Structural Colours through Photonic Crystals

Transcription

1 Structural Colours through Photonic Crystals R.C. McPhedran, N. Nicorovici, D. R. McKenzie, G. Rouse and M.Large, University of Sydney, L.C. Botten, University of Technology Sydney, A. Parker, V. Welch, Oxford University, V. Vardeny, M. Wohlgennant, University of Utah.

2 Overview Optical biomimetics: the study of naturally occurring optical systems to learn evolutionary approaches to design We present details of three living systems- one crawling, one swimming, one flying- using photonic bandgap designs to achieve structural colouration. The sea mouse-aphrodita sp. (Polychaeta: Aphroditidae)- incorporates a remarkably fine photonic crystal in its lower body spines. We present electronmicrographs, optical meaurements and diffraction calculations explaining its iridescence. A jelly fish- Bolinopsis- has a photonic crystal with a parallelogram unit cell. We show this results in strong angular dependence of its iridescence. A butterfly- Teinopalus Imperialis- incorporates threedimensional photonic crystal designs into its wing scales. One is a structural black, offering remarkably low reflectance in the visible. One is a structural green, for camouflage purposes.

3 Pigment versus Structural Colour Living systems have two ways of making colour: pigment and structural colour. Pigment relies on differential absorption of different wavelengths. Pigment can be difficult to make to achieve a desired colour effect, can be costly in terms of energy requirements, and is subject to photobleaching. Structural colours rely on physical structuring to achieve colour displays through interference or diffraction. Advantage: colouration is long lasting, and can be very apparent even in low light levels. Disadvantage: not all creatures have evolved the molecular assembly techniques to create structures on a similar scale to light. Remark 1: structural colours have been around a long time in Nature- e.g., fossils in Burgess shale show natural diffraction gratings. Remark 2: structural colour has obvious technological potential- e.g., permanent colour records.

4 TheSeaMouse(1) The sea mouse is a widely dispersed, bottom-dwelling sea scavenger, found at depths from a few metres to 1000 metres. Classified by Linnaeus. Figure 1: Figure 2: Two views of the sea mouse. Note the colourful lower body hairs.

5 TheSeaMouse(2) The sea mouse has bristles (spines) and finer hairs (felt). Both can exhibit strong colouration. Figure 3: Figure 4: Sea mouse felt and spines illuminated by white light.

6 TheSeaMouse(3) Electronmicrographs of a spine and a felt hair. Both contain close packed voids of water in chitin (refractive index 1.52). The spine has 88 layers of holes with a spacing of 0.51 µm. Figure 5: Figure 6: Sea mouse spine (above) and felt hair (below).

7 TheSeaMouse(4) At least one type (Pectinaria) of sea mouse creates the spines by extrusion. Presumably the holes are created by an extrusion template. The template itself must be created by molecular assembly. Figure 7: Figure 8: The extrusion factory.

8 Optical Modelling (1) The sea mouse spine was modelled by rigorous electromagnetic calculations of the reflectance and transmittance through a stack of 88 gratings, with the parameters for each layer taken from the electronmicrograph. Calculations were made for both polarizations of light: E polarization (electric vector of the incident wave along the axes of the cylinders), and H polarization (magnetic field vector of the incident wave along the axes). A multipole method was used to calculate the scattering matrix for each layer, and a transfer matrix method was used to calculate the properties of the stack. Also, a multipole method was used to calculate the photonic band diagram for an idealized model corresponding to an array whose geometric parameters correspond to the average from the elctronmicrograph.

9 Optical Modelling (2) Above: E polarization (red) and H polarization (green) normal incidence reflectance calculated for the 88 layer spine structure. Below: band diagram for the sea mouse spine structure in E polarization R λ[µm] Figure 9: kd/ π M K 1 Γ K Γ M K Figure 10: Optical modelling of the sea mouse spine.

10 Optical Measurements Optical measurements of reflectance were made at the University of Utah on a green-gold sea mouse hair. The measurements are quite difficult- the spot size of the beam used is comparable with the hair diameter, so that there is inevitable variation of the angle of incidence due to geometric effects. The results are shown in the figure below. Note the significant polarization difference in the reflectance curves, and the sharpness of the reflectance peaks. The peaks move with angle of incidence, so in fact the measurement technique may have somewhat broadened them Reflectance λ [nm] Figure 11: Microreflectance measurements on a green-gold sea mouse hair.

11 Bolinopsis (1) As our second case study, we take a jelly fish- Bolinopsis infundibulum. The antennae of this jellyfish show iridescent colour. Electronmicrographs show this colour to be due to a photonic crystal structure (photonic jelly crystals). The crystal has the structure of the oblique lattice, which means its optical properties vary strongly with incident direction. Figure 12: Two views of Bolinopsis. Note the red colour patches in the right image.

12 Electronmicrographs We show an electronmicrograph of a colour patch region of the antennae. Sample preparation for this was quite difficult. Figure 13: TEM image of a section of the antennae of Bolinopsis. We performed an optical analysis based on the region in the right of the figure. Its idealized structure is shown below. y x Figure 14: Structural model for Bolinopsis. The parameters chosen were: longer period d 1 =0.972µm, shorter period d 2 =0.664µm, angle between period axes 72, hole radius 0.307µm, refractive index of matrix 1.33, refractive index of cylinder 1.52.

13 Optical Analysis We show reflectance curves below for normally incident radiation, coming in at right angles to the longer period R λ [ nm] Figure 15: Normal incidence reflectance for the longer side of the unit cell of Bolinopsis. The red and blue curves are for E and H polarizations respectively. Note the plethora of reflectance peaks across the visible, with those for E polarization wider than those for H polarization R λ [ nm] Figure 16: Normal incidence reflectance for the shorter period. Naturally, there are fewer peaks for incidence on the shorter side, with the peak in the red a lot more prominent.

14 Band Diagrams (1) We show band diagrams below for E and H polarization, and for the longer period. kd / 2π λ [ nm] β0 d Figure 17: Band diagram for E polarization and incidence on the long side of the unit cell. kd / 2π λ [ nm] β0 d Figure 18: Band diagram for H polarization and incidence on the long side of the unit cell. The low symmetry of the unit cell and the long period result in many bands and mini-gaps, which cause the many reflectance peaks.

15 Band Diagrams (2) We show band diagrams below for E and H polarization, now for the shorter period. kd / 2π λ [ nm] β0 d Figure 19: Band diagram for E polarization and incidence on the short side of the unit cell. kd / 2π λ [ nm] β0 d Figure 20: Band diagram for H polarization and incidence on the long side of the unit cell. There are fewer bands than for the coarser period, but the diagrams are still more complicated than for hexagonal or square unit cells.

16 A Green Butterfly (1) As our final case study, we take a butterfly- Teinopalus Imperialus. The wing of this butterfly has a dappled green yellow colouration. This coluration in fact is due to a complicated threedimensional photonic crystal structure. The crystal has a labyrinthine element, which guides light onto a colour reflector, with the labyrinth taking the form of a distorted silicon lattice. Figure 21: An SEM cross section of a scale of the butterfly Teinopalus Imperialus. Note the dappled green colouration.

17 A Green Butterfly (2) The micrograph shows one view of a wing scale- note the overlying feed structure and the crystalline region. Figure 22: An SEM cross section of a scale of Teinopalus Imperialus. Careful image analysis has resulted in the reconstruction below of the photonic crystal in the wing scale. Figure 23: The photonic crystal structure in the wig scale of Teinopalus Imperialus.

18 Conclusions Nature s laboratory has assembled an interesting collection of photonic band gap structures. The refractive index contrasts available are not large enough to enable complete band gaps to be formed. Nevertheless, the structures formed have minigaps which provide striking structural colour effects. This is obviously a rich field for scientists to explore. We might also learn tricks with technological payoffs, particularly in the area of molecular assembly and permanent colour records.