Optical Length Scales Research Articles

Relationship between spatially heterogeneous reaction dynamics and photochemical kinetics in single crystals of anthracene derivatives.

Understanding physicochemical property changes based on reaction kinetics is required to design materials exhibiting desired functions at arbitrary timings. In this work, we investigated the photodimerization of anthracene derivatives in single crystals. Single crystals of 9-cyanoanthracene (9CA) and 9-anthraldehyde (9AA) exhibited reaction front propagation on the optical length scale, while 9-methylanthracene and 9-acetylanthracene crystals underwent spatially homogeneous conversion. Moreover, the sigmoidal behavior in the absorbance change associated with the reaction was much pronounced in the case of 9CA and 9AA and correlated with the observation of heterogeneous reaction progress. A kinetic analysis based on the Finke-Watzky model showed that the effective quantum yield of the photochemical reaction changes by more than an order of magnitude during the course of the reaction in 9CA and 9AA. Both the reaction front propagation and nonlinear kinetic behavior could be rationalized in terms of the difference in the cooperativity of the reactions. We propose a plausible mechanism for the heterogeneous reaction progress in single crystals that depends on the magnitude of the conformational change required for reaction. Our results provide useful information to understand the connection between photochemical reaction progress in the crystalline phase and the dynamic changes in the physicochemical properties.

Deconstructed beetles: Bilayered composite materials produce green coloration with remarkably high near-infrared reflectance

Beetle elytra (hardened forewings) are a promising source of inspiration to develop or enhance the performance of human-fabricated composite materials. The structures responsible for optical properties in the ultra-violet to visible spectrum (300–700 nm) have been extensively characterised, but we have limited knowledge of optical properties and their physical origin in the near-infrared (NIR; 700–1700 nm). We examined the elytra of three species of green scarab beetles (Xylonichus eucalypti, Anoplognathus prasinus and Paraschizognathus olivaceus) with very high NIR reflectance. We manually separated layers in the elytra to disambiguate their contributions to the overall optical response. We show that unlike other scarabs, nanostructures within the cuticle layer do not produce notable reflectance. Instead, the cuticle resembles a pigment-based filter with 50% transmittance in the NIR and absorption in the visible spectrum contributing to the green appearance. Each species has a layer below the cuticle that appears white to the naked eye and produces broadband reflectance, particularly in the near-infrared; however, the structure of the white underlay differs markedly between the three species. In A. prasinus and P. olivaceus, the structure is disordered (no regular, repeated elements at optical length scales); whereas in Xylonichus eucalypti, the white underlay was notably thinner and comprised quasi-ordered hollow cylindrical structures embedded in a chitin matrix. We modelled the coherent scattering produced by this structure to demonstrate that it is responsible for broadband visible and NIR reflectance. We discuss biological implications and technological applications of the composite structure of beetle elytra.

Structural color from solid-state polymerization-induced phase separation

Structural colors are produced by wavelength-dependent scattering of light from nanostructures. While living organisms often exploit phase separation to directly assemble structurally colored materials from macromolecules, synthetic structural colors are typically produced in a two-step process involving the sequential synthesis and assembly of building blocks. Phase separation is attractive for its simplicity, but applications are limited due to a lack of robust methods for its control. A central challenge is to arrest phase separation at the desired length scale. Here, we show that solid-state polymerization-induced phase separation can produce stable structures at optical length scales. In this process, a polymeric solid is swollen and softened with a second monomer. During its polymerization, the two polymers become immiscible and phase separate. As free monomer is depleted, the host matrix resolidifies and arrests coarsening. The resulting polymeric composites have a blue or white appearance. We compare these biomimetic nanostructures to those in structurally-colored feather barbs, and demonstrate the flexibility of this approach by producing structural color in filaments and large sheets.

GlobalToperator bounds on electromagnetic scattering: Upper bounds on far-field cross sections

We present a method based on the scattering T operator, and conservation of net real and reactive power, to provide physical bounds on any electromagnetic design objective that can be framed as a net radiative emission, scattering or absorption process. Application of this approach to plane-wave scattering from an arbitrarily shaped, compact body of homogeneous electric susceptibility χ is found to predictively quantify and differentiate the relative performance of dielectric and metallic materials across all optical length scales. When the size of a device is restricted to be much smaller than the wavelength (a subwavelength cavity, antenna, nanoparticle, etc.), the maximum cross-section enhancement that may be achieved via material structuring is found to be much weaker than prior predictions: the response of strong metals (Reχ≪0) exhibits a diluted (homogenized) effective medium scaling ∝χ/Imχ; below a threshold size inversely proportional to the index of refraction (consistent with the half-wavelength resonance condition), the maximum cross-section enhancement possible with dielectrics (Reχ>0) shows the same material dependence as Rayleigh scattering. In the limit of a bounding volume much larger than the wavelength in all dimensions, achievable scattering interactions asymptote to the geometric area, as predicted by ray optics. For representative metal and dielectric materials, geometries capable of scattering power from an incident plane wave within an order of magnitude (typically a factor of two) of the bound are discovered by inverse design. The basis of the method rests entirely on scattering theory and can thus likely be applied to acoustics, quantum mechanics, and other wave physics.Received 27 March 2020Revised 17 June 2020Accepted 10 July 2020DOI:https://doi.org/10.1103/PhysRevResearch.2.033172Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetismGeometrical & wave opticsLight propagation, transmission & absorptionMetamaterialsMie scatteringPhotonicsPlasmonicsScattering theoryAtomic, Molecular & OpticalGeneral Physics

Nanoscale Atomic Density Microscopy.

Quantum simulations with ultracold atoms typically create atomic wavefunctions with structures at optical length scales, where direct imaging suffers from the diffraction limit. In analogy to advances in optical microscopy for biological applications, we use a non-linear atomic response to surpass the diffraction limit. Exploiting quantum interference, we demonstrate imaging with super-resolution of λ/50 and excellent temporal resolution of 500 ns. We characterize our microscope's performance by measuring the ensemble averaged probability density of atoms within the unit cells of an optical lattice, and observe the dynamics of atoms excited into motion. This approach can be readily applied to image any atomic or molecular system, as long as it hosts a three-level system.

Photonic hyperuniform networks obtained by silicon double inversion of polymer templates

Hyperuniform disordered networks belong to a peculiar class of structured materials predicted to possess partial and complete photonic bandgaps for relatively moderate refractive index contrasts. The practical realization of such photonic designer materials is challenging however, as it requires control over a multi-step fabcrication process on optical length scales. Here we report the direct-laser writing of hyperuniform polymeric templates followed by a silicon double inversion procedure leading to high quality network structures made of polycrystalline silicon. We observe a pronounced gap in the shortwave infrared centered at a wavelength of $\lambda_{\text{Gap}}\simeq $ 2.5 $\mu$m, in nearly quantitative agreement with numerical simulations. In the experiments the typical structural length scale of the seed pattern can be varied between 2 $\mu$m and 1.54 $\mu$m leading to a blue-shift of the gap accompanied by an increase of the silicon volume filling fraction.

Colloidal crystals with diamond symmetry at optical lengthscales

Future optical materials promise to do for photonics what semiconductors did for electronics, but the challenge has long been in creating the structure they require—a regular, three-dimensional array of transparent microspheres arranged like the atoms in a diamond crystal. Here we demonstrate a simple approach for spontaneously growing double-diamond (or B32) crystals that contain a suitable diamond structure, using DNA to direct the self-assembly process. While diamond symmetry crystals have been grown from much smaller nanoparticles, none of those previous methods suffice for the larger particles needed for photonic applications, whose size must be comparable to the wavelength of visible light. Intriguingly, the crystals we observe do not readily form in previously validated simulations; nor have they been predicted theoretically. This finding suggests that other unexpected microstructures may be accessible using this approach and bodes well for future efforts to inexpensively mass-produce metamaterials for an array of photonic applications.

Structural Diversity of Arthropod Biophotonic Nanostructures Spans Amphiphilic Phase-Space.

Many organisms, especially arthropods, produce vivid interference colors using diverse mesoscopic (100-350 nm) integumentary biophotonic nanostructures that are increasingly being investigated for technological applications. Despite a century of interest, precise structural knowledge of many biophotonic nanostructures and the mechanisms controlling their development remain tentative, when such knowledge can open novel biomimetic routes to facilely self-assemble tunable, multifunctional materials. Here, we use synchrotron small-angle X-ray scattering and electron microscopy to characterize the photonic nanostructure of 140 integumentary scales and setae from ∼127 species of terrestrial arthropods in 85 genera from 5 orders. We report a rich nanostructural diversity, including triply periodic bicontinuous networks, close-packed spheres, inverse columnar, perforated lamellar, and disordered spongelike morphologies, commonly observed as stable phases of amphiphilic surfactants, block copolymer, and lyotropic lipid-water systems. Diverse arthropod lineages appear to have independently evolved to utilize the self-assembly of infolding lipid-bilayer membranes to develop biophotonic nanostructures that span the phase-space of amphiphilic morphologies, but at optical length scales.

Self-Assembly in Phospholipid DNA - Protein Mixtures With Applications To Complex Formation in Cationic Liposome-Chromatin Systems

We prepared complexes between cationic liposomes (CL) from a mixture of the neutral (DOPC) and the cationic (DOTAP) lipids with either nucleosome core particles (NCP) or chromatin arrays, prepared by in vitro over-expression. Microscopy showed the presence of distinct globules. Fluorescence labelling of the histone protein, DNA and lipid components showed complete co-localization under many conditions at the optical length scale, while separation of the histones from the DNA and lipids was sometimes found. Cryo-EM confirmed array aggregates with excess of positively charged lipid forming ordered complexes of multilamellar lipids. For complexes with lower cationic charge (50% DOTAP and 50% neutral DOPC), there is an indication of less order. In most Cryo-EM samples the complexes seems to have a “subunit size” on the order of 100-200 nm, consistent with DLS data. Synchrotron SAXS measurements were performed. The X-ray diffraction pattern demonstrates the lamellar Bragg peaks corresponding to an inter-lamellar separation of about 6-8 nm and sometimes presence of an in-plane DNA-DNA peak. This confirms a remarkably interesting phase behaviour for the system of DNA/protein (NCP or chromatin) with lipids. The SAXS and Cryo-TEM data clearly shows the formation of multilamellar aggregates with DNA sandwiched in between. This means that the DNA and the protein histone-octamer complex of the NCP and the chromatin arrays have dissociated. Hence, the question arises where the histone proteins are located, given the demonstrated co-localization at the length scale of the multilamellar complexes (a few hundred nm). One hypothesis is that the histone proteins are partially embedded in the bilayer and partially between the DNA domains. Alternatively, a few 50-100 nm size DNA-lipid complexes cluster, with histones associated in between, a type of associative phase separation.

Optical studies of the nematic phase of an oxazole-derived bent-core liquid crystal

Various optical and dynamic light scattering studies have been conducted on the nematic phase of an oxazole-derived bent-core liquid crystal. At optical length scales and in the absence of applied fields, homogeneously aligned samples of this material, which has an oxazole heterocyclic ring in the central core, are found to behave more like a conventional straight-core nematic than a previously investigated ester-based class of bent-core molecules that have a benzene ring as the core linkage between the two arms of the bow-shaped molecule. In particular, the nematic refractive indices of the oxazole compound combine in the standard way [i.e., square root of ((2no2+ne2)/3)] to match the isotropic value throughout the nematic range, and the observed director fluctuation modes have relaxation rates comparable to those of the usual thermotropics. However, polarized light scattering data reveal evidence of weak biaxial fluctuations, and indications of electric-field-induced biaxiality are observed in the refractive index measurements.

Coherent soft X-ray magnetic scattering

(2001). Coherent soft X-ray magnetic scattering. Synchrotron Radiation News: Vol. 14, No. 2, pp. 11-19.

Synthesis of thin film photonic crystals

We describe a synthesis procedure for fabricating thin film photonic crystals at optical length scales from a ceramic slurry of nanocrystalline titania and polystyrene spheres. Self assembly of the spheres is performed simultaneously with the introduction of the dielectric background. Thin film photonic crystals are grown on glass or semiconductor substrates. The photonic crystals are characterized with both surface and cross-sectional scanning electron microscope images. The photonic crystals exhibit a reflection peak and transmission dip at the wavelength of the first stop band.

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.