EXPRESS: Incidence-side-dependent terahertz refractive index response of opal–chlorite bilayer composites

Layered mineral composites may exhibit incidence-side-dependent terahertz responses because interfacial reflection and unequal optical paths modify wave propagation through the structure. In this study, terahertz time-domain spectroscopy (THz-TDS) was employed to investigate the effective refractive index response of opal-chlorite bilayer composites. The results show that the refractive index difference between opposite incidence sides decreases and gradually approaches a stable level with increasing chlorite thickness. TMM inversion retrieved the chlorite refractive index spectrum in good agreement with the directly measured result. Proton irradiation applied from the opal side enhanced the asymmetry mainly in the thinnest sample, whereas this effect was rapidly suppressed as the chlorite thickness increased. In addition, the measurements under the investigated environmental conditions showed that the refractive index difference remained comparatively stable. These results provide an experimental basis for analyzing incidence-side-dependent terahertz responses in layered mineral bilayers.