Mid-infrared (MIR) spectroscopy serves as a powerful tool for molecular structure identification and chemical composition analysis. However, conventional Fourier-transform infrared spectrometers (FTIR) demonstrate compromised signal-to-noise ratios (SNR) under low-light conditions, which severely limit their performance in high-sensitivity applications. Although MIR superconducting nanowire single-photon detectors (SNSPDs) offer high detection efficiency and low timing jitter, their performance is hindered by background dark counts caused by room-temperature blackbody radiation. In this work, we present a quantum-correlated absorption spectroscopy platform that combines MIR-SNSPDs with entangled photon pairs to overcome these limitations. Leveraging the intrinsic temporal and spectral correlations of the photon pairs, coincidence detection effectively distinguishes signal photons from dominant thermal noise, achieving a remarkable improvement of two orders of magnitude in SNR under ultralow pump power conditions. The system achieves a spectral coverage of 3350-3540 nm with a resolution of 3.7 cm -1 at 3.4 µm, operating at an ultralow illumination photon flux of 4.4 × 10 6 photons/s. Validation experiments using polystyrene and polyethylene samples successfully detected CH 2 vibrational modes, demonstrating the platform’s capability for ultrasensitive biochemical analysis and material characterization. By integrating quantum correlation with MIR-SNSPDs, this work establishes a new paradigm for high-performance MIR spectroscopy in low-light scenarios.
Mid-infrared (MIR) spectroscopy serves as a powerful tool for molecular structure identification and chemical composition analysis. However, conventional Fourier-transform infrared spectrometers (FTIR) demonstrate compromised signal-to-noise ratios (SNR) under low-light conditions, which severely limit their performance in high-sensitivity applications. Although MIR superconducting nanowire single-photon detectors (SNSPDs) offer high detection efficiency and low timing jitter, their performance is hindered by background dark counts caused by room-temperature blackbody radiation. In this work, we present a quantum-correlated absorption spectroscopy platform that combines MIR-SNSPDs with entangled photon pairs to overcome these limitations. Leveraging the intrinsic temporal and spectral correlations of the photon pairs, coincidence detection effectively distinguishes signal photons from dominant thermal noise, achieving a remarkable improvement of two orders of magnitude in SNR under ultralow pump power conditions. The system achieves a spectral coverage of 3350-3540 nm with a resolution of 3.7 cm -1 at 3.4 µm, operating at an ultralow illumination photon flux of 4.4 × 10 6 photons/s. Validation experiments using polystyrene and polyethylene samples successfully detected CH 2 vibrational modes, demonstrating the platform’s capability for ultrasensitive biochemical analysis and material characterization. By integrating quantum correlation with MIR-SNSPDs, this work establishes a new paradigm for high-performance MIR spectroscopy in low-light scenarios.
Promoting the sensitivity of mid-infrared (MIR) spectroscopy to the single-photon level is a critical need for investigating photosensitive biological samples and chemical reactions. MIR spectroscopy based on frequency upconversion is a compelling pioneer allowing high-efficiency MIR spectral measurement with well-developed single-photon detectors, which overcomes the main limitations of high thermal noise of current MIR detectors. However, noise from other nonlinear processes caused by strong pump fields hinders the development of the upconversion-based MIR spectroscopy to reach the single-photon level. Here, a broadband MIR single-photon frequency upconversion spectroscopy is demonstrated based on the temporal-spectral quantum correlation of non-degenerate photon pairs, which is well preserved in the frequency upconversion process and is fully used in extracting the signals from tremendous noise caused by the strong pump. A correlation spectrum broader than 660 nm is achieved and applied for the demonstration of sample identification under a low incident photon flux of 0.09 average photons per pulse. The system is featured with non-destructive and robust operation, which makes single-photon-level MIR spectroscopy an appealing option in biochemical applications.
Superconducting nanowire single-photon detectors (SNSPDs) in the mid-infrared (MIR) have the potential to open up numerous opportunities in fields such as exoplanet searches, direct dark matter detection, physical chemistry, and remote sensing. One challenge in pushing SNSPD sensitivity to the MIR is a decrease in the signal-to-noise ratio (SNR) of the readout signal, as the critical currents become increasingly smaller. We overcome this trade-off with a device architecture that employs impedance matching tapers and superconducting nanowire avalanche photodetectors to demonstrate increased SNR while maintaining saturated internal detection efficiency at 7.4 μm and approaching saturation at 10.6 μm. This work provides a platform for pushing SNSPD sensitivity to longer wavelengths while enabling the scalability to large arrays.
Mid-infrared (MIR) single-photon detection is emerging as an important technology for various applications. Superconducting nanowire single photon detectors (SNSPDs) fabricated with superconducting films with energy gaps of a few meV are natural broadband single-photon detectors. Recently, extending SNSPDs’ operation wavelengths into the MIR region is highly attractive. γ-Nb4N3 has a reduced N content and lower energy gap than the commonly used δ-NbN, making SNSPDs based on γ-Nb4N3 film more sensitive to low energy photons. We report on a Nb4N3-SNSPD based on 62-nm wide nanowire, with an optical absorption enhancement design and an optimized device package for efficient ZBLAN fiber coupling and dark count filtering. The developed device has a unity intrinsic detection efficiency (IDE) in the 1.5–4 µm wavelength region, and the device detection efficiency at 2.95 µm was measured to be 32.5%, with an uncertainty of 12.7%. Furthermore, we reduced the device geometry, and measured 3–10 µm photon response of a device based on 5-nm film and 42-nm nanowire, with an IDE of 95%, 81%, 40%, and 6% for 4.8, 6, 8, and 10 µm, respectively.
Single-photon counting has become an essential tool in quantum optics experiments, as well as remote sensing and life science applications. However conventional technologies such as single-photon avalanche diodes, as well as the availability of standard telecom optical components, has limited much of this work to the near infrared/telecom wavelength range. Superconducting nanowire single photon detectors (SNSPDs) have emerged in recent years as the gold standard in photon counting applications due to their low dark count rates, fast timing resolution and high efficiency [1]. SNSPDs have also demonstrated photon counting efficiency out to much greater wavelengths which enables us to explore new experimental possibilities in the mid-infrared [2]. In this work we design and fabricate mid-infrared SNSPDs and deploy them in a variety of photon counting experiments [3,4]. The devices are based on a NbTiN superconducting film integrated into an optical cavity to enhance absorption in the mid-infrared. We characterise these devices using an optical parametric oscillator, tuneable between 1.5 m and 4.2 m. We then deploy these in a proof-of-principle tabletop light detection and ranging (LIDAR) experiment at 2.3 m. LIDAR in the mid-infrared is attractive due to spectral regions of low atmospheric absorption and reduced solar background photon flux, when compared to telecom wavelengths. We also present results from a photon-pair source operating at 2 m. This is a key resource for extending quantum optics and quantum secure communications to the mid infrared domain. Pairs are generated using a custom lithium niobate crystal and detected using SNSPDs. We demonstrate two-photon interference and polarisation entanglement of the photon pairs at 2 m. This work opens the pathway to future development of quantum optics and quantum technologies in the mid-infrared spectral region. References [1] Gol’tsman et al Applied Physics Letters 79 705 (2001) [2] Marisli et al Nano Letters 12 (9) 4799 (2012) [3] G. G. Taylor et al Optics Express 26 (27) 38147 (2018) [4] S. Prabhakar et al Science Advances 6 (13) eaay5195 (2019)
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 ( < 0.8 eV ) 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 > 70 % 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.
Superconducting Nanowire Single-Photon Detectors (SNSPDs) are the premier single-photon counting technology in the near-infrared regime. Operating via a photon-induced phase transition from the superconducting to the normal state, SNSPDs act as highly sensitive digital detectors that now play a critical role in quantum communication, deep-space optical communications, and direct dark matter detection. As the demand for these detectors scales, there is a pressing need to extend their performance capabilities toward longer wavelengths, larger active areas, and higher operating temperatures. However, advancing these metrics requires navigating complex physical trade-offs inherent to the detection mechanism. This thesis explores the material optimizations, novel detector architectures, and fundamental device physics required to push SNSPD technology beyond its current limits. The first thrust of this work focuses on extending SNSPD sensitivity into the mid-infrared (MIR) and far-infrared regimes. By developing a low-Tc (sub-1 K) WSi film engineered to reduce the characteristic detection energy, the threshold for long-wavelength detection is significantly lowered. Coupled with a novel detector architecture utilizing impedance-matching tapers and superconducting nanowire avalanche photon detectors (SNAPs), sensitivity is initially demonstrated at wavelengths up to 29um using a blackbody thermal source. Expanding beyond this limit, the implementation of quantum cascade lasers enables the demonstration of single-photon counting at wavelengths up to 88um, marking a major improvement over the current state of the art. To address the practical MIR detection challenges of low signal-to-noise ratio and restricted fill factors, this work also demonstrates a 1 mm x 1mm array of superconducting microwire single-photon detectors (SMSPDs) sensitive out to 3.7um, establishing a pathway for future antenna-coupled detectors. The second thrust investigates the development of micrometer-scale wires to simultaneously increase active area and operating temperature. Moving to microwire geometries raises fundamental questions regarding intrinsic detection mechanisms. Using a scanning lens setup, this work demonstrates the first direct observation of transverse coordinate effects in microwires, where the precise location of photon incidence across the wire width significantly alters the detector's slew rate and timing performance. To counteract this, new rectification schemes are introduced to produce a more uniform detection landscape. Specifically, the integration of a superconducting rail alongside the main detector is shown to successfully mitigate the transverse coordinate effect, improve timing performance, and increase the viable operating temperature of the detector. The third thrust explores the use of unconventional superconductors, namely MgB2 to achieve an appreciably higher-temperature single-photon detection pathway. This section details the distinct challenges associated with the growth and fabrication of detectors using air-sensitive materials and evaluates their long-term viability for the SNSPD platform. Ultimately, this thesis highlights the critical material and architectural parameters required to advance the field of single-photon detection. By elucidating the metrics and mechanisms that govern ideal detector performance, this work aims to broaden the understanding and future application of SNSPDs across diverse scientific disciplines. All the data presented here is my own unless otherwise mentioned.