The production of high-quality low-defect single-domain flexible polymer opals which possess fundamental photonic bandgaps tuneable across the visible and near-infrared regions is demonstrated in an industrially-scalable process. Incorporating sub-50nm nanoparticles into the interstices of the fcc lattice dramatically changes the perceived color without affecting the lattice quality. Contrary to iridescence based on Bragg diffraction, color generation arises through spectrally-resonant scattering inside the 3D photonic crystal. Viewing angles widen beyond 40 masculine removing the strong dependence of the perceived color on the position of light sources, greatly enhancing the color appearance. This opens up a range of decorative, sensing, security and photonic applications, and suggests an origin for structural colors in Nature.

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A promising fabrication route to produce absorbing flexible photonic crystals is presented, which exploits self-assembly during the shear processing of multi-shelled polymer spheres. When absorbing material is incorporated in the interstitial space surrounding high-refractive-index spheres, a dramatic enhancement in the transmission edge on the short-wavelength side of the band gap is observed. This effect originates from the shifting optical field spatial distribution as the incident wavelength is tuned around the band gap, and results in a contrast up to 100 times better than similar but nonabsorbing photonic crystals. An order-of-magnitude improvement in strain sensitivity is shown, suggesting the use of these thin films in photonic sensors. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2032590͔ Synthesis of three-dimensional ͑3D͒ photonic crystals based on synthetic opals has been well developed over the past decade, 1-3 with a particular focus on in-filling with highrefractive-index media to create true 3D band gaps.1,4 However, the application of 3D photonic crystals has been restricted compared to the advanced technology built on twodimensional photonic crystals fabricated from patterned planar waveguides.5-7 Here we present an alternative approach to fabricating useful photonic crystals based on the extrusion self-assembly of low-contrast flexible photonic nanomaterials. Tuning of the band gaps with angle and strain is clearly observed in these films. However, as is typical in polymer-based photonic crystals, the dielectric nanostructures exhibit a low on/off contrast at the edges of the Bragg scattering peaks making their incorporation into sensors problematic. We show that by introducing an absorbing material into the surroundings of the polymer spheres ͑which is very easy to achieve in this process͒, the transmission contrast can be increased by a factor of Ͼ100, leading to prospective applications in compact vibration and thermal sensors. We analyze this behavior in terms of the optical field distribution on either side of the band gap, and show that absorbing 3D photonic crystals inherently improve on the performance of absorbing one-dimensional ͑1D͒ photonic crystals ͑Bragg mirrors͒.Our sample precursors consist of hard polystyrene ͑PS͒ cores, covered by a poly͑methylmethacrylate͒ ͑PMMA͒ interlayer, and a polyethylacrylate ͑PEA͒ shell. Self-assembly processes occur during the shearing by uniaxial compression of the precursor melt resulting in fcc crystallization of the PS-PMMA cores, with the soft PEA shell material filling the spaces between the PS-PMMA lattice sites thus forming an elastic film. Hence the ͑111͒ plane of the fcc lattice is parallel to the sample surface. A detailed description of the precursors and the manufacturing is reported elsewhere. 8 The final samples used here are disks with a diameter of 10 cm and a thickness of around 250 m ͓Fig. 1͑a͔͒.Angle-dependent reflection measurements were carried out to identify the final lattice pitch and average refractive index. Monoch...

Elastomeric films were prepared, by uniaxial compression, from core‐shell latex particles with a rigid thermoplastic core and a soft elastomeric shell. The films are rubbery, yet well ordered. The latex spheres form a fcc lattice, the 111 plane of which is oriented parallel to the film plane. This colloidal crystallinity is also found in opals. The films are colored due to selective reflection of the light wavelength corresponding to the lattice spacing, but they are not opaque. The crystallization process is surprisingly simple and fast because the core‐shell latex spheres flow in the melt practically like a regular polymer melt. Under uniaxial compression, this flowing melt deposits crystalline layers of the latex spheres along the plates of the press. The order in the films was characterized with transmission electron microscopy and UV spectroscopy. Deformation of the films results in a shift of the reflected light wavelength.

Synthesis of 3D opaline photonic crystals has developed into a standard procedure during the last decade. [1][2][3][4][5] However, the conventional methods suffer from multiple drawbacks, with cracking and polycrystallinity [6][7][8][9] leading to degradation of the optical properties of these photonic crystals through, e.g., scattering. These special difficulties in 3D photonic crystal fabrication have hindered the utilization of the technology in commercial applications, and such photonic information technology [10] is still in its infancy. Clearly there is a need for industrial-scale, high-yield methods for producing functional photonic crystals. A novel cost-effective large-scale technique to produce flexible opals through shear-ordering during compression, utilizing a core/shell approach based on polymers, has recently been developed [11,12] and further demonstrated to have possible applications, e.g., sensor, security, and structural color applications. [13] In this Communication we present a key analysis of the 3D rheologically derived properties of shear-ordered opaline thinfilm photonic crystals using optical tracking of the strain-induced anisotropy. Probing UV-surface diffraction combined with band-gap measurements reveals a complete picture of the unit cell changes under strain. The results demonstrate that our polymer opals consist of a coherently ordered ''super-domain'' characterized by a radial director vector and show anisotropic photonic behavior depending on the relative vectorial orientation of strain and director.Shear-ordering of colloidal suspensions has been studied extensively in recent years. [14][15][16][17] In these systems the crystal ordering is dependent on both the applied shear profile and the strength of shear, and with suitable conditions long-range ordering is achieved, possibly with some dislocations or stacking faults. Our approach relies on the compression-induced shear-flow ordering of core/shell polymer particles resulting in highly-ordered solid photonic crystals with spectacular structural color features (Fig. 1a). We start with precursor core/shell particles composed of a polystyrene-polymethylmethacrylate (PS-PMMA) core and polyethylacrylate (PEA) shell. The detailed precursor preparation is described elsewhere.[11] By uniaxially compressing the precursor powder between two heated plates (Fig. 1b), we create a viscous shear flow in the polymer melt forcing the spheres to assemble into an fcclattice. The resulting structure formed here is a circular thin (ca. 300 mm) film disk (diameter 15 cm) of low-refractiveindex-contrast fcc-crystal, with the PS-PMMA cores forming the lattice and the PEA filling the interstitial sites. In this photonic crystal film the (111) planes are oriented parallel to the compression plates. [11][12][13] The (111)-plane resonance wavelength can be tuned by varying the precursor PS-PMMA particle size, and the sample can be doped with nanoparticles leading to interesting photonic behavior. [18] In this paper we show results obtained from opals us...