Atomic Layer Deposition-Enhanced Far-to-Mid Infrared Camera Coating

The ALD-Enhanced Far-to-Mid IR Camera Coating is fabricated by first applying a conductively loaded epoxy binder ~500 microns thick onto a conductive metal substrate (e.g., Cu, Al). This serves to provide high absorptance and low reflectance at the longest wavelength of interest, as well as to provide a mechanical buffer layer to reduce coating stress. Borosilicate glass microspheres are coated with a thin film metal via ALD, essentially turning the microspheres into resonators. That film is optically thin in the far infrared and approximates a resistive (~200 ohms per square) coating. Light trapped in the borosilicate glass microspheres is reflected back and forth within the glass–at each contact point, the light is attenuated by 50%. A monolayer of thin metal film-coated borosilicate glass microspheres is applied to the epoxy binder and cured, forming a robust mechanical structure that can be grounded to prevent deep dielectric charging by ionizing radiation in space. Once cured, the far-to-mid IR absorber structure can be coated with a traditional ~20-to-50 microns “black” absorptive paint to enhance the absorption band at short wavelengths, or a “white” diffusive paint to reject optical radiation. At this thickness and broad tolerance, the longwave response of the coating is preserved. Tailoring the electromagnetic properties of the coating layers and geometry enables realization of a broad band absorption response where the mass required per unit area has been minimized. While NASA originally developed the ALD-Enhanced Far-to-Mid IR Camera Coating for the Stratospheric Observatory for Infrared Astronomy mission, its robustness, absorptive qualities, and optical performance make it a significant addition to IR and terahertz imaging systems. The IR camera coating is at Technology Readiness Level (TRL) 3 (experimental proof-of-concept) and is available for patent licensing. Infrared (IR) cameras for space missions require robust coatings that are cryogenically cooled to reduce thermal noise and are low-reflectance absorptive to mitigate image artifacts such as glinting, ghosting, and internal reflections. Thermal noise reduction improves as temperatures decrease (e.g., below 30 K). However, capturing the far-to-mid IR spectrum typically requires thick layers of common 'super black' coatings, which often use free-standing nanotubes or foam-like structures that become fragile below 120-150 K. In response, engineers at Goddard Space Flight Center developed an Atomic Layer Deposition (ALD)-Enhanced Far-to-Mid IR Camera Coating that can be cooled to and operate at less than 30 K while maintaining low-reflection absorption. The coating results in enhanced optical performance, increased strength, reduced total mass, and suitable ionizing radiation control.