Femtosecond-laser nanostructuring of silicon thin films for optical information encryption

Pulsed laser processing matures as a powerful technique for flexible and high-performing nanostructuration. Rich physics behind the light-matter interaction allows to produce unique surface nanotextures with desired properties, beneficial for various practically-relevant applications such as optical sensing, anti-counterfeiting, realization of nanophotonic platforms and so on. Here we have applied direct femtosecond-laser printing to locally fabricate nanostructures on glass-supported amorphous-silicon thin film for realization of high-resolution (up to 60 000 dots per inch) security labels offering full-optical information encryption in several ways. Since the proposed tag represents array of close-packed amorphous-silicon hemispherical nanoparticles, the first approach for information encryption is to selectively crystallize some of nanoparticles by continuous-wave laser irradiation without morphological changes. Thus, crystalized nanoparticles ordered in a user-defined way indicate encrypted information that can be read using Raman signal intensity mapping at frequency of 519 cm−1, corresponding to the main phonon mode of crystalline silicon. The second way is to hide Mie-resonant nanoparticles between non-resonant ones. Since the latter haven’t proper size to resonantly interact with pump radiation during Raman signal mapping, encrypted information can be revealed via evident variation in Raman yield between resonant and non-resonant nanoparticles. Thereby, we demonstrated facile single step printing of anti-counterfeiting labels at resolution up to 60 000 dots per inch justifying the applicability of the developed approach for optical information encryption.