JP2008151619A - Electromagnetic wave modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electromagnetic wave modulator having low electromagnetic polarization direction dependence and changing the propagation state of an electromagnetic wave without requiring mechanical operation. <P>SOLUTION: The electromagnetic wave modulator is constituted so as to change the propagation state of the electromagnetic wave 24 by forming spatial electron density distribution changed in the electron density of plasma in at least one direction within a wave front 26 in a propagation route containing the surface receiving the wave front 26 of the electromagnetic wave 24 and has a producing means 23 for producing the plasma 28 and electron density distribution regulating means 21 and 23 for regulating the spatial electron density distribution of the plasma. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a technique for arbitrarily modulating a propagation state such as an electromagnetic wave propagation direction and a cross-sectional pattern using plasma. In particular, the present invention relates to an electromagnetic wave modulation device that arbitrarily changes a propagation direction of an electromagnetic wave (referred to as a terahertz wave in this specification) within a frequency range of 30 GHz to 30 THz using plasma.

In recent years, technological development using terahertz waves has been active. In particular, attempts have been made to perform fluoroscopic imaging using the transmission of terahertz waves to various substances, and spectroscopic imaging that combines spectroscopy and imaging of substances using terahertz waves has been performed.

With respect to such a technique, the following is carried out for the purpose of simultaneously analyzing the components and imaging of the inherent matter without opening the mail or the like. In other words, a terahertz wave is focused on one point of an object (in this case, a postal item, etc.), and a spectral image is obtained by mechanically scanning the object while dispersing the object (see Patent Document 1). ).

A method for obtaining a two-dimensional image using a two-dimensional terahertz wave detecting element such as a microbolometer array is also known. However, a terahertz wave light source capable of obtaining sufficient intensity and a two-dimensional terahertz wave detecting element having sufficient sensitivity are not readily available. For these reasons, a method of obtaining an image by focusing a terahertz wave at one point and mechanically scanning an object is widely used. A method of obtaining an image by changing the propagation direction of the terahertz wave (that is, deflecting the terahertz wave beam) and scanning the focal point on the object is also used.

The deflection of the terahertz wave beam can be achieved by reflecting or refracting the terahertz wave beam with a mirror or prism and rotating the mirror or prism. If the mirror or prism is rotated about two axes, the terahertz wave beam can be deflected in an arbitrary direction.

On the other hand, in the infrared region having a shorter wavelength, beam deflection by a phased array method using liquid crystal is known (see Patent Document 2). In the passive phased array system described here, a plurality of phase shifters arranged with a period equal to or shorter than the wavelength of the incident electromagnetic wave generate an appropriate phase delay in each part of the electromagnetic wave with respect to the electromagnetic wave incident in the same phase. And this changes the propagation direction of electromagnetic waves using the property that each part of the electromagnetic waves emitted from each phase shifter propagates in the most intensifying direction.

The device of Patent Document 2 is composed of liquid crystal cells arranged two-dimensionally with a half-wavelength period, and an electric field can be applied independently to the liquid crystal in each cell. Then, an infrared beam is transmitted through the liquid crystal cell, and the orientation of the liquid crystal in each cell is changed appropriately, thereby giving an arbitrary phase delay to the infrared light transmitted through each cell. Thus, the propagation direction of the infrared beam transmitted through the liquid crystal cell is changed by the phase difference of the electromagnetic wave emitted from each cell. This achieves infrared beam deflection without requiring mechanical drive. Further, by appropriately adjusting the phase difference generated in each cell, it is possible not only to deflect the infrared beam in an arbitrary direction but also to draw an arbitrary pattern by the infrared beam. Furthermore, it is possible to collect and diverge an infrared beam.
JP 2004-286716 A JP-A-5-66427

The above-described method of deflecting the terahertz wave beam by rotating the mirror or the prism involves a mechanical operation of rotating the mirror or the prism and increases the size of the apparatus. In particular, since a terahertz wave has a wavelength of 300 μm at 1 THz, a thin beam (for example, 1 mm) spreads by a diffraction effect as it propagates. In order to suppress the spread of the beam, it is desirable to use a thick beam (for example, 10 mm or more). However, in this case, the mirror or prism to be used becomes larger than the beam diameter, and when this is rotated, the volume occupied by the mirror or prism increases. In addition, there is a limit to the operating speed as long as it involves mechanical motion.

On the other hand, in the method of Patent Document 2 that is a phased array system using liquid crystal, the characteristics strongly depend on the polarization direction of incident electromagnetic waves. The reason is that only the component of the incident electromagnetic wave that matches the alignment direction of the liquid crystal can be transmitted, and the polarization component perpendicular to the alignment direction of the liquid crystal has a large attenuation.

In view of the above problems, the electromagnetic wave modulation device of the present invention forms a spatial electron density distribution in which the electron density of plasma changes in at least one direction within the wavefront in a propagation path including a surface that receives the wavefront of the electromagnetic wave. It is characterized by changing the propagation state of electromagnetic waves. The electromagnetic wave modulation device of the present invention has a generating means for generating plasma and an electron density distribution adjusting means for adjusting the spatial electron density distribution of the plasma. The electromagnetic wave is typically a terahertz wave.

Further, in view of the above problems, the imaging apparatus of the present invention irradiates an object with electromagnetic waves, moves the irradiation position of the electromagnetic waves using the electromagnetic wave modulation device, and detects the transmitted waves, reflected waves, or scattered waves of the electromagnetic waves. In this way, information on the image of the object is obtained.

According to the present invention, the propagation state of electromagnetic waves such as terahertz waves is arbitrarily changed by adjusting the spatial electron density distribution in which the electron density of plasma changes as described above. Therefore, it is possible to realize a small-sized electromagnetic wave modulation device capable of high-speed operation by having characteristics with small dependency on the electromagnetic wave polarization direction and requiring no mechanical operation.

Hereinafter, after explaining the principle of the concept of the present invention, embodiments of the electromagnetic wave modulation device of the present invention will be described. Although the terahertz wave will be described, the principle is the same even for electromagnetic waves that are slightly out of the range.

Discharge plasma generated in the space regions of the order of the wavelength of the terahertz wave (e.g. space of the order of μm from mm), the electron density reaches about 10 ¹⁶ cm ^-3, plasma frequency reaches about 1 THz (Applied Physics Volume 75 Issue 4 Pages 399-410
(2006)). This is called microplasma that generates plasma using gas discharge, and the electron density can reach 10 ^{13 to} 10 ¹⁸ cm ^-3 , and the corresponding plasma frequency is 10 GHz to 10 THz. Can be. At this time, the plasma behaves as a dielectric with respect to the terahertz wave.

It is possible to adjust the electron density in the plasma by adjusting the discharge voltage and discharge current of the plasma. Therefore, it is possible to adjust the plasma frequency. That is, it is possible to adjust the dielectric constant of the dielectric with respect to the terahertz wave. The dielectric constant adjustment function for the terahertz wave is mainly used in the present invention. However, the present invention does not exclude partially utilizing the phenomenon described later in which plasma behaves like a metal with respect to incident electromagnetic waves. For example, when configuring a spatial optical modulator that projects an incident electromagnetic wave in an arbitrary cross-sectional pattern, a part of the plasma behaves like a metal to form an outgoing electromagnetic wave with an arbitrary cross-sectional pattern. It can happen.

The dielectric constant adjustment function will be described. When the electron density n _e in the plasma are given plasma frequency f _pe is given by the following equation (1).

Here, e is the elementary charge, ε ₀ is the vacuum dielectric constant, and _me is the static mass of the electrons. The plasma behaves like a metal with respect to an electromagnetic wave having a frequency lower than the plasma frequency, and the incident electromagnetic wave is reflected. On the other hand, for electromagnetic waves having a frequency equal to or higher than the plasma frequency, the plasma behaves like a dielectric, and incident electromagnetic waves pass through the plasma.

The electron density in the plasma is variable by the following means. For example, means such as electrodes for changing the discharge voltage or discharge current, means for changing the plasma medium density, means for changing the gas pressure, means for adjusting the electron density of the plasma by ionization with visible light or ultraviolet light irradiated from the outside There is. Another example is a means for accelerating ionization by accelerating electrons in the plasma using electromagnetic waves irradiated from outside that are different from the electromagnetic waves propagating in the plasma. Therefore, a desired plasma frequency can be obtained by appropriately adjusting the discharge voltage, the gas pressure, and the like.

In addition, the dispersion relational expression of the electromagnetic wave propagating in the plasma is obtained by the following expression (2).

Here, ω ₀ is the angular frequency of the incident electromagnetic wave, ω _pe is the plasma angular frequency, k is the wave number of the electromagnetic wave propagating in the plasma, and c is the speed of light in vacuum. There is a relationship of ω ₀ = 2πf ₀ between the angular frequency of the incident electromagnetic wave and the frequency of the incident electromagnetic wave, and there is a relationship of ω _pe = 2πf _pe between the plasma angular frequency and the plasma frequency.

Accordingly, a phase difference Δφ given by the following equation (3) is generated between the electromagnetic wave propagated through the plasma by the distance L and the electromagnetic wave propagated through the vacuum by the distance L.

Therefore, as shown in FIG. 1, if the electron density in the plasma 10 is non-uniform (the shading in FIG. 1 indicates the non-uniformity of the electron density), the electromagnetic wave propagating in the plasma 10 has an electron density distribution. A corresponding phase difference is generated. This electron density distribution is a spatial electron density distribution in which the electron density of plasma changes in at least one direction in the wavefront in a propagation path including a surface that receives the wavefront 26 of the incident electromagnetic wave 24. Here, when no magnetic field is applied to the plasma from the outside, the polarization direction dependence of the plasma on the electromagnetic wave is small. In FIG. 1, 25 is an output terahertz, and 27 is an output terahertz wavefront.

The electromagnetic wave modulation device of the present invention provides a phase difference to a terahertz wave by appropriately adjusting the electron density distribution of the plasma to give an appropriate phase difference to any different part of the same wavefront in the electromagnetic wave propagating in the plasma. Gives the distribution of. Thus, the propagation state of the terahertz wave is changed. Specifically, the propagation direction of the terahertz wave beam is deflected, condensed or diverged, or an arbitrary pattern is formed.

Embodiments of the present invention based on the principle described above will be described below with reference to the drawings.

As described above, the present invention realizes electromagnetic wave modulation by giving a distribution to the electron density in the plasma. That is, in the propagation path including the surface that receives the wavefront of the electromagnetic wave, the propagation state of the electromagnetic wave is changed by forming a spatial electron density distribution of plasma in at least one direction within the wavefront. As a configuration for that purpose, it has generating means for generating plasma and electron density distribution adjusting means for adjusting the spatial electron density distribution of the plasma. The generating means and the electron density distribution adjusting means may be provided so as to function independently of each other, or a part or all of them may be common. For example, an electrode to be described later can be configured as a means for performing both functions.

Therefore, in one embodiment of the present invention, a large number of small plasmas (spatial size is less than or equal to the wavelength of electromagnetic waves) are individually generated, and the electron density of each plasma is adjusted to an appropriate value as a whole. Realize electron density distribution in plasma. Typically, a plurality of hollow partitions arranged adjacent to each other are provided along a surface that receives the wavefront of the electromagnetic wave. The generating means includes a plurality of partition portion generating means (for example, electrodes to be described later) that generate plasma in the plurality of partition portions, respectively. Further, the electron density distribution adjusting means includes a plurality of partition parts (for example, a plasma cell described later) and a plurality of electron density adjusting means (for example, an electrode described later) for adjusting the electron density of the plasma therein. Constitute.

FIG. 2 is a schematic diagram of the electromagnetic wave modulation device, that is, the terahertz wave beam modulation device of the present embodiment. On the substrate 20 forming the device, a sealed space separated from others (this space does not necessarily have to be sealed and may communicate with each other) along the surface receiving the wavefront 26 of the electromagnetic wave 24, Many are arranged in parallel. This is called a plasma cell 21 in order to generate plasma 28 in this sealed space. Although eight plasma cells 21 are shown in FIG. 2, only the leftmost cell is numbered with a lead line for convenience. In other drawings, when there are a plurality of other parts, the numbers corresponding to these are given. For the substrate 20, for example, a quartz substrate is used.

The plasma cell 21 penetrates the substrate 20 and is arranged with a period of about half the wavelength of the terahertz wave 24 to be used (it may be a period shorter than the wavelength of the electromagnetic wave). The plasma cell 21 is sealed by a window member 22 that is attached to both surfaces of the substrate 20 and transmits the terahertz wave 24. For the window member 22, for example, a quartz substrate may be used similarly to the substrate 20, or a sapphire substrate may be used. However, the window member 22 is preferably made of a material that transmits the terahertz wave 24 well.

Although the detailed configuration of each plasma cell 21 will be described later with reference to FIG. 3, it includes at least a pair of electrodes 23. The electrode 23 is formed to extend substantially perpendicular to the surface of the window member 22. That is, when the terahertz wave 24 is incident substantially perpendicular to the surface of the window member 22, the electrode 23 is provided so as not to face the wave surface 26. As described above, a plurality of electrodes are provided, and at least one of the distances between the plurality of electrodes is set to be equal to or less than the wavelength of the electromagnetic wave.

Gas is sealed in the plasma cell 21. Examples of the type of gas include a rare gas such as argon. The gas pressure is, for example, about 10 ^{4 to} 10 ⁷ Pa.

The size of the plasma cell 21 (the distance between the pair of electrodes 23) is preferably about half the wavelength of the electromagnetic wave 24 as described above. The thickness of the substrate 20 is preferably about the same as the wavelength of the electromagnetic wave 24, but the effect of the terahertz wave beam modulation is exhibited regardless of whether it is larger or smaller than the wavelength.

Details of the plasma cell 21 will be described with reference to FIG. As shown in FIG. 3, electrodes 23 are provided on both end surfaces (which are substantially perpendicular to the surface of the window member 22) of a hole (a space in the plasma cell 21) formed so as to penetrate the substrate 20. . The inner wall of the plasma cell 21 is coated with a dielectric 30 (for example, MgO) so as to cover the electrode 23. By coating, it is possible to prevent erosion of the electrode 23 by plasma (for example, due to a sputtering effect).

By applying an AC voltage, for example, to the electrode 23 by the power source 29, a pulse discharge occurs in the plasma cell 21, and a dielectric barrier discharge plasma 28 is generated. The applied voltage is, for example, 1 kV. The frequency of the AC voltage is 10 kHz, for example. Alternatively, a pulse voltage with a duty ratio of about 0.05 may be alternately applied. By modulating this voltage, the electron density of the plasma 28 can be adjusted.

At this time, the plasma when the scale of the plasma 28 is in the order of mm or less and the internal gas pressure is in the order of 10 ^{4 to} 10 ⁷ Pa, as described above, is usually called microplasma, and the electrons in the plasma The density reaches 10 ^{14 to} 10 ¹⁶ cm ^-3 .

With the configuration as described above, the spatial electron density distribution of plasma is formed to be adjustable in at least one direction within the wavefront of the incident electromagnetic wave. Thus, the propagation state of the terahertz wave beam is changed, and terahertz wave beam modulation occurs. This plasma's spatial electron density distribution has low polarization direction dependence with respect to electromagnetic waves, and can be generated and adjusted without requiring mechanical operation. A direction-dependent electromagnetic wave modulation device can be realized.

Examples will be described below with specific numerical examples.

(Example 1)
Example 1 will be described. As mentioned above, in general, when the electron density n _e in the plasma are given plasma frequency f _pe is given by equation (1). From the above-mentioned formulas (1) and (2), the phase difference given by the above formula (3) is generated between the electromagnetic wave propagated in the plasma by the distance L and the electromagnetic wave propagated in the vacuum by the distance L. To do.

For example, consider a case where a terahertz wave having a frequency of 0.5 THz is incident on plasma 28 having a plasma frequency of 0.3 THz. It is assumed that plasma is generated in one of the two plasma cells 21 and no plasma is generated in the other. The thickness of the plasma 28 (the terahertz wave propagation direction) is about 0.5 mm.

At this time, the phase difference between the terahertz wave transmitted through the plasma cell 21 where the plasma 28 is generated and the terahertz wave transmitted through the plasma cell 21 where the plasma is not generated is approximately π / 3. Thereby, the phase of the terahertz wave transmitted through the plasma 28 advances. If the distance between the centers of the two plasma cells 21 is half the wavelength (and hence 300 μm), the terahertz waves emitted from the two plasma cells 21 are radiated in a strengthening direction due to the phase difference due to interference and diffraction effects. The

If a large number of such plasma cells 21 are arranged as shown in FIG. 2 (or FIG. 4), and terahertz waves transmitted through adjacent plasma cells 21 have different phases by π / 3, the terahertz wave beam is caused by interference. It is deflected in a direction that is bent about 19 °. For example, the incident terahertz wave 24 shown in FIG. 2 is transmitted through a large number of plasma cells 21 so that its traveling direction is deflected as indicated by the outgoing terahertz wave 25.

However, at this time, the phase difference Δφ exceeds 2π every certain period (in this case, six plasma cells). Since the phase difference 2π is the same as the phase difference zero, the electron density of the plasma 28 may be adjusted so that the phase difference 2π can be achieved with Δφ−2π.

Further, if the arrangement period of the plasma cells 21 is equal to or greater than the wavelength of the terahertz wave, a plurality of diffracted terahertz waves are generated. Depending on the application, this higher-order diffracted light is unnecessary, and in this case, the arrangement period of the plasma cell 21 should be equal to or less than the wavelength of the terahertz wave. A configuration in which a plurality of diffracted terahertz waves are generated can be used as a beam splitter, for example.

A case will be described in which a large number of plasma cells 21 are arranged two-dimensionally in parallel as shown in FIG. Assume that a terahertz beam is incident from the −Z direction to the + Z direction. If the terahertz beam is deflected in the Y direction, the electron density in each plasma cell 21 is uniform in the X direction. Then, the electron density of the plasma 28 may be adjusted so that an appropriate phase difference can be generated in the Y direction as described above.

Similarly, if the terahertz wave beam is deflected in the X direction, the electron density in each plasma cell 21 is made uniform in the Y direction. In the X direction, the electron density may be adjusted so that an appropriate phase difference can be generated as described above.

In addition, when a large number of two-dimensional plasma cells 21 are arranged as shown in FIG. 4, the electron density of each plasma cell can be adjusted so that the electron density distribution is concentric in the entire terahertz beam modulator. . Thereby, the terahertz wave beam can be condensed or diverged.

For example, as shown in FIG. 5, among the two-dimensionally arranged plasma cells 21, the electron density in the plasma cell 21 near the center is lowered (that is, the plasma frequency is lowered), and the plasma cell 21 moves outward. Accordingly, the electron density is increased (that is, the plasma frequency is increased). By doing so, a phase difference is generated between the terahertz wave that has passed through the outer plasma cell 21 and the terahertz wave that has passed through the inner plasma cell (the phase of the terahertz wave that has passed through the outer side advances). From the principle of interference / diffraction, the terahertz wave beam is emitted strongly in the direction in which the phase difference is eliminated, that is, near the center. This corresponds to the collection of the terahertz wave beam. The Fresnel zone plate and lens also generate a phase difference between the center and the outside to condense (or diverge) the beam, and are in principle the same as these Fresnel zone plate and lens.

In addition to focusing the terahertz wave at a single point by giving an appropriate phase difference using a terahertz wave beam modulator, it forms and projects an arbitrary pattern such as a cross, line, or concentric circle Is also possible. A terahertz wave having such a wavefront intensity distribution pattern is used, for example, to observe unevenness of an irradiated object.

Further, by providing an appropriate phase difference using a terahertz beam modulating device, it is possible to deflect the beam as if having a negative refractive index. For example, the angle formed by the normal line of the terahertz wave beam modulator and the incident terahertz wave beam is defined as the incident angle, and the angle formed by the normal line of the device and the output terahertz wave beam is defined as the output angle. The exit angle is positive in the direction in which the incident terahertz beam is extended. At this time, if an appropriate phase difference is given using a terahertz wave modulation device, the emission angle can be made negative. This behaves like an element having a negative refractive index and enables large beam deflection.

In FIG. 4, for convenience, 25 5 × 5 plasma cells 21 are arranged in FIG. 4. However, as the number of plasma cells 21 increases, the modulated beam approaches a desired shaping pattern. For example, when the diameter of the incident terahertz wave beam is 10 mm, if each plasma cell 21 is arranged with a period of 300 μm, about 33 plasma cells on one side can be arranged.

For example, when a terahertz wave beam is deflected and the number of plasma cells 21 is 25 as shown in FIG. 4, the deflected beam diverges as it propagates. On the other hand, if a beam modulation device having 1000 plasma cells 21 is used, the beam divergence accompanying the propagation of the deflected beam is small.

In the terahertz beam modulating apparatus of the present embodiment, the frame portion of each plasma cell 21 is in a lattice shape, and a conductive material such as an electrode 23 and electric wiring is provided on the frame portion. In general, an object in which regular holes are provided in a dielectric or metal plate-like object against an electromagnetic wave is called a mesh filter and has a property of strongly transmitting an electromagnetic wave having a specific frequency.

The terahertz beam modulator of the present embodiment may function as a mesh filter depending on the shape and arrangement of the plasma cell. Therefore, the above items need to be taken into consideration when selecting the wavelength of the terahertz wave to be used. If the wavelength is appropriately selected, the function of the mesh filter can also be used to perform both the filter and deflection functions.

Since the lifetime of plasma is generally about 1 ns to 100 ns (nanoseconds), in dielectric barrier discharge, the plasma flickers depending on the frequency of applied voltage and the duty ratio. While the plasma is lit (generated / sustained), the plasma behaves as a dielectric with respect to the terahertz wave, causing a phase delay. However, while the plasma is extinguished (extinguished), the plasma cell is merely a gas, and therefore a phase delay cannot be brought about with respect to the terahertz wave. As a means for coping with this, there is a means for synchronizing the incident terahertz wave with the generation of the plasma so that the terahertz wave is incident on the plasma cell only while the plasma is generated and sustained.

In addition, two plasma cells are installed adjacent to the propagation direction of the terahertz wave, and when the plasma of one plasma cell is turned on, the applied voltage and the like so that the plasma of the other plasma cell is turned off. There is also a means for adjusting the phase of. This makes it possible to create a plasma that is virtually always lit against terahertz waves. In this way, it is possible to realize an electromagnetic wave modulation device that always reliably performs the modulation action on the electromagnetic wave.

(Example 2)
A second embodiment of the present invention will be described with reference to FIG. The terahertz beam modulating apparatus shown in FIG. 6 has a structure in which a plurality of electrode pairs 23 are installed in a plasma cell 21 and a large number of plasmas 28 can be generated in one plasma cell 21.

When the terahertz wave is incident from the -Z direction and has a purpose of deflecting the beam in the X direction and no purpose of deflecting the beam in the Y direction, a large plasma common to the Y direction as in this embodiment Using the cell 21 makes it easier to obtain plasma uniformity in the Y direction. This is because the gas pressure is always constant in the Y direction. Since the uniformity of the plasma in the Y direction is easy to obtain, more precise beam deflection is possible when deflecting the beam in the X direction. In addition, this configuration makes it easy to suppress the disturbance of the wave front of the deflected beam.

(Example 3)
A third embodiment of the present invention will be described with reference to FIG. The waveguide type terahertz beam modulating apparatus of the present embodiment has a structure in which a large number of waveguides 71 are bundled as a plurality of partition portions. The plurality of waveguides 71 are arranged approximately in the in-plane direction of the wavefront 26 of the incident terahertz wave 24 with a half wavelength period of the incident terahertz wave.

Each waveguide 71 is provided with a large number of electrode pairs 23 along the propagation direction of electromagnetic waves. As described in Example 1, these electrode pairs are coated with a dielectric (eg, MgO). Also, both end faces of the waveguide 71 in the propagation direction of the incident terahertz wave 24 are sealed with a window material (for example, a quartz substrate), as described in the first embodiment. A gas is sealed in each waveguide 71.

A voltage can be applied to each electrode pair 23 independently, and plasma can be partially generated in the waveguide 71. In Example 1, the dielectric constant of the plasma was adjusted by adjusting the electron density in the plasma. In contrast, in this embodiment, the optical path length of the waveguide 71 is adjusted by adjusting the number of plasma generations, and the phase of the terahertz wave 25 emitted from the emission side of the waveguide 71 is adjusted.

In the method of this embodiment, the phase can be adjusted by the number of plasma generations, so that the phase difference between each part of the terahertz wave can be adjusted more precisely. The waveguide 71 has various cross-sectional types such as a circle and a rectangle, and any of them may be used.

(Example 4)
A fourth embodiment of the present invention will be described with reference to FIG. In the waveguide-type terahertz wave modulation device shown in the third embodiment, there may be a case where a certain period of time is required for plasma generation / extinction and beam modulation cannot be performed at a high speed within the certain period.

Therefore, electrodes are arranged in each waveguide 80 as shown in FIG. 8 so that plasma generation and extinction can be repeated at higher speed. That is, a preliminary discharge is always performed by applying a voltage to the electrode pair 81 provided close to the same side surface of the waveguide 80, and a small discharge plasma 83 is generated. An auxiliary electrode 82 is provided at a position facing the electrode pair 81, and a voltage is also applied to the auxiliary electrode 82 when plasma is generated. Electrons in the small discharge plasma 83 due to the preliminary discharge are accelerated by the electric field generated by the auxiliary electrode 82 and can immediately grow into a large plasma 84.

By the method of the present embodiment, the waveguide type terahertz wave modulation device can be operated at higher speed.

(Example 5)
A fifth embodiment of the present invention will be described with reference to FIGS. In the above embodiment, the plurality of electrodes and the power source serve as the generating means and the electron density distribution adjusting means. In the present embodiment and the next embodiment, the electrode pair and the power source serve as the generating means and the electron density distribution adjusting means, but the electron density distribution adjusting means further includes other functional elements.

In the embodiment of FIG. 9, a Langmuir probe 100 such as a metal needle is always inserted into the plasma cell 21. The Langmuir probe 100 is arranged so as not to hinder the propagation of the terahertz wave in the plasma cell 21 as much as possible. By measuring the current flowing through the external circuit 101 through the Langmuir probe 100, the electron density in the plasma 28 can be measured. Feedback control is performed using the measured electron density in the plasma 28, and the voltage applied to the plasma 28 is adjusted so as to reach an electron density that can generate a desired phase difference. Adjustment of voltage and the like is performed by the electrode pair 23.

In the embodiment shown in FIG. 10, the optical fiber 110 traverses the plasma cell 21. The optical fiber 110 is devised such as thinning the clad (or removing the clad) so that light leaks out in the plasma cell 21.

There are various levels of atoms in the plasma 28. For example, in the case of argon discharge plasma, there exists a metastable excited atom with a level of 1s3, which absorbs light having a wavelength of about 772 nm and makes a transition to the 2p2 state. Since the density of the metastable excited atoms and the electron density in the plasma 28 are closely related, the electron density can be calculated and estimated by measuring the density of the metastable excited atoms, and feedback control can be performed.

Based on the above principle, 772 nm laser light is guided to the optical fiber 110. In the plasma cell 21, the 772 nm laser light leaks out from the optical fiber 110 as evanescent light. When the plasma 28 is generated in the plasma cell 21, the argon metastable excited atoms in the plasma 28 partially absorb the 772 nm laser light that has exuded from the optical fiber 110. Therefore, the intensity of the laser light transmitted through the plasma cell 21 is detected by a detector (not shown) (plasma spectroscopy). Thus, it is possible to calculate the density of the argon metastable excited atoms from the detected intensity, calculate the electron density from the calculation means, and adjust the discharge voltage and the like so as to obtain an appropriate electron density based on the result. it can. This discharge voltage adjustment is performed by the electrode pair 23.

As described above, in this embodiment, the electron density distribution adjusting means including the Langmuir probe 100 or the optical fiber 110 monitors the electron density of plasma by the Langmuir probe method or plasma spectroscopy. Then, feedback control of the electron density distribution of the plasma is performed based on the monitoring result.

According to the method of this embodiment, the electron density distribution of plasma can be adjusted more accurately, and beam modulation with better controllability can be achieved.

(Example 6)
A sixth embodiment of the present invention will be described with reference to FIG. FIG. 11 is a cross-sectional view of the plasma cell 21. FIG. In two places of the plasma cell 21 of the present embodiment, a flow path 120 for supplying and discharging gas to the plasma cell 21 and a micro valve 121 are installed. The gas pressure in the plasma cell 21 is adjusted using these. Since the electron density in the plasma 28 is closely related to the gas pressure, if the pressure in the plasma cell 21 can be adjusted in addition to the adjustment of the discharge voltage and the like, the electron density in the plasma 28 can be adjusted more finely.

In this embodiment, the electrode 23 and the power source 29 constitute the partition part generating means constituting the generating means, and the flow path 120 and the microvalve 121 constitute the electron density adjusting means constituting the electron density distribution adjusting means. To do. Of course, the electrode 23 and the power source 29 may constitute an electron density adjusting means together with the flow path 120 and the microvalve 121, and may also serve as the electron density adjusting means. In the first embodiment and the like, the plurality of electrodes 23 and the power source 29 provided in each plasma cell 21 serve as both the generating means and the electron density distribution adjusting means. Can be designed flexibly.

The present embodiment can be used for applications in which terahertz beam modulation does not require high speed but requires accuracy. For example, it can be used as a lens having a variable focal length.

(Example 7)
A seventh embodiment of the present invention will be described with reference to FIG. In this embodiment, imaging is performed using terahertz waves. While the target object 94 is fixed, the position of the condensing point is moved using the terahertz wave beam modulator 93 while condensing the terahertz wave at one point.

In this embodiment, the terahertz wave 91 emitted from the terahertz wave light source 90 is shaped into a condensed beam by the first lens 92. The terahertz wave that has become the focused beam is incident on the terahertz wave beam modulator 93. The terahertz wave modulation device 93 moves the position of the condensing point on the target object 94 from right to left by deflecting the direction of the condensing beam from right to left, for example.

Further, the terahertz wave condensed at one point on the target object 94 and reflected or scattered is condensed by another condensing lens 95 and detected by the terahertz wave detector 96. A terahertz wave image of the target object 94 can be constructed from the intensity of the detected terahertz wave and the condensing position of the terahertz wave.

Alternatively, the terahertz wave that has passed through the target object 94 may be collected by another condensing lens and detected by a terahertz wave detector.

Thus, in the imaging apparatus of the present embodiment, the object is irradiated with electromagnetic waves, and the electromagnetic wave modulation apparatus according to the present invention is used to move the electromagnetic wave irradiation position to detect the transmitted wave, reflected wave, or scattered wave of the electromagnetic wave. To obtain information for constructing an image of the object.

Using the beam modulation apparatus of the present invention, terahertz wave imaging can be performed in a space-saving and high-speed manner and with low dependence on the polarization direction of the terahertz wave. In addition, by providing an appropriate phase with the terahertz wave modulation device 93, it is possible not only to collect the terahertz wave at one point but also to project an arbitrary pattern such as a cross or a concentric circle onto the object. .

The schematic diagram of the electromagnetic wave modulation apparatus explaining the concept of this invention. BRIEF DESCRIPTION OF THE DRAWINGS Schematic explaining the terahertz wave beam modulation apparatus of embodiment and 1st Example of this invention. Sectional drawing which shows the plasma cell of the terahertz wave beam modulation apparatus of embodiment and 1st Example of this invention 1 is a perspective view for explaining a terahertz beam modulating apparatus according to a first embodiment of the present invention. 1 is a schematic diagram illustrating a terahertz beam modulating apparatus according to a first embodiment of the present invention. The perspective view explaining the terahertz wave beam modulation apparatus of the 2nd Example of this invention. FIG. 6 is a schematic diagram for explaining a waveguide type terahertz beam modulating apparatus according to a third embodiment of the present invention. Sectional drawing explaining the modification of the waveguide plasma cell of the waveguide type terahertz wave beam modulation apparatus of the 3rd Example of this invention. FIG. 10 is a schematic diagram for explaining a terahertz beam modulating apparatus that performs feedback control using a Langmuir probe according to a fourth embodiment of the present invention. FIG. 9 is a schematic diagram for explaining a terahertz beam modulating apparatus that performs feedback control using a laser beam according to a fifth embodiment of the present invention. FIG. 10 is a schematic diagram illustrating a terahertz beam modulating apparatus that performs gas pressure adjustment according to a sixth embodiment of the present invention. The perspective view explaining the terahertz wave imaging using the electromagnetic wave modulation apparatus of this invention.

Explanation of symbols

10 Plasma with electron density distribution
21, 71, 80 Electron density distribution adjustment means (compartments, plasma cells, waveguides)
23 Generating means, electron density distribution adjusting means (electron density adjusting means, electrodes)
24 Incident electromagnetic wave (incident terahertz wave)
25 Outgoing electromagnetic wave (outgoing terahertz wave)
26 Wavefront of incident electromagnetic wave (wavefront of incident terahertz wave)
27 Wavefront of outgoing electromagnetic wave (wavefront of outgoing terahertz wave)
28 Plasma
29 Generation means, electron density distribution adjustment means (electron density adjustment means, power supply)
81 Generation means, electron density distribution adjustment means (electron density adjustment means, electrode pair)
82 Generating means, electron density distribution adjusting means (electron density adjusting means, auxiliary electrode)
93 Terahertz beam modulator
94 objects
96 terahertz detector
100 Electron density distribution control means (Electron density control means, Langmuir probe)
101 Electron density distribution adjustment means (electron density adjustment means, external circuit)
110 Electron density distribution adjustment means (electron density adjustment means, optical fiber)
120 Electron density distribution adjusting means (electron density adjusting means, flow path)
121 Electron density distribution adjusting means (Electron density adjusting means, micro valve)

Claims (10)

An electromagnetic wave modulation device that changes a propagation state of an electromagnetic wave by forming a spatial electron density distribution in which the electron density of plasma changes in at least one direction in the wave front in a propagation path including a surface that receives the wave front of the electromagnetic wave. ,
An electromagnetic wave modulation device comprising: generating means for generating the plasma; and electron density distribution adjusting means for adjusting a spatial electron density distribution of the plasma. A plurality of compartments arranged adjacent to each other along the surface receiving the wavefront of the electromagnetic wave,
The generating means includes a plurality of compartment generating means for generating plasma in the plurality of compartments, and the electron density distribution adjusting means is a plurality of electrons for adjusting the electron density of the plasma in the plurality of compartments. 2. The electromagnetic wave modulation device according to claim 1, further comprising density adjusting means. 3. The electromagnetic wave modulation device according to claim 2, wherein the partition portion is a waveguide. 4. The electromagnetic wave modulation device according to claim 2, wherein a plurality of the partition portions are arranged at a period equal to or shorter than the wavelength of the electromagnetic wave along a surface that receives the wave front of the electromagnetic wave. 3. The plurality of electrodes are provided inside each partition part, and the plurality of electrodes in each partition part constitute both the generation unit or the generation unit and the electron density distribution adjustment unit. 5. The electromagnetic wave modulator according to any one of 4 to 4. 6. The electromagnetic wave modulation device according to claim 5, wherein at least one inter-electrode distance among the plurality of electrodes is equal to or less than a wavelength of the electromagnetic wave. 7. The electromagnetic wave modulation device according to claim 1, wherein the generation unit generates the plasma by adjusting a plasma frequency of the plasma to be equal to or lower than a frequency of the electromagnetic wave. 8. The electron density distribution adjusting means monitors the electron density of the plasma by Langmuir probe method or plasma spectroscopy, and feedback-controls the electron density distribution of the plasma based on the monitoring result. The electromagnetic wave modulation device according to any one of the above. 9. The electromagnetic wave modulation device according to claim 1, wherein the electromagnetic wave is a terahertz wave. An object is irradiated with electromagnetic waves, and the electromagnetic wave modulation device according to any one of claims 1 to 9 is used to move the irradiation position of the electromagnetic waves, thereby detecting transmitted waves, reflected waves, or scattered waves of the electromagnetic waves. An imaging apparatus characterized by obtaining information on an image of the object.

Images