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Review
, 2 (2), 1-17

A Geological History of Reflecting Optics

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Review

A Geological History of Reflecting Optics

Andrew Richard Parker. J R Soc Interface.

Abstract

Optical reflectors in animals are diverse and ancient. The first image-forming eye appeared around 543 million years ago. This introduced vision as a selection pressure in the evolution of animals, and consequently the evolution of adapted optical devices. The earliest known optical reflectors--diffraction gratings--are 515 Myr old. The subsequent fossil record preserves multilayer reflectors, including liquid crystals and mirrors, 'white' and 'blue' scattering structures, antireflective surfaces and the very latest addition to optical physics--photonic crystals. The aim of this article is to reveal the diversity of reflecting optics in nature, introducing the first appearance of some reflector types as they appear in the fossil record as it stands (which includes many new records) and backdating others in geological time through evolutionary analyses. This article also reveals the commercial potential for these optical devices, in terms of lessons from their nano-level designs and the possible emulation of their engineering processes--molecular self-assembly.

Figures

Figure 1
Figure 1
Eyes in the Cambrian period. The head of Waptia fieldensis, a shrimp-like arthropod a few centimetres long, showing well-developed, stalked compound eyes.
Figure 2
Figure 2
Micrographs of the Burgess stem-group polychaete Canadia spinosa at increasing magnification—from ×10 to ×4000. The top picture shows the anterior half of the animal, the middle pictures show details of paleae (spines). The bottom picture shows the surface of a palea as removed from the rock matrix, revealing the remnants of a diffraction grating with a ridge spacing of 900 nm.
Figure 3
Figure 3
Reflection-type diffraction grating dividing white light into spectra.
Figure 4
Figure 4
Diagrammatic fibrils selected from ‘layers’ within an exoskeleton so that a spiral shape is formed. This acts as a complete ‘pitch’ (2d) of an optical system.
Figure 5
Figure 5
Part of the multilayer reflector in an 80 Ma ammonite ×10 000. The sharp horizontal lines represent the boundary layer pairs (where each pair contains a high and low refractive index layer, represented by dark and light shading, respectively).
Figure 6
Figure 6
Part of a 70 Ma ammonite that has undergone a change in chemical composition and ultrastructure during the fossilization to form ‘Ammolite’.
Figure 7
Figure 7
A preserved, flattened beetle with wing covers spread to the sides, 49 Ma, from Messel in Germany, showing original structural colours.
Figure 8
Figure 8
Elytron of an unidentified beetle from Messel, 50 Ma. (a) Light photograph of the specimen examined, embedded in rock. (b) Graphs showing the experimental optical (blue, solid line) and predicted (red, dashed line) reflectance from the elytron. (c) Transmission electron micrograph of the outer surface of a 60‐nm‐wide section of elytron, showing the fine lamination (running vertically in the picture) in a cross-section. The surface of the elytron has become wrinkled; the bend shown here represents a wrinkle. ‘Horizontal’ lines are an artefact of cutting. (d) Scanning electron micrograph of the outer surface of the elytron, perpendicular view, showing internal fine layers where outer layers are broken. Scale bars, a, 1 mm; c, 0.5 μm; d, 5 μm. From Parker & McKenzie (2003).
Figure 9
Figure 9
(a) Light rays affected by a single thin layer, such as a fly's wing, in air. The layer is shown in cross-section; the light-ray path and wave profiles are illustrated as solid lines (incoming light) and dashed lines (reflected light). (b) A narrow-band (‘ideal’) multilayer reflector and its effect on light. Reflected rays are in phase when all the layers are approximately a quarter of their wavelength in optical thickness.
Figure 10
Figure 10
Multilayer reflectors causing colour in the body and corneas of a long-legged fly (Dolichopodidae).
Figure 11
Figure 11
Micrograph of part of a putative iridophore, which causes a silver, mirror-like reflection, from the placoderm Groenlandaspis sp., ×4000 magnification.
Figure 12
Figure 12
Myctophum prolaternatum a Miocene fossil fish with bioluminescent organs (‘dots’), found near Oran.
Figure 13
Figure 13
Three ways of achieving a broad-band wavelength-independent reflector in a multilayer reflector (high refractive index material is shown shaded; Parker et al. 1998c). (a) Three quarter-wave (narrow-band) stacks, each tuned to a different wavelength, such as a ‘red’, ‘green’ and ‘blue’. (b) A ‘chirped’ stack, where layer thickness, and consequently the wavelength reflected in phase, decreases systematically with depth in the stack. (c) A ‘chaotic’ stack, where layers of different thickness are arranged randomly within the stack. Type a can be found in the herring (Denton 1970); type b in a lysianassoid amphipod (Crustacea; Parker 1999b); type c in many other silvery fishes.
Figure 14
Figure 14
The gold (‘mirrored’) beetle Anopognathus parvulus from Australia. The gold colour results from a (chirped) broad-band multilayer reflector that covers the entire body, but in unidirectional light provides limited colouration over the beetles' ‘hemispherical’ shape.
Figure 15
Figure 15
An ultraviolet photograph of a silvery fish, demonstrating that its ‘chaotic’ broad-band reflector extends into the ultraviolet (note the strong reflection where angles are accommodating).
Figure 16
Figure 16
Scanning electron micrograph of the corneal surfaces of four ommatidia from the compound eye of a 45 Ma dolichopodid fly preserved in Baltic amber, showing antireflective gratings. Micrograph courtesy of P. Mierzejewski. Scale bar, 3 μm.
Figure 17
Figure 17
Reflection measurements from photoresist material with (a) a smooth surface and (b) the fly-eye grating surface, showing both polarizations, over a range of angles of incidence with respect to the surface normal.
Figure 18
Figure 18
The zero-order grating (fine, parallel lines) on the surface of an antennule hair of Bathynomus immanis. Scale bar, 5 μm.
Figure 19
Figure 19
The corneal surface of the compound eye of the butterfly Vanessa kershawi, showing the antireflective surface of protuberances arranged in a hexagonal array. Scale bar, 1 μm.
Figure 20
Figure 20
A 110 Myr opalized bivalve (Mollusca) shell from Australia.
Figure 21
Figure 21
The weevil Metapocyrtus sp. can achieve a ‘metallic’ coloured appearance, of relatively unvarying colour, from scales positioned anywhere on its body, regardless that the light in its environment is strongly directional.
Figure 22
Figure 22
An opal analogue positioned within the flattened, coloured scales on the weevil Metapocyrtus sp. White scale bar, 1 μm.
Figure 23
Figure 23
Left: scanning electron micrograph of a ‘photonic crystal’ in the sea mouse Aphrodita sp. (Polychaeta). This shows a cross-section through the wall of a tube, or spine, constructed of nano-tubes, close-packed hexagonally. Internal diameters of the individual nano-tubes increase systematically with depth in the stack. Scale bar, 8 μm. Centre and right: light micrographs of a section of the spine showing the different colours obtained when the orientation in the horizontal plane varies (by 90°) with respect to the direction of the light source.
Figure 24
Figure 24
A wing feather of the sulphur crested cockatoo Cacatua galerita where randomly arranged air spaces within a sponge-like lattice cause scattering.
Figure 25
Figure 25
The fossil bill of Cacatua sp. from the Early to Middle Miocene of Riversleigh, Australia (left), and the skull of the extant Cacatua roseicapilla (right), showing similarities in bill morphology.
Figure 26
Figure 26
Feather of the Moa Megalapteryx (Cromwell, South Island, New Zealand). This giant, flightless bird inhabited an Alpine environment well over 1000 m above sea level.
Figure 27
Figure 27
Mammoth hair.
Figure 28
Figure 28
Diagrammatic representation of a scattering system.
Figure 29
Figure 29
The head of the Oxford dodo specimen. Above: right, lateral (external) view of the skin attached to the skull (note that the end of the bill is bare). Below: medial (internal) view of the skin from the left side of the head, without the skull (note the blue colouration near the end of the lower bill region, arrowed).
Figure 30
Figure 30
The dragonfly Orthetrum caledonicum (Libellulidae), male. The blue colouration results from Tyndall scattering.
Figure 31
Figure 31
The peacock; the subject of the first study of animal reflectors.
Figure 32
Figure 32
The iridescent effect from the first-antenna of the seed-shrimp Azygocypridina lowryi (left). Scanning electron micrograph of the diffraction grating on a single halophore (right); ridge spacing=600 nm.
Figure 33
Figure 33
Frame from a video recording of a mating pair of the cypridinid ostracod Skogsbergia sp. The male is above and has released its iridescent hairs (arrowed) from within its shell, and consequently iridescence is displayed to the female (below).
Figure 34
Figure 34
The jumping spider Cosmophasis thalassina possesses a combination of a first-order diffraction grating and an underlying broad-band multilayer reflector. The diffraction grating causes one spectral component to be selectively directed away from the direction of incident illumination. The laminar structure beneath produces a broad-band (‘white’) reflectance, and with a significant proportion of the green light being removed from the incident white light by the grating, the reflectance observed at most angles is magenta (white minus green). From Parker & Hegedus (2003).
Figure 35
Figure 35
A cross-section of a mammoth tusk, showing layered construction to enhance mechanical strength. The layers here, however, are too thick to have an optical effect, but on a smaller scale could form a multilayer reflector.
Figure 36
Figure 36
The epidermal (exoskeletal) cell of a beetle in the midst of secreting an unusual addition to a multilayer reflector, giving the structure a unique, three-dimensional quality. It is clues to the manufacturing process that are becoming the most important, at least novel, aspect of the study of animal structural colours. Magnification: ×13 000.

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