Microwave fabrication and design techniques are commonly employed in the terahertz (THz) domain. However, a characterization of commercially available microwave dielectric materials is usually ...lacking at sub-THz and THz frequencies. In this work, we characterized four substrates by Rogers and an Ordyl dry resist between 0.2 and 2 THz, in terms of relative permittivity and loss tangent. The reflectance spectra of the investigated materials were retrieved by means of THz time-domain spectroscopy in reflection mode and post-processed according to a transmission-line model in which the materials’ parameters are fit by means of the Havriliak–Negami variation of the Debye model. The relative permittivity of the investigated materials showed negligible frequency dispersion in the sub-THz and in the THz range. In terms of the loss tangent, the Rogers substrates revealed a more pronounced frequency-dispersive behavior among different materials, as dictated by the Havriliak–Negami model. The Ordyl resist was dispersive in the 0.2–1.2 THz range and presented a nearly constant loss tangent value between 1.2 and 2 THz. These results may represent a reference for the development of innovative components for THz and sub-THz emerging applications.
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.
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•Gold nanoparticles (GNPs) clusters are created by multi-photon direct laser writing, through multiple exposures.•Tuning the waiting time (WT) between consecutive exposures allows to ...control the size and density of GNPs clusters.•The role of chloroauric ion and water diffusion on the morphology of the GNPs aggregates is pointed out.•If WT is set in the order of the ionic Diffusion Characteristic Time (DCT), compact homogenous GNPs clusters are created.•If WT compares to the water DCT, a pressure wave is generated at each exposure, producing wider inhomogeneous GNPs clusters.
Multi-photon direct laser writing (MP-DLW) in polymeric matrices doped with a tetrachloroauric acid (HAuCl4) water solution allows creating clusters of gold nanoparticles (GNPs) inside the focus figure of a tightly focused ultrafast laser beam. The key physical phenomena involved in the process are analyzed, with the aim to assess the limits and potential of this promising technology. Multi Photon Absorption (MPA) triggers the photo-reduction of AuCl4– ions and the consequent creation of GNPs in the spotted volume. Thermal electronic decays lead to a local abrupt increase of the temperature, which influences the morphology of the created structures. At the same time, two different effects take place, related to the dehydration of the polymeric matrix, and the concentration gradient of the gold precursor upon localized photoreduction. Given their different timescales, these phenomena allow for controlling the GNPs density and size dispersity when a given energy dose is delivered in multiple exposures, tuning the delay between consecutive laser exposures. A simple yet effective experiment to estimate the temperature distribution at the micron-scale is also proposed.
A novel technique is developed to improve the resolution of two‐photon direct laser writing lithography. Thanks to the high collimation enabled by extraordinary εNZ (near‐zero) metamaterial features, ...ultrathin dielectric hyper‐resolute nanostructures are within reach. With respect to the standard direct laser writing approach, a size reduction of 89% and 50%, in height and width respectively, is achieved with the height of the structures adjustable between 5 and 50 nm. The retrieved 2D fabrication parameters are exploited for realizing extremely thin all‐dielectric metalenses tailored through deep machine learning codes. The hyper‐resolution achieved in the writing process enables the fabrication of a highly detailed dielectric 3D bas‐relief (with full height of 500 nm) of Da Vinci's “Lady with an Ermine”. The proof‐of‐concept results show intriguing cues for the current and trendsetting research scenario in anti‐counterfeiting applications and ultracompact photonics, paving the way for the realization of all‐dielectric and apochromatic ultrathin imaging systems.
Two‐photon direct laser writing is enhanced to hyper‐resolution by leveraging on the extraordinary laser writing collimation enabled by εNZ metamaterials. The realization of ultrathin all‐dielectric metalenses and a highly detailed Da Vinci's “Lady with an Ermine” bas‐relief (500 nm full height) confirms full freedom in nanofabrication. These results show intriguing cues in trendsetting research scenarios and industry application.
Gold nanoparticles (GNPs) can be patterned on specific positions and substrates by Multi-Photon Direct Laser Writing (MP-DLW) in wet polymeric matrices doped with tetrachloroauric acid (HAuCl 4 ) as ...gold precursor. The Hamaker constants describing the GNP-GNP interaction and the interaction between the GNPs and the surrounding materials is described, defining the role of the polymeric matrix; thus, its limits and the advantages are thoroughly analysed. The Multi-Photon Photo-Reduction (MPPR) process, leading to the GNPs creation, triggers a local temperature rising, which can ablate the polymer and influences the particle distribution, size and density. The GNPs polydispersity also depends on the water content in the film, variable because of the vaporization. A protocol to perform MP-DLW of GNPs, without the use of the polymer is illustrated: by treating the surface with a surfactant, it is possible to make the particles stick to the substrate, control their size and reduce the diffusive and convective effects generated by the MPPR.
Biomaterial surface modification through the introduction of defined and repeated patterns of topography helps study cell behavior in response to defined geometrical cues. The lithographic molding ...technique is widely used for conferring biomaterial surface microscale cues and enhancing the performance of biomedical devices. In this work, different master molds made by UV mask lithography were used to prepare poly (D,L-lactide-co-glycolide) - PLGA micropatterned membranes to present different features of topography at the cellular interface: channels, circular pillars, rectangular pillars, and pits. The effects of geometrical cues were investigated on different cell sources, such as neuronal cells, myoblasts, and stem cells. Morphological evaluation revealed a peculiar cell arrangement in response to a specific topographical stimulus sensed over the membrane surface. Cells seeded on linear-grooved membranes showed that this cue promoted elongated cell morphology. Rectangular and circular pillars act instead as discontinuous cues at the cell-membrane interface, inducing cell growth in multiple directions. The array of pits over the surface also highlighted the precise spatiotemporal organization of the cell; they grew between the interconnected membrane space within the pits, avoiding the microscale hole. The overall approach allowed the evaluation of the responses of different cell types adhered to various surface patterns, build-up on the same polymeric membrane, and disclosing the effect of specific topographical features. We explored how various microtopographic signals play distinct roles in different cells, thus affecting cell adhesion, migration, differentiation, cell-cell interactions, and other metabolic activities.
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•Biomimetic microenvironment for the development of innovative tools for tissue engineering.•Surface geometry is able to evoke a specific cellular arrangement and different cell behavior.•Effects of membrane topographic features on cells from different source.•The topographical stimuli affect cell orientation in a different way.
This paper reports on a new strategy for obtaining homogeneous dispersion of grafted quantum dots (QDs) in a photopolymer matrix and their use for the integration of single-photon sources by ...two-photon polymerization (TPP) with nanoscale precision. The method is based on phase transfer of QDs from organic solvents to an acrylic matrix. The detailed protocol is described, and the corresponding mechanism is investigated and revealed. The phase transfer is done by ligand exchange through the introduction of mono-2-(methacryloyloxy) ethyl succinate (MES) that replaces oleic acid (OA). Infrared (IR) measurements show the replacement of OA on the QD surface by MES after ligand exchange. This allows QDs to move from the hexane phase to the pentaerythritol triacrylate (PETA) phase. The QDs that are homogeneously dispersed in the photopolymer without any clusterization do not show any significant broadening in their photoluminescence spectra even after more than 3 years. The ability of the hybrid photopolymer to create micro- and nanostructures by two-photon polymerization is demonstrated. The homogeneity of emission from 2D and 3D microstructures is confirmed by confocal photoluminescence microscopy. The fabrication and integration of a single-photon source in a spatially controlled manner by TPP is achieved and confirmed by auto-correlation measurements.
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.
Full‐color, single‐step, microfabrication is demonstrated by two‐photon direct laser writing in cholesteric liquid crystalline photoresists. The highly localized photoexcitation allows to regulate the chiral photonic band gap over the whole visible range, with microscale spatial resolution, thus adding a fourth dimension to the 3D additive manufacturing technique. A 4D quick response (QR) tag is reported as proof‐of‐concept.
Abstract 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.
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