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. Ia (3) over bard - O-h(10), a = 12.4952(1) angstrom, 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 I-5(7) -> I-5(8) Ho3+ transition, the maximum stimulated-emission cross-section sigma(SE) is 0.47 x 10(-20) cm(2) at 2080.7 nm. The gain bandwidth of Tm,Ho:CNGG at similar to 2 mu m 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. (c) 2020 Elsevier B.V. All rights reserved.

Automated summary

This study presents the growth, structure refinement, optical spectroscopy, and laser operation of a Tm3+,Ho3+-codoped disordered CNGG crystal. The crystal exhibits a random distribution of Ga3+ and Nb5+ cations over octahedral and tetrahedral sites in a cubic structure. Laser operation includes continuous wave generation and femtosecond mode-locked pulses.