Quenching of Fluorescence and Broadband Emission in Yb 3+ :Y 2 O 3 and Yb 3+ :Lu 2 O 3

1. 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 for Laser Science, University of Electro-Communications, Tokyo, Japan 2Institute 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)

2. Quenching of Fluorescence and Broadband Emission in Yb3+ and Yb3+:Lu2O3 J.-F. Bisson1, S. T. Fredrich-Thornton1 , 2, D. Kouznetsov1, K. Ueda1 1 Institute for Laser Science, University of Electro-Communications, Tokyo, Japan 2Institute 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. 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.

3. 02 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

4. Bisson et al., Appl. Phys. Lett. 90 (20), 201901, 2007, Fig. 2 Evolution of Emission at Various Wavelengths Pump power =~5 W on a 200-μm spot (~14 kW/cm2) during 2.8s pulse duration. Saturation intensity: 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

5. Pump 940 nm (4.5 W on 200-μm spot during 8s,i.e.,~14kW/cm2) Si photodiode 632.8 nm HeNe laser sample Transparency vs Time 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)

6. Photoconductivity Experiments oscilloscope Rosc Copper electrodes V V V V R V V V V V V V V sample pump beam Vin • 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

7. Evolution of Photocurrent Bisson et al., Appl. Phys. Lett. 90 (20), 201901, 2007, Fig. 5 • 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.

8. Extinction of Yb3+ Luminescence With increase of the pump power, the luminescence increase, 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

9. Absolute Measurement of Emitted Power Diaphragm dia.=2 mm Al mirror R=100 mm Sample (Yb3+:Y2O3) MM collecting fiber dia.=62.5 μm f=60mm Optical response of the optical system Laser diode (913 nm, 940 nm) f=50 mm CO2 laser Al mirror Ando AQ-6315A OSA Total response

10. Luminescence Signal and that we would expect from the blackbody

11. What is the difference in spectra between pumping and just heating? Pumping with LD 940 nm Irradiation with a CO2 laser 2.7 W 2.3 W 2 W 2.7 W: just below threshold of phase transition 10

12. view of a sample (20%Yb:Y2O3) Before the exposition After the exposition 1mm scale Scale up The crater 11

13. 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.

14. 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. Conclusions

15. Repetition rate 2 Hz Threshold versus duration. Pump power, W Solid lines: 0.8 W*(500ms)/duration And 1.1W*(500ms)/duration Increase duration Reduce duration cv Duration, ms