One-order-higher Cr⁴⁺ conversion efficiency in Cr⁴⁺:YAG transparent ceramics for a high-frequency passively Q-switched laser

The Cr4+-doped yttrium aluminum garnet (Cr4+:YAG) saturable absorber, a new generation of passively Q-switched solid-state laser material, faces a significant obstacle of low conversion rate of chromium from trivalent to tetravalent, degrading efficiency in a passively Q-switched laser. In this paper, highly transparent Cr4+:YAG ceramics were fabricated and committed to compare the laser performance with Cr4+:YAG crystals on a 1 μm passively Q-switched laser. Thanks to the grain boundary effect, the Cr4+ conversion efficiency of 0.05 at.% Cr4+:YAG transparent ceramics coated with high transparency (HT) films (T=86.46% at 1064 nm) was nine times higher than that of 0.1 at.% Cr4+:YAG single crystals coated with HT films (T=84.00% at 1064 nm). Differing from the counterpart Cr4+:YAG crystals, no absorption saturation tendency was observed for the 0.05 at.% Cr:YAG ceramics when the pump power exceeded ∼1900  mW. Furthermore, the repetition frequency reached 217 kHz for 0.05 at.% Cr:YAG ceramics, which was a three-fold factor increase from that of the corresponding single crystal. The advantages of transparent ceramics over single crystals were proved through laser performance for the first time, to the best of our knowledge. This study also provided compelling evidence for replacing single crystals with ceramics for ultrafast dynamics.