Designing High-Performance Broadband Mirrors: A Multiscale Mathematical Framework for Multilayer Thin-Film Optimization

Thin-film optical coatings, including single or multilayer packs characterized by their refractive indices, absorption coefficients, and thicknesses, allow accurate management of the reflectance, transmittance, and absorptance. This work reports on the design of dielectric thin-film coatings aimed to build high-reflectance mirrors with broadband spectral yield that have the capability of direct relevance to optical instruments such as spectrophotometers, laser resonators and cavities, optical metrology setups, and hyperspectral and multispectral instruments that are essential in optical imaging and astronomical observations. Commonly used metallic reflectors—fabricated from silver, aluminum, or gold—show crucial limitations involving unavoidable optical absorption, exposure to oxidation under adverse environmental circumstances, and wavelength-dependent reflectivity. Most dielectric multilayer reflectors avoid these disadvantages by proposing low absorption, widening the effective lifetime, and better environmental stability. A series of mirror configurations, each containing a substrate with sustaining periodically regular manner of low- and high-refractive-index dielectric layers, is suggested and numerically estimated after mathematical modeling that contains all parameters for effectively describing the proposed design. The calculated spectral reflectance is compared with previously reported experimental data to justify the proposed design methodology. Results display that period alternating, linked with adjacent stack designs, significantly increases the high-reflectance bandwidth while maintaining the low-loss and stability benefits underlying to all-dielectric designs.