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Excitons and traps in rare-earth materials probed by a free-electron laser
Michael F ReidMichael F Reid
Jon Wells, Pubudu Senanayake Jon Wells, Pubudu Senanayake
Alex Salkeld, Roger ReevesAlex Salkeld, Roger Reeves
Giel Berden, FELIXGiel Berden, FELIX
Andries Meijerink, UtrechtAndries Meijerink, Utrecht
Chang-Kui Duan, USTC, ChinaChang-Kui Duan, USTC, China
NZIP, Wellington, October 18, 2011
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Outline
4fN, 4fN-15d, and exciton states FELIX FEL Excited state absorption with UV + IR Yb2+ in CaF2, SrF2
Marsden Fund
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Lanthanides (rare earth) materials
Generally form 3+ or 2+ ions
• Valence electrons are 4f.
• Chemically very similar since 4f electrons are close to nucleus and shielded by 5s and 5p electrons.
• N = 1..14 means optical and magnetic properties can be tuned.
• Widely used in phosphors, amplifiers, lasers, etc...
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s
sp
df
Filling of orbitals
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Lanthanides: 4fN, 4fN-15d, Excitons
4fN
No configuration shift Sharp lines Long lifetimes
4fN-15d Different bond length Broad absorption bands from 4fN
Broad emission bands Short lifetimes
Excitons
Excited electron can become delocalized, giving an excitonic state
Large bond-length change
Very broad, red-shifted, emission bands
Long lifetimes
e-
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4fN: Sharp-line spectra
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Bonds are like springs
Equilibrium
Change in electronic state can change spring constant
New equilibrium
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Quantized Vibration Version
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Conduction Band, Free Electrons, Excitons
Conduction Band
Valence Band
4f
5d
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Excitons: Can be “free”… Ours are “bound”
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Excited-state geometry: BaF2:Ce3+
Pascual, Schamps, Barandiaran, Seijo, PRB 74, 104105 (2006)BaF2:Ce3+ cubic sites.
Potential surfaces:
5d E is contracted
5d T2 is expanded
As bond length contracts 6s orbital becomes delocalized.
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E
T2
EnergyCe3+:CaF
2 4f1 5d1
Ce3+ : 4f1 5d1
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SrCl2:Yb2+: Sánchez-Sanz et al. J. Chem. Phys. 133, 114509 (2010)
Yb2+
Cl-/F-
Sr2+/Ca2+
SrCl2:Yb2+ / CaF
2:Yb2+
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SrCl2:Yb2+
4f14
4f135d (mixed)
4f136s
Sánchez-Sanz et al. J. Chem. Phys. 133, 114509 2010
bond length
4f135d (E)
30000 cm-1
Exciton state forms as excited electron
becomes delocalized and bonds shorten
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SrCl2:Yb2+
Absorption“Normal”Emission
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CaF2:Yb2+Absorption
“anomalous”Emission
4f14
4f135d
4f13+e
??
τrad
=15ms
τrad
=260μs
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FEL Excited State Absorption
UV Laser
Exciton emission
IR FEL
4f135d
4f14
4f13+e
40cm-1
τrad
=15ms
τrad
=260μs
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FELIX Nieuwegein, Netherlands
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FELIXSynchonized UV laser + FEL
UV IR
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UV + FEL + Emission Spectrometer
UVIR
Emission
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1kHz ps UV 10 Hz 6μs IRmacropulse
UV
IR
Emission
365 nm
12.1 µm825 cm-1
Lowest state τ
rad=15ms !
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10K
Time-resolved spectrum
Shift similar to temperatureProbably same emission
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12.1 µm825 cm-1
16 µm625 cm-1
Faster emission from higher exciton state
More sites radiating
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Scan IR 12.1 µm825 cm-1
16 µm625 cm-1
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Integrate over time to obtain spectrum
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Sharp lines
The sharp lines can be explained by transitions within the 4f13 hole.
Not all transitions are allowed.
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Yb3+ crystal field
Exchange Splitting
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Electron Trap Liberation?
Long-time enhancement must be trap liberation
Coulomb trap model
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Applications of TrapsX-ray storage phosphors Persistent Luminescence
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SrF2:Yb2+ Larger lattice, lower energy.
SrF2:Yb2+
CaF2:Yb2+
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CaF2:Yb2+ SrF
2:Yb2+
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Trap Liberation in SrF2:Yb2+
Effective even after exciton decay
UV IR
Exciton ESA+ Trap Liberation
Only Trap Liberation
200μs
400μs
600μs
800μs
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Conclusion
• FEL experiments give us a unique tool to investigate:• Excitonic states• Trap states
• More experiments and analysis• FEL• Synchrotron• Local laser experiments• Detailed modeling