Spectrally Tunable Reflectance with Invariant Transmittance via Weakly Coupled Dual Cavities

Weakly coupled dual cavities (WCDCs) are demonstrated to enable adjustable reflectance while maintaining invariant transmittance, capabilities that surpass those of conventional thin-film optical coatings. The structure consists of two vertically stacked optical cavities sharing a thin metallic interlayer: the upper cavity comprises a lossy metal, a transparent dielectric, and a reflective metal, while the lower cavity is formed by sandwiching a transparent dielectric between two highly reflective metals. While both cavities influence the reflectance, the transmittance is predominantly governed by the lower cavity. Comprehensive analyses of cavity thicknesses, coupling strength, and interference conditions reveal how spectral reflectance profiles can be tailored without altering the overall transmittance. To the best of our knowledge, this decoupled spectral behavior, tunable reflectance with fixed transmittance, is experimentally demonstrated here for the first time. Experimental demonstrations using three devices with varying upper cavity thicknesses confirm this tunability, showing consistent transmittance and markedly different reflectance spectra. These findings deepen our understanding of optical interference in multilayer structures and highlight the versatility of this design approach for a wide range of applications, including transflective displays, holography, optical isolation, and information encryption.