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REVIEW 3 major objections 4 minor 17 references

Glow reduction of ultra-low noise LmAPDs: towards photon counting infrared arrays

T0 review · 3 major / 4 minor · reviewed 2026-08-11 · deepseek-v4-flash

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Pith's one-line read A redesigned infrared avalanche photodiode array cuts its self-generated glow sevenfold and detects individual photons at high gain.

desk verdict Solid glow and dark-current measurements on a redesigned LmAPD; the photon-counting claim is plausible but under-verified. read the letter →

arxiv 2412.09735 v1 pith:C5Q5XOQM submitted 2024-12-12 astro-ph.IM

Guillaume Huber , Michael Bottom , Charles-Antoine Claveau , Shane Jacobson , Matthew Newland , Ian Baker , Keith Barnes , Matthew Hicks This is my paper · ORCID
classification astro-ph.IM
keywords infrareddetectorsavalanchephotodiodesphotoncountingdarkcurrentROICglowHgCdTehigh-contrastimagingLmAPDarrays
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper reports that a redesigned linear-mode avalanche photodiode (LmAPD) array, intended for ultra-low-background infrared astronomy, has its readout glow reduced to 0.012 e-/pixel/frame, about seven times lower than the previous chip. With that glow removed, the intrinsic dark current is below 0.1 e-/pixel/kilosecond at 50 K, effectively indistinguishable from zero over normal exposure times. At a higher bias voltage, the array produces light curves with discrete upward jumps that the authors identify as individual photoelectrons, which would make it the first mature near-infrared imaging array capable of photon counting in a low-noise regime. The result matters because future missions to image Earth-like exoplanets require detectors with dark current below 1 e-/pixel/kilosecond and read noise below 1 e-/pixel/frame at infrared wavelengths, where no flight-qualified technology currently meets both.

What carries the argument

The device is a HgCdTe linear-mode avalanche photodiode array grown by MOVPE, in which photoelectrons created in a P-type absorber diffuse through a p-n junction and are multiplied in a high-field region before being collected by a readout integrated circuit (ROIC). The specific fix at the pixel level is an added metal layer that blocks the suspected glow path: light emitted by a source-follower JFET leaking through a gap in the metal architecture onto the photosensitive region. Measurements separate glow from dark current by comparing two datasets that read the array in a Fowler pattern, one clocked with drop frames and one left unclocked, so that glow appears only in the first. For the photon-counting demonstration, the load-bearing mechanism is the unusually low excess noise factor of HgCdTe avalanche multiplication, which lets a single photoelectron's amplified signal exceed the read noise; a changepoint algorithm then finds the jumps in each pixel's light curve and fits their amplitudes.

What would settle it

Expose the array to a calibrated, steadily attenuated light source and record light curves; if the jumps are photons, the detected jump rate must rise linearly with the input flux and the per-jump amplitude must remain constant. If the jump rate stays flat while flux changes, or if jump amplitudes scale with something other than the photoelectron signal, the photon interpretation fails.

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Extended reading notes

Core claim

The central claim is that the glow produced by the readout integrated circuit when the array is clocked can be blocked at the pixel level, and that doing so exposes an essentially zero intrinsic dark current and enables single-photon detection. The paper measures a per-frame glow of $0.012 \pm 0.001$ e-/pixel/frame, a factor of about seven below the $0.08$ e-/pixel/frame reported for the previous chip, and a dark current of $0.07 \pm 0.03$ e-/pixel/kilosecond over a 20-hour test. At 12 V bias, where the effective read noise is below 1 e-/pixel/frame, the authors use a changepoint detection routine on up-the-ramp light curves to identify discrete jumps that they attribute to individual photoelectrons; roughly 80% of pixels visually show such photon-detection behavior, and the few jumps detected under a dark mask are smaller and about an order of magnitude rarer than expected for fully amplified glow. The implication, stated by the authors, is that glow photons may be only partially amplified and therefore distinguishable from signal photons.

Load-bearing premise

The paper's photon-counting claim rests on the assumption that the discrete jumps it detects in the light curves are single photoelectrons, not random telegraph noise, single-event upsets, or partially amplified glow photons.

Editorial extensions

If this is right

  • If the measured glow and dark current hold at array scale, LmAPD arrays meet the dark-current and read-noise budget that has been identified for coronagraphic exoplanet imaging in the near-infrared.
  • Because the glow is per-frame rather than per-second, high-frequency up-the-ramp readout suppresses its accumulated contribution, making fast sampling the natural operating mode for ultra-low-background observations.
  • The observation that glow-induced jumps under the mask are rarer and smaller than signal jumps implies that residual glow may not fully enter the multiplication region, so it may contribute less effective noise than its raw count suggests.
  • If single-photon jumps can be counted reliably, the same changepoint measurements provide a new way to measure gain, excess noise factor, glow, and dark current from the quiet portions and jump amplitudes of individual light curves.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • A direct test of the photon interpretation would be to measure the jump rate while varying a calibrated input flux: if the jumps are single photoelectrons, the rate should scale linearly with flux and the amplitude distribution should be independent of flux.
  • The same light-curve jump analysis could be turned into a general-purpose detector characterization tool that extracts gain and excess noise factor per pixel, something the paper suggests but does not implement.
  • If the partially amplified glow hypothesis is correct, glow suppression may be achievable by engineering the electric field profile so that parasitic photons are absorbed after the multiplication region, not just by blocking their emission.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 4 minor

Summary. The paper reports the characterization of a redesigned linear-mode avalanche photodiode (LmAPD) array with reduced readout integrated circuit (ROIC) glow. Using two matched Fowler-sampled datasets, one with drop frames and one without, the authors separate the ROIC glow from the intrinsic dark current and report a glow level of 0.012 e-/pixel/frame and an intrinsic dark current of 0.07 e-/pixel/kilosecond, about seven times lower than their previous chip. In a separate high-bias-voltage test at 12 V, the authors claim individual photon detection, identifying step-like jumps in light curves with a changepoint detection algorithm. They further argue, based on a qualitative comparison between masked and illuminated pixels, that glow photons are only partially amplified and can therefore be distinguished from signal photons.

Significance. If the photon-counting claim holds, this would be the first demonstration of single-photon detection in a mature, large-format near-infrared imaging array, with direct relevance to future space missions such as the Habitable World Observatory. The glow and dark-current measurements are well designed and credible: the matched datasets with and without drop frames cleanly separate the two contributions, and the 20-hour dark-current test is consistent with a null intrinsic dark current. The reported glow reduction from 0.08 to 0.012 e-/pixel/frame is itself a useful engineering milestone. However, the photon-counting evidence as presented is incomplete and requires additional calibration and statistical analysis before the central claim can be accepted.

major comments (3)
  1. [Section 4.1] The photon-counting claim rests on the identification of step-like jumps as individual photoelectrons, but the manuscript provides no calibrated photon flux, no detection efficiency estimate, no amplitude histogram, and no false-positive analysis for the changepoint algorithm. Without such verification, the jumps could be due to random telegraph noise, single-event upsets, or other non-photon artifacts. Please provide a quantitative validation, for example by varying the incident flux and showing that the jump rate scales linearly, or by demonstrating a resolved single-photon amplitude peak.
  2. [Section 4.2] The mask comparison used to argue that glow photons are only partially amplified is purely qualitative: the text reports 'very few jump detections' and 'smaller amplitudes' without giving the actual counts in masked versus illuminated pixels, the expected number of glow events based on the measured glow rate, or the statistical uncertainties. This is load-bearing for the photon-counting interpretation because it is the only evidence that glow events do not appear as full-amplitude jumps. Please report the jump rates and amplitude distributions in both regions and compare them with a quantitative prediction.
  3. [Table 1 and Section 4.1] The conversion gain reported in Table 1 (1.54 ± 0.01 e-/ADU) is measured at 4 V, but the photon-counting tests are performed at 12 V and no conversion gain for this operating condition is given. Without a high-bias conversion gain, the jump amplitudes cannot be expressed in electrons, and the claim that photoelectron signals are 'notably above the read noise level' cannot be checked quantitatively. Please provide the conversion gain at 12 V or otherwise specify how the jump amplitudes in electrons were obtained.
minor comments (4)
  1. [Abstract] Typographical error: 'essentally' should be 'essentially'.
  2. [Figure 5 caption] Typographical error: 'Righ' should be 'Right'.
  3. [Section 4.1] The phrase 'Detectors with this ability have been know for decades' contains a spelling error: 'know' should be 'known'.
  4. [Section 4.1] The sentence 'We believe this approach can be a new way to measure many parameters...' is speculative and does not directly support the central claim; consider moving it to the future-work section.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the paper's key results are independent calibrated measurements, and self-citations to prior work serve only as baseline comparisons.

full rationale

The paper is an experimental characterization report rather than a derivation. The central measurements, ROIC glow of 0.012 e-/pixel/frame and intrinsic dark current below 0.1 e-/pixel/kilosecond, are obtained from direct Fowler-sampled test sequences with the conversion gain measured independently via a photon transfer curve, and the dark-current test is explicitly separated from the glow test by running the array without clocking. The comparison with the prior chip's values from C22 is a baseline reference, not a fitted target or a load-bearing derivation, so the self-citations are not circular. The photon-counting interpretation in Section 4 relies on a changepoint detection routine and on the assumption that the observed jumps are photoelectrons, and the mask comparison in Section 4.2 is qualitative; however, an unverified interpretive assumption is a correctness or verification concern, not a circular reduction of the kind in which a claimed result is equivalent to its inputs by construction. No equation, calibration constant, or fitted parameter is reused as its own prediction. Therefore no specific circular step can be exhibited, and the appropriate finding is no significant circularity.

Assumptions & free parameters 1 free parameters · 3 assumptions · 0 invented entities

The paper introduces no new particles or physical entities. All inputs are standard laboratory measurements and a device design change (the added metal layer). The only fitted parameter is the conversion gain, which is a calibration rather than a theory parameter.

free parameters (1)
  • conversion gain = 1.54 ± 0.01 e-/ADU
    Measured via photon transfer curve at 50 K and 4 V bias, used to convert all ADU-based measurements into electrons. This is a standard calibration, but it is a fitted value that the reported glow and dark current depend on.
assumptions (3)
  • domain assumption The 'no drop frames' test isolates dark current from readout glow; clocking the array does not introduce any signal other than the per-frame glow.
    Section 3 defines the two datasets and interprets the difference between them as solely due to glow, assuming no other clock-dependent artifacts.
  • domain assumption The observed glow reduction is caused by the added metal layer blocking JFET leakage; no other chip-to-chip differences affect the comparison with C22.
    Section 3 states that the physical origin of glow is suspected to be JFET leakage through a metal gap and that the new chip adds a blocking layer, but the attribution relies on a correlation across two different chips, not a controlled before/after experiment.
  • domain assumption The dark mask fully blocks light, so pixels under it receive zero illumination and any jumps there are due to glow rather than external photons.
    Sections 2.3 and 4.2 use the masked pixels as a control to distinguish photon jumps from glow-related jumps; this assumes the mask is optically opaque and correctly aligned.

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Cite this review

Pith. "Pith review of Glow reduction of ultra-low noise LmAPDs: towards photon counting infrared arrays." pith.science (2026). https://pith.science/paper/C5Q5XOQM

@misc{pith2026241209735,
  author       = {Pith},
  title        = {Pith review of: Glow reduction of ultra-low noise LmAPDs: towards photon counting infrared arrays},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/C5Q5XOQM}},
  note         = {Machine review of arXiv:2412.09735}
}
read the original abstract

Spectroscopy and direct-imaging of ultra-faint targets such as Earth-like exoplanets and high redshift galaxies are among the primary goals of upcoming large scale astronomy projects like the Habitable World Observatory (HWO). Such objectives pose extreme instrumental challenges, in particular on detectors where dark currents lower than 1 e-/pixel/kilosecond and read noise less than 1 e-/pixel/frame will have to be achieved on large format arrays. Some technologies meet these requirements at optical wavelengths, but none do in the infrared. With this goal in mind, the University of Hawaii has partnered with Leonardo to develop linear-mode avalanche photodiodes (LmAPDs). In this paper, we report recent tests performed on LmAPDs, where we measure a ROIC glow of approximately 0.01 e-/pixel/frame, without which the intrinsic dark current is essentally zero (< 0.1 e- /pixel/kilosecond). We show that at high gain, these devices are capable of detecting single photons

Figures

Figures reproduced from arXiv: 2412.09735 by the authors.

Figure 1
Figure 1. Potential energy diagram illustrating the history-dependent avalanche process. [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Schematic of the diode structure illustrating the conversion of a photon into a photoelectron and then [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Pictures of the detector cryogenic vacuum chamber. [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figures from the paper (6 more)
Figure 4
Figure 4. Figure 4: Potential path for glow propagation within the pixel. [PITH_FULL_IMAGE:figures/full_fig_p004_4.png]
Figure 5
Figure 5. Figure 5: Fowler-sampling measurement of the glow and DC signals generated during an 11.000 frame test (here [PITH_FULL_IMAGE:figures/full_fig_p005_5.png]
Figure 6
Figure 6. Figure 6: Glow histogram of the entire array. • In the first dataset illustrated for one pixel on Fig.5, we acquire drop frames during the exposure, which means the array is clocked and periodically probed during the acquisition as it would be if read frames were taken like in a…
Figure 7
Figure 7. Figure 7: Glow heatmaps with same intensity scales. [PITH_FULL_IMAGE:figures/full_fig_p006_7.png]
Figure 8
Figure 8. Figure 8: Dark-current histogram of the entire array. Over a 20 hours test, the mean DC signal is only 5 ADU [PITH_FULL_IMAGE:figures/full_fig_p006_8.png]
Figure 9
Figure 9. Figure 9: Light curve of a pixel at ultra-low flux. The overall trend is dark-current and glow, the fluctuations [PITH_FULL_IMAGE:figures/full_fig_p007_9.png]

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