Tweak reactive mesogens properties by two-photon lithography

Two-Photon Lithography (TPL) in liquid crystalline photo-curable resins (i.e. reactive mesogens, RMs) allows to design 3D complex polymeric structures inheriting the optical characteristics of liquid crystals and sub-micrometric resolution. When high peak power infrared femtosecond laser is focused in RMs, two-photon absorption process triggers free-radical polymerization which crosslinks the resin in a small volume ("voxel") inside the laser focus. By scanning the laser focus, fine LC structures are created in 3D. Here we demonstrate how it is possible to take advantage of the various TPL parameters to conveniently adjust the intrinsic RMs properties (i.e. order, alignment, etc.), without the need of other external controls (i.e. temperature, voltage, etc.). We discuss both the nematic and chiral nematic (CLC) cases. In particular, we show that the birefringence and optical axis of a nematic RMs film, induced by a homogenous aligning substrate, could be preserved or deliberately altered on the microscale upon TPL, depending on the laser focus scanning geometrical and physical parameters (i.e., direction, spacing, speed, etc.). Also, we prove that the photonic band gap of a photo-curable CLC mixture can be finely tuned across the whole visible range, in a single-step fabrication session, by regulating the delivered energy dose. In this way, multicolor 3D structures can be fabricated and a 25x25 elements 4D QR code, with cuboids of different heights and selective reflection bands, is presented as a proof-of-concept of a novel security tag [1]. References: [1] Ritacco, T. D. M. Aceti, G. De Domenico, M. Giocondo, A. Mazzulla, G. Cipparrone and P. Pagliusi (2022). Tuning Cholesteric Selective Reflection in Situ upon Two-Photon Polymerization Enables Structural Multicolor 4D Microfabrication. Adv. Opt. Mater. 10, 2101526.