JP2011164068A - Superconductive photodetector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a superconductive photodetector that solves a problem of a conventional technology, namely low light detection efficiency, and then has small polarization dependency and high light detection efficiency. <P>SOLUTION: In this superconductive photodetector, a plurality of superconductive thin lines of a meandering (meandering, lightning pattern, or zigzag) shape are installed via the front and back of a crystal substrate, insulating film, or adhesive so that the thin lines are close to each other and the directions of the thin lines are orthogonal to each other. As a result of to this arrangement and constitution, a photon that has not been detected because it has polarization rectangular to the first meandering thin line can be detected because it has polarization parallel to the next meandering thin line. In other words, any photon is decomposed into polarizations consisting of mutually rectangular straight lines, so that a photon having any polarization is detected. A failure due to polarization is eliminated thereby, so that the polarization dependency is small and the light detection efficiency is increased by about twice comparing with the conventional example. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a photodetection element using superconductivity, and more particularly to a superconductivity photodetection element for detecting a single photon in the infrared region without polarization dependence.

  Quantum cryptography communication that integrates an encryption system that applies the principles of quantum mechanics and an information transmission system is the ultimate communication that cannot be tapped, and is expected to be put to practical use as the next generation of communications. Quantum cryptocommunication uses extremely weak single photons as a unit of transmission information, and single quantum photon detectors with high quantum efficiency, high speed, and low noise are required for long-distance transmission using optical fibers. is required. At present, various single photon detectors have been developed. Recently, single photon detectors using superconductivity have been realized and are attracting attention (Patent Documents 1, 2, and 3). This single photon detector consists of a meander-shaped superconducting wire, can operate at high speed (GHz) and has many advantages that cannot be realized by other detectors such as low dark count rate. . However, the single photon detector shows a polarization dependency that the light detection efficiency is high for the polarization parallel to the meander wire, but the light detection efficiency is low for the polarization perpendicular to the meander wire. . The polarization state of light in the optical fiber is usually random, and a photodetector having no polarization dependency is desirable for efficient detection.

JP 2008-71908 A US Pat. No. 6,812,464 US Pat. No. 7,049,593

S. N. Dorenbos, et al., 'Superconducting single photon detectors with minimized polarization dependence', Applied Physics Letters, Vol. 93, 161102, 2008.

  However, in the prior art, in order to reduce the polarization dependency, the meander shape is changed and the longitudinal meander wire and the transverse meander wire are placed side by side on the same plane to receive light, or Attempts have been made to change the meander wire to a spiral curve (Non-Patent Document 1). However, in this conventional method, the light that has not been detected with the polarization different from the meander direction is transmitted through the detection element, so that there is a problem to be solved that the light detection efficiency is low.

  An object of the present invention is to provide a superconducting photodetecting element that eliminates the above-described problems in the prior art, has a small polarization dependency, and has high photodetection efficiency.

  In order to achieve the above object, the superconducting photodetecting element of the present invention has a plurality of meander-shaped fine lines close to each other in the vertical direction, and the arrangement direction of the other fine line adjacent to the arrangement direction of one fine line is It is characterized by being placed one on top of the other so as to form a right angle. A photon that is not detected because it has a polarization perpendicular to the first meander wire is detectable because it has a polarization parallel to the next meander wire. Since arbitrary photons can be decomposed into polarized waves composed of straight lines perpendicular to each other, this arrangement makes it possible to detect photons having arbitrary polarized waves. With this arrangement, according to the present invention, since there is no loss due to polarization, the dependence on polarization is reduced, and the light detection efficiency is improved about twice as much as that of the prior art in which a meandering thin line is provided only on the surface.

  More specifically, in one embodiment of the present invention, for example, a meander-shaped first fine wire made of a superconducting material is formed on the surface of the crystal substrate, and then the back surface of the crystal substrate is formed. Then, a meander-shaped second fine wire made of a superconducting material is formed in a direction perpendicular to the first fine wire on the surface. As a result, the second fine line is formed at a position perpendicular to the first fine line and at a position where the arrangement direction of the second fine lines is perpendicular to the arrangement direction of the first fine lines. In this case, photons having a polarization perpendicular to the first thin line on the surface of the crystal substrate are not absorbed by the first thin line on the surface, but are irradiated onto the second thin line on the back surface of the crystal substrate. Since this photon has a polarization parallel to the second thin line on the back surface, it is absorbed by the second thin line on the back surface. These thin lines in the meander shape on the front and back surfaces can be regarded as a single thin line if their ends are combined, so photon detection that can detect single photons by connecting an electrical circuit for readout detection to the combined thin lines A device is obtained. Further, even if the first and second thin wires on the front and back surfaces of the crystal substrate are made independent and a readout detection electric circuit is connected to each thin wire, photons can be detected.

  In order to prevent a decrease in light detection efficiency due to the spread of the spot diameter of incident light, the distance between the first thin wire on the front surface and the second thin wire on the back surface (that is, the thickness of the crystal substrate) is preferably 10 μm or more and 100 μm or less. Install in close proximity.

  In another aspect of the present invention, for example, after forming a meander-shaped first fine wire made of a superconducting material on a crystal substrate and forming a first insulating film on the crystal substrate and the first fine wire Then, by further forming a meander-shaped second fine wire made of a superconducting material on the first insulating film in a direction perpendicular to the first fine wire, similarly to the above-described aspect, Since there is no missing, the dependence on polarization is reduced, and the light detection efficiency is improved about twice as compared with the conventional technique in which the meander wire is only on the surface. Further, a second insulating film is formed on the surface of the first insulating film and the second thin wire, and a meander-shaped third thin wire made of a superconducting material is formed on the surface of the second insulating film. Forming a third insulating film on the surface of the second insulating film and the third fine wire, and forming a meander-shaped fourth fine wire made of a superconductive material on the surface of the third insulating film. An even-numbered one or two is formed in a multilayer film structure in which N + 1 (N is an integer of 3 or more) insulating films and a meander-shaped N + 1 or more fine wire made of a superconducting material are repeatedly formed. The above-mentioned fine lines are formed at a position perpendicular to the adjacent odd-numbered fine lines and at a position where the even-numbered fine lines are arranged at right angles to the odd-numbered fine lines. Can be further improved. In addition, in order to prevent a decrease in light detection efficiency due to an increase in the spot diameter of incident light, the thickness of each insulating film is preferably 10 nm to 100 μm.

  In still another aspect of the present invention, for example, a meander-shaped first fine wire made of a superconducting material is formed on a first crystal substrate, and a meander-shape made of a superconducting material is formed on a second crystal substrate. Forming a second fine wire, and arranging the second fine wire through an adhesive or an insulating film and an adhesive in a direction in which the arrangement direction of the second fine wire is perpendicular to the arrangement direction of the first fine wire. By adhering to the thin wire, as in the other embodiments described above, the loss due to the polarization is eliminated, so that the polarization dependency is reduced, and the light detection efficiency is 2 as compared with the conventional technology in which the meander thin wire is only on the surface. It improves about twice. Here, in order to prevent a decrease in light detection efficiency due to the spread of the spot diameter of the incident light, the interval between the first thin wire and the second thin wire is preferably 1 μm or more and 100 μm or less.

  In addition, for each of the above-described configurations of the present invention, a non-reflective coating disposed on the surface side of the superconducting photodetection element on the light incident side, and a rearview mirror disposed on the back side of the superconducting photodetection element By further adding a cavity by installing at least one of the above, it is possible to further improve the light detection efficiency.

  Further, for each of the above-described configurations of the present invention, in each of the meander-shaped fine wires, the size of the meander shape is 1 μm square or more and 10 μm square or less, and the line width of the thin line is 10 nm or more and 200 nm or less. The thickness of the fine wire can be 1 nm or more and 5 nm or less.

Further, for each of the above configurations of the present invention, the superconductive material is NbN and the crystal substrate and the insulating film are MgO, AlN, or aluminum oxide, or the superconductive material is MgB ₂ and the crystal substrate and the insulating film are insulated. The film can be SiC or AlN or aluminum oxide, or the superconducting material can be a copper oxide superconductor, and the crystal substrate and the insulating film can be NdGaO _3, SrLaGaO _4, or LaSrAlO ₄ .

  As described above, according to the present invention, according to the present invention, the incident photon is always irradiated on the meander thin line having high polarization dependency with high light detection efficiency regardless of the polarization direction. A photodetecting element with high detection efficiency can be obtained.

1 shows a schematic configuration of a superconducting photodetection element according to a first embodiment to which the present invention is applied, wherein (a) is a top view (plan view), (b) is a back view (back view), and (c) is a top view. It is sectional drawing which follows the cutting line AA 'line of (a) and (b). 1 shows a schematic configuration of a superconducting light detection element according to a second embodiment to which the present invention is applied, wherein (a) is a top view (plan view) of the element, (b) is a top view of a crystal substrate, and (c). These are sectional drawings which follow the cutting line BB 'of (a) and (b). It is sectional drawing of the superconducting photodetection element of 3rd Embodiment to which this invention was applied. It is sectional drawing of the superconducting photodetection element of 4th Embodiment to which this invention was applied. It is sectional drawing of the superconducting photodetection element of 5th Embodiment to which this invention was applied.

  Hereinafter, embodiments of the present invention will be described in detail.

  FIG. 1 shows a schematic configuration of a superconducting photodetection element according to a first embodiment to which the present invention is applied. FIG. 1 (a) is a top view (plan view), and FIG. 1 (b) is a back view (back view). ) And FIG. 1C are cross-sectional views taken along line AA ′ in FIGS. 1A and 1B.

  The superconducting light detection element of this embodiment will be described in the order of the manufacturing process. First, a 4 nm thick NbN superconducting thin film is formed on the surface of the MgO crystal substrate 101 by reactive sputtering.

  Subsequently, the NbN thin film is formed on a meander-shaped thin wire 102 as shown in FIG. 1A by electron beam lithography (EB) and reactive ion etching (RIE). The meander size is 10 μm × 10 μm, the line width is 100 nm, and the line interval (distance from the center of one line to the center of the next line, the same applies hereinafter) is 200 nm (aperture ratio 50%). The “gap” between the lines is 100 nm.

  Next, the back surface of the MgO crystal substrate 101 is polished until the thickness of the substrate reaches about 50 μm. Here, in order to prevent substrate cracking due to polishing, another MgO substrate may be attached to the surface of the substrate before polishing. Alternatively, a crystal substrate having a thickness of 50 μm can be used from the beginning. The thickness of the crystal substrate is preferably small in order to suppress the spread of the spot diameter of incident light. However, if the thickness is too thin, the crystal substrate cannot be self-supporting, and is preferably 10 μm or more and 100 μm or less.

  Further, a 4 nm thick NbN superconducting thin film is formed on the back surface of the MgO crystal substrate 101 by reactive sputtering.

  Subsequently, an NbN thin film on the back surface is formed on a meander-shaped thin wire 103 as shown in FIG. 1B by electron beam lithography (EB) and reactive ion etching (RIE). At this time, the meandering fine wire 103 on the back surface is formed at a position where it overlaps perpendicularly with respect to the meandering thin wire 102 on the front surface, and the arrangement direction of the meandering fine wires is perpendicular to each other.

A cross section of the superconducting photodetection element thus formed is shown in FIG. From FIG. 1C, it can be seen that the upper and lower meander-shaped thin wires 102 and 103 intersect each other at right angles when viewed from above the element in the light irradiation direction. Consider a case where very weak light (indicated by an arrow 104) is irradiated from above the element and a single photon is incident. The polarization dependence of a single photon can be broken down into a component parallel to the upper meander wire and a component perpendicular to it. Photons having a component parallel to the upper meandering wire 102 are detected by the upper meandering wire 102 and detected as an electric signal. On the other hand, a photon having a component perpendicular to the upper meander wire 102 is transmitted through the upper meander wire 102, but is irradiated to the lower meander wire 103 and detected by the meander wire 103. Accordingly, the presence of the orthogonal meander wires 102 and 103 on both surfaces of the crystal substrate improves the light detection efficiency by a factor of two compared to the prior art in which the meander wires are only on the surface.

The materials, dimensions, etc. described above are only specific examples, and the present invention is not limited to these. For example, the thin wires 102 and 103 are made of a superconducting material. As this superconducting material, MgB ₂ and copper oxide can be used in addition to the above NbN. When NbN is used as the superconducting material of the thin wires 102 and 103, aluminum oxide or AlN can be used for the crystal substrate 101 in addition to the above MgO. Further, when MgB ₂ is used as the superconducting material of the thin wires 102 and 103, SiC, AlN or aluminum oxide can be used for the crystal substrate 101. On the other hand, when a copper oxide superconductor is used as the superconductive material of the thin wires 102 and 103, NdGaO _3, SrLaGaO _4, or SrLaAlO ₄ can be used as the crystal substrate 101. Furthermore, it is desirable that the cross section of the thin line is small in order to be able to detect single photons, the line width of the thin lines 102 and 103 is 10 nm or more and 200 nm or less, and the thickness of the thin lines 102 and 103 is 1 nm or more and 5 nm or less. Is preferred. Further, the size of the meander is preferably 1 μm square or more and 10 μm square or less.

  The meander fine wire 102 on the front surface of the crystal substrate 101 and the meander fine wire 103 on the back surface can be regarded as one fine wire if one end of the fine wire is joined to each other. Therefore, an electrical circuit (not shown) for reading and detecting is connected to the joined fine wire. If connected, an optical device capable of detecting single photons can be obtained. Further, the meander thin wire 102 on the front surface and the meander thin wire 103 on the back surface of the crystal substrate 101 are made independent, and an electric circuit (not shown) for reading detection is connected to each of the thin wires 102 and 103 separately. , Can detect photons.

  FIG. 2 shows a schematic configuration of a superconducting photodetection element according to a second embodiment to which the present invention is applied. FIG. 2 (a) is a top view (plan view) of the element, and FIG. 2 (b) is a crystal substrate. FIG. 2C and FIG. 2C are cross-sectional views taken along the cutting line BB ′ in FIGS.

  The superconducting light detection element of this embodiment will be described in the order of the manufacturing process. First, a 4 nm thick NbN superconducting thin film is formed on the MgO crystal substrate 201 by reactive sputtering.

  The NbN thin film is formed on the first thin wire 202 having a meander shape as shown in FIG. 2B by electron beam lithography (EB) and reactive ion etching (RIE). The meander size is 10 μm × 10 μm, the line width is 100 nm, and the line interval is 200 nm (aperture ratio 50%).

  Next, an MgO insulator thin film 203 is formed to a thickness of 1 μm on the crystal substrate 201 and the first thin wire 202 by reactive sputtering. Thereafter, the surface of the MgO insulator thin film 203 is planarized by chemical mechanical polishing (CMP). Here, the MgO thin film may be produced by a vapor deposition method. Further, since the MgO thin film 203 is a thin film, it can be made thinner than the case of using the substrate of the first embodiment described above, and is preferably 10 nm or more and 100 μm or less.

  A 4 nm thick NbN superconducting thin film is formed on the surface of the planarized MgO insulator thin film 203 by reactive sputtering.

  By means of electron beam lithography (EB) and reactive ion etching (RIE), an NbN thin film on the surface of the MgO insulator thin film is formed on a second thin wire 204 having a meander shape as shown in FIG. At this time, the second meander thin line 204 is formed at a position where it overlaps perpendicularly to the first meander thin line 202 on the MgO substrate 201 and at a position where the arrangement direction of the meander thin lines is perpendicular to each other.

  A cross section of the superconducting photodetection element thus formed is shown in FIG. From FIG. 2 (c), it can be seen that the upper and lower meander-shaped thin wires 202 and 204 intersect each other at right angles when viewed from above the element in the light irradiation direction. Consider a case in which extremely weak light (indicated by an arrow 205) is irradiated from above the element and a single photon is incident. The polarization dependence of a single photon can be decomposed into a component parallel to the upper meander wire 204 and a component perpendicular thereto. Photons having a component parallel to the upper meander thin line 204 are detected by the upper meander thin line 204 and detected as an electric signal. On the other hand, a photon having a component perpendicular to the upper meander thin line 204 is transmitted through the upper meander thin line 204, but is irradiated to the lower meander thin line 202 and detected by the meander thin line 202. Therefore, since the orthogonal meander wires 202.204 are above and below, the light detection efficiency is improved twice as compared with the case of the prior art in which the meander wires are only on the surface.

The materials, dimensions, etc. described above are only specific examples, and the present invention is not limited to these. For example, the thin wires 202 and 204 are formed of a superconducting material. As this superconducting material, MgB ₂ and copper oxide can be used in addition to the above NbN. When NbN is used as the superconducting material of the thin wires 202 and 204, aluminum oxide or AlN can be used for the crystal substrate 201 and the insulating film 203 in addition to the above MgO. Further, when MgB ₂ is used as the superconducting material of the thin wires 202 and 204, SiC, AlN, or aluminum oxide can be used for the crystal substrate 201 and the insulating film 203. On the other hand, when a copper oxide superconductor is used as the superconducting material of the thin wires 202 and 204, NdGaO _3, SrLaGaO ₄ or SrLaAlO ₄ can be used as the crystal substrate 201 and the insulating film 203. Furthermore, it is preferable that the line widths of the thin wires 202 and 204 are 10 nm or more and 200 nm or less, and the thicknesses of the thin wires 202 and 204 are 1 nm or more and 5 nm or less. Further, the size of the meander is preferably 1 μm square or more and 10 μm square or less.

  Since the connection state between the thin wires 202 and 204 of the present embodiment and the electrical circuit for reading detection (not shown) is the same as that of the first embodiment, the description thereof is omitted.

  FIG. 3 is a sectional view of a superconducting photodetecting element according to a third embodiment to which the present invention is applied.

  When light is applied to the NbN superconducting meander wire, in the configuration of FIG. 2, light that passes through the second meander wire 204 and the first meander wire 202 may actually be present. 3, the second insulating film 305 is formed on the second meander thin line 204, and the third meander thin line 306 is formed on the insulating layer 305 in the same direction as the first meander thin line 202. Further, a third insulating film 307 is formed on the third meander thin line 306, and a fourth meander thin line 308 is formed on the third insulating layer in the same direction as the second meander thin line 204. To do. By using such a multilayer structure, the light detection efficiency is further improved. The thickness of each insulating film is preferably 10 nm or more and 100 μm or less, as in the second embodiment described above.

  In addition, the order of the positional relationship of the arrangement of the fine lines in each layer and the number of layers may be arbitrarily determined as necessary. For example, the present embodiment is a superconducting photodetection element having a multilayer structure in which N (N is an integer of 3 or more) insulating films and meander-shaped N + 1 or more fine wires made of a superconducting material are alternately and repeatedly stacked. The same applies as above. Other configurations and modifications are the same as those of the second embodiment described above, and thus the description thereof is omitted.

  FIG. 4 is a sectional view of a superconducting photodetecting element according to a fourth embodiment to which the present invention is applied.

  The first meander wire 202 produced on the first crystal substrate 201 and the second meander wire 204 produced on the second crystal substrate 401 are connected to each other through an adhesive 402. A superconducting photodetecting element is formed by pasting in opposite directions so as to cross each other at right angles. The “adhesive” used in this example is an optical non-conductive adhesive that is transparent to light used for optical fiber connection and the like. The thickness of the adhesive 402 is preferably reduced in order to suppress the spread of the spot diameter of incident light, and is preferably 1 μm or more and 100 μm or less. With such a structure, a photon detection device capable of detecting a single photon having no polarization dependency can be obtained. In addition, after an insulating film (not shown) is formed on each of the first meander wire 202 and the second meander wire 204, the insulating films may be bonded to each other with an adhesive 402. Other configurations and modifications are the same as those of the second embodiment described above, and thus the description thereof is omitted.

  FIG. 5 is a sectional view of a superconducting photodetecting element according to a fifth embodiment to which the present invention is applied.

  In the fifth embodiment, there is no effect on the surface (light incident side) of the superconducting photodetection element of the present invention as shown in FIG. 1 (c), FIG. 2 (c), FIG. 3, or FIG. A reflective coating 501 is installed, and a rearview mirror 502 is installed on the back surface of the element. For example, a gold film is applied as the rearview mirror 502. As described above, by adding a cavity by installing a non-reflective coating or a rearview mirror, it is possible to further improve the light detection efficiency.

(Other examples)
In the above, the preferred embodiment of the present invention has been described by way of example. However, the embodiment of the present invention is not limited to the above-described example, and the constituent members thereof are within the scope of the claims. Various modifications such as replacement, change, addition, increase / decrease in number, change in shape design, and the like are all included in the embodiment of the present invention.

101 crystal substrate 102 surface superconducting meander wire (first wire)
103 Backside superconducting meander wire (second wire)
104 Incident light 201 First crystal substrate 202 First superconducting meander wire 203 Insulating film (first insulating film)
204 Second superconducting meander wire 205 Incident light 305 Second insulating film (insulator thin film)
306 Third superconducting meander wire 307 Third insulating film 308 Fourth superconducting meander wire 401 Second crystal substrate 402 Adhesive or adhesive and insulating film 501 Non-reflective coating 502 Rearview mirror

Claims (10)

A meander-shaped first fine wire made of a superconducting material formed on the surface of the crystal substrate;
A meander-shaped second fine wire made of the superconducting material formed on the back surface of the crystal substrate;
The second thin line is formed at a position perpendicular to the first thin line, and the arrangement direction of the second thin line is perpendicular to the arrangement direction of the first thin line. A superconducting photodetection element.   The superconducting photodetecting element according to claim 1, wherein the thickness of the crystal substrate is 10 µm or more and 100 µm or less. A meander-shaped first fine wire made of a superconducting material formed on the surface of the crystal substrate;
A first insulating film formed on the crystal substrate and the surface of the first thin wire;
A meander-shaped second fine wire made of the superconducting material formed on the surface of the first insulating film,
The second thin line is formed at a position perpendicular to the first thin line, and the arrangement direction of the second thin line is perpendicular to the arrangement direction of the first thin line. A superconducting photodetection element. A second insulating film formed on the surface of the first insulating film and the second thin wire;
A meander-shaped third fine wire made of a superconductive material formed on the surface of the second insulating film;
A third insulating film formed on the surface of the second insulating film and the third thin wire;
A meander made of a superconducting material and an insulating film of N (N is an integer of 3 or more) having at least a fourth meander-shaped thin wire made of a superconducting material formed on the surface of the third insulating film. A superconducting photodetecting element having a multilayer structure in which N + 1 or more fine wires having a shape are alternately and repeatedly laminated,
The even-numbered one or more fine lines are perpendicular to the adjacent odd-numbered fine lines, and the arrangement direction of the even-numbered fine lines is perpendicular to the arrangement direction of the odd-numbered thin lines. The superconducting photodetection element according to claim 3, wherein the superconducting photodetection element is formed at a position where   The superconducting photodetection element according to claim 3 or 4, wherein each insulating film has a thickness of 10 nm or more and 100 µm or less. A meander-shaped first fine wire made of a superconducting material formed on the surface of the first crystal substrate;
A meander-shaped second fine wire made of the superconducting material formed on the surface of the second crystal substrate,
The second fine wire is positioned perpendicular to the first fine wire via an adhesive or an insulating film and an adhesive, and the arrangement direction of the second fine wire is in the arrangement direction of the first fine wire. A superconducting photodetecting element, wherein the superconducting photodetecting element is formed by bonding to the first thin wire at a position perpendicular to the first thin wire.   The superconducting photodetection element according to claim 6, wherein an interval between the first fine wire and the second fine wire is 1 µm or more and 100 µm or less.   It further has at least one of a non-reflective coating disposed on the surface side of the superconducting light detection element that is a light incident side, and a rearview mirror disposed on the back surface side of the superconducting light detection element. The superconducting photodetection element according to any one of claims 1 to 7.   In each meander-shaped thin wire, the size of the meander shape is not less than 1 μm square and not more than 10 μm square, the line width of the fine line is not less than 10 nm and not more than 200 nm, and the thickness of the fine line is not less than 1 nm and not more than 5 nm The superconducting photodetecting element according to claim 1, wherein the superconducting photodetecting element is provided. The superconductive material is NbN, and the crystal substrate and the insulating film are MgO, AlN, or aluminum oxide, or the superconductive material is MgB ₂ , and the crystal substrate and the insulating film are SiC, AlN, or oxide. The superconducting material is a copper oxide superconductor, or the crystal substrate and the insulating film are NdGaO _3, SrLaGaO _4, or LaSrAlO _4. 8. The superconducting photodetecting element according to any one of 1 to 7.

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