Transcript 00 - University of Electro

Quenching of Fluorescence
and Broadband Emission
in Yb3+:Y2O3 and Yb3+:Lu2O3
J.-F. Bisson1, S. T. Fredrich-Thornton1 , 2, D. Kouznetsov1, K. Ueda1
1 Institute
2
for Laser Science, University of Electro-Communications, Tokyo, Japan
Institute for Laser-Physics, University of Hamburg, Germany
Quenching of the green fluorescence and avalanche-like broadband
emission is observed in highly-doped ceramics and crystals pumped
at 940 nm.
Darkening at 632.4 nm and
jump of the electric conductivity are observed.
3rd Laser Ceramics Symposium : International Symposium on Transparent Ceramics for
Photonic Applications (8-10 October 2007)
Quenching of Fluorescence and Broadband Emission in Yb3+ and Yb3+:Lu2O3
J.-F. Bisson1, S. T. Fredrich-Thornton1 , 2, D. Kouznetsov1, K. Ueda1
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1 Institute
2
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for Laser Science, University of Electro-Communications, Tokyo, Japan
Institute for Laser-Physics, University of Hamburg, Germany
We studied the influence of temperature on the fluorescence of
Yb:doped materials.
The samples were illuminated both LD and CO2 laser.
The non-radiative quenching dramatically increases when the
temperature increases.
The increase of heat generation with temperature in the case
of pumping with LD shows the avalanche-like behavior.
Observation of Broadband Emission
02
• Pump wavelength:
940 nm fiber-coupled LD
• Pump spot size:
200 μm diameter
• highly doped
20%Yb:YAG
15%-20%Yb:Y2O3
Evolution of Emission at Various Wavelengths
Bisson et al., Appl. Phys. Lett. 90 (20), 201901, 2007, Fig. 2
Pump power =~5 W on a 200-μm
spot (~14 kW/cm2) during 2.8s
pulse duration.
Saturation intensity:
I sat, pump 
 p
 abs 
Isat, pump= 65 kW/cm2
•
•
•
Signal at λ=1030 nm arises from excited Yb3+ ions
Gradual increase of emission at λ=850 nm (why?) followed by a jump at t=2.15 s
Measurable signal at λ=650 nm and λ=1250 nm after the jump of emission
02
Transparency vs Time
Bisson et al., Appl. Phys. Lett. 90 (20), 201901, 2007, Fig. 1
Pump 940 nm (4.5 W on 200-μm spot during 8s,i.e.,~14kW/cm2)
Si photodiode
632.8 nm
HeNe laser
sample
The sudden change in emission is synchronized with a sudden drop of transmitted
signal at 632.8 nm
Photoconductivity Experiments
oscilloscope
R
VVVVVV
Rosc
Copper
electrodes
sample
Vin
pump beam
•Pumping source: fiber-coupled LD at 940 nm (cw
and Q-cw) up to 25 WVin=30V, R=2MΩ
•Pump spot: 200 μm
•Sample thickness: ~200 μm
1
1

Rosc R
S
V in
1
V
Evolution of Photocurrent
photovoltage
signal 1100 nm
Bisson et al., Appl. Phys. Lett. 90 (20), 201901, 2007, Fig. 5
4
Amplitude (a.u.)
3
2
1
0
-1
0
2
4
6
8
10
12
14
Time (s)
The surge of emission is synchronized with the jump of conductivity.
Hypothesis:
–when the sample becomes liquid, it also becomes electrically conductive
–the sample resolidifies after the pump is cut, allow the process to
reproduce over thousands of shots.
16
Extinction of Yb3+ Luminescence
20% Yb:Y2O3
Absolute Power (pW/nm)
3000
pumping wavelength (~ 940 nm)
100 mW
500 mW
1W
1.5 W
2W
2.3 W
2.5 W
2500
2000
1500
With increase of
the pump power,
the luminescence
increase, then drops.
1000
500
0
850
900
950
1000
1050
1100
1150
1200
Thermal effect?
Wavelength (nm)
extinction of luminescence from Yb3+ when the pumping power reaches
2.3 W (7 kW/cm2
•Pump absorption saturation intensity is about Isat=65 kW/cm2 at
• 940 nm for the ytterbium-doped sesquioxides
•The luminescence signal reappears when the sample is cooled down
Absolute Measurement of Emitted Power
Diaphragm
dia.=2 mm
Sample
(Yb3+:Y2O3)
MM collecting
fiber dia.=62.5 μm
Al mirror R=100 mm
f=60mm
Optical response of the optical system
Ando AQ6315A OSA
Collection efficiency
1.2
Laser diode
(913 nm, 940 nm)
1.0
0.8
f=50 mm
CO2 laser
0.6
0.4
Al mirror
Total response
0.2
0.0
400
600
800
1000 1200 1400 1600 1800
Wavelength (nm)
2hc 2
I  S  R
5
1
e xp hc


k
T
b 

Luminescence Signal and
that we would expect from the blackbody
140
Amplitude (pW/nm)
120
100
BB 2530ooC
BB 2430 oC
BB 2330 C
80
2.5 W
60
40
2.7 W
2.3 W
20
0
800
976
1200
1400
Wavelength (nm)
1600
What is the difference in spectra between
pumping and just heating?
30
Pumping with LD 940 nm
30
25
20
pW/nm
pW/nm
25
2.7 W
20
Irradiation
with a CO2 laser
1.8%Yb:Y2O3
2.3 W
15
15
10
10
5
2W
5
0
800
976
1200
1400
1600
Wavelength (nm)
2.7 W: just below threshold of phase transition
0
800
1000
1200
1400
1600
Wavelength (nm)
Col 1 vs W109 9A (quenched)
Col 1 vs W110 9.3A (quenched)
Col 1 vs W111 9.4 A
Col 1 vs W112 9.5A
10
view of a sample (20%Yb:Y2O3)
Before the exposition
After the exposition
1mm scale
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Scale up
The crater
11
Hypotheses
The heating efficiency of Yb3+, i.e., the fraction of absorbed
pumping power converted to heat, increases very rapidly at high
temperature and approaches 100%, which is enough to melt the
sample.
At present, no theory exists to describe such a rapid
enhancement of non-radiative relaxation with temperature
Sudden change of the optical properties due to the melting of the
sample. The solid-> liquid phase transition is accompanied by a
jump of absorption and emissivity over a broad spectral domain,
and a jump of electrical conductivity.
Only one report of jump of emissivity of oxides, including YAG,
at the melting point: D.O. Nason et al., J. Cryst. Growth, 106,
221-226, 1990.
Conclusions
Jump of broadband absorption, emission and electric conductivity
Is observed in Yb doped laser materials with concentration above
15% pumped at 940nm above 5 W focused into a spot of size of
order of 200 micron.
Before the jump, the quenching of luminescence at 1030nm and
Green takes place.
Emission at the energies corresponding the energy of conduction
bandgap (300-400nm) should be confirmed or negated.
The spectra of emission are similar to those observed at the
heating of the sample with CO2 laser. Absolute measurements of
the total flux arw also consistent with the hypothesis of the
thermal emission.
The phenomenon can be basically explained by the melting of the
surface of the sample.
Threshold versus duration.
Repetition rate 2 Hz
Pump
power,
W
Solid lines: 0.8 W*(500ms)/duration
And
1.1W*(500ms)/duration
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Increase duration
Reduce duration
cv
Duration, ms