Light-induced modulation of viscoelastic properties in azobenzene polymers.

Photo-induced isomerization of azobenzene molecules drives mass migrations in azopolymer samples. The resulting macroscopic directional photo-deformation of the material morphology has found many applications in literature, although the fundamental mechanisms behind this mass transfer are still under debate. Hence, it is of paramount importance to find quantitative observables that could drive the community toward a better understanding of this phenomenon. In this regard, azopolymer mechanical properties have been intensively studied, but the lack of a nanoscale technique capable of quantitative viscoelastic measurements has delayed the progress in the field. Here, we use bimodal atomic force microscopy (AFM) as a powerful technique for nanomechanical characterizations of azopolymers. With this multifrequency AFM approach, we map the azopolymer local elasticity and viscosity, with high resolution, after irradiation. We find that, while in the (previously) illuminated region, a general photo-softening is measured; locally, the Young modulus and the viscosity depend upon the inner structuring of the illuminating light spot. We then propose a possible interpretation based on a light-induced expansion plus a local alignment of the polymer chains (directional hole-burning effect), which explains the experimental observations. The possibility to access, in a reliable and quantitative way, both Young modulus and viscosity could trigger new theoretical-numerical investigations on the azopolymer mass migration dynamics since, as we show, both parameters can be considered measurable. Furthermore, our results provide a route for engineering the nanomechanical properties of azopolymers, which could find interesting applications in cell mechanobiology research.

Wavelength-Dependent Shaping of Azopolymer Micropillars for Three-Dimensional Structure Control.

Surfaces endowed with three-dimensional (3D) mesostructures, showing features in the nanometer to micrometer range, are critical for applications in several fields of science and technology. Finding a fabrication method that is simultaneously inexpensive, simple, fast, versatile, highly scalable, and capable of producing complex 3D shapes is still a challenge. Herein, we characterize the photoreconfiguration of a micropillar array of an azobenzene-containing polymer at different light wavelengths and demonstrate the tailoring of the surface geometry and its related functionality only using light. By changing the irradiated light wavelength and its polarization, we demonstrate the fabrication of various complex isotropic and anisotropic 3D mesostructures from a single original pristine geometry. Quantitative morphological analyses revealed an interplay between the decay rate of absorbed light intensity, micropillar volume preservation, and the cohesive forces between the azopolymer chains as the origin of distinctive wavelength-dependent 3D structural remorphing. Finally, we show the potentialities of this method in surface engineering by photoreshaping a single original micropillar surface into two sets of different mesostructured surfaces exhibiting tunable hydrophobicity in a wide water contact angle range. Our study opens up a new paradigm for fabricating functional 3D mesostructures in a simple, low-cost, fast, and scalable manner.

Polariton nanophotonics using phase-change materials.

Polaritons formed by the coupling of light and material excitations enable light-matter interactions at the nanoscale beyond what is currently possible with conventional optics. However, novel techniques are required to control the propagation of polaritons at the nanoscale and to implement the first practical devices. Here we report the experimental realization of polariton refractive and meta-optics in the mid-infrared by exploiting the properties of low-loss phonon polaritons in isotopically pure hexagonal boron nitride interacting with the surrounding dielectric environment comprising the low-loss phase change material Ge3Sb2Te6. We demonstrate rewritable waveguides, refractive optical elements such as lenses, prisms, and metalenses, which allow for polariton wavefront engineering and sub-wavelength focusing. This method will enable the realization of programmable miniaturized integrated optoelectronic devices and on-demand biosensors based on high quality phonon resonators.

A mid-infrared biaxial hyperbolic van der Waals crystal.

Hyperbolic media have attracted much attention in the photonics community due to their ability to confine light to arbitrarily small volumes and their potential applications to super-resolution technologies. The two-dimensional counterparts of these media can be achieved with hyperbolic metasurfaces that support in-plane hyperbolic guided modes upon nanopatterning, which, however, poses notable fabrication challenges and limits the achievable confinement. We show that thin flakes of a van der Waals crystal, α-MoO3, can support naturally in-plane hyperbolic polariton guided modes at mid-infrared frequencies without the need for patterning. This is possible because α-MoO3 is a biaxial hyperbolic crystal with three different Reststrahlen bands, each corresponding to a different crystalline axis. These findings can pave the way toward a new paradigm to manipulate and confine light in planar photonic devices.

Structural Basis of Polydopamine Film Formation: Probing 5,6-Dihydroxyindole-Based Eumelanin Type Units and the Porphyrin Issue.

The role of 5,6-dihydroxyindole (DHI)-based oligomers, including porphyrin-like tetramers, in polydopamine (PDA) film formation was addressed by a comparative structural investigation against model polymers from DHI and its 2,7'-dimer. MALDI-MS data showed that (a) PDA is structurally different from DHI melanin and does not contain species compatible with DHI-based oligomers as primary building blocks; (b) PDA films and precipitate display a single main peak at m/ z 402 in common; (c) no species matching the range of m/ z values expected for cyclic porphyrin-type tetramers was detected in DHI melanin produced in the presence or in the absence of folic acid (FA) as templating agent, nor by oxidation of the 2,7'-dimer of DHI as putative precursor. 15N NMR resonances and Raman spectra predicted by extensive DFT calculations on porphyrin-type structures at various oxidation levels did not match spectral data for PDA or DHI melanin. Notably, unlike PDA, which gave structurally homogeneous films on quartz on atomic force microscopy (AFM) and micro-Raman spectroscopy, DHI melanin did not form any adhesive deposit after as long as 24 h. It is concluded that PDA film deposition involves structural components unrelated to DHI-based oligomers or porphyrin-type tetramers, which, on mechanism-based analysis, may arise by quinone-amine conjugation leading to polycyclic systems with extensive chain breakdown.