How to Tailor Structural Colors for Extended Visibility and White Light Generation Employing Direct Laser Interference Patterning

The appearance of a surface can be controlled by creating periodic microstructures designed to diffract light and produce structural colors. Nevertheless, since structural coloration is based on diffraction, the produced colors have a strong dependence on the viewing angle and absence of coloration takes place while tilting the samples. In this work direct laser interference patterning is used to firstly provide transparent polymer sheets a structural coloration with a high‐range observation angle, and secondly to demonstrate the combination of structural colors, producing a white coloring effect. The employed approaches are based on the fabrication of micro‐gratings with multiple periods in the same structured area and on the engineering of the diffraction orders of the diffraction spectrum. The patterned surfaces are characterized by confocal microscopy and angular spectrometry in reflection mode. The morphological characterization shows homogeneous surface patterns, while the spectral results demonstrate that combining four spatial periods on a single patterned surface, a white appearance is obtained over an angular observation range higher than 30°. The experimental results are supported by theoretical predictions by means of generalized formulas based on the diffraction of light.     

Direct Sub‐Micrometer‐Patterning of Conjugated Polymers and Polymer Light‐Emitting Devices by Electron Beam Lithography

The lateral pattering of polymer light emitting devices (PLEDs) on the micro‐ and even sub‐micrometer level is a challenging task. Being able to fabricate devices with sub‐micrometer active device dimensions will, however, open new possibilities for fundamental studies as well as enable improvements in device performance. Therefore, in this study an electron beam‐based patterning method for conjugated polymers is evaluated, where the structuring is achieved by deliberate degradation and alteration of the molecular structure accompanied by a change in the emission properties. We find that the typical feature size accomplished with this method is approx. 2 µm for the active line width in PLEDs, while the in‐depth investigation of the structures shows that the lateral resolution is found to be approx. 500 nm.

     

Low-symmetry nonlocal transport in microstructured squares of delafossite metals

Intense work studying the ballistic regime of electron transport in two-dimensional systems based on semiconductors and graphene had been thought to have established most of the key experimental facts of the field. In recent years, however, additional forms of ballistic transport have become accessible in the quasi–two-dimensional delafossite metals, whose Fermi wavelength is a factor of 100 shorter than those typically studied in the previous work and whose Fermi surfaces are nearly hexagonal in shape and therefore strongly faceted. This has some profound consequences for results obtained from the classic ballistic transport experiment of studying bend and Hall resistances in mesoscopic squares fabricated from delafossite single crystals. We observe pronounced anisotropies in bend resistances and even a Hall voltage that is strongly asymmetric in magnetic field. Although some of our observations are nonintuitive at first sight, we show that they can be understood within a nonlocal Landauer-Büttiker analysis tailored to the symmetries of the square/hexagonal geometries of our combined device/Fermi surface system. Signatures of nonlocal transport can be resolved for squares of linear dimension of nearly 100 µm, approximately a factor of 15 larger than the bulk mean free path of the crystal from which the device was fabricated.

The ballistic regime, in which the electron mean free path is larger than a characteristic geometric length scale, is an unconventional regime of nonlocal electrical transport that occurs only within ultrapure materials. A common method to examine materials in this regime is to study electrical transport in four-terminal junctions that are smaller than the electron mean free path. Within cross- or square-shaped devices, unconventional effects can occur, such as a negative nonlocal “bend” resistance (1–3) and an enhanced, suppressed, or even negative Hall resistance (4–7). Estimates of the characteristic length scales of the ballistic regime can also be made by examining the suppression of these effects with increasing device size (8–11).The majority of four-terminal ballistic regime studies, however, have concentrated on semiconductor heterostructures (6, 8, 12, 13) or monolayer graphene (14–17) rather than metals, primarily because the electronic mean free path in quasi–two-dimensional metals is typically less than 100 nm. In recent years, however, it has become clear that the delafossite metals PdCoO₂ and PtCoO₂ (18–20) have the potential to be ideal hosts for the study of nonlocal transport effects. Almost uniquely among oxide metals, they have an extremely high purity as grown, with defect levels as low as 1 in 10⁵ in the conducting Pd/Pt layers (21). They also bring the benefit of simplicity, because only one highly dispersive band crosses the Fermi level, giving a single, quasi–two-dimensional Fermi surface as established experimentally by angle-resolved photoemission and measurements of the de Haas–van Alphen effect (22–25). Due to the high purity, momentum-relaxing mean free paths as long as 20 µm can be achieved at low temperature (19, 23, 24, 26), enabling the observation of multiple unconventional transport and quantum interference effects in mesoscopic samples (27–29).For various nonlocal in-plane transport properties, the shape of the Fermi surface of PdCoO₂ has been demonstrated to strongly influence observations. In particular, its nearly hexagonal cross-section leads to a high level of directionality of transport properties once the nonlocal regime is entered at low temperatures (30, 31). This pronounced Fermi surface faceting is one way in which nonlocal transport measurements on delafossite metals differ from those on semiconductor two-dimensional electron gases or monolayer graphene, for both of which Fermi surfaces are close to circular. A second noteworthy feature is that, since the carrier density per layer in the delafossites is metallic (1.5 × 10¹⁵ cm^{– 2}), the Fermi wavelength is two orders of magnitude shorter than that of typical doped semiconductors, placing the delafossites in a new regime of two-dimensional transport. For these reasons it is interesting to make contact between the two fields by performing nonlocal transport measurements in some of the “classic” device geometries of mesoscopic physics, to investigate similarities and differences between the behavior of the delafossite metals and the previously studied low carrier density systems. In this paper we report on experiments designed to do this, studying the bend and Hall resistances of four-terminal microstructured squares.     

Kinetics of Holographic Recording and Spontaneous Erasure Processes in Light-Sensitive Liquid Crystal Elastomers

The optical mechanism for imprinting one-dimensional grating structures into thin films of a light-sensitive monodomain liquid crystal elastomer is investigated by analyzing the time dependence of optical diffraction properties. The recording kinetics shows an irregular oscillatory behavior, which is most expressed at small grating spacings and at temperatures close to the nematic-isotropic phase transition. The oscillations are attributed to the opto-mechanical response of the film, i.e., to contraction of the film during the recording process. At temperatures far below the nematic-isotropic phase transition, the spontaneous erasure kinetics exhibits exponential relaxation with relaxation time following the Arrhenius activation law. However, at temperatures close to the nematic-isotropic phase transition, the erasure process shows an interesting nonmonotonic behavior that we attribute to the non-linear relation between the concentration of the photo-transformed chemical groups and the nematic order parameter.     

Two-photon polymerization technique for microfabrication of CAD-designed 3D scaffolds from commercially available photosensitive materials

We report on recent advances in the fabrication of three-dimensional (3D) scaffolds for tissue engineering and regenerative medicine constructs using a two-photon polymerization technique (2PP). 2PP is a novel CAD/CAM technology allowing the fabrication of any computer-designed 3D structure from a photosensitive polymeric material. The flexibility of this technology and the ability to precisely define 3D construct geometry allows issues associated with vascularization and patient-specific tissue fabrication to be directly addressed. The fabrication of reproducible scaffold structures by 2PP is important for systematic studies of cellular processes and better understanding of in vitro tissue formation. In this study, 2PP was applied for the generation of 3D scaffold-like structures, using the photosensitive organic-inorganic hybrid polymer ORMOCER (ORganically MOdified CERamics) and epoxy-based SU8 materials. By comparing the proliferation rates of cells grown on flat material surfaces and under control conditions, it was demonstrated that ORMOCER and SU8 are not cytotoxic. Additional tests show that the DNA strand breaking of GFSHR-17 granulosa cells was not affected by the presence of ORMOCER. Furthermore, gap junction conductance measurements revealed that ORMOCER did not alter the formation of cell-cell junctions, critical for functional tissue growth. The possibilities of seeding 3D structures with cells were analysed. These studies demonstrate the great potential of 2PP technique for the manufacturing of scaffolds with controlled topology and properties.