Investigation of the multiphoton excitation process of the 4f 25d configuration in LiYF4 and LiLuF4 crystals doped with trivalent neodymium ion

Abstract

Ultraviolet (UV) fluorescence of Nd3+ ions induced by multistep laser excitation was investigated in Nd-doped LiYF4 and LiLuF4 crystals using a technique of time-resolved spectroscopy. The observed UV luminescence was due to transitions between the bottom of 4f25d configuration and the 4f3 states of Nd3+ ions. The first absorption band of 4f25d configuration, which starts around 56 700 cm−1, was excited by three stepwise absorptions of photons in the green (500–535 nm) from a short pulse laser excitation leading to broad emission bands in UV range (180–280 nm). An excitation band in the blue (468–486 nm) was observed due to the excitation of the second absorption band of 4f25d configuration around 63 000 cm−1 according to the photon absorption sequence: I9/24+hν(480 nm)→2G(1)9/2+hν(480 nm)→2F(2)7/2+hν(480 nm)→4f25d(second). The observed UV emissions (180–280 nm) from the bottom of the 4f25d configuration (first state) have a lifetime of 35 ns (parity allowed) and are broadband in contrast to UV emissions from 4f3 configuration, which are also present in the luminescence investigation but having a longer lifetime (8.5 μs) and structures composed of narrow lines. The excitation spectrum of fast UV luminescence exhibited different structures depending on the excitation geometry (σ or π) with respect to the c axis of the crystal. We observed two emissions from the first state of 4f25d configuration with peaks at 535 and 595 nm modifying the luminescence branching ratio of the bottom of the 4f25d configuration around 55 500 cm−1. The equivalent cross section of three and two excitation processes was estimated at 510 nm by solving the rate equations of the system under short laser excitation, which shows that is possible to have laser action under pulsed laser pumping with intensity below the crystal damage threshold.