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Nanocrystals for optical amplification

S. Janssens1,2, G. Williams2, D. Clarke1 and S. G. Raymond1

1Industrial Research Ltd, P.O. Box 31310, Lower Hutt, New Zealand.

2Victoria University, P.O. Box 600, Wellington, New Zealand.

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Nanoparticles

• Optical amplifiers• Necessary part for optical systems• Compensate losses• PMMA bands

• Why nanoparticles?• Incorporating in polymers → ease of processability• Limited scattering• Doped with luminescent ions → low phonon energy

host matrix → decreases quenching• Optical properties size dependent• Stable

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NanoparticlesRare earth ions:• Used in many applications:

• Optical amplifiers (EDFA)• Lasers• Phosphors• Scintillators• Etc.

• Luminescence of trivalent rare earth characterised by:

• Long lifetimes (μs-ms)• Low oscillator strength• Narrow emission bands• Independent of size

Quantum dots:• Still being developed:

• Optical amplifiers• Lasers• Phosphors• Bio-markers• Etc.

• Luminescence characterised by:

• Short lifetimes (ns-μs)• High oscillator strength• Broad emission bands• Tuneable emission

• Problems:

• Auger recombination• Transient absorption

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• Semiconductor nanocrystals

• Properties size and shape dependent– Exciton bohr radius > QD radius– Quantum confinement of e and h in 3 dimensions– Discrete atomic like energy levels– Larger bandgap

Quantum dots

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Quantum dots

• CdSxSe1-x

• Tunable– Size – Composition– x larger at surface

→ gradient • h confined in core • Overlap e and h

wavefunction → splitting dark-bright excitons

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Structural Characterization

Sample x (nominal)

x(EDS)

Particle size(nm)

A 0.96 0.69 3.07

B  0.98 0.65  2.3

C 0.980 0.78 4.65

D 0.992 0.90 4.0

E 0.993 0.91 5.11

F 1 1 4.48

• Zinc-blende structure

• Size XRD and TEM comparable

• PMMA composites 0.25% wt

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Optical properties

• Quantum confined states • Scattering and absorption in PMMA composites

• Blue shift with sulfur concentration

• Composition effect > size effect

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Optical properties in solution

• QY decreases with x

τexp increases than decreases

• Combined effect of increase in τrad and decrease in τnrad

• τnrad decreases due to decreasing energy barrier • longer τrad

τexp= QY τrad = (τ-1rad+τ-1

nrad )-1

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Optical properties in PMMA

• Red shift in time in PMMA• Not in solution• Forster Energy transfer• Clustering• x larger → shift smaller → transfer between QD with different composition

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BaMgF4 nanoparticles

• BaMgF4 ferroelectric crystal

• 2nd order nonlinear material

• Doped with luminescent ions

• Synthesised using reverse microemulsion

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XRD and TEM

Orthorhombic BaMgF4

Scherrer equation → 12 nm

Clusters → 0.5-2 μm long and 0.2-0.3 μm wide

Rods → 50-80 nm long and 10-15 nm wide

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Clustering

• Particle have permanent dipole moment

• Align along electric field lines of dipole

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Optical properties

• Doped with luminescent ions

-TM ions-RE ions

• Good luminescence

• QY Eu3+ 45% at RT

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Poling• Transparent PMMA films

• usefull 2nd order material → non centro-symmetric

• Random orientation → Centro-symmetric → no 2nd order nonlinear effects → Aligning necessary

• Applying high electric field (~50V/μm) → Heating to Tg

• Relative change in diffraction intensities

• (h00) lines stronger, (00l) lines weaker→ partial alignment → clusters hinder alignment

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Conclusion

• QY of QD ~30% and decreases with x• Shift in wavelength mainly due to change in composition• Clustering in PMMA composites, but still good

transparancy

• Synthesizing BaMgF4 nanoparticles• Good luminescence for doped BaMgF4 particles• Possible to partially align particles using electric field• Potential for optical amplification and EO devices

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Thanks for your attention

Questions?