Patent Publication Number: US-7898668-B2

Title: Terahertz spectrometer

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present application claims priority to Japanese Patent Application JP2008-053802 filed in the Japanese Patent Office on Mar. 4, 2008, the entire contents of which is being incorporated herein by reference. 
     BACKGROUND 
     The present application relates to a technique of using electromagnetic waves in a band of about 0.1×10 12  THz to 100×10 12  THz. 
     A known technique of generating or detecting terahertz waves is terahertz time-domain spectroscopy (THz-TDS). As known in the art, the terahertz time-domain spectroscopy is suitable for use in imaging samples because the THz-TDS utilizes terahertz waves that define ultra-short pulses, as short as about 100 femtoseconds. Therefore, the terahertz time-domain spectroscopy attracts attention in various technical fields such as industry, medical care, biotechnology, agriculture and security. 
     In the terahertz time-domain spectroscopy, a pulse light beam emitted from an ultra-short laser source is split into a pump beam and a probe beam. The pump beam is focused on a terahertz-wave generating element. In the terahertz-wave generating element, a current flow or electrical polarization lasts for about subpico seconds, generating a terahertz wave having field amplitude proportional to the time derivative. The terahertz wave is focused by an optical system on a terahertz-wave detecting element. At this point, the probe beam is applied to the terahertz-wave detecting element. Then, the terahertz-wave detecting element generates a carrier. The carrier is accelerated by the electrical field of the terahertz wave and changed to an electric signal. The time at which the probe beam reaches the terahertz-wave detecting element is delayed, thereby measuring the time waveform the terahertz wave has in the amplitude electric field. The time waveform is Fourier-transformed, thereby determining the spectrum of the terahertz wave. 
     In an apparatus that performs the terahertz time-domain spectroscopy, an optical system guides, to the sample, the terahertz wave generated in the terahertz-wave generating element, and guides, to the terahertz-wave detecting element, the terahertz wave generated in the sample. In most cases, as shown in  FIG. 1 , this optical system has four parabolic mirrors PM 1  to PM 4 . 
     This optical system is a transmission type that detects the terahertz wave that has passed though the sample. A reflection-type optical system is available, which detects the terahertz wave reflected by the sample (see, for example, Jpn. Pat. Appln. Laid-Open Publication No. 2004-191302). The parabolic mirrors may be replaced by small-aperture optical lenses, rendering the lens system small (see, for example, PCT National Publication No. 2003-518617). 
     SUMMARY 
     If the optical system incorporates small-aperture lenses, however, the terahertz wave emanating from the sample will travel off the lens diameter. In this case, the terahertz wave will not be guided to the terahertz-wave detecting element if the angle of incidence of the terahertz wave applied through the lenses deviates even a little. It is not known whether this problem is due to a change in the refractive index of the sample or to the shape of the sample. The value measured of the sample is inevitably poor in reliability. 
     In consideration of the foregoing, the present application has been made. It is desirable to propose a terahertz spectrometer that can measure samples at higher accuracy than before possible. 
     In an embodiment, a terahertz spectrometer includes a terahertz-wave generating element, an elliptical mirror, an optical lens, and a terahertz-wave detecting element. The optical lens applies a terahertz wave generated by the terahertz-wave generating element, obliquely to the first focusing plane of the elliptical mirror. The terahertz-wave detecting element is arranged at second focus of the elliptical mirror. 
     The present application in an embodiment can provide a terahertz spectrometer in which a terahertz wave is applied obliquely to an object, such as a sample, if the object is arranged in the focusing plane having the first focus of the elliptical mirror. The terahertz wave is reflected by the object and guided by the elliptical mirror to the terahertz-wave detecting element. Hence, the terahertz-wave detecting element can detect the terahertz wave reflected by the object even if the transmission path of the terahertz wave reflected by the object changes because the object inclines or because the refection surface of the object has depressions and projections. The terahertz spectrometer according to the present invention can therefore measure a sample at an increased accuracy. 
     The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters. 
     Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a diagram explaining a conventional optical system for guiding a terahertz wave to a sample; 
         FIG. 2  is a diagram showing the overall configuration of a terahertz spectrometer according to an embodiment; 
         FIG. 3  is a schematic sectional view showing the configuration of the terahertz-wave transmitting unit of the terahertz spectrometer; 
         FIG. 4  is a diagram explaining how a beam travels if the reflection surface of the object is parallel to the focusing plane having the first focus; 
         FIG. 5  is a diagram explaining how the beam travels if the reflection surface of the object inclines to the focusing plane having the first focus; 
         FIG. 6  is a block diagram illustrating the configuration of a computer; and 
         FIG. 7  is a schematic top view illustrating the configuration of the terahertz-wave transmitting unit of another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment will be described with reference to the accompanying drawings. 
     (1) Overall Configuration of the Terahertz Spectrometer 
       FIG. 2  shows the overall configuration of a terahertz spectrometer  10  according to an embodiment of the present invention. The terahertz spectrometer  10  includes an ultra-short laser  11 , a dispersion compensation unit  12 , a beam splitter  13 , a terahertz-wave generating element  14 , a time delay element  15 , a terahertz-wave detecting element  16 , and a computer  17 . 
     The ultra-short laser  11  emits a pulse beam having, for example, a pulse width of about 100 fs, a pulse repetition frequency of about 80 MHz and a central wavelength of about 780 nm. An ultra-short laser source LS is, in practice, a titanium laser or a sapphire laser, emitting femtosecond pulses. 
     The dispersion compensation unit  12  adjusts the pulse width of the pulse beam coming from the ultra-short laser  11 , in a specific direction to achieve inverse correction of the pulse width spread that has resulted from the wavelength-dependency of optical fibers BF 1  and BF 2 . 
     The beam splitter  13  splits the pulse beam coming from the dispersion compensation unit  12  into an excited pulse beam and a detected pulse beam. The excited pulse beam is guided by a focusing optical unit OU 1  to the optical fiber BF 1  and thence to the terahertz-wave generating element  14  provided in a terahertz-wave transmission unit TMB. Meanwhile, the detected pulse beam passes through the time delay element  15 , is then guided by a focusing optical unit OU 2  to the optical fiber BF 2  and guided through the optical fiber BF 2  to the terahertz-wave detecting element  16  provided in the terahertz-wave transmission unit TMB. 
     The terahertz-wave generating element  14  generates a terahertz wave that has field amplitude proportional to the time derivative of the excited pulse beam. The terahertz-wave generating element  14  is, for example, a photoconductive antenna or a nonlinear optical crystal such as ZnTe. Note that the photoconductive antenna includes a semiconductor substrate of semi-insulating GaAs or the like, a pair of electrodes formed on the semiconductor substrate and a voltage applying unit for applying a bias voltage between the electrodes. 
     The time delay element  15  delays the time at which the detected pulse beam reaches the terahertz-wave detecting element  16 . In this embodiment, the time delay element  15  has a pair of mirrors  15   a  and  15   b . One or both of the mirrors  15   a  and  15   b  are arranged movable. The optical path that extends between the mirrors  15   a  and  15   b  can therefore be changed in length to adjust the time at which the detected pulse beam reaches the terahertz-wave detecting element  16 . 
     The terahertz-wave detecting element  16  detects the terahertz wave generated in terahertz-wave generating element  14  and guided through the terahertz-wave transmission path provided in terahertz-wave transmission unit TMB. The terahertz-wave detecting element  16  generates an electric field equivalent to the terahertz wave supplied to the element  16  through the terahertz-wave transmission path provided in the terahertz-wave transmission unit TMB. When the terahertz-wave detecting element  16  receives the detected pulse beam from the time delay element  15 , the terahertz-wave detecting element  16  generates a signal that has a time waveform of the electric field intensity equivalent to the terahertz wave. Like the terahertz-wave generating element  14 , the terahertz-wave detecting element  16  is, for example, a photoconductive antenna or a nonlinear optical crystal such as ZnTe. 
     The computer  17  is configured to acquire information from the signal generated by the terahertz-wave detecting element  16 . The information represents the condition of the sample arranged in the transmission path provided in the terahertz-wave transmission unit TMB. Further, the computer  17  is configured to appropriately control the time delay element  15  and the terahertz-wave transmission unit TMB. 
     (2) Configuration of the Terahertz-Wave Transmission Unit 
     The configuration of the terahertz-wave transmission unit TMB will be described with reference to  FIG. 3 . The terahertz-wave transmission unit TMB has a base plate  21 . On the base plate  21 , a movable stage  22  is arranged. The movable stage  22  can move in a horizontal plane (in x-axis direction and y-axis direction) parallel to the base plate  12  and in the vertical direction (z-axis direction) perpendicular to the base plate  12 . 
     That part of the movable stage  22 , which does not face the base plate  21  has a passage port  23  for the terahertz wave. A mount plate is provided below the passage port  23 . Placed on the mount plate is an object RB (i.e., sample SPL or measurement reference of the sample (control)) to be irradiated with the terahertz wave. 
     On the back of the movable stage  22 , a distance sensor  24  is provided to detect the distance from the object RB. A shield  25  is provided, extending over the front of the movable stage  22  and, of course, the passage port  23 . A shielded space is provided between the inner surface of the shield  25  and the front of the movable stage  22 . In the shielded space, the terahertz-wave generating element  14 , the terahertz-wave detecting element  16 , three optical lenses, i.e., aspheric lens  31 , elliptical mirror  32  and semispherical lens  33 , and a charge coupled device (CCD)  34  are arranged, able to move with respect to the movable state  22 , while maintaining a specific positional relationship. 
     The aspheric lens  31  is made of silicon material, either single crystal or polycrystal. The aspheric lens  31  is provided on that surface of the terahertz-wave generating element  14 , from which the terahertz wave emanates. The aspheric lens  31  guides the terahertz wave diverging from the terahertz-wave generating element  14 , obliquely to the plane in which the first focus of the elliptical mirror  32  lies. 
     The object RB may be placed on the mount plate lying below the passage port  23 . In this case, the computer  17  adjusts the height of the movable stage  22  so that the surface having the first focus of the elliptical mirror  32  and the reflection surface of the object RB (the surface of the sample SPL or control, or the deep layer of the sample) may lie in the same plane. 
     The elliptical mirror  32  focuses the terahertz wave reflected from the object RB located on the surface having the first focus, at the detection surface of the terahertz-wave detecting element  16 . In this embodiment, the elliptical mirror  32  is arranged to apply the terahertz wave reflected by the object RB, back toward a position where the terahertz wave has been emitted. 
     More specifically, the components are arranged in two areas AR 1  and AR 2  demarked by a plane having a normal NL to the focusing plane FF having a first focus F 1 , as shown in  FIG. 4  wherein the component identical to those shown in  FIG. 3  are designated by the same reference numbers. That is, the terahertz-wave generating element  14 , aspheric lens  31 , terahertz-wave detecting element  16  and semispherical lens  33  are arranged in the area AR 1 , and the elliptical mirror  32  is arranged in the area AR 2 . More precisely, the elliptical mirror  32  is arranged to guide the terahertz wave, which has been reflected by the object RB, back to the terahertz-wave detecting element  16  provided on the movable stage  22  that lies above the focus plane FF. 
     The reflection surface of the object RB may be parallel to the focusing plane having the first focus. In this case, as shown in  FIG. 4 , an incidence angle θ 1  and a reflection angle θ 2  are equal in view of the law of reflection. The incidence angle θ 1  is defined by the axis ILA (axis of incidence) of the terahertz wave irradiated from the aspheric lens  31  and the normal NL passing the first focus F 1  in the focusing plane FF (i.e., reflection surface of the object RB). The reflection angle θ 2  is defined by the normal NL and the axis RLA (axis of reflection) of the terahertz wave reflected by the focusing plane FF (i.e., reflection surface of the object RB). Thus, the terahertz wave reflected by the reflection surface of the object RB is reflected in the center area of the reflection surface of the elliptical mirror  32  and eventually focused on the detection surface of the terahertz-wave detecting element  16 . 
     On the other hand, the reflection surface of the object RB may incline to the focusing plane FF. In this case, as shown in  FIG. 5 , the incidence angle θ 1  at which the terahertz wave is applied from the aspheric lens  31  and the reflection angle θ 2  at which the terahertz wave is reflected by the reflection surface of the object RB are different from each other. Therefore, the terahertz wave reflected by the reflection surface of the object RB is reflected by the peripheral area of the reflection surface of the elliptical mirror  32  and eventually focused on the detection surface of the terahertz-wave detecting element  16 . 
     Since the object RB inclines to the focusing plane having a first focus or the refection surface of the object RB has depressions and projections, the elliptical mirror  32  is designed to focus the terahertz wave reflected by the object RB, on the detection surface of the terahertz-wave detecting element  16 , even if the transmission path of the terahertz wave reflected by the object RB changes. 
     The semispherical lens  33  is made of silicon material, either single crystal or polycrystal. The lens  33  is provided on the detection surface of the terahertz-wave detecting element  16 . The semispherical lens  33  suppresses the reflection of the terahertz wave at the terahertz-wave detecting element  16 . 
     (3) Configuration of the Computer 
     The configuration of the computer  17  will be described. As shown in  FIG. 6 , the computer  17  has a central processing unit (CPU)  40 , a read only memory (ROM)  41 , a random access memory (RAM)  42 , an operation unit  43 , a storage unit  44 ,a display unit  45 , an interface  46 , and a bus  47 . The bus  47  connects the ROM  41 , RAM  42 , operation unit  43 , storage unit  44 , display unit  45  and interface  46  to the CPU  40 . The ROM  41  stores programs and preset information items. The RAM  42  is used as work memory for the CPU  40 . To the interface  46 , at least the time delay element  15  ( FIG. 2 ) and the movable stage  22  ( FIG. 3 ) are connected. 
     The CPU  40  reads the programs from the ROM  41  and expands them in the RAM  42 . In accordance with the programs, the CPU  40  controls, as needed, the storage unit  44 , display unit  45  and the interface  46 , which perform various processes. 
     The CPU  40 , which has thus expanded the programs in the RAM  42 , is considered to have a stage adjusting unit  51 , a sample measuring unit  52 , an image processing unit  53  and a time-delay adjusting unit  54  as illustrated in  FIG. 6 . 
     The stage adjusting unit  51  can move the movable stage  22  in accordance with a distance signal generated by the distance sensor  24  ( FIG. 3 ), setting the movable stage  22  at a prescribed height with respect to the object RB and holding the focusing plane having a first focus of the elliptical mirror  32  and the reflection surface of the object RB in the same plane. 
     Further, the stage adjusting unit  51  can move the movable stage  22  at a predetermined speed in a predetermined scanning direction, i.e., either the x-axis direction or the y-axis direction, thereby to change the position (focusing position), where the terahertz wave coming from the aspheric lens  31  should be focused on the focusing plane having a first focus of the elliptical mirror  32 . 
     Signals generated by the terahertz-wave detecting element  16  are input to the sample measuring unit  52 . More precisely, the sample measuring unit  52  receives two signals (hereinafter called “first detection signal” and “second detection signal”). The first detection signal is input from the terahertz-wave detecting element  16  while the object RB used as a sample SPL, remains arranged on the mount plate lying below the passage port  23 . The second detection signal is input from the terahertz-wave detecting element  16  while the object RB used as a control remains arranged on the mount plate lying below the passage port  23 . Note that the second detection signal may be stored in the storage unit  44  beforehand and may be acquired from the storage unit  44  whenever necessary. 
     After receiving the first and second detection signals, the sample measuring unit  52  extracts the amplitude data item and phase data item about the terahertz wave from the first signal, and the amplitude data item and phase data item about the terahertz wave from the second signal. From the difference between the amplitude data items and the difference between the phase data items, the sample measuring unit  52  generates information that represents the condition of the sample SPL. 
     On receiving the information representing the condition of the sample SPL, the image processing unit  53  causes the display unit  45  to display the condition of the sample SPL, which is represented by the information. The user can therefore visually recognize the condition of the sample SPL. 
     When the user operates the operation unit  43 , instructing the display unit  45  to display the condition of the sample SPL, the image processing unit  53  causes the display unit  45  to display the image represented by the image signal generated by the CCD  34 . The user can therefore visually recognize the passage port  23  and the object RB arranged below the passage port  23 . 
     The time-delay adjusting unit  54  is configured to determine a desired length of the optical path that extends between the mirrors  15   a  and  15   b  of the time delay element  15 , from the signal input at the operation unit  43  (not shown), and to move one or both of the mirrors  15   a  and  15   b , thereby to set the length of the optical path to the desired value. 
     (4) Operation and Effect 
     In the terahertz spectrometer  10  configured as described above, the terahertz-wave detecting element  16  is arranged at the second focus of the elliptical mirror  32 , and the terahertz wave generated by the terahertz-wave generating element  14  is applied obliquely to the plane in which the first focus of the elliptical mirror  32  lies. 
     In the terahertz spectrometer  10 , the terahertz-wave detecting element  16  can therefore detect the terahertz wave applied obliquely to the object RB and reflected by the object RB if the object RB is placed in the focusing plane FF (see  FIG. 3  and  FIG. 4 ). 
     Further, in the terahertz spectrometer  10 , the terahertz wave reflected by the focusing plane FF having a first focus F 1  of the elliptical mirror  32  is guided to the terahertz-wave detecting element  16  arranged at the second focus of the elliptical mirror  32 . The terahertz wave reflected by the object RB can be focused on the detection surface of the terahertz-wave detecting element  16  even if the transmission path of the terahertz wave reflected by the object RB changes because the object RB inclines with respect to the focusing plane FF or because the reflection surface of the object RB has depressions and projections (see  FIG. 5 ). 
     In the terahertz spectrometer  10 , the terahertz-wave generating element  14 , aspheric lens  31 , terahertz-wave detecting element  16  and semispherical lens  33  are arranged in the area AR 1 , and the elliptical mirror  32  is arranged in the area AR 2 , the areas AR 1  and AR 2  being demarked by a plane having a normal NL to the focusing plane FF having the first focus F 1  (see  FIG. 4  and  FIG. 5 ). 
     Thus, in the terahertz spectrometer  10 , the terahertz wave applied from the area AR 1 , reflected from the object RB and travelling toward the area AR 2  can be applied back toward a position where the terahertz wave has been emitted. The terahertz-wave generating element  14  and the terahertz-wave detecting element  16  can therefore be arranged close to each other. This helps to miniaturize the terahertz spectrometer  10 . Moreover, the terahertz-wave generating element  14  and the terahertz-wave detecting element  16  can be closer to each other than in the case where the terahertz wave applied to the object is applied back after passing through the object, even if no optical system is used to change the optical path with respect to the transmission path between the elliptical mirror  32  and the terahertz-wave detecting element  16 . 
     Further, in the terahertz spectrometer  10 , the terahertz-wave generating element  14 , aspheric lens  31 , terahertz-wave detecting element  16  and semispherical lens  33 , all provided in the area AR 1 , are arranged above the focusing plane FF having the first focus F 1  (see  FIG. 4  and  FIG. 5 ). 
     This means that the terahertz-wave generating element  14  and the terahertz-wave detecting element  16  can be arranged even closer to one another. The terahertz spectrometer  10  can therefore be made much smaller. For example, the terahertz-wave generating element  14  and the terahertz-wave detecting element  16  may be arrayed to provide a probe. Further, the movable stage  22  can be arranged to avoid the interference of the terahertz wave, more easily than in the case where, in the area AR 1 , the terahertz-wave generating element  14  and terahertz-wave detecting element  16  are arranged, respectively, above and below the focusing plane FF having the first focus F 1 . 
     In the terahertz spectrometer  10 , the aspheric lens  31 , which is made of silicon material, is used as an optical lens (see  FIGS. 3 to 5 ). The aspheric lens  31  can therefore be adjusted to the terahertz-wave generating element  14  in terms of refractive index. This can reduce the energy loss in the terahertz wave generated by the terahertz-wave generating element  14  and applied to the object RB, which results from the reflection of the terahertz wave at the optical lens. In addition, since a single lens can alone suppress the aberration, the number of lenses used and the sizes thereof are smaller than in the case where a spherical lens or a parabolic mirror is used. This serves to miniaturize the terahertz spectrometer  10 , too. No anti-reflection film is bonded to optical lenses for guiding terahertz waves in most cases to reduce the influence of multipath reflection, though anti-reflection film is bonded to the optical lenses for ordinary uses. In view of this, the use of a single lens is particularly useful. 
     Further, in the terahertz spectrometer  10 , the optically system provided between the terahertz-wave generating element  14  (terahertz-wave detecting element  16 ) and the sample SPL is composed of a single aspheric lens  31  (semispherical lens  33 ). This more reduces the number of interfaces in which unnecessary reflection may occur than in the case where the optical system is composed of a plurality of lenses. The terahertz wave can therefore be improved in intensity. Ultimately, the measuring accuracy can be enhanced. 
     In the terahertz spectrometer  10 , the computer  17  moves the movable stage  22  in accordance with the distance detected by the distance sensor  24 , i.e., the distance from the sample SPL placed at the first focus F 1 . The focusing plane FF having the first focus F 1  is thereby held in flush with the reflection surface of the sample SPL (see  FIG. 3  and  FIG. 6 ). 
     In the terahertz spectrometer  10 , the terahertz wave reflected by the sample SPL (i.e., object used as a sample or a control) can be guided to the terahertz-wave detecting element  16 , irrespective of the size or shape of the sample SPL. Thus, various kinds of samples can be measured with high accuracy. 
     In the terahertz spectrometer  10 , the computer  17  moves the movable stage  22 , changing the position at which the terahertz wave coming from the aspheric lens  31  is applied, with respect to the focusing plane FF having the first focus F 1 . The information about the position where the terahertz wave is applied to the sample SPL placed in the focusing plane FF is acquired from the signal generated by the terahertz-wave detecting element  16  (see  FIG. 3  and  FIG. 6 ). 
     The terahertz spectrometer  10  can therefore perform the imaging of a greater part of the sample, enhancing the measuring accuracy. The terahertz spectrometer  10  is advantageous over the Fourier spectroscopy in that the terahertz spectrometer  10  achieves a high S/N ratio and can acquire amplitude data and phase data at the same time. 
     In the configuration described above, the terahertz wave is applied obliquely to the sample SPL arranged in the focusing plane FF having the first focus F 1  of the elliptical mirror  32 . The sample SPL reflects the terahertz wave, which is detected by the terahertz-wave detecting element  16  provided at the second focus F 2  of the elliptical mirror  32 . The terahertz wave reflected by the sample SPL can therefore be focused on the detection surface of the terahertz-wave detecting element  16  even if the transmission path of the terahertz wave reflected by the sample SPL changes because the sample SPL inclines with respect to the focusing plane FF of the first focus or because the refection surface of the sample SPL has depressions and projections. Thus, the terahertz spectrometer  10  can measure the sample at an increased accuracy. 
     (5) Other Embodiments 
     The embodiment described above has an aspheric lens  31 . This invention is not limit to this, nevertheless. For example, a refractive lens composed of N lenses arranged in a line or such parabolic mirrors as shown in  FIG. 1  may be used. That is, one or more optical lenses of any type can be used if they can apply the terahertz wave generated by the terahertz-wave generating element  14 , obliquely to the focusing plane FF having the first focus F 1  of the elliptical mirror  32 . As specified above, the aspheric lens  31  is made of silicon material. Instead, the aspheric lens  31  may be made of any other material as long as the aspheric lens  31  can be adjusted to the terahertz-wave generating element  14  in terms of refractive index. 
     In the embodiment described above, one terahertz-wave generating element  14  and one aspheric lens  31  are arranged in the terahertz-wave transmission unit TMB. This invention is not limited to this configuration. Two or more terahertz-wave generating elements  14  and two or more aspheric lenses  31  may be arranged in the terahertz-wave transmission unit TMB. 
       FIG. 7  shows a terahertz-wave transmission unit having two terahertz-wave generating elements and two aspheric lenses. In  FIG. 7 , the components identical to those shown in  FIG. 3  are designated by the same reference numbers.  FIG. 7  shows the components as viewed from the shield  25  ( FIG. 3 ) toward the movable stage  22 . 
     In this terahertz-wave transmission unit TMB, two excited pulse beams are applied to two terahertz-wave generating elements  14 A and  14 B through optical fibers BF 1   a  and BF 1   b , respectively. The optical fibers BF 1   a  and BF 1   b  are independent fibers or fibers branched from one fiber. The terahertz-wave transmission unit TMB focuses the terahertz waves applied from the terahertz-wave generating elements  14 A and  14 B through the aspheric lenses  31 A and  31 B, respectively, to a sample SPL. The sample SPL reflects the terahertz waves. The terahertz waves thus reflected are reflected by an elliptical mirror  32 . A semispherical lens  33  focuses the terahertz waves on a terahertz-wave detecting element  16 . 
     One terahertz-wave detecting element is provided for all terahertz-wave generating elements in the terahertz-wave transmission unit TMB of  FIG. 7 . The terahertz-wave transmission unit TMB can therefore be smaller than in the case where terahertz-wave detecting elements are provided for the terahertz-wave generating elements, respectively. 
     In the terahertz-wave transmission unit TMB of  FIG. 7 , the optical lenses (i.e., elliptical mirror  32  and semispherical lens  33 ) are used to guide both terahertz waves from the sample SPL to the terahertz-wave detecting element  16 . The unit TMB can therefore be much smaller than in the case two elliptical mirrores and two semispherical lenses are used to guide the two terahertz waves, respectively. 
     Further, as shown in  FIG. 7 , the incidence angles at which the two terahertz waves are applied to the focusing plane FF having the first focus F 1  of the elliptical mirror  32  are equal or similar. Hence, if the aspheric lenses  31 A and  31 B have different numerical apertures, the spot plane each terahertz wave has with respect to the sample SPL can be altered merely by using a switching unit for switching the terahertz-wave generating element, from the element  14 A to the element  14 B, or vice versa. Therefore, information items representing different resolutions or different parts of the sample can be acquired for the sample SPL. This helps to increase the measuring accuracy. The switching unit may be one that switches the wavelength or pulse width of the excited pulse beam applied to the terahertz-wave generating elements  14 A and  14 B, or switches the element to which the excited pulse beam is applied, or switches not only the wavelength or pulse width, but also the element. 
     The aspheric lenses  31 A and  31 B may have different incidence angles with respect to the first focus F 1  of the elliptical mirror  32 , though they have the same numerical aperture or similar apertures. In addition, a switching unit may be used to switch the terahertz-wave generating element, from the element  14 A to the element  14 B, or vice versa. If this is the case, the depth to which the terahertz waves reach in the sample SPL can be changed to acquire accurate information about the layer structure of the sample. This will increase the measuring accuracy. 
     The aspheric lenses  31 A and  31 B may have the same incidence angle with respect to the first focus F 1  of the elliptical mirror  32 , and may have the same numerical aperture or similar numerical apertures. Using these aspheric lenses  31 A and  31 B, the terahertz waves may be focused at the same position. Then, the sample SPL will be irradiated with a terahertz wave that has an increased intensity (brightness). As a result, the measuring accuracy will be higher, and the components that cooperate to generate terahertz waves can be more durable, than in the case where only one terahertz-wave generating element is used. 
     Moreover, the various designs specified above can be combined in any possible way. 
     In the embodiment described above, the movable stage  22  is moved, moving the optical lenses  31  to  33 , terahertz-wave generating element  14  and terahertz-wave detecting element  16  without changing the positional relationship of these components, thereby to hold the focusing plane FF having the first focus F 1  in flush with the reflection surface of the sample SPL. This invention is not limited to this, nevertheless. Instead, the mount plate holding the sample SPL may be moved to hold the focusing plane FF having the first focus F 1  in flush with the reflection surface of the sample SPL. 
     In the embodiment described above, the movable stage  22  is moved, moving the optical lenses  31  to  33 , terahertz-wave generating element  14  and terahertz-wave detecting element  16  without changing the positional relationship of these components, thereby to change the position where the terahertz wave coming from the aspheric lens  31  is applied to the focusing plane FF having the first focus F 1 . This invention is not limited to this, nevertheless. Instead, the mount plate holding the sample SPL may be moved to change the position where the terahertz wave coming from the aspheric lens  31  is applied to the focusing plane FF having the first focus F 1 . 
     The present application can be utilized in various technical fields such as industry, medical care, biotechnology, agriculture, security, data communication and electronics. 
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.