Abstract
Fluorozirconate glasses, such as ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF), have a high infrared transparency and large rare-earth solubility, which makes them an attractive platform for highly efficient and compact mid-IR waveguide lasers. We investigate the structural changes within the glass network induced by high repetition rate femtosecond laser pulses and reveal the origin of the observed decrease in refractive index by using Raman microscopy. The high repetition rate pulse train causes local melting followed by rapid quenching of the glass network. This results in breaking of bridging bonds between neighboring zirconium fluoride polyhedra and as the glass resolidifies, a larger fraction of single bridging fluorine bonds relative to double bridging links are formed in comparison to the pristine glass. The distance between adjacent zirconium cations is larger for single bridging than double bridging links and consequently an expansion of the glass network occurs. The rarified glass network can be related to the experimentally observed decrease in refractive index via the Lorentz-Lorenz equation.
Highlights
The unique 3-dimensional and rapid prototyping capabilities of the femtosecond laser direct-write technique [1] have drawn much attention from a diverse range of fields like telecommunication [2], quantum photonics [3], microfluidics [4], astrophotonics [5] and waveguide lasers [6]
In phosphate glass, the change in refractive index was referred to a change in phosphorus-oxygen bond length and thereby densifaction/rarefaction of the glass network when using high repetition rate pulses [16], whereas after low repetition rate exposure, an increased number of Q1 phosphorus tetrahedra was found, that was assigned to a change in the polarizability of the glass network [17]
In this paper we report, to the best of our knowledge, on the first study on the morphology of fs laser induced structural modifications in bulk fluorozirconate glass (ZBLAN, ZrF4-BaF2LaF3-AlF3-NaF) using Raman microscopy
Summary
The unique 3-dimensional and rapid prototyping capabilities of the femtosecond (fs) laser direct-write technique [1] have drawn much attention from a diverse range of fields like telecommunication [2], quantum photonics [3], microfluidics [4], astrophotonics [5] and waveguide lasers [6]. A crucial requirement for the fs laser direct-write technique is a deeper understanding of the structural modifications induced by the fs laser pulses within the bulk dielectrics. This enables the optimization of the fabrication parameters and thereby the device performance [7], and allows for materials to be tailored to offer specific properties for the direct-write process [8]. In phosphate glass, the change in refractive index was referred to a change in phosphorus-oxygen bond length and thereby densifaction/rarefaction of the glass network when using high repetition rate pulses [16], whereas after low repetition rate exposure, an increased number of Q1 phosphorus tetrahedra (one bridging oxygen [20]) was found, that was assigned to a change in the polarizability of the glass network [17]
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