Superconducting nanowire single-photon detector based on optical Tamm state

Abstract

The invention discloses a superconducting nanowire single-photon detector based on an optical Tamm state. The superconducting nanowire single-photon detector is composed of an upper-layer photonic crystal I, a superconducting nanowire winding at an interface, a lower-layer photonic crystal II and a substrate from top to bottom. The photonic crystal is formed by periodically overlapping and arranging two dielectric layers with a high refractive index and a low refractive index respectively; the crystal I and the crystal II are two different types of photonic crystals, the contact dielectric layers of the two types of photonic crystals are respective high-refractive-index layers, and the two high-refractive-index layers are made of different materials; the thickness of each dielectric layer in the photonic crystal is 1 / 4 of the equivalent wavelength of incident light in the dielectric layer; the period number of the upper-layer photonic crystal I is 3-6; the period number of the lower-layer photonic crystal II is 8-14; when light enters from the front side, the optical Tamm state is excited at the interface of the two different photonic crystals. Due to the optical Tamm state at the interface of the two photonic crystals, the superconducting nanowire single-photon detector has the performance of narrow-band high absorption and polarization insensitivity.

Description

Technical Field

[0001] The present invention relates to the field of optoelectronic detection, and particularly to a superconducting nanowire single photon detector based on an optical Tamm state. Background Art

[0002] A superconducting nanowire single photon detector (SNSPD) is the single photon detector with the best comprehensive performance in the near-infrared band. It has excellent performances such as ultra-high system detection efficiency and extremely low time jitter, and has important application values in cutting-edge fields such as quantum key distribution, integrated circuit detection, and long-distance laser communication.

[0003] The photosensitive region of the SNSPD is composed of meandering nanowires with a line width in the range of dozens to hundreds of nanometers, and generally operates in a temperature range below 4K. Its working mechanism is as follows: when the superconducting nanowire absorbs an incident photon, the photon energy causes Cooper pairs to break, resulting in local normal state of the nanowire, becoming a resistive state. By applying a bias current I c slightly lower than the superconducting transition current I b , a voltage pulse can be output. After a recovery time, Cooper pairs reform, and the nanowire returns to the superconducting state, waiting to detect the next incident photon.

[0004] In some cases, although no photons are incident on the detector, the detector still outputs voltage pulses. Such miscounts are called "dark counts". The level of dark counts is one of the important indicators for measuring the performance of the SNSPD. The main sources of dark counts are intrinsic dark counts and background dark counts. The generation reason of intrinsic dark counts is not yet clear at present, while background dark counts come from environmental noises such as ambient light and thermal radiation introduced when optical fibers are coupled to the SNSPD. In order to reduce background dark counts, optical filtering can be adopted. The existing optical filtering methods include external fiber filters, etc., but this structure will cause greater signal attenuation; or electromagnetic shielding methods using aluminum foil and copper foil wrapping are adopted, but this method relies on manual wrapping and the effect is limited.

[0005] Another important indicator of the SNSPD is the absorption efficiency, which directly affects the detection efficiency of the device. At present, the absorption of nanowires can be improved by integrating an optical resonator and a metal mirror method. However, since the metal has absorption in the near-infrared band, this method will cause loss of some photons; there are also studies on improving the absorption by fabricating the device on a distributed Bragg reflector (DBR) substrate. Although the DBR basically has no loss, the absorption spectral bandwidth of the SNSPD based on this structure is relatively large, about 300 - 400 nanometers wide, and it cannot filter out background noises outside the target band, that is, the dark counts cannot be reduced, which is not conducive to applications such as single photon imaging.

[0006] Due to the anisotropy of its intrinsic geometric structure, the serpentine SNSPD has a problem of being sensitive to the polarization of incident light. This structure has a higher light absorption efficiency for light polarized parallel to the long axis direction of the nanowire (TE mode), while a lower light absorption efficiency for light polarized perpendicular to the long axis direction of the nanowire (TM mode), resulting in the detection mode efficiency varying with the change of the polarization state of the incident light. In some applications, such as free-space optical communication or light transmission in scattering media, the polarization state of light may be random or uncontrollable. Therefore, in these scenarios, the SNSPD needs to have polarization-insensitive characteristics. The following existing solutions can achieve polarization-insensitive regulation of SNSPD: changing the nanowire morphology, such as using a bent or wavy nanowire structure; arranging multiple nanowires in a cross pattern to form an orthogonal grid structure, etc. These methods can reduce polarization sensitivity, but may increase the process complexity. In addition, the polarization sensitivity of the nanowire can be reduced by the dielectric compensation method, which means coating the nanowire with a high-dielectric constant material such as Si to reduce the difference in dielectric constants between the nanowire and the surrounding medium, making the electric field distribution more uniform to improve the photon absorption efficiency in the TM mode. However, this method will affect the system detection efficiency of SNSPD because materials such as Si have absorption in the visible light and some near-infrared bands.

[0007] In view of the above, there is an urgent need to propose an SNSPD with high process feasibility, narrowband high absorption, and the ability to achieve polarization-insensitive effects.

[0008] Optical Tamm State (OTS) is a localized electromagnetic mode existing at the interface between two different photonic crystals or at the metal-photonic crystal interface, which can achieve local electromagnetic field enhancement. When the OTS is excited, the electromagnetic field will be localized near the interface, significantly enhanced, and exponentially oscillate and decay along the normal direction. The OTS existing at the interface between two photonic crystals will excite a sharp transmission peak near the center of the bandgaps of the two photonic crystals, allowing light to pass through. Compared with surface plasmon modes excited by structures such as metasurface modes and metal antennas, OTS is a planar structure that does not require patterning micro-nano processing techniques such as ultraviolet lithography and electron beam lithography, greatly reducing the process difficulty and cost. Due to its planar characteristics, OTS can also achieve polarization-insensitive excitation. In addition, OTS also has characteristics such as a narrow bandwidth of the excited transmission peak, and inherently has the advantage of being a narrowband filter; the local field enhancement effect existing at the two high-reflection interfaces also has a wide range of applications in improving material absorption. Summary of the Invention

[0009] Object of the Invention: The present invention aims to provide a narrowband high-absorption and polarization-insensitive superconducting nanowire single-photon detector based on optical Tamm state.

[0010] Technical solution: The superconducting nanowire single-photon detector based on the optical Tamm state of the present invention is composed of an upper photonic crystal I, a serpentine superconducting nanowire at the interface, a lower photonic crystal II, and a substrate from top to bottom;

[0011] The photonic crystal is formed by periodically overlapping two dielectric layers with high and low refractive indices respectively; the upper photonic crystal I and the lower photonic crystal II are two different types of photonic crystals, and the contact dielectric layers of the two photonic crystals are their respective high refractive index layers, and the materials of the two high refractive index layers are different; the thickness of each dielectric layer in the photonic crystal is one-fourth of the equivalent wavelength of the incident light in that layer; the number of periods of the upper photonic crystal I is 3 to 6; the number of periods of the lower photonic crystal II is 8 to 14; when light is incident from the front, an optical Tamm state is excited at the interface of the two different photonic crystals.

[0012] Further preferably, the number of periods of the upper photonic crystal I is 4 to 5.

[0013] Two different photonic crystals are selected, the thickness of the photonic crystal is controlled to be 1 / 4 of the equivalent wavelength of the incident light in that layer, and by adjusting the period number matching of the upper and lower photonic crystals, an optical Tamm state is excited at the interface of the photonic crystals when light is incident.

[0014] Preferably, the real part of the refractive index of the high refractive index layer dielectric material in the target band is 2 to 3.5; the real part of the refractive index of the low refractive index layer dielectric material in the target band is 1.4 to 1.7.

[0015] Preferably, the extinction coefficients of the high refractive index layer and the low refractive index layer dielectric materials in the target band are both less than 10 -3 .

[0016] Preferably, the photonic crystal I uses Ta2O5, Si3N4, Si or GaAs as the high refractive index layer dielectric material, and SiO2 or Al2O3 as the low refractive index layer dielectric material.

[0017] Preferably, the photonic crystal II uses Nb2O5, Nb2O5, Si3N4, Si or GaAs as the high refractive index layer dielectric material, and SiO2 or Al2O3 as the low refractive index layer dielectric material.

[0018] Preferably, the material of the superconducting nanowire is NbN, NbTiN, MoSi or WSi.

[0019] Preferably, the thin film thickness of the superconducting nanowire is 4 to 10 nanometers, the line width is 50 to 150 nanometers, and the period is 100 to 300 nanometers.

[0020] Preferably, the substrate is selected from Si, MgF2 or MgO.

[0021] Preferably, the thickness of the substrate is 300 microns to 1 millimeter.

[0022] Preferably, the dielectric layer in the photonic crystal can be prepared by magnetron sputtering, ion beam sputtering, electron beam evaporation, plasma enhanced chemical vapor deposition or the like.

[0023] Invention mechanism: For the superconducting nanowire single-photon detector based on the optical Tamm state of the present invention, when photons are incident from the front, optical Tamm states will be excited at the interfaces of two different photonic crystals, realizing local electromagnetic field enhancement, so that the absorption rate of the superconducting nanowire located there is significantly increased. In addition, due to the narrow transmission peak excited by the optical Tamm state, the nanowire can achieve high absorption of the target narrowband wavelength. The excitation of the Tamm state is not affected by the polarization direction of the incident light, and since the nanowire is wrapped by a high dielectric constant layer, based on the dielectric compensation principle, its polarization insensitivity can be further realized. The unique feature of the superconducting nanowire single-photon detector in the present invention is that it combines three characteristics: narrow response bandwidth, high absorption rate, and polarization insensitivity. It can effectively filter out the environmental blackbody radiation in non-target wavelength bands without connecting other optical filters, thereby reducing the background dark count; it can also achieve polarization insensitivity without changing the morphological structure of the nanowire or adding other polarization compensation structures. At the same time, the gain effect for incident light of different wavelengths can be achieved by adjusting the thickness of the dielectric layer of the photonic crystal. This has high industrial application value in the modern optical field.

[0024] Beneficial effects: Compared with the prior art, the present invention has the following remarkable advantages: (1) For the superconducting nanowire single-photon detector of the present invention, by forming an optical Tamm state at the interface of two photonic crystals, the detector has the performance of narrowband high absorption and polarization insensitivity; (2) In the target wavelength band, in the TE and TM polarization modes, the absorption rate of the nanowire can reach more than 95%, and the quality factor Q value of the absorption peak reaches more than 100. Description of the drawings

[0025] Figure 1 It is a structural diagram of the narrowband high-absorption and polarization-insensitive superconducting nanowire single-photon detector based on the optical Tamm state in the present invention;

[0026] Figure 2 It is a distribution diagram of the absolute value of the electric field along the normal direction after the combination of photonic crystal one and photonic crystal two in Example 1;

[0027] Figure 3 It is a simulation result diagram of the absorption rate of the detector in the 1550-nanometer wavelength band in Example 1;

[0028] Figure 4 It is a simulation result diagram of the absorption rate of the detector in the 1550-nanometer wavelength band in Example 2;

[0029] Figure 5 Structural diagram of a high-absorption superconducting nanowire single-photon detector based on a DBR substrate in Comparative Example 1;

[0030] Figure 6 Simulation result diagram of the absorption rate of the detector in Comparative Example 1. Specific implementation manners

[0031] The technical solution of the present invention will be further described below in conjunction with embodiments.

[0032] Embodiment 1

[0033] The superconducting nanowire single-photon detector based on the optical Tamm state of the present invention, as Figure 1 shown, includes Photonic Crystal 1, which is formed by periodically stacking a silicon dioxide (SiO2, n = 1.45 @ 1550 nm, k ≈ 0 @ 1550 nm, n is the real part of the refractive index, k is the extinction coefficient) dielectric layer 11 and a niobium pentoxide (Nb2O5, n = 2.21 @ 1550 nm, k ≈ 0 @ 1550 nm) dielectric layer 12; Photonic Crystal 2, which is formed by periodically stacking a silicon dioxide (SiO2, n = 1.45 @ 1550 nm) dielectric layer 31 and a tantalum pentoxide (Ta2O5, n = 2.08 @ 1550 nm, k ≈ 0 @ 1550 nm) dielectric layer 32; the superconducting nanowire 2 located at the interface of the high refractive index layers of the two photonic crystals is a serpentine niobium nitride (NbN, n = 5.23 @ 1550 nm, k ≈ 0 @ 1550 nm); the substrate 4 is silicon (Si).

[0034] In this embodiment, the wavelength of the incident photon is near-infrared 1550 nm, which is incident from the photonic crystal side of the device. In Photonic Crystal 1, the thickness of the SiO2 layer is 268 nm, the thickness of the Nb2O5 layer is 173 nm, and the number of periods is 5. In Photonic Crystal 2, the thickness of the SiO2 layer is 268 nm, the thickness of the Ta2O5 layer is 186 nm, and the number of periods is 14. The thickness of the NbN nanowire is 6 nm, the width is 100 nm, and the period is 200 nm. The thickness of the Si substrate is 400 μm.

[0035] The near-infrared 1550 nm is the target wavelength required for various applications. In order to achieve the gain effect at this wavelength, the thickness of the dielectric layer is set to λ / (4*n), where λ is the wavelength of the incident light and n is the refractive index of the dielectric layer at this wavelength. That is, the thickness of each dielectric layer in the photonic crystal is 1 / 4 of the equivalent wavelength of the incident light in this layer.

[0036] Figure 2In this embodiment, after the combination of Photonic Crystal 1 and Photonic Crystal 2, the absolute value distribution diagram of the electric field in the normal direction can be seen. The electromagnetic field is localized near the interface of the two crystals, the electric field intensity is significantly enhanced, and it decays exponentially on both sides in the normal direction, which conforms to the core characteristics of the optical Tamm state. This proves that the enhanced effect of the SNSPD in this structure is based on the optical Tamm state.

[0037] Figure 3 For the simulation result diagram of the absorption rate of the NbN nanowire in this embodiment under the finite-difference time-domain simulation. The solid line indicates that when light polarized in the direction parallel to the long axis of the nanowire (TE mode) is incident, the nanowire can reach an absorption efficiency of 98.8% at a wavelength of 1548 nanometers; the dashed line indicates that when light polarized in the direction perpendicular to the long axis of the nanowire (TM mode) is incident, the nanowire can reach an absorption efficiency of 94.8% at 1551 nanometers. The asterisk indicates that in both polarization modes, the nanowire can reach an absorption efficiency of 94.4% at a wavelength of 1550 nanometers, that is, this detector can achieve polarization insensitivity at 1550 nanometers. The fitted full width at half maximum (FWHM) of the nanowire absorption peak is 16 nanometers, and the quality factor Q value reaches approximately 100. Compared with the conventional DBR substrate detector with a half width at half maximum of about 400 nanometers, this structure improves the Q value by an order of magnitude, which can effectively filter out stray light in non-target wavelength bands and reduce background dark counts.

[0038] Embodiment 2

[0039] The device structure of this embodiment is basically the same as that of Embodiment 1. The difference lies in the change of the overlapping material and the number of periods in Photonic Crystal 1. Among them, the high refractive index material 12 is changed to silicon nitride (Si3N4, n = 2.46 @ 1550 nanometers, k ≈ 0 @ 1550 nanometers), with a thickness of 157 nanometers, and the low refractive index material 11 is silicon dioxide (SiO2, n = 1.45 @ 1550 nanometers, k ≈ 0 @ 1550 nanometers), with a thickness of 268 nanometers, and its number of periods is 4.

[0040] Figure 4 For the simulation result diagram of the absorption rate of the NbN nanowire in this embodiment under the finite-difference time-domain simulation. The solid line indicates that when light polarized in the direction parallel to the long axis of the nanowire (TE mode) is incident, the nanowire can reach an absorption efficiency of 99.1% at a wavelength of 1547 nanometers; the dashed line indicates that when light polarized in the direction perpendicular to the long axis of the nanowire (TM mode) is incident, the nanowire can reach an absorption efficiency of 95.9% at 1551 nanometers. The asterisk indicates that in both polarization modes, the nanowire can reach an absorption efficiency of 94.6% at a wavelength of 1550 nanometers, that is, this detector can achieve polarization insensitivity at 1550 nanometers. The fitted full width at half maximum (FWHM) of the nanowire absorption peak is 20 nanometers, and the quality factor Q value reaches approximately 77.5.

[0041] Comparative Example 1

[0042] As a comparison with the detector in the present invention, a high-absorption SNSPD based on a DBR substrate is given in this example. As Figure 5 shown, the difference in its structure from that in Embodiment 1 is that it does not have the upper photonic crystal I, and the lower photonic crystal II is formed by the overlapping of a silicon oxide dielectric layer 2 and a tantalum oxide dielectric layer 3, with a period number of 13.5, that is, the uppermost and lowermost dielectric layers are both silicon oxide; the nanowire 1 is located above the photonic crystal II, and the substrate 4 is located below the photonic crystal II.

[0043] Figure 6 This is the simulation result diagram of the absorption rate of the nanowire in this embodiment under the finite-difference time-domain simulation. In the 1550-nanometer band, although the absorption rate can also reach 99%, the full width at half maximum of its absorption peak is as wide as about 380 nanometers, and the Q value is only 4, which is one twenty-fifth of that in Embodiment 1 of the present invention, and it does not have the function of filtering the background dark count in the target band or improving the signal-to-noise ratio.

Claims

1. A superconducting nanowire single-photon detector based on optical Tamm state, characterized in that: From top to bottom, it consists of an upper photonic crystal 1, a winding superconducting nanowire at the interface, a lower photonic crystal 2 and a substrate; the photonic crystal is composed of two medium layers with high refractive index and low refractive index respectively, which are arranged in a periodic overlapping manner; the upper photonic crystal 1 and the lower photonic crystal 2 are two different types of photonic crystals, and the contact medium layers of the two photonic crystals are their own high refractive index layers, and the two high refractive index layers are made of different materials; the thickness of each medium layer in the photonic crystal is one quarter of the equivalent wavelength of the incident light in the layer; the period number of the upper photonic crystal 1 is 3 to 6; the period number of the lower photonic crystal 2 is 8 to 14; when light is incident from the front, optical Tamm states are excited at the interfaces of two different photonic crystals.

2. The superconducting nanowire single-photon detector based on optical Tamm state according to claim 1, characterized in that: The real part of the refractive index of the high refractive index layer dielectric material in the target waveband is 2 to 3.5; the real part of the refractive index of the low refractive index layer dielectric material in the target waveband is 1.4 to 1.

7.

3. The superconducting nanowire single-photon detector based on optical Tamm state according to claim 1, characterized in that: The extinction coefficients of the high refractive index layer and the low refractive index layer dielectric materials in the target wavelength band are both less than 10 -3 .

4. The superconducting nanowire single-photon detector based on optical Tamm state according to claim 1, characterized in that: The photonic crystal 1 uses Ta2O5, Si3N4, Si or GaAs as the medium material of the high refractive index layer, and SiO2 or Al2O3 as the medium material of the low refractive index layer.

5. The superconducting nanowire single-photon detector based on optical Tamm state according to claim 1, characterized in that: The photonic crystal 2 uses Nb2O5, Nb2O5, Si3N4, Si or GaAs as the medium material of the high refractive index layer, and SiO2 or Al2O3 as the medium material of the low refractive index layer.

6. The superconducting nanowire single-photon detector based on optical Tamm state according to claim 1, characterized in that: The material of the superconducting nanowire is NbN, NbTiN, MoSi or WSi.

7. The superconducting nanowire single-photon detector based on optical Tamm state according to claim 1, characterized in that: The film thickness of the superconducting nanowire is 4 to 10 nanometers, the line width is 50 to 150 nanometers, and the period is 100 to 300 nanometers.

8. The superconducting nanowire single-photon detector based on optical Tamm state according to claim 1, characterized in that: The substrate is selected from Si, MgF2 or MgO.

9. The superconducting nanowire single-photon detector based on optical Tamm state according to claim 1, characterized in that: The thickness of the substrate is 300 micrometers to 1 millimeter.

10. The superconducting nanowire single-photon detector based on optical Tamm state according to claim 1, characterized in that: The dielectric layer in the photonic crystal can be prepared by magnetron sputtering, ion beam sputtering, electron beam evaporation or plasma enhanced chemical vapor deposition.

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