Optical microscopy is a valuable tool for in vivo monitoring of biological structures and functions because of its noninvasiveness. However, imaging deep into biological tissues is challenging due to the scattering and absorption of light. Previous research has shown that the two optimal wavelength windows for high-resolution deep mouse brain imaging are around 1300 and 1700 nm. However, one-photon fluorescence imaging in the wavelength region has been highly challenging due to the poor detection efficiency of currently available detectors. To fully utilize this wavelength advantage, we demonstrated here one-photon confocal fluorescence imaging of deep mouse brains with an excitation wavelength of 1310 nm and an emission wavelength within the 1700 nm window. Fluorescence emission at 1700 nm was detected by a custom-built superconducting nanowire single-photon detector (SNSPD) optimized for detection between 1600 nm and 2000 nm with low detection noise and high detection efficiency. With the PEGylated quantum dots and SNSPD both positioned at the optimal imaging window for deep tissue penetration, we demonstrated in vivo one-photon confocal fluorescence imaging at approximately 1.7 mm below the surface of the mouse brain, through the entire cortical column and into the hippocampus region with a low-cost continuous-wave laser source and low excitation power. We further discussed the significance of the staining inhomogeneity in determining the depth limit of one-photon confocal fluorescence imaging. Our work may motivate the further development of long wavelength fluorescent probes, and inspire innovations in high-efficiency, high-gain, and low-noise long wavelength detectors for biological imaging.

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Short-Wave Infrared Confocal Fluorescence Imaging of Deep Mouse Brain with a Superconducting Nanowire Single-Photon Detector

Shortly after their inception, superconducting nanowire single-photon detectors (SNSPDs) became the leading quantum light detection technology. With the capability of detecting single-photons with near-unity efficiency, high time resolution, low dark count rate, and fast recovery time, SNSPDs outperform conventional single-photon detection techniques. However, detecting lower energy single photons ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m1"> <mml:mrow> <mml:mo form="prefix">&lt;</mml:mo> <mml:mn>0.8</mml:mn> <mml:mtext> </mml:mtext> <mml:mi>eV</mml:mi> </mml:mrow> </mml:math> ) with high efficiency and low timing jitter has remained a challenge. To achieve unity internal efficiency at mid-infrared wavelengths, previous works used amorphous superconducting materials with low energy gaps at the expense of reduced time resolution (close to a nanosecond), and by operating them in complex milliKelvin (mK) dilution refrigerators. In this work, we provide an alternative approach with SNSPDs fabricated from 5 to 9.5 nm thick NbTiN superconducting films and devices operated in conventional Gifford-McMahon cryocoolers. By optimizing the superconducting film deposition process, film thickness, and nanowire design, our fiber-coupled devices achieved <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m2"> <mml:mrow> <mml:mo form="prefix">&gt;</mml:mo> <mml:mn>70</mml:mn> <mml:mi>%</mml:mi> </mml:mrow> </mml:math> system detection efficiency (SDE) at 2 μm and sub-15 ps timing jitter. Furthermore, detectors from the same batch demonstrated unity internal detection efficiency at 3 μm and 80% internal efficiency at 4 μm, paving the road for an efficient mid-infrared single-photon detection technology with unparalleled time resolution and without mK cooling requirements. We also systematically studied the dark count rates (DCRs) of our detectors coupled to different types of mid-infrared optical fibers and blackbody radiation filters. This offers insight into the trade-off between bandwidth and DCRs for mid-infrared SNSPDs. To conclude, this paper significantly extends the working wavelength range for SNSPDs made from polycrystalline NbTiN to 1.5–4 μm, and we expect quantum optics experiments and applications in the mid-infrared range to benefit from this far-reaching technology. 

/pdf/efficient-mid-infrared-single-photon-detection-using-1euotjoa.pdf

Efficient mid-infrared single-photon detection using superconducting NbTiN nanowires with high time resolution in a Gifford-McMahon cryocooler

One of the highest performing single-photon detectors in the visible and near-infrared regions is the superconducting nanostrip photon detector (SNSPD or SSPD), which usually uses NbN or NbTiN as the superconductor. Using other superconductors may significantly improve, for example, the operating temperature and count rate characteristics. This paper briefly reviews the current state of the potential, characteristics, thin film growth, and nanofabrication process of SNSPD using various superconductors. key words: single-photon detector, SNSPD, SSPD, superconductor

Review of Superconducting Nanostrip Photon Detectors using Various Superconductors

The near-unity system detection efficiency (SDE) and excellent timing resolution of superconducting nanowire single-photon detectors (SNSPDs), combined with their other merits, have enabled many classical and quantum photonic applications. However, the prevalent design based on meandering nanowires makes SDE dependent on the polarization states of the incident photons; for unpolarized light, the major merit of high SDE would get compromised, which could be detrimental for photon-starved applications. Here, we create SNSPDs with an arced fractal geometry that almost completely eliminates this polarization dependence of the SDE, and we experimentally demonstrate 84$\pm$3$\%$ SDE, 1.02$^{+0.06}_{-0.02}$ polarization sensitivity at the wavelength of 1575 nm, and 20.8 ps timing jitter in a 0.1-W closed-cycle Gifford-McMahon cryocooler, at the base temperature of 2.0 K. This demonstration provides a novel, practical device structure of SNSPDs, allowing for operation in the visible, near-, and mid-infrared spectral ranges, and paves the way for polarization-insensitive single-photon detection with high SDE and high timing resolution 

/pdf/fractal-superconducting-nanowires-detect-infrared-single-bgs2zd0g.pdf

Fractal Superconducting Nanowires Detect Infrared Single Photons with 84% System Detection Efficiency, 1.02 Polarization Sensitivity, and 20.8 ps Timing Resolution

Lidar with superconducting nanowire single-photon detectors: Recent advances and developments

Mid-infrared single photon detector with superconductor Mo0.8Si0.2 nanowire

Amorphous materials are attractive candidates for fabricating the superconducting nanowire single-photon detectors (SNSPDs) due to their superior tolerance and scalability over crystalline niobium nitride. However, the reduced superconducting transition temperature degenerates both operating temperature and saturation efficiency. Herein, the SNSPD (6.5 nm thickness and 50 nm width) based on the amorphous Mo0.8Si0.2 film with a high optical absorption coefficient demonstrates close-to-unity intrinsic detection efficiency for 1550 nm photons from 75 mK to 2.2 K. Further, a high-performance array SNSPD with optimized 90 nm-width wires is also demonstrated. As-fabricated uniform 4-pixel SNSPD exhibits a saturation plateau for the photon counts at 2.2 K, which overcomes the limitation of operation at low temperature (<1  K) for traditional amorphous SNSPDs. Coupled with superior intrinsic quantum efficiency, highly efficient photon counts, and low dark count ratio, this detector paves a way for achieving high efficiency and superior yield for large array systems.

Saturation efficiency for detecting 1550 nm photons with a 2 × 2 array of Mo 0.8 Si 0.2 nanowires at 2.2 K

The niobium nitride (NbN) nanowires fabricated with the high-quality ultra-thin NbN film with a thickness of 3 nm–6 nm were widely used for single photon detectors. These nanowires had a low aspect ratio, less than 1:20. However, increasing the thickness and the aspect ratio of highly-uniformed NbN nanowires without reducing the superconductivity is crucial for the device in detecting high-energy photons. In this paper, a high-quality superconducting nanowire with aspect ratio of 1:1 was fabricated with optimized process, which produced a superconducting critical current of 550 μA and a hysteresis of 36 μA at 2.2 K. With the optimization of the electron beam lithography process of AR-P6200.13 and the adjustion of the chamber pressure, the discharge power, as well as the auxiliary gas in the process of reactive ion etching (RIE), the meandered NbN nanowire structure with the minimum width of 80 nm, the duty cycle of 1:1 and the depth of 100 nm were finally obtained on the silicon nitride substrate. Simultaneously, the sidewall of nanowire was vertical and smooth, and the corresponding depth-width ratio was more than 1:1. The fabricated NbN nanowire will be applied to the detection of soft X-ray photon emitted from pulsars with a sub-10 ps time resolution.

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Fabrication of superconducting niobium nitride nanowire with high aspect ratio for X-ray photon detection

A traditional concept proposes that the suppression of the transition temperature ${T}_{c}$ in an amorphous nanobelt is driven by enhanced disorder, which accounts for localized Cooper pairs. However, in this paper, we observe ${T}_{c}$ suppression in an amorphous molybdenum-silicide (MoSi) nanobelt, which scales as the inverse square of the width but contradicts disorder theory. Instead, the transition regime can be well described by Cooper pair diffusion in the proximity effect. Both the nonlinear reduction of the switching current density and the abnormal increase of the effective retrapping current density with the reduction of the width further verify the proximity-induced relation. Therefore, we attribute the main size dependence of the suppressed superconducting properties in the MoSi nanobelt to the proximity effect rather than disorder. We speculate that the competition between superconductivity and disorder only appears at the two narrow edge bands rather than the entire nanobelt. Subsequently, the reduction in width does not produce a significant impact on superconductivity for disorder, and only the proximity effect plays an overwhelming role in the MoSi nanobelt. 

Suppression of superconductivity dominated by proximity effect in amorphous MoSi nanobelts

Superconducting nanowire single photon detector response to X-ray photon was demonstrated using a laser-plasma subps X-ray radiation, to the best of our knowledge. The time jitter was measured to be 248.2 ps, which is larger than ordinary visible or NIR SNSPDs and its efficiency is relatively lower, but the results pave the way for a new competitive X-ray detector with ultrahigh count rates, ultralow timing jitter, ultrahigh sensitivity and negligible dark counts.

Hang Han

Papers

Mid-infrared single photon detector with superconductor Mo0.8Si0.2 nanowire

Saturation efficiency for detecting 1550 nm photons with a 2 × 2 array of Mo 0.8 Si 0.2 nanowires at 2.2 K

Fabrication of superconducting niobium nitride nanowire with high aspect ratio for X-ray photon detection

Suppression of superconductivity dominated by proximity effect in amorphous MoSi nanobelts

Temporal resolution research based on x-ray superconducting nanowire single-photon detectors