JP2018004444A - Superconductive single photon detector module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a superconductive single photon detector module which enables input of a photon entering from an optical fiber into a silicon fine wire optical waveguide with a small loss.SOLUTION: The superconductive single photon detector module includes a superconductive single photon detector and a quartz-based core optical waveguide 204 stacked on an under-clad 104 on a semiconductor substrate to be adjacent to each other. The superconductive single photon detector includes a silicon fine wire optical waveguide 106, a tapered spot-size converter 202 connected to the silicon fine wire optical waveguide at an end with a larger cross section, and a superconductive nanowire 108 on the silicon fine wire optical waveguide. The quartz system core optical waveguide is edge-connected to the optical fiber at an incident end, is connected to the spot-size converter, and is connected to the silicon fine wire optical waveguide by the spot-size converter.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a superconducting single photon detector module, and more particularly to a superconducting single photon detector module in which a superconducting single photon detector is integrated on an optical waveguide circuit.

  A superconducting single-photon detector (SSPD) is a light receiving element used for quantum information processing, quantum cryptography communication, and the like. In addition to the extremely low dark count rate (low noise), this device features a short excitation electron relaxation time, which makes it possible in principle to operate at a higher speed (on the order of GHz) than conventional InGaAs avalanche photodiodes. It is a point. The most promising structure is a thin-line optical waveguide coupling type SSPD. The thin-wire optical waveguide coupling type SSPD can realize a very small element by loading a superconducting film nanowire on an optical waveguide having strong optical confinement. As a result, an element structure with a small inductance is achieved, and high-speed operation is possible. The thin-line optical waveguide type SSPD is also characterized in that it can be integrated with an optical waveguide circuit.

  FIG. 1 is a schematic configuration diagram of a thin optical waveguide coupled SSPD. A fine-line optical waveguide coupling type SSPD 100 shown in FIG. 1 has an underclad 104 formed on a semiconductor substrate 102 and a fine-line optical waveguide 106 and a fine-line optical waveguide 106 formed on the underclad 104, respectively. It is an extremely small device including the loaded superconducting film nanowire 108 and the electrode 110 connected to the superconducting film nanowire 108.

Oliver Kahl, Simone Ferrari, Vadim Kovalyuk, Gregory N. Goltsman, Alexander Korneev & Wolfram H. P. Pernice, “Waveguide integrated superconducting single-photon detectors with high internal quantum efficiency at telecom wavelengths”, Scientific Reports, DOI: 10.1038 / srep10941 Hiroshi Fukuda, Koji Yamada, Tai Tsuchizawa, Toshifumi Watanabe, Hiroyuki Shino and Sei-ichi Itabashi, “Silicon photonic circuit with polarization diversity”, Optics express, Vol. 16, Issue 7, pp. 4872-4880- (2008). Ryoichi Kasahara, Yasuyuki Inoue, Motohashi Ishii, Hiroshi Takahashi, “AWG Pair Type Optical Bandpass Filter with High Rectangular Transmission Spectrum”, 2008 IEICE General Conference, C-3-23

  However, improvement of detection efficiency and noise reduction are problems in realizing a thin-line optical waveguide type SSPD.

  The detection efficiency of SSPD is limited mainly by the coupling loss of light between the optical fiber and the thin optical waveguide. Conventionally, there is an example in which a grating coupler is used for connection between an optical fiber and a thin optical waveguide (see, for example, Non-Patent Document 1). However, when a grating coupler is used for the connection between the optical fiber and the thin optical waveguide, the wavelength dependence of the coupling efficiency is large and the loss is also large.

  The main factor of dark count in SSPD is the energy of black body radiation incident on SSPD from a room temperature environment outside the freezer. There is a method of combining a band pass filter and SSPD for removing this noise component. However, when individual components are combined, the apparatus becomes large.

  Therefore, the filter and SSPD are monolithically integrated on the chip by taking advantage of the feature that can be integrated with the optical waveguide circuit, which is a feature of the thin-line optical waveguide type SSPD, and the device can be reduced in size and cost. .

  On the other hand, there is the following problem about monolithically integrating the filter and the SSPD on the chip.

  The effective refractive index of the thin optical waveguide is extremely sensitive to processing errors. Therefore, even if a thin optical waveguide having a square core with a cross section in the light propagation direction is designed, a large polarization dependence occurs in the effective refractive index due to a slight shape error during processing.

  A polarization-independent bandpass filter for cutting blackbody radiation on a chip can be monolithically integrated by using a thin optical waveguide, etc., but a polarization diversity circuit is required to prevent polarization dependence (For example, refer nonpatent literature 2). Therefore, excessive loss due to elements such as a polarization splitter and a polarization rotator is unavoidable, and as a result, detection efficiency is impaired.

  The present invention has been made in view of such problems, and the object of the present invention is to make it possible to input photons incident from an optical fiber into a silicon (Si) thin-wire optical waveguide with low loss. It is to provide a single photon detector module. It is another object of the present invention to provide a superconducting single-photon detector module that facilitates monolithic integration of polarization-independent filters.

  In order to achieve such an object, one aspect of the present invention is a superconducting single photon detector module. The superconducting single photon detector module includes a superconducting single photon detector and a silica-based core optical waveguide that are integrated adjacently on an underclad formed in a semiconductor substrate. The superconducting single-photon detector is a silicon wire optical waveguide formed on the underclad and a tapered spot size converter formed on the underclad. It comprises a spot size converter connected to the fine wire optical waveguide and a superconductor nanowire formed on the silicon fine wire optical waveguide. The quartz-based core optical waveguide is connected to the optical fiber at the incident end, is in contact with the spot size converter, and is connected to the silicon fine wire optical waveguide through the spot size converter.

  As described above, according to the present invention, it is possible to provide a superconducting single-photon detector module that allows incident photons from an optical fiber to be input to a silicon fine wire optical waveguide with low loss. . Furthermore, it is possible to provide a superconducting single photon detector module that facilitates monolithic integration of polarization independent filters.

It is a schematic block diagram of a thin-wire optical waveguide coupling type SSPD. (A) is a schematic block diagram of the thin wire | line optical waveguide coupling type | mold SSPD concerning one Embodiment of this invention, (b) is a thin wire | line optical waveguide coupling type | mold SSPD mounted in the high thermal conductivity material and end-face-coupled with the optical fiber. It is a figure which shows the superconducting single photon detector module which has. It is a figure which shows the experimental value of the fiber coupling efficiency in the Example of this invention. It is a figure which shows the relationship between the quantum efficiency and bias current in the chip | tip of SSPD measured using the superconducting single photon detector module in the Example of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The same or similar reference numerals in the drawings indicate the same or similar elements, and repeated description is omitted.

  The superconducting single photon detector module of the present embodiment has a thin optical waveguide coupled type SSPD mounted on a high thermal conductivity material and end face coupled with an optical fiber.

  FIG. 2A is a diagram showing a schematic configuration of a thin-line optical waveguide coupling type SSPD. The thin-wire optical waveguide coupling type SSPD 200 includes an underclad 104 formed on the semiconductor substrate 102, and a silicon (Si) thin-wire optical waveguide 106 and a Si thin-line optical waveguide 106 formed on the underclad 104, respectively. A superconductor nanowire 108 loaded on the electrode, an electrode 110 connected to the superconducting film nanowire 108, a silica-based core optical waveguide 204 end-connected to an optical fiber (not shown) through which input light propagates, and a silica-based core light A silicon spot size converter 202 that converts the mode field diameter of the input light incident on the waveguide 204 and couples it to the silicon thin-wire optical waveguide 106 is provided.

  The quartz-based core optical waveguide 204 is a waveguide having a core having a refractive index lower than that of Si. The quartz-based core optical waveguide is connected to an optical fiber at its end face, and low loss and a wide band in optical coupling between the optical fiber and the silicon fine wire optical waveguide 106 are possible.

  The silicon spot size converter 202 has a width (x direction) that expands from the tip toward the silicon fine wire optical waveguide 106 in the light propagation direction (y direction), and the silicon fine wire light guide is connected to the silicon fine wire optical waveguide 106. The taper shape is equal to the width of the waveguide 106 (reverse taper shape with respect to the light propagation direction (y direction)).

  The silica-based core optical waveguide 204 is formed so as to embed the silicon spot size converter 202, and input light that has entered from the optical fiber and propagated through the silica-based core optical waveguide 204 is coupled to the silicon spot size converter 202. The input light incident on the silicon spot size converter 202 has a mode field diameter converted and is coupled to the silicon fine wire optical waveguide 106. In this way, incident photons are coupled with low loss from the optical fiber to the silicon wire optical waveguide 106.

  In addition, since the mode field diameter of the silica-based core optical waveguide 204 is the same as the mode field diameter (core diameter) of the optical fiber, the polarization dependence of the effective refractive index due to the processing error of the silica-based core optical waveguide 204. Is extremely small. Therefore, a core with a square cross-section and a small polarization dependence can be easily created as the silica-based core optical waveguide 204, so a polarization-independent bandpass filter can be created without using a polarization diversity circuit. It becomes possible to do.

  The low refractive index waveguide as the quartz-based core optical waveguide 204 is obtained by, for example, a low temperature plasma CVD process used in a general CMOS back-end process, and an atomic composition ratio of silicon (Si), oxygen, and nitrogen, A dielectric film in which the atomic composition ratio of silicon (Si) and oxygen, or the atomic composition ratio of silicon (Si) and nitrogen can be adjusted, that is, the refractive index can be adjusted. By forming the dielectric film by a low-temperature plasma CVD process, it is possible to prevent thermal damage and oxidation to a very thin superconducting film (superconducting nanowire) having a thickness of several meters. Since the quantum efficiency of SSPD is affected by oxidation and defects of superconducting nanowires, it is preferable to form a dielectric film as the silica-based core optical waveguide 204 using the above-described material by a low temperature process.

  Since the superconductor 108 is formed on the silicon fine wire optical waveguide 106 of the semiconductor substrate 102, high thermal conductivity is required between the cooling stage and the chip. Therefore, the chip must be mounted on a package of high thermal conductivity material such as oxygen-free copper. Also, since the optical fiber is end-face coupled to the chip, the temperature is extremely low near the chip. Therefore, a high NA optical fiber that can guide input light with low loss even at extremely low temperatures is desirable. Specifically, it is a single mode fiber having a mode field diameter of less than 10 μm. In this case, the low index waveguide 204 is designed to match the mode field diameter of the high NA optical fiber.

(Example)
Examples are given below. The chip is manufactured using an SOI (Silicon on Insulator) substrate. On the chip, an Si thin-line optical waveguide 106, a tapered spot size converter (SSC) 202, and a silica-based core optical waveguide (SiOx waveguide) (3 μm square (core width 3 μm, core thickness 3 μm)) 204 are integrated. The Si thin-wire optical waveguide 106 and the SiOx waveguide 204 are connected by the SSC 202. After forming the Si thin-line optical waveguide 106 and the tapered spot size converter (SSC) 202 on the SOI substrate, superconductivity is formed on the Si thin-line optical waveguide 106, and the superconducting nanowire 108 is patterned. Thereafter, a chip can be manufactured by selectively growing a quartz-based core optical waveguide (SiOx waveguide) 204.

  The Si thin-line optical waveguide 106 has a core width of 400 nm (x direction) and a core thickness of 220 nm (z direction), and the core width (x direction) of the tapered tip of the SSC 202 is 80 nm. The SiOx waveguide (SiOx film) 204 is a Si-rich quartz-based film and is formed at a temperature of about 200 ° C. The superconducting nanowire 108 is an NbN (niobium nitride) film having a thickness of 7 nm (z direction), a thin line width of 60 nm, and a length of 50 μm, and has a pattern that is folded once on the Si thin line optical waveguide 106. The electrode 110 is made of NbN / Ti / Cr / Au. All the above elements are covered with a quartz overclad film (not shown).

  The refractive index of the 3 μm square SiOx waveguide 204 is designed to be mode-matched with a high NA optical fiber having a mode field diameter of 4.3 μm. The high NA optical fiber is connected at the end face (chip end face) of the SiOx waveguide 204 with resin.

  FIG. 2 (b) is a diagram showing a superconducting single photon detector module mounted on a high thermal conductivity material and having a thin-wire optical waveguide coupled SSPD. The chip 200 on which the thin-line optical waveguide coupling type SSPD is integrated is mounted (fixed) on a package 302 made of oxygen-free copper having high thermal conductivity, and an electric signal terminal (SMA connector) 304 of the package and the chip 200 are mounted on the chip 200. The electrodes 110 are electrically connected. The optical fiber is indicated by an optical fiber support material 308 (glass material) in which a V-groove is formed, and is coupled to the end surface of the thin-wire optical waveguide coupling type SSPD 200 (the end surface of the SiOx waveguide 204). The package 302 may be made of any metal material, not limited to oxygen-free copper, as long as the thermal conductivity is high. The thin-line optical waveguide coupling type SSPD 200 may or may not be sealed with a metal material.

  The SiOx waveguide 204 has a core diameter of about 10 times that of the Si thin-line optical waveguide 106, and the change in effective refractive index due to processing errors is extremely small.

  Table 1 shows the calculation result of the change in effective refractive index due to the core width variation per 1 nm of the SiOx waveguide 240 having a 3 μm square core of the Si fine-wire optical waveguide 106.

  The change in the effective refractive index with respect to the core width processing error of 1 nm in the core of the SiOx waveguide 240 is nearly two orders of magnitude smaller than that in the Si thin-line optical waveguide 106. Therefore, although the core of the Si thin-line optical waveguide 106 has a significant polarization dependency even with a slight shape error, the polarization dependency due to the processing error of the core in the core of the SiOx waveguide 204 becomes so small that it can be ignored. As a result, a polarization-independent bandpass filter or the like can be formed with the SiOx waveguide 204 without using a polarization diversity circuit or the like. It is known that a polarization-independent bandpass filter can be formed by using, for example, an AWG pair (see, for example, Non-Patent Document 3), but is not limited thereto. For example, by introducing a bent waveguide structure into the SiOx waveguide 204, noise light in the long wavelength region does not propagate, and the dark count rate can be lowered. If the waveguide size is adjusted so that confinement is weak, the function of the filter can be realized by the radius of curvature. Such a bending waveguide can be easily realized without adding a process step.

  The experimental value of the fiber coupling efficiency in this configuration is shown in FIG. The experimental value of the end face coupling efficiency of the high NA optical fiber 306 and the thin optical waveguide coupling type SSPD 200 at room temperature before and after performing the lattice detection experiment by installing the package 302 in the cryostat was 1.9 dB / facet. . This value includes the mounting resin between the optical fiber 306 and the SiOx waveguide 204, the loss of the SSC 202, and the like. It has been confirmed that this value hardly changes even under a cryogenic environment in the cryostat.

  FIG. 5 is a diagram showing the relationship between quantum efficiency and bias current in the SSPD module 300 produced in FIG. 4. The quantum efficiency of the chip 200 in which the thin-wire optical waveguide coupled SSPD is integrated is the net quantum of the SSPD on the chip 200 obtained by subtracting the coupling loss with the optical fiber and the loss of the Si waveguide 204 from the detection efficiency of the entire SSPD module 300. Efficiency. From the experimental results, the bias current is 7.6 μA or more, the quantum efficiency of SSPD is 90% or more in the experimental value, and the structure in which the SiOx waveguide 204 and the superconductor (superconducting nanowire) 108 are integrated is superconductive. It can be confirmed that there is very little oxidation and defects of the film, and there is almost no deterioration in the characteristics of the superconducting element. Further, it can be seen that superconductivity is exhibited on a package using oxygen-free copper, and good thermal conductivity is ensured between the cooling stage and the chip.

100,200 Fine line optical waveguide coupled SSPD
DESCRIPTION OF SYMBOLS 102 Semiconductor substrate 104 Under clad 106 Fine-line optical waveguide 108 Superconducting film nanowire 110 Electrode 202 Spot detail converter 204 Low refractive index waveguide 300 Superconducting single photon detector module 302 Package 304 Connector 306 Optical fiber 308 Optical fiber support material

Claims (7)

A superconducting single photon detector module comprising:
A superconducting single photon detector and a quartz-based core optical waveguide integrated adjacently on an underclad formed in a semiconductor substrate;
The superconducting single photon detector is
A silicon fine wire optical waveguide formed on the under cladding;
A taper-shaped spot size converter formed on the underclad, the spot size converter connected to the silicon thin-wire optical waveguide at the end of the larger cross section,
A superconductor nanowire formed on the silicon wire optical waveguide;
Consisting of
The silica-based core optical waveguide is
End face connection with optical fiber at the incident end,
In contact with the spot size converter, connected to the silicon fine wire optical waveguide through the spot size converter,
A superconducting single photon detector module.   2. The superconducting single photon detector module according to claim 1, wherein the quartz-based core optical waveguide is a dielectric film composed of silicon and at least one of oxygen and nitrogen.   The superconducting single photon detector module according to claim 1, wherein the optical fiber is a single mode fiber having a mode field diameter of less than 10 μm.   The superconducting single photon detector module according to any one of claims 1 to 3, wherein the quartz-based core optical waveguide constitutes a polarization-independent wavelength filter.   The superconducting single photon detector module according to claim 4, wherein the polarization-independent wavelength filter is an arrayed waveguide diffraction grating.   The superconducting single photon detector module according to claim 4, wherein the polarization-independent wavelength filter is a bent waveguide.   The superconducting single photon detector module according to claim 1, wherein the semiconductor substrate is fixed to a metal material.

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