What does this research mean for the field?

Femtosecond laser direct writing in aluminogermanate glass enables low-loss, programmable phase control for mid-IR optics with high transmission values and stable writing conditions. Novelty: ClaimNovelty.NOVEL_FINDING. Consensus alignment: ConsensusAlignment.NEUTRAL.

What question did this study set out to answer?

This research aims to explore femtosecond laser processing in mid-IR aluminogermanate glass to enhance optical properties.

February 19, 2026Photonics0 citationsOpen Access

Processing Maps and Nano-IR Diagnostics of Type I Modifications in Mid-IR Germanate-Based Optical Glass

PMPaul MathieuUniversité Paris-Saclay NSNadezhda ShchedrinaUniversité Paris-Saclay FBFlorence de la BarrièreUniversité Paris-Saclay

Key Points

This research aims to explore femtosecond laser processing in mid-IR aluminogermanate glass to enhance optical properties.
Mapped the processing landscape for laser writing in aluminogermanate glass.
Used quantitative-phase microscopy to measure phase changes and refractive index variations.
Employed transmission spectroscopy to assess optical losses across the near-IR and mid-IR spectrum.
Utilized FTIR to analyze structural changes in the glass.
Investigated local properties with s-SNOM to confirm changes in permittivity.
Identified different modification regimes: no modification, Type I index increase, and spatial broadening.
Achieved around 90% transmission up to 4 µm with low loss characteristics.
Observed a redshift in the Ge–O stretching envelope correlated with refractive index changes.
Confirmed local permittivity alterations consistent with densification-driven Type I modifications.

Abstract

Mid-IR flat/integrated optics require low-loss, programmable phase control. We investigate femtosecond laser direct writing (FLDW) in aluminogermanate glass (Corning 9754), first mapping the processing landscape to delineate no modification, Type I index increase, and spatial broadening regimes. We then operate in a non-accumulating regime that provides a broad, stable writing window. Quantitative-phase microscopy yields Δφ and a monotonic Δn with optically limited cross-sections compatible with low loss. Transmission spectroscopy shows high values (about 90% up to 4 µm) and no additional absorptions across the near-IR and mid-IR range. FTIR reveals a redshift of the Ge–O–(Ge/Al) stretching envelope from ≈1 µJ, correlating with the high Δn onset. s-SNOM at 925 cm−1 resolves the written line as reduced near-field amplitude and decreased phase, confirming a local complex permittivity change consistent with densification-driven Type I tracks. Together, these results define practical conditions for on-demand mid-IR flat/GRIN/Fresnel optics by FLDW in this commercial mid-IR transparent glass.

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