JP5283981B2 - Photoconductive element for terahertz wave generation and detection - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photoconductive element for generating or detecting a terahertz wave that can adjust the propagation state, especially the frequency characteristics, of the terahertz wave. <P>SOLUTION: The photoconductive element for generating or detecting a terahertz wave includees: a carrier generation layer 2801; an electrode 2802; antennas 2804; and two adjusting stubs 2806 for adjusting a propagation state of the terahertz wave generated or detected by the carrier. The two adjusting stubs 2806 are conductors having length of wavelength &lambda; or less of the terahertz wave generated by the carrier. The two adjusting stubs 2806 are connected to an extended line in the longitudinal direction of the electrode 2802. An end 2807 of the adjusting stub is placed within a range of the wavelength &lambda; or less from joint 2808 between the antenna 2804 and the electrode 2802. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

The present invention relates to a terahertz wave generating / detecting photoconductive element that can be used for at least one of generation and detection of terahertz waves, and an analysis apparatus, imaging apparatus, communication apparatus, and the like using the photoconductive element. In particular, the present invention relates to a technique for adjusting the propagation state of a terahertz wave by an electrode structure constituting a photoconductive element. In this specification, the terahertz wave is used as a term indicating an electromagnetic wave that occupies an arbitrary frequency band in a frequency band of 0.03 THz to 30 THz.

In the terahertz wave band, there are characteristic absorption bands derived from the structures and states of various substances including biomolecules. In order to make use of these characteristics, inspection techniques for non-destructive analysis and identification of substances have been developed. In addition, it is expected to be applied to safe imaging technology that replaces X-rays and high-speed communication technology.

As an element for generating and detecting terahertz waves, there is a photoconductive element. The photoconductive element has a semiconductor layer that generates carriers. The photoconductive element has a plurality of electrodes patterned on the semiconductor layer in order to apply a voltage to the semiconductor layer. Here, the electrodes have a gap. In order to improve the coupling efficiency with the space, an electrode structure may be provided in part of the electrode. Here, the antennas have a gap, and carriers are generated from the semiconductor layer by irradiating the gap with light. The carrier is accelerated by applying a voltage between the electrodes, and a terahertz wave is generated from the photoconductive element.

In general, the propagation characteristics of terahertz waves in a photoconductive element depend on the propagation state of carriers in addition to the antenna structure. Here, the carrier propagation state includes the behavior of the carrier and the influence of the member used (for example, phonon absorption peculiar to the semiconductor substrate). For example, in the case of an antenna for transmitting and receiving radio waves, a bowtie antenna has a wider band characteristic than a dipole antenna. However, in the case of a photoconductive element, since it depends on the propagation state of carriers, it is possible to obtain a terahertz wave having a frequency characteristic with a wider band than using a bow tie antenna when using a dipole antenna. The above content is disclosed in Non-Patent Document 1. Further, Patent Document 1 discloses a technique for controlling a propagation state of a terahertz wave generated from the carrier or detected by the carrier by applying a bow tie antenna to the photoconductive element.
JP 2006-010319 A Appl.Optics36,7853 (1997)

As in the technique disclosed in Patent Document 1, it is possible to adjust the propagation state of the terahertz wave generated from the carrier or detected by the carrier by changing the antenna structure itself of the photoconductive element. Widely done. However, although such an adjustment method is one of effective methods, a structure manufacturing process is separately required.

An object of the present invention is to provide a photoconductive element capable of adjusting a propagation state of the terahertz wave, particularly a frequency characteristic.

In view of the above problems, a photoconductive element for generating or detecting terahertz waves according to the present invention has the following characteristics.
A photoconductive element for generating or detecting terahertz waves according to the first aspect of the present invention,
A carrier generation layer for generating carriers by light irradiation;
Two electrode portions comprising a strip line arranged in parallel to each other, made of a conductor provided on one surface of the carrier generation layer,
Each consisting of a conductor in contact with each of the two electrode portions, two antenna portions disposed opposite to each other with a gap for irradiating the carrier generation layer with light, and
Each comprising a conductor in contact with each of the two electrode portions, and having two adjustment stubs for adjusting a propagation state of a terahertz wave generated or detected by the carrier,
The two adjustment stubs have a length equal to or shorter than an operating wavelength λ of a terahertz wave generated or detected by the carrier, and are not longer than the wavelength λ from a connection portion between the antenna portion and the electrode portion. It is located. That is, at least a part of each adjustment stub is in the range of the wavelength λ or less from the connection portion of the antenna portion and the electrode portion.

A photoconductive element for generating or detecting terahertz waves according to the second aspect of the present invention,
A carrier generation layer for generating carriers by light irradiation;
Two electrode portions comprising a strip line arranged in parallel to each other, made of a conductor provided on one surface of the carrier generation layer,
Each consisting of a conductor in contact with each of the two electrode portions, two antenna portions disposed opposite to each other with a gap for irradiating the carrier generation layer with light, and
Each comprising a conductor in contact with each of the two electrode portions, and having two adjustment stubs for adjusting a propagation state of a terahertz wave generated or detected by the carrier,
The two adjustment stubs are conductors having a length equal to or shorter than the operating wavelength λ of the terahertz wave generated or detected by the carrier, and are connected on an extension line in the longitudinal direction of the strip line,
The end of the adjusting stub, characterized in that from the connection portion of the antenna portion and the electrode portion within a range of the wavelength lambda.

A photoconductive element for generating or detecting terahertz waves according to the third aspect of the present invention,
A carrier generation layer for generating carriers by light irradiation;
Two electrode portions comprising a strip line arranged in parallel to each other, made of a conductor provided on one surface of the carrier generation layer,
Each consisting of a conductor in contact with each of the two electrode portions, two antenna portions disposed opposite to each other with a gap for irradiating the carrier generation layer with light, and
Each comprising a conductor in contact with each of the two electrode portions, and having two adjustment stubs for adjusting a propagation state of a terahertz wave generated or detected by the carrier,
The two adjustment stubs have a length equal to or shorter than an operating wavelength λ of a terahertz wave generated or detected by the carrier, and a connection portion between the antenna unit and the electrode unit along an outer edge of the strip line To the electrode portion within a wavelength λ. That is, at least a part of each adjustment stub is connected to the electrode unit within a wavelength λ from the connection portion of the antenna unit and the electrode unit.

A photoconductive element for generating or detecting a terahertz wave according to the fourth aspect of the present invention,
A carrier generation layer that generates carriers by irradiating light; and
A first electrode provided on the surface of the carrier generation layer;
A second electrode provided on the surface of the carrier generation layer and arranged in parallel with the first electrode;
The first electrode and the second electrode are each configured to include an antenna portion, and the irradiation position for irradiating the light is between the antenna portions of the first electrode and the second electrode. Yes,
At least one of the end portions of the first electrode and the second electrode is provided at a distance within approximately twice the distance d between the first electrode and the second electrode from the irradiation position. It is characterized by.

A photoconductive element for generating or detecting a terahertz wave according to the fifth aspect of the present invention,
A carrier generation layer that generates carriers by irradiating light, a first electrode provided on a surface of the carrier generation layer, a first electrode provided on the surface of the carrier generation layer, and the first electrode A second electrode arranged in parallel with,
The first electrode and the second electrode are each configured to include an antenna portion, and the irradiation position for irradiating the light is between the antenna portions of the first electrode and the second electrode. Yes,
The said first electrode and said second electrode, is adjusted stubs disposed so as to face each other, approximately twice the distance from the irradiation position, and the at least the first electrode and the second electrode It is characterized by being a conductor provided at a distance of within. That is, at least a part of each adjustment stub is at a distance within approximately twice the interval from the irradiation position.

A photoconductive element according to a sixth aspect of the present invention includes a carrier generation layer for generating carriers, an antenna portion, an electrode portion, and a pair of adjustment stubs. The antenna section is formed by arranging a conductor facing a carrier generation layer with a gap therebetween. The electrode part is formed including a strip line composed of two conductors, and controls the propagation state of carriers generated in the gap between the antenna parts. The one or more pairs of adjusting stubs adjust the state of the generated terahertz wave or the detected terahertz wave. The antenna section is sandwiched between two conductors constituting the strip line and connected to the conductor. Further, the one or more pairs of adjustment stubs are conductors having a length equal to or shorter than an operating wavelength λ defined by a distance between two conductors constituting the strip line, and a pair of first adjustment stubs. Including at least one of a pair of second adjustment stubs. The first adjustment stub extends on the longitudinal extension of the stripline, the tip thereof, with respect to the connection portion of the antenna portion and the electrode portion is in the range within the wavelength lambda. The second adjusting stubs, along the outer edge of the two conductors constituting a strip line to the connection portion of the antenna portion and the electrode portion are connected to within the wavelength lambda. That is, at least a portion of each second adjusting stubs are said antenna portion from the connecting portion of the electrode portion within a range of the wavelength lambda.

The photoconductive device according to the seventh aspect of the present invention has the following characteristics. That is, the photoconductive element is provided on a surface of the carrier generation layer, a carrier generation layer that generates carriers by irradiating light, a first electrode provided on the surface of the carrier generation layer, and And a second electrode arranged in parallel with the first electrode. Each of the first electrode and the second electrode includes an antenna portion, and the irradiation position for irradiating the light is between the antenna portions of the first electrode and the second electrode. Further, the adjustment stub disposed so as to face the first electrode and the second electrode is at least approximately twice the distance between the first electrode and the second electrode from the irradiation position. It is a conductor provided at a distance of. That is, at least a part of each adjustment stub is at a distance within approximately twice the interval.

Further, in view of the above problems, the imaging apparatus of the present invention includes the photoconductive element, an ultrashort pulse laser that generates a carrier by applying an output to a gap between the antenna portions, and the electrode that controls a propagation state of the generated carrier. And a drive driver connected to the unit. Then, the terahertz wave generated from the photoconductive element is irradiated to the measurement object, and the internal structure information of the measurement object is acquired from the terahertz wave reflected at the surface of the measurement object and the internal refractive index interface. Here, the frequency characteristic of the photoconductive element is desirably flattened.

In view of the above problems, the communication device according to the present invention includes the photoconductive element, an ultrashort pulse laser that generates a carrier by applying an output to the gap between the antenna units, and an electrode unit that controls a propagation state of the generated carrier. At least a drive driver and a modulation unit. The modulation unit modulates a signal supplied from the drive driver to the electrode unit or an output of the ultrashort pulse laser according to transmission information. Then, communication is performed using terahertz waves in which the frequency characteristics are unevenly distributed so as to reduce components in an absorption wavelength region unique to the terahertz wave band existing in the atmosphere.

According to the photoconductive element of the present invention, as described above, the terahertz generated from the carrier or detected by the carrier is provided by providing the adjustment stub with respect to the electrode portion or by arranging the end portion of at least one of the electrodes. The wave propagation state, particularly the frequency characteristics can be adjusted. In addition, the arrangement of the adjustment stub or the arrangement of the end of the electrode can provide a terahertz wave having frequency characteristics suitable for an application using a photoconductive element.

Embodiments capable of implementing the idea of the present invention will be described with reference to the drawings. The present invention is not limited to the embodiments and examples to be described later, and such forms are not excluded as long as they do not depart from the spirit of the present invention. Here, as described above, the terahertz wave is used as a term indicating an electromagnetic wave that occupies an arbitrary frequency band in a frequency band of 0.03 THz to 30 THz.

An embodiment of a photoconductive element for generating or detecting terahertz waves according to the present invention will be described.

First, a photoconductive element according to an embodiment has a carrier generation layer for generating carriers by light irradiation. As the carrier generation layer, low-temperature grown gallium arsenide (LT-GaAs) or indium gallium arsenide (InGaAs) can be used.

Next, the photoconductive element according to the present embodiment has two electrode portions including strip lines arranged in parallel to each other. Here, the electrode part is a conductor provided on one surface (surface irradiated with light) of the carrier generation layer. For example, the width of the strip lines can be 10 μm, and the distance between the strip lines can be 30 μm. In the present specification, the distance d between the strip lines may be referred to as an antenna length. At this time, a terahertz wave having a wavelength λ (= 2d) that is approximately twice the antenna length d is generated.

In addition, the photoconductive element according to the present embodiment has two antenna portions arranged to face each other with a gap for irradiating the carrier generation layer with light. Here, each of the antenna portions is a conductor in contact with the two electrode portions. The gap can be, for example, 5 μm, is wide enough to irradiate the carrier generation layer with light, and the carrier generated from the carrier generation layer is accelerated when a voltage is applied to the electrode portion. It is desirable to be narrow.

The photoconductive element according to this embodiment has two adjustment stubs for adjusting the propagation state of the terahertz wave generated or detected by the carrier. Here, the two adjustment stubs are conductors in contact with the two electrode portions, respectively.

Further, the two adjustment stubs have a length equal to or shorter than the wavelength λ of the terahertz wave generated by the carrier, and are positioned in a range equal to or smaller than the wavelength λ from a connection portion between the antenna unit and the electrode unit. Yes.

By changing the length and arrangement of the adjustment stub included in the photoconductive element according to the present embodiment, it is possible to adjust the propagation state such as flattening and uneven distribution of the frequency band of the terahertz wave. Here, in this specification, planarization is defined as follows. That is, the frequency characteristic of the terahertz wave distributed in a range in which the intensity of the terahertz wave is attenuated by −3 dB from the maximum value. Moreover, in this specification, uneven distribution is defined as follows. That is, the frequency characteristic of the terahertz wave in which the intensity of the terahertz wave is concentrated and distributed in a certain frequency band.

Next, the length and arrangement of the adjustment stub or the end of the electrode in order to control the propagation state will be described.

(First and second adjustment stubs)
The present embodiment relates to a technique for adjusting frequency characteristics of a photoconductive element for generating or detecting terahertz waves. Specifically, the frequency characteristics of the photoconductive element are adjusted by separately adding an adjustment stub to the electrodes (including the antenna structure and the control electrode) constituting the photoconductive element. This adjustment stub is a pair of the above-mentioned first adjustment stub or electrode end (2606 in FIG. 26 and 2806 in FIG. 28) and a second adjustment stub (2506 in FIG. 25 and FIG. 27). 2706). The first adjustment stub extends on the extension line in the longitudinal direction of the strip line as described in Example 1 described later, and the tip thereof is within the wavelength λ with respect to the connection portion between the antenna portion and the electrode portion. Is in range. The second adjustment stub is within the wavelength λ range with respect to the connection portion of the antenna portion and the electrode portion along the outer edges of the two conductors constituting the strip line, as described in Example 3 described later. Connected. When a photoconductive element is used as a generating element, the frequency energy distribution of the generated terahertz wave can be adjusted to provide a terahertz wave suitable for the application to be used.

The first and second adjustment stubs will be described in detail below.
(A) First Adjustment Stub A photoconductive element for generating or detecting a terahertz wave according to the present embodiment will be described with reference to FIG.

First, 2801 is a carrier generation layer for generating carriers by light irradiation. Next, reference numeral 2802 denotes two electrode portions including strip lines arranged in parallel to each other. Here, the electrode portion 2802 is a conductor provided on one surface (surface to which light is irradiated) of the carrier generation layer 2801. Reference numeral 2804 denotes two antenna portions arranged opposite to each other with a gap for irradiating the carrier generation layer 2801 with light. Here, the antenna portion 2804 is a conductor that is in contact with the two electrode portions 2802, respectively. Reference numeral 2806 denotes two adjustment stubs for adjusting the propagation state of the terahertz wave generated or detected by the carrier. Here, the two adjustment stubs 2806 are conductors in contact with the two electrode portions 2802, respectively.

Further, the two adjustment stubs 2806 are conductors having a length equal to or shorter than the wavelength λ of the terahertz wave generated by the carrier. Here, the wavelength λ is approximately twice as long as the distance d between the two strip lines (electrode portions 2802). The two adjustment stubs 2806 are connected to an extension line in the longitudinal direction of the strip line (electrode portion 2802). Furthermore, an end portion (or an end portion of the electrode portion 2802) 2807 of the adjustment stub is within a wavelength λ from a connection portion 2808 between the antenna portion 2804 and the electrode portion 2802.

(A-1) Flattening In the photoconductive element according to this embodiment according to another embodiment, a configuration in which the frequency characteristic of the terahertz wave generated from the carrier or detected by the carrier is flattened will be described below. First, the two adjustment stubs 2806 are connected to an extension line in the longitudinal direction of the strip line (electrode portion 2802). The lengths of the two adjustment stubs 2806 are 0.5λ to 0.8λ from the connection portion 2808 between the antenna portion 2804 and the electrode portion 2802. The flattening when the length of the adjustment stub is described in detail in Example 1 below.

(A-2) Uneven distribution In the photoconductive element according to this embodiment, the configuration in which the frequency characteristics of the terahertz wave generated from the carrier or detected by the carrier is unevenly distributed will be described below. First, the two adjustment stubs 2806 are connected to an extension line in the longitudinal direction of the strip line (electrode portion 2802). In addition, the lengths of the two adjustment stubs 2806 are the connection portions 2808 to 0.1λ to 0.5λ of the antenna portion 2804 and the electrode portion 2802. The uneven distribution when the length of the adjustment stub is described in detail in Example 2 below.

(A-3) A photoconductive element for generating or detecting a terahertz wave according to this embodiment, which is defined by the interval d between the electrodes, will be described with reference to FIG. Here, in the above-described embodiment, the length and position of the adjustment stub are defined by the wavelength λ, but here are defined by the distance d between the electrodes.

First, reference numeral 2601 denotes a carrier generation layer that generates carriers when irradiated with light. Next, reference numeral 2602 denotes a first electrode provided on the surface of the carrier generation layer 2601. Reference numeral 2603 denotes a second electrode provided on the surface of the carrier generation layer 2601 and arranged in parallel with the first electrode 2602. Each of the first electrode 2602 and the second electrode 2603 includes an antenna portion 2604. An irradiation position 2605 for irradiating light is located between the antenna portion 2604 included in the first electrode 2602 and the second electrode 2603. Further, at least one of the end portions 2607 of the first electrode 2602 and the second electrode 2603 has an interval d between the first electrode 2602 and the second electrode 2603 from the irradiation position 2605. It is provided at a distance within about twice.

(B) A photoconductive element for generating or detecting a terahertz wave according to this embodiment according to the second adjustment stub will be described with reference to FIG.

First, reference numeral 2701 denotes a carrier generation layer for generating carriers by light irradiation. Next, reference numeral 2702 denotes two electrode portions including strip lines arranged in parallel to each other. Here, each of the two electrode portions 2702 is a conductor provided on one surface of the carrier generation layer (a surface on which light is irradiated). Reference numeral 2704 denotes two antenna portions arranged opposite to each other with a gap for irradiating the carrier generation layer 2701 with light. Here, the antenna unit 2704 is a conductor in contact with the two electrode units 2702, respectively. Reference numeral 2706 denotes two adjustment stubs for adjusting the propagation state of the terahertz wave generated or detected by the carrier. Here, the two adjustment stubs 2706 are conductors in contact with the two electrode portions 2702, respectively. The two adjustment stubs 2706 have a length equal to or shorter than the wavelength λ of the terahertz wave generated by the carrier.

Further, the two adjustment stubs 2706 are arranged along the outer edge of the strip line (electrode part 2702) from the connection part 2708 between the antenna part 2704 and the electrode part 2702 within the wavelength λ. Connect to.

(B-1) Flattening In the photoconductive element according to this embodiment according to this embodiment, a configuration for flattening the frequency characteristic of the terahertz wave generated from the carrier or detected by the carrier will be described below. . First, the lengths of the two adjustment stubs 2706 are 0.1λ to 0.2λ. Further, the two adjustment stubs 2706 are located along the outer edge of the electrode portion 2702 (strip line) from the connection portion 2708 of the antenna portion 2704 and the electrode portion 2702 at a position equal to the length of the adjustment stub 2706. It is connected. The flattening at the length of the adjustment stub will be described in detail in Example 3 below.

(B-2) Uneven distribution In the photoconductive element according to this embodiment, a configuration for unevenly distributing the frequency characteristics of the terahertz wave generated from the carrier or detected by the carrier will be described below. . First, the length of the two adjustment stubs 2706 is 0.2λ to 0.5λ. Further, the two adjustment stubs 2706 are located along the outer edge of the electrode portion 2702 (strip line) from the connection portion 2708 of the antenna portion 2704 and the electrode portion 2702 at a position equal to the length of the adjustment stub 2706. It is connected. The uneven distribution when the length of the adjustment stub is described in detail in Example 4 below.

(B-3) A photoconductive element for generating or detecting a terahertz wave according to the present embodiment, which is defined by the interval d between the electrodes, will be described with reference to FIG. Here, in the above-described embodiment, the length and the position of the second adjustment stub are defined by the wavelength λ, but here are defined by the distance d between the electrodes.

This photoconductive element first has a carrier generation layer 2501 that generates carriers by irradiating light. The carrier generation layer 2501 will be described in detail in the following examples. Further, the first electrode 2502 provided on the surface of the carrier generation layer 2501 is provided. Further, a second electrode 2503 provided on the surface of the carrier generation layer 2501 and arranged in parallel with the first electrode 2502 is provided. The first electrode 2502 and the second electrode 2503 will be described in detail in the following examples. Each of the first electrode 2502 and the second electrode 2503 includes an antenna (antenna portion) 2504. The antenna (antenna unit) 2504 will also be described in detail in the following examples.

Further, the irradiation position 2505 for irradiating light is between the antenna portions of the first electrode 2502 and the second electrode 2503. In addition, a pair of adjustment stubs 2506 is provided. The adjustment stub 2506 is a conductor and is disposed on the first electrode 2502 and the second electrode 2503. The adjustment stubs 2506 are disposed so as to face each other. The adjustment stub 2506 is provided at a distance that is at least approximately twice the distance d between the first electrode 2502 and the second electrode 2503 from the irradiation position 2505. Here, the position where the adjustment stub is provided is not limited to the case where the adjustment stub is provided as shown in FIG. For example, the adjustment stub and the first electrode or the second electrode may be in a positional relationship as shown in FIG. As illustrated in FIG. 25, the wavelength λ according to the present photoconductive element is approximately twice the distance d. The adjustment stub 2506 is also described in detail in the following example.

Hereinafter, more specific embodiments will be described with reference to the drawings.
(Example 1: flattening with the first adjustment stub)
FIG. 1 is a diagram showing the configuration of Example 1 of a photoconductive element that can implement the present invention. Specifically, the present embodiment relates to a configuration example for flattening the frequency characteristics of the photoconductive element. As shown in FIG. 1, the photoconductive element of this embodiment includes an antenna unit 101, an adjustment stub 102 that is a first adjustment stub that forms a pair, a carrier generation layer 103, an electrode unit 104, and a substrate 105.

The carrier generation layer 103 has a function of generating carriers by excitation light irradiated from the outside (this is irradiated to a minute gap of the antenna unit 101). In this embodiment, low-temperature grown Gaurim arsenic (LT-GaAs) is used as the carrier generation layer 103. The carrier generation layer 103 is formed, for example, by molecular beam low-temperature epitaxial growth (growth temperature 250 ° C.) on a semi-insulating gallium arsenide (SI-GaAs) substrate. The material of the carrier generation layer 103 is not limited to this, and for example, a semiconductor material such as indium gallium arsenide (InGaAs) may be used.

The substrate 105 is a substrate that holds the carrier generation layer 103. In this embodiment, as the substrate 105, for example, the above-described SI-GaAs substrate is used as it is. In order to avoid unnecessary absorption of the terahertz wave by the substrate 105, a substrate transparent to the terahertz wave may be used. As the substrate 105, for example, high resistance silicon (Si) can be used. In this case, a method of removing the semiconductor substrate used for the growth of the carrier generation layer 103 by etching and fixing the carrier generation layer 103 to the substrate 105 by a bonding means such as an adhesive is used.

The antenna unit 101 is composed of a conductor having a minute gap. The above-described carrier generation layer 103 exists at least immediately below the minute gap of the antenna unit 101. The antenna unit 101 is a part that determines the approximate operating frequency and frequency characteristics of the photoconductive element. In the photoconductive element, the antenna length of the antenna unit 101 (equal to the distance d of the electrode unit 104) determines the operating frequency λ (approximately λ = 2d), and the conductor width and shape determine the frequency characteristics.

In this embodiment, a dipole antenna structure is used as the antenna unit 101. The antenna length is 30 μm. The conductor width is 10 μm. The minute gap is 5 μm. However, it is not limited to these numerical values, and can be changed according to the desired characteristics of the terahertz wave. The same applies to the antenna structure type.

The dipole antenna constitutes a half-wave resonator. Therefore, if the operating wavelength of the antenna is λ, the antenna length corresponds to approximately 1 / 2λ. In this specification, the operation wavelength λ assumed from this antenna length (which is also the distance d between the electrode portions 104) will be described as a reference. However, the actual frequency characteristics in the photoconductive element are influenced not only by the antenna length but also by the carrier behavior and the substrate characteristics. Therefore, the frequency characteristic assumed from the antenna shape may differ from the actual frequency characteristic of the photoconductive element.

The electrode section 104 is composed of a strip line 104b and an electrode pad 104a extending in parallel. The electrode pad is mainly used for connection with an external device. The tip of the strip line is configured such that two conductors constituting the strip line sandwich the antenna unit 101. As shown in FIG. 1, a pair of adjustment stubs 102 are connected in the longitudinal direction of two conductors constituting the strip line. In this embodiment, the conductor width of the strip line 104b is 10 μm, and the conductor interval d is 30 μm. The conductor distance d corresponds to the antenna length of the antenna unit 101 (that is, approximately 1 / 2λ).

These conductor structures are collectively patterned by a general process technique such as vapor deposition. Therefore, the adjustment stub 102 can be formed relatively easily.

When the photoconductive element of this embodiment is used as a terahertz wave generating element, the electrode unit 104 is used to apply a desired electric field to the gap of the antenna unit 101. When the photoconductive element is used as a terahertz wave detection element, the electrode unit 104 is used to detect a signal (specifically, current) that propagates through the electrode unit 104.

The adjustment stub 102 is used to adjust the frequency characteristic of the photoconductive element. In this embodiment, the frequency characteristics are flattened. The adjustment stub 102 flattens the frequency characteristic by causing a part of the terahertz wave propagating through the antenna unit 101 to interfere. In this specification, the flattening of the frequency characteristic is defined as a state where the intensity distribution of the terahertz wave is distributed over a wide range. More specifically, in this specification, flattening is defined as distributing more terahertz waves in a frequency range in which -3 dB is attenuated from the maximum intensity value of the terahertz waves. In this embodiment, the adjustment stub 102 is a linear line having a conductor width of 10 μm.

Summarizing the above configuration, the photoconductive element of this example has the following configuration. That is, the photoconductive element includes a carrier generation layer 103 for generating carriers, an antenna unit 101, an electrode unit 104, and a pair of adjustment stubs 101. The antenna unit 101 is formed by arranging a conductor facing the carrier generation layer 103 with a gap therebetween. The electrode unit 104 is formed including a strip line 104b composed of two conductors, and controls the propagation state of carriers generated in the gap between the antenna units 101. The pair of adjustment stubs 102 adjust the state of the generated terahertz wave or the detected terahertz wave. The antenna unit 101 is sandwiched between two conductors constituting the strip line 104b and connected to the conductor.

Furthermore, the adjustment stub 102 includes a first adjustment stub that forms a pair that is a conductor having a length L1 that is equal to or shorter than the wavelength λ defined by the antenna length of the antenna unit 101. The first adjustment stub extends on an extension line in the longitudinal direction of the strip line 104b, and the tip of the first adjustment stub is within a wavelength λ with respect to the connection portion between the antenna unit 101 and the electrode unit 104. Further, the end of the two conductors constituting the strip line 104b constituting the electrode part 104, the one having the antenna part 101 is located at a wavelength λ or less from the connection part of the antenna part 101 and the electrode part 104,

The operation of this embodiment will be described. The principle of operation is the same as that of a normal photoconductive element. That is, when this photoconductive element is used as a generating element, a desired electric field is applied to the electrode portion 104. In this embodiment, 10 V is biased. In this state, an ultrashort pulse laser is irradiated to the minute gap of the antenna unit 101. In this embodiment, a titanium sapphire laser having a pulse width of 50 fsec and a repetition frequency of 76 MHz is used as a light source. Carriers are generated from the carrier generation layer 103 when an ultrashort pulse is applied to the minute gap of the antenna portion 101. Dipole radiation obtained by accelerating the generated carriers by the electric field applied by the electrode unit 104 is used as a terahertz wave.

Further, when the present photoconductive element is used as a detection element, a current detector is connected to the electrode portion 104. In synchronization with the timing at which the terahertz wave enters the detection element, the ultra-short pulse laser is irradiated onto the minute gap of the antenna unit 101. At this time, carriers generated from the carrier generation layer 103 induced by the terahertz wave are detected by a current detector. More specifically, the time waveform of the terahertz wave is detected by sequentially shifting the incident timing of the terahertz wave and the ultrashort pulse laser and plotting the current value at each timing.

As described above, the adjustment stub 102 is used to flatten the frequency characteristics of the photoconductive element. FIG. 12 shows an electromagnetic field analysis result showing the effect of the adjustment stub 102. The stub length L1 of the adjustment stub 102 is 0.6λ. The broken line indicates the frequency characteristic of the photoconductive element when the adjustment stub 102 is not provided, and the solid line indicates the frequency characteristic of the photoconductive element when the adjustment stub 102 is connected. As a guideline for the flatness of the frequency characteristics, comparing the bandwidth at the position where the intensity is −3 dB, it can be seen that the flatness is improved about 1.2 times by connecting the adjustment stub 102. In this embodiment, the stub length L1 of the adjustment stub 102 is adjusted so that this bandwidth is widened.

FIG. 13 is an analysis result showing a change in bandwidth at a position of −3 dB when the stub length L1 of the adjustment stub 102 is changed from 0 to λ. In FIG. 13, 0λ corresponds to the case where the adjustment stub 102 is not provided. For example, based on this state, it can be seen that the flatness of the frequency characteristics is improved by changing the stub length L1 of the adjustment stub 102 from 0.5 to 0.8λ. That is, in the present embodiment, the adjustment stub 102 is connected to the connecting portion between the antenna portion and the electrode portion on the extension line in the longitudinal direction of the strip line, has a length of 0.5λ to 0.8λ, and has a photoconductive property. The frequency characteristics of the element are flattened.

In this embodiment, the adjustment stub 102 is handled as a straight line, but this is not restrictive. For example, a tapered stub can be used. The track width also changes depending on the situation. The point is that the structure of the adjustment stub 102 capable of adjusting the frequency characteristics of the photoconductive element does not depart from the gist of the present invention.

As described above, by adjusting the stub length of the adjustment stub 102, it is possible to easily adjust the frequency characteristics of the photoconductive element. Then, by flattening the frequency characteristics, it becomes easy to provide a terahertz wave having a shape closer to a monocycle.

Here, as a general circuit technique, Japanese Patent No. 3165653 discloses a method of adjusting the antenna characteristics by causing interference between radio waves by using a stub structure or the like for the antenna. This prior document only teaches the use of a stub structure for a so-called Yagi-Uta antenna, and there is no disclosure or suggestion regarding terahertz waves. There is no disclosure or suggestion regarding a photoconductive element for generating or detecting terahertz waves or an antenna in contact with an electrode deposited on a layer for generating carriers.

As described above, the propagation characteristic of the terahertz wave in the photoconductive element generally depends on the propagation state of the carrier in addition to the antenna structure. Here, the carrier propagation state includes the behavior of the carrier and the influence of the member used (for example, phonon absorption peculiar to the semiconductor substrate). For this reason, it is difficult to obtain a desired propagation state even if an existing circuit control technique (antenna technique or distributed constant circuit technique) used in millimeter waves or microwaves is simply applied to the terahertz wave technique. The adjustment stub according to the present invention is a device for applying to the terahertz wave technology, and solves a problem peculiar to the terahertz wave of controlling the propagation state of the carrier. Therefore, the conventional antenna for transmitting and receiving radio waves as in the above-mentioned prior art document is not simply diverted to the conventional photoconductive element as it is.

(Example 2: Uneven distribution with the first adjustment stub)
The present embodiment relates to the photoconductive element having the basic structure described in the first embodiment, but is an example in which frequency characteristics different from the first embodiment are unevenly distributed. The description of the same parts as those in the first embodiment is omitted. FIG. 1 also shows the configuration of a photoconductive element according to the second embodiment that can implement the present invention. However, the length L1 of the adjustment stub 102 is different from that of the first embodiment.

FIG. 14 is an electromagnetic field analysis result showing the effect of the adjustment stub 102. The stub length L1 of the adjustment stub 102 of the present embodiment is 0.5λ. The broken line indicates the frequency characteristic of the photoconductive element when the adjustment stub 102 is not provided, and the solid line indicates the frequency characteristic of the photoconductive element when the adjustment stub 102 is connected. In the present specification, the uneven distribution of frequency characteristics is defined as a state in which intensity is concentrated in a certain frequency band among substantially flat frequency characteristics. For example, in FIG. 14, a strong intensity distribution exists in the vicinity of 0.8 THz due to the adjustment stub 102.

FIG. 15 is an analysis result showing the shift amount of the center frequency of the strong intensity distribution that appears by providing the adjustment stub 102 with respect to the center frequency in a state where there is no adjustment stub 102 (that is, corresponding to the broken line in FIG. 14). As can be seen from FIG. 15, the frequency shift amount increases almost linearly with respect to the stub length L1 of the adjustment stub 102 from 0λ to 0.5λ. Also in this embodiment, the adjustment stub 102 is patterned by a vapor deposition process. When the process fabrication error is converted, the micro-order fabrication accuracy is an error of the order of 0.01λ. Considering this, in this embodiment, the region where the characteristics of the photoconductive element can be adjusted with high accuracy is defined as 0.1λ to 0.5λ. In other words, in this embodiment, the adjustment stub 102 is connected to the connecting portion between the antenna portion and the electrode portion on the extension line in the longitudinal direction of the strip line, has a length of 0.1λ to 0.5λ, and has a photoconductive property. The frequency characteristics of the elements are unevenly distributed.

In this embodiment, the adjustment stub 102 is handled as a straight line, but this is not restrictive. For example, a tapered stub can be used. The track width also changes depending on the situation. The point is that the structure of the adjustment stub 102 capable of adjusting the frequency characteristics of the photoconductive element does not depart from the gist of the present invention.

FIG. 21 shows the measurement results of the photoconductive element of this example. Here, LT-GaAs having a thickness of 2 μm is used as the carrier generation layer 103. An SI-GaAs substrate is used as the substrate 105. As the antenna unit 101, a dipole antenna structure is used. The antenna portion 101 has an antenna length of 30 μm and a conductor width of 10 μm. As described above, the antenna unit 101 has a minute gap, and the minute gap is 5 μm. The length of the strip line 104b constituting the electrode unit 104 is 2 mm. And the shape of the electrode pad 104a which comprises the electrode part 104 is a 500 micrometer x 500 micrometer pad. The stub length L1 of the adjustment stub 102 is 10 μm. Here, since the antenna length is 30 μm, the stub length L1 corresponds to approximately 0.15λ.

In FIG. 21, this photoconductive element is applied to a terahertz wave generating element, and the characteristics are measured (solid line). At this time, a photoconductive element having the same configuration but having no adjustment stub is used as the terahertz wave detecting element. Further, in FIG. 21, in order to verify the effect of the adjustment stub 102, the measurement results were compared using a terahertz wave detection element as a common element and a photoconductive element without the adjustment stub 102 as a terahertz wave generation element (broken line). According to FIG. 21, it can be seen that the intensity at the portion exceeding 1.2 THz decreases and the intensity from 0.5 THz to around 1.0 THz increases. FIG. 14 shows the analysis result when the stub length L1 is 0.5λ, and it can be seen that the same tendency as in FIG. 21 is shown. Further, FIG. 15 shows that when the stub length L1 is 0.16λ, the frequency characteristic of the terahertz wave is slightly uneven on the low frequency side by about 0.15 THz. Looking at the measurement results in FIG. 21, it can be confirmed that the produced photoconductive elements are shifted to the low frequency side in the same order.

As described above, by adjusting the stub length of the adjustment stub 102, it is possible to easily adjust the frequency characteristics of the photoconductive element. By selecting the structure of the adjustment stub 102 together with the size and structure of the antenna unit 101, it is easy to realize high-efficiency transmission using, for example, terahertz waves that avoid the characteristic absorption spot of water vapor It becomes.

(Example 3: Planarization with the second adjustment stub)
FIG. 2 is a diagram showing a configuration of Example 3 of a photoconductive element that can implement the present invention. Specifically, an example of flattening the frequency characteristics of the photoconductive element is shown. The description of the same parts as those in the above embodiment is omitted. What is different from the previous embodiments is the connection position of the adjustment stub 202. In the present embodiment, a pair of adjustment stubs 202 are connected at intervals of L2 along the outer edges of the two conductors constituting the strip line. The stub length of the adjustment stub 202 has the same length as the connection position interval L2.

In summary, the photoconductive element of this example has the following configuration. That is, the photoconductive element includes a carrier generation layer 103 for generating carriers, an antenna unit 101, an electrode unit 104, and an adjustment stub 202 that forms one or more pairs. The antenna unit 101 is formed by arranging a conductor facing the carrier generation layer 103 with a gap therebetween. The electrode unit 104 is formed including a strip line 104b composed of two conductors, and controls the propagation state of carriers generated in the gap between the antenna units 101. The pair of adjustment stubs 202 adjust the state of the generated terahertz wave or the detected terahertz wave. The antenna unit 101 is sandwiched between two conductors constituting the strip line 104b and connected to the conductor.

Furthermore, the adjustment stub 202 includes a second adjustment stub that forms a pair that is a conductor having a length equal to or shorter than the wavelength λ defined by the antenna length of the antenna unit 101 (the distance d between the electrode units 104). The second adjustment stub is connected to the connection portion of the antenna portion and the electrode portion within a wavelength λ along the outer edges of the two conductors constituting the strip line 104b. Further, the end of the two conductors constituting the strip line 104b constituting the electrode part 104, the one having the antenna part 101 is located at a wavelength λ or less from the connection part of the antenna part and the electrode part,

FIG. 16 is an electromagnetic field analysis result showing the effect of the adjustment stub 202. The connection position L2 and the stub length L2 of the adjustment stub 202 are 0.15λ. The broken line indicates the frequency characteristic of the photoconductive element when the adjustment stub 202 is not provided, and the solid line indicates the frequency characteristic of the photoconductive element when the adjustment stub 202 is connected. Similar to Example 1, when the bandwidths at positions where the intensity is −3 dB are compared, it is found that the flatness is improved by about 1.4 times by connecting the adjustment stub 202. Also in the present embodiment, the stub length L2 and the connection position L2 of the adjustment stub 202 are adjusted so that this bandwidth is widened.

FIG. 17 is an analysis result showing a change in bandwidth at a position of −3 dB when the stub length of the adjustment stub 202 and the connection position L2 are changed. As in the first embodiment, 0λ in FIG. 17 corresponds to the case where the adjustment stub 202 is not provided. For example, based on this state, it can be seen that the flatness of the frequency characteristics is improved by setting the stub length of the adjustment stub 202 and the connection position L2 to 0.2λ or less. Also in this embodiment, the adjustment stub 202 is patterned by a vapor deposition process. When the process fabrication error is converted, the micro-order fabrication accuracy is an error of the order of 0.01λ. Considering this, in this embodiment, the region where the characteristics of the photoconductive element can be adjusted with high precision is defined as 0.1λ to 0.2λ. That is, in this embodiment, the adjustment stub 202 as the second adjustment stub has a length of 0.1λ to 0.2λ. Then, along the outer edges of the two conductors constituting the strip line 104b, the connection portion of the antenna portion and the electrode portion is connected at a position equal to the length of the adjustment stub 202, and the frequency characteristic of the photoconductive element is flattened. It has become.

In this embodiment, the adjustment stub 202 is handled as a straight line, but this is not restrictive. For example, a tapered stub can be used. The track width also changes depending on the situation. In this embodiment, the line length and the connection position of the adjustment stub 202 are the same length, but this is not restrictive. The point is that the structure of the adjustment stub 202 capable of adjusting the frequency characteristics of the photoconductive element does not depart from the gist of the present invention.

FIG. 22 shows the measurement results of the photoconductive element of this example. Here, LT-GaAs having a thickness of 2 μm is used as the carrier generation layer 103. An SI-GaAs substrate is used as the substrate 105. As the antenna unit 101, a dipole antenna structure is used. The antenna portion 101 has an antenna length of 30 μm and a conductor width of 10 μm. As described above, the antenna unit 101 has a minute gap, and the minute gap is 5 μm. The length of the strip line 104b constituting the electrode unit 104 is 2 mm. And the shape of the electrode pad 104a which comprises the electrode part 104 is a 500 micrometer x 500 micrometer pad. The stub length L2 and the connection position L2 of the adjustment stub 202 are 10 μm. Here, since the antenna length is 30 μm, the stub length L2 and the connection position L2 correspond to approximately 0.15λ.

The configuration of the photoconductive element used in FIG. 22 is the same as the element configuration used in the analysis of FIG. As described above, when the bandwidths at positions where the intensity is −3 dB are compared, it can be confirmed that the band characteristics of the measurement results are improved by about 1.5 times in flatness and show the same tendency as the analysis results. FIG. 23 shows the measurement result of the time waveform of the terahertz wave generated from the photoconductive element used in FIG. As described above, by improving the flatness of the frequency characteristics by the adjustment stub 202, a terahertz wave having a shape closer to a monocycle can be acquired.

As described above, the frequency characteristic of the photoconductive element can be easily adjusted by adjusting the stub length of the adjustment stub 202 which is the second adjustment stub. By flattening the frequency characteristics, it becomes easy to provide a terahertz wave having a shape closer to a monocycle.

(Example 4: Uneven distribution with the second adjustment stub)
The present embodiment relates to the photoconductive element described in the third embodiment, and is an example in which frequency characteristics are unevenly distributed as in the second embodiment. The description of the same parts as those in the above embodiment is omitted. FIG. 2 also shows a configuration example of Embodiment 4 of the photoconductive element in which the present invention can be implemented. However, the length and connection position of the adjustment stub 202 are different from those in Embodiment 3.

FIG. 18 is an electromagnetic field analysis result showing the effect of the adjustment stub 202. The stub length and the connection position L2 of the adjustment stub 202 are 0.5λ. A broken line is a frequency characteristic when the adjustment stub 202 is not provided, and a solid line indicates a frequency characteristic of the photoconductive element when the adjustment stub 202 is connected. As defined in the second embodiment, in this specification, the uneven distribution of frequency characteristics is defined as a state in which intensity is concentrated only in a certain frequency band among substantially flat frequency characteristics. For example, in FIG. 18, it can be seen that the adjustment stub 202 suppresses the intensity near 0.7 THz and a strong intensity distribution exists near 0.4 THz.

FIG. 19 is an analysis result showing the shift amount of the center frequency of the strong intensity distribution that appears by the adjustment stub 202 with respect to the center frequency without the adjustment stub 102 (that is, corresponding to the broken line in FIG. 18). As shown in FIG. 19, it can be seen that the frequency shift amount tends to increase with respect to the stub length of the adjustment stub 202 from 0λ to 0.5λ and the connection position L2. As described in the third embodiment, the adjustment stub 202 greatly contributes to frequency flattening up to 0.2λ. Therefore, in this embodiment, the region where the characteristics of the photoconductive element can be accurately adjusted is defined as 0.2λ to 0.5λ. That is, in this embodiment, the adjustment stub 202 as the second adjustment stub has a length of 0.2λ to 0.5λ. Then, along the outer edges of the two conductors constituting the strip line 104b, the connection portion between the antenna portion and the electrode portion is connected at a position equal to the length of the adjustment stub 202, and the frequency characteristics of the photoconductive element are unevenly distributed. It has become.

In this embodiment, the adjustment stub 202 is handled as a straight line, but this is not restrictive. For example, a tapered stub can be used. The track width also changes depending on the situation. Also in this embodiment, the line length and the connection position of the adjustment stub 202 are the same length, but this is not restrictive. The point is that the structure of the adjustment stub 202 capable of adjusting the frequency characteristics of the photoconductive element does not depart from the gist of the present invention.

As described above, the frequency characteristics of the photoconductive element can be easily adjusted by adjusting the stub length and the connection position of the adjustment stub 202. By appropriately selecting the structure of the adjustment stub 202 together with the size and structure of the antenna unit 101, for example, the frequency characteristics are unevenly distributed in a bandwidth (atmosphere window) where the absorption of terahertz waves by the atmosphere is small, and high efficiency is achieved. It becomes easy to realize transmission.

Incidentally, in some cases, the first adjustment stub 102 and the second adjustment stub 202 described in the above embodiments can be used in combination.

(Example 5: Imaging device)
In this embodiment, an example of the configuration of an apparatus that can implement the present invention is shown. Specifically, it is an example of a configuration related to an imaging apparatus that acquires information in the depth direction of a sample using the photoconductive element described so far. In addition, description of the part which overlaps with the said Example is abbreviate | omitted.

FIG. 3 is a schematic configuration diagram of the imaging apparatus in the present embodiment. As shown in FIG. 3, the imaging apparatus includes a fiber laser 301, a generation photoconductive element 302, a detection photoconductive element 303, a drive driver 304, a detection unit 305, a beam splitter 306, and a delay unit 307. As shown in FIG. 3, the imaging apparatus constructs a reflection optical system for the sample 308, and acquires information in the depth direction of the sample 308 from the reflected signal of the terahertz wave. In this embodiment, a reflection type configuration example is shown, but a transmission type configuration may be adopted.

For example, as shown in FIG. 20, when a tablet is used as the sample 308, the terahertz wave incident on the tablet is reflected by the interface between the atmosphere and the coating film 2001 and the interface between the coating film 2001 and the drug 2002. By monitoring the relative time delay Δt of each reflected terahertz wave and the change in waveform, the film thickness of the coating film 2001, the physical properties of the drug 2002, and the like can be acquired. In order to detect each reflected terahertz wave with high accuracy, it is desired that each reflected terahertz wave is temporally separated and interference between each reflected terahertz wave is small. For this purpose, it is desirable that the waveform has a shape close to a monocycle with a narrow pulse width as a characteristic of the terahertz wave to be used.

The terahertz wave generated from the photoconductive element 302 is affected by the characteristics of the excitation light from the fiber laser 301 and the characteristics of the photoconductive element 302 itself. For example, if the excitation light has a waveform having a shape close to a monocycle with a narrow pulse width, and the frequency characteristic of the photoconductive element 302 has a wide band characteristic, it is easy to separate each reflected terahertz wave in the time domain. Become.

The fiber laser 301 is a small and stable ultrashort pulse laser source that is configured almost by an optical fiber. A configuration example of the fiber laser 301 is shown in FIG. As shown in FIG. 4, the fiber laser 301 has the following configuration. That is, it is configured by a femtosecond fiber laser 401, half-wave plates 402 and 406, an amplification unit 403, an isolator 404, a dispersion compensation unit 405, a polarization beam splitter 407, a PPLN 408, a green cut filter 409, and a dichroic mirror 410. PPLN means Periodically-Poled-Lithium-Niobate, which is a highly efficient wavelength conversion element. The femtosecond fiber laser 401 uses an optical fiber as a laser oscillation medium. The center wavelength is 1558 nm, the average intensity is 5 mW, the pulse width is 300 fsec, and the repetition frequency is 48 MHz. Such a fiber type femtosecond fiber laser 401 is smaller and more stable than a solid-state laser.

The half-wave plates 402 and 406 are used to adjust the polarization. The amplifying unit 403 is a part that amplifies the intensity of the optical pulse of the femtosecond fiber laser 401. The optical pulse whose intensity is amplified by the amplification unit 403 is shortened by the dispersion compensation unit 405. The PPLN 408 is a part that generates a 780 nm component that is a second harmonic component with respect to the optical pulse that has been shortened. Thereafter, using the green cut filter 409 and the dichroic mirror 410, the harmonic component 780 nm is output at a desired branching ratio together with the reference wave component 1550 nm. This harmonic component corresponds to the absorption wavelength of LT-GaAs, and is used as excitation light for the photoconductive elements 302 and 303 in this embodiment.

When indium gallium arsenide (InGaAs) is used as the carrier generation layer 103 used in the photoconductive element, a reference wave component can also be used as excitation light for exciting the carriers. In this case, an optical system that generates and extracts harmonics can be omitted.

Hereinafter, the amplification unit 403 and the dispersion compensation unit 405 will be described in detail.
FIG. 5 shows a configuration example of the amplification unit 403. As shown in FIG. 5, the amplification unit 403 includes the following components. The constituent elements are three laser diodes (denoted by LD in FIG. 5), a single mode fiber 501, WDM couplers 502 and 505, a polarization controller 503, an Er (erbium) doped fiber 504, and a polarization beam combiner 506. WDM means Wavelength-Division-Multiplexing.

The single-mode fiber 501 has a second-order group velocity dispersion of -21.4 ps ² / km, a mode field diameter of 9.3 μm, a nonlinear coefficient of 1.89 W ^-1 km ^-1 for a wavelength of 1.56 μm, and a fiber length of 4.5 m. is there. The Er-doped fiber 504 has a second-order group velocity dispersion of 6.44 ps ² / km, a mode field diameter of 8.0 μm, a nonlinear coefficient of 2.55 W ⁻¹ km ⁻¹ for a wavelength of 1.56 μm, and a fiber length of 6.0 m. . The three LDs have a wavelength of 1480 nm and an intensity of 400 mW. As shown in FIG. 5, one of the LDs is used for forward excitation and two are used for backward excitation.

The pulse width of the optical pulse incident from the femtosecond fiber laser 401 is expanded in the single mode fiber 501 due to the influence of group velocity dispersion. Thereby, the peak intensity of the light pulse is temporarily suppressed. As a result, an excessive non-linear effect when the optical pulse propagates through the Er-doped fiber 504 can be suppressed, so that efficient energy amplification is possible. According to this configuration, the average intensity of the light pulse can be expected to be about 20 dB.

FIG. 6 shows a configuration example of the dispersion compensation unit 405. The dispersion compensation unit 405 has a dispersion characteristic opposite to the dispersion characteristic generated in the amplification unit 403. The optical pulse output from the amplifying unit 403 tends to have a wide band due to the influence of self-phase modulation generated in the Er-doped fiber 504. Therefore, the dispersion compensation unit 405 acquires a pulse shorter than the pulse width of the femtosecond fiber laser 401 by compensating for dispersion at each wavelength. As shown in FIG. 6, in this embodiment, a dispersion compensating fiber 601 is used as the dispersion compensating unit 405. Specifically, a large-diameter photonic crystal fiber is used as the dispersion compensation fiber 601. The dispersion compensating fiber 601 used in this example has a second-order group velocity dispersion of −30.3 ps ² / km, a mode field diameter of 26 μm, a nonlinear coefficient of 0.182 W ⁻¹ km ⁻¹ for a wavelength of 1.56 μm, and the length of the fiber The height is 0.42m. According to this configuration, it is possible to expect a pulse width of the obtained optical pulse of about 55 fsec and an average intensity of about 280 mW.

As described above, in this embodiment, since the photoconductive elements 302 and 303 using LT-GaAs are used for the carrier generation layer 103, the second harmonic is generated by the PPLN 408 and used as excitation light. In PPLN 408, since the reference wave component (1550 nm) is output in addition to the harmonic component (780 nm), the dichroic mirror 410 is used for separation. In addition, in addition to the second harmonic, the PPLN 408 generates a slight amount of green light that is the third harmonic, and is thus configured to be removed by the green cut filter 409. According to such a configuration, it is possible to expect a pulse width of an optical pulse in the 780 nm band of about 58 fsec and an average intensity of about 60 mW. In addition, the pulse width of an optical pulse in the 1550 nm band can be expected to be about 64 fsec, and the average intensity can be about 170 mW.

Further, as described above, the pulse width of the fiber laser 301 used for the excitation light affects the resolution in the depth direction of the imaging apparatus. Therefore, in some cases, it is possible to perform pulse compression using a highly nonlinear fiber as shown in FIG. FIG. 7 (a) shows a configuration diagram for compressing an optical pulse in the 1550 nm band. FIG. 7B shows a configuration diagram for compressing a light pulse in the 780 nm band. In addition, these structures are only one form, and the method of performing pulse compression is not limited to this.

In FIG. 7 (a), a single mode fiber 701 and a highly nonlinear fiber 702 are used to perform pulse compression in the 1550 nm band. The single mode fiber 701 has a second-order group velocity dispersion of −21.4 ps ² / km, a nonlinear coefficient of 1.89 W ⁻¹ km ⁻¹ for a wavelength of 1.56 μm, and a fiber length of 0.115 m. The highly nonlinear fiber 702 has a second-order group velocity dispersion of −14.6 ps ² / km, a nonlinear coefficient of 4.53 W ⁻¹ km ⁻¹ for a wavelength of 1.56 μm, and the length of the fiber is 0.04 m. In addition, the optical pulse output from the fiber is collimated using a parabolic mirror in order to avoid pulse spreading due to dispersion in the lens. According to such a configuration, it is possible to expect a pulse width of the obtained optical pulse of about 22 fsec and an average intensity of about 120 mW.

In FIG. 7B, a highly nonlinear fiber 702 and a chirped mirror 703 are used to perform pulse compression in the 780 nm band. The chirp mirror 703 is a negative dispersion chirp mirror, and each time the mirror is reflected once, a dispersion of about −35 fs ² is added. The pulse compression is performed by reflecting the light pulse between the chirp mirror 703 a plurality of times. Here, 1 m of highly nonlinear fiber 702 is used. According to such a configuration, the pulse width of the obtained optical pulse can be expected to be approximately 37 fsec, and the average intensity can be expected to be approximately 30 mW. In this embodiment, this light pulse is used as excitation light for the photoconductive element. The specific configuration and parameters of the fiber laser 301 are not limited to those described above, and are appropriately selected according to various purposes.

Returning to the description of FIG. 3, at least the photoconductive element 302 on the generation side has the configuration described in the first or third embodiment. That is, the photoconductive element 302 having a broadband frequency characteristic is used. As the photoconductive element 303 on the detection side, an element similar to that on the generation side may be used, or a general element used in a terahertz time domain spectroscopy (THz-TDS) apparatus may be used. The drive driver 304 is a part that applies a desired voltage to the photoconductive element 302. The applied voltage may be DC or periodic.

The beam splitter 306 is a part that branches the output of the fiber laser 301 having the above-described configuration, and leads one to the photoconductive element 302 and the other to the delay unit 307.

The delay unit 307 is configured by a general delay optical system, and changes the irradiation timing of the light pulse incident on the photoconductive element 303 with time. The detection unit 305 detects a signal output from the photoconductive element 303 and stores a signal corresponding to the light pulse irradiation timing set by the delay unit 307. As a result, the time waveform of the terahertz wave reflected from the sample 308 is reconstructed, and information on the depth direction regarding the sample 308 is acquired.

In FIG. 3, a structure for separately changing the relative position between the sample 308 and the terahertz wave applied to the sample 308 may be provided to perform three-dimensional imaging of the sample 308. Further, as described above, the configuration of the present embodiment can also be applied to a transmissive or reflective THz-TDS apparatus.

In the configuration of FIG. 3, the photoconductive element 302 has a configuration in which excitation light is incident from the antenna unit 101 side and a terahertz wave is extracted from the substrate 105. However, the present invention is not limited to such a system configuration. For example, as shown in FIGS. 24A and 24B, a configuration in which excitation light is incident from the antenna unit 101 side and terahertz waves are extracted from the antenna unit 101 side is also possible. is there. At this time, the excitation light used to generate the terahertz wave propagates along the propagation path of the terahertz wave as shown in FIG. In FIG. 24, in order to remove such an unnecessary signal, a filter 2409 is inserted in the propagation path of the terahertz wave. The filter 2409 has characteristics of removing excitation light and transmitting terahertz waves. For example, when a reference wave component (1550 nm) is used as excitation light, a germanium (Ge) substrate can be applied. In addition, when a harmonic component (780 nm) is used as excitation light, a Si substrate can be applied. Note that the configuration of the filter 2409 is not limited to this. When such a configuration is applied, the absorption of the terahertz wave by the substrate 105 used for the photoconductive element 302 can be reduced, so that a wider band terahertz wave can be provided. For detection, a configuration in which terahertz waves and excitation light are incident from the antenna unit 101 can be applied.

As described above, the imaging apparatus of the present embodiment controls the photoconductive element of the present invention, the ultrashort pulse laser that generates a carrier by applying an output to the gap of the antenna section, and the propagation state of the generated carrier. And at least a drive driver connected to the electrode portion. Then, the measurement target (sample) is irradiated with the terahertz wave generated from the photoconductive element, and the internal structure information of the measurement target is acquired from the terahertz wave reflected at the surface of the measurement target and the internal refractive index interface. Here, the frequency characteristic of the photoconductive element is desirably flattened.

With the above-described configuration, it is possible to use a terahertz wave having a shape close to a monocycle for an imaging apparatus. Therefore, depth direction imaging with higher resolution becomes possible. Further, by using a fiber laser as an excitation light source for the photoconductive element, it is possible to provide a small, stable, and relatively inexpensive device.

(Example 6: Communication device)
In this embodiment, an example of the configuration of an apparatus that can implement the present invention is shown. Specifically, it is a configuration example related to a communication apparatus that transmits information using the photoconductive element and the fiber laser described so far. The description of the same parts as those in the above embodiment is omitted.

This embodiment particularly relates to a transmission part of a communication device. In the present embodiment, terahertz waves are unevenly distributed in a band corresponding to an atmospheric window and used as a carrier wave. Here, the atmospheric window is a wavelength band in which the influence of absorption by the atmosphere is small and the light transmittance is high.

FIG. 8 shows an example of the configuration relating to the transmission part of the communication device in the present embodiment. As shown in FIG. 8, the transmission part of this communication apparatus is composed of a fiber laser 301, a photoconductive element 302, a drive driver 304, and a modulation unit 801. The photoconductive element 302 has the configuration described in the second or fourth embodiment. For example, as an example of an atmospheric window, it is known to exist in the vicinity of 0.38 THz, 0.71 THz, 0.86 THz, 1.05 THz, and 1.38 THz. In the present embodiment, the adjustment stub 202 is selected so that the frequency characteristics are unevenly distributed at locations corresponding to these atmospheric windows.

Modulation section 801 controls drive driver 304 with a signal component corresponding to information to be transmitted, and modulates the terahertz wave radiated from photoconductive element 302. As a modulation method, as shown in FIG. 11A, there is a method of switching a voltage applied to the photoconductive element 302 from the drive driver 304 and transmitting a signal by ON / OFF of a terahertz wave. Further, as shown in FIG. 11 (b), there is a method of modulating the amplitude of the terahertz wave by the value of the voltage applied to the photoconductive element 302.

FIG. 9 shows another configuration example relating to the transmission part of the communication device. In FIG. 9, the modulation unit 901 modulates the emission timing and intensity of the fiber laser 301, not the drive driver 304. This makes it possible to modulate the phase (delay time) of the terahertz wave as shown in FIG. 11 (c) in addition to the modulation schemes of FIGS. 11 (a) and 11 (b). These modulation units 801 and 901 are configured by a normal circuit or a computer.

FIG. 10 shows another configuration example related to the transmission part of the communication device. In FIG. 10, the modulation unit 1001 controls the light pulse output from the fiber laser 301. For example, the timing at which the light pulse reaches the photoconductive element 302 is controlled by mechanically stretching the fiber. Further, the intensity of the optical pulse is modulated using an attenuator. Alternatively, the light pulse may be turned ON / OFF using a switch. Note that the configuration and control target of these modulation units are not limited to this. For example, the generated terahertz wave itself may be modulated. As an example, the modulation state shown in FIG. 11C can be realized by changing the position of the photoconductive element 302 with respect to the emission direction of the terahertz wave. The point is that the frequency characteristic of the terahertz wave is unevenly distributed in a band corresponding to the atmospheric window and a signal is superimposed on the frequency characteristic.

In this embodiment, the atmospheric window is described as a guide for unevenly distributing the frequency characteristics of the terahertz wave, but the present invention is not limited to this. For example, a form that avoids absorption of the atmosphere surrounding the apparatus and a form that unevenly distributes terahertz waves so as to avoid absorption of characteristic members are possible. In addition, the configuration of this embodiment can be applied to a transmissive or reflective THz-TDS apparatus. As in the sixth embodiment, the output of the fiber laser 301 can be selected from either the 1550 nm band or the 780 nm band. In this case, the configuration of the carrier generation layer 103 of the photoconductive element 302 is appropriately selected according to the output waveband of the fiber laser 301.

As described above, the communication device of this embodiment controls the propagation state of the generated carrier, the photoconductive element of the present invention, the ultrashort pulse laser that generates a carrier by applying an output to the gap of the antenna section, and the like. At least a drive driver connected to the electrode portion to be modulated and a modulation portion. The modulation unit modulates a signal supplied from the drive driver to the electrode unit or an output of the ultrashort pulse laser according to transmission information. Then, for example, communication is performed using terahertz waves in which frequency characteristics are unevenly distributed so as to reduce components in an absorption wavelength region unique to the terahertz wave band existing in the atmosphere.

By having the above configuration, it is possible to avoid the characteristic absorption of the atmosphere or the like and to propagate the terahertz wave efficiently. Further, by using a fiber laser as an excitation light source for the photoconductive element, it is possible to provide a small, stable, and relatively inexpensive device.

The figure explaining the structure of embodiment thru | or Example of a photoconductive element. The figure explaining the structure of another embodiment thru | or an Example of a photoconductive element. 3A and 3B illustrate a configuration in which a photoconductive element is applied to an imaging apparatus. The figure explaining the structure of a fiber laser. The figure explaining the amplification part of a fiber laser. The figure explaining the dispersion compensation part of a fiber laser. The figure explaining the structure which performs pulse compression. 2A and 2B illustrate a structure in which a photoconductive element is applied to a communication device. The figure explaining another structure which applied the photoconductive element to the communication apparatus. The figure explaining another structure which applied the photoconductive element to the communication apparatus. The figure explaining operation | movement of the modulation | alteration part in a communication apparatus. FIG. 3 is a diagram for explaining the electromagnetic field analysis result of the photoconductive element of Example 1. FIG. 6 is a diagram illustrating the effect of the adjustment stub of the photoconductive element of Example 1. FIG. 6 is a diagram for explaining an electromagnetic field analysis result of the photoconductive element of Example 2. FIG. 10 is a diagram for explaining the effect of the adjustment stub of the photoconductive element of Example 2. FIG. 6 is a diagram for explaining an electromagnetic field analysis result of the photoconductive element of Example 3. FIG. 10 is a diagram illustrating the effect of the adjustment stub of the photoconductive element of Example 3. FIG. 6 is a diagram for explaining an electromagnetic field analysis result of the photoconductive element of Example 4. FIG. 10 is a diagram for explaining the effect of the adjustment stub of the photoconductive element of Example 4. The figure explaining the example of 1 application of an imaging device. FIG. 6 is a diagram for explaining the measurement results of the photoconductive element of Example 2. FIG. 6 is a diagram for explaining the measurement results of the photoconductive element of Example 3. FIG. 6 is a diagram for explaining the measurement results of the photoconductive element of Example 3. The figure explaining the other example of the generation method of a terahertz wave. The figure explaining the structure of one Embodiment (2nd adjustment stub) of a photoconductive element. The figure explaining the structure of one Embodiment (1st adjustment stub) of a photoconductive element. The figure explaining the structure of one Embodiment (2nd adjustment stub) of a photoconductive element. The figure explaining the structure of one Embodiment (1st adjustment stub) of a photoconductive element

Explanation of symbols

101, 2504, 2604, 2704, 2804 Antenna section (antenna)
102, 2606, 2806 First adjustment stub
103, 2501, 2601, 2701, 2801 Carrier generation layer
104, 2502, 2503, 2602, 2603, 2702, 2802 Electrode (electrode)
104b stripline
202, 2506, 2706 Second adjustment stub
301 Ultrashort pulse laser (fiber laser)
302 Photoconductive element (for generation)
303 Photoconductive element (for detection)
304 drive driver
305 Detector
308 Measurement target (sample)
801, 901, 1001 Modulator
2409 Filter
2505, 2605 Irradiation position
2607, 2807 End of first adjustment stub (tip of first adjustment stub, end of electrode)
2708, 2808 connection part

Claims (8)

A photoconductive element for generating or detecting a terahertz wave pulse,
A carrier generation layer for generating carriers by light irradiation;
Two electrode parts comprising a strip line which is made of a conductor provided on one surface of the carrier generation layer and arranged in parallel to each other;
Two antenna portions each of which is made of a conductor in contact with the two electrode portions, and is disposed opposite to each other with a gap for irradiating the carrier generation layer with light;
Two first adjustment stubs each comprising a conductor in contact with each of the two electrode portions, for adjusting a propagation state of a terahertz wave generated or detected by the carrier;
Have
The two first adjustment stubs are positioned within a range of an operating wavelength λ or less of a terahertz wave generated or detected by the carrier from a connection portion of the antenna unit and the electrode unit, respectively, and the length of the strip line A conductor with a length, connected on the extension of the direction,
Each of the two first adjustment stubs has a length of 0.5λ to 0.8λ from a connection portion of the antenna unit and the electrode unit, so that the terahertz wave generated from the carrier or detected by the carrier is used. photoconductive device frequency characteristic of you, characterized in that flattening. A photoconductive element for generating or detecting a terahertz wave pulse,
A carrier generation layer for generating carriers by light irradiation;
Two electrode parts comprising a strip line which is made of a conductor provided on one surface of the carrier generation layer and arranged in parallel to each other;
Two antenna portions each of which is made of a conductor in contact with the two electrode portions, and is disposed opposite to each other with a gap for irradiating the carrier generation layer with light;
Two first adjustment stubs each comprising a conductor in contact with each of the two electrode portions, for adjusting a propagation state of a terahertz wave generated or detected by the carrier;
Have
The two first adjustment stubs are positioned within a range of an operating wavelength λ or less of a terahertz wave generated or detected by the carrier from a connection portion of the antenna unit and the electrode unit, respectively, and the length of the strip line A conductor with a length, connected on the extension of the direction,
The two first adjustment stubs are generated from the carrier or detected by the carrier by having a length of 0.1λ to 0.5λ from a connection portion of the antenna portion and the electrode portion, respectively. photoconductive element characterized in that the frequency characteristic of the terahertz wave is unevenly distributed. A photoconductive element for generating or detecting a terahertz wave pulse,
A carrier generation layer for generating carriers by light irradiation;
Two electrode parts comprising a strip line which is made of a conductor provided on one surface of the carrier generation layer and arranged in parallel to each other;
Two antenna portions each of which is made of a conductor in contact with the two electrode portions, and is disposed opposite to each other with a gap for irradiating the carrier generation layer with light;
Two second adjustment stubs, each comprising a conductor in contact with the two electrode portions, for adjusting the propagation state of the terahertz wave generated or detected by the carrier;
Have
The two second adjustment stubs are located within a range of the operating wavelength λ or less of the terahertz wave generated or detected by the carrier from the connection portion of the antenna unit and the electrode unit, respectively, and the outer edge of the strip line A conductor to be connected at a distance from a connection portion of the antenna part and the electrode part along
The two second adjustment stubs each have a length of 0.1λ to 0.2λ, and extend from the connection portion of the antenna portion and the electrode portion along the outer edge of the stripline. by being connected to the position equal to the length of the adjusting stub photoconductive element you characterized in that the frequency characteristic of the terahertz wave or to the carrier generated from the carrier is detected is flattened. A photoconductive element for generating or detecting a terahertz wave pulse,
A carrier generation layer for generating carriers by light irradiation;
Two electrode parts comprising a strip line which is made of a conductor provided on one surface of the carrier generation layer and arranged in parallel to each other;
Two antenna portions each of which is made of a conductor in contact with the two electrode portions, and is disposed opposite to each other with a gap for irradiating the carrier generation layer with light;
Two second adjustment stubs, each comprising a conductor in contact with the two electrode portions, for adjusting the propagation state of the terahertz wave generated or detected by the carrier;
Have
The two second adjustment stubs are located within a range of the operating wavelength λ or less of the terahertz wave generated or detected by the carrier from the connection portion of the antenna unit and the electrode unit, respectively, and the outer edge of the strip line A conductor to be connected at a distance from a connection portion of the antenna unit and the electrode unit along
The two second adjustment stubs have a length of 0.2λ to 0.5λ, respectively, and extend along the outer edge of the stripline from the connection portion of the antenna portion and the electrode portion. by being connected to the position equal to the length of the adjusting stub photoconductive element you characterized by or to the carrier generated from the carrier frequency characteristic uneven distribution of the terahertz wave detecting. The wavelength λ is photoconductive device according to any one of claims 1 to 4, characterized in that twice the length of the interval d of each other the two strip lines. The position where the light irradiation is performed is between the two antenna units,
The two first adjustment stubs or the two second adjustment stubs are respectively provided at a distance within twice the distance d between the two electrodes from the irradiation position. 5. The photoconductive element according to any one of 1 to 4 . An imaging device,
The photoconductive element according to any one of claims 1 to 6 ,
An ultrashort pulse laser that generates a carrier by applying an output to the gap of the antenna unit;
A drive driver connected to the electrode unit for controlling the propagation state of the generated carriers,
An imaging apparatus, wherein a terahertz wave is generated from the photoconductive element, and internal structure information of the measurement target is acquired from the terahertz wave reflected at the surface of the measurement target and the internal refractive index interface. A communication device,
The photoconductive element according to claim 2 or 4 ,
An ultrashort pulse laser that generates a carrier by applying an output to the gap of the antenna unit;
A drive driver connected to the electrode unit for controlling the propagation state of the generated carriers;
A signal that the drive driver supplies to the electrode unit, or a modulation unit that modulates the output of the ultrashort pulse laser according to transmission information,
A communication apparatus that performs communication using a terahertz wave having a frequency characteristic unevenly distributed so as to reduce a component in an absorption wavelength range peculiar to a frequency band of a terahertz wave existing in the atmosphere.

Images