CN112798116B - Intermediate infrared superconducting nanowire single photon detector - Google Patents
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Abstract
The invention discloses a middle infrared superconducting nanowire single photon detector, which adopts an electron beam lithography technology and a reactive ion etching technology to prepare an amorphous or polycrystalline superconducting thin film containing Mo and Si into a superconducting nanowire which is used as a photosensitive surface of the detector, thereby realizing the effective preparation of a middle infrared SNSPD; the detector is composed of a mid-infrared light source, an adjustable attenuator, a collimator, a band-pass filter, a dilution refrigerator, a photosurface, a biaser, an amplifier and a counter, and the difficulty of fiber coupling mid-infrared SNSPD is effectively solved by adopting a free space coupling technology; and photons of the intermediate infrared band are emitted and received, the number of photons reaching the photosensitive surface in unit time is calculated, and a foundation is laid for effectively calculating the intermediate infrared SNSPD quantum detection efficiency.
Description
Technical Field
The invention relates to the technical field of photon detection, in particular to an infrared photon detection technology.
Background
Mid-infrared radiation is defined as wavelengths in the range of 2.5-25 μm or 400-4000cm-1The wave number electromagnetic wave and mid-infrared single photon detection technology is one of the key technologies in the infrared field. Many biomolecules radiate fluorescent light at a wavelength of mid-redAnd on the outer wave band, the signal is extremely weak, and almost reaches the single photon magnitude. In the field of infrared astronomical detection, the noise equivalent power requirement of the internationally established latest infrared astronomical telescope, such as the origin number astronomical telescope, on the mid-infrared detector is improved to 10-25W/Hz1/2Magnitude. Therefore, the mid-infrared single-photon detector has extremely important application value and considerable development prospect in the fields of biomolecule spectral analysis, chemical research, astronomical observation and the like.
Conventional semiconductor single photon detectors, such as Si PMTs, can only operate in the visible wavelength range. Although the operating cut-off wavelengths of Si SPADs and InGaAs SPADs can be extended to near infrared, the detection efficiency is not high, and the dark noise is large. The third generation semiconductor photoelectric detector HgCdTe, quantum well and II type superlattice can work in medium wave infrared and even long wave infrared, but it is difficult to realize single photon detection. Although the frequency up-conversion single photon detector can carry out frequency doubling on mid-infrared photons to convert the wavelength of the mid-infrared photons into near infrared, and then the efficiency of detecting the single photons can reach 20% by adopting a system integrated avalanche photodiode such as InGaAs SPAD, the background noise is large.
In the past 20 years, a Superconducting Nanowire Single Photon Detector (SNSPD) shows very excellent working performance in the ultraviolet to near-infrared spectrum band: the detection efficiency is high, and the communication wavelength is 98% @1.55 mu m; the response speed is high, and dozens of MHz or even higher; the noise, i.e., the dark count is low,<10-3hz; the time precision is high, and the time precision is high,<3 ps. The SNSPD has extremely important application value in the scientific fields of quantum optics, satellite laser radar ranging, deep space communication, quantum key distribution, Bell inequality verification and the like.
Because the SNSPD has a low superconducting energy gap which is generally several meV, the longest response wavelength of the SNSPD can be extended to a terahertz wave band, and the SNSPD also has great application potential in the fields of mid-infrared single-photon application, such as biological fluorescence spectrum detection, astronomical exploration and the like. Although SNSPDs in recent years have a completely open head corner in the field of mid-infrared detection, the related technology is still not mature enough and needs to be continuously enriched and developed, such as materials, structures and the like. Compared with near infrared photons, the energy of the intermediate infrared photons is lower, higher detection efficiency and sensitivity are more difficult to obtain, and the development of SNSPD in the field of intermediate infrared detection is limited. How to effectively improve the quantum efficiency and the sensitivity of the intermediate infrared SNSPD is a big difficulty in the field of intermediate infrared superconducting single photon detectors.
The threshold model of the superconducting nanowire responding to the middle infrared single photon is
Where N is the number of superconducting Cooper pairs on a superconducting nanowire actually destroyed by a mid-infrared photon, EλThe energy of the middle infrared single photon absorbed by the nano wire, f (delta) is a variable factor which is in positive correlation with the superconducting energy gap, L is the quasi-particle diffusion length, wd is the sectional area of the nano wire, n0Is the superconducting electron density at zero bias current, IBAnd ICRespectively the bias current and the superconducting critical current of the nanowire, and alpha is a constant factor. Under different light response models, such as a "hot spot" model, a diffusion "hot spot" model, a magnetic flux crossing model, and a magnetic flux nucleation model, α has different values. The right side of the symbol represents the number of the damaged superconducting Cooper pairs required for causing the superconducting nanowire to generate complete superconducting phase change, so that the effective method for improving the photon response sensitivity of the intermediate infrared SNSPD can be developed from the following aspects.
1. The low energy gap superconducting material is adopted to reduce f (delta), so that infrared photons with certain energy can destroy more Cooper pairs on the superconducting nanowire to generate quasi-particles with resistance states, and the superconducting thin film for preparing the intermediate infrared SNSPD only comprises NbN, WSi and NbTiN.
2. By reducing carrier concentration, i.e. by reducing n0(1-IB/Ic)αThe photon energy is distributed to a smaller number of quasi-particles, increasing the temperature of the quasi-particles, and also increasing the thermal resistance on the nanowires.
3. The sectional area (wd) of the superconducting nanowire is reduced, so that the photon energy density absorbed on the sectional area is increased, the heat conduction along the longitudinal area of the nanowire is reduced, and finally, the probability of generating hot spots is greatly improved. In fact, because the thickness of a superconducting nanowire is usually around its GL coherence length, further reducing d will reduce the superconducting properties and absorption efficiency of the nanowire, the main method of reducing the cross-sectional area in the experiment is to reduce the nanowire width w.
In general, the intermediate infrared SNSPD has wide application prospect, but the research technology is still in the starting stage. On one hand, the low-gap superconducting nanowire with narrow line width less than 50nm is beneficial to improving the sensitivity of the intermediate infrared SNSPD, but the number of reported superconducting thin film materials for preparing the intermediate infrared SNSPD is still less. On the other hand, the optical fiber coupling SNSPD technology is widely applied to near infrared bands, but is difficult to apply to intermediate infrared bands. Therefore, the development of a new optical coupling technology is another necessary way for the research and development of the mid-infrared SNSPD.
Disclosure of Invention
The invention provides a mid-infrared superconducting nanowire single-photon detector, aiming at solving the problems that the superconducting thin film material of a superconducting nanowire single-photon detector is less in research and the optical fiber coupling SNSPD technology is difficult to be applied to mid-infrared SNSPD in the prior art, and adopting the following technical scheme in order to achieve the aim.
Preparing an amorphous or polycrystalline superconducting thin film containing Mo and Si into a superconducting nanowire serving as a photosensitive surface of the detector by adopting an electron beam lithography technology and a reactive ion etching technology; the detector is composed of a mid-infrared light source, an adjustable attenuator, a collimator, a band-pass filter, a dilution refrigerator, a photosensitive surface, a bias device, an amplifier and a counter, photons of a mid-infrared band are emitted and received by adopting a free space coupling technology, and the number of photons reaching the photosensitive surface in unit time is calculated.
The superconducting film comprises a substrate, a superconducting nano layer and an anti-oxidation layer, wherein the superconducting nano layer is positioned on the surface of the substrate, and the anti-oxidation layer is positioned on the surface of the superconducting nano layer; the thickness of the substrate is 300-650 mu m, silicon or silicon nitride or silicon oxide or magnesium oxide is adopted, the thickness of the super-nano layer is 3-10nm, and the thickness of the anti-oxidation layer is 0.5-20nm by adopting an insulating film with the resistivity larger than 10 omega cm.
Further, the substrate employs a surface roughness RMS<1nm and a thickness of 350 μm; the material component ratio of Mo and Si of the super-nano layer is 80:20, and the thickness is 6.08 nm; the oxidation preventing layer is Nb5N6The thickness was 3 nm.
The electron beam lithography technique includes: and writing the superconducting nanowire pattern by adopting an electron beam lithography system, a high-resolution negative electron beam anti-etching agent and a writing beam current of 0.1-1.0 nA.
Furthermore, the solute concentration of the anti-etching agent is 2%, the pre-baking temperature is 90 ℃, the baking time is 4min, the spin coating thickness is 20-50nm, the electron energy of an electron beam lithography system works at 100keV, and the beam spot size is 2 nm.
Further, TMAH developer with a solute concentration of 2.38% was used for development at 23 ℃ for 3 min.
The reactive ion etching technology comprises the following steps: using CF4Or SF6Or O2Or CHF3Or Ar or the mixed gas of the Ar and the Ar is used as etching gas to transfer the nanowire pattern to the MoSi film.
Further, CF is used4The flow rate of the etching gas was 20sccm, the gas pressure was 1.2Pa, the etching power was 50W, and the etching time was 65 s.
The thickness of the superconductive nanowire is 3-10nm, the line width is 20-50nm, the period is 50-300nm, the superconductive nanowire is in a winding structure, and the photosensitive area is 1-10000 μm2。
Free space coupling techniques include: the mid-infrared light source emits wide-spectrum mid-infrared photons, the incident wavelength is controlled by a band-pass filter through an adjustable attenuator and a collimator in a free space to obtain mid-infrared photons with specific wavelength, the mid-infrared photons are coupled into a window of the dilution refrigerator and coupled to a photosensitive surface arranged in the dilution refrigerator, the size of a light spot is not smaller than that of the photosensitive surface, electric pulse signals generated by the photons are detected, and the photons are accessed into a counter to read the number of the photons through a biaser and an amplifier.
Further, the transmission attenuation of the mid-infrared photons through the window of the dilution refrigerator and the coupling loss of the photons reaching the photosensitive surface are defined as a fixed attenuator arranged between the window of the dilution refrigerator and the detector, the attenuation multiplying power is set to be A, and the adjustable attenuator and the fixed attenuator attenuate the number of the infrared photons reaching the photosensitive surface in unit time to a single photon magnitude.
Placing a power probe at the outermost window position of the dilution refrigerator, placing the power probe on an optical axis in close contact with the window, and measuring the power of photons with various incident wavelengths at the outermost window position of the dilution refrigerator, wherein the power is defined as P (lambda); setting photon energy h ν, attenuating A by a fixed attenuator to reach a photosensitive surface, and calculating photon density according to a formula ρ (λ) ═ P (λ)/h ν) x A, wherein ρ (λ) is defined; the number of photons reaching the photosurface per second is estimated in combination with the area of the photosurface.
Furthermore, a COOL-RED medium infrared black body radiation source is adopted as the medium infrared light source, the working temperature is 1500K, and the radiation wavelength covers 0.5-10 mu m; the power of the radiated light is adjusted by adopting an adjustable neutral density filter; the center of the band-pass filter is arranged on an optical axis, is tightly attached to the outermost window sheet of the dilution refrigerator, and is cooled to be below 300 mK; center wavelengths are set to 1.55, 2.00, 2.25, 3.00, 4.00, 4.26 and 5.07 μm respectively, and filters are replaced to respond to the photon rate of different incident wavelengths; a low-noise voltage source is connected in series with a 100k omega resistor to provide a detector bias current; a low-noise normal-temperature amplifier with the gain of 50dB, the bandwidth of 1.5GHz and the cut-off frequency of 1MHz is adopted; detecting infrared radiation power with the wavelength of 1.55-2.25 mu m by adopting an InGaAs photodiode power meter probe S148C Thorlabs; an HgCdTe integrating sphere photodiode power probe S180C Thorlabs is used to detect the infrared radiation power with a wavelength of 3.00-5.07 μm.
The technical scheme provided by the invention has the beneficial effects that: the narrow nanowire is prepared by adopting the MoSi low-gap superconducting film, so that the effective preparation of the intermediate infrared SNSPD is realized; the free space coupling SNSPD technology is adopted to couple the mid-infrared photons to the photosurface, so that the problem of coupling the mid-infrared SNSPD by optical fibers is effectively solved; the representation of the number of photons reaching the photosensitive surface of the nanowire every second lays a foundation for effectively calculating the detection efficiency of the mid-infrared SNSPD quantum.
Drawings
FIG. 1 is a schematic cross-sectional structure of a superconducting nanowire, FIG. 2 is an SEM image of an SNSPD device, FIG. 3 is a schematic size diagram of a nanowire, and FIG. 4 is Nb5N6I-V characteristic of the film at 80mK working temperature, FIG. 5 is a free space coupling mid-infrared SNSPD measuring system structure diagram, FIG. 6 is dark count rate DCR with bias current I under 4K background radiationBA variation relation graph, and FIG. 7 shows the intrinsic detection efficiency eta of the mid-infrared SNSPDinternalDependent on the bias current IBAnd (3) a change relation diagram, wherein FIG. 8 is a relation diagram of quantum detection efficiency QE and detection wavelength lambda of a mid-infrared SNSPD system, and FIG. 9 is a relation diagram of noise equivalent power and detection wavelength lambda of the mid-infrared SNSPD.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention clearer, the following detailed description of the technical solutions of the present invention is made with reference to the accompanying drawings.
The superconducting nanowire structure is shown in fig. 1 and comprises a substrate, a superconducting nanowire and an anti-oxidation layer, wherein the superconducting nanowire is positioned on the surface of the substrate, and the anti-oxidation layer is positioned on the surface of the superconducting nanowire; the substrate is made of silicon or silicon nitride or silicon oxide or magnesium oxide, the thickness is 300-650 μm, the embodiment is made of silicon nitride, and the thickness is 350 μm; the superconductive nanowire is made of Mo and Si compounds, has a thickness of 3-10nm, a line width of 20-50nm, a period of 50-300nm, a meandering structure, and a photosensitive area of 1-10000 μm2。
In this embodiment, as shown in FIGS. 2 and 3, the material composition ratio of Mo and Si is 80:20, the nanowire thickness is 6.08nm, the line width is 30nm, the period is 150nm, and the photosensitive area is 100 μm2(ii) a The oxidation preventing layer adopts alpha-Si or Nb5N6The thickness is 2-10nm, and Nb is adopted in the embodiment5N6The thickness is 3.16nm, the superconducting nanowire is conductive at normal temperature, and 80mK is similar to an insulator at low temperature, so that the electrical characteristics of the superconducting nanowire at low temperature are not influenced as shown in FIG. 4.
The high-resolution negative electron beam anti-etching agent HSQ is adopted, and a 100pA writing beam is adopted to write the superconducting nanowire pattern through an electron beam lithography EBL system. In the embodiment, the solute concentration of the HSQ is 2%, the spin coating thickness on the MoSi surface is 20-50nm, the pre-baking temperature of the HSQ is 90 ℃, the baking time is 4min, the EBL electron energy is adjusted to work at 100keV, the beam spot size of the 100pA beam is reduced to 5nm, and the EBL writing resolution and the electron beam spot size are set to be 2 nm.
Developing at room temperature by using TMAH developer, wherein the solute concentration of the TMAH developer is 2.38%, the developing temperature is 23 ℃, and the substrate is prevented from shaking violently in the developing process for 3min, so as to prevent pattern drift; transferring the nanowire pattern to the MoSi film by Reactive Ion Etching (RIE), and etching with CF4Or SF6Or O2Or CHF3Or Ar or their mixture, in this embodiment, CF is used4The flow rate is 20sccm, the gas pressure is 1.2Pa, the etching power is 50W, and the etching time is 65 s.
The SNSPD measuring system is structurally shown in figure 5 and comprises a mid-infrared light source, an adjustable attenuator, a collimator, a band-pass filter, a dilution refrigerator, a photosensitive surface and a counter, wherein in a free space, the mid-infrared light source generates wide-spectrum mid-infrared photons, the incident wavelength of the detector is controlled by the adjustable attenuator, the collimator and the band-pass filter to obtain mid-infrared photons with specific wavelengths, the mid-infrared photons are coupled into a window of the dilution refrigerator and reach the mid-infrared SNSPD photosensitive surface arranged in the dilution refrigerator, light spots completely cover the mid-infrared SNSPD photosensitive surface, the temperature is reduced to be below 300mK, electric pulse signals generated by the photons are detected, and the detection signals are read by a Bias-Tee amplifier and a counter or an oscilloscope.
The intermediate infrared photons reach the photosensitive surface through the transmission attenuation of the dilution refrigerator window, the coupling loss exists, the intermediate infrared photons are defined as a fixed attenuator arranged between the dilution refrigerator window and the detector, and the attenuation multiplying factor is defined as A.
In the embodiment, a COOL-RED medium infrared black body radiation source is adopted as the medium infrared light source, the working temperature is 1500K, the radiation wavelength covers 0.5-10 mu m, an adjustable neutral density filter is adopted to adjust the power of radiation light, the center of a band-pass filter is arranged on an optical axis and is tightly attached to the outermost window sheet of a dilution refrigerator, the central wavelengths are 1.55, 2.00, 2.25, 3.00, 4.00, 4.26 and 5.07 mu m respectively, and the filter sheet is replaced to respond to the single photon rates of different incident wavelengths.
In the embodiment, a low-noise voltage source is connected in series with a 100k omega resistor to provide a detector bias current, and a low-noise normal-temperature amplifier with the gain of 50dB, the bandwidth of 1.5GHz and the cut-off frequency of 1MHz is adopted; an InGaAs photodiode power meter probe S148C Thorlabs is used to detect the infrared radiation power with the wavelength of 1.55-2.25 μm, and an HgCdTe integrating sphere photodiode power probe S180C Thorlabs is used to detect the infrared radiation power with the wavelength of 3.00-5.07 μm.
The power probe is placed at the outermost window position of the dilution refrigerator and is tightly attached to the window, the center of an incident diaphragm of the probe is placed on the optical axis of a system, the power P (lambda) of each incident wavelength at the outermost window position of the dilution refrigerator is detected, the power density is calculated by combining the area of the incident diaphragm of the power probe, and the power density is attenuated by a fixed attenuator A and reaches the photosensitive surface of the intermediate infrared SNSPD.
The optical power density of each area on the photosensitive surface is approximately equal, the optical power density is divided by the single photon energy h v to obtain the photon density on the intermediate infrared photosensitive surface, the photon density is defined as rho (lambda), the photon density is calculated according to a formula rho (lambda) ═ P (lambda)/h v). times.A, and the number of photons reaching the intermediate infrared SNSPD photosensitive surface per second is estimated by combining the area of the intermediate infrared SNSPD photosensitive surface.
Measuring the dark mark rate of the intermediate infrared SNSPD system under the 4K environment and under different bias currents, as shown in FIG. 6, measuring the photon count rate of the intermediate infrared SNSPD system under the incident condition of infrared photons with different wavelengths to obtain the intrinsic detection efficiency of the intermediate infrared SNSPD, as shown in FIG. 7, estimating the photon number reaching the photosensitive surface of the nanowire every second, combining the photon count rate, calculating the quantum detection efficiency of the system, as shown in FIG. 8, combining the dark mark rate and the quantum detection efficiency of the intermediate infrared SNSPD, and calculating the bias current IBThe noise equivalent power of the mid-infrared SNSPD when it reaches 0.9 times the superconducting critical transition current of the mid-infrared SNSPD is shown in fig. 9.
The above-described embodiments are not intended to limit the present invention, and any modifications, equivalents, improvements, etc. made within the spirit and principle of the present invention are included in the scope of the present invention.
Claims (3)
1. A middle infrared superconducting nanowire single photon detector adopts an electron beam lithography technology and a reactive ion etching technology to prepare an amorphous or polycrystalline superconducting thin film containing Mo and Si into a superconducting nanowire which is used as a photosensitive surface of the detector; the detector is composed of a mid-infrared light source, an adjustable attenuator, a collimator, a band-pass filter, a dilution refrigerator, a photosensitive surface, a bias device, an amplifier and a counter, photons of a mid-infrared band are emitted and received by adopting a free space coupling technology, and the number of photons reaching the photosensitive surface in unit time is calculated;
the superconducting film comprises a substrate, a superconducting nano layer and an anti-oxidation layer, wherein the superconducting nano layer is positioned on the surface of the substrate, and the anti-oxidation layer is positioned on the surface of the superconducting nano layer; the thickness of the substrate was 350 μm and the surface roughness RMS was<1nm, silicon or silicon nitride or silicon oxide or magnesium oxide is adopted; the material component ratio of Mo and Si of the super-nano layer is 80:20, and the thickness is 6.08 nm; the oxidation preventing layer is Nb with resistivity larger than 10 omega cm5N6An insulating film having a thickness of 3 nm;
the electron beam lithography technology adopts an electron beam lithography system, a high-resolution negative electron beam anti-etching agent and a writing beam current of 0.1-1.0nA to write a superconducting nanowire pattern; the solute concentration of the anti-etching agent is 2%, the pre-baking temperature is 90 ℃, the baking time is 4min, and the spin coating thickness is 20-50 nm; the electron energy of the electron beam lithography system works at 100keV, and the beam spot size is 2 nm;
the reactive ion etching technique adopts CF4Or SF6Or O2Or CHF3Or Ar or the mixed gas of Ar and Ar is used as etching gas to transfer the nanowire pattern to the MoSi film; developing the superconducting nanowire pattern at 23 deg.C for 3min by using TMAH developer with solute concentration of 2.38% to obtain superconducting nanowire with thickness of 3-10nm, line width of 20-50nm, period of 50-300nm, serpentine structure, and photosensitive area of 1-10000 μm2(ii) a Etching gas adopts CF4The flow is 20sccm, the air pressure is 1.2Pa, the etching power is 50W, and the etching time is 65 s;
it is characterized by comprising:
the free space coupling technology adopts a medium infrared light source to emit wide-spectrum medium infrared photons, in a free space, the incident wavelength is controlled by a band-pass filter through an adjustable attenuator and a collimator to obtain medium infrared photons with specific wavelength, the medium infrared photons are coupled into a window of a dilution refrigerator and are coupled to a photosensitive surface arranged in the dilution refrigerator, the size of a light spot is not smaller than that of the photosensitive surface, the transmission attenuation of the medium infrared photons through the window of the dilution refrigerator and the coupling loss of the photons reaching the photosensitive surface are defined as a fixed attenuator arranged between the window of the dilution refrigerator and a detector, the attenuation multiplying factor is set to be A, an electric pulse signal generated by the detected photons is attenuated to a single photon magnitude by an offset device, an amplifier, the adjustable attenuator and the fixed attenuator in unit time, and the number of the photons is read by an access counter.
2. The mid-infrared superconducting nanowire single photon detector of claim 1, wherein the calculating the number of photons reaching the photosurface per unit time comprises: placing a power probe at the outermost window position of the dilution refrigerator, placing the power probe on an optical axis in close contact with the window, and measuring the power of photons with various incident wavelengths at the outermost window position of the dilution refrigerator, wherein the power is defined as P (lambda); setting photon energy h ν, attenuating A by a fixed attenuator to reach a photosensitive surface, and calculating photon density according to a formula ρ (λ) ═ P (λ)/h ν) x A, wherein ρ (λ) is defined; the number of photons arriving per second is estimated in combination with the area of the photosurface.
3. The mid-infrared superconducting nanowire single photon detector of claim 1 or 2, comprising: the middle infrared light source adopts a COOL-RED middle infrared black body radiation source, the working temperature is 1500K, and the radiation wavelength covers 0.5-10 μm; the power of the radiated light is adjusted by adopting an adjustable neutral density filter; the center of the band-pass filter is arranged on an optical axis, is tightly attached to the outermost window sheet of the dilution refrigerator, and is cooled to be below 300 mK; center wavelengths are set to 1.55, 2.00, 2.25, 3.00, 4.00, 4.26 and 5.07 μm respectively, and filters are replaced to respond to the photon rate of different incident wavelengths; a low-noise voltage source is connected in series with a 100k omega resistor to provide a detector bias current; a low-noise normal-temperature amplifier with the gain of 50dB, the bandwidth of 1.5GHz and the cut-off frequency of 1MHz is adopted; detecting infrared radiation power with the wavelength of 1.55-2.25 mu m by adopting an InGaAs photodiode power meter probe S148C Thorlabs; an HgCdTe integrating sphere photodiode power probe S180C Thorlabs is used to detect the infrared radiation power with a wavelength of 3.00-5.07 μm.
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