Augustine Urbas

Tunable Block Copolymer/Homopolymer Photonic Crystals

Interest in the photonic properties of dielectrically struc-tured materials is growing rapidly. This is fueled by the potential application of such materials in a broad range of fields. Much recent work has focused on lithographically fabricated photonic materials, [1,2] and on self-assembled colloidal crystals [3,4] and inverse opal structures. [5±7] The former need to be fabricated on small-area flat surfaces, the latter tend to have long timescales inherent in the fabrication and both have limited tunability with respect to the precursor materials. Our recent work [8,9] has shown that self-assembled block copolymer systems can exhibit a one-dimensionally periodic lamellar structure with component domains having widths large enough (l/4n) to act as visible light photonic materials. Their use in photonics would bring many advantages in terms of materials properties, process-ability, as well as cost, and would enable the fabrication of large-area conformable photonic materials. In this communication we show that multilayer photonic crystals active within a broad range of wavelengths in the visible spectrum can easily be produced from a simple system comprised of a block copolymer and two homopolymers. Ternary blends of a poly(styrene-b-isoprene) (S/I) di-block copolymer, polystyrene, and polyisoprene were solution cast from cumene. As the samples begin to dry they selectively reflect light, giving the appearance of green or red color. After drying, each sample showed a single well-defined peak in reflectivity in the visible wavelength range (350±600 nm) and a corresponding drop in the transmission profile. This implies that the reflection was fairly efficient and that the color observed in the samples was not due to significant absorption. In Figure 1a, we show typical reflec-tivity curves for several blend samples. The relative width of the reflectivity peaks (or transmission dips), Dl/l, varies approximately from 0.15 to 0.25, widening as the fraction of homopolymer in the blend increases. The wavelength of the reflectivity peak versus the homopolymer composition is plotted in Figure 1b. There is a monotonic increase in the peak reflective wavelength with the fraction of homopoly-mer. Scanning electron microscopy (SEM) of the samples confirmed that a lamellar morphology was present in the samples , with typical grain sizes of the order of several micro-meters. The backscattered electron imaging (BEI) image of a sample containing 40 % homopolymer is shown in Figure 2a. The bright regions correspond to the OsO 4 stained polyisoprene domains and the dark regions are the un-stained polystyrene. The lamellar repeat from the …

Block copolymers as photonic bandgap materials

Block copolymers self-assemble into one-, two-, and three-dimensional periodic equilibrium structures, which can exhibit photonic bandgaps. This paper outlines a methodology for producing photonic crystals at optical length scales from block copolymers. Techniques for enhancing the intrinsic dielectric contrast between the block copolymer domains, as well as increasing the characteristic microdomain distances, and controlling defects are presented. To demonstrate the applicability of this methodology, a self-assembled one-dimensional periodic structure has been fabricated that reflects visible light. The wealth of structures into which block copolymers can assemble and the multiple degrees of freedom that can be built into these materials on the molecular level offer a large parameter space for tailoring new types of photonic crystals at optical length scales.

Light-driven nanoscale chiral molecular switch: reversible dynamic full range color phototuning

A light-driven nanoscale chiral molecular switch was found to impart its chirality to an achiral liquid crystal host to form a self-organized, optically tunable helical superstructure capable of fast and reversible phototuning of the structural reflection across the entire visible region.

Reversible Visible-Light Tuning of Self-Organized Helical Superstructures Enabled by Unprecedented Light-Driven Axially Chiral Molecular Switches

Two enantiomeric light-driven azo molecular switches with axial chirality and extended conjugation were found to exhibit unprecedented reversible photoisomerization in both organic-solvent and liquid-crystal media only upon visible-light irradiation. When doped in an achiral liquid crystal with a different concentration, the chiral switch was able either to immediately induce an optically tunable helical superstructure or to retain an achiral liquid-crystal phase whose helical superstructure was induced and tuned reversibly upon visible-light irradiation. Furthermore, reversible dynamic red, green, and blue reflection achieved only by using visible light was demonstrated.

Mechanochromic Photonic Gels

Polymer gels are remarkable materials with physical structures that can adapt significantly and quite rapidly with changes in the local environment, such as temperature, light intensity, electrochemistry, and mechanical force. An interesting phenomenon observed in certain polymer gel systems is mechanochromism - a change in color due to a mechanical deformation. Mechanochromic photonic gels are periodically structured gels engineered with a photonic stopband that can be tuned by mechanical forces to reflect specific colors. These materials have potential as mechanochromic sensors because both the mechanical and optical properties are highly tailorable via incorporation of diluents, solvents, nanoparticles, or polymers, or the application of stimuli such as temperature, pH, or electric or strain fields. Recent advances in photonic gels that display strain-dependent optical properties are discussed. In particular, this discussion focuses primarily on polymer-based photonic gels that are directly or indirectly fabricated via self-assembly, as these materials are promising soft material platforms for scalable mechanochromic sensors.

Reversible Light-Directed Red, Green, and Blue Reflection with Thermal Stability Enabled by a Self-Organized Helical Superstructure

Adding external, remote, and dynamic control to self-organized superstructures with desired properties is an important leap necessary in leveraging the fascinating molecular subsystems for employment in applications. Here two novel light-driven dithienylethene chiral molecular switches possessing remarkable changes in helical twisting power during photoisomerization as well as very high helical twisting powers were found to experience photochemically reversible isomerization with thermal stability in both isotropic organic solvents and anisotropic liquid crystal media. When doped into a commercially available achiral liquid crystal host, the chiral switch was able to either immediately induce an optically tunable helical superstructure or retain an achiral photoresponsive liquid crystal phase whose helical superstructure was induced and tuned reversibly upon light irradiation. Moreover, reversible light-directed red, green, and blue reflection colors with thermal stability in a single thin film were demonstrated.

Subwavelength direct laser patterning of conductive gold nanostructures by simultaneous photopolymerization and photoreduction.

This article presents a new method for fabricating highly conductive gold nanostructures within a polymeric matrix with subwavelength resolution. The nanostructures are directly written in a gold precursor-doped photoresist using a femtosecond pulsed laser. The laser energy is absorbed by a two-photon dye, which induces simultaneous reduction of gold in the precursor and polymerization of the negative photoresist. This results in gold nanoparticle-doped polymeric lines that exhibit both plasmonic effects, due to the constituent gold nanoparticles, and relatively high conductivity (within an order of magnitude of the bulk metal), due to the high density of particles within these lines. Line widths from 150 to 1000 nm have been achieved with this method. Various optically functional structures have been prepared, and their structural and optical properties have been characterized. The influence of laser intensity and scan speed on feature size have been studied and found to be in agreement with predictions of a mathematical model of the process.

Photodynamic Chiral Molecular Switches with Thermal Stability: From Reflection Wavelength Tuning to Handedness Inversion of Self‐Organized Helical Superstructures

A good turn: Three compounds that bear two axially chiral bridged binaphthyl units were developed as photodynamic chiral dopants for nematic liquid crystals. For compounds with suitable bridge lengths, a change in the dihedral angle induced a switch of the binaphthyl units from the cisoid to the transoid form upon UV irradiation, which led to an inversion of the handedness of the helices.

Light-Driven Molecular Switches with Tetrahedral and Axial Chirality

Two light-driven molecular switches with tetrahedral and axial chirality were synthesized, which can induce a helical superstructure in an achiral liquid crystal host and dynamically phototune it to achieve reversible reflection color.

Super‐resolution microscopy by movable thin‐films with embedded microspheres: Resolution analysis

Microsphere-assisted imaging emerged as a surprisingly simple way of achieving optical super-resolution imaging. In this work, we use movable PDMS thin films with embedded high-index barium titanate glass microspheres a sample scanning capability was developed, thus removing the main limitation of this technology based on its small field-of-view.