Research 3: S Janssens

1. Nanocrystals for optical
amplification
S. Janssens1,2, G. Williams2, D. Clarke1 and S. G. Raymond1
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Industrial Research Ltd, P.O. Box 31310, Lower Hutt, New Zealand.
Victoria University, P.O. Box 600, Wellington, New Zealand.
2
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2. 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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3. Nanoparticles
Rare earth ions: Quantum dots:
• Used in many applications: • Still being developed:
• Optical amplifiers (EDFA) • Optical amplifiers
• Lasers • Lasers
• Phosphors • Phosphors
• Scintillators • Bio-markers
• Etc. • Etc.
• Luminescence of trivalent rare • Luminescence characterised
earth characterised by: by:
• Long lifetimes (μs-ms) • Short lifetimes (ns-μs)
• Low oscillator strength • High oscillator strength
• Narrow emission bands • Broad emission bands
• Independent of size • Tuneable emission
• Problems:
• Auger recombination
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• Transient absorption

4. Quantum dots
• 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
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5. 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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6. Structural Characterization
Sample x x Particle size
(nominal) (EDS) (nm) • Zinc-blende structure
A 0.96 0.69 3.07
• Size XRD and TEM comparable
B 0.98 0.65 2.3
C 0.980 0.78 4.65 • PMMA composites 0.25% wt
D 0.992 0.90 4.0
E 0.993 0.91 5.11
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F 1 1 4.48

7. Optical properties
• Quantum confined states
• Scattering and absorption in
PMMA composites
• Blue shift with sulfur concentration
• Composition effect > size effect
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8. 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-1nrad )-1
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9. 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 9

10. BaMgF4 nanoparticles
• BaMgF4 ferroelectric crystal
• 2nd order nonlinear material
• Doped with luminescent ions
• Synthesised using reverse microemulsion
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11. XRD and TEM
Orthorhombic BaMgF4
Scherrer equation → 12 nm
Clusters → 0.5-2 μm long and 0.2-0.3 μm wide
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Rods → 50-80 nm long and 10-15 nm wide

12. Clustering
• Particle have permanent dipole moment
• Align along electric field lines of dipole
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13. Optical properties
• Doped with luminescent ions
-TM ions
-RE ions
• Good luminescence
• QY Eu3+ 45% at RT
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14. 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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15. 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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16. Thanks for your attention
Questions?
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