Easy 4D Microprinting by Tweaking Structural Color and Optical Axis in Reactive Mesogens

Over the last two decades, considerable progress has been made in the field of twophoton polymerization lithography (TPL) [1-5], which has resulted in a wide range of applicationsin biotechnology, microfluidics, microrobotics, photonics and antiforgery. One of the currentchallenges of TPL is to impart the objects with further functionalities (often called "4D printing"),such as the ability to dynamically respond to external stimuli (i.e., temperature, light, pH,humidity, etc.) [3, 4] or to spatially modulate their properties (i.e., optical, mechanical, etc.) atthe microscale [5-7].Here we report a simple feasible strategy to provide the "color dimension" to 3D microobjects,adopting a dye-free structural color printing. We take advantage of the self-organized supramolecular helical structure of a photonic photoresist, namely cholesteric reactive mesogens (CRMs),and of the "gray-tone" TPL approach [3], which allows for fine tuning the intrinsic photonicband gap (PBG) of the CRMs, at the microscale, during the fabrication by adjusting the laserexposure dose [7]. Altering the crosslinking density of the CRMs polymer network, the shrinkage of polymer is exploited to control the effective helix pitch, still guaranteeing compact solidstructures. Thus, full color mapping across the whole visible range is demonstrated in a rapid,single-step TPL fabrication session. In addition, we show that the effective birefringence and/oroptical axis of a nematic RMs microstructure could be preserved or deliberately altered uponTPL, depending on the laser focus scanning geometrical and physical parameters (i.e., direction,spacing, speed, etc.).In view of the potential application impact in the fields of miniaturized optics, lasers, and anticounterfeiting, a 4D quick response (QR) code, made of 25 × 25 micron-sized square cuboidelements with different heights and selective reflection color bands, is reported as a novel paradigmof secure authentication code.REFERENCES1. Kawata, S., et al., "Finer features for functional microdevices," Nature, Vol. 412, 697, 2001.2. Harinarayana, V., et al., "Two-photon lithography for three-dimensional fabrication in micro/nanoscale regime: A comprehensive review," Optics & Laser Technology, Vol. 142, 107180,2021.3. Spiegel, C. A., et al., "4D printing at the microscale," Adv. Funct. Mater., Vol. 30, 1907615,2020.4. Del Pozo, M., et al., "Direct laser writing of four-dimensional structural color microactuatorsusing a photonic photoresist," ACS Nano, Vol. 14, 9832, 2020.5. Liu, Y., et al., "Structural color three-dimensional printing by shrinking photonic crystals,"Nat. Comm., Vol. 10, 4340, 2019.6. Nocentini, S., et al., "Structured optical materials controlled by light," Adv. Optical Mater.,Vol. 6, 1800167, 2018.7. Ritacco, T., et al., "Tuning cholesteric selective reflection in situ upon two-photon polymerization enables structural multicolor 4D microfabrication," Adv. Optical Mater., Vol. 10, 2101526,2022.