Disordered Tm3+,Ho3+-codoped CNGG garnet crystal: Towards efficient laser materials for ultrashort pulse generation at ∼2 μm.

We report on the growth, structure refinement, optical spectroscopy, continuous wave and femtosecond mode-locked laser operation of a Tm3+,Ho3+-codoped disordered calcium niobium gallium garnet (CNGG) crystal. The 2.64 at.% Tm, 0.55 at.% Ho:CNGG is grown by the Czochralski method. Its cubic structure, sp. gr. I a 3 ¯ d - O10 h , a = 12.4952(1) Å, is refined by the Rietveld method revealing a random distribution of Ga3+ and Nb5+ cations over octahedral and tetrahedral sites. The Ho3+ transition probabilities are determined within the Judd-Ofelt theory accounting for an intermediate configuration interaction (ICI). For the 5I 7 → 5I 8 Ho3+ transition, the maximum stimulated-emission cross-section σ SE is 0.47 × 10−20 cm2 at 2080.7 nm. The gain bandwidth of Tm,Ho:CNGG at ∼2 μm is > 150 nm and the thermal equilibrium decay time - 6.80 ms. The Tm3+ ↔ Ho3+ energy transfer parameters are determined. A diode-pumped Tm,Ho:CNGG microchip laser generated 413 mW at 2088.4 nm with a slope efficiency of 15.9%. A continuous wavelength tuning between 1940.3 and 2144.6 nm is demonstrated. Ultrashort pulses as short as 73 fs are achieved at 2061 nm from a Tm,Ho:CNGG laser mode-locked by a GaSb semiconductor saturable absorber mirror at a repetition rate of 89.3 MHz. • Tm3+,Ho3+-codoped disordered calcium niobium gallium garnet (CNGG) crystal. • Cubic structure, sp. gr. I a 3 ¯ d - O10 h , a = 12.4952(1) Å, is refined by the Rietveld method. • Transition probabilities, emission cross-section and Tm3+↔Ho3+ energy-transfer. • Continuous-wave Tm,Ho:CNGG laser is tuned between 1940.3 and 2144.6 nm. • Tm,Ho:CNGG laser is mode-locked by a GaSb-based SESAM generating 73 fs pulses. [ABSTRACT FROM AUTHOR]