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Register a new userRise times of voltage pulses in NbN superconducting single-photon detectors
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Added by
Alexander Divochiy Mr
Publication date
2016
fields
Physics
and research's language is
English
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We have found experimentally that the rise times of voltage pulses in NbN superconducting single photon detectors increase nonlinearly with increasing detector length. We fabricated superconducting single photon detectors based on NbN thin films with a meander-like sensitive region of area from 2x2um2 to 11x11um2. The effect is connected with the dependence of the detector resistance, which appears after photon absorption, on its kinetic inductance and hence on detector length. This conclusion is confirmed by our calculations in the framework of the two-temperature model.
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We investigate thermal properties of a NbN single-photon detector capable of unit internal detection efficiency. Using an independent calibration of the coupling losses we determine the absolute optical power absorbed by the NbN film and, via a resistive superconductor thermometry, the thermal resistance Z(T) of the NbN film in dependence of temperature. In principle, this approach permits a simultaneous measurement of the electron-phonon and phonon-escape contributions to the energy relaxation, which in our case is ambiguous for their similar temperature dependencies. We analyze the Z(T) within the two-temperature model and impose an upper bound on the ratio of electron and phonon heat capacities in NbN, which is surprisingly close to a recent theoretical lower bound for the same quantity in similar devices.
We measure the maximal distance at which two absorbed photons can jointly trigger a detection event in NbN nanowire superconducting single photon detector (SSPD) microbridges by comparing the one-photon and two-photon efficiency of bridges of different overall lengths, from 0 to 400 nm. We find a length of $23 pm 2$ nm. This value is in good agreement with to size of the quasiparticle cloud at the time of the detection event.
We investigated the suitability of AlN as a buffer layer for NbN superconducting nanowire single-photon detectors (SNSPDs) on GaAs. The NbN films with a thickness of 3.3 nm to 20 nm deposited onto GaAs substrates with AlN buffer layer, demonstrate a higher critical temperature, critical current density and lower residual resistivity in comparison to films deposited onto bare substrates. Unfortunately, the thermal coupling of the NbN film to the substrate weakens. SNSPDs made of 4.9 nm thick NbN films on buffered substrates (in comparison to detectors made from NbN films on bare GaAs) demonstrate three orders of magnitude lower dark count rates and about ten times higher detection efficiency at 900 nm being measured at 90% of the critical current. The system timing jitter of SNSPDs on buffered substrates is 72 ps which is 36 ps lower than those on bare substrate. However, a weaker thermal coupling of NbN nanowire to the buffered substrate leads to a latching effect at bias currents > 0.97 IC.
We investigate the role of electrothermal feedback in the operation of superconducting nanowire single-photon detectors (SNSPDs). It is found that the desired mode of operation for SNSPDs is only achieved if this feedback is unstable, which happens naturally through the slow electrical response associated with their relatively large kinetic inductance. If this response is sped up in an effort to increase the device count rate, the electrothermal feedback becomes stable and results in an effect known as latching, where the device is locked in a resistive state and can no longer detect photons. We present a set of experiments which elucidate this effect, and a simple model which quantitatively explains the results.
Using two-temperature model coupled with modified time-dependent Ginzburg-Landau equation we calculate the delay time $tau_d$ in appearance of growing normal domain in the current-biased superconducting strip after absorption of the single photon. We demonstrate that $tau_d$ depends on the place in the strip where photon is absorbed and monotonically decreases with increasing of the current. We argue, that the variation of $tau_d$ (timing jitter), connected either with position-dependent response or Fano fluctuations could be as small as the lowest relaxation time of the superconducting order parameter $sim hbar/k_BT_c$ ($T_c$ is the critical temperature of the superconductor) when the current approaches the depairing current.
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