We demonstrate a 64-pixel single-photon imager based on superconducting nanowire single-photon detectors (SNSPDs) capable of counting single photons up to a wavelength of 10μm. This technology could be useful in future space telescopes in applications such as exoplanet transit spectroscopy.
State readout of trapped-ion qubits is usually achieved by observing qubit-state-dependent fluorescence from the ion while driving an optical cycling transition with laser light. The integration of photon detectors for fluorescence detection into the ion trap itself may benefit the development of many-qubit setups for applications like quantum computing. Superconducting nanowire single-photon detectors (SNSPDs) are promising candidates for trap-integrated detectors for high-fidelity trapped-ion qubit state readout. However, the strong oscillating electromagnetic fields that are typically used to trap and manipulate ions can affect the function of the SNSPDs significantly. In this work, we demonstrate an improved design to integrate SNSPDs into linear rf ion traps that reduces the susceptibility of the SNSPDs to applied rf trapping potentials. Our measurement results represent an improvement in rf tolerance by an order of magnitude with an increase in operation temperature from 3.5K to 6K compared to previous work.
We will discuss recently-developed approaches to improve sensitivity of superconducting nanowire single photon detectors in the mid-infrared, showing saturated internal detection efficiency up to a wavelength of 10 microns. We will also show preliminary data from small 64-element SNSPD arrays with high internal detection efficiency in the midinfrared at 3.5 μm, and will discuss calibration techniques we are developing for measuring system detection efficiency in this region of the spectrum.
The Origins Space Telescope mission concept includes an exoplanet transit spectrometer that requires detector arrays with ultrahigh pixel-to-pixel stability. Superconducting nanowire single-photon detectors, or SNSPDs, have the potential to meet these stringent stability requirements due to their digital-like output. Traditionally used for applications at near-IR telecom wavelengths, SNSPDs have demonstrated near-unity detection efficiencies, ultralow dark-count rates, and high dynamic ranges. Until recently, however, SNSPD operation at the mid-infrared (mid-IR) wavelengths of interest for Origins had not been demonstrated, and SNSPD formats were limited to small arrays and active areas. Recent advances in SNSPD fabrication techniques have pushed SNSPD sensitivity to wavelengths beyond 7 μm and have enabled millimeter-scale active areas and kilopixel arrays. We report here on this progress and the outlook toward developing arrays of ultrastable superconducting nanowire single-photon detectors for mid-IR astronomy applications.
Superconducting nanowire single-photon detectors (SNSPDs) are excellent single-photon detectors from the ultraviolet to the near-infrared. System detection efficiencies of ~ 90% are typical, with jitters on the order of 100 ps and maximum count rates of a few MHz. Recently we have begun exploring the use of SNSPDs for the detection of single mid-infrared photons in the 2 - 11 μm wavelength range for applications in astronomy and chemical sensing. In particular, we are developing arrays of SNSPDs which could potentially be used for exoplanet spectroscopy in order to identify elements in the atmospheres of exoplanets outside our solar system. Improved sensitivity for these low-energy photons has been made possible by the recent development of amorphous WSi which is now used in the fabrication of superconducting nanowire detectors. I will discuss the optimization of these detectors to enhance their detection efficiency in the midinfrared, with the ultimate goal of building a single-photon focal plane array of SNSPDs in the 2 - 11 μm band.
Photonic integration is an enabling technology for photonic quantum science, offering greater
scalability, stability, and functionality than traditional bulk optics. Here, we describe a scalable,
heterogeneous III-V/silicon integration platform to produce Si3N4 photonic circuits incorporating
GaAs-based nanophotonic devices containing self-assembled InAs/GaAs quantum dots. We
demonstrate pure single-photon emission from individual quantum dots in GaAs waveguides
and cavities - where strong control of spontaneous emission rate is observed - directly launched
into Si3N4 waveguides with > 90 % efficiency through evanescent coupling. To date, InAs/GaAs
quantum dots constitute the most promising solid state triggered single-photon sources, offering
bright, pure and indistinguishable emission that can be electrically and optically controlled.
Si3N4 waveguides offer low-loss propagation, tailorable dispersion and high Kerr nonlinearities,
desirable for linear and nonlinear optical signal processing down to the quantum level. We
combine these two in an integration platform that will enable a new class of scalable, efficient
and versatile integrated quantum photonic devices.
This paper reports progress on a type of ultrafast photoconductive source that can be driven at 1550 nm but exhibits the robustness of GaAs (e.g., low-temperature-grown GaAs) driven at 780 nm. The approach is GaAs doped heavily with Er (≈4x1020 cm-3 or 2% atomic-Er-to-Ga fraction) such that ErAs nanoparticles form spontaneously during epitaxial growth by MBE. The nanoparticles are mostly spherical with a diameter of a few nm while the packing density is estimated as high as ~2.2x1019/cm3. Yet, the Er-doped GaAs epilayer maintains excellent structural quality and smooth surface morphology. A photoconductive switch coupled to a 4-turn square spiral antenna is fabricated and characterized. At least ~40 μW average THz power is generated when the device is biased at 75 V and pumped with a 1550-nm 90-fs-short pulsed laser having average power ~85 mW. This research is significant for 1550-nm-technologycompatible, cost-effective THz sources.
We present measurements of sub-bandgap photoconductivity and photoconductive switches using GaAs doped heavily with Er such that nanoparticles of ErAs are formed. In addition to strong resonant absorption centered around 1550 nm, the material provides strong sub-bandgap photoconductivity and >> μW average power levels when fabricated into an efficient (square spiral) THz antenna and driven by a 1550- nm ultrafast fiber laser. Photo-Hall measurements prove that the predominant photocarrier is the electron and the linearity of the 1550-nm photocurrent (with laser power) suggests that the photoconductivity is “extrinsic”, not other possible mechanisms, such as two-photon absorption. These results have immediate relevance to the use of GaAs:Er switches as the transmitter in 1550-nm-driven THz imaging systems such as the “impulse imager” that we have successfully used for biomedical imaging applications.
We demonstrate a 64-pixel free-space-coupled array of superconducting nanowire single photon detectors optimized for high detection efficiency in the near-infrared range. An integrated, readily scalable, multiplexed readout scheme is employed to reduce the number of readout lines to 16. The cryogenic, optical, and electronic packaging to read out the array, as well as characterization measurements are discussed.
We demonstrate a 64-pixel free-space-coupled array of superconducting nanowire single photon detectors optimized for high detection efficiency in the near-infrared range. An integrated, readily scalable, multiplexed readout scheme is employed to reduce the number of readout lines to 16. The cryogenic, optical, and electronic packaging to read out the array, as well as characterization measurements are discussed.
We report on the photoresponse mapping of nanowire superconducting single-photon detectors using a focal spot
significantly smaller than the device area (10 μm x 10 μm). Using a solid immersion lens we achieve a spot size of 320
nm full-width half maximum onto the device at 470 nm wavelength. We compare the response maps of two devices: the
higher detection efficiency device gives a uniform response whereas the lower detection efficiency device is limited by a
single defect or constriction. A second optical setup is used to simultaneously image and measure the photoresponse of
the lower detection efficiency device, allowing the constriction location to be pinpointed.
We demonstrate photon-number discrimination using a novel semiconductor detector that utilizes a layer of self-assembled
InGaAs quantum dots (QDs) as an optically addressable floating gate in a GaAs/AlGaAs δ-doped field-effect
transistor. When the QDOGFET (quantum dot, optically gated, field-effect transistor) is illuminated, the internal gate
field directs the holes generated in the dedicated absorption layer of the structure to the QDs, where they are trapped.
The positively charged holes are confined to the dots and screen the internal gate field, causing a persistent change in the
channel current that is proportional to the total number of holes trapped in the QD ensemble. We use highly attenuated
laser pulses to characterize the response of the QDOGFET cooled to 4 K. We demonstrate that different photon-number
states produce well resolved changes in the channel current, where the responses of the detector reflect the Poisson
statistics of the laser light. For a mean photon number of 1.1, we show that decision regions can be defined such that
the QDOGFET determines the number (0, 1, 2, or ≥3) of detected photons with a probability of accuracy ≥83 % in a
single-shot measurement.
We report use of a niobium nitride superconducting single-photon detector in a time-correlated single-photon counting experiment. The detector has a timing jitter of 68 ± 3 ps full width at half maximum with a Gaussian temporal profile. The detector's dark count rate and detection efficiency can be tuned by adjusting the bias current applied to the device. Typical values include a detection efficiency of ~1-2% and a dark count rate below 100 Hz. We use this detector to measure time-resolved photoluminescence at wavelengths up to 1650 nm, well beyond the range of conventional silicon detectors. We also use this superconducting detector to measure the emission of a quantum dot single-photon source.
Coupling between InGaAs/GaAs quantum dots is investigated using differential transmission spectroscopy. Degenerate measurements show an initial carrier relaxation time that is relatively independent of carrier density. Two-color pump-probe techniques are used to spectrally resolve the carrier dynamics, revealing transfer between quantum dots and a homogeneous linewidth of 12 nm at room temperature. The time constant for carrier escape is shown to increase from 35 ps at room temperature to 130 ps at 230 K. We then employ a rate equation model to simulate the performance of a semiconductor optical amplifier with QDs as the active region.
We demonstrate InGaAs/GaAs quantum dot lasers with multimode lasing at room temperature immediately above threshold. The lasing modes are separated by about ten times the Fabry-Perot mode spacing, with several dark modes in between the lasing modes. Rate equation simulations indicate that this multimode behavior can be explained by a homogeneous broadening that is on the order of the mode spacing.
We designed, fabricated and characterized AlxGa1- xAs/GaAs p-i-n resonant cavity enhanced (RCE) photodetectors with near-unity quantum efficiency. The peak wavelength is in the 780 - 830 nm region and post-process adjustable by recessing the top surface. Transit time limited bandwidth for these devices is in excess of 50 GHz. Possible applications of these detectors include conventional measurements of low light levels, quantum optical experiments that use pulsed sources and short-haul high speed communications.
In this paper, we review our research efforts on RCE high- speed high-efficiency p-i-n and Schottky photodiodes. Using a microwave compatible planar fabrication process, we have designed and fabricated GaAs based RCE photodiodes. For RCE Schottky photodiodes, we have achieved a peak quantum efficiency of 50% along with a 3-dB bandwidth of 100 GHz. The tunability of the detectors via a recess etch is also demonstrated. For p-i-n type photodiodes, we have fabricated and tested widely tunable devices with near 100% quantum efficiencies, along with a 3-dB bandwidth of 50 GHz. Both of these results correspond to the fastest RCE photodetectors published in scientific literature.
Resonant cavity enhanced (RCE) photodiodes are promising candidates for applications in optical communications and interconnects where ultrafast high-efficiency detection is very desirable. In RCE structures, the electrical function of the photodiode is largely unchanged, but optically it is subject to the effects of the cavity, mainly wavelength selectivity and a large enhancement of the resonant optical field. The increased optical field allows photodetectors to be made thinner and therefor faster in the transit-time limited operation, while simultaneously maintaining a high quantum efficiency at the resonant wavelengths. The combination of RCE detection scheme with Schottky photodiodes allows for fabrication of high-performance photodetectors with relatively simple material structure and fabrication process. In RCE Schottky photodiodes, a semi-transparent metalization can be used simultaneously as the electrical contact and the top reflector for the resonant cavity. Device performance is optimized by varying the thickness of the Schottky metalization and utilizing a dielectric matching layer. We present theoretical and experimental results on spectral and high-speed properties. We have demonstrated RCE Schottky photodiodes in (Al, In)GaAs/GaAs material system with temporal response of 10 ps full-width-at-half-maximum. These results were measurement setup limited and a conservative estimation of the bandwidth corresponds to more than 100 GHz. The photodiodes were designed and fabricated for 900 nm and 840 nm resonant wavelengths. The best measured quantum efficiency is around 50% which is slightly less than the theoretical prediction for these devices.
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