Influence of gold nanoparticles on the nonlinear optical and photoluminescence properties of Eu₂O₃ doped alkali borate glasses

Alkali borate glasses activated with trivalent europium ions and rooted with gold (Au) nanoparticles (NPs) were synthesised through a melt quenching process involving a selective thermochemical reduction and their applicability as photonic materials was assessed in detail. Non-linear optical (NLO) measurements were performed using a Z-scan approach in the wavelength range of 700-1000 nm. The open aperture Z-scan signatures for the Eu³⁺-containing glasses embedded with and without the Au NPs established a reverse saturable absorption (RSA) at all of the studied wavelengths ascribed to the two-photon absorption (2PA). Surprisingly, the nonlinear optical absorption switched to a saturable absorption (SA) with an increase in the concentration of AuCl₃. With the incorporation of the Au NPs, the UV excited photoluminescence (PL) intensity of the Eu³⁺-doped glasses increased first as a consequence of the local field enhancement by the Au NPs, and subsequently decreased at a higher concentration of AuCl₃ due to the reverse energy transfer from the Eu³⁺ ion to the Au⁰ NPs. The electronic polarization effect of the host glass enhanced the ⁵D₀ → ⁷F₄ transition intensity on the incorporation of the gold NPs owing to the gold NP-embedded glasses showing a deep-red emission. The NLO and PL studies suggested that the investigated glasses containing a 0.01 mol% of AuCl₃ is practically appropriate for photonic applications.