Abstract Superconducting nanowire single-photon detectors (SNSPDs) offer high system detection efficiency (SDE), low dark counts, and excellent timing resolution, making them indispensable for applications from quantum optics to deep-space optical communications. Although near-unity SDE and intrinsic detection efficiency (IDE) have been achieved at near-infrared wavelengths through advanced fabrication and optical design, further improving IDE at the material and device levels remains essential for achieving broader bandwidths, higher operating temperatures, and simplified processing. Here, we explored helium ion irradiation as a post-fabrication method to modify the thermal properties of NbN SNSPDs without altering their geometry or material stack. With increasing fluence (0–1.1 × 10 17 ions cm −2 ), the thermal boundary conductance decreases by 57% from 127 W m −2 K −4 to 54 W m −2 K −4 , and saturates near 9 × 10 16 ions cm −2 . At this fluence, the minimum hotspot relaxation time increases by 41%, rising from 17 ps to 24 ps, while the electron–phonon interaction time decreases, with the magnitude of change depending on temperature and sample batch. Transmission electron microscopy reveals interfacial disorder near the NbN/SiO₂ interface (6–8 nm) and localized defects in the SiO₂ layer (30–260 nm), correlating with thermal transport suppression. These thermal modifications have a significant impact on relaxation dynamics and contribute to IDE enhancement. Our findings establish ion irradiation as an effective strategy for thermal tuning in SNSPDs, providing new insights into defect engineering for superconducting optoelectronic devices.
Abstract Superconducting nanowire single-photon detectors (SNSPDs) offer high system detection efficiency (SDE), low dark counts, and excellent timing resolution, making them indispensable for applications from quantum optics to deep-space optical communications. Although near-unity SDE and intrinsic detection efficiency (IDE) have been achieved at near-infrared wavelengths through advanced fabrication and optical design, further improving IDE at the material and device levels remains essential for achieving broader bandwidths, higher operating temperatures, and simplified processing. Here, we explored helium ion irradiation as a post-fabrication method to modify the thermal properties of NbN SNSPDs without altering their geometry or material stack. With increasing fluence (0–1.1 × 10 17 ions cm −2 ), the thermal boundary conductance decreases by 57% from 127 W m −2 K −4 to 54 W m −2 K −4 , and saturates near 9 × 10 16 ions cm −2 . At this fluence, the minimum hotspot relaxation time increases by 41%, rising from 17 ps to 24 ps, while the electron–phonon interaction time decreases, with the magnitude of change depending on temperature and sample batch. Transmission electron microscopy reveals interfacial disorder near the NbN/SiO₂ interface (6–8 nm) and localized defects in the SiO₂ layer (30–260 nm), correlating with thermal transport suppression. These thermal modifications have a significant impact on relaxation dynamics and contribute to IDE enhancement. Our findings establish ion irradiation as an effective strategy for thermal tuning in SNSPDs, providing new insights into defect engineering for superconducting optoelectronic devices.
Superconducting nanowire single-photon detectors (SNSPDs) are indispensable in fields such as quantum science and technology, astronomy, and biomedical imaging, where high detection efficiency, low dark count rates, and high timing accuracy are required. Recently, helium (He) ion irradiation was shown to be a promising method to enhance SNSPD performance. Here, we study how changes in the underlying superconducting NbTiN film and the SiO2/Si substrate affect device performance. While irradiated and unirradiated NbTiN films show similar crystallinity, we observe He bubble formation below the SiO2/Si interface and an amorphization of the Si substrate. Both reduce the thermal conductance between the superconducting thin film and the substrate from 210 to 70 W m−2 K−4 after irradiation with 2000 ions nm−2. This effect, combined with the lateral straggle of He ions in the substrate, allows the modification of the superconductor-to-substrate thermal conductance of an SNSPD by selectively irradiating only the regions around the nanowire. With this approach, we achieved a broader bias current range (9.8 μA vs 3.7 µA) in which the detector operates at its maximum detection efficiency, which is beneficial for reducing dark counts while maintaining high sensitivity. Moreover, the photon-assisted critical current remained similar to that of the unirradiated reference device (59.0 µA vs 60.1 µA), while full irradiation reduced it to 22.4 µA. Our results suggest that the irradiation-induced reduction of the thermal conductance significantly enhances SNSPD sensitivity, offering a novel approach to locally engineer substrate properties for improved detector performance.
Superconducting nanowire single-photon detectors (SNSPDs) with high system detection efficiency (SDE) would enable remarkable experiments in quantum information processing. However, realizing a NbN SNSPD with saturated intrinsic detection efficiency (IDE) that retains high SDE at near-infrared wavelengths is challenging, due to the high critical temperature of NbN. This study uses defect engineering by helium-ion irradiation to enhance the IDE of NbN SNSPDs to saturation without sacrificing SDE. This technique also allows direct comparison of irradiation-induced changes in detector performance, making it a useful tool for studying the physics of superconducting devices.
Abstract Achieving homogeneous performance metrics between nominally identical pixels is challenging for the operation of arrays of superconducting nanowire single‐photon detectors (SNSPDs). Here, local helium ion irradiation is utilized to post‐process and tune single‐photon detection efficiency, switching current, and critical temperature of individual devices on the same chip. For 12 nm thick highly absorptive SNSPDs, which are barely sensitive to single photons with a wavelength of 780 nm prior to He ion irradiation, an increase of the system detection efficiency from <0.05% to (55.3 1.1)% is observed following irradiation. Moreover, the internal detection efficiency saturates at a temperature of 4.5 K after irradiation with 1800 ions nm −2 . Compared to 8 nm SNSPDs of similar detection efficiency, a doubling of the switching current (to 20 µA) is observed for irradiated 10 nm thick detectors, increasing the amplitude of detection voltage pulses. Investigations of the scaling of superconducting thin film properties with irradiation up to a fluence of 2600 ions nm −2 revealed an increase of sheet resistance and a decrease of critical temperature towards high fluences. A physical model accounting for defect generation and sputtering during helium ion irradiation is presented and shows good qualitative agreement with experiments.
The hotspot dynamics is one of the key process involved in the detection mechanism in superconducting nanowires single photon detectors (SNSPDs). The energy delivered by the photon absorption creates a non-equilibrium region where Cooper pairs break into quasi-particles, which undergo a thermalization process involving hot-electron–phonon interactions within the superconducting film. Eventually, phonons dissipate the excess energy into the substrate and relax to the base temperature. Hence, intrinsic reset time, count rate and possibly other parameters are conceivably affected by thermal coupling between the substrates and the superconducting film. Thus, a detailed understanding of the thermal boundary conductance is crucial for achieving the best possible performances. Even though recent studies have demonstrated that the substrate plays an important role in defining the hotspot relaxation time, no systematic study on the influence on detector performance metrics has been carried out yet. In this work, we investigated the effect of different substrates on SNSPDs performance. Meandered detectors and single nanowires with various dimensions have been successfully fabricated on ultrathin NbN film sputtered on six substrates: SiO2, Al2O3, SiC, SiN, AlN and GaN. We characterized the deposited films measuring Tc, ρ, ΔTc and RRR. Lattice mismatch is known to play an important role in determining the resulting film quality. SiO2 and SiN samples, in which the mismatch is pronounced, showed lower Tc and higher ρ compared to the other substrates. We extracted the critical and retrapping current densities (Jc, Jr respectively) for all the nanowire widths, observing a peculiar behavior in SiO2 and SiN samples which needs further investigation. Optical response was characterized in terms of reset time (τ), dark count rate (DCR) and photon count rate (PCR) with a 1550 nm wavelength laser at 3K. Single photon regime was verified in all the measured detectors. Reset time has been confirmed to be limited by kinetic inductance Lk for all the substrates. DCR behavior appeared to be correlated to the difference in film properties. Only detector fabricated on SiN and SiO2 substrates showed saturation in the photon count rate, while the highest count rate was achieved by Al2O3 sample. The different behavior of photon counts response on the substrates suggests a correlation with the measured thermal conductance β and needs further study. Nevertheless, the comparison between the extracted thermal boundary conductances and the theoretical ones highlighted relevant discrepancies between theory and experimental data. In conclusion, devices fabricated on NbN deposited on different substrates showed a peculiar detector photo-response behavior. A deeper evaluation of NbN disorder grade and a better thermal characterization of the substrate-thin film interface are needed as future work.
Superconducting nanowire single-photon detectors (SNSPDs) play a pivotal role in infrared single-photon-detection applications owing to their free-running mode, near-unity detection efficiency, and excellent time resolution. However, conventional niobium nitride- (NbN) SNSPDs often have a trade-off between optimal detection efficiency and time resolution. We herein demonstrate NbN SNSPDs in the combination of high efficiency and time resolution by using the helium-ion-irradiated film of 10 nm and a homemade cryogenic amplifier to improve the signal-to-noise ratio. The proposed 18-μm-diameter device operated at 850 mK shows a saturated system detection efficiency of ∼90.5% at 1550 nm wavelength with a dark count rate below 50 counts per second (cps), and minimum system timing jitter of ∼14.7 ps. Meanwhile, the recovery time is 21.2 ns owing to its lower kinetic inductance. The time-of-flight light detection and ranging with this device was further carried out indoors, which demonstrates a submillimeter (<0.5 mm) depth resolution, as well as good depth and intensity reconstruction of a low-signature object even under the dwell time of 0.5 ms. It indicates that our device has a great advantage and potential in high-time-resolution and weak-signal-detection applications, such as real-time imaging, fluorescence lifetime detection, and remote laser sensing.
Superconducting nanowire single-photon detectors (SNSPDs) with high detection efficiency have played significant role in numerous applications for photon detection from visible light to mid-infrared band. However, it is still a challenge to easily promote the sensitivity of SNSPD without compromising its other performance. We report a method to improve the detection sensitivity of devices via suspended SiO2 membrane, and the intrinsic detection efficiency is significantly enhanced from 78% to 100% for a 1550 nm wavelength. The electro-thermal simulation and hotspot relaxation time measurement indicate that a suspended SiO2 membrane can significantly decrease the cooling efficiency between the hotspot and the substrate, and the relaxation time is enlarged twice than before. Meanwhile, we note that the proposed method does not affect other properties of devices, such as dark count rate, detection speed, and timing jitter. Because the thermal dissipation process can be controlled continuously (by varying the size of the suspended area), the proposed method may also help to obtain a deeper understanding of the thermal diffusion and detection process of SNSPDs.