Origin of 4 µm transmission peak in 700 µm sapphire-sphere composite

Determine the physical origin of the second smaller transmission peak near 4 µm observed in the infrared spectrum of composite samples comprising single-diameter 700 µm sapphire spheres embedded in UHU plus Endfest 300 epoxy resin, in order to explain the mechanism responsible for this feature.

Background

The paper measures infrared transmission of epoxy composites loaded with sapphire spheres at various diameters and thicknesses to evaluate materials for cryogenic infrared filtering. For single-diameter sphere samples, the authors report a distinct spectral feature in the 700 µm sphere composite: a second smaller peak near 4 µm appears in the transmission data.

Immediately after noting this feature, the authors state that the origin of the peak is unclear and speculate it may arise from a path entirely inside sapphire due to direct contact between large spheres. They also describe thickness-dependent behavior supporting this hypothesis, but do not establish a definitive mechanism, motivating the explicit open question about the peak’s origin.

References

For the \SI{700}{\micro\meter} spheres, a second smaller peak near \SI{4}{\micro\meter} is observed; its origin is unclear.

— Low-loss Material for Infrared Protection of Cryogenic Quantum Applications  (2601.05147 - Griedel et al., 8 Jan 2026) in Supplementary Material, Section "IR spectroscopy transmission data" (after discussion of single-diameter sphere samples and Fig. \ref{fig:sapphire}(b))