Structural environment influence on Faraday effect in Tb3+ and Pr3+ co-doped fluorophosphate glass and glass-ceramics containing TbOF nanocrystals (2023)

The Faraday effect (FE) consists in the rotation of polarization of the light across a magneto-optically sensitive medium submitted to an external magnetic field. This effect is of strong interest for optical isolators in telecommunication and high-power laser fields [1]. The Verdet constant is used to characterize this phenomenon considering the size of the magneto-optical material, the angle of rotation and the value of the magnetic field applied. The higher this Verdet constant is in absolute values, the higher the FE is. From visible to near infrared ranges, lanthanide ions are the most efficient paramagnetic elements to use. In the lanthanide family, Tb³⁺ is the most efficient ion in reason of its transition 4f⁸ $\to$ 4 f⁷ 5d, and its strong magnetic susceptibility (J = 6, g = 1.46) [2], [3].

The Faraday material mostly used commercially is a garnet single crystal composed of terbium and gallium oxides named TGG (Tb₃Ga₅O₁₂) which shows a Verdet constant of −134 rad.T^-1.m^-1 at 632 nm [4]. Some oxide glasses mainly based on silicate and germanate glass matrix have been studied with a maximal Verdet constant reported of −168.6 rad.T^-1.m^-1 at 632 nm for a borogermanate glass with 45 mol.% of Tb₂O₃ [3], [5], [6], [7], [8], [9], [10]. In our recent work on fluorophosphate glasses, a glass of composition 35 NaPO₃ – 15 BaF₂ – 50 TbF₃ has been successfully incorporated in the network without observing any devitrification, with a maximal Verdet constant measured for this glass of −80 rad.T^-1.m^-1 at 632 nm [11]. To go beyond these limits, substituting a small part of terbium by another lanthanide ion might be a promising avenue. Indeed, an increase of variable amplitude according to the selected lanthanide has been recently demonstrated in TGG single crystals by partly substituting Tb³⁺ with Nd³⁺, Er³⁺, Tm³⁺, Dy³⁺, Ce³⁺, Pr³⁺ or Ho³⁺ comparing to pure TGG single crystals. Among these lanthanide ions, the weakest effect is observed by co-doping with Nd³⁺ and Er³⁺ ions with an increasing Verdet constant of −145 and −145.3 rad.T^-1.m^-1 at 632 nm, respectively [12], [13]. An intermediate increase of the Verdet constant is then reported for Ce³⁺, Dy³⁺, Tm³⁺ and Pr³⁺ substitution in TGG single crystal with a Verdet constant of −157.3, −178.6, −178.6 and −200.1 rad.T^-1.m^-1 at 632 nm, respectively [14], [15], [16], [17]. The strongest amelioration of Verdet constant via co-doping is reported for Ho³⁺ substitution with a Verdet constant of −214.9 rad.T^-1.m^-1 at 632 nm [18]. This phenomenon is still not well understood. It is observed in a wide variety of crystalline materials as TSAG (Tb₃Sc₂Al₃O₁₂) [19], TbVO₄ [20], [21], TAG (Tb₃Al₅O₁₂) [16], [22] and Tb₂O₃ [23] based material with rare-earth co-doping. The attribution to superexchange interaction between two cationic sites of different nature is the most accepted explanation to date, but further investigation is required [12], [13], [14], [15], [16], [17], [18]. The superexchange interaction corresponds to the exchange of an electron between two cations via the orbitals of an intermediate anion. This effect is sensitive to the angle of the bond between the cations.

Recently, in our research group, the observation of this effect for three lanthanide cations Pr³⁺, Ho³⁺ and Dy³⁺ co-doped terbium fluorophosphate glasses has been reported [24]. The control of all the parameters of influence on the Faraday Effect as the temperature, the sample thickness, the magnetic field, the wavelength of measurement and the ionic density were considered to validate the co-doping origin of this Verdet constant increase. The Tb³⁺ single-doped glass sample shows a Verdet constant of −81.9 rad.T^-1.m^-1 at 600 nm while Pr³⁺, Ho³⁺ and Dy³⁺ co-doped glass sample exhibit respectively a Verdet constant of −84.2, −84.5 and − 84.5 rad.T^-1.m^-1 at 600 nm. However, the increase observed seems to be lower in glassy materials compared to single crystal. This effect have also been reported in germanate glasses co-doped with Tb³⁺ and Ho³⁺ with a maximal Verdet constant value of −172.65 rad.T^-1.m^-1 at 633 nm [25].

To help understanding the origin of this phenomenon, which is closely related to the lanthanide cation local environment and inter-cationic distances, the controlled crystallization of co-doped glasses of appropriate compositions could provide useful information. Indeed, previous studies carried out on rare-earth co- or triply- doped transparent oxyfluoride (including fluorophosphate) glass-ceramics have evidenced the influence of heat-treatment on the distribution of rare-earth ions and their integration within the nanocrystalline phase [26], [27], [28], [29], [30], [31], [32]. More recently, glass-ceramics with V₂O₅ nanorods show the influence of atomic environment on Faraday effect properties [33].

In the first part of this work, the crystallization process of fluorophosphate glasses singly doped with terbium of composition law 35 NaPO₃ – 15 BaF₂ – 50 TbF₃ is investigated. For an additional comprehension of co-doping effect, as the main difference between glass and single crystal is related to the local environment of cations, the controlled crystallization of these materials could provide crucial information. The fine control of the crystallization process is a key point to reach a maximal crystalline fraction and a minimal impact on the optical material transparency, which are the two most important parameters to monitor the structural modification of lanthanide environment and measuring the Faraday Effect, respectively. This crystallization process will be investigated by thermal, optical, structural and morphological analyses. In a second part, fluorophosphate glass-ceramics elaborated from glass with the following composition law 35 NaPO₃ – 15 BaF₂ – 0.5 ((100-x) TbF₃ – x PrF₃) with x = 0, 1, 2, 3, 4 and 5 are investigated. The influence of praseodymium on the crystallization process and the final properties of the glass-ceramics is discussed. Then, the magneto-optical properties of glass and glass-ceramics from same composition are compared.