Refractive Indexes and Spectroscopic Properties to Design Er3+-Doped SiO2-Ta2O5 Films as Multifunctional Planar Waveguide Platforms for Optical Sensors and Amplifiers

Jefferson L Ferrari; Karmel de O Lima; Rogéria R Gonçalves

doi:10.1021/acsomega.0c05351

Refractive Indexes and Spectroscopic Properties to Design Er³⁺-Doped SiO₂-Ta₂O₅ Films as Multifunctional Planar Waveguide Platforms for Optical Sensors and Amplifiers

ACS Omega. 2021 Mar 25;6(13):8784-8796. doi: 10.1021/acsomega.0c05351. eCollection 2021 Apr 6.

Authors

Jefferson L Ferrari^{1

2}, Karmel de O Lima¹, Rogéria R Gonçalves¹

Affiliations

¹ Laboratório de Materiais Luminescentes Micro e Nanoestruturados-Mater Lumen, Departamento de Química, FFCLRP, Universidade de São Paulo, Av. Bandeirantes, 3900-Campus da Usp, Ribeirão Preto-SP, 14040-900 São Paulo, Brazil.
² Desenvolvimento de Materiais Inorgânicos com Terras Raras-DeMITeR, Laboratório de Materiais Fotoluminescentes (LAMAF), Instituto de Química-(IQ), Universidade Federal de Uberlândia-(UFU), Av. João Naves de Ávila, 2121-Bairro Santa Mônica, CEP, 38400-902 Uberlândia, Minas Gerais, Brazil.

Abstract

This paper reports on the news about refractive index measurements and spectroscopic features of thin films, which can be applied as optical planar waveguides, focusing on their manufacturing processes, designs, and possible applications as optical amplifiers and sensors. Er³⁺-doped SiO₂-Ta₂O₅ planar waveguides, with Si/Ta ratios of 90:10, 80:20, 70:30, 60:40, and 50:50, were prepared by a soft sol-gel process. Multilayer films were deposited by the dip-coating technique onto 10 μm SiO₂-Si (100) p-type silicon and Si (100) silicon easily and successfully. The mechanisms of the densification process, porosity, and hydroxy group or water molecule occurrence have been accompanied by m-line and vibrational spectroscopy analyses. The thickness and refractive index values were used to understand better the influence of temperature and annealing time on the densification of the bulk films and the reduction of the pore volume as the tantalum oxide concentration increases. The refractive index shows the density of the films, and by the atomic force microscopy (AFM) technique, the films showed low surface roughness, achieving relatively high light confinement within the waveguide structure, and negligible optical loss due to surface scattering. Nanoparticle crystallization of Ta₂O₅ with size distribution ranging from 2.0 to 15.0 nm embedded in SiO₂ was observed with size depending on annealing time and tantalum concentration. Intense and broadband emission positioned at 1550 nm, which is attributed to the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions, was observed for all planar waveguides under excitation at 271, 272, and 278 nm. Depending on the porosity degree, the adsorption of H₂O molecules occurs, changing the refractive index and contributing to the deactivation of excited states of Er³⁺ ions, making them an optical platform for use as an optical sensor for different species. Besides, the densified waveguides containing 20 or 30 mol % Ta exhibit high potential for applications as broadband optical amplifiers for wavelength division multiplexing (WDM), optical sensing, or augmented reality.