Abstract
We report on the development of a novel laser crystal with broadband emission properties at ∼2 µm – a Tm3+,Li+-codoped calcium tantalum gallium garnet (Tm:CLTGG). The crystal is grown by the Czochralski method. Its structure (cubic, sp. gr. I a 3 ¯ d , a = 12.5158(0) Å) is refined by the Rietveld method. Tm:CLTGG exhibits a relatively high thermal conductivity of 4.33 Wm-1K-1. Raman spectroscopy confirms a weak concentration of vacancies due to the charge compensation provided by Li+ codoping. The transition probabilities of Tm3+ ions are determined using the modified Judd-Ofelt theory yielding the intensity parameters Ω2 = 5.185, Ω4 = 0.650, Ω6 = 1.068 [10−20 cm2] and α = 0.171 [10−4 cm]. The crystal-field splitting of the Tm3+ multiplets is revealed at 10 K. The first diode-pumped Tm:CLTGG laser generates 1.08 W at ∼2 µm with a slope efficiency of 23.8%. The Tm3+ ions in CLTGG exhibit significant inhomogeneous spectral broadening due to the structure disorder (a random distribution of Ta5+ and Ga3+ cations over octahedral and tetrahedral lattice sites) leading to smooth and broad gain profiles (bandwidth: 130 nm) extending well above 2 µm and rendering Tm:CLTGG suitable for femtosecond pulse generation.
Highlights
An ordered crystal is an ideal solid state material exhibiting lattice periodicity in all directions.The certain arrangement of atoms in the unit-cell is repeated forming a translationally invariant crystal structure in the long-range
We reveal the potential of Tm3+, Li+-codoped calcium tantalum gallium garnet (CTGG)
In the tantalum gallium garnet (Tm):CLTGG structure, each [Ca|TmO8] dodecahedron shares edges with four other dodecahedra where the shortest interatomic distance Ca|Tm―Ca|Tm is 3.8321(1) Å, and it is surrounded by four corner-sharing octahedra (Ta5+ and Ga3+ at 16a sites) and by six tetrahedra (Ga3+, Ta5+ and Li+ at 24d sites), two of them sharing edges, and the remaining four being connected by shared corners
Summary
An ordered crystal is an ideal solid state material exhibiting lattice periodicity in all directions. The structure disorder of CNGG originates from a random distribution of Nb5+ and Ga3+ cations over [B] and (C) lattice sites leading to a significant inhomogeneous broadening of the emission bands of the RE3+ dopant ions The latter are expected to replace for the divalent Ca2+ cations; the charge compensation is provided by vacancies [6] or by intentional codoping by univalent alkali ions (Na+, Li+ or their combination) [7,8] further affecting the spectral broadening [9]. Zhao et al achieved shorter pulses (67 fs, i.e., 10 optical cycles at 2083 nm) from a Tm,Ho,Li:CNGG (Tm,Ho:CLNGG) laser ML by the same SA utilizing the combined gain bandwidths from both active ions [13] There exists another disordered multicomponent garnet similar to CNGG, namely calcium tantalum gallium garnet (CTGG) [14]. Tm:CLTGG) as a broadband laser gain material at ~2 μm and beyond
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