Cr³⁺ doped ZnGa₂O₄ micro-flowers as stable saturable absorber media

Abstract Micro-flowers of Cr 3+ doped ZnGa 2 O 4 , a spinel compound with high chemical and thermal stability is synthesized by hydrothermal method. Flower-like structures in the micrometer range made up of secondary nanoflakes are evident from the field emission scanning electron microscope images. X-ray photoelectron spectroscopy studies confirm the presence of Cr 3+ dopants. The steady-state optical characterizations show higher absorbance of the samples in the ultraviolet (UV) region. Due to introduction of the d–d electron transition states of Cr 3+ ions, ZnGa 2 O 4 :Cr 3+ exhibits a prominent red near infrared (NIR) luminescence. Optical nonlinearity studies show that Cr 3+ doping changes the nonlinear absorption from reverse saturable absorption to saturable absorption. This is due to the introduction of an excitation band owing to 4 T 1 - 4 A 2 transition of Cr 3+ ion.