D.J. Brink

Structural colours from the feathers of the bird Bostrychia hagedash

Iridescence in the bird Bostrychia hagedash is interesting in the sense that instead of the usual single, fairly narrow, reflection peak seen for many birds and insects, up to six strong peaks were found covering the spectral range from the near ultraviolet (UV) to the near infrared (IR). By recording reflectance spectra over this range and comparing it to model calculations based on a structural analysis using electron microscopy, the mechanism for iridescence as well as the optical constants of the materials comprising the coloured feathers, could be established. It was discovered that iridescence is mainly based on a relatively thick but uniform keratin outer layer on the feather barbules. In contrast to iridescence in most other birds, the melanin-containing platelets inside the barbules only serve to define the layer thickness of the keratin without contributing to the iridescence in any other way. Unlike previous work where keratin was assumed to be transparent, it was established that both keratin and melanin are absorbing and that keratin exhibits an anomalous dispersion in the green, blue and near UV.

Unusual coloration in scarabaeid beetles

In this paper we investigate the reflection of circularly polarized light from the exocuticle of the scarabaeid beetle Gymnopleurus virens. Reflection spectra are deeply modulated, exhibiting a number of relatively narrow well-defined peaks, which differ from previously studied specimens. By comparing model calculations and electron microscopy work with the recorded spectra, we can propose the presence of specific structural defects responsible for the unusual spectra.

Iridescent colors on seashells: an optical and structural investigation of Helcion pruinosus

Many animal species display exceptionally bright iridescent coloration caused by interference or diffraction from a periodic surface microstructure. Although many mollusks are colored, only few utilize such a form of structural coloration. We are not referring to the well-known pearly appearance that is due to the nacreous layer found on the inner surfaces of most shells, but to small brightly colored spots on the outer surface. The Helcion pruinosus is one such example. We show by optical measurements and scanning electron microscopy (SEM) that coloration in this shell is indeed of a structural nature based on thin-film interference from a layered quarter-wave stack tilted by approximately 24 degrees with respect to the outer surface. The microstructure is embedded in the transparent top layer of the shell approximately 50 microm below the surface. By comparing the SEM and optical measurements, we were able to establish that the layered structure is made from a birefringent material (crystalline aragonite) giving slightly different spectral peaks for S- and P-type reflections.

An investigation of green iridescence on the mollusc Patella granatina

In this paper we investigate the relatively rare phenomenon of iridescence on the outer surface of seashells (not the well known pearly inner surfaces). Using reflection spectroscopy and scanning electron microscopy we show that rows of iridescent green spots on the mollusc Patella granatina are caused by a thin-film stack buried about 100 µm below the rough outer surface of the shell. The high-density layers in the stack seem to be made of crystalline aragonite, but according to Raman spectroscopy and ellipsometry measurements the low-density layers as well as the bulk of the shell wall are a mixture of porous aragonite and organic materials such as carotenoids.

Confined blue iridescence by a diffracting microstructure: an optical investigation of the Cynandra opis butterfly.

When illuminated and viewed along certain well-defined directions, segments on the wings of the butterfly Cynandra opis shows a striking violet-blue to blue-green. We quantify the spectral and the directional properties of these areas of the wings of the insect. Electron microscopy shows that wing scales from these iridescent regions of the wings contain two gratinglike microstructures crossed at right angles. Application of the diffraction theory, as formulated by the Stratton-Silver-Chu integral, to the microstructure can explain all the important features observed experimentally.

Ellipsometry of diffractive insect reflectors

Scales on the wings of certain insects, such as Trichoplusia orichalcea, exhibit a surface micro structure resembling a fine diffraction grating. Diffraction of incident light by this structure is responsible for many of the optical properties of the wings of this moth, such as the metallic yellow color and the almost-specular reflection and polarization properties of the scattered radiation. It is shown that by the use of null ellipsometry the polarization characteristics can be used to obtain the optical constants of the scale material. Theoretical considerations and suitable experimental conditions are discussed and evaluated.

Optical diffraction by the microstructure of the wing of a moth

On the wing of the moth Trichoplusia orichalcea a prominent, apparently highly reflective, golden spot can be seen. Scales from this area of the wing exhibit a regular microstructure resembling a submicrometer herringbone pattern. We show that a diffraction process from this structure is responsible for the observed optical properties, such as directionality, brightness variations, polarization, and color.

Process Optimisation for 4H-SiC MOSFET Applications

We report on 4H-SiC MOSFET devices implemented on p-type <11-20>-oriented epitaxial layers, using a two-step procedure for gate oxide formation. First is a thin, dry, thermal SiO2 layer grown at 1050°C for 1 hour. Next, is a thick (50 nm) layer of complementary oxide deposited by PECVD using TEOS as gas precursor. With respect to the standard thermal oxidation process, this results in much improvement of the field effect mobility. For the best samples, we find a peak value in the range of 330 cm2/Vs while, on the full wafer, an average mobility of about 160 cm2/Vs is found. Up to now, this is one of the best results ever reported for 4H-SiC MOSFETs.

Optical Properties of as-Grown and Proton Irradiated ZnO

Conventional photoluminescence spectroscopy (PL) shows several high energy optical transitions within the region 3.072 to 3.4345 eV from as-grown ZnO excited with a 257 nm UV line. Performing inelastic light scattering experiments (ILS), we probed some previously reported modes. Proton implantation caused drastic changes in the excitonic region and generates a broad band centered at about 650 nm (1.9eV) In addition, we observed from both sides of the ZnO sample implantation-induced high-energy vibrational modes centered at about 1350 and 1600 cm -1 (0.167 and 0.198 eV, respectively).

Depth profiling of high-energy hydrogen-implanted 6H-SiC

The results of implanting silicon carbide with a 1-MeV proton beam at a dose of 1 x 10(17) cm(-2) are presented. Using high-resolution confocal Raman spectroscopy, we analyzed the depth profile of the implantation damage before and after thermal annealing. When it is applied to a high-refractive-index medium, such as SiC, this technique requires careful manipulation to ensure the correct interpretation of results. To this end we discuss a simple ray-tracking model that includes the effects of additional spherical aberration and of the Gaussian intensity profile of the excitation beam. In addition, infrared reflectance measurements show evidence of a well-defined step in the refractive-index profile at the expected implantation depth.