Biomedical Engineering Reference
In-Depth Information
(a)
(b)
FIGURE 25.6
Electron.microscopy.images.showing.the.substructure.of.the.scales.in.
P. nireus
..The.upper.surface.has.an.array.
of. hollow. cylinders. that. act. as. a. 2D. photonic. crystal. to. enhance. emission. from. the. structure. in. the. upward.
direction..Any.emission.that.occurs.in.the.downward.direction.is.relected.back.upward.by.a.three-layer.relec-
tor.in.the.substructure..(a).SEM.image.of.the.upper.2D.photonic.crystal..(b).TEM.image.of.a.cross.section.from.
P.
sesostris
.bisecting.the.cylinders.and.showing.the.lower.structure.that.separates.the.2D.photonic.crystal.from.the.
emitter..Scale.bars:.1.μm..(From.Vukusic,.P.,.and.Hooper,.I.,.
Science
,.310,.1151,.2005..With.permission.)
light.and.the.structural.color.of.the.material..However,.recent.studies.have.demonstrated.
that.the.blue.color.is.the.result.of.a.luorescent.pigment,.and.that.both.
P. zalmoxis
.and.
P. nireus
.
contain.a.structure.that.acts.in.a.manner.similar.to.that.of.recent.LED.structures.
23,24
The.structure.in.
P. nireus
.(examined.by.electron.microscopy.in.Figure 25.6).consists.of.
two.sections..At.the.top.is.a.series.of.approximately.vertical.tubes.around.240.nm.in.diam-
eter. that. extend. 2. μm. from. the. upper. to. lower. surface. of. the. scales.. The. cylinders. are.
approximately.340.nm.long.and.were.shown.by.Fourier.analysis.to.be.not.spatially.inde-
pendent.of.one.another,.which.is.a.requirement.for.incoherent.effects.such.as.Tyndall.scat-
tering..Fluorescent.emission.occurs.in.the.2D.photonic.crystal.formed.by.the.cylinders.
At.the.bottom.of.the.structure.is.a.distributed.Bragg.relector.separated.from.the.tube.
array. by. a. 1.5. μm. gap.. Any. emission. that. is. initially. directed. downward. from. the. tube.
array.is.relected.upward.by.the.relecting.structure..This.produces.a.highly.eficient.direc-
tional.luorescent.source.
23
25.2 Three-DimensionalCrystals
Electronic.band.engineering.has.achieved.great.success.in.controlling.and.designing.the.
electronic.properties.of.materials..One.of.the.ultimate.goals.in.the.ield.of.photonic.mate-
rials.engineering.is.to.deine.completely.the.optical.properties.of.a.crystal.by.controlling.
and.designing.its.photonic.bands,.thereby.deining.its.optical.properties..The.process.of.
deining.electronic.band.gaps.is.now.well.understood,.and.advances.within.this.ield.have.
come. by. reducing. the. dimensionality. of. the. structures. from. operating. in. three. dimen-
sions.to.sizes.suficiently.small.to.remove.the.effects.of.the.extra.dimensions..In.contrast,.
advances.in.optical.systems.have.come.from.increasing.the.dimensionality,.starting.with.
simple.solids.and.multilayers,.such.as.those.used.in.thin-ilm.ilters.and.iber.optics,.and.
moving.toward.more.elaborate.systems.
The.same.is.true.in.nature;.in.the.majority.of.iridescent.insects,.the.structural.color.is.
produced.by.1D.photonic.crystals..However,.there.is.a.minority.that.has.a.common.trait.
