Search

The origin of the structural colors from several different examples of the weevil and longhorn families (Curculionidae and Cerambycidae, respectively) was investigated by structural and optical characterization techniques. A range of interesting three-dimensional photonic crystal structures operating at visible wavelengths was discovered, including both disordered and ordered non-close-packed lattices of cuticular spheres and bicontinuous diamond-based architectures. The discovered photonic structures display a large variation in lattice constants and dielectric filling fractions and thereby create optical reflectance colors spanning the entire visible range. To transform these bio-polymeric photonic crystals into heat and photo-stable inorganic structures, a low-temperature bio-templating method was developed. Using organic–inorganic hybrid silica sol–gel infiltration–templation chemistry combined with acid-etching template removal, stable inverse photonic structures were fabricated. The inverse structures display good structural quality and vivid reflection properties.

We developed a simple sol−gel chemistry-based opal infiltration method for the fabrication of high-quality planar titania photonic crystals. Polystyrene synthetic opals were infiltrated with a hydrophobic and air/moisture-stable liquid titania precursor. The high stability and hydrophobicity of the titania precursor enabled a new “lift-off/turn-over” backfilling technique that after calcination gives planar titania inverse opals with a flat and completely open surface, without the need for additional processing steps to remove any excess surface coating of the backfilling material. We also compare two different infiltration strategies and their influence on the structural and photonic properties. The obtained inverse opals display excellent photonic properties as evidenced by the presence of first-, second-, and third-order Bragg reflection peaks in accordance with theoretical photonic band structure calculations.

The world of insects is full of structural colors, lending butterfly wings, bird feathers, and beetle scales their strikingly iridescent appearance. Since many of the structures behind these optical effects are so-called photonic crystals, they have recently attracted the interest of optical engineers and scientists as unique structural templates. This chapter aims to discuss recent developments in replicating biological structures into oxide-based photonic crystals operating at visible frequencies. Following a brief introduction into the physics and properties of photonic crystals, this chapter will focus on the following topics: (1) recent advances and limitations of top-down and bottom-up photonic crystal engineering routes; (2) examples and properties of biological photonic crystals; and (3) conversion of biological structures into oxide-based replicas by deposition methods (atomic layer deposition and conformal-evaporated-film-by-rotation) and sol–gel chemistry routes. The chapter will conclude with an outlook on the potential of biotemplating for achieving complete photonic band gaps at visible frequencies.

We demonstrate high resolution transmission microscopy in a conventional two-photon wide-field fluorescence microscope by exploiting nonlinear white light generation from clusters of silver nanoparticles placed beneath the specimen. Surface-enhanced two-photon luminescence occurs at nanoparticle hot spots in the form of spectrally broad, spatially confined light which can be exploited to determine the transmission properties of a sample placed on the silver nanoparticles. We demonstrate the versatility of the technique by revealing individual crystalline domains formed in the diffuse biological photonic crystals of the scales of a beetle. We can identify submicron changes between photonic crystal facets as well as the occurrence of stacked domains invisible to surface-sensitive methods. Control over wavelength, polarization, and pulse shape promises selective addressing of hot spots in nanoparticle assemblies for motionless spatial scanning of the transmission properties with subdiffraction resolution.

We investigated the photonic crystal structure inside iridescent scales of the weevil Lamprocyphus augustus. By combining a high-resolution structure analysis technique based on sequential focused ion beam milling and scanning electron microscopy imaging with theoretical modeling and photonic band-structure calculations, we discovered a natural three-dimensional photonic structure with a diamond-based crystal lattice operating at visible wavelengths. Moreover, we found that within individual scales, the diamond-based structure is assembled in the form of differently oriented single-crystalline micrometer-sized pixels with only selected lattice planes facing the scales' top surface. A comparison of results obtained from optical microreflectance measurements with photonic band-structure calculations reveals that it is this sophisticated microassembly of the diamond-based crystal lattice that lends Lamprocyphus augustus its macroscopically near angle-independent green coloration.