JP4759770B2 - Terahertz electromagnetic wave generating element - Google Patents

Description

  The present invention relates to a terahertz electromagnetic wave generating element that efficiently generates a terahertz electromagnetic wave.

  The frequency of the terahertz electromagnetic wave ranges from 0.1 THz to 10 THz, and the wavelength is from 30 μm to 3 mm. That is, the frequency region of the terahertz electromagnetic wave is a frequency region that hits the boundary between the light wave and the radio wave. For this reason, terahertz electromagnetic waves have a property of transmitting a substance like radio waves and going straight like light. Information on molecular structure and motion state can be obtained from the frequency band of terahertz electromagnetic waves. Therefore, biomedical such as information communication, non-destructive inspection, security field such as detection of dangerous and concealed objects, and imaging of malignant tumors. Applications in a wide range of fields such as agriculture, industry, environment and space are expected.

  As a method for generating a terahertz electromagnetic wave, for example, a method in which a nonlinear optical crystal is irradiated with a femtosecond pulse laser to generate the terahertz electromagnetic wave is known (for example, Patent Document 1). When the nonlinear optical crystal is irradiated with strong light such as laser light by a femtosecond pulse laser, polarization is generated and disappears in the nonlinear optical crystal, which is equivalent to effectively generating a pulse current. As a result, the nonlinear optical crystal emits terahertz electromagnetic waves.

  However, in the frequency region of the terahertz electromagnetic wave, the refractive index of the substance increases due to dispersion. For this reason, when the terahertz electromagnetic wave passes through the generating element, a component that is reflected or absorbed increases, and the output of the terahertz electromagnetic wave decreases. In order to prevent a decrease in output of the terahertz electromagnetic wave, an attempt has been made to reduce reflection on the surface by providing a thin film layer on the surface of the nonlinear optical crystal. However, since there is no thin film material effective in the frequency region of the terahertz electromagnetic wave and the thin film itself has dispersibility, it has not been realized that the energy conversion efficiency to the terahertz electromagnetic wave is greatly improved.

  In addition, it is known that certain substances easily transmit terahertz electromagnetic waves in a specific frequency band. Utilizing this property, inspection systems that detect specific substances such as prohibited drugs and explosives using terahertz electromagnetic waves have been studied. In order to detect a specific substance as described above, it is necessary to generate a terahertz electromagnetic wave having a predetermined frequency band determined by the specific substance with the highest possible energy conversion efficiency.

JP 2005-99453 A

  Therefore, there is a need for a terahertz electromagnetic wave generating element that can generate terahertz electromagnetic waves in a predetermined frequency band with as high energy efficiency as possible.

  A terahertz electromagnetic wave generating element according to the present invention includes a generation layer made of a nonlinear optical material that emits a terahertz electromagnetic wave from the other surface when light is irradiated on one surface, and a plurality of pairs of layer structures provided on both sides of the generation layer And. Each of the plurality of pairs of layer structures includes a first layer, a second layer provided on a side of the first layer opposite to the generation layer, and a side of the generation layer of the second layer. A first grating having a first period and a first height smaller than the wavelength of the terahertz electromagnetic wave to be used, and a surface of the second layer opposite to the generation layer. And a second grating having a first period and a first height. The refractive index (first refractive index) of the medium of the first layer is lower than the refractive index (second refractive index) of the medium of the second layer. The first and second gratings are configured such that a refractive index between the first layer and the second layer gradually changes between the first and second refractive indexes. Yes. The thicknesses of the first and second layers, the first period, and the first frequency so as to generate a terahertz electromagnetic wave having a desired bandwidth with respect to the center wavelength of the terahertz electromagnetic wave generated by the generation layer. 1 height is defined.

  According to the terahertz electromagnetic wave generating element according to the present invention, the energy of the terahertz electromagnetic wave can be concentrated in a desired bandwidth. Therefore, a terahertz electromagnetic wave having a desired bandwidth can be generated with high energy efficiency.

  In the terahertz electromagnetic wave generating element according to the embodiment of the present invention, the first period is determined so as to remove as much as possible the terahertz electromagnetic wave having a frequency equal to or higher than the upper limit value of the desired bandwidth.

  According to this embodiment, the terahertz electromagnetic wave having a frequency equal to or higher than the upper limit value of the desired bandwidth is diffracted and removed by the first and second gratings provided on both sides of the second layer.

  In the terahertz electromagnetic wave generating element according to the embodiment of the present invention, the convex portions of the first and second gratings are conical.

  According to this embodiment, since the convex portions of the first and second gratings are conical, the refractive index can be gradually changed between the first layer and the second layer. .

  In the terahertz electromagnetic wave generating element according to the embodiment of the present invention, the convex portions of the first and second gratings are conical.

  According to this embodiment, since the convex portions of the first and second gratings are conical, the refractive index can be gradually changed between the first layer and the second layer. .

  In the terahertz electromagnetic wave generating element according to the embodiment of the present invention, the convex portions of the first and second gratings have a pyramid shape.

  According to this embodiment, since the convex portions of the first and second gratings are pyramidal, the refractive index can be gradually changed between the first layer and the second layer. .

  In the terahertz electromagnetic wave generating element according to the embodiment of the present invention, the convex portions of the first and second gratings are made of the same material as that of the second layer.

  According to the present embodiment, since the convex portions of the first and second gratings are made of the same material as that of the second layer, the refractive index is continuously maintained between the first layer and the second layer. Can be changed.

  In the terahertz electromagnetic wave generating element according to the embodiment of the present invention, the first and second gratings are arranged one-dimensionally.

  According to this embodiment, the output of the terahertz electromagnetic wave having a predetermined polarization plane can be increased by the one-dimensionally arranged grating. Specifically, it is possible to prevent the attenuation of the terahertz electromagnetic wave having a polarization plane orthogonal to the longitudinal direction of the protrusions of the lattice arranged in one dimension.

  In the terahertz electromagnetic wave generating element according to the embodiment of the present invention, the first and second gratings are two-dimensionally arranged.

  According to this embodiment, the output of the terahertz electromagnetic wave can be increased by the two-dimensionally arranged grating regardless of the polarization plane.

  In the terahertz electromagnetic wave generating element according to the embodiment of the present invention, the medium of the first layer is air.

  According to this embodiment, since air is used as a medium, the first layer can be easily formed.

  The terahertz electromagnetic wave generating element according to the embodiment of the present invention includes a frame-shaped support structure so as to form the first layer.

  According to this embodiment, the first layer whose medium is air can be formed by providing the frame-shaped support structure.

  In the terahertz electromagnetic wave generating element according to the embodiment of the present invention, the second layer and the frame-shaped support structure are integrally formed.

  According to this embodiment, since the second layer and the frame-shaped support structure are integrally formed, the manufacturing process is simplified.

  In the terahertz electromagnetic wave generating element according to the embodiment of the present invention, the medium of the second layer is polypropylene.

  In the terahertz electromagnetic wave generating element according to the embodiment of the present invention, the nonlinear optical material is a crystal of zinc telluride (ZnTe).

  An object inspection system according to an aspect of the present invention includes a terahertz electromagnetic wave generating element according to the present invention.

  In the object inspection system according to this aspect, since the output of the terahertz electromagnetic wave generating element is large, the object can be inspected with higher accuracy.

It is a figure which shows the structure of the object inspection system containing a terahertz electromagnetic wave generating element. It is a figure for demonstrating the structure of the terahertz electromagnetic wave generating element by one Embodiment of this invention. It is a figure which shows the structure of the terahertz electromagnetic wave generation element provided with 5 pairs of layer structure by one Embodiment of this invention. It is a figure for demonstrating a duty ratio. It is a figure which shows the structure of the grating | lattice arranged in one dimension. It is a figure which shows the two-dimensional arrangement | sequence grating | lattice in which the projection part of the quadrangular pyramid was arranged in the square lattice shape. It is a figure which shows the two-dimensional arrangement | sequence grating | lattice in which the projection part of the cone was arranged in square lattice shape. It is a figure which shows the two-dimensional array lattice by which the projection part of the quadrangular pyramid was arranged in regular hexagonal lattice shape. It is a figure which shows the two-dimensional arrangement | sequence grid in which the projection part of the cone was arranged in the regular hexagonal lattice form. It is a flowchart for demonstrating the design method of the nonlinear optical crystal which generates the terahertz electromagnetic wave of a desired center wavelength. 5 is a flowchart for explaining a design method of a terahertz electromagnetic wave generating element that generates a terahertz electromagnetic wave having a desired bandwidth with respect to the center wavelength of the terahertz electromagnetic wave at λ _THz . It is a figure which shows the relationship between the frequency and utilization efficiency of the terahertz generating element of the 1st Example calculated | required by RCWA. It is a figure which shows the relationship between the frequency and utilization efficiency of the terahertz generating element of the 2nd Example calculated | required by RCWA. It is a figure which shows the relationship between the frequency and utilization efficiency of the terahertz generating element of the 3rd Example calculated | required by RCWA. It is a figure which shows the relationship between the frequency and utilization efficiency of the terahertz generating element of the 4th Example calculated | required by RCWA. It is a figure explaining the method of manufacturing the 2nd layer provided with the grating | lattice by the imprint method. It is a figure explaining the method of manufacturing the 2nd layer provided with the grating | lattice by injection molding. It is a figure for demonstrating the method to manufacture a terahertz electromagnetic wave generating element using a 2nd layer. It is a figure explaining the method to manufacture integrally the 2nd layer provided with the grating | lattice, and the frame for forming the 1st layer. It is a figure for demonstrating the method of manufacturing a terahertz electromagnetic wave generating element using the 2nd layer and frame formed as integral.

  FIG. 1 is a diagram illustrating a configuration of an object inspection system including a terahertz electromagnetic wave generating element. In FIG. 1, the laser beam emitted by the femtosecond laser 201 is reflected by the reflecting mirror 203 and reaches the beam splitter 205. The laser beam pulse is branched into two by the beam splitter 205, and one is incident on the terahertz electromagnetic wave generating element 100. The terahertz electromagnetic wave generating element 100 that has received the laser beam pulse generates and emits a terahertz electromagnetic wave. The terahertz electromagnetic wave reaches the concave reflecting mirror 207 and is reflected and converted into a plane wave. The terahertz electromagnetic wave converted into the plane wave is condensed by the reflecting mirror 207a and the concave reflecting mirror 207b so that sufficient intensity can be obtained at the inspection target object 301. The terahertz electromagnetic wave passes through the object to be inspected 301, reaches the concave reflecting mirror 209 through the concave reflecting mirror 209 b and the reflecting mirror 209 a, is reflected, and then collected on the terahertz detector 211.

  The specifications of the femtosecond laser are, for example, a center wavelength of 800 nm, a pulse width of 60 to 100 fs (femtosecond), a repetition frequency of 70 to 80 MHz, and an output power of 1 W. A titanium / sapphire laser may be used instead of the femtosecond laser.

  The other laser beam pulse branched by the beam splitter 205 passes through the optical delay stage 213, is reflected by the reflecting mirror 215, and reaches the terahertz detector 211. The terahertz detector 211 detects the electric field intensity of the terahertz electromagnetic wave incident on the terahertz detector 211 when the other laser beam pulse is irradiated. The optical delay stage 213 adjusts the optical path length by moving the mirror, and adjusts the time for the other laser beam pulse to reach the terahertz detector 211. The time change of the electric field strength of the terahertz electromagnetic wave is measured while adjusting the time for the other laser beam pulse to reach the terahertz detector 211. Finally, if the time change of the electric field strength is Fourier transformed, the transition of the electric field strength with respect to the frequency can be obtained.

  FIG. 2A is a diagram for explaining the configuration of the terahertz electromagnetic wave generating element 100 according to an embodiment of the present invention.

  A laser beam is incident on one surface of the terahertz electromagnetic wave generating element 100, and a terahertz electromagnetic wave is emitted from the other surface. Hereinafter, a surface on which a laser beam is incident is referred to as an incident surface, and a surface from which a terahertz electromagnetic wave is emitted is referred to as an emission surface. The terahertz electromagnetic wave generating element 100 includes a nonlinear optical crystal 101, a layer structure 103A provided on the incident surface side of the nonlinear optical crystal 101, and a layer structure 103B provided on the emission surface side of the nonlinear optical crystal 101. The layer structure 103A includes a first layer 105, a second layer 1073 provided on the side opposite to the nonlinear optical crystal 101 of the first layer 105, and a side of the first layer 105 of the second layer 1073. And a grating 1075 provided on the surface of the second layer 1073 opposite to the first layer 105. The layer structure 103B has the same structure as the layer structure 103A. The layer structure 103A and the layer structure 103B are referred to as a pair of layer structures.

The nonlinear optical crystal 101 is, for example, a zinc telluride (ZnTe) crystal. In addition, crystals such as inorganic nonlinear optical crystals such as lithium niobate (LiNbO ₃ ), gallium phosphide (GaP), and gallium selenide (GaSe) may be used. Furthermore, DAST (4-dimethylamino-N-methyl-4-stilbazolium tosylate) crystal may be used as the organic nonlinear crystal.

  The refractive index of the medium of the first layer 105 is lower than the refractive index of the medium of the second layer 1073. That is, the first layer 105 is a layer made of a low refractive index material, and the second layer 1073 is a layer made of a high refractive index material. By providing an appropriate thickness layer made of a low refractive index material and an appropriate thickness layer made of a high refractive index material, the nonlinear optical crystal 101 of the terahertz electromagnetic wave generated in the nonlinear optical crystal 101 The reflected wave at the boundary surface between the first layer 105 and the reflected wave at the boundary surface between the second layer 1073 and the surrounding area cancel each other, so that a decrease in output due to reflection of the terahertz electromagnetic wave can be prevented.

Here, air is selected as the medium of the first layer 105. In order to prevent a decrease in radiation intensity due to reflection on the surface of the nonlinear optical crystal 101, it is necessary to select a medium having an appropriate refractive index as the medium of the second layer 1073. However, the refractive index of a substance tends to increase in the frequency region of terahertz electromagnetic waves. For example, the refractive indexes of aluminum oxide (Al ₂ O ₃ ) and titanium dioxide (TiO ₂ ), which are typical thin films for visible light, are 1.7 and 2.0 for visible light, respectively. For frequency 0.45 THz (wavelength 670 μm), they are 3.3 and 10.5, respectively. Therefore, it is not easy to find a medium having an appropriate refractive index in the terahertz electromagnetic wave band. As described above, when the refractive index of the first layer 105 and the refractive index of the second layer 1073 are greatly different, reflection at the interface between the first layer 105 and the second layer 1073 increases, and the terahertz electromagnetic wave is increased. Output decreases.

  Therefore, in the present embodiment, the first grating 1071 is provided on the surface of the second layer 1073 on the nonlinear optical crystal 101 side, and the second layer 1073 is disposed on the surface opposite to the nonlinear optical crystal 101. Two grids 1075 are provided. Here, the grating period of the first grating 1071 and the second grating 1075 is made smaller than the wavelength of the terahertz electromagnetic wave used. By comprising in this way, generation | occurrence | production of reflected diffracted light and transmitted diffracted light is suppressed, and only 0th order light can be generated.

ここで、ＴＥ偏光に対する平均屈折率をn_TE、ＴＭ偏光に対する平均屈折率をn_TMで表す。また、格子突起部の、第２の層１０７３の面からの高さをh、高さhにおける格子突起部の屈折率をn₂(h)、格子突起部の周辺の媒質（空気）の屈折率をn₁(h)、格子周期をΛ、格子突起部の頂点を含み、第２の層１０７３の、格子突起部が配置される面に垂直な断面において、高さhにおける周期内に占める格子突起部の比率（デューティー比）をF(h)=f(h)/Λ、光の入射角をθで表す。As shown in FIG. 2A, the space between the protruding portions of the first grating 1071 is filled with air that is the medium of the first layer. The average refractive index of the protruding portion of the first grating 1071 or the second layer 1073 and the portion made of air is approximately given by the following expression.

Here, the average refractive index for _TE polarized light is represented by n _TE , and the average refractive index for TM polarized light is represented by n _TM . Further, the height of the lattice protrusion from the surface of the second layer 1073 is h, the refractive index of the lattice protrusion at the height h is n ₂ (h), and the refraction of the medium (air) around the lattice protrusion is The ratio is n ₁ (h), the lattice period is Λ, the vertex of the lattice protrusion is included, and the second layer 1073 occupies the period at the height h in the cross section perpendicular to the surface on which the lattice protrusion is disposed. The ratio (duty ratio) of the grating protrusions is represented by F (h) = f (h) / Λ, and the incident angle of light is represented by θ.

  FIG. 3 is a diagram for explaining the duty ratio. When h = 0, that is, on the surface on which the lattice projections of the second layer 1073 are arranged, the duty ratio is 1. The duty ratio is 0 at the height of the apex of the lattice projection. While h changes from the height (H) of the apex of the protrusion to 0, the duty ratio continuously changes from 0 to 1. In FIG. 3, the shape of the lattice protrusion in the cross section perpendicular to the surface of the second layer 1073 on which the lattice protrusion is arranged, including the apex, is a triangle. In general, the shape of the lattice protrusion in the cross section is an arbitrary shape in which the duty ratio continuously changes from 0 to 1 while h changes from the height (H) of the vertex of the protrusion to 0. It may be.

According to Equation (1), while the duty ratio continuously changes from 0 to 1, the average refractive index n _TE for TE polarized light changes from n ₁ (H) to n ₂ (0). Further, according to the equation (2), the average refractive index n _TM for TM polarized light is continuously changed from n ₁ (H) to n ₂ (0) while the duty ratio continuously changes from 0 to 1. Change.

  Thus, by providing the grating, the average refractive index continuously changes from the refractive index of the medium of the first layer 105 to the refractive index of the medium of the second layer 1073. Accordingly, reflection at the boundary surface between the first layer 105 and the second layer 1073 can be prevented.

  In the above description, the material of the protruding portion of the lattice is the same as the material of the second layer 1073. Alternatively, the material of the protruding portion of the lattice may be different from the material of the second layer 1073. In that case, the refractive index of the material of the protrusion of the grating is close to the refractive index of the material of the second layer 1073, and the refractive index gradually changes from the first layer 105 to the second layer 1073. That is, it is configured not to change rapidly.

  As an example, polypropylene is used for the second layer 1073. The reason why polypropylene is used for the second layer 1073 is that the hygroscopic property is small, a large-area thin film can be easily manufactured, and the refractive index in the terahertz electromagnetic wave band is relatively small compared to other plastics. The refractive index of polypropylene with an electromagnetic wave having a frequency of 1.5 THz (wavelength 200 μm) is 1.48. In addition, as a plastic that is easy to process and has a relatively low refractive index in the terahertz electromagnetic wave band, polymethylmethacrylate (refractive index for electromagnetic waves of the above frequency is about 2.5), polyethylene (refractive index for electromagnetic waves of the above frequency is about 2.4), polycarbonate (refractive index for electromagnetic waves of the above frequency is about 2.6), polystyrene (refractive index for electromagnetic waves of the above frequency is about 2.6), and the like may be used.

  The lattice is arranged in one or two dimensions on the surface of the second layer 1073.

  FIG. 4 is a diagram showing a configuration of a lattice arranged in one dimension. The output of the terahertz electromagnetic wave having a predetermined polarization plane can be increased by the one-dimensionally arranged grating. Specifically, it is possible to prevent the attenuation of the terahertz electromagnetic wave having a polarization plane orthogonal to the longitudinal direction of the protrusions of the lattice arranged in one dimension.

  5A to 5D are diagrams showing the configuration of a lattice arranged in two dimensions. Regardless of the plane of polarization, the output of terahertz electromagnetic waves can be increased by the two-dimensionally arranged grating. FIG. 5A is a diagram showing a two-dimensional array lattice in which quadrangular pyramid protrusions are arrayed in a square lattice pattern. FIG. 5B is a diagram showing a two-dimensional array lattice in which conical protrusions are arrayed in a square lattice pattern. FIG. 5C is a diagram showing a two-dimensional array lattice in which the projections of the quadrangular pyramids are arrayed in a regular hexagonal lattice shape. FIG. 5D is a diagram showing a two-dimensional array lattice in which conical protrusions are arrayed in a regular hexagonal lattice shape.

  In order to simplify the description above, it is assumed that the terahertz electromagnetic wave generating element 100 has a pair of layer structures. Actually, the terahertz electromagnetic wave generating element 100 has a plurality of pairs of layer structures.

  FIG. 2B is a diagram illustrating a configuration of the terahertz electromagnetic wave generating element 100 including five pairs of layer structures according to an embodiment of the present invention.

  Next, a method for designing a terahertz electromagnetic wave generating element according to the present invention will be described. First, a method for designing a nonlinear optical crystal that generates a terahertz electromagnetic wave having a desired center wavelength is described. Next, a method for designing a terahertz electromagnetic wave generating element that generates a terahertz electromagnetic wave having a desired bandwidth around the center wavelength is described. explain.

  FIG. 6 is a flowchart for explaining a design method of a nonlinear optical crystal that generates a terahertz electromagnetic wave having a desired center wavelength.

  In step S1010 of FIG. 6, the center wavelength of the terahertz electromagnetic wave desired to be generated by the terahertz electromagnetic wave generating element is determined.

  In step S1020 of FIG. 6, the refractive index of the nonlinear optical crystal at the center wavelength of the terahertz electromagnetic wave and the group refractive index of the nonlinear optical crystal at the wavelength of the excitation pulse laser are obtained.

ここで非線形光学結晶の厚さをL、テラヘルツ電磁波の中心波長をλ_THz、中心波長λ_THzにおける非線形光学結晶の屈折率をn_THz、励起パルスレーザーの波長における非線形光学結晶の群屈折率をn_gで表している。In step S1030 of FIG. 6, the thickness of the nonlinear optical crystal is determined from the values of the center wavelength, the refractive index, and the group refractive index according to the following equation.

Here, the thickness of the nonlinear optical crystal is L, the center wavelength of the terahertz electromagnetic wave is λ _THz , the refractive index of the nonlinear optical crystal at the center wavelength λ _THz is n _THz , and the group refractive index of the nonlinear optical crystal at the wavelength of the excitation pulse laser is n Indicated by _g .

FIG. 7 is a flowchart for explaining a design method of a terahertz electromagnetic wave generating element that generates a terahertz electromagnetic wave having a desired bandwidth with respect to the center wavelength of the terahertz electromagnetic wave as λ _THz .

すなわち、第１の層１０５の厚さH₁及び第２の層１０７３の厚さH₂は、中心波長のテラヘルツ電磁波の位相がπ／２またはその倍数だけ変化するように定められる。中心波長テラヘルツ電磁波の位相は、両側に備わる第１の層１０５及び第２の層１０７３を２回通過することにより２πだけ変化する。この結果、テラヘルツ電磁波の中心波長λ_THz以外の電磁波は、入射波とその反射光による位相が干渉効果の作用で打ち消しあって消滅し、テラヘルツ電磁波の中心波長λ_THzを有する電磁波のみが透過光として生じる。In step S2010 of FIG. 7, the thickness H ₂ of the thickness H ₁ and a second layer 1073 of the first layer 105 defines the following equation. Here, the refractive index of the first layer 105 is n _105, and the refractive index of the second layer 1073 is n ₁₀₇₃ .

That is, the thickness H ₂ of the thickness H ₁ and a second layer 1073 of the first layer 105, the terahertz wave in the phase of the central wavelength is determined so as to change by [pi / 2 or a multiple thereof. The phase of the center wavelength terahertz electromagnetic wave changes by 2π by passing through the first layer 105 and the second layer 1073 provided on both sides twice. As a result, electromagnetic waves other than the center wavelength λ _THz of the terahertz electromagnetic wave are extinguished by canceling out the phase of the incident wave and its reflected light due to the interference effect, and only the electromagnetic wave having the center wavelength λ _THz of the terahertz electromagnetic wave is transmitted Arise.

ここで、第１の層及び第２の層の屈折率または格子突起部の屈折率をn_min及びn_maxで表している。In step S2020 of FIG. 7, the average refractive indexes of the first layer and the second layer are obtained according to the following formula.

Here, the refractive index of the first layer and the second layer or the refractive index of the grating protrusion is represented by n _min and n _max .

式（７）はテラヘルツ電磁波が、格子高さh、屈折率の平均値

を有する格子部を通過した際の光路長の変化により、周波数の帯域幅が広がることを示している。In step S2030 of FIG. 7, the grating height h and period Λ for generating a terahertz electromagnetic wave having a desired bandwidth W _THz are determined according to the following formula with respect to the center wavelength of the terahertz electromagnetic wave as λ _THz .

Equation (7) indicates that the terahertz electromagnetic wave has a lattice height h and an average refractive index.

It shows that the bandwidth of the frequency is widened due to the change in the optical path length when passing through the grating portion having λ.

Here, the grating height h and the period Λ need to satisfy the following expressions.

  Equation (8) means that the grating period Λ is smaller than the lower limit of the bandwidth frequency. When the grating period Λ is equal to or greater than the lower limit of the bandwidth frequency, diffracted light is generated, causing a decrease in transmittance.

  Equation (9) is obtained by eliminating the grating period Λ from equation (8) using equation (7). If the expression (9) is not satisfied, high-order diffracted light is generated as in the case where the expression (8) is not satisfied, and the transmittance is reduced.

  The inventor has shown that the terahertz electromagnetic wave generating element having a plurality of pairs of layer structures in FIG. 2B generates a terahertz electromagnetic wave having a bandwidth given by the equation (7). In the graph (frequency spectrum) showing the relationship with the utilization efficiency, new knowledge has been obtained that a rectangular shape with sharp both sides of the band can be obtained. In the frequency spectrum, the reason why a rectangular shape with sharp both sides of the band is obtained is as follows.

  In FIG. 2A, it is assumed that the second layer 1073 does not include a lattice. For example, in the case of an electromagnetic wave having a high frequency such as an infrared region or a visible region, the center wavelength (frequency) is determined by determining the thicknesses of the first layer and the second layer so as to satisfy the equations (4) and (5). A narrow-band transmission spectrum having a peak at can be obtained. However, since the wavelength of terahertz electromagnetic waves is extremely long, it is necessary to increase the layer thickness in order to perform phase control. As a result, the distance that the terahertz electromagnetic wave passes through the medium also increases. The increase in the passing distance in the medium causes transmission loss and reflection loss as in the case of conventional visible light and infrared light, and a phase shift occurs due to an increase in the absorption effect associated with the passage of the terahertz electromagnetic wave in the medium. . As a result, the peak frequency shifts. As a result, every time the layer structure on both sides of the generation layer goes back and forth, the frequency band that can interfere gradually shifts, so the approximate shape of the obtained frequency spectrum does not have a rectangular distribution, and the utilization efficiency changes gradually with respect to the frequency. Gaussian distribution.

分の位相シフトが生じる。この位相シフトを与えることにより、ピーク周波数が前記光路長分だけずれる。さらに第２層１０７３の両側への格子付加と発生素子１０１の両側への層構造の配置により、位相シフト量に係数

の補正が与えられ、全体で帯域幅W_THzが定められる（式（７））。Therefore, in the present invention, the gratings 1071 and 1075 are provided on both sides of the second layer 1073. When the gratings 1071 and 1075 are provided on both sides of the second layer 1073, the optical path length of the formula (7) is used for the grating alone.

A phase shift of minutes occurs. By giving this phase shift, the peak frequency is shifted by the optical path length. Furthermore, the addition of a lattice on both sides of the second layer 1073 and the arrangement of the layer structure on both sides of the generating element 101 make a coefficient in the phase shift amount.

And a total bandwidth W _THz is determined (Equation (7)).

In this system, the shift amount of the peak frequency is modulated every time it passes through each layer, and a narrow-band spectrum corresponding to the shift amount is generated. However, since each layer passes through at random, the obtained spectrum is radiated in a state where narrowband spectra having an arbitrary shift amount are randomly overlapped. At this time, the modulation amount of the peak frequency given at random is limited by the phase modulation by the lattice structure, and does not exceed the bandwidth W _THz shown in the equation (7).

  As a result of the above phenomenon, a frequency spectrum having a rectangular distribution in which narrow-band spectra are finally overlapped is obtained.

  In the following, the effect of the present invention is verified by simulation. For the simulation, the Rigorous Coupled Wave Analysis (RCWA) method was used. In the simulation, the polarization direction of the electromagnetic wave was TE polarization, and a lattice arranged in one dimension was used. Similar results are obtained with TM polarized light or a two-dimensionally arranged grating.

First Embodiment The first embodiment is a terahertz electromagnetic wave generating element having five pairs of layer structures. The center wavelength of the terahertz electromagnetic wave is determined to be 136 μm and the center frequency is determined to be 2.2 THz (step S1010 in FIG. 6).

The refractive index n _{THz at} the center frequency of the nonlinear optical crystal and the group refractive index _{ng at} the wavelength of the excitation pulse laser are as follows (step S1020 in FIG. 6).
n _THz = 2.92
n _g = 5.876
Therefore, from the equation (3), the thickness of the nonlinear optical crystal is as follows (step S1030 in FIG. 6).

Since the refractive index of the first layer 105 is n ₁₀₅ = 1 and the refractive index of the second layer 1073 is n ₁₀₅ = 1.48, the thickness of the first layer 105 can be calculated from the equations (4) and (5). The thickness H ₁ and the thickness H ₂ of the second layer 1073 are 34 μm and 23 μm, respectively.

Here, the bandwidth (wavelength width) is assumed to be 38 μm. In this case, the design bandwidth is as follows.
Wavelength band 117μm-155μm
Frequency band 1.94THz-2.56THz
The grating period Λ is determined to be 90 μm so as to satisfy Expression (6).

Substituting the refractive index of the first layer (refractive index of air 1.0) and the refractive index of the second layer (refractive index of polypropylene 1.48) into the formula (4), the following formula is obtained (FIG. 7 step S2010).

Further, when the grating period, the center wavelength, and the average refractive index are substituted into Expression (5), the band wavelength width becomes 38 μm when the grating height is 37 μm as follows (step S2020 in FIG. 7).

Table 1 is a table | surface which shows the specification of the terahertz generating element of a present Example.

となる。ここで、ε0は真空の誘電率、χ(2)は非線形光学結晶の2次非線形感受率、Ｅ及びＥ^*は電場を表す。FIG. 8 is a diagram showing the relationship between the frequency and the utilization efficiency of the terahertz generating element of this example obtained by RCWA. Here, the utilization efficiency represents the ratio of the emission intensity of the terahertz electromagnetic wave to the theoretical maximum emission intensity of the terahertz electromagnetic wave radiated from the nonlinear optical crystal, that is, the utilization efficiency of generation of the terahertz electromagnetic wave. Here, the theoretical maximum emission intensity is expressed by the second-order nonlinear polarization P ⁽²⁾ (ω3) at the frequency ω3 of the terahertz electromagnetic wave when laser light with the frequencies ω1 and ω2 is incident on the nonlinear optical crystal from the femtosecond laser. And

It becomes. Here, ε0 represents the dielectric constant of vacuum, χ (2) represents the second-order nonlinear susceptibility of the nonlinear optical crystal, and E and E ^* represent the electric field.

  In FIG. 8, the utilization efficiency is almost 1 in the frequency band from 1.94 THz to 2.45 THz (the wavelength band from 122 μm to 155 μm), and a value close to the design value of the above bandwidth is obtained.

  As described above, after determining the thicknesses of the first and second layers, the grating period (first period), and the grating height (first height) by the design method shown in FIG. While performing a simulation by the RCWA method, the above value may be adjusted so as to obtain a desired bandwidth.

  According to the present embodiment, the energy of the terahertz wave can be concentrated on a desired bandwidth. Therefore, for example, the detection accuracy of the object inspection system using the terahertz wave generating element shown in FIG. 1 can be improved. Specifically, the prohibited drug cocaine has a transmittance of about 50% near 2.3 THz, and the prohibited drug ecgonine has a transmittance of about 30% near 2.1 THz. The explosive TNT explosive has a transmittance of about 70% at around 2.2 THz. Therefore, these substances can be detected with high accuracy by the object inspection system by concentrating the energy of the terahertz wave in the bandwidth of the present embodiment.

  In FIG. 8, some terahertz waves are generated in a frequency region of 1.75 THz or less in addition to a desired bandwidth. Such a terahertz electromagnetic wave in a frequency region lower than the desired bandwidth can be removed from the optical system by changing the traveling direction as diffracted light using a diffraction grating.

所望の帯域幅のテラヘルツ電磁波は、上記の周期の回折格子を０次光として透過する。除去すべき帯域のテラヘルツ電磁波は、回折光として進行方向を変えるので光学系から除去することができる。このように、所望の帯域幅の上限値以上の周波数のテラヘルツ電磁波を除去するには、格子周期を上記の式の下限値に近づければよい。The band to be removed is a band of 1.75 THz or less in terms of frequency and a band of 171.5 μm or more in terms of wavelength. On the other hand, the desired band is a band of 1.94 THz or more in frequency and a band of 155 μm or less in wavelength. Accordingly, the period Λ _{d of the} diffraction grating is set to the following range.

A terahertz electromagnetic wave having a desired bandwidth passes through the diffraction grating having the above-described period as zero-order light. The terahertz electromagnetic wave in the band to be removed can be removed from the optical system because the traveling direction is changed as diffracted light. Thus, in order to remove terahertz electromagnetic waves having a frequency equal to or higher than the upper limit value of the desired bandwidth, the grating period may be made closer to the lower limit value of the above formula.

Second Embodiment A second embodiment is a terahertz electromagnetic wave generating element having nine pairs of layer structures. As in the first embodiment, the center wavelength of the terahertz electromagnetic wave is 136 μm, the center frequency is 2.2 THz, the thickness of the first layer 105 is 34 μm, and the thickness of the second layer 1073 is 23 μm.

Table 2 is a table | surface which shows the specification of the terahertz generating element of a present Example. Other than the number of pairs, the specification is the same as that of the first embodiment.

  FIG. 9 is a diagram showing the relationship between the frequency and the utilization efficiency of the terahertz generator of the present example obtained by RCWA.

  In FIG. 9, the utilization efficiency is almost 1 in the frequency band of 1.94 THz to 2.45 THz (the wavelength band of 122 μm to 155 μm), and a value close to the designed frequency band (1.94 THz−2.56 THz) is obtained. Thus, a terahertz wave having a desired bandwidth can be generated by a terahertz generating element having a plurality of pairs of layer structures. In general, as the number of pairs increases, a rectangular shape with steep sides on the both sides of the band is obtained in a graph showing the relationship between frequency and utilization efficiency.

Third Embodiment A third embodiment is a terahertz electromagnetic wave generating element having three pairs of layer structures. As in the first embodiment, the center wavelength of the terahertz electromagnetic wave is 136 μm, the center frequency is 2.2 THz, the thickness of the first layer 105 is 34 μm, and the thickness of the second layer 1073 is 23 μm.

Table 3 is a table | surface which shows the specification of the terahertz generating element of a present Example. Other than the number of pairs, the specification is the same as that of the first embodiment.

  FIG. 10 is a diagram showing the relationship between the frequency and the utilization efficiency of the terahertz generating element of this example obtained by RCWA.

  In FIG. 10, the utilization efficiency is almost 1 in the frequency band of 1.95 THz to 2.45 THz (the wavelength band of 122 μm to 154 μm), and a value close to the designed frequency band (1.94 THz−2.56 THz) is obtained. Thus, a terahertz wave having a desired bandwidth can be generated by a terahertz generating element having a plurality of pairs of layer structures.

Fourth Embodiment The fourth embodiment is a terahertz electromagnetic wave generating element having five pairs of layer structures. As in the first embodiment, the center wavelength of the terahertz electromagnetic wave is 136 μm, the center frequency is 2.2 THz, the thickness of the first layer 105 is 34 μm, and the thickness of the second layer 1073 is 23 μm.

Here, the bandwidth (wavelength width) is assumed to be 6 μm. In this case, the design bandwidth is as follows.
Wavelength band 133μm-139μm
Frequency band 2.16 THz-2.25 THz
The grating period Λ is determined to be 90 μm so as to satisfy Expression (6).

Substituting the refractive index of the first layer (refractive index of air 1.0) and the refractive index of the second layer (refractive index of polypropylene 1.48) into the formula (4), the following formula is obtained (FIG. 7 step S2010).

Further, when the grating period, the center wavelength, and the average refractive index are substituted into Expression (5), the band wavelength width becomes 6.18 μm when the grating height is 6 μm as follows (step S2020 in FIG. 7). .

Table 4 is a table | surface which shows the specification of the terahertz generating element of a present Example.

  FIG. 11 is a diagram showing the relationship between the frequency and the utilization efficiency of the terahertz generating element of this example obtained by RCWA.

  In FIG. 11, the utilization efficiency is almost 1 in the frequency band of 2.13 THz to 2.25 THz (the wavelength band of 133 μm to 141 μm), and a value close to the designed frequency band (2.16 THz−2.25 THz) can be obtained.

  Comparing Example 1 with this example, it can be seen that the desired bandwidth can be changed by changing the grating height.

Method for Manufacturing Terahertz Electromagnetic Wave Generator First, a method for manufacturing a grating will be described. When the grating is manufactured, a grating mold is manufactured by cutting or lithography. When a lattice mold is manufactured by cutting, a metal mold or glass mold substrate is cut into a predetermined concavo-convex shape using a diamond tool. Since the period of the diffraction grating applied to the terahertz electromagnetic wave is between 30 μm and 3 mm, it can be sufficiently handled by cutting. When a grating mold is manufactured by lithography, a resist is applied on a mold substrate, and the resist is irradiated with an electron beam or light while the exposure amount is modulated according to a predetermined uneven shape. When the irradiated resist is developed, the resist is removed according to the exposure amount modulation, and an uneven shape is formed by the resist. Finally, by performing etching, the uneven shape due to the resist is transferred to the mold substrate, and a predetermined uneven shape can be obtained on the mold substrate.

  12A and 12B are views for explaining a method of manufacturing a grating incorporated in a terahertz electromagnetic wave generating element using the mold manufactured as described above.

  FIG. 11A is a diagram for explaining a method of manufacturing a second layer having a lattice by an imprint method. In the case of manufacturing the second layer having the grating by the imprint method, the molds 4011 and 4013 are pushed onto the substrate 1070 made of a high refractive index medium such as polypropylene, and heat is applied to the mold (heat imprinting). The second layer 107 provided with the lattice is manufactured by transferring the lattice onto the substrate 1070 by irradiating with light (printing method) or light (photoimprinting method). In the imprint method, a large-area diffraction grating structure can be manufactured by forming the substrate 1070 into a film shape. Furthermore, a large amount can be manufactured by using a method called a roll imprint method in which the molds 4011 and 4013 are arranged in a roll shape and the mold is rotated while the film-like substrate 1070 is pushed in.

  FIG. 11B is a diagram for explaining a method of manufacturing a second layer having a lattice by injection molding. In the case of manufacturing a diffraction grating by injection molding, the second layer 107 provided with the grating can be manufactured by injecting a resin into the mold 501 and releasing the mold.

  FIG. 13 is a diagram for explaining a method of manufacturing a terahertz electromagnetic wave generating element using the second layer 107 manufactured as described above.

  A frame 109 for forming the first layer 105 using air as a medium is attached to the surface of the nonlinear optical crystal plate 101. Further, the second layer 107 is attached to the surface of the frame 109 opposite to the surface to which the nonlinear optical crystal plate 101 is attached. A terahertz electromagnetic wave generating element having a plurality of layer structures as shown in FIG. 11 can be manufactured by stacking and laminating a layer structure composed of a frame 109 and a second layer 107.

  FIG. 14 is a diagram for explaining a method of integrally manufacturing the second layer provided with the lattice and the frame for forming the first layer by the imprint method. Molds 5011 and 5013 having a lattice and frame shape are pressed onto a substrate 1070 made of a high refractive index medium such as polypropylene, and heat is applied to the mold (thermal imprint method) or light is irradiated (light By imprinting), the lattice is transferred onto the substrate 1070, and the second layer 107 provided with the lattice and the frame for forming the first layer 105 are integrally manufactured.

  FIG. 15 is a diagram for explaining a method of manufacturing a terahertz electromagnetic wave generating element using the second layer and the frame 111 formed integrally as described above.

  The second layer formed integrally and the frame portion of the frame 111 are attached to the surface of the nonlinear optical crystal plate 101. A terahertz electromagnetic wave generating element having a plurality of layer structures as shown in FIG. 11 can be manufactured by stacking and laminating a second layer and a frame 111, which are integrally formed, corresponding to the layer structure.

Claims (10)

A generation layer made of a nonlinear optical material that emits terahertz electromagnetic waves from the other surface when light is irradiated on one surface;
A terahertz electromagnetic wave generating element provided with a plurality of pairs of layer structures provided on both sides of the generation layer, each of the plurality of pairs of layer structures,
A first layer;
A second layer provided on the opposite side of the first layer from the generation layer;
A first grating having a first period and a first height smaller than the wavelength of the terahertz electromagnetic wave to be used, provided on a surface of the second layer on the generation layer side;
A second grating having a first period and a first height provided on a surface of the second layer opposite to the generation layer,
The refractive index of the medium of the first layer (first refractive index) is lower than the refractive index of the medium of the second layer (second refractive index), and the first and second gratings are The refractive index between the first layer and the second layer is configured to gradually change between the first and second refractive indexes, and the center of the terahertz electromagnetic wave generated by the generation layer A terahertz electromagnetic wave generating element in which the thicknesses of the first and second layers and the first height are determined so as to generate a terahertz electromagnetic wave having a desired bandwidth with respect to a wavelength.   The terahertz electromagnetic wave generating element according to claim 1, wherein the convex portions of the first and second gratings are made of the same material as that of the second layer.   The terahertz electromagnetic wave generating element according to claim 1, wherein the first and second gratings are arranged one-dimensionally.   The terahertz electromagnetic wave generating element according to claim 1, wherein the first and second gratings are two-dimensionally arranged.   The terahertz electromagnetic wave generating element according to claim 1, wherein the medium of the first layer is air.   The terahertz electromagnetic wave generating element according to claim 5, comprising a frame-shaped support structure so as to form the first layer. The terahertz electromagnetic wave generating element according to claim 6 , wherein the second layer and the frame-shaped support structure are integrally formed.   The terahertz electromagnetic wave generating element according to claim 1, wherein the medium of the second layer is polypropylene.   The terahertz electromagnetic wave generating element according to claim 1, wherein the nonlinear optical material is a crystal of zinc telluride (ZnTe).   An object inspection system comprising the terahertz electromagnetic wave generating element according to claim 1.

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