Quenching of Fluorescence and Broadband Emission in Yb3+:Y2O3 and Yb3+:Lu2O3

3rd Laser Ceramics Symposium : International Symposium on Transparent Ceramics for Photonic Applications (8-10 October 2007)

1 Institute for Laser Science, University of Electro-Communications, Tokyo, Japan

2 Institute for Laser-Physics, University of Hamburg, Germany

J.-F. Bisson1, S. T. Fredrich-Thornton1 , 2, D. Kouznetsov1, K. Ueda1

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.

Quenching of Fluorescence and Broadband Emission in Yb3+ and Yb3+:Lu2O3

1 Institute for Laser Science, University of Electro-Communications, Tokyo, Japan

2 Institute for Laser-Physics, University of Hamburg, Germany

J.-F. Bisson1, S. T. Fredrich-Thornton1 , 2, D. Kouznetsov1, K. Ueda1

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We studied the influence of temperature on the fluorescence ofYb:doped materials.

The samples were illuminated both LD and CO2 laser.

The non-radiative quenching dramatically increases when thetemperature increases.The increase of heat generation with temperature in the case of pumping with LD shows the avalanche-like behavior.

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Observation of Broadband Emission

• Pump wavelength:

940 nm fiber-coupled LD

• Pump spot size:

200 μm diameter

• highly doped

20%Yb:YAG

15%-20%Yb:Y2O3

02

Evolution of Emission at Various Wavelengths

• 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

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:

τσ

ω=

abs

ppumpsat,

hI

Isat, pump= 65 kW/cm2

02

Transparency vs Time

sample632.8 nm HeNe laser

Si photodiode

Pump 940 nm (4.5 W on 200-μm spot during 8s,i.e.,~14kW/cm2)

Bisson et al., Appl. Phys. Lett. 90 (20), 201901, 2007, Fig. 1

The sudden change in emission is synchronized with a sudden drop of transmitted signal at 632.8 nm

Jump of absorption <=> jump of emissivity (Kirchhoff’s law)

Photoconductivity Experiments

VVVV

VVVV

VVVV

Vin

R

sample

oscilloscope

Copper electrodes

pump beam

1

11

−

+=

VV

RRS

in

osc

Rosc

•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

Bisson et al., Appl. Phys. Lett. 90 (20), 201901, 2007, Fig. 5

Evolution of Photocurrent

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.

Extinction of Yb3+ Luminescence20% Yb:Y2O3

Wavelength (nm)

850 900 950 1000 1050 1100 1150 1200

Absolute Power (pW/nm)

0

500

1000

1500

2000

2500

3000100 mW500 mW1 W1.5 W2 W2.3 W2.5 W

pumping wavelength (~ 940 nm) With increase of the pump power,the luminescenceincrease, then drops.

Thermal effect?

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

MM collecting fiber dia.=62.5 μm

CO2 laser

Laser diode (913 nm, 940 nm)

f=50 mm

f=60mm

Al mirror R=100 mmSample (Yb3+:Y2O3)

Diaphragm dia.=2 mm

Ando AQ-6315A OSA

Wavelength (nm)

400 600 800 1000 1200 1400 1600 1800

Collection efficiency

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Optical response of the optical system

Al mirror

Total response

1exp

25

2

−⎟⎠⎞

⎜⎝⎛

λ

λΩ=ν

Tkhc

hcRSI

b

Luminescence Signal and that we would expect from the blackbody

What is the difference in spectra between pumping and just heating?

Irradiation with a CO2 laserPumping with LD 940 nm

2 W

2.3 W

2.7 W

2.7 W: just below threshold of phase transition

10

Before the exposition After the exposition

11

view of a sample (20%Yb:Y2O3)

1mm scaleQuickTime™ and a

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Scale upThe crater

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.

ConclusionsJump of broadband absorption, emission and electric conductivityIs observed in Yb doped laser materials with concentration above15% pumped at 940nm above 5 W focused into a spot of size oforder of 200 micron. Before the jump, the quenching of luminescence at 1030nm andGreen takes place.

Emission at the energies corresponding the energy of conductionbandgap (300-400nm) should be confirmed or negated.

The spectra of emission are similar to those observed at theheating of the sample with CO2 laser. Absolute measurements ofthe total flux arw also consistent with the hypothesis of thethermal emission.

The phenomenon can be basically explained by the melting of thesurface of the sample.

Threshold versus duration.

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Pumppower, W

Repetition rate 2 Hz

Duration, ms

cv

Reduce duration

Increase duration

Solid lines: 0.8 W*(500ms)/duration And 1.1W*(500ms)/duration