Anti‐Counterfeiting Microstructures Induced by Ultrashort Laser Pulses

Abstract

Anti‐counterfeiting microstructures are induced by ultrashort laser pulses in four kinds of materials. 1) Computer‐generated holograms of “SIOM” are written pixel‐by‐pixel using focused laser pulses by evaporation of aluminum films on silica substrates. Pixels are well‐defined holes, which can be reconstructed by He–Ne laser. 2) On stainless steel, surface ripples are induced with its orientation perpendicular to laser polarization. With “S‐O” and “I‐M” written with orthogonal laser polarizations, selective display of “SIOM” is achieved. Spatially overlapped “E” and “F” are also selectively displayed. 3) Luminous point defect of non‐bridging oxygen hole center (NBOHC) is generated in hydroxyl‐fused silica by ultrashort laser pulses. Due to photoluminescence (PL) of NBOHC, a barcode fabricated in hydroxyl‐fused silica is invisible in sunlight, while can be read out by 254 nm UV light. 4) Long persistent luminescence (LPL) is a phenomenon that PL continues for an obvious length of time when excitation source is removed. In homemade Mn2+‐doped germanate glass, LPL is observed from characters of “SIOM” written by laser pulses. “SIOM” then act as luminous centers. An exposure of 365 nm UV light enables the reappearance of LPL in “SIOM.” These phenomena can enable anti‐counterfeiting applications, and the mechanisms are also discussed.