Effective optical elements with tailored properties often rely on the capability to tune the material's structure at the nanoscale. Thanks to their self‐organized 1D helical arrangement, cholesteric liquid crystals represent a beautiful example of optical materials whose properties are governed by their supramolecular structure. According to the Bragg's law, selective reflection of circular polarized light occurs for wavelengths within the photonic band gap, which, beside the refractive indices, depends on the helix pitch. Here, polymeric microstructures with tailored PBG are demonstrated by two‐photon polymerization direct laser writing in cholesteric reactive mesogens. Turning a renowned disadvantage into an opportunity, the shrinkage upon the layer‐by‐layer photopolymerization is exploited to control the effective helix pitch. Starting from a PBG of the precursor in the near infra‐red, micro‐cylinders which exhibit Bragg selective reflection at lower wavelength ranges, down to the opposite end of the visible spectrum, are fabricated in a single‐step process by tuning the laser exposure parameters. As a proof‐of‐concept, a 4D quick response (QR) micro‐tag, which adds the polarization‐selective structural color and the height of the blocks to the usual 2D black/white QR codes, is demonstrated as a novel paradigm of optical anti‐counterfeiting microdevice.

This work investigates the heat delivered by a mono-layer 1D grating of gold nanoparticles (GNPs) created by photo-reduction through two-photon direct laser writing (2P-DLW) in a poly-vinyl alcohol (PVA) matrix doped with HAuCl₄, under resonant laser radiation. We drop cast a film of a PVA + HAuCl₄ mixture onto a glass substrate, in which we create gratings of 1 mm² made by stripes of GNPs characterized by high polydispersivity. We demonstrate that, by controlling the pitch of the GNP stripes, we obtain different values of the photo-induced temperature variations. In the framework of thermo-plasmonics, the experimental investigation of the heat generation from a monolayer of gold nanoparticles represents a key element as a starting point to design thermo-smart platforms for sensing, solar energy harvesting and thermo-catalysis.

We report about the study on the physical features of gold nano-particles (GNPs) created by 2-photons photo-reduction Direct Laser Writing in a Poly-Vinyl Alcohol (PVA) matrix doped with HAuCl₄. We drop cast a film of the PVA+ HAuCl₄ onto a glass substrate, in which we create 1D gratings made by stripes of GNPs with a single laser sweep. We show that the stripe width increases with the laser power and the exposure time. We also analyse the influence of the exposure time over the created nano-particles size distribution and density and we show that by suitably adjusting the exposure time it is possible to maximize the frequency of a given diameter. By comparing the experimental results with a polymerization “voxel” model, we are able to evaluate the effective cross section for 2-photons absorption of our material.

Multi‐level anticounterfeiting tags have been developed using a combination of different materials. Polyvinyl alcohol (PVA) mixed with titanium dioxide (TiO ₂ ) is used to produce flexible substrates. Fluorescent Opuntia Ficus‐indica (OFI) extract dissolved with polymethyl methacrylate (PMMA) is then sprayed over the substrate to create a random, yet unique deposition of droplets. Photographs of the tags are taken under UV illumination at different angles and analyzed through the scale‐invariant feature transform (SIFT) algorithm to extract their unique features. The SIFT analysis reveals hundreds to thousands of matched features when a given tag is compared with itself, whereas this number drops to tens for different tags. To enhance the security of the tags, ITO is sputtered onto one of them in the form of a pattern formed by a patch array exhibiting a specific fingerprint at terahertz (THz) frequencies. The evaluation of ITO reflectance shows that each patch array has a unique and unpredictable response stemming from its distinct electro‐optical characteristics. The non‐deterministic response of sprayed dye droplets and ITO patches enables the realization of two‐level authentication, which is difficult to replicate at a reasonable cost. The simple manufacturing process and inexpensive materials involved make the proposed tags easily integrable into packaging.