Abstract

We report the realization of monocrystalline nanofilms of Nd³⁺-doped fluoride on LiYF₄ (YLF) substrates by pulsed laser deposition (PLD). Both the bulk Nd³⁺:LiYF₄ (Nd:YLF) crystal and the YLF substrates were grown by using a conventional Czochralski furnace with conventional resistive heating. The films were obtained with the pulsed laser ablation and film deposition facility, by ablating bulk YLF crystals doped with Nd³⁺ ions at 1.5% atomic concentration (at.) with 355 nm laser photons, in a UHV chamber. The average film thickness was obtained by an in situ interferometric technique, while the film optical characteristics were analyzed ex situ via laser induced fluorescence (LIF) spectroscopy following the IR excitation of the fundamental Nd³⁺ absorption band in the 800 nm range. Lifetime measurements of the Nd³⁺ ion ⁴F_3/2 manifold in the film were also performed by the temporal analysis of the fundamental Nd³⁺ ion transition. All the results were compared with those obtained, in the same experimental conditions, in a Nd:YLF bulk crystal. We deposited several films whose thickness varies between 100 and 400 nm in high vacuum (7.7×10⁻⁵ Pa) or in a 1 Pa Ne atmosphere by changing important parameters such as the ablating laser fluency, the number of ablation shots and the temperature of the substrate. We obtained monocrystalline Nd:YLF films on the YLF substrates as testified by the comparison between the film and the bulk fluorescence spectra following excitation around 800 nm, an example of which is shown in fig. 1 where the spectra corresponding to the emission from the ⁴F₃/2 level to the ⁴In_/2 one are plotted both for the bulk crystal (gray line) and for the film (black line). The latter was obtained by ablating a monocrystalline 1.5 % at. Nd:YLF crystal with 355 nm laser pulses from a Nd:YAG laser whose duration was 13 ns at 10 Hz repetition rate. The ablation beam was focused to an 800 pm diameter; the laser fluency was set at 4 J/cm². The temperature of the subs (rale was 650 °C and the film growth was produced in a 1 Pa Ne atmosphere with 42000 laser shots. The match between the spectra is evident and the presence of well resolved, narrow lines in the film spectrum is a mark of its single crystalline nature. The lifetime of the ⁴F_3/2 manifold measured in this film gives a value (272 + 5 ps) shorter than that in the bulk (466 + 1 ps) even if the difference reduces significantly in the case of films grown with a higher laser fluency (10 J/cm²) and a shorter number of shots (12000) in UHV. By modifying some deposition parameters, the growth of films of different Nd³⁺ doped crystals such as Nd³⁺: YF₃, is also possible. A morphological study of the depositions has just begun via SEM analysis which shows a rather homogeneous distribution of the film.

© 2007 IEEE