Faraday rotation and photoluminescence in heavily Tb(3+)-doped GeO2-B2O3-Al2O3-Ga2O3 glasses for fiber-integrated magneto-optics

Abstract

We report on the magneto-optical (MO) properties of heavily Tb(3+)-doped GeO2-B2O3-Al2O3-Ga2O3 glasses towards fiber-integrated paramagnetic MO devices. For a Tb(3+) ion concentration of up to 9.7 × 10(21) cm(-3), the reported glass exhibits an absolute negative Faraday rotation of ~120 rad/T/m at 632.8 nm. The optimum spectral ratio between Verdet constant and light transmittance over the spectral window of 400-1500 nm is found for a Tb(3+) concentration of ~6.5 × 10(21) cm(-3). For this glass, the crystallization stability, expressed as the difference between glass transition temperature and onset temperature of melt crystallization exceeds 100 K, which is a prerequisite for fiber drawing. In addition, a high activation energy of crystallization is achieved at this composition. Optical absorption occurs in the NUV and blue spectral region, accompanied by Tb(3+) photoluminescence. In the heavily doped materials, a UV/blue-to-green photo-conversion gain of ~43% is achieved. The lifetime of photoluminescence is ~2.2 ms at a stimulated emission cross-section σem of ~1.1 × 10(-21) cm(2) for ~ 5.0 × 10(21) cm(-3) Tb(3+). This results in an optical gain parameter σem*τ of ~2.5 × 10(-24) cm(2)s, what could be of interest for implementation of a Tb(3+) fiber laser.

Figure 1. Magneto-optical properties of GBAG-xTb glasses.

(a) Variation of the Verdet constant with wavelength for GBAG-xTb glasses as a function of Tb₂O₃ concentration at room-temperature. The solid lines represent a fit of the data to the power function y = a(1-x)^b. (b) Dependence of V_B on Tb³⁺ ion concentration and comparison to other reported data glasses at a fixed wavelength of 632.8 nm. (c) Van Vleck-plot of the inverse V_B (V_B⁻¹) over the square wavelength (λ²). The solid lines in (c) represents a linear fit of the data. The inset of (c) shows the value of the transition wavelength λ_t versus Tb₂O₃ concentration. In (d), the UV-VIS-NIR optical absorption spectra are given, from which the spectral MO figure of merit is obtained (shown in (e)). The inset of (d) exemplarily shows a zoom at the absorption spectrum in the spectral region of 260–550 nm for GBAG-14Tb.

Figure 2. Photoluminescence of GBAG-xTb glasses.

Static (a) PLE and (b) PL spectra, and (c) normalized dynamic decay curves of photoluminescence from GBAG-xTb as a function of Tb₂O₃ concentration at room-temperature. (d) is a zoom (by a factor of 3000) into the PLE spectra at the spectra region of 360–470 nm. (e) represents the energy level diagram of Tb³⁺. The labels in (a–b) indicate the respective band assignment.

Figure 3. Thermal properties of GBAG-xTb glasses.

(a) Physical properties density and refractive index of GBAG-xTb as a function of Tb₂O₃ concentration. (b) DSC curves of GBAG-xTb as dependent on Tb₂O₃ concentration. From (b), the variation of the glass transition temperature T_g and the onset temperature of crystallization T_c are extracted (c). (d–e) show the glass stability parameter ΔT and the apparent activation energy of crystallization, respectively, of GBAG-xTb as a function of Tb₂O₃ concentration. Solid lines are drawn as guides for the eye.