What are the key optical properties of theophylline crystals, and how do they contribute to their application in optical metamaterials?

One study explores the synthesis and characterization of highly uniform single crystal films of theophylline, a xanthine derivative, highlighting their unique optical characteristics, such as high birefringence. These properties are crucial for their potential use in advanced optical devices.

Theophylline crystalline sheets exhibit high optical anisotropy, with a refractive index difference (Δn) of approximately 0.25 and a refractive index along the slow axis (n) of approximately 1.7. Experimentally, the in-plane refractive indices for theophylline were determined to be n fast =1.44±0.01 and n slow =1.69±0.01, resulting in an in-plane birefringence of about 0.25, or 15%. This birefringence is significantly higher than that of inorganic crystals like calcite (Δn≈0.16, ≈10%).
These properties make theophylline crystals suitable for optical metamaterial applications. The crystals can function as polarization-dependent flat optical elements, effectively acting as zero-order wave plates. Furthermore, the ability to pattern these crystalline stripes with tailored periodic gratings allows for the creation of thin organic polarization-dependent diffractive meta-surfaces, opening possibilities for fabricating various optical devices from small-molecule based organic dielectric crystals.