Pr3 +–Yb3 +‐codoped lanthanum fluorozirconate glasses and waveguides for visible laser emission

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Abstract

Pr3 +–Yb3 +‐codoped fluoride glass waveguides have been synthesized by Physical Vapor Deposition (PVD). A study of the evaporation of ternary mixture of rare earth fluorides LaF3–PrF3–YbF3 has been necessary to control the doping of the evaporated glass. Optical and spectroscopic studies have been performed in both bulk and waveguide configuration. Red, orange, green and blue emissions in Pr3 +–Yb3 +-codoped lanthanum flurozirconate glasses called ZLAG have been investigated, by exciting in the blue or in the infra-red at 980 nm. Bulk samples with different dopant concentrations (0.25–3 mol% for Pr3 + and 0–5 mol% for Yb3 +) have been studied in order to optimize the Pr3 + emission. It has been shown than the luminescence is similar in bulk and waveguide upon excitation at 980 nm.

Highlights

► We fabricated Pr3 +–Yb3 +‐codoped fluoride glass waveguides by PVD. ► The blue, green, orange and red luminescence of Pr3 + was studied by direct and indirect pumping schemas. ► The up-converted luminescence is similar in bulk and waveguide upon excitation at 980 nm.

Introduction

Frequency up-conversion of infra-red to visible light in rare-earth (RE) doped materials has attracted much interest because of the large number of potential applications already known. One of major interests is the operation of visible laser pumped with infra-red commercially available diode laser. The fluoride glasses are well adapted for this kind of application, because of their wide optical transmission from 0.2 to 8 μm, their low phonon energy (600 cm 1) and the ability to accept high RE content [1]. In particular, the Pr3 +–Yb3 +‐codoped fluoride system is interesting, since laser action in the blue, green, orange and red have been obtained by up-conversion in ZBLAN glass fiber, BaY2F8 and LiYF4 hosts using the same pumping scheme at 850 nm [2]. An energy level diagram is shown on Fig. 1; the interesting properties of the system are the strong infra-red absorption of Yb3 +, the efficient energy transfer (ET) from Yb3 + to Pr3 + [3], [4] and the low non-radiative probability for the 1G4  3F3,4 transition of Pr3 +.

Among the different fluoride glasses with different network formers the new family of lanthanum fluorozirconate glass (ZLAG and ZLA) [5], [6] excluding glass modifier (i.e. NaF, BaF2) emerged as a promising host since this glass is also the precursor of transparent glass ceramics, obtained after an adequate thermal treatment. These glasses are characterized by a high LaF3 content (23 mol%) in comparison to the well known ZBLAN or ZBLA fluoride glasses with 5 mol% LaF3 [7], [8]. Last but not the least, RE‐doped ZLAG glasses (with RE = Er, Yb, Ce…) have been obtained as thin films and deposited on CaF2 and SiO2/Si substrates [9], [10].

Thin film waveguides made from up-converting fluoride glasses may offer a way to combine the attractive aspect of light producing single crystal or fibers while offering a very compact light source. Moreover, the ability to deposit these waveguides on semiconductor substrates would allow also direct integration with the pump source. In this scope, the ZLAG glass is a fluoride material of main interest. Prior to this work, other techniques have been used to obtain RE-doped fluorozirconate glass planar waveguides: ionic F/Cl exchange for Pr3 +-ZBLA [11] and pulsed laser deposition for Pr3 +-ZBLAN [12]; only the ionic exchange gave waveguides with the required optical properties.

In this paper, we present the optical and spectroscopic characterizations of Pr3 +–Yb3 +-codoped ZLAG bulk glasses for visible emission of Pr3 + in the blue, green and red; direct pumping in the 3P2 level of Pr3 + and undirect pumping via ET are compared. The results of the first blue and infrared pumped Pr3 +–Yb3 +-codoped glassy waveguides that produce visible light are also reported.

Section snippets

Bulk glass synthesis

Two series of bulk ZLAG glass — with composition:

  • 70ZrF4(23.5  x)LaF30.5AlF36GaF3xPrF3 with 0 < x < 3 mol%

  • 70ZrF4(23  y)LaF30.5AlF36GaF30.5PrF3yYbF3 with 0 < y < 5 mol%

were prepared as follows; stoichiometric quantities of high purity fluorides (purity > 99.9%) – 4 g – were melted at 875 °C for 15 min in inert atmosphere. The temperature was shortly taken to 900 °C before casting into a brass mold preheated at 240 °C. The heating at 900 °C reduces the viscosity of the melt while the short duration of this step

Bulk glass synthesis

Table 1 gathers the thermal characteristics of the glasses; the Tg slightly increases with YbF3 content. The synthesis of ZLAG glass with high YbF3 content (> 5 mol%) was unsuccessful, probably because La3 + and Yb3 + ions are too different in atomic radius. In order to improve the glass stability, we added small quantity of glass modifier BaF2 to ZLAG composition. To find the optimal composition that gives ability to prepare transparent GC, we chose to mix the ZLAG glass with ZBLA glass (57ZrF4

Conclusion

We have investigated the visible luminescence in Pr3 +–Yb3 +-codoped lanthanum fluorozirconate glasses. Blue, green, orange and red emissions have been obtained by direct pumping in the blue or through frequency up-conversion by using a diode laser operating at 980 nm. Our result show that energy transfer occurs from Yb3 + to Pr3 + after absorption of Yb3 + at 980 nm. Planar waveguides of 2 μm thickness have been fabricated by PVD with Pr3 +–Yb3 +‐codoping. Visible emissions similar to the bulk have been

Acknowledgments

We are grateful to Jean-Luc Adam and Virginie Nazabal (Sciences Chimiques Rennes — Equipe Verres et Céramiques, Université de Rennes) for making available their spectroscopic facilities, and Melinda Olivier for her help in lifetime measurements.

The technical assistance and the special skill of Alessandro Carpentiero (IFN-CNR) are gratefully acknowledged.

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    On the other hand, the relationship between structural and optical properties has been investigated by different authors [28,29] focusing on up-conversion (UC) mechanisms of Er3+ and Er3+-Yb3+ doped β-NaGdF4 GCs; they concluded that the UC luminescent intensity of the glass ceramic is several times stronger respecting to the precursor glass due to the incorporation of Er3+/Yb3+ ions into the nanocrystalline phase. Among lanthanide ions, Pr3+ is an important optical activator which offers the possibility of simultaneous blue, green, and red UC emissions for laser action as well as infrared (IR) emission for optical amplification at 1.3 µm [30–32]. Moreover, the Yb3+–Pr3+ system has received large attention because of the efficient energy transfer processes from Yb3+ to Pr3+ ions which give rise to up-conversion luminescence and/or IR emission by quantum cutting processes [33–37].

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1

Laboratoire de Physico-Chimie des Matériaux Luminescents, UMR CNRS 5620, Université Claude Bernard Lyon1, bâtiment Alfred Kastler, 10 rue Ada Byron, 69622 Villeurbanne cedex, France.

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