Crypto-Imaging in Photoluminescent Metastructures

Combining an advanced additive manufacturing technique, the Two-Photon Direct Laser Writing (TP-DLW), with the peculiar electro-optical properties of Quantum Dots (QDs) allows the creation of hybrid metastructures consisting in 3D photo-luminescent objects featuring sub-millimetric details. The study of the the interaction between the QDs and the UV-sensitive photo-resist during the TP-DLW allows the fine tuning of the emitted photo-luminescence. During the polymerization, the cross-linking of the monomers expels the majority of QDs from the irradiated volume, but they get excited and trigger a surrounding thermal polymerization, which changes the structure resolution. Here, the various steps of the TP-DLW procedures of such hybrid systems are illustrated in detail. The study defines the best conditions to modulate the QDs density in the created nanostructures to be integrated in optical devices, almost down to the single QDs. Moreover, the combined use of TP-DLW and single photon lithography introduces a further degree of freedom in the local QDs density control, allowing for the fabrication of even more complex object, such as the already mentioned hybrid metastructures. As a possible application of this technique, anti-counterfeiting is discussed with the fabrication of a 3D fluorescent design hidden in a polymeric microstructure. The rapidity of the process makes it suitable for industrial applications.