Goal: Development of kinetic-inductance single-photon detectors for the visible, near and far IR ranges based on TiN films, study of energy relaxation time in superconducting films of boron-doped diamond, development and study of superconducting single-photon detectors based on NbN nanowires for coherent detection of weak signals.
Objectives of research:
Optimization of ultra-thin films of TiN and other superconductors to create kinetic-inductance single-photon detectors for the visible and near IR ranges. In this regard, the research objectives are the following:
a) Fabrication of superconducting films of TiN and study of the temperature dependence of their resistivity.
b) Study of the time of recombination of nonequilibrium quasiparticles in films of TiN and determination of the energy-relaxation time.
c) Improvement of the method of determining the resistance relaxation time, based on the absorption of the amplitude-modulated sub-THz radiation.
d) Development of mechanisms of energy relaxation in disordered thin superconducting films.
d) Fabrication of superconducting films of boron-doped diamond on silicon substrates by chemical vapor deposition.
e) Study of the energy-relaxation time and its temperature dependence in the diamond films in the temperature range from 1.8 to 2.7 K.
f) Development and research of the possibility of establishing a heterodyne receiver based on superconducting single-photon detectors, integrated with an optical waveguide.
The methods used are: TiN superconducting films were fabricated by magnetron sputtering, and by atomic layer deposition. Superconducting boron-doped diamond films on silicon substrates were fabricated by vapor deposition in the presence of plasma. The relaxation time was determined directly from the frequency dependence of the response of the films to amplitude-modulated electromagnetic radiation. Coherent-detection method with digital signal processing and with high temporal resolution.
The empirical base of the research:
Superconducting films on dielectric substrates are key elements of practical detectors of electromagnetic radiation, such as superconducting single-photon detectors SSPDs, microwave kinetic-inductance detectors (MKIDs) and hot-electron bolometers (HEBs). The optimal operation of such detectors is determined by the balance between radiation absorption and cooling in the detector material. In the superconducting state the relaxation processes that occur after the redistribution of energy between the quasiparticles are determined by recombination processes in the Cooper pairs. The recombination time increases exponentially with decreasing temperature due to the reduced availability of quasiparticles. The scale is determined by the recombination time of the electron-phonon interaction. One strategy to optimize the characteristics of the detectors is the choice of the appropriate material. Selection of a suitable material is a complex problem, and needs to start with the selection of a suitable material of the electron-phonon interaction. However, requirements differ for different types of detectors. SSPDs need materials with a fast electron-phonon relaxation to ensure a rapid response and materials with similar properties are required for HEB mixers to ensure a wide IF band. MKIDs and nano-bolometers based on HEBs require materials with a long electron-phonon relaxation, which provides high sensitivity and low noise. Another important parameter which determines noise properties and geometry of the detector is the density of states of electrons, and materials with a low density of states to optimize noise performance detectors are needed. The final choice determines the reliable technology for creating thin films with given T and resistance. For sensitive superconducting detectors it is very important to achieve a controlled adjustment of critical temperature of the material to the desired value.
a) in the theory:
1) An analytical model for the relaxation mechanisms in disordered thin films of WSi considering the effect of the phonon "bottleneck."
b) the development of methodology:
1) Technologies for fabrication of nanostructures on the basis of samples of ultrathin superconducting films of TiN and boron-doped diamond on silicon substrates.
2) Optimized method for determining the resistance relaxation time, based on the absorption of the amplitude-modulated sub-THz radiation.
3) A method for measuring the sensitivity of superconducting single-photon detectors in the coherent mode based on the detection of the beat frequency of the two lasers, which leads to the dependence of the probability of detection of photocounts.
c) new empirical knowledge:
1) we have investigated superconducting characteristics and material parameters of TiN nanostructures: the surface resistance, the critical temperature, the width of the superconducting transition, the critical current density.
2) we have investigated the characteristic energy-relaxation time in a wide temperature range in TiN nanostructures. Under the absorption of amplitude-modulated sub-THz radiation by disordered TiN films the relaxation time of the resistive state, created by the external magnetic field, similar in magnitude to the second critical at this temperature, is determined by the electron-phonon interaction. The value of the energy-relaxation time is about 90 ns at a temperature of 1.7 K. The electron-phonon interaction in the TiN films follows the temperature dependence T-3. This dependence is observed in films of TiN, made by magnetron sputtering (with a mean value of the resistivity ρ≈100 mO · cm), and in the films of TiN, obtained by atomic layer deposition (in the range of resistivity ρ≈120 -250 mO · cm).
3) We have studied the energy-relaxation time in superconducting films of boron-doped diamond grown on silicon substrates by chemical vapor deposition. The time ranges from 160 ns at a temperature of 2.7 K to 410 ns at a temperature of 1.8 K. The dependence is described as T-2. The results are consistent with the values of the electron-phonon interaction primarily measured on samples of single-crystal epitaxial boron-doped diamond grown on diamond substrates.
4) We have developed and investigated a heterodyne receiver based on superconducting single-photon detector. The receiver is a chip detector based on a thin NbN film in the form of a meander together with single-mode optical waveguide. The NbN nanowire is located on nanophotonic waveguide made of Si3N4, with a length necessary for the effective absorption of incoming photons. The measured required LO power was approximately 1 mW, which provides the detection mode in the conditions of the quantum noise limit.
The degree of implementation, recommendations for the implementation or results of the implementation of research results (to be completed in case of the possibility of practical use of the results)
Superconducting TiN films and boron-doped diamond films are promising for multi-element arrays of far-infrared kinetic-inductance detectors, which is currently one of the major applications in projects of space and ground-based astronomy. In addition, these films have a high resistivity in the normal state and are key elements in hybrid superconductor / spin systems for quantum computing and devices related to the study of Majorana physics in which a superconductor is used in conjunction with a large magnetic field.
Superconducting single-photon detectors based on NbN nanowires for coherent detection of weak signals has a high quantum efficiency, which makes them attractive for a large number of practical applications in which the performance of the detector must be combined with a high sensitivity. Using devices of integrated optics will create in the future a new technology for quantum communication and the development of the quantum computer.