Electronic changes of refractive index in intensively pumped Nd:YAG laser crystals

Refractive index changes accompanying changes in population of electron levels of Nd³⁺ ions in a Nd:YAG laser crystal under intensive diode and laser pumping have been studied using a highly sensitive polarization interferometer. An electronic change of the index due to population of the ⁴F₃2/ level is measured to be high (comparable with the thermal component) in the crystal under QCW diode-stack pumping (at 808 nm). The electronic component increased dramatically under pumping by an additional laser (at 266 nm) due to population of a higher-lying level ²F(2)₅2/. Analytical estimation reveals a predominating contribution of the well-allowed 4f-5d inter-shell transitions in polarizability of the excited levels ⁴F₃2/ and ²F(2)₅2/ both at a testing wavelength of 633 nm and at a wavelength of the strongest laser transition 1064.2 nm.