Absorption and generation of light with silicon nanocrystals in SiO 2 Amsterdam Master of Physics...
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Transcript of Absorption and generation of light with silicon nanocrystals in SiO 2 Amsterdam Master of Physics...
Absorption and generation of light with silicon nanocrystals in SiO2
Amsterdam Master of Physics Symposium 2008
Dolf Timmerman
Van der Waals-Zeeman InstituteUniversity of Amsterdam
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Silicon
Indirect Bandgap
Ene
rgy
Momentum
Valence Band
Conduction Band
Phonon assisted transition
Silicon is a poor light-emitter
Indirect bandgap
Transitions need to overcome Δk
Radiative transitions phonon assisted
Amsterdam Master of Physics Symposium 2008
Photonics
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Nanocrystals
• Bandgap modification induced by quantum confinement
• Bands quantized energy levels• Relaxation of k-vector conservation rule, in
relation to indirect bandgap (Heisenberg)• Tuning optical properties
Silicon
4.3 nmSi NC
Amsterdam Master of Physics Symposium 2008
SampleV
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SiO2
3.1 nm
Optical active layer containing silicon nanocrystals (and erbium)
Sample preparation, Kobe University
2 μm
Si - NC
Amsterdam Master of Physics Symposium 2008
σ = 14%
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ND:YAG Laser OPO
Spectro-meter
Sample
λ = 420 – 660 nmλ = 354 nm
5 ns 10 HzRoom-temperature
Setup
Amsterdam Master of Physics Symposium 2008
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PL measurements
Si-NC
PL
Amsterdam Master of Physics Symposium 2008
PhotoluminescenceV
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Time integrated PL-spectrum
Amsterdam Master of Physics Symposium 2008
Erbium V
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SiO2:(Er3+ +Si NC)
NC absorbs photon
Energy transfer to erbium
Erbium emits at 1.5 μm
Si-NC
1.5 μm
Er3+
Amsterdam Master of Physics Symposium 2008
One photon in one photon out
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Energy transfer
Amsterdam Master of Physics Symposium 2008
Photoluminescence Er3+
)]exp(1[*
**)(
*
*
tNN
NNN
dt
dN
NI
PLEr
ErPL
PL
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Time integrated erbium PL flux-dependence Rate equations:
In the linear regime:
tNN PLEr *
The nr. of emitted photons is proportional to σPL
σPLis determined from PL intensity flux-dependence
Amsterdam Master of Physics Symposium 2008
NEr
NEr
Absorption
Linear absorption
Absorbed fraction:
Absorbed photons:
Emitted photons:
Define relative quantum efficiency:
AbsNh
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I0I1
0
10
I
IIAbs
PLN *
PLh
Abs
N
N
*
Amsterdam Master of Physics Symposium 2008
Relative quantum efficiency (η)V
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- For larger photon energies a second excitation mechanism takes place
Q.E. for different wavelengths in visible and near UV.
Amsterdam Master of Physics Symposium 2008
Quantum Cutting detected with Er3+V
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Si-NC
Er3+
0.8 eV
Er3+
Amsterdam Master of Physics Symposium 2008
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Si-NC
Er3+Er3+
1 in 2 out
Amsterdam Master of Physics Symposium 2008
Quantum Cutting detected with Er3+
η for Er3+V
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The quantum cutting process is less efficient for sample with larger NC-Er3+ distance
Two samples
- Identical Er3+ concentration and NC size
- Different NC concentration
- lower NC concentration larger average NC-Er3+ distance
Amsterdam Master of Physics Symposium 2008
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Amsterdam Master of Physics Symposium 2008
Relative quantum efficiency (η)
- η is constant up to a photon energy threshold of ≈ 2Eg
- For larger photon energies quantum cutting takes place.
Q.E. for different wavelengths in visible and near UV.
Eex > 2EG
Quantum cutting with Si NCsV
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Eex < 2EG
Space-separated quantum cutting (SSQC)
Nature Photonics 2, 105-109 (2008)
Amsterdam Master of Physics Symposium 2008
Conclusions Si NCs can be used to split energy quanta Erbium ions and neighboring NCs are used to detect
this Space separation in NCs gives long lifetime of excitons,
which is desired for applications
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Amsterdam Master of Physics Symposium 2008
Van
der
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ls-Z
eem
an In
stitu
te |
Uni
vers
ity o
f Am
ster
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Dol
f Tim
mer
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Amsterdam Master of Physics Symposium 2008