Femtosecond laser writing of photonic devices in borate glasses compositionally designed to be laser writable

Abstract

The design and performance of borate glass samples compositionally pre-designed to be femtosecond laser writable via laser-induced ion migration is reported in this Letter. It is demonstrated that borate glasses modified on purpose with small amounts of LaO and NaO can be straightforwardly used to produce high-index contrast (Δn) waveguides by femtosecond-laser-assisted ion migration. The positive Δn of the waveguides is caused by the local enrichment of LaO in the guiding region with a slope of 8·10(mol.%). The value of this is consistent with numerical aperture measurements of the waveguides and local compositional measurements at the guiding region performed by energy-dispersive x-ray micro-analysis. The maximum achievable Δn values can be controlled through the initial LaO content of the glass. Maximum Δn values >10 for samples with just 5.5 mol. % of LaO have been produced. This compositional design approach could be potentially used to produce highly efficient femtosecond laser writeable glasses in other glass families.

Role of the La/K Compositional Ratio in the Properties of Waveguides Written by Fs-Laser Induced Element Redistribution in Phosphate-Based Glasses

“…written at 695 nJ in PS03. This additional migration pole has been also observed in other glasses (borates) upon waveguide writing via FLIER [17]. Figure 2a.…”

“…However, higher contents of La 2 O 3 will produce a lack of enough terminal oxygens to coordinate all La 3+ , giving rise to formation of La-O-La bonds that will affect density and refractive index non-linearly. This index modification mechanism has also been confirmed in the case of a borate glass matrix, where, due to the different initial matrix density, the local index increase versus the local La 3+ content is much stronger [17].…”

“…Indeed, the ultimate reason making different species migrate in opposite directions is far from being understood. Above a certain threshold, in the case of phosphates [7,8,16] and borates [17] that are modified with alkali and rare earth (RE) oxides, the glass former ions remain relatively stable in their positions, while alkali and RE move in opposite directions. In the case of silicates, the general trend is to observe the glass former ions to move to the hottest region of the interaction region, while modifiers move outwards to the colder one [6,13].…”

“…However, the role of the initial composition of the glass on the final performance of the waveguides that are produced by FLIER has not yet been analyzed in detail. From the early work of Brow and coworkers on La-phosphate [18] and La-Al-phosphate [19] glasses, and the works on FLIER in La-K-phosphate glasses by Fernandez, Hoyo, Moreno-Zarate et al [7,8,20], and in La-Na-borate glasses by Dias et al [17], it is more or less clear that in the 0-10 mol•% range of La 2 O 3 content in the glass, the local enrichment of a given region in La 2 O 3 should give rise to a refractive index increase nearly proportional to the induced enrichment. At such low levels of La 2 O 3 in the glass, La 3+ ions will be fully coordinated by the terminal oxygens of the Q n units that form the phosphate network [18], and the glass density will be directly proportional to the lanthanum addition.…”

Imaging Ellipsometry Determination of the Refractive Index Contrast and Dispersion of Channel Waveguides Inscribed by fs‐Laser Induced Ion‐Migration

“…It is worth noting that fs‐laser induced local modification of the glass composition has been shown to provide an extremely efficient alternative for the production of high performance passive waveguides, waveguide optical amplifiers and lasers . We have shown that in several glass families including phosphates, tellurites, and borates, it is feasible producing high contrast waveguides by inducing the migration of an index carrier element (La, Te, …) toward the guiding region by fs‐laser irradiation at high repetition rate. The migration of the carrier index element occurs along with the cross migration of a fast diffusing element, normally an alkaline (Na, K).…”