focussed onto the front surface of Terahertz receiver 77 by tertiary focussing member 81 which is an aspheric glass lens.

The reflected Terahertz radiation and the incoming probe radiation set up a current between the electrodes (not shown) at Terahertz receiver 77 and thus output a current which is related to the strength of the Terahertz signal received.

Figure 5 shows a variation on the probe of Figure 4. To avoid unnecessary repetition, like reference numerals will be used to denote like features.

Probe body 51 of Figure 5 has a focussing member 91 which has a convex focussing surface 93 and an opposing planar sample surface 95. The convex focussing surface 93 faces towards the convex surfaces of first and second secondary lenses 73 and 75.

Focussing member 91 is of the type described with reference to Figure 2. Collimated Terahertz radiation received from first secondary lens 73 impinges on the convex focussing surface 93 and is focussed by focussing member 91 to a point 97 which is beyond the end of lens 91. Thus, the sample surface 95 of lens 91 does not touch the sample 99.

In this particular example, the planar sample surface 95 is flush with the end of probe casing 51. Radiation reflected from object 99 is then collected by lens 91 and collimated. The collimated radiation leaves through convex focussing surface 93 and is collected by second secondary ellipsoid lens 75 which is configured and operated in the same manner as described with reference to Figure 4. How the radiation is transmitted to the probe and how the radiation is collected by the probe is the same as described with reference to Figure 4.

Figure 6 a yet further variation on the probe design of Figure 3. In the probe of Figure 6, a large focussing member 11 is provided which has an ellipsoidal focussing surface 113 provided at one end and a sample surface 115 provided at the opposing end.

Ellipsoidal focussing surface 113 is of the type described with reference to Figure 1.

The focussing member 111 comprises a first region 117 which has a constant cross- section and a second region 119 which is integral with the first section 117. The focusing member 111 is machined from a single piece of material which is typically silicon. The first section 117 has a constant cross-section and the second section 119 tapers away from the ellipsoidal surface 113 towards sample surface 115.

In the same manner as described with reference to Figure 4, the probe comprises first 121 and second 123 secondary focussing members.

First and second secondary focussing members 121 and 123 are also ellipsoidal lenses and the convex surface of the lenses 121 and 123 faces the convex focussing surface 113 of primary focusing member 111. As described with reference to Figure 4, a Terahertz emitter 125 is provided on the planar surface of first secondary focussing member 121. The Terahertz emitter 125 is of the photoconducting antenna type. The radiation is carried to the probe-unit along first optical fibre 127 and this radiation is focussed onto Terahertz emitter 125 by tertiary focussing member 129 which is an aspheric lens.

Similar to Figure 4, a Terahertz detector 131 is provided on the planar surface of ellipsoidal secondary focussing member 123. The probe beam for Terahertz receive 131 is provided through secondary fibre optic cable 133 which carries radiation to tertiary focussing member 135 which then focuses the radiation onto Terahertz receiver 131.

The first and second secondary focussing members 121 and 123 are provided adjacent to each other and both facing the focussing surface 113 of primary focussing member 111. Thus, radiation generated by Terahertz emitter 125 is transmitted along one side of the primary focussing member 11 and reflected back along the other side of primary focussing member 111. The focussing member 11 is flattened at its top and its bottom in order to conserve space since no radiation flows through these parts of the member.

The primary focussing member 111 is fairly large and it also serves as the probe body itself. First and second secondary lens 121 and 123 are bonded within a metal enclosure onto elongate focussing member 111. Similarly, tertiary focussing members 129 and 135 are also provided in the metal enclosure (not shown).

Figure 7 schematically illustrates a perspective view of the probe of Figure 6. To avoid unnecessary repetition, like reference numerals will be used to denote like features. The upper flattened surface of primary focussing member 111 can easily be seen. Again, the metal mounting block which mounts both the secondary 121 and 123 and tertiary 129 and 135 lenses to primary focussing member 111 is not shown.

Since the Terahertz path length is substantially in silicon, very little atmospheric water- absorption is experience due to the small gap between the lenses. Further, the size advantage of a primarily silicon THz probe is maintained due to the high refractive index of silicon.

In the description of the probes of figures 4 to 7, THz radiation is directed through the primary focussing member along one path and reflected back through the primary focussing member along a second path. The probe of figure 8 is a probe where both incident radiation and reflected radiation is transmitted along the same path through the primary member.

To avoid unnecessary repetition, like reference numerals of the probe shown in figure 8 will be used to denote like features of the probes of figures 4 and 5.

The primary focussing member 61 is a focussing member of the type described with reference to either of figures 1 and 2.

Incident THz radiation generated by emitter 71 is collimated by first ellipsoidal lens 73 and directed onto mirror 141. Mirror 141 reflects the incident radiation through 90° onto beam splitter 143 which then reflects the radiation through 90° onto primary focussing member 61.

In the same manner as described before, radiation is then reflected from object 63 and collected by primary focussing member 61. The reflected radiation leaves primary focussing member 61 and enters beam splitter 143. Beam splitter 143 allows passage of this radiation through the beam splitter to second ellipsoidal lens 75. The radiation is then processed in the same manner as previously described.

The advantage of this arrangement is that the THz beam (including both the incident and the reflected beams together) covers a lower range of angles about the direction normal to the object surface, because the incident and reflected beams are fully overlapped. This results in a somewhat improved spatial resolution for an equivalent THz numerical aperture.

Figure 9 schematically illustrates a so-called scanning probe.

The scanning probe comprises a focussing member having a demountable first section 151, a fixed second section 153 and a translating third section 155. The translating third section 155 is partially located within translating head unit 157.

Demountable section 151 fixes to fixed section 153. Demountable section 151 is provided abutting or close to said object 159 for studying said object.

Demountable section 151 has an exit surface 161 provided at is free end and connects to fixed silicon section 153 at the opposing end. Demountable section 151 has a tapering cross-section towards the exit surface 161. Demountable section 151 also has a relatively large exit surface 161 and can be used for studying large area samples. Fixed section 153 essentially provides an intermediate stage between demountable section with sample surface 161 and translating section 155.

Translating section 155 is provided mounted within translating head unit 157.

Translating head unit 157 comprises a body 163, first 165 and second 167 secondary lenses which are both ellipsoidal lenses of the type described with reference to Figure 1.

Both of the first and second 165 and 167 secondary lenses are provided next to each other and at the same distance from the translating focussing section 155.

The first secondary member 165 directs largely collimated Terahertz radiation onto the translating section 155 and the second secondary member is configured to collect radiation coming from focussing member 155. First fibre optic cable 169 directs radiation into translating head unit 157. This radiation is focussed using first tertiary focussing member 171 onto Terahertz emitter 173 which is provided on the planar side of first secondary focussing member 165. Second fibre optic cable 175 carries the probe radiation into translating head unit 157. This radiation is focussed by second tertiary focussing member 177 onto Terahertz receiver 179 which is of the type described with reference to Figure 4.

Translating focussing section 155 is of the type described with reference to Figure 3 and comprises a convex focussing surface 181 which faces the focussing surfaces of secondary focussing members 165 and 167 and a planar surface 183. Radiation entering translating focussing section 155 is transmitted through translating focussing section 155 into fixed section 153 and into demountable section 151 where it is focussed to a point 187 located on exit surface 161 or just beyond exit surface 161.

This radiation is then reflected by object 189 back through demountable section 151 through fixed section 153 and through translating section 155 to receiving secondary focussing member 167 where it is focussed onto Terahertz receiver 179.

As translating head unit 157 moves, translating focussing section 155 moves across the surface of fixed section 153. This causes the beam focus 187 to be scanned across the area of sample surface 161 thus allowing imaging of a larger area of object 189. The translating section may move in one direction or two directions.

Figure 10 schematically illustrates the scanning probe head of Figure 9 with its control.

To avoid unnecessary repetition, like reference numerals will be used to denote like features. The demountable focussing section 151 is mounted on fixed section 153.

Fixed section 153 and demountable section 151 are mounted on probe head casing 191.

Probe head casing 191 contains translating head unit 157 which is fixed to a two-axis linear translation stage. The linear translation stage allows the translating head to be moved in both the x and y direction across the planar surface 185 of fixed section 153.

A connector 193 is mounted on the opposing side of the scanning probe head 191 to fixed focussing section 153. This connector 193 directs both the optical fibres and electrical output from translating head unit 153 down an umbilical 195 which returns the electrical connections to a control PC. Using the control PC the movement of the head can be controlled. The optical fibres are connected to an optical system as described previously in GB 2 371 618.

In Figures 9 and 10, a demountable silicon focussing section 151 is shown with a relatively large sample area 161. Figure 1 la illustrates this system and Figure 1 lb illustrates the system of Figure 1 la where the large area demountable section 151 has been replaced with small area demountable section 201. Small area demountable section 201 has a much smaller sample surface 203. The control system may be operated such that translating head unit 157 translates by the correct amount to scan the whole of the sample surface for both large area demountable section and small area demountable section 201.

Figure 12 is a perspective view of the scanning probe head of Figures 9 to 11. The probe is shown with the small area demountable section 201. Small area demountable section 201 is a frustrum with the smaller area frustated surface forming the sample surface 203. This is mounted to fixed section 153. Fixed section 153 is substantially disc shaped. Translating section 155 is also substantially disc shaped with a planar surface which abuts the lower planar surface of fixed section 153 and a convex surface of the type described with reference to either of Figures 1 or 2.

The convex surface is located within translating probe head and is not visible in Figure 12. Disc shaped translating member 155 is then moved in the x and y directions by linear actuators 205 and 207. The details of the translating head and connectors have been described with reference to Figure 8. The flat top surface of the translating section is biased against the lower flat surface of the fixed section 153 by a flexure (springy) mount. The primary focussing member is made from silicon, the silicon-silicon interface between translating section 155 and fixed section 153 is a low friction interface. A dry lubricant such as PTFE spray may be used to reduce the interface friction further.