Patent Publication Number: US-2021165029-A1

Title: Superconducting electromagnetic wave sensor

Description:
FIELD OF THE INVENTION 
     The present invention relates to a sensor of electromagnetic radiation comprising superconducting layers that can be used in a wide variety of applications requiring extremely high sensitivity. 
     In particular, the invention relates to an electromagnetic sensor that can be used for measuring power and characteristics of incident electromagnetic radiation. 
     DESCRIPTION OF THE PRIOR ART 
     Accurate wide-band electromagnetic sensors are known that are based on superconducting heterostructures. Sensors of this kind, in particular Transition Edge Sensors also known as TES, are described for instance in U.S. Pat. Nos. 5,880,468 and 5,090,819, and can be used in applications requiring extremely high sensitivity, for example for detection of the cosmic microwave background, for generic-purpose terahertz radiation sensing, like security imaging, and in materials analysis via detection of x-rays scattered from target materials. 
     As shown in A. V. Sergeev, V. V. Mitin, and B. S. Karasik,  Appl. Phys. Lett.  80, 817 (2002), a Kinetic Inductance Detector (KID) was proposed as an alternative to TES sensors. 
     However, both TES and KID sensors operating at sub Kelvin temperatures have the disadvantage of requiring a separate voltage bias line, which is a structural constraint especially in the case of an array of such sensors. Moreover, bias lines tend to heat up the system and introduce noise, which can be relevant in case of extremely high sensitivity measurements. Furthermore, they are based on the measurement of an impedance of the sensor as a result of a response to a probe signal. 
     Potential functionalities based on superconducting heterostructures not requiring probe signals have been described, exploiting the known thermoelectric effect, namely a direct conversion of temperature differences into an electric voltage and vice versa. 
     In A. Ozaeta, P. Virtanen, F. S. Bergeret, and T. T. Heikkilä, Phys. Rev. Lett. 112, 5 (2014), thermoelectric effects in a junction, between a conventional superconductor, in the presence of an exchange field and a ferromagnet with polarization, have been theoretically described. In particular, a structure comprising a ferromagnet coupled via a tunneling contact to a superconductor, whose density of states is modified by an exchange field, has been described. 
     In F. Giazotto, P. Solinas, A. Braggio, and F. S. Bergeret,  Phys. Rev. App. American Physical Society,  4, 4 (2015) a thermometer based on superconducting tunnel junction has been disclosed. In particular, the authors analysed thermoelectric transport of charge carriers in a thermoelectric element based on a metal-ferromagnetic insulator-superconductor heterostructure, implemented as an ultra-sensitive thermometer. The thermoelectric element is included in a circuit formed by superconducting wires, a load element having a predetermined resistance, and a Josephson junction. Current in the circuit is responsive to the difference of temperature between the metal and the superconductor of the thermoelectric element. 
     In STRAMBINI E ET AL: “Revealing the magnetic proximity effect in EuS/AI bilayers through superconducting tunneling spectroscopy”, arXiv: 1705.04795˜2 [cond-mat. supr-con], 22 May 2017 (2017-05-22), XP055470963, and in GIAZOTTO F ET AL: “Ferromagnetic-Insulator-Based Superconducting Junctions as Sensitive Electron Thermometers”, PHYSICAL REVIEW APPLIED, vol. 4, no. 4, 044016, 26 Oct. 2015 (2015-10-26), XP055479037, superconducting tunnel junctions are disclosed comprising a (superconducting) metal electrode, a superconducting layer arranged to carry an exchange field for providing a spin splitting effect of charge carriers (electrons (N, e.g. Al), and an insulating layer between the superconducting layer and the metal electrode, in such a way to form a spin filter junction using magnetic proximity effect by a ferromagnetic insulator as underlayer or as tunnel barrier layer, as thermometric sensors. 
     In WO2012/038596A1 a detector for quantum computing is disclosed that is able to detect single photons propagating in a collector as input and an electric measurement arrangement in the form of an RF reflection measurement circuit and amplifier as output coupled to a superconducting tunnel junction detector element. 
     SUMMARY OF THE INVENTION 
     It is a feature of the present invention to provide an electromagnetic sensor not requiring a probe signal or any extra bias power to be applied into the electromagnetic sensor. 
     It is a particular feature of the present invention to provide such an electromagnetic sensor that can provide the intensity of an incoming radiation. 
     It is another particular feature of the present invention to provide such an electromagnetic sensor that can provide also the frequency of an incoming radiation. 
     It is a feature of the present invention to provide an electromagnetic sensor that, during its operation, can achieve a high-energy efficiency versus dissipated heat and absorbed energy. 
     It is still a feature of the present invention to provide an electromagnetic sensor with good measurement properties under a wide range of operating temperatures. 
     It is still a feature of the present invention to provide an electromagnetic sensor having a large signal to noise ratio. 
     It is another feature of the present invention to provide such an electromagnetic sensor that can be arranged as an array of sensors providing a plurality of pixels, to achieve a broad detection bandwidth. 
     It is a particular feature of the present invention to provide such an electromagnetic sensor that can be arranged as an array of sensors providing a plurality of pixels, to achieve broad spatial and angular resolution. 
     These and other objects are achieved by an electromagnetic sensor comprising,
         a superconducting layer, arranged to carry an exchange field for providing a spin splitting effect of charge carriers in said superconducting layer,   a metal electrode,   an insulating layer arranged between said superconducting layer and said metal electrode, in such a way to form a spin filter junction between said superconducting layer and said metal electrode,   an antenna comprising a wave collecting element, arranged in contact with said superconducting layer to convey into said superconducting layer electromagnetic waves collected by said antenna and generated by an external source, said electromagnetic waves having an amplitude and a frequency,   an electric measurement device arranged in contact with said metal electrode and configured to measure an electric current or voltage caused by said spin splitted charge carrier flow from said superconducting layer through said spin filter junction into said metal electrode and to provide an output signal responsive to said amplitude and frequency of said external electromagnetic waves collected by said antenna.       

     This way, electromagnetic waves, conveyed by the wave-collecting element, are absorbed by the spin-split superconductor, thus creating a strong non-equilibrium state of spin-split quasiparticles, namely charge carriers, within the superconductor. This non-equilibrium state can relax in various ways, like via quasiparticle-quasiparticle collisions, via processes such as the quasiparticle-phonon relaxation, via escapes of quasiparticles back through the wave-collecting element, and via the escape of the quasiparticles to the metal electrode via the spin filter junction, the last process yielding the output signal sought by the present invention. 
     By means of the electric measurement device, arranged in contact with metal electrode to measure quasiparticles flow through the spin filter junction into the metal electrode, an output signal responsive to the amplitude of the external electromagnetic waves is obtained which can be measured for example with the use of a regular amplifier. 
     This way, the incoming electromagnetic waves determine the output signal, and no separate power source is needed except from the electric measurement device, which can be remote or sufficiently far from the superconducting layer, where radiation is absorbed. Moreover, the ratio between the intensity of the output signal and the incoming radiation is very high, and therefore the sensor can be used also for measurements that would require hypersensitivity, without oversizing the sensors. 
     With respect to the prior art radiation sensors like TES or KID, the present invention does not require extra power to be applied on the electromagnetic sensor, obtaining a reduced power consumption. Moreover, a separate voltage bias line is not required, reducing structural constraints, which would be very encumbering in the case of an array of such sensors. In addition, the absence of bias lines that would heat up the system and introduce noise, is very relevant, and permits making extremely high sensitivity measurements, owing to a very low signal to noise ratio. 
     The superconductor can be of any known superconducting metal. Preferably, the superconductor can be of a metal selected from the group consisting of: Al, V, Ti. In fact, these materials would have lower critical temperature, and therefore less noise, and lower magnitude of spin relaxation within the material in such a way to increase the sensor sensitivity. 
     In possible embodiments, the superconducting layer is arranged to carry an exchange field for providing the sought spin splitting effect via the proximity to a ferromagnetic material. Alternatively, the superconducting layer is arranged to carry an exchange field for providing the sought spin splitting effect by an externally applied magnetic field. 
     In a possible embodiment, the superconducting layer of the electromagnetic sensor is in contact with the insulating layer on a first face, and a ferromagnetic insulating layer is provided arranged in contact with the superconducting layer on the second face opposite to the first face, said ferromagnetic insulating layer arranged to provide the spin splitting field. 
     This way, the ferromagnetic insulating layer in contact with the superconducting layer determines in a simple way a magnetic proximity effect on the superconducting layer resulting into a spin-splitting exchange field h inside the superconducting layer. 
     In a further possible embodiment, the insulating layer is a ferromagnetic insulating layer and said superconducting layer is in contact with the insulating layer on the first face, the ferromagnetic insulating layer arranged to provide the spin splitting exchange field. 
     This way, the ferromagnetic insulating layer in contact with the superconducting layer has a double function of supplying the exchange field for the spin splitting of the charge carriers, owing to its ferromagnetic nature, and of providing a tunneling spin filter function, owing to its spin-dependent insulating properties. This double function enhances the possibility of achieving high density of integration. 
     Advantageously, the ferromagnetic insulating layer can be an insulator selected from the group consisting of: EuS, EuO, GdN. The use of these materials allows generating effects of magnetic proximity in a particularly efficient way. 
     In an alternative embodiment, an external magnetic field parallel to the superconducting layer provides the spin-split status in the superconducting layer. This situation would not require the ferromagnetic insulating layer to be achieved. In a possible arrangement, the external magnetic field is provided by a coil oriented right to the superconducting layer in such a way to provide a magnetic field parallel to the superconducting layer. In this case, the contact between the superconducting layer and the electrode still needs to be a spin filter contact. This functionality can be obtained by either using a ferromagnetic insulating layer for this contact, or using a non-magnetic insulating layer and a ferromagnetic electrode. 
     In possible alternative embodiments, the metal electrode is selected from the group consisting of: a paramagnetic metal, a ferromagnetic metal, a superconducting metal. 
     In case the metal electrode is a paramagnetic metal, it can be selected from the group consisting of: Cu, Ag, Au, Pt. 
     In case the metal electrode is a superconductor, it can be of be any known superconducting metal. Preferably, the superconducting metal electrode is selected from the group consisting of: Al, V, Sn, Nb, Pb, In, Ta, La, Hg, Ti, Cd, Zn, Mo, Ga, Re, Pd. 
     In case the metal electrode is a ferromagnetic metal, it can be selected from the group consisting of: Fe, Co, Ni. 
     In a possible embodiment, in particular where metal electrode is a ferromagnetic metal, the insulating layer is made of an aluminum oxide, preferably Al 2 O 3 . This material for the insulating layer can be very thin, even a few atomic layers, without losing the insulating properties between said superconducting layer and said metal electrode, and the capability of forming together with the ferromagnetic metal a spin filter tunneling junction. 
     In a possible embodiment, said antenna is a superconductive antenna that is in contact with said superconducting layer. 
     In particular, to prevent heat leaking out from the superconducting layer, the wave-collecting element is manufactured from a material with a superconducting gap Δ higher than the gap of the material selected for the superconducting layer Δ. For example, the wave-collecting element can be an Nb antenna and said superconducting layer can be made of Al. 
     Advantageously, the superconducting layer has a normal state resistance, as it can be seen by the radiation at frequencies higher than Δ/h—where h is Planck&#39;s constant—that is matched to the specific impedance of the antenna, typically somewhat below the vacuum impedance. 
     In a possible embodiment, said wave collecting elements can also be an absorber, in particular a thicker film of a material with higher atomic number than said superconducting layer and in contact with said superconducting layer, in order to be capable of absorbing high frequency electromagnetic radiation. 
     In a possible embodiment of the invention, the electric measurement device comprises an amplifier. In particular, said amplifier can be a current or voltage amplifier, such as a superconducting quantum interference device (SQUID) amplifier or field effect transistor, such as a high electron mobility transistor (HEMT). 
     In a possible alternative embodiment, the electromagnetic sensing apparatus comprises an array of electromagnetic sensors in any of the embodiments above defined. This way, the array of sensors, would have a broader detection bandwidth and spatial/angular resolution than a single sensor. Moreover, thanks to the absence of bias lines, a detector of electromagnetic radiation with high integration density can be attained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further characteristics and/or advantages of the present invention will be made clearer with the following description of an exemplary embodiment thereof, exemplifying but not limitative, with reference to the attached drawings in which: 
         FIG. 1  diagrammatically shows a first exemplary embodiment of a single electromagnetic sensor, according to the present invention; 
         FIG. 2  diagrammatically shows a second exemplary embodiment of a single electromagnetic sensor, starting from the sensor of  FIG. 1 , with the addition of a ferromagnetic insulating layer according to the present invention on a second face of the superconducting layer; 
         FIG. 3  diagrammatically shows a third exemplary embodiment of a single electromagnetic sensor, according to the present invention, starting from the element of  FIG. 1 , with a ferromagnetic insulating layer configured as insulating layer; 
         FIG. 4  diagrammatically shows a fourth exemplary embodiment of a single electromagnetic sensor, according to the present invention, comprising two superconducting layers separated by a ferromagnetic insulating layer; 
         FIG. 5  diagrammatically shows a fifth exemplary embodiment of a single electromagnetic sensor, according to the present invention, where the magnetic field is provided by an external magnetic element; 
         FIG. 6  diagrammatically shows a perspective schematic view of an electromagnetic sensor according to the invention with a possible embodiment of an electric measurement device. 
     
    
    
     DESCRIPTION OF SOME PREFERRED EXEMPLARY EMBODIMENTS 
     With reference to  FIG. 1 , in a first possible embodiment, an electromagnetic sensor  1  comprises a spin filter junction consisting of an insulating layer  30 , in particular a ferromagnetic insulator  30  arranged between a superconducting layer  10  and a metal electrode  20 , or an insulator  30  arranged between a superconducting layer  10  and a ferromagnetic metal electrode  20 . Insulating layer  30  can be very thin, sufficiently to form the spin filter junction between superconducting layer  10  and metal electrode  20 . 
     A wave collecting element  60  is arranged in contact with superconducting layer  10  to convey into superconducting layer  10  external electromagnetic waves  61  generated by an external source. In particular, the electromagnetic waves  61  have an amplitude and a frequency. 
     Superconducting layer  10  comprises a face  11  or  12  arranged to be exposed to a magnetic field (not shown) in order to provide, by either Zeeman effect or magnetic proximity, a spin splitting effect  63 ,  64  of charge carriers in superconducting layer  10 . The magnetic field  62  can be parallel to any of faces  11  or  12 . The spin splitting  63 ,  64  has been symbolically depicted as charge carriers in the body of superconducting layer  10 , like spin-up  63  and spin-down  64  electrons. 
     In particular, as above described, electromagnetic waves  61 , conveyed by wave collecting element  60 , are absorbed by the spin-split superconductor  10  where a strong nonequilibrium state of spin-split quasiparticles is created, which can relax in various ways, like via quasiparticle-quasiparticle collisions, via spurious processes such as the quasiparticle-phonon relaxation, via escapes of quasiparticles back through the wave collecting element, and via the escape of the quasiparticles to the metal electrode  20  via the spin filter junction, which is the sought effect. The last process yields to the output signal  71  detected by an electric measurement device  70 , arranged in contact with the metal electrode  20 . Electric measurement device  70  is arranged to measure quasiparticle flow through the spin filter junction into the metal electrode  20  and by providing an output signal  71  responsive to the amplitude of the external electromagnetic waves  61 . Electric measurement device  70  can also preferably be arranged to provide the frequency of the incoming external electromagnetic waves  61 . 
     Also, the quasiparticle-quasiparticle collisions relax the initially non-thermal nonequilibrium energy distribution into a thermal one, but with an increased temperature. 
     Superconducting layer  10  can be for example Al, V, Ti, in order to obtain, by magnetic proximity or by an external magnetic field, the desired spin splitting effect  63 , 64  on the charge carriers present in superconducting layer  10 . 
     Advantageously, the wave collecting element  60  is an antenna. In particular, it can be a superconductive antenna  60  in good contact with superconducting layer  10 . 
     Measurement device  70  can comprise an amplifier. In particular, the amplifier can be a current or voltage amplifier, such as a superconducting quantum interference device (SQUID) amplifier or field effect transistor, such as a high electron mobility transistor (HEMT). 
     To prevent heat leaking out from the superconducting layer  10 , the superconductor used in the antenna  60  should be manufactured from a material with a higher superconducting gap Δ A  than gap Δ of the material selected for the superconducting layer  10 . 
     One possible combination could be an Nb antenna  60  and an Al absorber  10 . For optimal quantum efficiency, the normal state resistance of the absorber (seen by the radiation at frequencies higher than Δ/h, where h is Planck&#39;s constant) should be matched to the specific impedance of the antenna, typically somewhat below the vacuum impedance. For example, considering an Al film thickness of 10 nm of superconducting layer  10 , it would have a typical sheet resistance is 5-10 Ωm. Hence, for wave collecting element  60  a 1 μm wide film with length L=10 μm would have the resistance Rγ=50-100Ω seen by the radiation, thereby matching well with typical microwave antennas. 
     With reference to  FIG. 2 , in a second possible embodiment, similarly to  FIG. 1 , an electromagnetic sensor  1  comprises a spin filter junction consisting of an insulating layer  30  arranged between a superconducting layer  10  and a metal electrode  20 , in particular a ferromagnetic insulator  30  arranged between a superconducting layer  10  and a metal electrode  20 , or an insulator  30  arranged between a superconducting layer  10  and a ferromagnetic metal electrode  20 , and is configured to permit passage of current of quasiparticles. Superconducting layer  10  is in contact with the insulating layer  30  on first face  11 . 
     Differently from  FIG. 1 , a ferromagnetic insulating layer  40  is provided arranged in contact with superconducting layer  10  on a second face  12  opposite to first face  11 . The ferromagnetic layer  40  is arranged to provide the magnetic field. In particular, ferromagnetic insulating layer  40  in contact with the superconducting layer  10  provides, by magnetic proximity effect a spin-splitting exchange field in superconducting layer  10 . 
     In this embodiment the insulating layer  30  can be made of an aluminum oxide, for example Al 2 O 3 , which can be very thin, even a few atomic layers, without losing the insulating properties between said superconducting layer and electrode  20 , and the capability of forming a spin filter junction. In this embodiment, it is particularly advantageous that the electrode  20  is a ferromagnetic electrode, for example selected among Ni, Co, Fe, for achieving maximum spin filter efficiency. 
     Concerning the generation of the electric signal and its measurement, the embodiment of  FIG. 2  operates in the same way as described for the embodiment of  FIG. 1 . 
     With reference to  FIG. 3 , in a third possible embodiment, similarly to  FIG. 1 , an electromagnetic sensor  1  comprises a spin filter junction consisting of an insulating layer  30  arranged between a superconducting layer  10  and a metal electrode  20  and is configured to permit passage of current of quasiparticles. Superconducting layer  10  is in contact with the insulating layer  30  on first face  11 . 
     As a particular embodiment derived from the general embodiment  FIG. 1 , the insulating layer  30  is a ferromagnetic insulating layer and superconducting layer  10  is in contact with the insulating layer  30  on a first face  11 . The ferromagnetic insulating layer  30  in contact with the superconducting layer  10  has a double function of supplying the magnetic field for the spin splitting of the charge carriers resulting from the electromagnetic waves  61 , owing to its ferromagnetic nature and, owing to its insulating properties, of providing spin filter junction, which carries out tunneling spin filter function. This double function enhances the possibility of achieving high density of integration. 
     In this embodiment, the ferromagnetic insulating layer  30  can be EuS, EuO, GdN, which are ferromagnetic insulators capable of providing both functions. In particular, the metal electrode  20  can be a metal like Cu, Ag, Au, Pt. 
     It is also possible that also the metal electrode  20  is a ferromagnetic metal like ferromagnetic electrode Ni, Co, Fe. 
     Concerning the generation of the electric signal and its measurement, the embodiment of  FIG. 3  operates in the same way as described for the embodiment of  FIG. 1 . 
     With reference to  FIG. 4 , in a fourth possible embodiment the insulating layer  30  is a ferromagnetic insulating layer, and the spin filter junction is obtained by ferromagnetic insulating layer arranged between first superconducting layer  10  and a second superconducting layer  10 ′ as metal electrode. 
     The advantage of having a superconducting metal electrode is in the possibility of obtaining a larger thermoelectric effect, and in the absence of any spurious dissipation of the read-out signal. 
     Concerning the generation of the electric signal and its measurement, the embodiment of  FIG. 4  operates in the same way as described for the embodiments of  FIGS. 1 and 3 . 
     With reference to  FIG. 5 , in a fifth possible embodiment an electromagnetic sensor  1  comprises a spin filter junction consisting of an insulating layer  30  arranged between a superconducting layer  10  and a metal electrode  20 , in particular a ferromagnetic metal electrode, for example selected among Ni, Co, Fe, for achieving maximum spin filter efficiency. Differently from  FIGS. 1-4  an external magnetic element  50  can be arranged to provide the magnetic field, like a metal coil or a large permanent magnet. 
     Superconducting layer  10  is exposed to a magnetic field in order to provide, by Zeeman effect, a spin splitting effect of charge carriers in superconducting layer  10 . 
     With reference to  FIG. 6 , a perspective view of an electromagnetic sensor  1 , similar to the embodiment of  FIG. 2 , is shown. In particular, the current injected to the metal electrode  20  is detected via an electric measurement device  70  comprising an amplifier  92 . Also a capacitor  91  and/or an inductor  90  can be provided, in particular in case of multi-pixel readout systems, for example in case multiple sensor are arranged as an array and multiplexing or multi-pixel readout is needed. 
     In this connection, in a way not shown but that can be implemented by a person skilled in the art of manufacturing integrated circuits, an array of electromagnetic sensors can be provided without finding structural constraints like the prior art. 
     The foregoing description of some exemplary specific embodiments can show the invention from a conceptual viewpoint so that other, by applying current knowledge, will be able to modify and/or adapt in various applications the specific exemplary embodiments without further research and without parting from the invention, and, accordingly, it is meant that such adaptations and modifications will have to be considered as equivalent to the specific embodiments. The means and the materials to perform the different functions described herein could have a different nature without, for this reason, departing from the field of the invention. It is to be understood that the phraseology or terminology that is employed herein is for the purpose of description and not of limitation.