JP2008008862A - Electromagnetic wave measuring instrument - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technique for small realizing an electromagnetic wave measuring instrument capable of acquiring the space data and time data of a moving measuring target or a measuring target during movement in a real time. <P>SOLUTION: The electromagnetic wave measuring instrument is equipped with a delay time modulation element 21 having a plurality of regions different in optical length within the plane vertical to a light advancing direction with respect to passing light and constituted so that probe light 138 enters a plurality of the regions and a plurality of time lag beams of probe light 139 different in delay are outputted from the respective regions, an electrooptical element 116 on which a plurality of the time lag beams of probe light 139 and a terahertz wave 136 are incident in a superposed state and which modulates the time lag beams of probe light 139 corresponding to the electric field of the terahertz wave 136 and an image sensor 119 for detecting the respective time lag beams of probe light 139 after modulation by respective different pixels. The delay time modulation element has a plurality of regions in a plurality of parts of magnitude contained in one wavelength of the terahertz wave. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a technique for measuring the shape and material of an object to be measured using electromagnetic waves.

  In recent years, research and development of electromagnetic waves having a frequency of 0.1 to 10 THz, so-called terahertz waves, have been actively conducted. Terahertz waves have the transparency of radio waves and the light condensing property of light, and can be used for transmission analysis that is safer than X-rays. Since many materials have an intrinsic absorption spectrum in the frequency band of 0.1 to 10 THz, it is possible to specify materials and materials using terahertz waves. Terahertz waves are expected to be widely used in the future in industrial application fields such as security, biotechnology, medical treatment, food processing, and freshness analysis.

  Imaging an object to be measured using terahertz waves is highly useful because the characteristics of the object to be measured can be understood at a glance. In principle, this imaging is performed using the measurement system shown in FIG. First, this measurement system will be described.

  A short light pulse (center wavelength: 800 nm, spectrum width (FWHM) of about 20 nm, pulse width: 100 fs) 131 emitted from a femtosecond laser apparatus 101 using a titanium sapphire crystal is pumped by terahertz wave excitation by a beam splitter 102. 132 and the probe pulse light 133 for detecting the spatial distribution of modulation of the terahertz wave.

  The direction of the pump pulse light 132 is changed by the mirror 103 and is incident on the terahertz emitter 112. For example, when a (110) ZnTe crystal is used as the emitter, an optical rectification action is generated due to the nonlinearity of ZnTe, and a terahertz wave 134 is generated. The terahertz wave 134 has a pulse width of about several ps. The frequency component has a wide range from 0.1 to several THz, and the peak wavelength is around 1 THz.

  The terahertz wave 134 is collimated by the polyethylene lens 113 to become a collimated terahertz wave 135. When the terahertz wave 135 passes through the object to be measured 114, the terahertz wave 135 undergoes modulation depending on the spatial position according to the spatial distribution of characteristics such as absorption and reflection with respect to the terahertz wave in the object to be measured 114. Hereinafter, such modulation is simply referred to as spatial modulation.

  The spatially modulated terahertz wave 136 passes through the silicon mirror 115 and enters the electro-optic element 116. As this electro-optical element, (110) ZnTe is used. In the electro-optic element crystal, a Pockels effect depending on the spatial position is generated according to the electric field intensity distribution of the spatially modulated terahertz wave 136.

  On the other hand, the probe pulse light 133 passes through a delay line composed of the mirrors 104, 105, 106, and 107. The mirrors 105 and 106 are placed on a stage 108, and each time the stage 108 moves about 150 microns in the vertical direction of the drawing, the time required for the probe pulse light 133 to arrive at the subsequent optical element can be changed by 1 ps. .

  After the probe pulse light 137 that has passed through the delay line is reflected by the mirror 109, the probe pulse light 137 is expanded to a spot diameter of the collimated terahertz wave 135 by a beam expander including a concave lens 110 and a convex lens 111. The expanded probe pulse light 138 is adjusted in polarization direction by the polarizing plate 117, reflected by the silicon mirror 115, and guided to the electro-optic element 116.

  In the electro-optic element 116, the birefringence changes depending on the spatial position due to the Pockels effect described above. As a result, the polarization direction of the probe pulse light 138 is spatially modulated, and the amount of probe pulse light passing through the polarizing plate 118 is spatially modulated.

  Since the probe pulse light has a wavelength of 800 nm, it can be detected by a general Si photodiode. For example, a commonly used Si image sensor 119 can detect and image the spatial distribution of probe pulse light, that is, the spatial distribution of the terahertz wave absorption and reflection characteristics in the object 114 to be measured. it can.

  By moving the stage 108 of the delay line, the relative relationship between the arrival time of the terahertz wave 136 and the probe pulse light 138 of the electro-optic element 116 can be changed. Since the pulse width of the probe pulse light 138 is only about one tenth of the pulse width of the terahertz wave 136, the electric field intensity at a desired time position of the terahertz wave can be sampled with the probe light.

  By using a plurality of probe pulse lights having different relative relations of arrival times, it is possible to know a temporal change in the electric field intensity of the terahertz wave 136. Since the refractive index of the object to be measured is known from the peak position of the time change, and the terahertz band absorption spectrum of the object to be measured is known from the Fourier spectrum of the time change, it is possible to specify what the object is to be measured.

  However, this measurement system samples the electric field strength at different time positions of the terahertz wave 136 as the amount of light on the image sensor while mechanically moving the stage 108 of the delay line, and thus moves within the moving time of the stage 108. The object to be measured cannot be imaged. That is, it is not suitable for an application of imaging a moving object to be measured, for example, a living body, and moving an object to be measured, for example, on a moving belt conveyor.

  Then, another prior art suitable for such a use is proposed by patent document 1, for example. As shown in FIG. 16, the measurement system according to this technique includes a pulse stretcher 401 and an optical fiber bundle 402 instead of the delay line. The pulse stretcher 401 includes a reflective diffraction grating, a concave mirror, a plane reflecting mirror, a retroreflecting mirror, and the like.

  As is generally known, femtosecond pulses have a spectral broadening. In the pulse stretcher 401, the chirped probe light 347 is obtained by delaying the shorter wavelength component of the components contained in the probe light (femtosecond pulse) with respect to the longer wavelength component. The delay time is set to be approximately the same as the pulse width of the terahertz wave 136, for example, about 10 psec.

  By doing so, the shorter wavelength component of the chirped probe light 438 that has passed through the electro-optic element 116 and the polarizing plate 118 is sampled at a temporally slower position of the terahertz wave 136. Can be converted to the wavelength axis.

  Then, the optical fiber bundle 402 also has a dispersion characteristic in which the shorter wavelength component is delayed in time, thereby further expanding the difference in delay time and expanding the pulse width to 10 nsec. When the pulse width is about this level, it is possible to perform time resolution using a general light receiving element.

As described above, in this other conventional technique, the electric field strengths at different times of the terahertz wave 136 are conveyed by different frequency components of the probe light, and each frequency component can be time-resolved by a general light receiving element. By giving a delay difference, two-dimensional information and its time change information are detected.
JP 2004-020352 A

However, even if such a conventional technique is used, there are the following problems.
First, since the probe light is enlarged and measured in the time direction, time information cannot be obtained in real time.

  Secondly, the optical fiber bundle requires as many optical fibers as the number of pixels to be imaged in order to maintain spatial information. In addition, in order to give a sufficient delay time difference to different frequency components of the probe light, each optical fiber needs to have a length of several kilometers. For these reasons, the optical fiber bundle becomes very large.

  The present invention has been made in view of such a problem, and is a compact electromagnetic wave measuring device that can acquire a moving object to be measured or a space object and a time information of the object to be measured in real time while moving the object to be measured. The purpose is to provide technology to be realized.

  In order to achieve the above object, the electromagnetic wave measuring apparatus of the present invention has a plurality of regions having different optical lengths in a plane perpendicular to the traveling direction of light passing therethrough, and probe light in the plurality of regions. , A delay time modulation element that outputs a plurality of time difference probe lights having different delays from each region, a superposition optical element that superimposes the plurality of time difference probe lights and the electromagnetic wave to be measured, and the plurality of superposition time differences An electro-optical element that receives the probe light and the electromagnetic wave to be measured and modulates each time difference probe light according to the electric field of the electromagnetic wave to be measured, and an image sensor that detects each time difference probe light after the modulation by different pixels. With.

  Here, the superposition optical element may superimpose both of the time difference probe light and the electromagnetic wave to be measured by reflecting one and transmitting the other.

  The optical length is an amount obtained by multiplying the optical path length by the refractive index. The plurality of regions may have different thicknesses in the light passing direction so that the plurality of regions have different optical lengths, and the plurality of regions have different refractive indexes. Also good.

  According to this configuration, the measured electromagnetic wave can be sampled at a plurality of different time positions by the plurality of time difference probe lights, and the sampling result can be obtained from different pixels of the image sensor.

  Since this configuration does not include a long and long optical fiber bundle for enlarging the delay time difference, it can be realized in a small size. Further, unlike the conventional case, since the probe light is not time-extended after sampling, the object to be measured can be measured strictly in real time.

  Further, in the delay time modulation element, it is preferable that the plurality of regions are provided periodically, and the period is smaller than the center wavelength of the electromagnetic wave to be measured.

  According to this configuration, as is generally known, since the spatial resolution of the electromagnetic wave is almost one wavelength, a plurality of signals output from a plurality of regions within one period smaller than the center wavelength of the electromagnetic wave to be measured. The time difference probe light samples the measured electromagnetic waves having the same spatial information at different time positions. That is, the maximum spatial resolution due to the electromagnetic wave to be measured is exhibited with time sampling.

  The probe light is a part of laser light having a center wavelength in the visible light to infrared light band and a pulse width of 300 femtoseconds or less, and the measured electromagnetic wave is the other of the laser light. The part is preferably a terahertz wave radiated by being incident on the terahertz emitter.

  According to this configuration, it is possible to perform excellent transmission analysis on the measurement object based on unique characteristics such as absorption and reflection of various measurement objects with respect to the terahertz wave. And the detection of the probe light for that purpose can be performed using the general image sensor which has sensitivity in visible light thru | or infrared light band.

  Further, the electromagnetic wave measuring apparatus further includes the time difference probe light after the modulation between the electro-optic element and the image sensor and the image sensor of the image sensor with respect to the arrangement interval of the region of the delay time modulation element. You may provide the reduction optical system which reduces by the ratio of the arrangement | sequence space | interval of a pixel.

  According to this configuration, one time difference probe light is detected by exactly one pixel of the image sensor, and the pixel can be effectively used. When the arrangement interval of the pixels of the current general image sensor is several μm, the center frequency of the electromagnetic wave to be measured is 1 THz (center wavelength 300 μm), and as an example, time sampling of 30 points is performed. The arrangement interval of the regions is 10 μm. When the arrangement interval of the pixels is smaller than the arrangement interval of the regions, by providing such a reduction optical system, probe light bearing the same time-space information is detected by only one pixel, so that waste is not caused. Absent.

  In particular, if at least one of the light entrance surface and the light exit surface of the delay time modulation element is formed in a step shape corresponding to the thickness of each region, the cross section is symmetrical, the other two or more steps are combined. The creation process is facilitated because it is not necessary to create a steep step that leads to the connection.

  Further, if both the light incident surface and the light exit surface of the delay time modulation element are formed in a step shape corresponding to the thickness of each region, the time delay can be gained on both surfaces of the delay time modulation element.

  In addition, it is preferable that an antireflection film is formed on at least one of the light incident surface and the light emitting surface of the delay time modulation element with respect to the center wavelength of the probe light. Furthermore, it is preferable that a protective film covering the surface irregularities is formed.

  By using the present invention, data necessary for shape determination and material determination of an object to be measured using electromagnetic waves can be measured simultaneously and in real time, and a high-speed electromagnetic wave measuring apparatus can be realized in a small size.

<Configuration of electromagnetic wave measuring device>
FIG. 1 is a configuration diagram illustrating an example of an electromagnetic wave measuring apparatus according to an embodiment of the present invention.

  A short light pulse (center wavelength: 800 nm, spectral width (FWHM) of about 20 nm, pulse width: 100 fs) 131 emitted from a femtosecond laser apparatus 101 using a titanium sapphire crystal is pumped by a beam splitter 102 and pump pulse light 132 and probe pulse light 133. Fork.

  The direction of the pump pulse light 132 is changed by the mirror 103 and is incident on the terahertz emitter 112. As the terahertz emitter 112, for example, a (110) ZnTe crystal having a thickness of 3 mm is used. For the terahertz emitter 112, a photoconductive switch can be used in addition to ZnTe.

  When the pump pulse light 132 is incident, the terahertz emitter 112 emits a terahertz wave 134 having a center frequency of 1 THz (a center wavelength of 300 μm) corresponding to the pulse width.

  Note that the pulse width of the pump pulse light is not limited to 100 fs described above. For example, a terahertz wave can be obtained using pump pulse light having a pulse width of 300 fs.

  The terahertz wave 134 is collimated by the polyethylene lens 113 and becomes a collimated terahertz wave 135. When the terahertz wave 135 passes through the object to be measured 114, the terahertz wave 135 is modulated according to the spatial distribution of characteristics such as absorption and reflection with respect to the terahertz wave in the object to be measured 114. As the DUT 114, an aluminum 52 stripe pattern (width 100 μm, pitch 400 μm) formed on a 120 μm plastic plate 51 as shown in FIG. 2 is used.

  The spatially modulated terahertz wave 136 passes through the silicon mirror 115 and enters the electro-optic element 116. As the electro-optic element, (110) ZnTe having a thickness of 3 mm is used. In the electro-optic element crystal, a Pockels effect depending on the spatial position and time occurs in accordance with the spatial distribution and temporal change of the electric field intensity of the terahertz wave 136.

  On the other hand, after the probe pulse light 133 is reflected by the mirror 109, the probe pulse light 133 is expanded to the same size as the spot diameter of the collimated terahertz wave 135 by a beam expander including the concave lens 110 and the convex lens 111. The expanded probe pulse light 138 passes through the delay time modulation element 21.

  As will be described in detail later, the delay time modulation element 21 has a plurality of regions having different optical lengths in a plane perpendicular to the traveling direction of light passing therethrough, and gives a plurality of different delays to the incident probe pulse light 138. As a result, a plurality of time difference probe pulse lights 139 are output.

  Here, the delay time modulation element 21 has the plurality of regions described above in a plurality of portions having a size included in one wavelength of the terahertz wave 136, and the arrangement interval of each region is larger than the center wavelength of the probe pulse light 138. Is also wide enough.

  The plurality of time-difference probe pulse lights 139 output from the delay time modulation element 21 are adjusted by the polarizing plate 117 and then reflected by the silicon mirror 115 so that they are superimposed on the spatially modulated terahertz wave 136 and electrically Guided to the optical element 116.

  Here, the silicon mirror 115 is an example of a superimposing optical element. On the other hand, a superimposed optical element can be realized using glass with ITO (Indium Tin Oxide) that transmits the time difference probe pulse light 139 and reflects the terahertz wave 136.

  In the electro-optic element 116, as described above, the Pockels effect depending on the spatial position and time caused by the spatially modulated terahertz wave is generated, and a plurality of the Pockels effects are sampled in the time direction with the probe light.

  More specifically, a change in the polarization direction that occurs in each of the plurality of time difference probe pulse lights 139 due to the Pockels effect is replaced with a change in the amount of light by the polarizing plates 117 and 118, and is detected by each pixel of the image sensor 119.

  Note that a reduction optical system using a lens is installed between the electro-optical element 116 and the image sensor 119, and a plurality of time-difference probe pulse lights 139 are applied to the pixels of the image sensor 119 with respect to the arrangement interval of the regions in the delay time modulation element 21. You may reduce by the ratio of arrangement | sequence space | interval.

  As the image sensor, for example, a 1/4 inch QVGA image sensor having 320 horizontal pixels and 240 vertical pixels can be used. As a type of image sensor, a CCD, a MOS sensor or the like can be applied.

<Delay time modulation element>
FIG. 3 is a cross-sectional view showing an example of a cross section of the delay time modulation element 21. Note that a similar shape may appear in a cross section orthogonal to the paper surface, or a uniform shape may appear. When a similar shape appears, the step is provided in the two-dimensional direction, and when a uniform shape appears, the step is provided only in the one-dimensional direction. The surface of the delay time modulation element 21 is formed in a step shape represented by such a cross section. One step of this step shape corresponds to one of the aforementioned areas.

  As the delay time modulation element 21, for example, an n-type GaN substrate having a thickness of about 1 mm can be used. Since GaN has a band gap of 3.4 eV (corresponding to a wavelength of 365 nm), it is transparent to probe pulsed light 138 having a wavelength of 800 nm.

  As an example, the delay time modulation element 21 repeatedly includes five types of regions having different thicknesses, and the surface thereof is formed in a shape that repeats five steps according to the thickness of each region. Here, for example, if the width of each step is 10 μm and the step is 43 μm, the refractive index of GaN is 2.4. Therefore, one unit of this repetition consists of five time difference probe pulse lights with a time difference of 0.2 psec, The light is emitted over a time of 1 psec and a space width of 50 μm.

  One unit having a space width of 50 μm is an example of a portion having a size included in one wavelength of 300 μm of the terahertz wave 136. A step width of 10 μm, which is an arrangement interval of each region, is sufficiently wider than the central wavelength of 800 nm of the probe pulse light 138.

  The number and size of the steps shown here are examples. As long as the repetition unit is included in one wavelength of the terahertz wave 136 and the step width is wider than the center wavelength of the probe pulse light 138, steps of other numbers and sizes may be used.

  For example, the inventors conducted an experiment using the delay time modulation element 21 formed in a shape in which 10 steps having a width of 10 μm and a step of 43 μm were repeated, and obtained good results. This result will be described in detail later.

Next, the operation of the delay time modulation element 21 will be described in detail.
FIG. 3 shows an optical path of the probe pulse light 138 incident on each step of the delay time modulation element 21. Here, each step is indicated by a number from 0 to 4. For example, the probe pulse light incident on step 3 travels along the optical path 13. The width of the i-th step is denoted as Wi, and the step between the i-th and j-th steps is denoted as _dij . For example, for the case of i = 3, j = 4, the step width is W3, the step is d _34.

  Here, Wi is set to a value sufficiently larger than the wavelength λp of the probe light 138. In order to reduce the influence of scattering or the like at the boundary, Wi is preferably 5 times or more of λp. Thereby, the wavefront of the probe pulse light is substantially maintained near the center in each step and is emitted in the same direction as the incident direction.

  On the other hand, the light near the boundary of each step changes direction due to refraction, scattering, etc. of the boundary. However, since Wi is sufficiently larger than λp, the light whose direction is changed is small compared to the light traveling straight, and most of the light whose direction is changed is out of the optical path reaching the image sensor 119. It doesn't matter.

The probe pulse light 138 passes through the inside of the delay time modulation element 21, is given a plurality of different delays, and is output as a plurality of time difference probe pulse lights 139. For example, the optical path 14 passes through the inside of the delay time modulation element 21 by d ₃₄ more than the optical path 13. As a result, a time difference ΔT ₃₄ is generated between the time difference probe pulse light output along the optical path 13 from the delay time modulation element 21 and the time difference probe pulse light output along the optical path 14.

If the speed of light is c and the refractive index of the delay time modulation element 21 is n, ΔT ₃₄ is ΔT ₃₄ = (n−1) × d ₃₄ / c
Given in. Similar time differences ΔT ₀₁ , ΔT ₁₂ , and ΔT ₂₃ also occur in the time difference probe pulse light output along the other two adjacent optical paths.

As shown in FIG. 4, the time difference probe pulse light 139 emitted from the delay time modulation element 21 is superimposed on the terahertz wave 136 by the silicon mirror 115 and is incident on the electro-optic element 116. At this time, when W = ΣWi and the wavelength of the terahertz wave 136 are λ, the width on the delay time modulation element 21 that outputs a set of time difference probe pulsed light 139 with different delays,
λ> W
Is satisfied, each time-difference probe pulse light 139 within the unit width W is converted into terahertz waves having the same spatial information (T1, T2, T3 in FIG. 4), considering that the spatial resolution of the terahertz wave is about the wavelength λ. ...) are sampled at a plurality of different time positions.

  Here, the condition of λ> W is equivalent to the fact that the repeating unit of the plurality of regions described above is included in one wavelength of the terahertz wave.

  These time difference probe pulse lights are incident on different pixels of the image sensor 119. In the example of FIG. 4, each pixel detects the following information with the sampling time by the earliest time difference probe pulse light as time T.

Pixel P1: the information pixel at the time T of the terahertz wave T1 P2: the information pixel at the time T + [Delta] T ₀₁ of the terahertz wave T1 P3: information of the time T + ΔT ₀₁ + ΔT ₁₂ of the terahertz wave T1
...
Pixel P6: the information pixel at the time T of the terahertz wave T2 P7: the information pixel at the time T + [Delta] T ₀₁ of the terahertz wave T2 P8: information of the time T + ΔT ₀₁ + ΔT ₁₂ of the terahertz wave T2
...

  As shown in FIG. 1, the terahertz wave is spatially modulated by the measurement object 114. Therefore, the image sensor 119 can acquire both terahertz spatial information and temporal information.

<Experimental result>
The inventors of the present invention, together with the delay time modulation element 21 having the above-mentioned 10 steps of 10 μm width and 43 μm step as a repeating unit (unit width 100 μm), 1/4 inch of 320 horizontal pixels and 240 vertical pixels. An experiment for measuring the object to be measured shown in FIG. 2 was performed using a QVGA image sensor.

  The measurement results obtained from this experiment are shown in FIGS. FIG. 5 shows a measurement result when the measurement object 114 is attached, and FIG. 6 shows a measurement result when the measurement object 114 is detached. 5 and 6 show the signals on the 120th line of the image sensor (horizontal parallel near the center of the screen). In this experiment, since the measurement object uniform in the vertical direction shown in FIG. 4 was used, substantially the same signal was obtained in other rows of the image sensor.

  5 and 6, the value of the horizontal pixel number i + 10j (i = 1, 2, 3... 10, j = 0, 1, 2, 3,... 31) is the horizontal position j × 100 μm ± 50 μm of the object to be measured. Represents a value obtained by sampling the terahertz wave transmitted through the wave at reference time + i × 0.2 psec.

  For example, the imaging data after 0.6 psec from the reference time is obtained as an image as shown in FIG. 7, for example, by plotting the value of the horizontal pixel number 3 + 10j. Note that the image in FIG. 7 is subjected to signal processing so that the boundary becomes smooth. The part that appears white is the part through which the terahertz wave passes, and the part through which the black part does not pass. It can be seen that a pattern with a pitch of 400 μm can be imaged.

On the other hand, in order to perform the time-domain spectroscopy of the portion where the terahertz wave is transmitted, for example, the value of the horizontal pixel number i + 50 may be plotted, and the result shown in FIG. 8 is obtained. It can be seen that the peak position is shifted by 0.2 psec depending on the presence or absence of the object to be measured. When the thickness of the plastic plate is d, the refractive index is n, the speed of light is c, and the peak position change with and without the object to be measured is ΔT,
ΔT = (n−1) d / c
Therefore, the refractive index of this plastic plate is about 1.5, and the material can be evaluated. Further, if the Fourier transform of FIG. 8 is performed, an absorption spectrum can be obtained from a comparison with / without the object to be measured. In general, since there are many materials having a specific absorption spectrum in the terahertz band, the plastic material can be specified more accurately.

  Thus, when the present invention is used, imaging and material evaluation can be performed simultaneously.

  The inventors produced the delay time modulation element 21 used in this experiment by general semiconductor photolithography and chlorine-based dry etching.

  In addition, disturbance of the probe light by the delay time modulation element 21 (for example, a loss generated when passing through the delay time modulation element varies depending on the delay amount) may be corrected by signal processing for each pixel.

<Manufacture of delay time modulation element and other shape examples>
FIG. 9 is a cross-sectional view showing another shape example of the delay time modulation element according to the embodiment of the present invention. Unlike FIG. 3, it forms so that the cross section may become a symmetrical mountain shape. By making such a shape, it is not necessary to create a steep step where two or more other steps are connected to one, so that the creation process is facilitated. As a result, the yield is improved.

FIGS. 10A to 10D are diagrams showing this creation process. (A) A Ni mask 302 is formed on the GaN substrate 301 by electron beam evaporation and photolithography. (B) Next, a first convex portion is formed by dry etching using Cl ₂ gas. The Ni mask is removed with HCl. Similarly, a Ni mask 303 is formed by electron beam evaporation and photolithography. (C) Next, a second convex portion is formed by dry etching using Cl ₂ gas. The Ni mask is removed with HCl. Similarly, a Ni mask 304 is formed by electron beam evaporation and photolithography. (D) Finally, the second convex portion is formed by dry etching using Cl ₂ gas, and the Ni mask is removed by HCl to complete.

  Note that the mountain shape of the cross section is not necessarily symmetrical. For example, a shape in which the steps between adjacent regions are all the same size can be created by the above process.

  FIG. 11 is a cross-sectional view showing still another example of the shape of the delay time modulation element 21. FIG. 11A shows a shape in which the pattern of FIG. 2 is formed symmetrically on both surfaces of the substrate, and FIG. 11B shows a shape in which the pattern of FIG. 9 is formed symmetrically on both surfaces of the substrate. In either case, a time delay can be earned on both sides of the delay time modulation element 21.

  As a result, if the level difference is the same, the time delay can be given to the passing light twice as compared with the case where it is formed on one side, and the temporal measurement range is widened. Alternatively, if the same time measurement range is acceptable, the step difference of each step can be halved compared to the case where the steps are formed on one side, and the process becomes easy.

  Note that the shapes of the front surface and the back surface are not necessarily plane-symmetric. You may form the shape of a surface and a back surface in the relationship where the difference of the thickness of each area | region is emphasized simply.

<Modification of delay time modulation element>
FIG. 12 is a cross-sectional view showing the structure of the delay time modulation element 21 in the modification. In this example, alumina dielectric films 22 and 23 are formed on the surface of the delay time modulation element 21 to form an antireflection film. In general, when the refractive index of a dielectric is n and the target wavelength is λ, antireflection can be performed when the dielectric thickness is λ / 4n.

  In the above-described experiment, specifically, by using a delay time modulation element in which 1200 mm of alumina having a refractive index of 1.65 is formed, the loss when probe pulse light having a center wavelength of 800 nm enters and exits the delay time modulation element 21. Reduced. Needless to say, another dielectric film (a material having a refractive index different from the refractive index of the step portion of the delay time modulation element) may be used.

  FIG. 13 is a cross-sectional view showing the structure of the delay time modulation element 21 in another modification. A protective material 24 having a refractive index different from the step portion refractive index of the delay time modulation element is embedded in the surface of the delay time modulation element 21. In the inventors' experiments, BPSG (Boro-phospho silicate glass) was used as the protective material 24. BPSG is a kind of oxide film and can be formed by spin coating. As a result, the refractive index of the step portion of the delay time modulation element can be protected, and handling of the delay time modulation element becomes easy.

  FIG. 14 is a cross-sectional view showing the structure of the delay time modulation element 21 in still another modification. In the delay time modulation element 30, regions 32, 33, 34, 35, and 36 having different refractive indexes are periodically provided on a GaN substrate 31 that is transparent to probe light. If the refractive index is different, the optical length is also different. Therefore, the probe pulse light passing through each region arrives at the image sensor with different delay times.

The inventors conducted experiments using a resin (so-called nanocomposite) in which fine TiO ₂ was dissolved as means for changing the refractive index, and confirmed that a plurality of different delays were given to the probe pulse light. The resin can be to change the refractive index by the content of TiO _2. The regions 32, 33, 34, 35, and 36 may be made of different materials, for example, dielectrics and semiconductors (eg, AlGaN) having different compositions, instead of nanocomposites.

  The present invention can be applied to an electromagnetic wave measurement device capable of high-speed measurement, particularly a terahertz measurement device, and can be used for nondestructive measurement in various fields such as medical treatment, biotechnology, agriculture, food, environment, and security.

The block diagram which shows an example of the electromagnetic wave measuring device in embodiment of this invention A diagram showing an example of a device under test Sectional drawing which shows an example of the cross-sectional shape of a delay time modulation element The figure explaining the principle by which space information and time information are acquired simultaneously The figure which shows an example of the image sensor output when the measurement object is attached The figure which shows an example of the image sensor output when the measurement object is removed A diagram showing an example of the image of the measurement object The figure which shows an example of the time change of the terahertz wave Sectional drawing which shows an example of the other cross-sectional shape of a delay time modulation element (A)-(d) The figure which shows an example of the manufacturing process of a delay time modulation element (A), (b) Sectional drawing which shows an example of other cross-sectional shape of a delay time modulation | alteration element Sectional drawing which shows the structure of the delay time modulation element in a modification Sectional drawing which shows the structure of the delay time modulation element in another modification Sectional drawing which shows the structure of the delay time modulation element in other modification Configuration diagram showing an example of a conventional terahertz imaging device Configuration diagram showing an example of a conventional terahertz imaging device using chirped probe light

Explanation of symbols

21, 30 Delay time modulation element 22, 23 Alumina dielectric film 24 Protective material 31 GaN substrate 32, 33, 34, 35, 36 Regions with different refractive indexes 51 Plastic plate 52 Aluminum 101 Femtosecond laser device 102 Beam splitter 103, 104 , 105, 106, 107, 109 Mirror 108 Stage 110 Concave lens 111 Convex lens 112 Terahertz emitter 113 Polyethylene lens 114 Measured object 115 Silicon mirror 116 Electro-optic crystal 117, 118 Polarizer 119 Image sensor 131 Short light pulse 132 Pump pulse light 133, 137, 138 Probe pulse light 134, 135, 136 Terahertz wave 139 Time difference probe pulse light 301 GaN substrate 302, 303, 304 Ni mask 401 Pulse stretcher 402 Optical fiber bundle 437, 438 Chirped probe light

Claims (11)

A plurality of time-difference probe lights having a plurality of regions having different optical lengths in a plane perpendicular to the traveling direction of the light passing therethrough, the probe light being incident on the plurality of regions, and delays being different from each region A delay time modulation element that outputs
A superposition optical element that superimposes the plurality of time difference probe lights and the electromagnetic wave to be measured;
An electro-optical element that receives the plurality of superimposed time difference probe lights and the electromagnetic wave to be measured, and modulates each time difference probe light according to the electric field of the electromagnetic wave to be measured;
An electromagnetic wave measuring apparatus comprising: an image sensor that detects each time-difference probe light after the modulation with different pixels. The electromagnetic wave measuring apparatus according to claim 1, wherein in the delay time modulation element, the plurality of regions are periodically provided, and the period is smaller than a center wavelength of the electromagnetic wave to be measured. The probe light is a part of laser light having a center wavelength included in a visible light to infrared light band and a pulse width of 300 femtoseconds or less.
The electromagnetic wave measuring apparatus according to claim 1, wherein the electromagnetic wave to be measured is a terahertz wave emitted when another part of the laser beam is incident on a terahertz emitter. And a reduction optical system for reducing each modulated time-difference probe light between the electro-optic element and the image sensor at a ratio of the pixel arrangement interval to the region arrangement interval. The electromagnetic wave measuring device according to claim 1. The electromagnetic wave measuring apparatus according to claim 1, wherein the superimposing optical element superimposes the time difference probe light and the electromagnetic wave to be measured by reflecting one of them and transmitting the other. The electromagnetic wave measuring apparatus according to claim 1, wherein the plurality of regions have different thicknesses in the light passing direction. The at least one of the light entrance surface and the light exit surface of the delay time modulation element is formed in a step shape corresponding to the thickness of each region, the cross section being symmetric. Electromagnetic wave measuring device. The electromagnetic wave measuring apparatus according to claim 6, wherein both the light incident surface and the light emitting surface of the delay time modulation element are formed in a step shape according to the thickness of each region. The electromagnetic wave measuring apparatus according to claim 6, wherein an antireflection film is formed on at least one of the light incident surface and the light emitting surface of the delay time modulation element with respect to the center wavelength of the probe light. The electromagnetic wave measuring apparatus according to claim 6, wherein a protective film that covers the surface irregularities is formed on the delay time modulation element. The electromagnetic wave measuring apparatus according to claim 1, wherein the plurality of regions have different refractive indexes.