JP6440304B2 - Optical design method for superconducting single photon detector - Google Patents

Description

  The present invention relates to a superconducting single photon detector and an optical design method for the superconducting single photon detector.

  Superconducting single photon detectors (hereinafter abbreviated as SSPD as needed) that can detect photons one by one are attracting attention in a wide range of fields such as quantum communication, quantum optics, and life sciences. It is a key device. For example, by using SSPD in quantum cryptography key distribution tests, quantum entanglement experiments, etc., it has become possible to obtain extremely accurate results that cannot be achieved with conventional photon detectors such as semiconductor avalanche photodiodes. .

  By the way, various techniques for improving the light detection efficiency of such SSPD have already been proposed.

  For example, Patent Document 1 and Patent Document 2 propose an SSPD including an optical cavity layer between a nanowire used in a superconducting state and a substrate. Patent Document 1 describes an SSPD having a front side light incident structure (a structure in which light irradiation is performed from the surface side of a substrate). Patent Document 2 describes a backside light incident structure (a structure in which light irradiation is performed from the back side of a substrate). These have higher optical coupling efficiency to the nanowire and higher light absorption rate of the nanowire than SSPD having a single-layer structure that does not include the optical cavity layer, thereby improving the light detection efficiency of the SSPD.

JP 2009-38190 A JP 2010-258208 A

  However, in the conventional example, SSPD intended to have an arbitrary wavelength spectrum characteristic has not been studied.

  An aspect of the present invention has been made in view of such circumstances, and provides an SSPD that can have various wavelength spectrum characteristics as compared with the conventional one.

  Another aspect of the present invention provides an SSPD optical design method capable of appropriately performing the optical design of the SSPD.

  The SSPD of one embodiment of the present invention includes a substrate, a dielectric multilayer film provided on the substrate, and a nanowire provided on the dielectric multilayer film and used in a superconducting state, One photon is detected at a time based on a change in resistance of the nanowire when light in a wavelength band specified by the dielectric multilayer film is incident on the nanowire.

  According to one embodiment of the present invention, an SSPD that can have various wavelength spectrum characteristics can be obtained as compared with the related art.

FIG. 1A is a diagram illustrating an example of a superconducting single photon detector according to an embodiment. FIG. 1B is a diagram illustrating an example of the superconducting single photon detector according to the embodiment. FIG. 2 is a diagram showing an example of a two-dimensional cell unit corresponding to a model of a superconducting single photon detector for numerical simulation. FIG. 3A is a diagram illustrating an example of a light absorption spectrum in a solid film obtained by numerical simulation. FIG. 3B is a diagram illustrating an example of a light absorption spectrum in a nanowire obtained by numerical simulation. FIG. 3C is a diagram illustrating an example of a light absorption spectrum in a nanowire obtained by numerical simulation. FIG. 4 is a diagram illustrating an example of a light absorption spectrum in a solid film and a nanowire obtained by numerical simulation. FIG. 5 is a diagram illustrating an example of a light absorption spectrum in a solid film and a nanowire obtained by numerical simulation.

  In the conventional example, by optimizing the material and film thickness of the optical cavity layer, it is possible to improve the light detection efficiency of the SSPD for light of a specific wavelength. However, the optical cavity structure of the conventional example cannot give any wavelength spectrum characteristics to SSPD. Specifically, in the conventional example, it is difficult to optically design the SSPD in accordance with various requirements such as widening the wavelength band of light absorption of the nanowire, or conversely, extremely narrowing the wavelength band. Therefore, in the conventional example, for example, SSPD having a high optical absorptance in a wide wavelength band, and SSPD having a filter effect by providing a cutoff wavelength are disadvantageous for practical use, and various restrictions are imposed on the application of SSPD. There is.

  Accordingly, the inventors diligently studied the configuration of SSPD that can have various wavelength spectrum characteristics, and arrived at the idea of providing a dielectric multilayer film on the substrate instead of the optical cavity layer. At this time, considering light absorption at the substrate (eg, visible light absorption at the silicon substrate), it is determined that the SSPD having the front side light incident structure is more advantageous than the SSPD having the back side light incident structure. did.

  That is, the SSPD of this embodiment includes a substrate, a dielectric multilayer film provided on the substrate, and nanowires provided on the dielectric multilayer film and used in a superconducting state, One photon is detected at a time based on the resistance change of the nanowire when light in the wavelength band specified by the multilayer film enters the nanowire.

  According to such a configuration, an SSPD that can have various wavelength spectrum characteristics as compared with the conventional one can be obtained. In other words, since the SSPD of this embodiment includes a dielectric multilayer film, light in various wavelength bands can be converted into nanowires by changing the film structure (for example, material, film thickness, number of layers, etc.) of the dielectric multilayer film. Can be configured to absorb.

  In addition, the SSPD optical design method of the present embodiment uses a multilayer calculation software when it is assumed that a layer of the same material and the same film thickness as the nanowire is provided on the dielectric multilayer film. The material, film thickness, and number of layers of the dielectric multilayer film are derived so that the light absorption spectrum in the solid layer becomes the desired spectrum, and the dielectric material, film thickness, and number of layers derived by the multilayer film calculation software are derived. An optical absorption spectrum in a nanowire is obtained by performing a numerical simulation by assigning data when light of a predetermined wavelength is incident from above the nanowire to an SSPD model when a nanowire is provided on a body multilayer film. To derive.

  In addition, the electric field strength in the nanowire caused by light incidence is derived by numerical simulation, and the light absorption spectrum in the nanowire is derived based on the light absorption rate obtained from the electric field strength and the occupation rate of the nanowire.

  As described above, the material, film thickness, and number of layers of the dielectric multilayer film can be easily derived using the multilayer film calculation software so that the light absorption spectrum in the solid layer becomes a desired spectrum.

  In addition, by the numerical simulation, it is possible to optimally design the optical spectrum of the SSPD having nanowires that could not be handled by the multilayer film calculation software. At this time, the light absorption spectrum can be appropriately derived by considering the electric field strength and the occupancy of the nanowire in the nanowire.

  Hereinafter, specific examples of the present embodiment will be described with reference to the drawings. In the following description, the same or corresponding elements are denoted by the same reference symbols throughout the drawings, and redundant description is omitted.

[Device configuration]
1A and 1B are diagrams illustrating an example of a superconducting single photon detector according to an embodiment. FIG. 1A is a plan view of a superconducting single photon detector 100 (hereinafter, abbreviated as SSPD 100 if necessary). 1B is a cross-sectional view taken along the line BB in FIG. 1A.

  As shown in FIGS. 1A and 1B, the SSPD 100 includes a substrate 10, a dielectric multilayer film (DML) 11, and a wiring 12 made of meandering niobium nitride (NbN) (hereinafter, abbreviated as nanowire 12 if necessary). And). For example, silicon (Si) can be used as the material of the substrate 10. The SSPD 100 has a front side light incident structure for the above reasons.

The dielectric multilayer film 11 is provided on the substrate 10. In the present embodiment, the dielectric multilayer film 11 is formed by alternately stacking a TiO ₂ film having a high refractive index and a SiO ₂ film having a low refractive index on the substrate 10. As a result, by changing the film structure of the dielectric multilayer film 11 (for example, the thickness of these TiO ₂ films and SiO ₂ films, the number of layers, etc.), light in various wavelength bands is absorbed by the nanowires 12. Can be configured.

  The nanowire 12 is provided on the dielectric multilayer film 11 and is used in a superconducting state. And the area | region of the nanowire 12 comprises the photon detection part of SSPD100. For example, the nanowire 12 has a meander shape having a predetermined line width (for example, a line width of about 80 nm to 200 nm) by patterning a niobium nitride layer having an appropriate thickness (for example, about 10 nm) using an electron beam or the like. Can be manufactured by forming. At this time, the size of the light detection unit of the SSPD 100 may be set to about 10 μm to 20 μm square, for example.

  Here, the nanowire 12 is cooled by an appropriate cooling means (not shown) so that it can be used in a superconducting state. Both ends of the nanowire 12 are connected to output terminals of a bias source (not shown) via the transmission path 13 so that a desired bias current slightly below the critical current flows through the nanowire 12. The transmission path 13 also functions as a path for flowing a bias current through the nanowire 12. The transmission path 13 is often made of the same material (niobium nitride) as the niobium nitride layer for the purpose of making the SSPD 100 difficult to break at the contact surface with the niobium nitride layer.

  Further, in the SSPD 100, a single photon can be detected one by one based on a change in resistance of the nanowire 12 when the single photon is incident on the nanowire 12 from above the nanowire 12. Specifically, when a photon is incident on the nanowire 12, the cap energy is exceeded at the position where the photon is incident, and as a result, a normal conduction region (resistance region) called a hot spot is generated. At this time, the current flowing through the nanowire 12 flows intensively in the portions of the nanowire 12 on both sides of the region so as to bypass the resistance region. Then, the current flowing around the region exceeds the critical current, the portions on both sides of the region are also in the normal conduction state, and the region in the normal conduction state temporarily spreads over the entire width direction of the nanowire 12. In this way, one photon entering the nanowire 12 as a voltage signal is generated based on the resistance change over the entire width direction of the nanowire 12 in the process of generating the normal conduction region and restoring the normal conduction region to the superconducting state. Each voltage signal is appropriately detected, and the voltage signal is taken out from the transmission path 13.

  In addition, the detection method itself of SSPD which detects such a photon one by one is well-known (for example, refer to "IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL.11, NO.1, MARCH 2001 P574-577").

  As described above, in the present embodiment, the SSPD 100 that can have various wavelength spectrum characteristics as compared with the prior art is obtained. That is, since the SSPD 100 of the present embodiment includes the dielectric multilayer film 11, light in various wavelength bands can be obtained by changing the film structure (for example, material, film thickness, number of layers, etc.) of the dielectric multilayer film 11. The nanowire 12 can be configured to absorb. Therefore, by using such SSPD 100, SSPD having a high light absorption rate in a wide wavelength band, and SSPD having a filter effect by providing a cutoff wavelength can be put into practical use.

The above SSPD 100 is an example, and is not limited to this example. For example, the superconducting material used for the nanowire 12 may be NbTiN, Nb, TaN, MgB ₂ or the like in addition to NbN. In addition to Si, the material of the substrate 10 may be MgO, quartz (quartz), sapphire, or the like. In addition to the TiO ₂ film, an Nb ₂ O ₅ film, a Ta ₂ O ₅ film, or the like is used as the high refractive index film of the dielectric multilayer film 11, and an Al ₂ O film other than the SiO ₂ film is used as the low refractive index film. _Three films or the like may be used.

[SSPD optical design method]
Hereinafter, an optical design method of the SSPD 100 of the present embodiment will be described with reference to the drawings.

First, assuming that a niobium nitride layer having the same material and the same film thickness as the nanowire 12 is provided on the dielectric multilayer film 11 in a solid form, this solid form is obtained using appropriate multilayer film calculation software. The material, film thickness, and number of layers of the dielectric multilayer film 11 are derived so that the light absorption spectrum in the niobium nitride layer becomes a desired spectrum. Here, as described above, since the TiO ₂ film is used as the high refractive index film of the dielectric multilayer film 11 and the SiO ₂ film is used as the low refractive index film, the light absorption spectrum in the niobium nitride layer is desired. The film thickness and the number of layers of these TiO ₂ film and SiO ₂ film are derived so that In the present embodiment, the dielectric is calculated by the multilayer calculation software so that the light absorption rate in the niobium nitride layer takes a flat maximum value when the light wavelength is around 650 nm-900 nm (that is, the light wavelength is near the center of 800 nm). The film thickness and the number of layers of the multilayer film 11 (TiO ₂ film and SiO ₂ film) are set.

  Various methods for calculating the transmittance and reflectance of the dielectric multilayer film 11 are already known, and many commercially available multilayer film calculation software are also provided. Therefore, detailed description is omitted.

  Next, a model of the SSPD 100 in the case where the nanowire 12 is provided on the dielectric multilayer film 11 having the material, film thickness, and number of layers derived by the multilayer film calculation software is created on a computer. Specifically, the model is reproduced on a computer by dividing an area corresponding to the model into fine elements.

  FIG. 2 is a diagram showing an example of a two-dimensional cell unit corresponding to the model of SSPD 100 for numerical simulation. In the present embodiment, the thickness T of the nanowire 12 is set to 10 nm, and the width W and the gap S of the nanowire 12 are each set to 100 nm. Therefore, the cell unit period P is 200 nm. In addition, as boundary conditions for the two-dimensional cell unit CU, periodic boundary conditions are set for the meshes on both the left and right sides of the cell unit CU, and calculation is performed. This makes it possible to reduce the number of meshes of the SSPD 100 model for numerical simulation, thereby saving the storage capacity of the computer and shortening the calculation time.

  Then, a light absorption spectrum in the nanowire 12 is derived by assigning data when light of a predetermined wavelength is incident from above the nanowire 12 to the above model and performing a numerical simulation. Specifically, Maxwell, which is a basic equation that describes the behavior of the electromagnetic field using finite element method software (for example, COSMOL Multiphysics 4.4, RF module) by assigning physical property conditions and the above data to each element of the model. The equation is solved and the light absorption rate in the nanowire 12 is numerically simulated.

At this time, the electric field strength in the nanowire 12 generated by light incidence is derived by numerical simulation, and the light in the nanowire 12 is based on the light absorptivity obtained from parameters such as the electric field strength and the occupancy (filling factor FF) of the nanowire 12. An absorption spectrum is derived. Specifically, the light absorption rate A can be formulated by the following formula (1), and is approximately proportional to the product of the average electric field strength E _avg and the filling factor FF in the nanowire 12.

なお、式（１）のフィリングファクターＦＦは、ナノワイヤ１２の幅Ｗ及びセルユニット周期Ｐの値から１／２（ＦＦ＝Ｗ／Ｐ＝１／２）となる。

In addition, the filling factor FF of Expression (1) is ½ (FF = W / P = ½) from the value of the width W of the nanowire 12 and the cell unit period P.

  Therefore, by calculating the optical absorptance A in the nanowire 12 using the equation (1), the optical absorptance in the nanowire 12 can be calculated for light of a certain wavelength, and this calculation is performed while slightly changing the wavelength of the light. When it is repeated, the light absorption spectrum in the nanowire 12 can be derived.

  FIG. 3A is a diagram illustrating an example of a light absorption spectrum in a solid film obtained by numerical simulation. 3B and 3C are diagrams illustrating an example of a light absorption spectrum in a nanowire obtained by numerical simulation. Specifically, FIG. 3A shows an example of a light absorption spectrum in a solid niobium nitride layer. The reason why the light absorption spectrum of FIG. 3A is calculated is to check whether the light absorption spectrum calculated by the multilayer film calculation software can be reproduced by numerical simulation.

  On the other hand, FIG. 3B shows an example of an absorption spectrum of TE polarized light in the nanowire 12. FIG. 3C shows an example of an absorption spectrum of TM polarized light in the nanowire 12. Note that TE polarized light refers to light whose electric field vibrates in parallel with the extending direction of the nanowire 12, and TM polarized light refers to light whose electric field vibrates perpendicularly to the extending direction of the nanowire 12.

  3A, 3B, and 3C, it can be seen that the light absorption spectrum in the niobium nitride layer does not match the absorption spectrum of the TE-polarized light and the absorption spectrum of the TM-polarized light in the nanowire 12. This fact indicates that the conventional multilayer calculation software cannot handle the optical design of the SSPD 100 having a grid structure corresponding to the nanowire 12 portion.

  That is, the nanowire 12 that is a light absorber in the SSPD 100 needs to be regarded as a grid structure (uneven structure) as shown in FIG. 2 instead of a solid film, and correct optical design cannot be performed by the multilayer film calculation software.

  On the other hand, in the optical design of the SSPD 100 of the present embodiment, the optical spectrum of the SSPD 100 can be optimally designed by combining numerical simulation such as the finite element method with respect to the grid structure corresponding to the nanowire 12 portion. .

  As described above, in the optical design method of the SSPD 100 according to the present embodiment, the material, film thickness, and layer of the dielectric multilayer film 11 are set using the multilayer film calculation software so that the light absorption spectrum in the niobium nitride layer becomes a desired spectrum. Numbers can be derived easily.

  In addition, the numerical simulation enables optimal design of the optical spectrum of the SSPD 100 including the nanowire 12 that could not be handled by the multilayer film calculation software. At this time, the light absorption spectrum in the nanowire 12 can be appropriately derived by considering the electric field strength and the filling factor FF in the nanowire 12.

  Note that the optical design method of the SSPD 100 described above is an example, and is not limited to this example. For example, rigorous coupled-wave analysis (RCWA) or the like may be used as an optical design method for the SSPD 100 other than the numerical simulation based on the finite element method.

In the optical design of the above SSPD 100, the dielectric multilayer film 11 (TiO ₂ film and SiO ₂ film is formed so that the light absorption rate in the niobium nitride layer takes a flat maximum value when the wavelength of light is around 650 nm to 900 nm. However, the present invention is not limited to this.

  For example, the material of the dielectric multilayer film 11 so that the light absorption rate in the niobium nitride layer takes a flat maximum value within a predetermined range (for example, 440 nm to 560 nm; the center of the light wavelength is about 500 nm), The film thickness and the number of layers may be set. Then, as illustrated in FIG. 4, it becomes possible to put SSPD having a bandpass filter effect with a wide band wavelength into practical use.

  In addition, when the wavelength of light is around a predetermined value (for example, 1550 nm), the light absorptance in the niobium nitride layer has a maximum value, and when the wavelength of light is at a predetermined value (for example, 1600 nm) or more, the light absorption in the niobium nitride layer is The material, film thickness, and number of layers of the dielectric multilayer film 11 may be set so as to cut off. Then, as illustrated in FIG. 5, it is possible to put SSPD having a filter effect into practical use by providing a cutoff wavelength.

  From the foregoing description, many modifications and other embodiments of the present invention are obvious to one skilled in the art. Accordingly, the foregoing description should be construed as illustrative only and is provided for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details of the structure and / or function may be substantially changed without departing from the spirit of the invention.

  One embodiment of the present invention provides an SSPD that can have various wavelength spectrum characteristics as compared with the related art. Thus, one embodiment of the present invention can be used for SSPDs that are attracting attention in a wide range of fields such as quantum communication, quantum optics, and life science.

10 Substrate 11 Dielectric Multilayer 12 Nanowire (Niobium Nitride Wiring)
13 Transmission path 100 Superconducting single photon detector (SSPD)

Claims (2)

A substrate,
A dielectric multilayer film provided on the substrate;
A nanowire provided on the dielectric multilayer and used in a superconducting state;
With
Optical design of a superconducting single photon detector in which photons are detected one by one based on a resistance change of the nanowire when light in a wavelength band specified by the dielectric multilayer film is incident on the nanowire A method,
When it is assumed that a layer of the same material and the same film thickness as the nanowire is provided in a solid shape on the dielectric multilayer film, a light absorption spectrum in the solid layer is calculated using a multilayer film calculation software. Deriving the material, film thickness and number of layers of the dielectric multilayer so that the desired spectrum is obtained,
The superconducting single-photon detector model when the nanowire is provided on the dielectric multilayer film of the material, film thickness and number of layers derived by the multilayer film calculation software has a predetermined wavelength from above the nanowire. An optical design method for deriving a light absorption spectrum in the nanowire by performing numerical simulation with data when light of the incident light enters. Directing an electric field intensity in the nanowire produced by light incident at the numerical simulations, the electric field intensity and the optical of claim 1 to derive the light absorption spectrum in the nanowire based on light absorption resulting from the occupation ratio of the nanowire Design method.

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