Biomedical Engineering Reference
In-Depth Information
45°
90°
135°
45°
90°
135°
180°
225°
270°
315°
180°
225°
270°
315°
(a)
(b)
FIGURE 25.8
White.illuminated.microscope.images.of.an.individual.scale.from. E. magniicus while.it.is.rotated.under.static.
crossed.polarizers..Images.shown.for.(a).a.blue.scale.from.approximately.0°.to.315°.at.approximately.45°.incre-
ments,.and.(b).a.green.scale.at.similar.orientations..In.both.sets.it.is.possible.to.see.domains.that.fade.in.and.out..
While.the.majority.of.the.blue.domains.in.both.scales.fade.in.this.manner,.there.is.also.an.orange.region.clearly.
visible.in.the.upper.region.of.the.(a).270°.that.also.changes.in.intensity.
of.incident.light.by.90°..When.the.sample.was.rotated.(Figure 25.8),.the.relected.intensity.was.
observed.to.fade.in.and.out.for.the.majority.of.the.domains.at.90°.intervals..This.demonstrates.
that.the.relection.was.caused.by.a.double.relection.within.the.crystal..This.requires.that.two.
optical.planes.within.the.crystal.relect.at.right.angles.to.the.incident.light.and.are.oriented.at.
right.angles.to.one.another..This.way,.the.relected.light.undergoes.a.second.relection.from.
the.other.plane.and.is.relected.with.the.polarization.rotated.by.90°..While.this.effect.is.compa-
rable.to.those.observed.in.the. Papilio .butterly, 16 .the.continuous.nature.of.the.photonic.crystal.
allows.for.a.more.uniform.relection.that.is.only.subject.to.the.domaining.of.the.scales.
There.appears.to.be.no.change.in.hue.as.the.sample.is.rotated,.with.only.the.intensity.
appearing. to. change.. Other. domains. within. the. scale. can. be. observed. through. crossed.
polarizers.to.relect.over.the.full.rotation,.thereby.indicating.that.some.domains.consis-
tently.scatter.light.with.a.converted.polarization.state..As.there.is.only.one.structure.type.
that. has. been. observed. to. display. a. color,. it. is. unlikely. that. there. are. domains. with. a.
signiicantly.different.structure.that.would.be.responsible.for.this.feature..It.is.therefore.
more.credible.that.these.observations.are.caused.by.the.scattering.of.light.from.the.struc-
ture.with.a.random.polarization,.or.that.the.rotational.symmetry.is.of.a.higher.order..By.
relecting.light.with.a.randomized.polarization.state,.the.domain.will.consistently.relect.
light.of.all.polarizations,.regardless.of.the.illumination..Alternatively,.by.having.a.higher.
degree.of.rotational.symmetry,.any.right-angle-oriented.planes.capable.of.producing.the.
double-bounce. effect. will. be. evident. at. most. angles,. such. that. the. variation. in. intensity.
appears.constant.
By. taking. optical. images. of. the. samples. immediately. before. the. deposition. of. the. gold.
layer,.it.was.possible.to.match.the.structure.to.the.colors.in.the.regions.under.observation..In.
a.scale.from. E. schoenherri pettiti ,.which.was.imaged.with.both.optical.microscopy.and.SEM,.
shown.in.Figure 25.9,.it.is.demonstrated.that.the.observed.colors.are.not.directly.linked.to.
a.change.in.the.lattice.dimensions..In.this.example,.there.are.three.different.colors.present.
in.a.single.scale..The.periodicities.in.each.section.were.measured.using.the.image.process-
ing.package.ImageJ.over.a.number.of.periods.to.reduce.the.random.error.natural.variation..
Multiple.measurements.were.taken.from.the.image,.and.the.maximum.variations.in.these.
measurements. are. quoted. 25 . The. periodicity. seen. in. the. yellow. region. (Figure  25.9b). was.
measured.to.be.283.±.24.nm,.the.periodicity.within.the.green.area.(Figure 25.9c).was.mea-
sured.to.be.288.±.28.nm,.and.within.the.blue.region.(Figure 25.9d).the.periodicity.was.330.±.
31.nm,.with.the.air.gap.diameter.in.each.region.being.150.±.30.nm.
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