Near-Infrared Emission and Photon Energy Upconversion of Two-Dimensional Copper Silicates

BaCuSi₄O₁₀ (Han blue), CaCuSi₄O₁₀ (Egyptian blue), and SrCuSi₄O₁₀ are pigments found in many ancient artifacts all over the world. Behind their brilliant color, we demonstrate here that these ancient pigments are strong candidates for photonic materials due to their bright Stokes and anti-Stokes emissions. These pigments give near-infrared emissions (NIR) from Cu²⁺ centered at around 930 nm under excitation of 440–800 nm light. This NIR emission can also be produced by pumping using a NIR laser diode. With the rise of pumping density, the emission bandwidth increases notably and stretches to the visible region, giving rise to bright and broadband photon upconversion (UC). This photon UC process is interpreted in terms of laser-driven blackbody radiation from the ancient pigments.