BRDF measurements of ultra-black materials with THz quantum cascade lasers

This study, conducted by researchers at the National Institute of Standards and Technology (NIST), explores the use of a terahertz (THz) quantum cascade laser (QCL) to perform bi-directional reflectance distribution function (BRDF) measurements of ultra-black materials. When the main goal is to improve the characterization of super-black materials used in detector devices.
Another goal is to develop measurement protocols for the darkest black absorbers. Specifically for applications in THz bolometers, such as those using vertically aligned carbon nanotube (VACNT) structures.

The researchers emphasize the need for accurate measurement tools in the THz range, where traditional methods, such as integrating spheres used in the visible and infrared (IR) spectrum, fall short. Although these methods are typically used to evaluate IR coatings, their limitations in the THz regime necessitate alternative approaches. The team’s innovative BRDF setup allows for the precise angular characterization of reflected radiation, which is critical for assessing black coating performance in reducing reflection uncertainties and enabling effective stray light suppression.

The experimental setup includes the use of a quantum cascade laser QCL which operates on a single-mode distributed feedback (DFB) at approximately 3.9 THz. The reflections are captured by a custom black coated VACNT bolometer, optimized for low thermal noise and high sensitivity at cryogenic temperatures. These detectors are responsive across a broad spectral range and are essential for understanding how stray light behaves across different angles. Because they enable accurate measurement of scattering and reflectance.

The research demonstrates the feasibility of capturing angular reflectance data for a simple anodized aluminum sample, using it as a baseline to validate the method. Future work focuses on advancing µ-BRDF measurements by developing miniaturized BRDF techniques that will enable the characterization of micro-scale ultra black materials. These developments are especially relevant for remote sensing applications that require reliable control of stray light and precise thermal regulation.

Acktar’s technology plays a critical role in similar measurement environments, especially where stray light suppression and the darkest black finishes are crucial. The study’s innovations set the stage for broader adoption of BRDF methods to characterize super black materials and black coatings, such as those used by Acktar.

Patrick McArdle, Bradley Pelz, Chris Yung, Nathan Tomlin, John Lehman, and Michelle Stephens “BRDF measurements of ultrablack materials with THz quantum cascade lasers”, Proc. SPIE 13365, Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications XVIII, 133650M (19 March 2025); https://doi.org/10.1117/12.3043443