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arxiv: 2606.22863 · v1 · pith:QQZHIP4Fnew · submitted 2026-06-22 · ⚛️ physics.optics

Free-Running Waveguide-Integrated Single-Photon Avalanche Detectors for Visible Light

Aswin Alexander , Anirudh R. Ramaseshan , Soe M. Thar , Thomas Y. L. Ang , Jing Zhou , Alexander Ling , Victor Leong This is my paper

Pith reviewed 2026-06-26 07:29 UTC · model grok-4.3

classification ⚛️ physics.optics
keywords waveguide-integrated SPADsingle-photon avalanche diodevisible lightsilicon nitridefree-running modephoton detection efficiencypassive quenchingroom temperature
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The pith

A laterally-doped p-i-n+ SPAD end-fire coupled to a SiN waveguide achieves 1.95% PDE at 685 nm in free-running room-temperature mode.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper establishes that a silicon diode integrated with a silicon nitride waveguide can function as a single-photon avalanche detector for visible light while running at room temperature without gating or cryogenic cooling. Different lateral and vertical doping profiles were tested with a simple passive quenching circuit, and the best performer reached a photon detection efficiency of 1.95 percent at 685 nm when biased 1.5 V above its 15 V breakdown voltage. A sympathetic reader would care because this removes major barriers to building compact, scalable photonic circuits for quantum technologies and low-light sensing. The work centers on practical end-fire coupling and stable free-running operation rather than maximum efficiency.

Core claim

The optimal device is a laterally-doped p-i-n+ SPAD with a maximum photon detection efficiency (PDE) of 1.95 +/- 0.32% for input light at 685 nm wavelength, when reverse-biased at an excess of 1.5 V beyond the breakdown voltage of 15.0 V. The device is based on a doped silicon diode end-fire-coupled to a silicon nitride photonic integrated circuit and operates in free-running mode with a current-mode passive quenching circuit.

What carries the argument

The laterally-doped p-i-n+ SPAD diode end-fire-coupled to the SiN waveguide, which places the avalanche region directly in the optical path of visible photons delivered by the photonic circuit.

If this is right

  • The device operates in free-running mode without external gating electronics.
  • Room-temperature function removes the need for cryogenic systems in extreme-low-light applications.
  • End-fire integration with a SiN photonic circuit supports scalable on-chip photon detection platforms.
  • Identified avenues for doping and circuit improvements could extend use to quantum technologies, low-light imaging, and visible-wavelength communications.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same lateral doping approach could be adapted to other visible wavelengths by shifting the absorption region.
  • Combining the SPAD with additional SiN components such as waveguides or resonators on the same chip would enable more complex integrated quantum sensors.
  • Free-running passive-quenched operation may allow higher sustained count rates than gated alternatives in continuous monitoring tasks.

Load-bearing premise

The end-fire coupling from the SiN waveguide to the doped silicon diode is efficient enough and the passive quenching circuit keeps afterpulsing and dark counts low enough to support stable free-running single-photon detection at the reported bias point.

What would settle it

A measurement showing photon detection efficiency consistently below 1 percent or unstable free-running operation with high afterpulsing rates at 1.5 V excess bias over 15 V breakdown would falsify the reported performance.

Figures

Figures reproduced from arXiv: 2606.22863 by Alexander Ling, Anirudh R. Ramaseshan, Aswin Alexander, Jing Zhou, Soe M. Thar, Thomas Y. L. Ang, Victor Leong.

Figure 1
Figure 1. Figure 1: (a) Device schematics showing the geometry of the Si SPADs end [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: (a) Setup schematic. Input light is coupled via a polarization-maintaining lensed fiber to an on-chip photonic waveguide, and then to the waveguide [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Measured SPAD performance at varying input optical powers [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Comparison of SPAD performance across all doping profiles (16 [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Distribution of the output pulse amplitudes after the RF amplifier for a lateral SPAD with [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗
read the original abstract

Waveguide-integrated single-photon avalanche detectors (SPADs) are essential components of integrated photonics platforms for scalable extreme-low-light applications without the use of cryogenics. Here, we demonstrate an integrated SPAD for visible light operating at room temperature in a free-running mode without gating. The device is based on a doped silicon diode end-fire-coupled to a silicon nitride (SiN) photonic integrated circuit (PIC). We investigate a range of lateral and vertical doping profile designs, and operate the devices with a simple current-mode passive quenching circuit. The optimal device is a laterally-doped p-i-n+ SPAD with a maximum photon detection efficiency (PDE) of 1.95 +/- 0.32% for input light at 685 nm wavelength, when reverse-biased at an excess of 1.5 V beyond the breakdown voltage of 15.0 V. We identify promising avenues for improving device performance, which would enable such integrated SPADs to be an attractive choice for cutting-edge integrated photonics solutions in quantum technologies, low-light imaging, and high-speed communications at visible wavelengths.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The manuscript demonstrates waveguide-integrated single-photon avalanche detectors (SPADs) for visible light on a SiN PIC platform. The devices operate in free-running mode at room temperature using a passive quenching circuit. Various lateral and vertical doping profiles are investigated, with the optimal laterally-doped p-i-n+ SPAD achieving a photon detection efficiency (PDE) of 1.95 ± 0.32% at 685 nm under 1.5 V excess bias above the 15 V breakdown voltage.

Significance. If the measurements are fully validated, the work provides an experimental demonstration of room-temperature, free-running waveguide-integrated SPADs at visible wavelengths, which addresses a practical need in scalable integrated photonics for quantum technologies and low-light applications. The systematic comparison of doping profiles is a constructive element of the study.

major comments (2)
  1. [Abstract] Abstract: The headline PDE of 1.95 ± 0.32% is an end-to-end system efficiency that necessarily incorporates the end-fire coupling efficiency from the SiN waveguide into the active volume of the silicon diode. No separate transmission measurement, reference detector calibration, or test-structure data is described to quantify or bound the coupling loss, so any unaccounted insertion loss directly scales the reported value and prevents separation of internal avalanche probability from optical losses.
  2. [Abstract] Abstract: No numerical dark-count rate, afterpulsing probability, or measurement protocol (e.g., how PDE was extracted from count rates at the stated 1.5 V excess bias) is supplied. These quantities are required to substantiate stable free-running operation and to confirm that the passive quenching circuit maintains acceptable performance at the reported bias point.
minor comments (1)
  1. The manuscript would be strengthened by the inclusion of device micrographs, doping-profile schematics, and raw count-rate versus bias curves to allow readers to assess the experimental conditions directly.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive feedback and recommendation for major revision. We address each comment point by point below, with plans to revise the manuscript accordingly where the points identify areas for clarification or additional detail.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The headline PDE of 1.95 ± 0.32% is an end-to-end system efficiency that necessarily incorporates the end-fire coupling efficiency from the SiN waveguide into the active volume of the silicon diode. No separate transmission measurement, reference detector calibration, or test-structure data is described to quantify or bound the coupling loss, so any unaccounted insertion loss directly scales the reported value and prevents separation of internal avalanche probability from optical losses.

    Authors: We agree that the reported PDE represents the end-to-end system efficiency, incorporating coupling losses from the SiN waveguide to the silicon diode. This metric is the relevant one for integrated photonic applications. The original manuscript emphasizes the integrated device performance rather than isolating internal quantum efficiency. In revision, we will explicitly note this in the abstract and add a discussion (with supporting mode-overlap simulations) to bound the coupling efficiency and estimate the internal avalanche probability. revision: yes

  2. Referee: [Abstract] Abstract: No numerical dark-count rate, afterpulsing probability, or measurement protocol (e.g., how PDE was extracted from count rates at the stated 1.5 V excess bias) is supplied. These quantities are required to substantiate stable free-running operation and to confirm that the passive quenching circuit maintains acceptable performance at the reported bias point.

    Authors: The referee is correct that explicit numerical values and a detailed protocol were not supplied. The manuscript describes the passive quenching circuit and free-running operation at 1.5 V excess bias, but we will revise the methods and results sections to include the measured dark-count rate, afterpulsing probability at the operating point, and a clear step-by-step description of how PDE was extracted from the illuminated and dark count rates. revision: yes

Circularity Check

0 steps flagged

No circularity: experimental device demonstration

full rationale

The paper reports fabrication, doping profiles, and direct experimental characterization of waveguide-integrated SPADs, including measured PDE at specific bias and wavelength. No derivation chain, model equations, fitted parameters renamed as predictions, or self-citation load-bearing steps exist. The central claim is an end-to-end measured efficiency on fabricated devices; any calibration concerns are correctness issues, not circularity per the rules. The work is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Experimental demonstration paper; no free parameters, mathematical axioms, or invented entities are invoked in the central claim.

pith-pipeline@v0.9.1-grok · 5752 in / 987 out tokens · 16030 ms · 2026-06-26T07:29:24.385336+00:00 · methodology

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Reference graph

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