Abstract
The multiphoton ultraviolet and visible upconversion luminescence of Tm<sup>3+</sup>Yb<sup>3+</sup> codoped ZBLAN fluoride glass as excited by a 975nm diode laser was studied. Two typical ultraviolet 290.6nm <sup>1</sup>I<sub>6</sub> → <sup>3</sup>H<sub>6</sub> and 362.0nm <sup>1</sup>D<sub>2</sub> → <sup>3</sup>H<sub>6</sub> upconversion luminescence lines were found. The careful measurement of the variation of upconversion luminescence intensity F as a function of the 975nm pumping laser power P has proven that the 290.6nm <sup>1</sup>I<sub>6</sub> → <sup>3</sup>H<sub>6</sub> and 362.0nm <sup>1</sup>D<sub>2</sub> → <sup>3</sup>H<sub>6</sub> upconversion luminescences are a six-photon and a five-photon upconversion luminescence respectively. Several visible upconversion luminescence lines at 450.5nm, 473.9nm, 648.5nm, (687.3nm, 696.2nm) and (793.5nm, 800.7nm) were found also, which result from the fluorescence transitions of <sup>1</sup>D<sub>2</sub> → <sup>3</sup>F<sub>4</sub>, <sup>1</sup>G<sub>4</sub> → <sup>3</sup>H6, <sup>1</sup>G<sub>4</sub> → <sup>3</sup>F<sub>4</sub>, <sup>3</sup>F<sub>3</sub> → <sup>3</sup>H<sub>6</sub> and <sup>3</sup>H<sub>4</sub> → <sup>3</sup>H<sub>6</sub> of Tm<sup>3+</sup> ion respectively. It has been proved that the upconversion luminescence of <sup>1</sup>G<sub>4</sub> state is a three-photon upconversion process, while that of <sup>3</sup>F<sub>3</sub> or <sup>3</sup>H<sub>4</sub> state is a two-photon upconversion process. The theoretical analysis suggests that the upconversion mechanism of the 362.0nm <sup>1</sup>D<sub>2</sub> → Tm<sup>3+</sup> upconversion luminescence is the cross energy transfer of {<sup>3</sup>H<sub>4</sub>(Tm<sup>3+</sup>) → <sup>3</sup>F<sub>4</sub>(Tm<sup>3+</sup>), <sup>1</sup>G<sub>4</sub>(Tm<sup>3+</sup>) → 1D<sub>2</sub>(Tm<sup>3+</sup>)} and {<sup>1</sup>G<sub>4</sub>(Tm<sup>3+</sup>) → <sup>3</sup>F<sub>4</sub>(Tm<sup>3+</sup>), <sup>3</sup>H<sub>4</sub>(Tm<sup>3+</sup>) → <sup>1</sup>D<sub>2</sub>(Tm<sup>3+</sup>)} between Tm<sup>3+</sup> ions, whereas the mechanism of the 290.6nm <sup>1</sup>I<sub>6</sub> → <sup>3</sup>H<sub>6</sub> upconversion luminescence is the sequential energy transfer of {<sup>2</sup>F<sub>5</sub>/2(Yb<sup>3+</sup>) → <sup>2</sup>F<sub>7</sub>/2(Yb<sub>3+</sub>), <sup>1</sup>D<sub>2</sub>(Tm<sup>3+</sup>) → <sup>3</sup>P<sub>1</sub>(Tm<sup>3+</sup>)} and {<sup>2</sup>F<sub>5/2</sub>(Yb3+) → <sup>2</sup>F<sub>7/2</sub>(Yb<sup>3+</sup>), <sup>1</sup>D<sub>2</sub>(Tm<sup>3+</sup>) → <sup>3</sup>P<sub>2</sub>(Tm<sup>3+</sup>)} from Yb<sup>3+</sup> ions to Tm<sup>3+</sup> ions. In addition, the upconversion luminescence of G<sub>4</sub> and <sup>3</sup>H<sub>4</sub> state also results from the sequential energy transfer {<sup>2</sup>F<sub>5/2</sub>(Yb<sup>3+</sup>) → <sup>2</sup>F<sub>7/2</sub>(Yb<sup>3+</sup>), <sup>3</sup>H<sub>4</sub>(Tm<sup>3+</sup>) → <sup>1</sup>G<sub>4</sub>(Tm<sup>3+</sup>)} and {<sup>2</sup>F<sub>5/2</sub>(Yb<sup>3+</sup>) → <sup>2</sup>F<sub>7/2</sub>(Yb<sup>3+</sup>), <sup>3</sup>F<sub>4</sub>(Tm<sup>3+</sup>) → <sup>3</sup>F<sub>2</sub>(Tm<sub>3+</sub>)} from Yb<sup>3+</sup> ions to Tm<sup>3+</sup> ions respectively.
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