Biomedical Engineering Reference
In-Depth Information
resolution.of.the.(N-V)
−
.centers.in.bulk.diamond..The.width.of.the.image.is.reported.as.
small.as.7.6.nm,.which.is.far.below.the.diffraction.limit.
Han.et.al..have.explored.other.long-lived.dark.states.of.the.(N-V)
−
.center.for.the.GSD.
microscopy. (Han. et. al.. 2010).. A. red. doughnut-shaped. laser. beam. at. the. wavelength. of.
638.nm.is.applied.on.the.bulk.diamond.sample.to.excite.the.(N-V)
−
.center.from.
3
A.to.
3
E.
and.then.eficiently.transfer.the.populations.to.a.metastable.dark.state,.thereby.depleting.
the. ground. state.. An. additional. blue. excitation. laser. beam. is. then. co-aligned. with. the.
green.Gaussian.laser.beam,.with.the.wavelength.of.592.nm,.to.improve.the.luorescence.
intensity..Compared.with.that.of.the.STED,.the.laser.power.of.the.doughnut.beam.used.is.
much.reduced.(down.to.10.mW)..The.resolution.can.be.as.high.as.12.nm.under.low-power.
laser.excitation,.which.is.applicable.to.live.cell.imaging.
A.unique.feature.of.the.(N-V)
−
.center.is.that.it.has.a.magnetically.sensitive.ground.state.
with.a.spin-zero.level.and.two.degenerate.spin-one.levels,.as.illustrated.in.Figure.25.10..Due.
to.the.presence.of.diamond.crystal.ield,.the.spin-zero.and.spin-one.levels.are.separated.
by. a. microwave. transition. of. 2.87.GHz,. which. can. be. manipulated. by. an. electron. spin.
resonance.(ESR).microwave.source.(McGuinness.et.al..2011)..Similar.to.GSD,.a.low-power.
doughnut-shaped. laser. beam. in. combination. with. an. ESR. technique. has. been. applied.
to.achieve.super-resolution.imaging.of.the.(N-V)
−
.center.in.bulk.diamond.(McGuinness.
et. al.. 2011).. Optical. cycling. of. the. population. between. the. ground. state. and. the. excited.
state. are. spin-conserving,. but. the. intersystem. crossing. rates. to. an. intermediate. singlet.
state.are.strongly.spin-dependent..The.intersystem.crossing.from.the.
m
s
.=.±1.excited.states.
to. the. intermediate. singlet. state. is. much. higher. than. the.
m
s
.=.0. excited. state.. Moreover,.
when.trapped.in.the.intermediate.singlet.state,.the.(N-V)
−
.center.cannot.undergo.optical.
cycles. and. remains. dark. for. 250.ns.. Hence,. the. luorescence. due. to. excitation. of. the.
m
s
.=.±1.ground.state.is.weaker.than.that.of.the.
m
s
.=.0.ground.state..Finally,.the.population.
returns.from.the.intermediate.singlet.state.to.the.ground.state.by.nonradiative.decay.and.
preferentially. ends. up. in. the.
m
s
.=.0. ground. state,. leading. to. a. strong. polarization. of. the.
electron. spin. under. optical. excitation. (Jelezko. and. Wrachtrup. 2006).. Taking. advantage.
of.this.spin.property,.Maurer.et.al..irst.polarize.all.(N-V)
−
.centers.into.the.
m
s
.=.0.ground.
state. by. using. a. focused. Gaussian. laser. beam. and. drive. the. ESR. spin. transitions. to. the.
m
s
.=.±1.ground.state.(McGuinness.et.al..2011)..Then.an.optical.doughnut.beam.is.applied.
to.selectively.re-polarize.the.spins.of.the.nearby.region..Hence,.the.(N-V)
−
.centers.located.
at.the.doughnut.region.of.high.intensity.are.optically.pumped.to.the.
m
s
.=.0.ground.state,.
whereas. an. (N-V)
−
. center. located. at. the. zero-intensity. region. remains. unaffected. and.
maintains. its. original. level.. The. spin. state. is. inally. determined. by. conventional. optical.
readout.of.the.luorescence..Since.the.(N-V)
−
.centers.that.are.not.re-polarized.are.darker.
than.those.re-polarized,.scanning.the.sample.with.respect.to.the.beams.and.repeating.the.
earlier.procedure.allows.sub-diffraction.imaging.of.the.centers.in.the.bulk.diamond..This.
technique.can.be.further.applied.for.high-resolution.imaging.of.FNDs.in.biological.cells.
25.4.4 Optically Detected Magnetic resonance imaging
Having.the.spin.states,.the.(N-V)
−
.center.is.amenable.to.ultrasensitive.detection.under.ambient.
conditions.with.optically.detected.magnetic.resonance.(ODMR).(Balasubramanian.et.al..2008,.
Maze.et.al..2008)..As.the.spins.of.the.center.are.very.sensitive.to.their.magnetic.environment,.
even.a.slight.variation.in.the.magnetic.ield.leads.to.a.shift.in.the.spin.resonance.frequency,.
which.can.be.detected.based.on.the.luorescence.signal..Balasubramanian.et.al..have.scanned.
a. nanoscale. magnetic. tip. over. an. immobilized. ND,. hosting. a. single. (N-V)
−
. center,. and.
located.the.position.of.the.particle.with.nanometric.precision.(Balasubramanian.et.al..2008)..
