Holmium GSAG:Cr3+:Tm3+:Ho3+ crystal laser (λ = 2.09 μm) operating at room temperature

The Czochralski method was used to synthesize a gadolinium scandium aluminum garnet (GSAG) single crystal activated with chromium (C_Cr = 2 X 10²⁰ cm ^– 3), thulium (C_Tm = 8 X 10²⁰ cm ^– 3), and holmium (C_Ho = 5 X 10¹⁹ cm ^– 3) ions. The luminescence spectra and lasing properties were investigated. The experimental results indicated efficient energy transfer from the Cr³⁺ ions ( ²E and ⁴T₂ states) to the Tm³⁺ ions ( ³F₄ state), fast cross-relaxation sharing of the excitation of the Tm³⁺ ions (³F₄ → ³H₄, ³H₆ → ³H₄), and efficient energy transfer from the Tm³⁺ to the Ho³⁺ ions (³H₄ → ³H₆Tm³⁺, ⁵I₈ → ⁵I₇Ho³⁺). A GSAG:Cr³⁺:Tm³⁺:Ho³⁺ crystal in the form of a rod 4 mm in diameter and 36 mm long (with the length of the illuminated region 31 mm) was subjected to flashlamp pumping resulting in lasing at 2.09 μm. When the reflection coefficient of the exit mirror was R = 75% and the pump pulse energy was 100 J, the output radiation energy was 1.4 J, the differential efficiency amounted to 1.9%, and the threshold pump energy was less than 20 J. Such a laser may be useful in those cases when a high pulse repetition frequency is required.