NASA and the Rochester Institute of Technology (RIT) are accelerating development of single-photon sensing CMOS detectors to enable future exoplanet biosignature missions. The effort focuses on hardening and optimizing photon-counting image sensors for the Habitable Worlds Observatory (HWO), a proposed flagship mission that would search for atmospheric signatures of life in ultraviolet, optical, and near-infrared (NIR) light. The program is funded by NASA’s Astrophysics Division through Strategic Astrophysics Technology and by the Space Technology Mission Directorate’s Early Stage Innovations initiatives. Source: NASA.
Why photon-counting matters for biosignatures
Biosignature features from distant exoplanets are exceptionally faint and easily masked by detector noise. RIT’s Single-Photon Sensing CMOS (SPSCMOS) devices are engineered for near-zero read noise, low dark current, and radiation tolerance. When cooled to about 250 K, measured dark current falls to roughly one electron per pixel every 30 minutes, improving sensitivity to weak spectral lines from molecules such as oxygen, methane, and carbon dioxide.
How SPSCMOS works
SPSCMOS sensors build on conventional CMOS architectures but use a low-capacitance floating diffusion node to produce large, resolvable voltage steps per collected electron. That enables photon counting and, in some cases, photon-number resolution. Current devices demonstrate multi-megapixel formats, including 9.4 and 16.7 Mpixel arrays, supporting high-resolution imaging and spectroscopy concepts relevant to HWO-class instruments.
Testing in space-like conditions
The RIT Center for Detectors characterizes performance before, during, and after exposure to high-energy radiation. A vacuum Dewar and thermally controlled mounts replicate space-relevant temperatures and stability, allowing systematic evaluation of dark current, quantum efficiency (QE), and read noise over a broad operating range. Results include photon-counting histograms that resolve individual photon events and dark-current trends measured pre- and post-irradiation (e.g., 50 krad with 60 MeV protons).
Radiation effects and readout strategies
HWO’s radiation environment can induce transient upsets and cumulative degradation. To preserve science data, the team is developing custom readout modes not found in commercial CMOS devices. A ramp-sampling acquisition enables detection and removal of cosmic-ray artifacts; when an event is identified, the system segments the time series and selectively averages the unaffected portions to reconstruct the original signal. A real-time data acquisition system monitors power consumption for signs of damage progression and can adjust detector biases to maintain nominal operation. These strategies will be validated at ground-based radiation facilities to map failure modes and refine mitigation.
Extending to the near-infrared
While today’s SPSCMOS devices target visible wavelengths, RIT is designing the first NIR single-photon photodiode based on the same low-capacitance architecture. Using TCAD simulations, the team models 2D/3D device structures in silicon and mercury cadmium telluride (HgCdTe), tracking charge generation, storage, transfer, and output response. This virtual prototyping guides pixel design choices and foundry process parameters before fabrication, reducing technical risk for NIR photon-counting detectors crucial to exoplanet spectroscopy.
On-sky validation
Laboratory results are being cross-checked on a ground-based telescope to capture conditions that are hard to replicate indoors. In January 2025, an SPSCMOS-based camera at RIT’s C.E.K. Mees Observatory observed the open cluster M36 and the Bubble Nebula through an H-alpha filter. Measured dark current and read noise matched laboratory values, and QE estimates from photometric reference stars agreed with bench calibrations. A passing satellite (STARLINK-32727) produced a brief streak, but measured persistence was negligible at approximately 0.03 e− per pixel, below both sky background and read noise.
Key technical takeaways
- Photon-counting CMOS architecture with low-capacitance floating diffusion enables single-electron sensitivity and photon-number resolving histograms.
- Dark current as low as ~1 e− per pixel per 30 minutes at ~250 K supports detection of ultra-faint spectral lines.
- Radiation-hardening by design plus custom ramp readouts mitigate cosmic rays and long-term damage effects.
- TCAD-driven NIR pixel design explores silicon and HgCdTe implementations for extending sensitivity beyond the visible.
- On-sky tests confirm lab-measured noise and QE, with negligible image persistence after bright events.
What’s next
Upcoming radiation campaigns will benchmark device robustness and refine bias-control algorithms for lifetime stability. The NIR pixel design is progressing toward a foundry path to demonstrate HgCdTe-based, single-photon capability. If successful, SPSCMOS could advance detector readiness for HWO and other astrophysics missions requiring extreme sensitivity, stability, and radiation tolerance, from exoplanet spectroscopy to faint-object imaging.
