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Journal of Applied Physics

Analysis of quantum superconducting kinetic inductance photodetectors

Nathan Bluzer

Published Dec 15, 1995• 27 citations
Publisher

Abstract

Analysis of a quantum superconducting kinetic inductance photodetector (QSKIP) structure is presented for operation in a low background environment. We project the QSKIP’s spectral response to be limited by the Cooper pair binding energy, 2Δ or about 32 μm for YBCO. The QSKIP response and sensitivity are computed from the minimum of the Hamiltonian energy functional and linearized Rothwarf–Taylor equations. Photoresponse and sensitivity expressions are computed in terms of the quasiparticle lifetime and indicate background limited infrared performance at very low photon flux levels. At low temperatures and under background limited infrared performance conditions, the photoresponse is proportional to the number of absorbed photons. Operating the QSKIP in the Meissner state and below 50% of the critical current limits the noise sources to fluctuations in the condensate population. With the noise and photoresponse equations, the QSKIP noise equivalent power is computed in terms of the characteristic lifetimes of: quasiparticle generation; quasiparticle recombination; anharmonic phonon decay; and phonon trapping. The calculated noise equivalent power for a YBCO QSKIP is about 2.5×10−15 W/√Hz at 9 K.

DOI
10.1063/1.360383

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Analysis of a quantum superconducting kinetic inductance photodetector (QSKIP) structure is presented for operation in a low background environment. We project the QSKIP’s spectral response to be limited by the Cooper pair binding energy, 2Δ or about 32 μm for YBCO. The QSKIP response and sensitivity are computed from the minimum of the Hamiltonian energy functional and linearized Rothwarf–Taylor equations. Photoresponse and sensitivity expressions are computed in terms of the quasiparticle lifetime and indicate background limited infrared performance at very low photon flux levels. At low temperatures and under background limited infrared performance conditions, the photoresponse is proportional to the number of absorbed photons. Operating the QSKIP in the Meissner state and below 50% of the critical current limits the noise sources to fluctuations in the condensate population. With the noise and photoresponse equations, the QSKIP noise equivalent power is computed in terms of the characteristic lifetimes of: quasiparticle generation; quasiparticle recombination; anharmonic phonon decay; and phonon trapping. The calculated noise equivalent power for a YBCO QSKIP is about 2.5×10−15 W/√Hz at 9 K.
A superconducting quantum detector structure in YBCO is described with a directly coupled SQUID read out circuit. The detector geometry is optimized for maximum photoresponse with the use of a serpentine pattern. The serpentine pattern reduces quasiparticle diffusion effects and maximizes the photoinduced changes in the kinetic inductance. The operation of this sensor is analyzed in terms of geometry and quasiparticle lifetime to obtain expressions for the signal and noise of this detector. The background limited infrared performance of this detector is calculated to be about 5/spl times/10/sup -17/ NEP. The photoresponse is expected to be limited by the superconducting energy gap, about 30-40 /spl mu/m.>
While experimental studies of kinetic-inductance sensors have been limited so far by the temperature range near the superconducting transition, these detectors can be very sensitivity at temperatures well below the transition, where the number of equilibrium quasiparticles is exponentially small. In this regime, a shift of the quasiparticle chemical potential under radiation results in the change of the kinetic inductance, which can be measured by a sensitive SQUID readout. We modeled the kinetic inductance response of detectors made from disordered superconducting Nb, NbC, and MoRe films. Low phonon transparency of the interface between the superconductor and the substrate causes substantial re-trapping of phonons providing high quantum efficiency and the operating time of approximately 1 ms at 1 K. Due to the small number of quasiparticles, the noise equivalent power of the detector determined by the quasiparticle generation-recombination noise can be as small as approximately 10(exp -19) W/Hz(exp 1/2) at He4 temperatures.
While experimental studies of kinetic-inductance sensors have been limited so far by the temperature range near the superconducting transition, these detectors can be very sensitive at temperatures well below the transition, where the number of equilibrium quasiparticles is exponentially small. In this regime, a shift of the quasiparticle chemical potential under radiation results in the change of the kinetic inductance, which can be measured by a sensitive SQUID readout. We modeled the kinetic inductance response of detectors made from disordered superconducting Nb, NbC, and MoRe films. Low phonon transparency of the interface between the superconductor and the substrate causes substantial re-trapping of phonons providing high quantum efficiency and the operating time of ∼1 ms at ≈1 K. Due to the small number of quasiparticles, the noise equivalent power of the detector determined by the quasiparticle generation-recombination noise can be as small as ∼10−19 W/√Hz at He4 temperatures.
While most experimental studies of kinetic-inductance sensors have been limited so far by the temperature range near the superconducting transition, kinetic-inductance detectors can be very sensitive at temperatures well below the transition, where the number of equilibrium quasiparticles is exponentially small. In this regime, a shift of the quasiparticle chemical potential under radiation results in the change of the kinetic inductance. We modeled the noise characteristics of the kinetic-inductance detectors made from disordered superconducting Nb, NbC, and MoRe films. Low-phonon transparency of the interface between the superconductor and the substrate causes substantial retrapping of phonons providing high quantum efficiency and the operating time of ∼1 ms at ≈1 K. Due to the small number of quasiparticles, the noise equivalent power of the detector determined by the quasiparticle generation–recombination noise can be as small as ∼10−19 W/Hz at He4 temperatures.
Abstract The Cooper-pair breaking efficiency quantifies the effectiveness with which incident energy, such as that delivered by photons or phonons, is converted into quasiparticles—a process central to the response of superconducting pair-breaking detectors. We measure the pair-breaking efficiency in two far-infrared kinetic inductance detectors (KIDs) operating at 4.3 THz and 15 THz by comparing photon noise and thermal generation-recombination noise. Both KIDs are lumped-element devices consisting of a tantalum interdigitated capacitor in parallel with an aluminum inductor meandered line, which also acts as a low-volume absorber. The measured pair-breaking efficiencies are 0.42 and 0.07 for the 4.3 THz and 15 THz devices, respectively. The extremely low efficiency at 15 THz, a frequency well above the aluminum Debye frequency, is mainly attributed to rapid anharmonic decay of phonons, with additional phonon relaxation in the tantalum layer further reducing the available phonon population for pair-breaking. Furthermore, a relationship between the pair-breaking efficiency and detector resolving power is derived.
We analyze the ultimate performance of a superconducting quantum detector in order to meet requirements for applications in near-infrared astronomy and X-ray spectroscopy. The detector exploits a combined detection mechanism, in which avalanche quasiparticle multiplication and the supercurrent jointly produce a voltage response to a single absorbed photon via successive formation of a photon-induced and a current-induced normal hotspot in a narrow superconducting strip. The response time of the detector should increase with the photon energy providing energy resolution. Depending on the superconducting material and operation conditions, the cut-off wavelength for the single-photon detection regime varies from infrared waves to visible light. We simulated the performance of the background-limited infrared direct detector and X-ray photon counter utilizing the above mechanism. Low dark count rate and intrinsic low-frequency cut-off allow for realizing a background limited noise equivalent power of 10−20 W Hz−1/2 for a far-infrared direct detector exposed to 4-K background radiation. At low temperatures, the intrinsic response time of the counter is rather determined by diffusion of nonequilibrium electrons than by the rate of energy transfer to phonons. Therefore, thermal fluctuations do not hamper energy resolution of the X-ray photon counter that should be better than 10−3 for 6-keV photons. Comparison of new data obtained with a Nb based detector and previously reported results on NbN quantum detectors support our estimates of ultimate detector performance.