JP2008209493A - Optical device for terahertz wave and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new optical device for terahertz waves, which has a nonconventional constitution, is excellent in productivity and has various merits such as a high degree of freedom in shape; and to provide a method for producing the optical element. <P>SOLUTION: The optical device for terahertz waves includes a diamond base body in which a working part working optically on incident light of terahertz waves is formed inside. The working part has such a structure that columnar electrically-conductive portions formed by change of property of a diamond are arrayed cyclically. The optical element like this can be formed by irradiating the diamond base body with femtosecond laser. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a terahertz wave optical element including an action part that optically acts on a terahertz wave, for example, a diffraction part that diffracts a terahertz wave, or a spectroscopic part that splits a terahertz wave, and a manufacturing method thereof.

The terahertz wave is an electromagnetic wave that vibrates about 1 trillion (10 ¹² , that is, “T (tera)”) times per second. When the frequency (frequency) of radio waves becomes so high, the property as light is also taken. For this reason, the terahertz wave is said to have a property of passing through a substance like radio waves and going straight like light. However, a terahertz wave band corresponding to a frequency of 100 GHz to 100 THz is conventionally referred to as an undeveloped electromagnetic wave or an undeveloped optical because there has been no method for easily generating and detecting an electromagnetic wave included in this band. I came. In recent years, due to innovations such as laser technology, the spread of terahertz light sources and light receiving elements is expected for the first time. Therefore, industrial applications of terahertz waves will continue, but the use of electromagnetic waves that humankind has never obtained is a promising candidate for national core technology. Reports such as development and market size prediction with a clear awareness of application are beginning to be made, and the future is expected. Since a lot of information can be obtained from the terahertz wave band about the structure and motion state of molecules, the assumed application fields are not only information and communication fields but also safety and crime prevention fields (for example, dangerous and concealed materials). Detection), biomedical fields (eg, imaging of malignant tumors), agriculture, industry, environment and space.

  In order to actually apply terahertz waves to the industry, not only the light source but also optical elements (terahertz wave optical elements) that can be used for terahertz waves, such as diffraction elements, spectroscopic elements, and polarizing elements, are required. . For example, Patent Document 1 discloses, as such an optical element, a polarizing element composed of a wire grid in which a large number of metal wires having a thickness of several μm are arranged at intervals of about 10 μm (paragraph number [0025] and the like). .

Although not directly related to the terahertz wave optical element, Patent Document 2 discloses a method for forming a three-dimensional three-dimensional wiring structure on diamond.
JP 2003-14620 A JP 2005-294413 A

  A wire grid as disclosed in Patent Document 1 can efficiently split a terahertz wave by reflecting a polarization component parallel to the wire and transmitting a perpendicular polarization component. However, in the production, usually, a method in which fine metal wires having a diameter of μm order are stretched at a fixed interval on a frame coated with an adhesive, and the thin metal wires at the time of production and the arrangement interval of the fine wires are taken. Unevenness is likely to occur. For this reason, it is very difficult to stably manufacture the wire grid, which is a factor that the price of the optical element for terahertz waves is very expensive. In addition, with the expansion of the expected application field of terahertz waves, smaller optical elements are required. However, wire grids require a frame of a certain size or larger in order to stably stretch metal wires. For this reason, it is difficult to further reduce the size beyond the current level.

  Therefore, the present invention provides a novel terahertz wave optical element having various features such as an unconventional configuration, excellent productivity, and a high degree of freedom in shape, and a method for manufacturing the same. Objective.

  An optical element for a terahertz wave according to the present invention includes a diamond base in which an action part that can optically act on an incident terahertz wave is formed, and the action part has a columnar shape formed by alteration of the diamond. It has a structure in which conductive portions are periodically arranged.

  The manufacturing method of the present invention is a manufacturing method of a terahertz wave optical element including an action part that can optically act on an incident terahertz wave, and irradiates a femtosecond laser to a diamond substrate and irradiates the substrate A step of altering a portion near the focal point of the laser to make the portion conductive; In the manufacturing method of the present invention, in the step, by changing the focal position of the laser, two or more portions having a columnar shape are formed so that the portions are periodically arranged, and the inside of the substrate is formed. A working portion having a structure in which columnar conductive portions are periodically arranged is formed.

  The element of the present invention has an action portion that optically acts on an incident terahertz wave inside a diamond base, and the action portion is a periodic arrangement of columnar conductive portions formed by the alteration of diamond. It is comprised by. For this reason, the element of the present invention is composed of fine metal wires stretched at equal intervals, and is superior in strength and durability as an element compared to a wire grid in which the action part is exposed in the air. . In addition, since diamond is a dielectric having high transparency (having high transmittance) with respect to terahertz waves and small dispersion of refractive index (dielectric constant), the element of the present invention can exhibit high efficiency. .

  As shown in the manufacturing method of the present invention, the element of the present invention irradiates a diamond substrate with a femtosecond laser, and more specifically, a portion of the substrate irradiated with the laser, more specifically, near the focal point of the irradiated laser. The portion can be altered to make the portion conductive. At this time, by changing the focal position of the laser, it is possible to form two or more column-shaped portions (conductive portions) such that the portions are periodically arranged. In this method, the arrangement and the size of the substrate are not particularly limited as long as the arrangement of the columnar conductive portions can be easily controlled and laser irradiation can be performed. For this reason, the element of the present invention is excellent in productivity as compared with the wire grid, and the degree of freedom of the shape and size can be increased. Depending on the shape and size of the substrate, it may be integrated with a terahertz light source (oscillator) or a light receiving element.

  1 and 2 show an example of the optical element for terahertz waves of the present invention. The optical element 1 includes a diamond substrate 2 in which an action part 3 that optically acts on an incident terahertz wave is formed. The action part 3 has a structure in which columnar conductive parts 4 formed by alteration of diamond are arranged in parallel to each other. In other words, in the action part 3, the extending directions of the conductive portions 4 are parallel to each other. The extending direction of the conductive portions 4 is substantially perpendicular to the main surface of the substrate 2, and the interval between the adjacent conductive portions 4, that is, the arrangement period of the conductive portions 4 is substantially constant over the entire action portion 3. FIG. 2 is a plan view of the optical element 1 shown in FIG. 1 viewed from a direction parallel to the extending direction of the conductive portion 4.

  The optical element 1 has various optical functions by controlling the configuration of the action portion 3, for example, the average diameter or arrangement period of the conductive portions 4, or the extending direction of the conductive portions 4 with respect to the substrate 2. Can be granted. The optical element 1 includes, for example, a diffraction element that diffracts an incident terahertz wave, a spectroscopic element that selects an electromagnetic wave having a specific frequency included in the incident terahertz wave, and an incident terahertz wave according to the assigned function. It functions as at least one selected from polarization (polarization) elements that select specific polarization (polarization).

  The conductive portion 4 is a portion formed by alteration of diamond. Typically, the conductive portion 4 includes graphite, and optionally contains α-carbon, DLC (Diamond Like Carbon) or the like as a component other than graphite.

  The average diameter of the columnar conductive portions 4 (the average value of the diameters indicated by d in FIG. 2) is not particularly limited as long as the action portion 3 can act on the terahertz wave, but is typically 100 nm or more and 10 mm or less. Is several μm or more (for example, 1 μm or more), preferably 10 μm or more. If the average diameter is too small or too large, the function as a terahertz wave optical element may be lost. In addition, in the manufacturing method of this invention mentioned later for details, the electroconductive part 4 which has an average diameter of several micrometers or more can be formed accurately and efficiently.

  The arrangement period of the conductive portions 4 (for example, X or Y shown in FIG. 2) is not particularly limited as long as the action part 3 can act on the terahertz wave. Usually, the wavelength range of the terahertz wave (3 μm to 3 mm: this wavelength is The frequency may be about 100 GHz to 100 THz), and typically about several tens to several hundreds μm, for example, 10 to 100 μm. When the arrangement period is too small or too large, the function as a terahertz wave optical element may be lost. In the optical element 1 of the present invention, since the conductive portion 4 is formed in a diamond substrate, the wavelength of the terahertz wave and the arrangement period of the conductive portion 4 corresponding to the wavelength are 1: 1. It does not correspond. When obtaining one from the other, it is necessary to consider the refractive index of diamond (about 2.38).

  The arrangement of the conductive portions 4 is typically an arrangement in which the extending directions are parallel to each other as in the optical element 1 shown in FIGS. 1 and 2, but is not particularly limited as long as the action portion 3 can act on the terahertz wave. Not.

  When the conductive portions 4 are arranged such that the extending directions thereof are parallel to each other, the direction relative to the substrate 2 is not particularly limited as long as the action portion 3 can act on the terahertz wave. For example, the direction is parallel to the main surface of the substrate 2. It may be inclined at a certain angle with respect to the main surface of the substrate 2.

  It is sufficient that at least a part of the conductive portion 4 is periodically arranged in the action portion 3. As long as the conductive portion 4 can function as a terahertz wave optical element, the arrangement of the conductive portion 4 is partially disturbed. Good.

  The shape and size of the diamond substrate 2 are not particularly limited. As long as the action part 3 is formed inside and can function as an optical element for terahertz waves, it can be arbitrarily set according to the shape and size required for the element 1.

  The action part 3 of the optical element 1 shown in FIGS. 1 and 2 is formed on a diamond substrate, that is, a flat diamond base. However, the shape of the base is not limited to a flat plate, and is necessary for the element 1. It can be set arbitrarily according to the shape.

  The optical element 1 having a flat substrate may be manufactured by forming the action portion 3 on a diamond substrate that is flat in advance, that is, a diamond substrate, or the diamond having the action portion 3 formed thereon. You may produce by cutting a base | substrate into flat form.

  In the optical element of the present invention, an inexpensive industrial diamond can be used as the diamond used for the substrate (substrate). Industrial diamond appears yellowish to the naked eye due to the impurities contained therein, but generally has a high transmittance for terahertz waves, and therefore can be suitably used as a substrate.

  In the optical element 1 shown in FIGS. 1 and 2, the diamond substrate has a flat plate shape, and the conductive portion 4 extends in the thickness direction of the flat plate substrate (substrate 2). The element 1 having such a structure is easy to manufacture and can be an element having excellent productivity. The element 1 having such a structure can optically act on the terahertz wave incident from the side surface of the flat substrate (substrate).

  In the optical element of the present invention, the diamond substrate may have a flat plate shape, and the conductive portion 4 may extend in a direction parallel to the main surface of the flat plate substrate. An element having such a structure can be suitably used for, for example, a terahertz wave oscillator and a light receiving element described later. An element having such a structure can optically act on a terahertz wave incident from the main surface of a flat substrate.

  In the optical element of the present invention, the action part 3 may have two or more surfaces (array surfaces) on which the conductive portions 4 are arranged. For example, in the action part 3 of the optical element 1 shown in FIGS. 1 and 2, there are five arrangement surfaces of the conductive portions 4 including the surfaces α and β with respect to the side surface A of the substrate 2. The arrangement surface of the conductive portions 4 in the action portion 3 can also be considered as the surfaces γ and δ shown in FIG. 2. In this case, the action portion 3 of the optical element 1 shown in FIGS. There are nine arrangement surfaces of the conductive portions 4.

  In general, the arrangement surface of the conductive portions 4 may be considered based on the surface on which the terahertz wave is incident on the substrate. When the terahertz wave is incident from the side surface A in the optical element 1 shown in FIGS. Therefore, the number of arrangement surfaces of the conductive portions 4 in the action portion 3 is five.

  In a wire grid, which is a conventional optical element for terahertz waves, there is only one surface where metal thin wires are arranged at equal intervals with respect to one element, that is, an array surface. In addition to being difficult to improve, its functionality is also limited. On the other hand, in the element of the present invention, there may be two or more arrangement surfaces. By using such an element, the efficiency as an optical element can be further improved. For example, the element can be obtained as a spectroscopic element. Spectral peaks can be made sharper.

  Such an element can be said to be an optical element having two or more grids, that is, a grid group, when one arrangement surface is considered as one “grid”.

  When there are two or more array surfaces of the conductive portions 4 in the action part 3, the distance between adjacent conductive portions in one array surface (for example, the array period X shown in FIG. 2) and the adjacent array surfaces The distance (for example, the arrangement period Y shown in FIG. 2) may be the same or different. If they are the same, an optical element having higher efficiency can be obtained.

  Further, when there are two or more arrangement surfaces of the conductive portions 4 in the action part 3, at least one selected from the arrangement periods X and Y may be partially changed in the action part 3, and the form of the change Depending on the case, a higher function can be given to the element. As an optical element having such a high function, for example, a diffractive element capable of selecting only the first-order diffracted light is conceivable.

  The optical element of the present invention can be combined with a terahertz wave light source or a detection element depending on its configuration. An example of a combination is shown below.

  3A and 3B show an example of a combination of the optical element 1 of the present invention and a terahertz light source (oscillator). The terahertz wave oscillator 11 shown in FIGS. 3A and 3B has the same configuration as that of a general terahertz wave oscillator except for the element 1, specifically, photoconductivity made of a semiconductor on a substrate 12 made of Si, GaAs, or the like. The layer 13 and the pair of electrodes 14 and 15 are stacked. Each member such as the photoconductive layer 13 excluding the element 1 may be made of a material common as a terahertz wave oscillator. The element 1 of the present invention is disposed between the electrodes 14 and 15 on the photoconductive layer 13 and has a flat plate shape made of a diamond substrate on which an action portion is formed. In the action portion of the element 1, the conductive portions are formed so that their extending directions are parallel to each other and parallel to the main surface of the substrate. 3A is a cross-sectional view showing a cross section AA of the oscillator 11 in FIG. 3B.

  Here, when a predetermined voltage V is applied between the electrodes 14 and 15 and a region located between the electrodes 14 and 15 in the photoconductive layer 13 is irradiated with a femtosecond laser, carriers are generated inside the photoconductive layer 13. The electric field changes instantaneously with the generation and disappearance of, and a terahertz wave is generated. The terahertz wave generated in the photoconductive layer 13 passes through the element 1 and is oscillated outside the oscillator 11. At this time, the terahertz wave optically acted by the action part of the element 1 is oscillated. Although different depending on the configuration of the action unit, the oscillator 11 can oscillate, for example, a terahertz wave (polarized wave) and / or a terahertz wave including a specific frequency component.

  The oscillator 11 shown in FIGS. 3A and 3B can be a terahertz light receiving element as it is. When a terahertz wave enters a region located between the electrodes 14 and 15 in the photoconductive layer 13, a potential difference is generated between the electrodes 14 and 15. A terahertz wave can be detected by measuring the current A flowing in an external circuit connected to the electrodes 14 and 15 based on this potential difference or the generated potential difference. At this time, since the terahertz wave transmitted through the element 1 is incident on the region in the photoconductive layer 13, the oscillator 11 has, for example, polarization (polarization) of the terahertz wave and / or Alternatively, a terahertz wave composed of a specific frequency component can be detected.

  4A and 4B show another example of a combination of the element 1 of the present invention and a terahertz wave oscillator. The terahertz wave oscillator 21 shown in FIGS. 4A and 4B has a configuration in which a pair of electrodes 16 and 17 are further added on the photoconductive layer 13 in the oscillator 11 shown in FIGS. 3A and 3B. The electrodes 16 and 17 are arranged so that the direction of the electric field generated when a potential difference is applied between the electrodes is orthogonal to the direction of the electric field generated when a potential difference is applied between the electrodes 14 and 15. The element 1 of the present invention is disposed between the electrodes on the photoconductive layer 13, and the shape thereof is a flat plate made of a diamond substrate on which an action portion is formed. In the action portion of the element 1, the conductive portions are arranged so that the extending directions thereof are parallel to each other, parallel to the main surface of the substrate, and 45 ° to each electric field. 4A is a cross-sectional view showing a cross section AA of the oscillator 21 in FIG. 4B.

  Here, while applying predetermined voltages V1 and V2 between the electrodes 14 and 15 and between the electrodes 16 and 17, respectively, between the electrodes 14 and 15 and between the electrodes 16 and 17 in the photoconductive layer 13. When a femtosecond laser is irradiated to a region located between them, an instantaneous electric field change occurs due to the generation and disappearance of carriers inside the photoconductive layer 13, and a terahertz wave is generated. The terahertz wave generated in the photoconductive layer 13 passes through the element 1 and is oscillated to the outside of the oscillator 21. At this time, the terahertz wave optically acted by the action part of the element 1 is oscillated. Since the extending direction of the conductive portion in the action portion of the element 1 is as described above, the oscillator 21 can oscillate two polarized lights (polarized waves) simultaneously, for example. Similarly to the oscillator 11, the oscillator 21 can also oscillate a terahertz wave composed of a specific frequency component.

  The oscillator 21 shown in FIGS. 4A and 4B can be a terahertz light receiving element as it is. When a terahertz wave enters a region located between the electrodes in the photoconductive layer 13, a potential difference is generated between the electrodes 14 and 15 and between the electrodes 16 and 17. By measuring the current A1 flowing in the external circuit connected to the electrodes 14 and 15 and / or the current A2 flowing in the external circuit connected to the electrodes 16 and 17 based on this potential difference or the generated potential difference. Terahertz waves can be detected. At this time, since the terahertz wave transmitted through the element 1 is incident on the region in the photoconductive layer 13, the oscillator 21 can detect two polarized lights (polarized waves) at the same time, for example. The oscillator 21 can also detect a terahertz wave composed of a specific frequency component.

  The optical element 1 shown in FIGS. 1 and 2 includes a substrate 2 having an action portion 3 formed therein, but the element of the present invention may include a member other than the substrate (base) as necessary.

  The element of the present invention can be produced, for example, by the production method of the present invention shown below.

  According to the manufacturing method of the present invention, a diamond substrate is irradiated with a femtosecond laser, and a portion of the substrate near the focal point of the irradiated laser is altered to make the portion a conductive portion (conductive portion). be able to. At this time, by changing the focal position of the laser, two or more columnar portions are formed so that the portions are periodically arranged, and the columnar conductive portions are periodically formed inside the substrate. Action portions having an arrayed structure can be formed.

  In this method, by changing the focal position of the laser, the average diameter and arrangement period of the periodically arranged conductive portions can be easily controlled. Further, the shape and size of the substrate are not particularly limited as long as laser irradiation can be performed. For this reason, the manufacturing method of this invention is excellent in productivity. Further, according to the manufacturing method of the present invention, the degree of freedom of the optical function, shape, size, etc. of the optical element for terahertz wave to be formed can be increased.

The femtosecond laser is a pulse laser having a pulse width of femtosecond (10 ⁻¹³ to 10 ⁻¹⁵ seconds). In the production method of the present invention, it is considered that the high peak intensity can modify the diamond to form a conductive portion.

  The femtosecond laser with which the substrate is irradiated may be a general femtosecond laser. For example, the wavelength may be about 200 to 1600 nm. The oscillation source is not particularly limited. For example, a titanium sapphire laser that is common as a femtosecond laser may be used.

  Laser irradiation conditions for the substrate, for example, laser output, repetition frequency, etc. are appropriately set according to the type of substrate, the average diameter of the conductive portion to be formed, the depth of the conductive portion to be formed from the substrate surface, etc. do it.

  The configuration of the conductive portion formed by irradiation with the femtosecond laser may be the same as the configuration of the conductive portion 4 in the optical element of the present invention described above.

  In order to change the focal position of the laser, for example, the laser oscillation source and / or the diamond substrate may be fixed to a stage that can move in a three-dimensional direction, and the stage may be moved.

  At this time, it is preferable to form the conductive portion from a position far from the laser oscillation source to a position close to the substrate. A portion altered by laser irradiation is more likely to absorb laser energy than diamond, which is a component of the substrate. For this reason, by forming a conductive part from a position far from the laser oscillation source on the base to a position close to the laser, absorption of the laser by the altered part can be prevented, and a columnar conductive part is formed more efficiently. can do.

  Hereinafter, the present invention will be described in more detail with reference to examples. The present invention is not limited to the examples shown below.

(Example 1)
In Example 1, a diamond substrate was irradiated with a femtosecond laser to produce an optical element as shown in FIGS. 1 and 2, and its characteristics were evaluated. A manufacturing method of the evaluated element sample (sample 1) is shown below.

  A diamond substrate made of industrial diamond (manufactured by Sumitomo Electric Hardmetal Corp., UP303015: size is 3050 μm × 3050 μm, thickness 1500 μm) is fixed to a stage that can be arbitrarily moved in a three-dimensional direction, and the focal position of the substrate is fixed by the stage While changing inside, a femtosecond laser was irradiated from a direction perpendicular to the main surface of the substrate to form an element 1 (sample 1) as shown in FIGS. The average diameter of the conductive portions 4 in the sample 1 was 25 μm, and the distance (arrangement period) between the adjacent conductive portions 4 was 80 μm for both X and Y shown in FIG. In addition, the conductive portions 4 in the sample 1 were formed so that the number of arrangement surfaces was 10 when viewed from the side surface A shown in FIGS.

  Femtosecond laser irradiation conditions were a wavelength of 800 nm, a pulse width of 150 fs, a repetition frequency of 1 kHz, a pulse energy of 1 μJ / pulse, and an objective lens having a numerical aperture (NA) of 0.45 was used for laser irradiation. The columnar conductive portion 4 was formed while gradually moving the focal position of the laser inside the substrate from a position far from the light source at a scanning speed of 20 μm / s.

  FIG. 5 shows an optical microscope image of the sample 1 viewed from a direction perpendicular to the main surface. In FIG. 5, each portion that appears black and round is the conductive portion 4, and a portion that appears white is the diamond substrate. As shown in FIG. 5, it was confirmed that the conductive portions 4 arranged periodically were formed on the diamond substrate by the laser irradiation.

  An optical microscope image of the sample 1 viewed from the side surface A is shown in FIG. As shown in FIG. 6, it was confirmed that columnar conductive portions 4 extending in parallel with each other and extending from one main surface to the other main surface of the substrate were formed on the diamond substrate by the laser irradiation. did it.

  The following evaluation was performed on the sample 1 thus manufactured.

[Verification of substances contained in conductive part 4]
When the Raman spectrum of the sample 1 and the diamond substrate before laser irradiation was evaluated, the results shown in FIG. 7 were obtained. As shown in FIG. 7, in the Raman spectrum for Sample 1, a broad peak that was considered to be derived from α-carbon, DLC, and graphite that did not appear on the diamond substrate before laser irradiation appeared. From this, it was found that a conductive portion containing graphite was formed in the diamond substrate by laser irradiation.

[Influence of polarization on transmittance]
Terahertz parallel polarized light (polarized light) and vertical polarized light (polarized light) were incident from side A of sample 1 while changing the frequency, and the transmission spectrum of each polarized light in sample 1 was evaluated. The frequency of the terahertz wave incident on the sample 1 was in the range of 0.5 to 2.7 THz. The parallel polarized light is an electromagnetic wave whose electric field vibrates in a direction parallel to the extending direction of the conductive portion 4, and the vertical polarized light is an electromagnetic wave whose electric field vibrates in a direction perpendicular to the extending direction of the conductive portion 4. is there.

  The evaluation was performed by an optical system in which a known terahertz wave oscillator and detection device and a wire grid for obtaining polarized light incident on the sample 1 were combined.

  FIG. 8 shows the transmission spectrum (vacuum reference) of sample 1 together with the transmission spectrum (vacuum reference) of the diamond substrate before laser irradiation. In addition, FIG. 9 shows the transmission spectrum of sample 1 (with reference to the diamond substrate).

  As shown in FIGS. 8 and 9, when vertically polarized light is incident on the sample 1, the transmittance of the terahertz wave decreases with respect to the diamond substrate without depending on the frequency, and the degree of the decrease is determined by the frequency of the terahertz wave. It became bigger as it became higher.

  On the other hand, in the transmission spectrum when parallel polarized light is incident on the sample 1, a frequency of 1.59 THz (corresponding to the arrangement period X, Y (= 80 μm) when the refractive index of diamond is about 2.38 ( In FIGS. 8 and 9, a peak with a diamond-based transmittance exceeding 0.9 was observed in the vicinity of the solid line rising perpendicularly to the x-axis). Considering the arrangement period X, Y (= 80 μm) of the conductive portions 4, the geometric transmittance with respect to the tiremond substrate in the sample 1 is 0.68, so that the frequency 1. The above phenomenon in which the transmittance near 59 THz exceeds 0.9 is considered to be the same as the abnormal transmission phenomenon seen in the metal photonic crystal. Further, in the frequency range higher than this frequency, the transmittance of the terahertz wave was higher than that when vertically polarized light was incident.

  Thus, it was found that the sample 1 can split a terahertz wave having a specific polarization and frequency.

  Although further investigation is necessary for elucidation of the details, electrons are excited in the conductive portion 4 by the incidence of the terahertz wave at the peak of the transmission spectrum observed when the parallel polarized light is incident on the sample 1. Thus, there is a possibility that an effect of generating a current in the extending direction of the conductive portion 4 or an enhancement effect by surface plasmon generated in the conductive portion 4 due to incidence of the terahertz wave may be involved.

  FIG. 10 shows the relationship between the transmission spectrum of sample 1 evaluated as described above (parallel polarized light incidence, diamond substrate reference) and the phase change of the terahertz wave.

  Since diamond is a dielectric that has high transparency (having high transmittance) with respect to terahertz waves and small dispersion of refractive index (dielectric constant), the phase change is inherent to the frequency of terahertz waves. Should change linearly. However, as shown in FIG. 10, it has been found that the phase change with respect to the frequency of the terahertz wave does not change linearly and becomes a minimum at the peak rise (around 1.5 THz) in the transmission spectrum. The phase change linearly changed at the transmittance peak (approximately in the range of 1.5 to 1.8 THz). It was found that the same phase change as above was observed even in the vicinity of 2.0 THz in frequency.

The length of the region where the conductive portion 4 is formed with respect to the traveling direction of the terahertz wave is L, the wave number of the terahertz wave propagating through the sample 1 on the vacuum reference is k _s , and the conductive portion 4 on the vacuum reference is formed. If the wave number of the terahertz wave propagating in a non-diamond substrate (reference sample) is k _{r and} the angular frequency of the terahertz wave is ω, the phase change Δφ of the terahertz wave can be expressed by the following equation (1).

Furthermore, the relationship shown in the following formula (2) is established between the group velocity when the terahertz wave propagates through the sample 1 and the phase change Δφ.

In Equation (2), v _gs is the group velocity of the terahertz wave propagating through the sample 1, and v _gr is the group velocity of the terahertz wave propagating through the reference sample. Since diamond has almost no dispersion of refractive index (dielectric constant) with respect to terahertz waves, v _gr is constant. Therefore, the group velocity v _gs when the terahertz wave propagates through the sample 1 depends only on the slope of the phase change with respect to the frequency. According to Equation (2), it can be seen that in the transmission spectrum of Sample 1, the slope of the phase change is large and the group velocity v _gs is small in the frequency region where the transmittance increases.

  In addition, although future examination is necessary for elucidation of details, as shown in FIG. 10, at the rising edge of the transmission spectrum obtained by injecting parallel polarized light into Sample 1 (about 1.5 THz), The phenomenon in which the phase change of the terahertz wave is minimized may correspond to the existence of the end of the photonic band gap. Further, the phenomenon in which the phase change linearly changes at the transmittance peak (in the range of about 1.5 to 1.8 THz) is the phenomenon of the phase change due to the photonic band gap as observed in the photonic crystal ( There is a possibility that light is localized by repeating multiple scattering. According to these theories, the interaction between the terahertz wave and the conductive portion 4 can be enhanced by the multiple scattering of the terahertz wave in the region where the conductive portions 4 in the sample 1 are periodically arranged. Since the interaction between the surface plasmon and the terahertz wave generated in the conductive portion 4 can also be enhanced, in the sample 1, the terahertz wave having a specific polarization and frequency corresponding to the arrangement period of the conductive portions 4 can be selectively dispersed. It will be possible.

[Effect of array period on transmittance]
Next, in the same manner as the preparation of Sample 1, Sample 2 in which arrangement periods X and Y were set to 60 μm and Sample 3 in which arrangement periods X and Y were set to 100 μm were prepared. The configurations of Samples 2 and 3 other than the arrangement periods X and Y were the same as those of Sample 1.

  In the same manner as in Sample 1, Terahertz parallel polarization was incident on Samples 2 and 3 while changing the frequency, and the transmission spectrum was evaluated. However, in this evaluation, a terahertz wave that passed through a pinhole (circular, inner diameter 1 mm) was incident on each sample in order to make the area through which the terahertz wave transmitted in each sample constant. The evaluation results are shown in FIG. 11 together with the results of Sample 1 measured using pinholes in the same manner as Samples 2 and 3. In addition, the transmission spectrum of each sample shown in FIG. 11 is a vacuum reference | standard.

  As shown in FIG. 11, in any of samples 1 to 3, a transmittance peak is observed in the vicinity of the frequency corresponding to the arrangement periods X and Y when the refractive index of diamond is about 2.38 in the transmission spectrum. Observed. In FIG. 11, a solid line rising perpendicular to the x-axis indicates the frequency, and specific values thereof are 1.59 THz for sample 1, 2.11 THz for sample 2, and 1.26 THz for sample 3.

  Further, in any of samples 1 to 3, from the above-mentioned peak, even in the vicinity of twice the frequency corresponding to the arrangement periods X and Y when the refractive index of diamond is about 2.38 in the transmission spectrum. Although the peak intensity was small, the peak of transmittance was confirmed. In FIG. 11, the solid line rising perpendicular to the x-axis indicates the frequency, and the specific numerical values are 0.79 THz for sample 1, 1.05 THz for sample 2, and 0.63 THz for sample 3.

  From the results shown in FIG. 11, it was found that in samples 1 to 3, the frequency of the terahertz wave to be dispersed can be controlled by controlling the arrangement period of the conductive portions 4.

[Effects by increasing the number of arrangement surfaces of the conductive portions 4]
Next, in the same manner as in the preparation of Sample 1, Sample 4 in which the arrangement periods X and Y were set to 70 μm and the conductive portions 4 were formed so that the number of the arrangement surfaces was 40 when viewed from the side A was produced. . The configuration other than the arrangement periods X and Y and the number of arrangement surfaces in the sample 4 was the same as that of the sample 1.

  Similarly to sample 1, the terahertz parallel polarized light and vertical polarized light were incident on sample 4 while changing the frequency, and the transmission spectrum of each polarized light in the sample was evaluated. The frequency of the terahertz wave incident on the sample 4 was in the range of 0.4 to 1.0 THz.

  The transmission spectrum (vacuum reference) of Sample 4 is shown in FIG.

  As shown in FIG. 12, in the frequency band of 0.9 THz or less, the transmittance is smaller when parallel polarized light is incident than when vertically polarized light is incident, and this tendency is reduced as the frequency is decreased. It became bigger. As shown in FIG. 8, considering that this tendency was not obtained in the sample 1 having 10 array surfaces, a new optical function can be imparted to the element 1 by increasing the number of array surfaces. I understood.

(Example 2)
In Example 2, the preferable irradiation conditions of the femtosecond laser in the case where the diamond substrate made of industrial diamond used in Example 1 was used as the base were examined.

[Repetition frequency]
The diamond substrate was irradiated with a femtosecond laser having a wavelength of 800 nm and a pulse width of 150 fs while changing the repetition frequency between 200 KHz and 1 kHz, and how the substrate was altered by each laser irradiation was evaluated. An objective lens with a numerical aperture of 0.90 was used for laser irradiation. Further, when irradiating a laser having a repetition frequency of 200 kHz, the pulse energy is set to 0.25, 0.50, 1.00, and 1.50 μJ / pulse (30, 60, 121, and 181 J as the laser fluence at the focal position). / Cm ² ) in four ways. When irradiating a laser with a repetition frequency of 1 kHz, the pulse energy is 1.1, 6.3, 8.8 and 50 μJ / pulse (125.3, 711.8, 996.5 as the laser fluence at the focal position). And 5694.5 J / cm ² ).

  As a result of the evaluation, under the condition where the repetition frequency is 200 kHz, ablation occurs on the surface of the substrate regardless of the value of the pulse energy, and an altered portion, that is, a conductive portion cannot be formed only inside the substrate. It was. That is, it was found that in the above diamond substrate, it is difficult to form a conductive portion only inside the substrate when the repetition frequency of the laser to be irradiated is 200 KHz.

On the other hand, when the repetition frequency is 1 kHz, when the pulse energy is 50 μJ / pulse (when the laser fluence at the focal position is 5694.5 J / cm ² ), an altered portion may be formed only inside the substrate. did it. However, even in this case, ablation occurred on the surface of the substrate when the focal position was less than 100 μm from the surface of the substrate. In order to form the conductive portion at a position less than 100 μm from the surface of the substrate, it is considered that some optical adjustment such as changing the numerical aperture of the objective lens is required.

[Pulse energy required for internal machining]
Mathematically calculated pulse energy of the laser irradiated on the substrate and the focal position of the laser when the numerical aperture (NA) of the objective lens is 0.30, 0.45, 0.80 and 0.90, respectively. The relationship with the laser fluence is as shown in FIG. FIG. 13 shows that, for example, when an objective lens having a numerical aperture of 0.45 is used, a pulse energy of 200 μJ / pulse is necessary to achieve 5694.5 J / cm ² as a laser fluence. .

[Relationship between the number of laser irradiation pulses and the average diameter of the conductive part]
A femtosecond laser with a wavelength of 800 nm, a pulse width of 150 fs, a repetition frequency of 1 kHz, a pulse energy of 50 μJ / pulse is used with the objective lens having a numerical aperture of 0.90, and the focal position is 100 μm from the surface of the substrate. Irradiation was carried out while changing the number of irradiation pulses. When the average diameter of the altered portion of the substrate formed by this irradiation was evaluated with an optical microscope, the result shown in FIG. 14 was obtained.

  As shown in FIG. 14, it was found that the average diameter of the altered portion formed by laser irradiation, that is, the conductive portion, can be increased by increasing the number of pulses of the laser irradiated to the same portion. From this figure, for example, in order to form a conductive portion having an average diameter of 100 μm or less under the above-mentioned substrate and irradiation conditions, it is necessary that the number of laser pulses irradiated to the same place be 30 pulses or less. I understand.

[Relationship between laser fluence and conductivity of conductive part]
Using a femtosecond laser with a wavelength of 800 nm, a pulse width of 150 fs, and a repetition frequency of 1 kHz, and an objective lens with a numerical aperture of 0.45, changing the pulse fluence, that is, changing the laser fluence at the focal position. Then, irradiation was performed at a scanning speed of 20 μm / s. When the conductivity of the altered portion formed by irradiation was evaluated by applying a conductive paste to the upper and lower surfaces of the diamond substrate and sandwiching the paste with a tester, the results shown in FIG. 15 were obtained.

  As shown in FIG. 15, it was found that increasing the laser fluence can increase the conductivity of the altered portion of the substrate formed by laser irradiation, that is, the conductive portion. This is presumably because the structure of the altered portion has changed from the crystal structure of diamond to an amorphous phase containing conductive α-carbon or graphite.

[Relationship between laser scanning speed and conductivity of conductive part]
The diamond substrate was irradiated with a femtosecond laser having a wavelength of 800 nm, a pulse width of 150 fs, a repetition frequency of 1 kHz, and a pulse energy of 1 μJ / pulse while changing the scanning speed using an objective lens having a numerical aperture of 0.45. The laser to be irradiated was linearly polarized light or circularly polarized light. In each case, the conductivity of the altered portion formed by irradiation was evaluated as described above, and the result shown in FIG. 16 was obtained.

  As shown in FIG. 16, in the case where the laser to be irradiated is either linearly polarized light or circularly polarized light, an appropriate scanning speed at which the conductivity of the altered portion of the substrate formed by laser irradiation, that is, the conductive portion is increased. Was found to exist. The speed was approximately 20 μm / s in the example shown in FIG.

  The reason why there is an appropriate scan speed for the conductivity of the conductive part is not clear, but when the scan speed is low, the laser is continuously irradiated at the same location, so that the formed conductive material such as graphite can be regenerated. A mechanism in which the crystallization progresses and the conductivity decreases may be considered. On the other hand, when the scanning speed is high, a mechanism in which laser irradiation is insufficient and a conductive substance is not sufficiently formed is conceivable.

  Further, as shown in FIG. 16, the conductivity of the formed conductive portion changed depending on whether the laser to be irradiated was linearly polarized light or circularly polarized light. For example, when the scanning speed was 20 μm / s, the conductivity when irradiated with linearly polarized light was 1.7 times that when irradiated with circularly polarized light.

  The reason why the conductivity of the conductive part is affected by the polarization state of the irradiating laser is not clear, but the polarization state of the irradiating laser contributes to the transformation of diamond into an amorphous phase containing α-carbon or graphite. There is a possibility. For example, when the laser is linearly polarized, the polarization direction when the laser propagates through the diamond is constant, so that the crystallites in the amorphous phase are formed in a substantially constant direction, whereas the laser is circularly polarized. In this case, since the polarization direction of the laser propagating through the diamond rotates, microcrystals in the amorphous phase are formed in random directions, resulting in differences in the ease of movement of electrons between the microcrystals. A mechanism that causes this is considered.

  In this evaluation, the focal position of the laser to be irradiated is gradually moved from the surface of the diamond substrate far from the laser oscillation source to the inside of the substrate, so that a femtosecond laser having a pulse energy of 1 μJ / pulse ( Even when a repetition frequency of 1 kHz was used, a conductive portion could be formed inside the diamond substrate. This is presumably because, on the surface of the substrate, the energy threshold for alteration by laser irradiation is smaller than that inside the substrate, and a conductive portion can be formed even by a femtosecond laser with low pulse energy.

  According to the present invention, it is possible to provide a novel terahertz wave optical element having various features such as an unconventional configuration, excellent productivity, and a high degree of freedom in shape, and a method for manufacturing the same.

It is a perspective view which shows typically an example of the optical element for terahertz waves of this invention. It is the top view which looked at the optical element shown in FIG. 1 from the upper direction. It is sectional drawing which shows typically an example of the terahertz wave oscillator which combined the optical element for terahertz waves of this invention. It is sectional drawing which shows typically an example of the terahertz wave oscillator which combined the optical element for terahertz waves of this invention. It is sectional drawing which shows typically an example of the terahertz wave oscillator which combined the optical element for terahertz waves of this invention. It is sectional drawing which shows typically an example of the terahertz wave oscillator which combined the optical element for terahertz waves of this invention. It is a figure which shows the optical microscope image which looked at the sample 1 produced in the Example from the direction perpendicular | vertical to the main surface. It is a figure which shows the optical microscope image which looked at the sample 1 produced in the Example from the one side surface. It is a figure which shows the Raman spectrum of the sample 1 produced in the Example, and the diamond substrate before laser irradiation. It is a figure which shows the transmission spectrum (vacuum reference | standard) of the sample 1 produced in the Example. It is a figure which shows the transmission spectrum (diamond board | substrate reference | standard) of the sample 1 produced in the Example. It is a figure which shows the relationship between the transmission spectrum of the sample 1 produced in the Example (at the time of parallel polarized light incidence, a diamond substrate reference), and the phase change of a terahertz wave. It is a figure which shows the transmission spectrum (at the time of parallel polarized light incidence, the vacuum reference | standard) of the samples 1-3 produced in the Example. It is a figure which shows the transmission spectrum (vacuum reference | standard) of the sample 4 produced in the Example. It is a figure which shows the relationship between the pulse energy of the laser irradiated to a board | substrate when changing the numerical aperture (NA) of the objective lens used for laser irradiation, and the laser fluence in the focus position of a laser. It is a figure which shows the relationship between the irradiation pulse number of the laser evaluated in Example 2, and the average diameter of the degenerated part (conductive part) of the board | substrate formed by laser irradiation. It is a figure which shows the relationship between the laser fluence evaluated in Example 2, and the electrical conductivity of the altered part (conductive part) of the board | substrate formed by laser irradiation. It is a figure which shows the relationship between the scanning speed of the laser evaluated in Example 2, and the electrical conductivity of the altered part (conductive part) of the board | substrate formed by laser irradiation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical element for terahertz waves 2 Substrate 3 Action part 4 Conductive part 11 Terahertz wave oscillator 12 Substrate 13 Photoconductive layer 14, 15, 16, 17 Electrode 21 Terahertz wave oscillator

Claims (12)

It has a diamond base formed inside with an action part capable of optically acting on the incident terahertz wave,
The action portion is a terahertz wave optical element having a structure in which columnar conductive portions formed by alteration of the diamond are periodically arranged.   The terahertz wave optical element according to claim 1, wherein the conductive portions are arranged in parallel to each other.   The optical element for terahertz waves according to claim 1, wherein the conductive portion includes graphite.   The optical element for terahertz waves according to claim 1, wherein an average diameter of the conductive portion is 100 nm or more and 10 mm or less.   The optical element for terahertz waves according to claim 1, wherein the arrangement period of the conductive portions is in a wavelength region of terahertz waves.   The optical element for terahertz waves according to claim 1, wherein the action portion has two or more surfaces on which the conductive portions are arranged.   The terahertz wave optical element according to claim 1, wherein the substrate has a flat plate shape.   The terahertz wave optical element according to claim 7, wherein the conductive portion extends in a direction parallel to a main surface of the flat substrate.   The optical element for a terahertz wave according to claim 7, wherein the action part optically acts on a terahertz wave incident from a main surface of the flat substrate.   The terahertz wave optical element according to claim 1, which is at least one selected from a diffraction element, a spectroscopic element, and a polarizing element. A method for manufacturing a terahertz wave optical element including an action part capable of optically acting on an incident terahertz wave,
Irradiating a diamond substrate with a femtosecond laser, altering a portion of the substrate near the focal point of the irradiated laser to make the portion conductive,
In the step, by changing the focal position of the laser, two or more columnar portions are formed so that the portions are periodically arranged, and a columnar conductive portion is formed inside the substrate. A method for manufacturing an optical element for a terahertz wave, which forms working portions having a periodically arranged structure.   The method for manufacturing an optical element for a terahertz wave according to claim 11, wherein in the step, the two or more portions having a columnar shape are formed such that the portions are arranged in parallel to each other.