We deposited niobium nitride (NbN) thin film by radio-frequency (RF) magnetron sputtering on MgO substrate at 800 ºC (with Tc ~15 K). To increase the photodetectivity, we design an Ag nanocube nanoresonator with a strong gap-plasmon resonance in the visible range to further enhance the photoresponsivity by engineering the optical response for the NbN superconducting devices. This is due to the superconducting states are broken down by a localizing strong electromagnetic field. To design the plasmonic nanostructures, we calculated the electromagnetic field distribution of Ag nanocube/Al2O3/NbN structure by finite-difference time-domain (FDTD). We observed a strong plasmonic resonance field which tightly confined in the Al2O3 layer between the Ag nanocube and NbN film at a resonant wavelength of 532 nm. In the end, we will also discuss the detailed working mechanism and the potential application of plasmon-enhanced photodetection in NbN superconducting photodetectors.
We deposited niobium nitride (NbN) thin film by radio-frequency (RF) magnetron sputtering on MgO substrate at 800 ºC (with Tc ~15 K). To increase the photodetectivity, we design an Ag nanocube nanoresonator with a strong gap-plasmon resonance in the visible range to further enhance the photoresponsivity by engineering the optical response for the NbN superconducting devices. This is due to the superconducting states are broken down by a localizing strong electromagnetic field. To design the plasmonic nanostructures, we calculated the electromagnetic field distribution of Ag nanocube/Al2O3/NbN structure by finite-difference time-domain (FDTD). We observed a strong plasmonic resonance field which tightly confined in the Al2O3 layer between the Ag nanocube and NbN film at a resonant wavelength of 532 nm. In the end, we will also discuss the detailed working mechanism and the potential application of plasmon-enhanced photodetection in NbN superconducting photodetectors.
Single-photon detectors have been widely studied for decades because of their unique capability to resolve photon numbers, enabling many applications in quantum information technologies. Compared to other types of single-photon detectors, niobium nitride (NbN) superconducting nanowire single-photon detectors are promising candidates and are commercially available today. However, the absorption coefficient of superconducting NbN in the visible range is typically low, resulting in low quantum efficiency and low signal-to-noise ratios. In the present work, we use a novel approach to enhance the visible-light photoresponse in NbN superconducting microwire photon detectors (SMPDs) by integrating them with gap plasmon resonators (GPRs). This talk describes how we observe the plasmonic NbN SMPDs can achieve a 233-fold enhancement in the phonon-electron interaction factor (γ) compared to pristine NbN SMPDs under resonant conditions with illumination at 532 nm. The nonlinear photoresponse in the visible region is attributed to the gap-plasmon-induced heating that breaks the superconducting state to normal. Our results open new opportunities for ultrasensitive single-photon detection for quantum information processing, quantum optics, imaging, and sensing at visible wavelengths. The detailed mechanisms and possible applications will be discussed. Finally, I will discuss emerging plasmonic platforms based on transition metal nitrides and their potential applications.
The ps optical response of ultrathin NbN photodetectors has been studied by electro-optic sampling. The detectors were fabricated by patterning ultrathin (3.5 nm thick) NbN films deposited on sapphire by reactive magnetron sputtering into either a 5×10 μm2 microbridge or 25 1 μm wide, 5 μm long strips connected in parallel. Both structures were placed at the center of a 4 mm long coplanar waveguide covered with Ti/Au. The photoresponse was studied at temperatures ranging from 2.15 K to 10 K, with the samples biased in the resistive (switched) state and illuminated with 100 fs wide laser pulses at 395 nm wavelength. At T=2.15 K, we obtained an approximately 100 ps wide transient, which corresponds to a NbN detector response time of 45 ps. The photoresponse can be attributed to the nonequilibrium electron heating effect, where the incident radiation increases the temperature of the electron subsystem, while the phonons act as the heat sink. The high-speed response of NbN devices makes them an excellent choice for an optoelectronic interface for superconducting digital circuits, as well as mixers for the terahertz regime. The multiple-strip detector showed a linear dependence on input optical power and a responsivity R=3.9 V/W.
A very promising way to increase the detection efficiency of nanowire superconducting single-photon detectors (SSPDs) consists in integrating them with advanced optical structures such as distributed Bragg reflectors (DBRs) and optical waveguides. This requires transferring the challenging SSPD technology from the usual substrates, i.e. sapphire and MgO, to an optical substrate like GaAs, on which DBRs and waveguides can be easily obtained. Therefore, we optimized the deposition process of few-nm thick superconducting NbN films on GaAs and AlAs/GaAs-based DBRs at low temperatures (substrate temperature T S = 400 °C), in order to prevent As evaporation. NbN films ranging from 150 to 3 nm in thickness were then deposited on single-crystal MgO, GaAs, MgO-buffered GaAs and DBRs by current-controlled DC magnetron sputtering (planar, circular, balanced configuration) of Nb in an Ar+N 2 plasma. 5.5 nm thick NbN films on GaAs exhibit T C = 10.7 K, Δ T C = 1.1 K and RRR = 0.7. The growth of such high quality thin NbN films on GaAs and DBRs has never been reported before.
Nitride superconductors and specially niobium nitride are key materials for developing high performance optoelectronic and digital circuits. We are presenting a way to achieve such high frequency devices on R-plane sapphire or MgO substrates. Deposition of thin and flat NbN films with Te above 10 K, low and reproducible penetration depth ( L 250 nm) and surface resistance (Rs) values up to I THz, is required and obtained by sputtering on a substrate heated in the 300-600 °C range. Simple sub-micrometer size HEB bridge structures where patterned even in a very thin (2-5 nm thick) NbN layers offering relaxation times below 30 ps. It is then possible to achieve fast optoelectronic data links and sensors on-chip with high clock frequency NbN RSFQ digital circuits.
Superconducting nanowire single photon detectors (SNSPDs) promise to combine near-unity quantum efficiency with >100 megacounts per second rates, picosecond timing jitter, and sensitivity ranging from x-ray to mid-infrared wavelengths. However, this promise is not yet fulfilled, as superior performance in all metrics is yet to be combined into one device. The highest single-pixel detection efficiency and the widest bias windows for saturated quantum efficiency have been achieved in SNSPDs based on amorphous materials, while the lowest timing jitter and highest counting rates were demonstrated in devices made from polycrystalline materials. Broadly speaking, the amorphous superconductors that have been used to make SNSPDs have higher resistivities and lower critical temperature (Tc) values than typical polycrystalline materials. Here, we demonstrate a method of preparing niobium nitride (NbN) that has lower-than-typical superconducting transition temperature and higher-than-typical resistivity. As we will show, NbN deposited onto unheated SiO2 has a low Tc and high resistivity but is too rough for fabricating unconstricted nanowires, and Tc is too low to yield SNSPDs that can operate well at liquid helium temperatures. By adding a 50 W RF bias to the substrate holder during sputtering, the Tc of the unheated NbN films was increased by up to 73%, and the roughness was substantially reduced. After optimizing the deposition for nitrogen flow rates, we obtained 5 nm thick NbN films with a Tc of 7.8 K and a resistivity of 253 μΩ cm. We used this bias sputtered room temperature NbN to fabricate SNSPDs. Measurements were performed at 2.5 K using 1550 nm light. Photon count rates appeared to saturate at bias currents approaching the critical current, indicating that the device's quantum efficiency was approaching unity. We measured a single-ended timing jitter of 38 ps. The optical coupling to these devices was not optimized; however, integration with front-side optical structures to improve absorption should be straightforward. This material preparation was further used to fabricate nanocryotrons and a large-area imager device, reported elsewhere. The simplicity of the preparation and promising device performance should enable future high-performance devices.
At the present time, ultrahigh performance superconducting nanowire single-photon detectors are the key elements in a variety of devices from biological research to quantum communications and computing. Accurate tuning of superconducting material properties is a powerful resource for fabricating single-photon detectors with desired properties. Here, we report on the major theoretical relations between ultrathin niobium nitride (NbN) film properties and superconducting nanowire single-photon detector characteristics, as well as the dependence of ultrathin NbN film properties on reactive magnetron sputtering recipes. Based on this study, we formulate the exact requirements for ultrathin NbN films for ultrahigh performance superconducting nanowire single-photon detectors. Then, we experimentally studied the properties of ultrathin NbN films (morphology, crystalline structure, critical temperature, and sheet resistance) on silicon, sapphire, silicon dioxide, and silicon nitride substrates sputtered with various recipes. We demonstrate ultrathin NbN films (obtained with more than 100 films deposition) with a wide range of critical temperature from 2.5 to 12.1 K and sheet resistance from 285 to 2000 Ω/sq and report a sheet resistance evolution of more than 40% within two years. Finally, we found out that one should use ultrathin NbN films with a specific critical temperature near 9.5 K and a sheet resistance of about 350 Ω/sq for ultrahigh performance state-of-the-art superconducting nanowire single-photon detectors at 1550 nm wavelength.