Designer Glasses — Future of Photonic Device Platforms

Introduction

A tightly focused femtosecond laser can induce a localized and permanent refractive index change in a wide range of transparent dielectrics. Ultrafast laser inscription enables the creation of integrated photonics circuits providing true rapid prototyping functionality. 

Glasses are often the substrates of choice. While they are easily accessible and have high optical quality, ultrafast laser inscription in these materials is often slow. Multicomponent silicate glasses, on the other hand, allow much faster inscriptions with ultrafast lasers at high repetition rates, but the optical quality of these glasses is often inadequate because they are contaminated with iron. 

This work attempts to provide a complete picture of the dynamics and mechanisms of waveguide formation during high-repetition femtosecond laser irradiation by shedding light on why multicomponent glasses are so well suited for the ultrafast laser inscription process. Waveguide formation in four glass families (borosilicate, aluminosilicate, boro-aluminosilicate, and soda-lime silicate) using fourteen different commercial glasses was studied. Based on these results, an optimized glass composition was devised.

Methodology

All waveguides in this study were written using a 5.1 MHz high-repetition-rate Ti:sapphire chirped-pulse femtosecond oscillator laser emitting 50 fs pulses and operating at a wavelength of 800 nm. Circularly polarized laser pulses were focused inside the glass using an Olympus UPLANSAPO 100 × oil immersion microscope objective (NA = 1.4). Waveguides were written at a depth of 170 μm using a set of 3-axis computer-controlled high-precision Aerotech air-bearing linear stages. The nominal composition of commercial glasses was determined using X-ray fluorescence (XRF) analysis.

All glasses were investigated under identical experimental conditions. Identical 30 μm diameter structures were inscribed in all glasses by adjusting the pulse energy for each feed rate.

The designer glasses were fabricated using the conventional melt-quench technique. The glasses were melted at a temperature of 1650 °C in platinum crucibles. Post waveguide inscription, the glass samples were cut to size and their end-face were ground and polished to optical quality. Images of the end-faces were recorded using an Olympus IX81 inverted optical microscope in differential interference contrast (DIC) mode. SEM imaging and electron probe microanalysis (EPMA) mapping of ion migration was carried out on a field-emission analyzer. 

Results

Ion migration in low aluminium and aluminium-free BK7 glass

Even though circular waveguides were formed, as shown in Figure 1, all the 30-μm diameter waveguides exhibited cracks around their periphery. Densified zones (positive index change) were found for >200 mm/min, whereas the first concentric ring had low density (negative index change). This behavior was reversed as soon as the feed rate dropped below 100 mm/min. 

EPMA showed that the positive index core at a feed rate of 500 to 3000 mm/min was mainly due to the migration of alkali. To our knowledge, this is the first report in which alkali forms a positive index core.

The inversion of migration directions of glass constituents for identical focusing conditions by just changing feed rate, as observed in BK7, challenges and adds new information to two existing theories on ion migration, which are 1) glass forming elements (silicon) migrate to the focal point (highest temperature) and 2) SiO₂ migrates to hot (cold) side in SiO₂-rich (poor) compositions due to an existing negative (positive) Soret coefficient and the reversal of the trend happened around 20 mol%.

Ion migrations in aluminium-rich silicate glasses

Alkali rich glasses 

For alkali-rich glasses, Asahi Dragontrail, Corning Gorilla-3, and Schott Xensation-3D glasses were selected. The backscattered electron (BSE) images of structures written between 200–3000 mm/min revealed a core that has a complex combination of high and low-density zones despite featuring an exclusive positive refractive index contrast.

For the alkali-rich but alkaline earth-deficient glasses Gorilla 3 and Xensation 3D, the positive index core produced at high feed rates was primarily associated with a change in polarizability due to non-bridging oxygen (NBOs) atoms. Dragontrail's composition was relatively rich in alkaline earth, which resulted in a densified high index core through the accumulation of alkaline earth albeit with a low index contrast. Alkali and alkaline earth are both glass modifiers.

Alkali-free, alkaline earth boro-aluminosilicate glasses 

The morphology of high-alkaline-earth-containing glasses with relatively low aluminium concentrations, such as high-barium 7059F and AF45, stood out from other high-alkaline-earth-content glasses. These glasses exhibited a vertically elongated, inhomogeneous core. Enrichment of alkaline earth elements in the core was common to all alkaline earth glasses, resulting in densification and an increased refractive index, while silicon migration to the low-index and low-density regions surrounding the positive-index core was observed.

Key factors for sustaining a positive index core with circular morphology

Aluminium tends to migrate towards densified zones in peraluminous glass compositions. For the alkaline earth boro-aluminosilicate glasses, a non-circular waveguide core was observed for AF45 and 7059F, whose Al:[AE+Alk] ratios are 0.376 and 0.4, respectively. Aluminium reverses its trend in these peralkaline compositions, where it migrates to low-density zones and is no longer a refractive index provider for wave guidance.

Dragontrail, Gorilla 3, and Xensation 3D glasses have ratios of 0.215, 0.405, and 0.41, respectively. The main physical property that is influenced by the Al:[AE + Alk] ratio is a sharp change in viscosity. The viscosity is at its maximum when the Al:[AE + Alk] molar ratio is 0.5, where there is a perfect balance between Al and charge-compensating modifiers. 

When the composition is in the peralkaline domain, the waveguide-forming region undergoes a sharp decrease in viscosity with high gradients owing to the segregated migration of element species. However, there was only a minor variation in the viscosity of the peraluminous composition. It could be that the former situation causes waveguide formation in Gorilla 3 and Xensation 3D in a more unpredictable fashion, depending on where aluminium migrates to. It is known that lowering the SiO₂ content reduces the viscosity of the glass melt at a particular temperature. Hence, during laser inscription, strong viscosity gradients owing to peralkaline compositions at lower viscosities produce highly asymmetric waveguide morphologies in AF45 and 7059F.

It was determined that the presence of aluminium in alkaline-earth boro-aluminosilicate glasses prevented the depletion of calcium and other alkaline earth elements in the core, possibly because of the additional role of Al as a charge compensator. 

Designer glass

The following formula (all in wt. %) for alkali-alkaline earth aluminosilicate glass was proposed:

 (100-(1+a₁+b₁)∙x) SiO₂∙(x) Al₂O₃∙(a₁∙x)MO∙(b₁∙x) R₂O 

Where MO is the alkaline earth metal oxide and R₂O is the alkali metal oxide. The preferred composition of Al₂O₃ was selected to be between x = 15–25 wt%. 

Two different compositions were prepared, based on calcium and barium. The composition of the glass was 57SiO₂-25Al₂O₃-9Na₂O-9MO where M was replaced by Ca and Ba. Glasses were fabricated using the conventional melt-quenching technique. The 30 μm structures were fabricated within these glasses, and the morphological effect of an increase in the Al:[AE+Alk] ratio value was observed in the DIC image (Fig. 2) as the composition from lighter Ca to heavier Ba was changed. The barium-based glass exhibited a more circular morphology within the same inscription window as compared to the keyhole morphologies of AF45 and 7059F.

Conclusion

The aluminium to alkaline earth + alkali ratio was found to be the prime parameter controlling the formation of circular and positive index waveguide cores. Aluminium, the sole glass intermediate in all of the silicate glasses investigated in this study, was found to assume the role of a glass former in peraluminous glass compositions, which tends to migrate towards the densified/positive index core exhibiting a contrary behavior in peralkaline compositions. Aluminium was also found to be the key element for maintaining a positive-index core and waveguide circularity. The results further show that the presence of alkaline earth elements as well as the addition of alkalis triggers densification in the core region. Heavier alkaline earth promotes an increase in the refractive index while ensuring that a peraluminous glass composition helps the waveguide core to maintain the desired circular morphology.

Reference

Fernandez, T. T., Gross, S., Privat, K., Johnston, B., Withford, M., Designer Glasses—Future of Photonic Device Platforms. Adv. Funct. Mater. 2022, 32, 2103103 https://doi.org/10.1002/adfm.202103103