Abstract

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° with respect to the outer surface. The microstructure is embedded in the transparent top layer of the shell approximately 50 µm 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.

© 2002 Optical Society of America

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References

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    [CrossRef]
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    [CrossRef]
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    [CrossRef] [PubMed]
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    [CrossRef]
  7. D. J. Brink, M. E. Lee, “Confined blue iridescence by a diffracting microstructure: an optical investigation of the Cynandra opis butterfly,” Appl. Opt. 38, 5282–5289 (1999).
    [CrossRef]
  8. M. F. Land, “The physics and biology of animal reflectors,” Prog. Biophys. Mol. Biol. 24, 75–106 (1972).
    [CrossRef] [PubMed]
  9. G. M. Branch, C. L. Griffiths, M. L. Branch, L. E. Beckley, Two Oceans—A Guide to the Marine Life of Southern Africa (David Philip, Cape Town, Johannesburg, 1994), pp. 136, 138.
  10. S. P. Dance, Shells (Dorling Kindersley, London, 1992), p. 34.
  11. R. Guenter, Modern Optics, 1st ed. (Wiley, New York, 1990), pp. 98–100.
  12. S. Weiner, L. Addadi, “Design strategies in mineralized biological materials,” J. Mater. Chem. 7, 689–702 (1997).
    [CrossRef]
  13. D. R. Linde, ed., Handbook of Chemistry and Physics (CRC Press, Boca Raton, Fla., 2000).
  14. A. N. Winchell, W. Winchell, Elements of Optical Mineralogy (Wiley, New York, 1967), p. 117–118.
  15. R. M. A. Azzam, N. M. Bashara, “Polarization characteristics of scattered radiation from a diffraction grating by ellipsometry with application to surface roughness,” Phys. Rev. B 5, 4721–4729 (1972).
    [CrossRef]

1999 (1)

1998 (1)

1997 (1)

S. Weiner, L. Addadi, “Design strategies in mineralized biological materials,” J. Mater. Chem. 7, 689–702 (1997).
[CrossRef]

1995 (1)

1991 (1)

1979 (1)

D. Mossakowski, “Reflection measurements used in the analysis of structural colors of beetles,” J. Microsc. (Oxford) 116, 351–364 (1979).
[CrossRef]

1972 (2)

M. F. Land, “The physics and biology of animal reflectors,” Prog. Biophys. Mol. Biol. 24, 75–106 (1972).
[CrossRef] [PubMed]

R. M. A. Azzam, N. M. Bashara, “Polarization characteristics of scattered radiation from a diffraction grating by ellipsometry with application to surface roughness,” Phys. Rev. B 5, 4721–4729 (1972).
[CrossRef]

1960 (1)

1942 (1)

T. F. Anderson, A. G. Richards, “An electron microscope study of some structural colors of insects,” J. Appl. Phys. 13, 748–758 (1942).
[CrossRef]

Addadi, L.

S. Weiner, L. Addadi, “Design strategies in mineralized biological materials,” J. Mater. Chem. 7, 689–702 (1997).
[CrossRef]

Anderson, T. F.

T. F. Anderson, A. G. Richards, “An electron microscope study of some structural colors of insects,” J. Appl. Phys. 13, 748–758 (1942).
[CrossRef]

Azzam, R. M. A.

R. M. A. Azzam, N. M. Bashara, “Polarization characteristics of scattered radiation from a diffraction grating by ellipsometry with application to surface roughness,” Phys. Rev. B 5, 4721–4729 (1972).
[CrossRef]

Beckley, L. E.

G. M. Branch, C. L. Griffiths, M. L. Branch, L. E. Beckley, Two Oceans—A Guide to the Marine Life of Southern Africa (David Philip, Cape Town, Johannesburg, 1994), pp. 136, 138.

Branch, G. M.

G. M. Branch, C. L. Griffiths, M. L. Branch, L. E. Beckley, Two Oceans—A Guide to the Marine Life of Southern Africa (David Philip, Cape Town, Johannesburg, 1994), pp. 136, 138.

Branch, M. L.

G. M. Branch, C. L. Griffiths, M. L. Branch, L. E. Beckley, Two Oceans—A Guide to the Marine Life of Southern Africa (David Philip, Cape Town, Johannesburg, 1994), pp. 136, 138.

Brandt, W.

Brink, D. J.

Dance, S. P.

S. P. Dance, Shells (Dorling Kindersley, London, 1992), p. 34.

Friel, D. D.

Ghiradella, H.

Greenwalt, C. H.

Griffiths, C. L.

G. M. Branch, C. L. Griffiths, M. L. Branch, L. E. Beckley, Two Oceans—A Guide to the Marine Life of Southern Africa (David Philip, Cape Town, Johannesburg, 1994), pp. 136, 138.

Guenter, R.

R. Guenter, Modern Optics, 1st ed. (Wiley, New York, 1990), pp. 98–100.

Land, M. F.

M. F. Land, “The physics and biology of animal reflectors,” Prog. Biophys. Mol. Biol. 24, 75–106 (1972).
[CrossRef] [PubMed]

Lee, M. E.

M. Bashara, N.

R. M. A. Azzam, N. M. Bashara, “Polarization characteristics of scattered radiation from a diffraction grating by ellipsometry with application to surface roughness,” Phys. Rev. B 5, 4721–4729 (1972).
[CrossRef]

Möller, A.

Mossakowski, D.

D. Mossakowski, “Reflection measurements used in the analysis of structural colors of beetles,” J. Microsc. (Oxford) 116, 351–364 (1979).
[CrossRef]

Richards, A. G.

T. F. Anderson, A. G. Richards, “An electron microscope study of some structural colors of insects,” J. Appl. Phys. 13, 748–758 (1942).
[CrossRef]

Smit, J. E.

Weiner, S.

S. Weiner, L. Addadi, “Design strategies in mineralized biological materials,” J. Mater. Chem. 7, 689–702 (1997).
[CrossRef]

Winchell, A. N.

A. N. Winchell, W. Winchell, Elements of Optical Mineralogy (Wiley, New York, 1967), p. 117–118.

Winchell, W.

A. N. Winchell, W. Winchell, Elements of Optical Mineralogy (Wiley, New York, 1967), p. 117–118.

Appl. Opt. (4)

J. Appl. Phys. (1)

T. F. Anderson, A. G. Richards, “An electron microscope study of some structural colors of insects,” J. Appl. Phys. 13, 748–758 (1942).
[CrossRef]

J. Mater. Chem. (1)

S. Weiner, L. Addadi, “Design strategies in mineralized biological materials,” J. Mater. Chem. 7, 689–702 (1997).
[CrossRef]

J. Microsc. (Oxford) (1)

D. Mossakowski, “Reflection measurements used in the analysis of structural colors of beetles,” J. Microsc. (Oxford) 116, 351–364 (1979).
[CrossRef]

J. Opt. Soc. Am. (1)

Phys. Rev. B (1)

R. M. A. Azzam, N. M. Bashara, “Polarization characteristics of scattered radiation from a diffraction grating by ellipsometry with application to surface roughness,” Phys. Rev. B 5, 4721–4729 (1972).
[CrossRef]

Prog. Biophys. Mol. Biol. (1)

M. F. Land, “The physics and biology of animal reflectors,” Prog. Biophys. Mol. Biol. 24, 75–106 (1972).
[CrossRef] [PubMed]

Other (5)

G. M. Branch, C. L. Griffiths, M. L. Branch, L. E. Beckley, Two Oceans—A Guide to the Marine Life of Southern Africa (David Philip, Cape Town, Johannesburg, 1994), pp. 136, 138.

S. P. Dance, Shells (Dorling Kindersley, London, 1992), p. 34.

R. Guenter, Modern Optics, 1st ed. (Wiley, New York, 1990), pp. 98–100.

D. R. Linde, ed., Handbook of Chemistry and Physics (CRC Press, Boca Raton, Fla., 2000).

A. N. Winchell, W. Winchell, Elements of Optical Mineralogy (Wiley, New York, 1967), p. 117–118.

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Figures (9)

Fig. 1
Fig. 1

Photograph of the Helcion pruinosus mollusk showing the iridescent (green) spots arranged along rays running from the apex to the perimeter of the shell. Shells are typically between 15 and 25 mm long.

Fig. 2
Fig. 2

Sketch of the proposed thin-film microstructure embedded in the wall of the shell. Directions of incident and outgoing light rays are shown as used in the experimental arrangements. The dashed lines indicate the directions of the surface normal and of the internal normal on the thin-film stack.

Fig. 3
Fig. 3

SEM micrographs of a thin section of the shell cut perpendicular to the thin-film stack. In (a) a low-magnification (230×) picture is shown. At approximately 50 µm below the surface a slight discontinuity can be seen (indicated by arrows) where the thin-film stack is located. A indicates the outside of the shell and B is in the direction of the inside of the shell. In (b) a high-magnification (14000×) picture is shown. The slanted parallel lines are the thin-film stack responsible for the structural color of the green spots.

Fig. 4
Fig. 4

Experimental arrangements used to obtain reflection spectra. (a) Separate lenses (L2 and L3) are used to focus the incoming light and to collect the outgoing beam. This arrangement allows recordings of reflection spectra over a large range of angles. (b) A single lens is used to focus the incident light and to collect the scattered beam. This arrangement allows measurement at small angles between incident and outgoing beams, and is necessary to determine the orientation of the thin-film stack. LS, tungsten source; L1, collimating lens; M, aluminum-coated mirror; P1 and P2, linear polarizers; MON, monochromator and detection setup; S, sample.

Fig. 5
Fig. 5

Reflection spectra recorded with the setup in Fig. 4(a). The solid curves represent spectra recorded at a direction close to specular reflection from the surface. The dotted curves are spectra recorded along directions close to specular reflection from the internal thin-film stack. P, polarization, which is shown as the finely dotted curve.

Fig. 6
Fig. 6

Intensity of scattered light versus angle of incidence recorded with the small-angle setup of Fig. 4(b). Intensity values were taken at the spectral peak in the range 450–650 nm. A maximum signal is obtained when a specular reflection occurs on the internal thin-film stack.

Fig. 7
Fig. 7

Spectral position of the reflection peak versus angle of incidence recorded with the small-angle setup of Fig. 4(b). A red extreme is obtained when a specular reflection occurs on the internal thin-film stack.

Fig. 8
Fig. 8

Reflection spectra recorded with the more versatile setup of Fig. 4(a). Solid curves represent recording for a deviation angle of 35°, and the dotted curves show recordings made for a deviation angle of 75°. In the latter case the outgoing beam is at a grazing angle to the shell surface.

Fig. 9
Fig. 9

Experimental measurements of the position of the spectral peak for various deviation angles between incoming and outgoing beams. The circles are for S polarization and the squares for P polarization. The solid curves represent a best fit of Eq. (1), with the index of refraction as a parameter, to the experimental data.

Equations (2)

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m+12λ0=2nd cos ϕini;  m=0, 1, 2,,
θni=sin-1sin θnen,

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