Stepwise three-photon absorption and vacuum ultraviolet fluorescence in neodymium-doped lithium yttrium fluoride

In this paper, stepwise three-photon excitation of the 4f²5d configuration in Nd³⁺:YLF is investigated with nanosecond laser pulses. Three-photon excitation of the 4f²5d configuration is first conducted. The first step populates the ⁴G7/2 + ⁴G9/2 + ²K13/2 levels, the absorption spectra of which overlap at room temperature. The second step excites the ²F(2)5/2 level, and finally, a third photon excites the 4f²5d configuration. In the experiment, 8-ns pulses from a frequency-doubled Q-switched Nd:YAG laser (λ = 532 nm) are focused into a Nd³⁺:YLF crystal along the a-axis (E∥c) resulting in an estimated laser intensity of 0.1 GW/cm². The fluorescence spectrum shows strong UV and VUV fluorescence in the range from 180-280 nm. The spectrum can be ascribed to transitions from the bottom of the 4f²5d band to the lower energy levels of the 4f³ configuration. The three-photon excitation efficiency of the 4f²5d configuration at 300 K is estimated to approx.10^-4. However, the excitation is far from being saturated at 0.1 GW/cm² and scaling the laser intensity to 1 GW/cm² yields a potential excitation efficiency of about 10%.